Spin Engineering of Dual‐Atom Site Catalysts for Efficient Electrochemical Energy Conversion

Abstract

Dual‐atom site catalysts (DASCs) provide more advantages than single‐atom systems in improving energy conversions, owing to their unique features. For example, the coupling effect may align the spin of two adjacent dual‐atom active centers in parallel or antiparallel via electron exchange interactions, thereby altering reaction mechanisms and overall efficiency. While numerous reviews have explored spin‐dependent electrocatalysis, there remains a lack of a comprehensive, spin‐focused framework for understanding the catalytic behavior of DASCs. This review emphasizes the role of spin in dual‐atom site centers for electrocatalysis research. First, spin fundamentals in electrocatalysts, including spin‐selective orbital occupation, spin ordering, and spin coupling, are comprehensively summarized to provide a solid foundation for subsequent discussions. Then, spin engineering strategies of DASCs are reviewed, including manipulating the spin configuration of the central atoms, modulating coordination environments, and tuning metal–support interactions. Next, recent developments in spin engineering of DASCs are reviewed, with a focus on structure–performance relationships. Furthermore, high‐throughput screening techniques integrated with machine learning are discussed for developing highly efficient DASCs based on spin engineering. The challenges and opportunities of DASCs and spin engineering are thoroughly discussed to promote the advancement of new energy applications.

This review highlights recent progress in spin engineering of dual‐atom site catalysts (DASCs), emphasizing how spin‐related properties enhance electrocatalytic activity, selectivity, and stability. It summarizes cutting‐edge developments in dual‐atom catalysis, discusses the underlying spin‐catalysis mechanisms and structure–performance relationships, and proposes high‐throughput screening strategies to guide the rational design of spin‐optimized DASCs for small‐molecule conversion reactions.

1. Introduction

Technologies for renewable energy conversion involve a broad range of chemical transformation processes, enabling sustainable and environmentally benign pathways for resolving the quandaries of energy paucity and environmental deterioration.^{[ 1 ]} Among these technologies, electrochemical conversion processes, have the potential to couple with renewable energy sources, enabling conversion of H₂O, O₂, CO₂, and N₂ into high‐value‐added products. These processes encompass systems including fuel cells and metal‐air batteries, associated with oxygen reduction (ORR), oxygen evolution (OER), and hydrogen oxidation (HOR) reactions,^{[ 2 ]} as well as water electrolysis (comprising OER and hydrogen evolution reaction HER),^{[ 3 ]} carbon dioxide reduction (CO₂RR) and nitrogen/nitrate reduction (NRR/NO₃ ^‐RR).^{[ 4 ]} At present, the most efficient platforms for such conversions rely predominantly on precious metal electrocatalysts from the platinum group (e.g., Pt, Ir, Ru).^{[ 5 ]} However, the limited natural abundance, high economic cost, susceptibility to poisoning by small organic molecules, and poor durability of Pt‐group precious metal catalysts severely impede large‐scale commercial application of these electrocatalytic energy conversion technologies.^{[ 6 ]} Therefore, developing high‐performance, cost‐effective alternatives with reduced or no Pt content has become essential for scaling these technologies.

Single‐atom catalysts (SACs), characterized by isolated metal atoms dispersed on supporting matrices, can not only reduce the cost by decreasing the amount of metal required for catalytic reactions but also provide more efficient reaction pathways due to the combined effect of their electronic structures and unsaturated coordination environments. These features help stabilize metal elements in specific oxidation states and spin configurations,^{[ 7 ]} thereby achieving functions that are not available in traditional homogeneous catalysts (molecular complexes) and heterogeneous nanoparticle catalysts.^{[ 8 ]} Although SACs possess uniform and well‐defined active sites, making them an ideal platform for activity and selectivity modulation,^{[ 9 ]} they face inherent limitations in optimizing the adsorption of all reaction intermediates. These challenges arise from linear scaling relationships (LSR) from a thermodynamic perspective, which has severely impeded the application of SACs in catalyzing reactions involving multi‐molecule cooperative activation and intermediates (such as ORR, CO₂RR).^{[ 10 ]}

To overcome the limitations of SACs, dual‐atom site catalysts (DASCs), have been introduced, featuring two adjacent metal atoms anchored on supports to form synergistic bimetallic centers. This configuration not only integrates the advantages of SACs and traditional nanoparticle systems but also enhances catalytic efficiency through cooperative metal–metal interactions.^{[ 11 ]} Additionally, the dual‐atom configuration provides greater flexibility in tuning electronic structures and spin states, thereby performing independent regulation of reaction intermediates and enhancing catalytic activity.^{[ 12 ]} In comparison with SACs, the advantages of DASCs can be summarized as^{[ 13 ]} 1) increased active sites: adjacent active sites provide additional positions for molecular activation and intermediate adsorption, allowing for the selective activation of different reaction intermediates and facilitating multiple reaction pathways; 2) breaking the LSR limitation: bimetallic sites enable independent tuning of the adsorption and binding energies of reactants and intermediate products, allowing for LSR decoupling; 3) prominent spin effect: electron exchange interactions between dual atom sites can align the spins of the adjacent active centers either parallel or antiparallel, allowing for precise control over the reaction pathways. Particularly, the significant role of spin effect in the thermodynamics and kinetics of various electrocatalytic processes has gained increasing recognition in the last decades.^{[ 14 ]}

A landmark contribution in this area was made by Shao‐Horn et al., who identified a “volcano‐like” correlation between OER activity and the electron population in the e_g orbitals across a wide range of perovskite oxide electrocatalysts.^{[ 15 ]} The underlying principle is that moderate adsorption is optimal for catalytic activity following the Sabatier principle, while the e_g orbitals are responsible for forming chemical adsorption with intermediates or reactants. Thus, the filling degree of the e_g orbitals determines the strength of adsorption/desorption, which depends on the spin configuration of active sites, rather than just the oxidation state. This work opened a new chapter in the study of spin effects in catalysis performance improvement, leading to a surge of subsequent research not only on the OER but also on various other electrocatalytic processes, such as ORR and CO₂RR.^{[ 16 ]} For example, Zou et al. applied straightforward adsorption–oxidation technique to immobilize Fe³⁺ species onto ultrathin TiO₂ nanobelts. This approach effectively modulated the spin state (adjusting e_g filling approximately to be 1.08) and strengthened the binding of oxygen intermediates.^{[ 17 ]} Consequently, the prepared atomically dispersed Fe³⁺ catalyst demonstrates excellent OER performance. Zhang et al. strategically developed a bimetallic Fe, Mn/N‐C catalyst with atomically isolated active centers.^{[ 18 ]} The neighboring Mn‐N motifs served to activate Fe³⁺ sites, facilitating an ideal electronic configuration in the FeN₄ structure characterized by a single e_g electron (t_2g ⁴e_g ¹). This spin configuration enabled the Fe center to efficiently interact with antibonding π‐orbitals of oxygen molecules, thereby achieving remarkable ORR performance in both acidic and alkaline conditions. Similarly, Wu et al. synthesized N‐doped carbon nanosheets featuring ultrathin, defect‐rich morphology, and axial N‐coordinated Fe‐N₅ sites.^{[ 19 ]} The axial N coordination altered the crystal field and local symmetry, weakening spin polarization at Fe centers. This adjustment enhanced the activation of adsorbed intermediates and improved dynamic equilibrium between COOH^* and CO^* species, leading to notable catalytic performance in CO₂RR. These selected examples illustrate that tuning the spin configuration and e_g orbital filling has become one of the most effective strategies and frequently employed strategies in enhancing the catalytic efficiency of various electrocatalytic processes.

Regulating spin states has also proven to be a highly effective approach in SACs, although it presents distinct challenges and opportunities compared with other methods. For instance, owing to the tunable nature of the atomic coordination and electronic configuration of SAC active sites,^{[ 20 ]} the inclusion of spin‐responsive centers is essential to sustaining their catalytic efficiency, especially in reactions involving spin‐flip mechanisms. Anchoring single metal atoms with unpaired electrons in their orbitals onto supports, such as carbon‐based materials,^{[ 8} , ^{21 ]} metal oxides,^{[ 10 ]} or transition metal dichalcogenides,^{[ 22 ]} yields a subclass of SACs termed single‐atom spin catalysts (SASCs), which are characterized by stable, well‐defined spin‐active sites effective in driving catalytic processes.^{[ 23 ]} SASCs can effectively surmount the spin‐forbidden reaction barriers, and transform inactive reactants into a spin‐active state, thereby facilitating spin‐related catalytic processes.^{[ 23 ]} Nevertheless, many catalytic reactions require the spins of neighboring atoms to be parallel or antiparallel. For instance, in OER, it is desired that the spin electrons of adjacent atoms are arranged in the same direction to facilitate the generation of triplet O₂.^{[ 3a ]} In SACs, where individual atoms are isolated without neighboring atoms, this spin alignment effect may not be fully reached. However, this becomes feasible in DASCs. For DASCs, in accordance with the Goodenough–Kanamori rule, the spin electrons of two adjacent metal atoms tend to be arranged in either parallel or antiparallel configurations, providing us with more opportunities to meet this spin‐related requirement.^{[ 24 ]} Consequently, spin‐related effects are expected to play an even more crucial role in DASCs than previously recognized. However, research examining the catalytic implications of spin behavior in DASCs remains relatively nascent. Prior reviews have primarily focused on one of these dimensions: 1) spin effects, highlighting advances in spin‐mediated catalysis, particularly in oxygen‐involved reactions,^{[ 14} , ^{15 ]} including mechanisms such as metal–ligand bond modulation and improved intermediate adsorption; and 2) catalyst optimization around transition metal oxides and SACs,^{[ 25 ]} summarizing synergistic dual‐atom effects such as d‐band tuning, spin modulation, and electronic coupling.^{[ 20} , ^{26 ]} At present, a comprehensive, spin‐focused framework for understanding the behavior of DASCs remains lacking.

Therefore, this review aims to elucidate the critical role of spin effect in dual‐atom catalytic centers during electrocatalysis. We place particular emphasis on understanding the key aspects of spin, including spin‐selected orbital occupation, spin ordering, and spin‐orbit‐charge‐lattice coupling. Based on this, spin regulation strategies of DASCs are expounded in detail, such as directly regulating the spin states of the central atoms, regulating via coordination environments, as well as regulating metal‐support interactions. A meticulous discussion on the latest progress of spin engineering of DASCs in electrocatalytic reactions is presented to reveal the structure‐activity relationship, encompassing ORR, OER, HER, HOR, CO₂RR, NRR, and NO₃ ^‐RR (Figure 1 ). Subsequently, high‐throughput screening techniques integrated with machine learning (ML) are discussed for developing highly efficient DASCs based on spin engineering. Finally, the existing challenges and emerging opportunities associated with DASCs and spin engineering in electrochemical energy conversion reactions are systematically proposed and discussed.

Figure 1.

Outline of the spin‐dependence in various electrocatalytic reactions.

2. Spin Related Physical Properties in Electrocatalysts

An electron has an inherent attribute of spin, and research on electron spin has been a frontier direction in the field of material science and other interdisciplinary fields.^{[ 27 ]} The establishment of the correlation between spin and electrocatalysis hinges upon three prominent theoretical frameworks: the molecular orbital theory (MOT) based on chemical bond theory, the ligand field theory (LFT) applied to transition metal compounds and complexes, and the crystal field theory (CFT).^{[ 28 ]} Unlike precious metal catalysts, which preponderantly display a closed‐shell electron structure, 3d metal complexes, such as those centered around iron, exhibit a relatively small crystal field splitting energy.^{[ 29 ]} This endows them with a propensity to form open‐shell structures and, moreover, facilitates the formation of complex potential energy surfaces via spin transition. Such complexes thus possess the potential to exhibit reactivity profiles that deviate substantially from those of traditional closed‐shell catalysts, thereby portending revolutionary progressions in the catalysis domain.^{[ 30 ]} Recent progress, both theoretical and experimental, has confirmed that spin effect substantially impacts the thermodynamics and kinetic aspects of electrochemical transformations. Accordingly, a deep understanding of the spin effect of open‐shell catalysis is critical for rational catalyst engineering in spin‐dependent electrochemical reactions. The development timeline of research on spin‐dependent electrocatalysis is shown in Figure 2 .

Figure 2.

Timescale for the recent development of spin catalytic correlation in electrocatalysis.

The discovery of the electron spin phenomenon can be historically traced back to 1922,^{[ 31 ]} when a series of atomic beam experiments first suggested its existence, followed by validation via the relativistic quantum framework in 1928.^{[ 32 ]} A pivotal moment came in 1984, when a definitive link was established between the theoretical construct of spin and its catalytic implications,^{[ 33 ]} demonstrating that enhanced OER activity in perovskite materials corresponded to elevated electron populations in the Mz‐OH antibonding orbitals. Subsequently, Berdinsky et al. further explored the theory and quantitative kinetics of spin catalysis in chemical reactions.^{[ 34 ]} This finding later supported computational modeling efforts to evaluate correlations between ORR performance and magnetic properties such as local moments.^{[ 35 ]} A descriptor based on e_g orbital filling was subsequently introduced, revealing a volcano‐like trend for oxygen intermediate adsorption at B‐site positions in perovskites.^{[ 27 ]} Because the e_g orbital can directly hybridize with oxygen species through σ bonds, its occupancy level serves as a direct indicator of OER performance. Similarly, in the case of ORR, both e_g orbital filling and the covalent bonding strength between B‐site transition metals and oxygen influence catalytic activity.^{[ 14} , ^{36 ]} Since 2015, spin catalysis has entered a developmental phase. Specifically, spin catalysis has been applied in the OER and ORR.^{[ 37 ]} During this period, the significance of e_g orbital occupancy in electrocatalytic OER/ORR was further solidified.^{[ 38 ]} Concurrently, strategies for spin‐state modulation were systematically proposed,^{[ 39 ]} while externally applied magnetic fields were harnessed to enhance water electrolysis efficiency.^{[ 40 ]} After 2019, research on spin catalysis entered an explosive period. The spin‐pinning effect,^{[ 2} , ^{41 ]} spin polarization with or without an externally applied magnetic field,^{[ 42 ]} spin coupling,^{[ 43 ]} and chiral‐induced spin selectivity,^{[ 44 ]} spin channels^{[ 45 ]} have been successively spotlighted and widely applied in different catalytic materials (such as ferromagnetic single atoms) and various electrocatalytic reactions (such as the NRR, NO₃ ^‐RR, and CO₂RR).^{[ 23} , ^{46 ]} An integrated understanding of these mechanisms is vital for decoding the complexity of spin‐governed electrochemical catalysis.

2.1. Spin‐Selected Orbital Occupation

Recent advancements in spin‐dependent electrocatalysis have predominantly concentrated on transition metal‐based catalysts, particularly cations with partially filled d orbitals. When the metal d orbitals interact with different ligands under the influence of electric field generated by the surrounding ligands, the fivefold degenerate d orbitals split into several groups of orbitals with different energy levels, namely $d_{xy},d_{xz},d_{yz},d_{z^2}$, and $d_{x^2-y^2}$.^{[ 47 ]} Generally, the electric fields formed by ligands are not spherically symmetric; they can have octahedral, tetrahedral, square planar, and other geometries. Depending on the negative charge of the coordinating ligands, as well as their spatial orientation relative to the d orbitals of the metal, the degree of orbital splitting varies. The ligands directly aligned with the d orbital orientation have a stronger repulsion, and the energy level of the d orbital will increase significantly. However, some ligands are not directly in line with the d orbital orientation, resulting in weaker repulsion and a smaller increase in energy level.^{[ 48 ]} In the case of octahedral coordination, the five d orbitals split into two high‐energy e_g orbitals ($d_{z^2}$ and $d_{x^2-y^2}$) and three low‐energy t_2g orbitals (d_xy , d_xz , d_yz ) (Figure 3a). The energy gap between these sets is termed crystal field splitting energy (ΔCFSE). When a metal center forms a complex and contains three or fewer d electrons, these electrons populate the t_2g orbitals with parallel spins, in accordance with Hund's rule and the Pauli exclusion principle. For systems with more than three electrons, whether the additional electrons pair within the t_2g orbitals or occupy the high‐energy e_g orbitals (n > 6) first depends on the competition between ΔCFSE and the electron pairing energy. If pairing energy is lower, a low‐spin state forms, with electrons filling t_2g first; if pairing energy is higher, electrons occupy the e_g orbitals before returning to t_2g, yielding an intermediate or high‐spin state.

Figure 3.

Spin‐selected orbital occupation. a) Illustration of how octahedral Co³⁺ ions yield different spin states through orbital population. b) Diagram showing the electron density distribution in an octahedral crystal field. c) Relationship between OER performance and e_g orbital occupation in B‐site transition metals of ABO₃ perovskites. Reproduced with permission.^{[ 27 ]} Copyright 2011, Science. d–f) Examples of e_g orbital filling in perovskite structures and associated catalytic activity. Reproduced with permission.^{[ 29 ]} Copyright 2017, Science.

The e_g orbitals display a more advantageous vertical alignment in contrast to the t_2g orbitals (Figure 3b), presenting enhanced orbital overlap or interaction with relevant adsorbates (e.g., with the O 2p orbital).^{[ 49 ]} Consequently, the occupancy of the e_g orbitals determines the binding strength to reactants or intermediates. According to the Sabatier principle, overly weak binding makes it difficult to activate reactants, while overly strong binding hinders the desorption of products.^{[ 5a ]} A moderate binding energy enables the catalyst to achieve the optimal reaction rate. Hence, e_g occupation is a critical factor in optimizing catalyst performance. In a systematic study spanning numerous transition metal oxides, Shao‐Horn et al. first revealed a volcano‐type relationship linking OER activity with e_g orbital occupation in transition metal cations.^{[ 15a ]} They demonstrated that OER efficiency peaks when e_g electron number approaches one, reflecting enhanced covalency in the metal–oxygen bond (Figure 3c–f). This established e_g occupancy as a dependable activity descriptor for oxygen‐related electrocatalysis.^{[ 15b ]}

The occupancy of e_g orbitals functions serves as a descriptor providing a straightforward metric for assessing catalytic activity. Researchers have exerted significant effort to optimize the activity of transition metals by modulating e_g occupancy of electrocatalysts via diverse strategies for prospective commercial utilization. For example, doping is employed to modify the crystal field and the oxidation state of active site ions (at the atomic/electronic scale), as well as to control the morphology and nanostructure of transition metals (at the microscopic scale). These modulation techniques optimize electrocatalytic activity by delicately adjusting the spin state of transition metal cations, inducing different e_g orbital occupancies. According to crystal field theory, the 3d orbital configuration and the spin state of Fe in FeN₄ structures can be influenced through coordination with ligands of varying field strengths, where strong‐field ligands (ethylenediamine (EN) > triethylamine (TEA) > thiocyanate ion (NCS^‐)) and weak‐field ligands (OH^‐ > F^‐ > Cl^‐ > Br^‐ > I^‐) exert different effects. Based on this principle, Sun et al. applied a set of ligands with varying field strengths for axial coordination with the Fe‐N₄ in poly(phthalocyanine iron).^{[ 50 ]} Axial coordination not only altered the 3d orbital arrangement of Fe but also lowered its orbital energy, leading to a high‐spin state. These results highlight the essential role of spin‐selected orbital occupation in electrocatalysis and demonstrate the value of tuning spin states to enhance catalytic performance.

2.2. Spin Ordering and Polarization

Spin ordering arises from exchange interactions between adjacent electron spins, typically mediated by direct or superexchange interactions, which can induce magnetic coupling between atomic sites. Direct exchange originates from the quantum mechanical exchange integral between overlapping wavefunctions of neighboring magnetic atoms, favoring either parallel or antiparallel spin alignment depending on the sign of the exchange energy. In DASCs, the two metal centers can exhibit various spin alignment patterns depending on their electronic configuration, spatial distance, and the surrounding ligand environment.^{[ 51 ]}

According to the alignment patterns of these spin ordering in long range, several types can be identified, such as ferromagnetic (FM) ordering (where the entire spins are aligned in parallel, possessing a strong spontaneous magnetization, Figure 4a), antiferromagnetic (AFM) ordering (where two adjacent sets of spins are aligned antiparallel, with opposite directions but equal numbers, and a net spontaneous magnetization equal to zero, Figure 4b), paramagnetic ordering, characterized by randomly oriented spins due to thermal agitation (Figure 4c); ferrimagnetic ordering, spin sets are oppositely aligned but unequal magnitudes, yielding a net magnetic moment (Figure 4d); and nonlinear spin arrangements like helical, sinusoidal, or conical patterns, which do not conform to simple parallel or antiparallel structures.

Figure 4.

Spin ordering. Diagrams illustrating a) ferromagnetic, b) antiferromagnetic, c) paramagnetic, and d) ferrimagnetic orderings in electrocatalysts.

In spin‐dependent electrocatalysis, the ordering of spins plays a crucial role in altering reaction pathways to lower energy barriers and thereby enhancing electrocatalytic performance.^{[ 52 ]} For example, Xu et al. revealed a spin‐pinning effect in cobalt‐based oxyhydroxides by implementing surface reconstruction on FM cobalt oxides.^{[ 41 ]} This spin‐pinning effect originates at the interface between the FM oxide core and the surface‐reconstructed oxyhydroxide layer. The interface stabilizes spin ordering within the active oxyhydroxide phase, and even a weak external magnetization can further enhance the effect. As a result, spin polarization of oxygen intermediates is strengthened, lowering the energy barrier for the O─O coupling step during the OER, ending up with significantly higher intrinsic activity than directly synthesized cobalt (iron) oxyhydroxides.

Beyond these long‐range spin alignments, the electrocatalytic activity of active sites is also influenced by spin polarization, the imbalance in spin‐up and spin‐down electronic states near the Fermi level, which directly governs the spin‐selective electron transfer. Wu et al. introduced compressive strain to adjust the spin polarization at Fe sites in FeN₄, thereby increasing its spin density.^{[ 53 ]} The high‐spin state of Fe and the shortening of Fe─N bonds contributed to the formation of broader spin‐associated channels that enhance charge mobility during the ORR. As Fe─N bonds contract, the spin density within FeN₄ increases significantly, indicating a stronger high‐spin configuration. Additionally, the spin‐orbit interaction between Fe and O₂, leading to higher bond order, allows the FeN₄ active centers with shorter Fe─N bonds to more effectively capture O₂ molecules. Zhao et al. theoretically proposed that FeNi dual‐atom sites display softened spin‐polarized conductive electrons.^{[ 42a ]} By establishing a bimetallic Fe‐Ni center and leveraging the interaction between their spins, the spin polarization in the 3d orbitals of Fe can be reduced. Furthermore, hydroxyl modification at the active center aids charge delocalization and facilitates improved electron transport, thereby contributing to enhanced ORR performance. Li et al., through theoretical screening, developed an Fe/Zn─N─C DASC and identified a distinctive half‐metallic electronic structure, arising from the cooperative behavior between Fe and Zn.^{[ 54 ]} The addition of nonmagnetic Zn²⁺ altered the electronic properties of Fe─N─C, inducing a transition from semiconducting to half‐metallic character and promoting spin‐polarized electron occupation near the Fermi level (E _f ), enhancing the center's capability to adsorb and activate O₂. These findings further substantiate the importance of spin ordering in regulating spin‐influenced catalytic processes.

2.3. Spin Orbit Coupling

In spin‐related electrocatalytic reactions, SOC and quantum spin exchange interaction (QSEI) are significant factors causing the overall alteration spin‐related properties of the catalyst (Figure 5 ). SOC profoundly influences the electronic properties of materials.^{[ 55 ]} It arises from the interaction between an electron's orbital magnetic moment, due to its movement around the nucleus, and its intrinsic spin magnetic moment. Disregarding the magnetic term leads to the formulation of the SOC effect. A central feature of SOC is that, even in the absence of an external magnetic field, an electron moving in an electric field will experience a momentum‐related magnetic‐field‐like effect, and this equivalent magnetic field interacts with the electron spin magnetic moment. Transition metal complexes are ideal models for studying SOC catalysis. These complexes usually possess different orbital and spin symmetries, enabling large orbital magnetic moments that facilitate variations in orbital angular momentum during spin‐state transitions in catalytic systems, thereby enhancing the SOC effect and overcoming spin‐forbidden limitations.^{[ 56 ]} For low‐cost transition metal oxides (TMOs) used for OER, the direct electron contribution from the M‐O band beneath the E _f causes M‐O bond instability, limiting performance improvements in TMOs.^{[ 57 ]} Given the tunability of the 4f level relative to E _f , variable 4f variable valence states, and strong SOC, rare earth (RE) elements are considered promising in forming electron‐donating buffer bands during the OER. Constructing RE‐O‐TM coordination motifs may help maintain TM‐O covalency while shifting electron depletion toward higher RE‐4f energy states.^{[ 58 ]} Fu et al. developed a cerium single‐atom cobalt oxide catalyst (P‐Ce SAs@CoO) through plasma‐assisted synthesis.^{[ 59 ]} The active interface formed by Ce(4f)‐O(2p)‐Co(3d) presented a finely tuned Co‐3d‐e_g orbital. Gradient‐level orbital coupling improved Co─O bond strength, suppressed bond breakage at the Co─O─Ce sites, optimized intermediate adsorption, and delivered favorable overpotential and electrochemical stability. This work offers mechanistic insight into designing RE‐TMO catalysts and demonstrates a viable strategy for f‐p‐d orbital coupling to enhance both activity and durability in spin‐dependent electrocatalytic systems.

Figure 5.

Spin‐orbit coupling. Spin textures of a) the Rashba SOC and b) the Dresselhaus SOC. c) Band splitting caused by Rashba or Dresselhaus SOC.

QSEI stems from the overlap of two‐electron wave functions. This overlap results in Coulomb interaction due to their charges and enables SOC‐driven exchange interactions between the electrons.^{[ 60 ]} When extended to the spatial domain, orbital ordering in coordinated FM materials becomes beneficial in modulating the binding energy of reactants through QSEI in spin‐selective steps. Therefore, QSEI plays a vital role in reducing electron repulsion within the d orbitals at catalytic active sites, thereby enhancing the spin‐mediated electron mobility.^{[ 61 ]} Specifically, QSEI can lower Coulombic repulsion between electrons with the same spin alignment,^{[ 62 ]} helping stabilize the catalyst structure, decrease the adsorption/desorption energy barrier at the surface, and reduce the activation energy required for bond cleavage or formation. The enhanced ORR activity seen in magnetic Pt alloys like Pt₃Co compared to Pt/C, is attributed to favorable oxygen intermediate chemisorption on the alloy surface. Here, QSEI and the interlayer magnetic coupling introduced by the cobalt‐rich interior layers are key factors that promote optimal binding of adsorbed oxygen at active centers. Graci et al. conducted theoretical modeling of AFM and FM Pt₃Co(111) nanostructures to assess these effects.^{[ 63 ]} Their calculations showed that chemisorption enthalpy values for O^* and H^* atoms on stable AFM (type a) and FM layers were lower than those for conventional Pt(111), suggesting weaker binding. The presence of cooperative spin potential from open‐shell orbital configurations contributes to reduced adsorption enthalpies. This highlights the need for precise evaluation of orbital magnetism when analyzing structure–activity relationships in heterogeneous catalysts. Spin‐dependent potentials are essential design parameters, and harnessing magnetically polarized orbitals in stable, strongly correlated systems presents promising avenues for improving catalytic activity and reducing nanomaterial costs.

2.4. Spin‐Charge‐Lattice Coupling

In inorganic solid‐state catalysts, factors such as active site density, intrinsic conductivity, and energy barriers for reactions are fundamentally tied to the material's lattice, charge distribution, and spin characteristics.^{[ 64 ]} Charge transfer and spin states evolution are inherent to electrocatalytic processes.^{[ 14d ]} Among them, charges distribution within particular spin states can facilitate the activation and adsorption of reactants, thereby accelerating reaction kinetics.^{[ 65 ]} Lattice coupling behavior indirectly affects charge mobility by modifying the structure and altering electron cloud overlap among atoms.^{[ 66 ]} For example, lattice stretching or compression may change the orbital hybridization of atoms, thereby affecting the delocalization of electrons and having a significant impact on electrocatalytic activity.^{[ 67 ]} The spin‐charge coupling can effectively alter the electronic structure and promote catalytic reactions. According to Dai et al.,^{[ 68 ]} it has been demonstrated that when heteroatoms are doped into carbon materials, spin polarization occurs in adjacent carbon atoms, causing changes in the distribution of their electron clouds. This affects the charge transfer path and efficiency and further alters the activity and selectivity of the catalyst. In OER, spin‐polarized electrons contribute to triplet O₂ formation through QSEI mechanisms.^{[ 42b ]} Additionally, modulating the band structure enhances both conductivity and reactant adsorption via improved charge transfer behavior. Energy transfer is also facilitated by spin‐charge coupling, enabling matching of spin states between catalysts and reactants, which is an important requirement for efficient catalytic progression.^{[ 14e ]} During this process, charge movement coincides with energy flow, which collectively reduces activation barriers and enhances overall catalytic effectiveness. In a recent study employing artificial intelligence‐assisted analysis, Jiang et al. observed that the interaction between Fe single‐atom catalysts anchored on C₂N substrates and O₂ molecules led to a shift in the spin magnetic moments of both Fe and O₂.^{[ 69 ]} This effect reflected electron transfer from Fe to O₂. Through intentional control of spin magnetic moments, the study revealed that catalytic behavior and spin interactions are governed by the redistribution of Fe d orbitals and the π‐bond molecular orbitals of O₂.

In electrocatalytic reactions, there exists a synergistic effect between spin and lattice. When the spin state changes, it may trigger minute lattice distortions.^{[ 70 ]} Conversely, lattice vibrations or structural alterations can also influence the spin state. This interdependence contributes to refined control over the adsorption and desorption of intermediate species during electrocatalytic steps. For instance, in reactions involving multi‐step electron transfer,^{[ 71 ]} the spin‐lattice synergy can ensure the smooth progression of reaction steps, preventing over‐adsorption or difficult desorption of intermediates, thereby enhancing the selectivity and stability of electrocatalysis. Lattice vibrations,^{[ 72 ]} acting as energy carriers, are capable of interacting with spins, transferring energy to reactants or extracting energy from the reaction system. This coupling aids in fine‐tuning activation energies and promotes faster reaction kinetics. In complex reactions such as the OER, properly engineered spin‐lattice coupling can reduce the reaction overpotential and increase energy conversion efficiency. Luo et al. introduced 3d transition metals into the CoOOH lattice,^{[ 73 ]} resulting in a CoMnOOH phase with enhanced lattice distortion. This structural modification minimized the energy separation between the d_xy and d_z ² orbitals, facilitating electron excitation into the e_g orbital, enabling spin state tuning, lowering the rate‐determining step (RDS) barrier, and improving OER kinetics. Maintaining structural integrity during harsh electrocatalytic conditions is essential, and lattice coupling plays a critical role in ensuring long‐term catalyst stability. The resilience of the lattice often dictates whether the catalyst retains its activity over time. The coupling interaction between spin and lattice can enhance durability by stabilizing the electron configuration and mitigating structural disruptions caused by charge movement. Sun et al. utilized the strong Jahn‐Teller effect of the e_g orbitals of Cu²⁺ to effectively induce and optimize the spin and electronic structure of Fe³⁺,^{[ 74 ]} achieving a transformation of the catalyst from ferrimagnetic to ferromagnetic. This facilitated O─O bond formation and accelerated the conversion of OH^‐ to O₂, endowing the Cu₁‐Ni₆Fe₂‐LDH catalyst with excellent stability and OER performance. Based on current research, it is evident that for the coupling of spin‐charge‐orbit‐lattice, adjusting any one of these parameters will trigger changes in the others. This coupling characteristic provides more degrees of freedom for spin regulation in the meticulous tuning of catalysts and the optimization of electrochemical catalytic reactions.^{[ 75 ]}

2.5. Fundamental Factors Influencing Spin States and Spin Descriptors

A review of developments related to spin‐dependent electrocatalysis reveals that the spin intrinsic of electrons should be carefully considered as the spin configuration of active sites has shown its importance in determining overall catalyst performance. The primary determinants of spin states of an electrocatalyst include the following: 1) Atomic and electronic structure. The electron arrangement within an atom or molecule plays a decisive role in determining its spin state. The presence of unpaired electrons contributes to spin generation, and the specific distribution and occupancy of electrons across orbitals determine the overall spin quantum number (S). 2) Crystal field effects. In crystalline environments, the electric field produced by nearby ligands or ions alters the electron orbital structure of central atoms,^{[ 24b ]} thereby influencing electron spin states. For example, in octahedral versus tetrahedral fields, d‐orbital splitting in transition metal ions varies significantly, producing distinct spin configurations. 3) Exchange interaction. Spin orientation is strongly affected by the exchange interactions between electrons in neighboring atoms or ions.^{[ 76 ]} In ferromagnetic systems, this interaction promotes parallel alignment of spins, resulting in collective magnetic behavior. 4) External conditions. Parameters such as temperature, pressure, and magnetic fields also influence spin arrangements.^{[ 77 ]} Thermal agitation can disrupt spin alignment; pressure can modulate the spin‐coupling strength by compressing the lattice and altering orbital overlap; and an externally magnetic field can induce spin polarization directly. 5) Electron correlation effects. In systems with strong electron correlation, Coulomb repulsion plays a dominant role in spin configuration, for example, in the spin‐ordered phases of Mott insulators.^{[ 78 ]} Additionally, structural features such as lattice symmetry or the presence of dopants, vacancies, or other defects can also affect spin behavior.^{[ 43} , ^{79 ]}

Spin descriptors are quantifiable parameters used to define and track spin states. The most frequently used descriptors include: 1) Spin multiplicity,^{[ 80 ]} represented by S, is used to describe the spin‐related properties of particles and is a fundamental quantum number. For example, singlet states, triplet states, etc., which directly reflect the number of unpaired electrons. 2) Magnetic moments,^{[ 69 ]} includes both atomic and total magnetic moments. The atomic magnetic moment arises from the spin and orbital angular momentum of unpaired electrons, whereas the total magnetic moment reflects macroscopic magnetization. 3) Spin polarizability,^{[ 81 ]} that is, describes the imbalance in spin‐up and spin‐down electron density near the Fermi surface. It is key for understanding spin‐related transport and magnetic properties. 4) Spin state,^{[ 82 ]} categorized into high‐spin, low‐spin, or intermediate‐spin configurations, depends on the interplay between crystal field strength and external perturbations. It is widely used in transition metal complex chemistry. 5) d‐band center,^{[ 67b ]} represents the average energy position of d‐orbital electrons and is closely associated with the electronic structure, chemical reactivity, and magnetic properties of the material. This value can be obtained via theoretical tools like density functional theory (DFT) or experimental methods such as photoelectron spectroscopy. Typically, a d‐band center positioned closer to the E _f correlates with enhanced catalytic activity, due to improved electron transfer and orbital overlap with reactant species. 6) The strength (λ) of SOC,^{[ 83 ]} such as Rashba and Dresselhaus effects, alter spin relaxation rates and influence electronic band structures. 7) Topological spin structures,^{[ 44b ]} including Skyrmions or spin waves, require advanced descriptors and models. Overall, spin regulation arises from a dynamic interplay between electron‐electron interactions, crystal field environments, and external perturbations. Quantitative evaluation of spin descriptors is vital for guiding the targeted development of spin‐functional materials in electrocatalysis.

3. Spin Engineering of DASCs

Research on DASCs can be traced back as early as the 1950s. In the wake of continuous technological innovation and the rapid advancement of surface chemistry, this field has now entered a phase of intensive exploration.^{[ 84 ]} The spin state of DASCs exhibits a profound correlation with its catalytic activity. Modifying the spin configuration at bimetallic centers can directly alter d‐orbital electron occupation, thereby influencing the binding strength and energy transfer between the active sites and intermediates. However, precise control over the interatomic distance between metal atoms and their surrounding coordination environment remains highly challenging. Unlike well‐defined ligand fields in molecular complexes, the coordination environment in supported catalysts lacks strict symmetry. Moreover, the electronic properties of the metal centers depend not only on their bonding with the support but also on the spacing between adjacent atoms. In this section, based on the electronic structure characteristics of DASCs, the spin engineering of these complexes is systematically and comprehensively summarized from three core dimensions: metal type and interatomic distance (central atom regulation), coordination environment, and metal‐support interactions.

3.1. Regulation of the Spin‐Related Properties of the Central Atoms in DASCs

The unique structural configuration and atomic spacing of dual‐metal centers significantly affect its spin‐related factors, which subsequently dictate the electronic structure and electrocatalytic activity of the catalyst.^{[ 13a ]} Therefore, we first discuss the arrangement of metal centers. Based on the metal active site configuration, DASCs can be divided into three groups: combination of two distinct single‐atom sites (Figure 6a), homonuclear DASCs (Figure 6b), and heteronuclear DASCs (Figure 6c,d).

Figure 6.

Classification and characteristics of dual‐atom site catalysts.

3.1.1. Combination of Two Distinct Single‐Atom Sites

This class of catalyst comprises randomly dispersed single‐atom species of two different metals across the support, referred to here as the combination of two independent single‐atom active sites. There is no strict spatial constraint on the separation between the two atoms (M and M′), making it difficult to form close‐proximity bimetallic configurations. However, owing to long‐range electronic interactions between the metal centers, such DASCs can still exhibit significant superiority over SACs in electrochemical energy conversion reactions.

Regarding synthesis, these configurations are more readily prepared than homonuclear or heteronuclear dual‐atom constructs. The prevalently adopted synthesis method is the wet chemical method, similar to that of SACs.^{[ 7a ]} Two metal precursors are mixed with carbonaceous materials or carbon‐containing precursors, and subsequent to high‐temperature carbonization treatment, the target product can be obtained. The two metals are randomly dispersed on the support, and it is difficult to generate dual‐atom configurations. Wang et al. first utilized DFT simulations to propose that a Pt‐N₄ site located near an FeN₄ center could facilitate O₂ activation,^{[ 85 ]} modify Fe d and O p orbital rehybridization, improve oxygen intermediate adsorption, and accelerate overall ORR kinetics. Based on the theoretical calculation, the authors adopted a spatial confinement strategy to encapsulate in situ the Fe and Pt source in the nanocavities of metal–organic frameworks (MOFs). After high‐temperature carbonization, Fe‐N₄/Pt‐N₄ units were uniformly anchored within a three‐dimensional porous carbon matrix, resulting in a composite denoted as Fe‐N₄/Pt‐N₄@NC (Figure 7a). Atomic‐resolution high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) revealed numerous isolated bright spots of different contrast across the carbon matrix. Due to the difference in atomic number, brighter spots corresponded to Pt atoms and dimmer ones to Fe atoms (Figure 7b,c). The intensity profile of HAADF‐STEM confirmed that the Fe and Pt atoms were typically spaced at ≈5.3 Å apart (Figure 7d,e). As a result, the Fe‐N₄/Pt‐N₄@NC catalyst exhibited an ultra‐high half‐wave potential of 0.93 V, outstanding electrocatalytic activity, enhanced durability, and excellent tolerance to methanol crossover—outperforming both commercial Pt/C and most reported SACs under similar conditions.

Figure 7.

Combination of two distinct single‐atom sites. a–e) Synthesis and characterization of Fe‐N₄/Pt‐N₄@NC. Reproduced with permission.^{[ 85 ]} Copyright 2021, Wiley‐VCH. f–i) Synthesis and characterization of Cu‐Co/NC. Reproduced with permission.^{[ 86 ]} Copyright 2023, Wiley‐VCH.

Li et al. employed a similar methodology.^{[ 86 ]} Copper acetate was incorporated into a Zn‐Co MOF with a Zn:Co molar ratio of 9:1. This composite was then coated with a N‐rich polymer. Subsequent high‐temperature pyrolysis under an inert atmosphere led to the acquisition of a Cu‐Co DASC (Cu‐Co/NC) (Figure 7f). Characterization by aberration‐corrected scanning transmission electron microscopy (AC‐STEM) and energy‐dispersive X‐ray spectroscopy (EDS) mapping revealed that most Cu and Co atoms appeared in paired distributions on the porous N‐C substrate (Figure 7g–i). Spectroscopic analysis of the N K‐edge and C K‐edge confirmed that the presence of Cu and Co modified, the electronic environments of N and C, with N predominantly forming metal‐pyridinic‐N bonds. Experimental observations combined with theoretical computations illustrated that the synergistic effect between Cu‐Co bimetallic sites possessing a metal‐N₄ coordination structure could instigate an asymmetric charge distribution, thereby conferring a moderate binding strength for oxygen intermediates and enhancing adsorption and desorption kinetics. As a result, the catalyst displayed excellent ORR/OER performance under both acidic and alkaline conditions. The structure and chemical makeup of the support can significantly influence the coordination configuration of the embedded metal atoms, thereby tuning the electrocatalytic behavior. Building on this concept, Zhao et al. exploited an unconventional approach, namely, the chemical transformation of Mn_0.43Fe_2.57O₄ nanocrystal supercrystals.^{[ 87 ]} These supercrystals, formed through self‐assembly of oleic acid‐capped Mn_0.43Fe_2.57O₄ nanocrystals, served as a precursor to synthesize Mn‐Fe‐N/S@mC. Upon annealing at 500 °C in argon, the oleic acid ligands were carbonized into conformal interconnected carbon shells. Subsequently, acid etching removed the core nanocrystals, leaving Fe and Mn atoms uniformly embedded in the newly generated carbon framework. A second heat treatment with thiourea at 900 °C produced the final Mn‐Fe‐N/S@mC catalyst, uniformly distributed over an ordered mesoporous graphitic carbon network. By varying the nanocrystals used in the assembly process, the composition of the anchored bimetallic centers could be tuned. Notably, the cooperative interaction between Fe and Mn resulted in a fivefold enhancement in catalytic performance, primarily due to the decreased energy barrier for OH^* reduction during the ORR. This efficiency boost was directly attributed to the dual‐metal site synergy.

However, when the spatial separation between two metal centers becomes too large, it poses considerable challenges in modulating the spin configuration of the active sites. The primary limitations and potential strategies for improvement include the following: 1) Diminished long‐range synergistic effects. Excessive site separation leads to a significant reduction in electron coupling or magnetic interactions, thereby weakening spin regulation efficiency. To address this, one potential remedy is the incorporation of π‐conjugated systems or magnetic carriers, which can enhance long‐range electron and spin coupling. 2) Challenges in dynamic spin synchronization. Under reactive conditions, spatially distant metal centers may respond independently to local variables such as pH or potential, making synchronized spin alignment (e.g., ferromagnetic or antiferromagnetic coupling) difficult to maintain. To address this, designing symmetric ligand environments can help synchronize spin‐state transitions. Alternatively, applying external stimuli such as magnetic or light fields may induce ordered spin configurations, supporting dynamic spin synchronization. 3) Carrier‐mediated effects are difficult to control. When spin or electron communication between distant sites relies on carrier materials (e.g., graphene, MXene), imperfections such as defects or uneven doping can interrupt these channels and impair spin polarization. This limitation can be mitigated by constructing conductive pathways within the carrier structure to support electron tunneling. Additionally, utilizing structural motifs like double vacancies or 5–8–5 ring defects in graphene can fix inter‐site spacing and strengthen electronic coupling, thereby enabling more precise control of spin transport. 4) Spin instability during synthesis and operation. Uneven atomic distribution can lead to abnormal local electron cloud density, which affects the stability of the spin configuration. This challenge can be addressed by tightly controlling the atom deposition sequence and concentration, thereby promoting uniform distribution of dual‐atom sites. Techniques such as atomic layer deposition and molecular beam epitaxy are particularly suitable. Surface modification methods, including the application of protective layers or promoters, may also enhance resistance to environmental perturbations and improve spin configuration preservation.

Despite the limitations associated with large inter‐site distances, it has been observed that even when two metal atoms are isolated and widely separated within a catalyst, they can influence each other through long‐range synergistic mechanisms. These include orbital hybridization, charge redistribution, and modulation of crystalline architecture, which collectively affect the spin configuration of the metal sites, While the spins of the two metal sites may not directly couple, this independence allows for greater flexibility in tuning the adsorption and desorption of catalytic intermediates, minimizes undesirable electronic interactions, and enhances adaptability toward diverse reaction pathways.

3.1.2. Homonuclear DASCs

The proposal of DASCs breaks the limitations of SACs in the concerted activation of multiple molecules and intermediates, endowing an additional degree of freedom for catalytic modulation. Homonuclear DASCs are constituted by two identical metal atom moieties. To date, three general strategies for synthesizing DASCs have been widely reported. One of the strategies involves using binuclear metal complexes as precursors. First, the precursors are anchored on the carbon support through π–π electrostatic interactions, then DASCs are obtained through pyrolysis. Xiong et al. added precursors in situ containing different numbers of Fe atoms during the synthesis of zeolitic imidazolate framework (ZIF‐8).^{[ 88 ]} As ZIF‐8 formed, the Fe precursors became embedded within its porous structure. After high‐temperature calcination, clusters with varying iron atom counts were generated, permitting the precise attainment of Fe₁, Fe₂, and Fe₃ clusters (Figure 8a). Among these, elemental distribution analysis corroborated the homogeneous dispersion of iron within the samples, with the mass fraction of iron in each of the three samples approximating 0.4%. AC‐STEM characterization could confirm the existence of Fe₂ clusters (Figure 8b). Notably, the Fe₂─N─C system exhibited the highest catalytic activity, comparable to commercial Pt/C, and demonstrated excellent durability in acidic media, exhibiting a decline of only 20 mV in half‐wave potential after 20 000 cycles. The authors employed low‐temperature infrared spectroscopy to ascertain the superoxide adsorption of O₂ at the Fe₁ site, while the adsorption at the Fe₂ and Fe₃ sites was per oxo. Compared with superoxide adsorption, the adsorption energy of oxygen was significantly enhanced during per oxo adsorption. Each oxygen atom coordinated with two iron atoms individually, and each iron atom donated a certain quantity of electrons to the p orbital of oxygen. This interaction weakened the O─O bond, thus promoting O₂ activation. This work represents the first report of a generalized synthesis strategy enabling the precise regulation of the number of Fe atoms per clusters, allowing for the precise fabrication of Fe₁, Fe₂, and Fe₃ species anchored on N‐doped carbon substrate. The enhanced electrocatalytic performance of these cluster‐based materials was attributed to spin‐charge coupling, which aligned the reaction energy at the Fe sites with that of the intermediates and changed the adsorption state on the single‐atom sites. Although this strategy offers advantages in terms of uniformity and atomic precision, the loading limitation precludes its application in reactions necessitating high throughput. At high loadings, there is a lack of space to accommodate the precursors, and aggregation occurs during the annealing process.

Figure 8.

Homonuclear DASCs. a,b) Synthesis and characterization of Fe₂‐N‐C. Reproduced with permission.^{[ 88 ]} Copyright 2019, Wiley‐VCH. c,d) Structural characterizations of Fe₂N₆. e) Proposed ORR mechanism over Fe₂N₆. Reproduced with permission.^{[ 89 ]} Copyright 2019, Elsevier. f–h) Structure of Ni₂NC. HAADF‐STEM of i) Pd₂NC, j) Mn₂NC, and k) Zn₂NC. Reproduced with permission.^{[ 90 ]} Copyright 2022, Springer Nature.

In an alternative approach derived from the SAC “bottom‐up” strategy, which begins with metal salts and organic precursors, increasing the metal loading augments the proximity among single atoms and affords the opportunity to form dual‐atom sites.^{[ 91 ]} However, the synthesized DASCs are invariably plagued by the coexistence of single‐atom and multi‐coordination configurations. A more precise strategy lies in the potential‐controlled selective adsorption. Through modulating the electrostatic interactions between the SAC substrate and the metal precursor, oxygen‐bridged DASCs can easily synthesized. Xie et al. synthesized Fe₂N₆ with a planar‐like structure by thermally migrating isolated FeN₄ units on a graphitized carbon support (KetjenBlack ECP‐600JD) (Figure 8c).^{[ 89 ]} HAADF‐STEM imaging confirmed the dense and uniform distribution of Fe atoms on the support (Figure 8d). Close‐proximity Fe atom pairs indicated the successful formation of Fe₂ dimers through thermally induced coupling of FeN₄ species. ICP‐OES analysis revealed a Fe content of 4.9 wt%. The Fe₂N₆ structure exhibited a redox transition from an initial Ox–Fe³⁺–Fe²⁺ state to a more reduced Fe²⁺–Fe²⁺ configuration. This redox behavior contributed to improved oxygen intermediate binding and facilitated O─O bond dissociation, expediting ORR kinetics while suppressing parasitic reactions (Figure 8e). This spin‐charge coupling effect (change in valence state) optimizes the catalytic reaction pathway, and the high‐density planar‐like Fe₂N₆ ameliorates the performance of PEMFCs.

DASCs bridge single‐atom and metal/alloy nanoparticle catalysts, which presents greater potential for enhancing the kinetics of electrocatalytic reactions. However, their advancement is still constrained by the ambiguous structural definition of dual‐atom configurations. Zhang et al. designed a nickel‐based Ni₂N₆ DASC via a “top‐down” approach by transforming nanoparticles into dual‐atom structures in situ.^{[ 90 ]} HAADF‐STEM characterization confirmed the formation of numerous Ni dual‐atom centers that were evenly distributed across the carbon substrate (Figure 8f). X‐ray absorption fine structure (XAFS) analysis verified that the dominant coordination structure was Ni₂N₆, confirming the successful creation of the intended dual‐site motif (Figure 8g,h). During the electrocatalytic CO₂ reduction process, the Ni₂N₆ site adsorbs OH^* (OH_ad) in the solution, forming a unique Ni₂N₆OH active center, which can effectively regulate the charge around the Ni atom and maintain the active center as an electron‐rich state. This configuration effectively tunes the local electron density around Ni atoms, preserving an electron‐rich environment at the active site. As a result, the adsorption of the key intermediate COOH^* is significantly improved, and the associated reaction energy barrier is lowered. The research team also synthesized Pd, Mn, and Zn DASCs with M₂N₆ coordination structures by employing a ligand‐mediated strategy, which attests to the generality of this synthesis method (Figure 8i–k). This method holds considerable promise for advancing the development of other transition metal DASCs and other electrocatalytic applications. Liu et al. reported a straightforward cascade DASC synthesis strategy based on precise electrostatic interaction control and adjacent vacancy construction (Figure 9a–c).^{[ 92 ]} The authors effected the bridging of additional Fe³⁺ ions with the Fe sites on Fe SAC containing N source molecules (such as melamine) through spontaneous electrostatic/complexation interactions. This is because the edge N sites in the N source molecules bear a negative charge, while Fe³⁺ ions and Fe sites possess a positive charge. In the ensuing high‐temperature annealing process, the corrosive gas NH₃ was released from the bridged N source molecules and adjacent vacancies were induced as the nearby carbon atoms were etched. These vacancies can readily capture additional Fe ions and give rise to Fe─N₂─Fe dual‐atom sites connected by two N bonds. The unique coordination structure of the Fe─N₂─Fe site optimizes the O₂ adsorption geometry, thereby reducing the O─O bond dissociation energy barrier. This strategy can be further extended to synthesize DASCs with diverse metal pairs and coordination environments, given the facile adjustability of metal precursors and bridging molecules. To further regulate the geometry of active centers and elucidate bimetallic synergy, Liu et al. also employed a pre‐encapsulation pyrolysis approach.^{[ 93 ]} A Co‐based Robson‐type dinuclear complex served as the molecular precursor for the synthesis of a dual‐Co single‐atom catalyst (Co‐DAC), wherein the Co atoms were fixed at angstrom‐scale distances (Figure 9d,e). The customized structure induces remarkable charge redistribution, which reduces the crystal‐field splitting energy. Consequently, high‐spin Co can more readily engage in the formation of σ and δ‐like bonds with oxygen intermediates (O^*), promoting flexible adsorption dynamics. Simultaneously, the enhanced Co─O SOC promotes electron transfer along the bridging‐O channel, forming a highly active Co─O─O─Co electronic chain for the synergistic adsorption of O^* and establishing a favorable reaction pathway. This study provides valuable insights into how precise geometric tailoring of active sites affects electron‐transfer behavior and catalytic efficiency.

Figure 9.

Homonuclear DASCs. a) Structural model of Fe‐N₂‐Fe. b) 3D atom‐overlap Gaussian fitting map of Fe‐N₂‐Fe. c) Atomic distance histogram of Fe atoms in Fe‐N₂‐Fe. Reproduced with permission.^{[ 92 ]} Copyright 2024, Wiley‐VCH. d) Structure of Co‐DAC. e) Aberration‐corrected HAADF‐STEM image of Co‐DAC (left). zoomed area (upper right); intensity profiles for regions D1 and D2 (lower right). Reproduced with permission.^{[ 93 ]} Copyright 2024, Wiley‐VCH.

Based on this, homonuclear DASCs have advantages in bridging the gap between single‐atom and metal/alloy catalysts, providing ample opportunities for improving electrocatalytic performance. At the same time, the latest developments also demonstrate that the precise regulation of spin states in homonuclear DASCs is an effective strategy in significantly improving catalytic activity. This has been shown to play a key role in optimizing adsorption/desorption steps and mediating the interactions between active metal sites and reactive intermediates. Different works have consistently revealed the importance of spin‐charge coupling and SOC in enhancing catalytic efficiency and stability in these homonuclear DASCs. Nonetheless, homonuclear DASCs still have some limitations in regulating the spin structure of metal centers and the catalytic reaction process. The specific deficiencies and potential strategies for improvement include the following: 1) Restricted diversity in spin state modulation. Due to the intrinsic electronic similarity of identical metal centers, the spin polarization tends to occur in a single direction, making it difficult to create energetically complementary adsorption sites. 2) Competition in charge localization across metal sites. Homonuclear metal centers tend to equilibrate their electron densities, which limits their ability to differentially activate specific intermediates. In reactions such as CO₂ reduction, this behavior diminishes the efficiency of C–C coupling. To mitigate this, introducing axial ligands (e.g., Cl^‐, OH^‐) selectively at one metal center may induce symmetry breaking. This adjustment facilitates spin splitting, thereby enabling multi‐dimensional spin control and promoting distinct charge environments at each site. The result is enhanced catalytic activity and reduced charge‐sharing competition. 3) Deficiencies in dynamic stability and SOC interference. Identical metal atoms are prone to migration or aggregation under operational conditions, particularly in reducing environments (e.g., HER) or oxidative ones (e.g., OER). In systems involving heavier metal species (e.g., Ru, Ir), strong SOC effects may trigger spin‐state transitions that deviate from adiabatic conditions, potentially compromising the continuity of the catalytic cycle. Possible remedies include stabilizing the metal centers using bidentate ligands or incorporating them into bimetallic‐organic frameworks to maintain controlled interatomic spacing. Another strategy involves introducing local strain in the catalyst support, which leverages lattice distortion to stabilize targeted spin states and preserve the coherence of catalytic function.

3.1.3. Heteronuclear DASCs

In contrast to homonuclear dual‐atoms, heteronuclear dual‐atoms consist of two disparate metal atom sites. The dual sites within heteronuclear DASCs not only manifest the synergistic effect characteristic of homonuclear dual sites but also possess adjustable cations, strong electronic coupling, high catalytic activity, and accurate catalytic selectivity.^{[ 94 ]} The presence of a second, different metal atom near a single‐atom site can cause coordination structure distortion and disrupt the local symmetry of the active center. This symmetry perturbation may lead to transitions in the spin state of the catalytic site. Heteronuclear DASC synthesis typically involves confinement or anchoring effects, enabled through techniques such as high‐temperature pyrolysis, wet‐chemical impregnation, or atomic layer deposition (ALD).^{[ 95 ]} For example, Lei et al. synthesized Fe‐Se atom pairs (Fe‐Se/NC) anchored on N‐doped carbon using a three‐step process.^{[ 96 ]} Initially, ZIF‐8, a material with a suitable pore size (3.4 Å) and cavity diameter (11.6 Å), was used to encapsulate Fe species (≈9.7 Å in size) within its porous framework. During the formation of Fe/ZIF‐8, both Fe and Zn ions were confined in the internal cavities. This intermediate was then thoroughly mixed with Se powder to ensure close proximity (Fe‐Se/ZIF‐8), followed by a high‐temperature (950 °C) treatment in argon to produce the Fe‐Se/NC catalyst (Figure 10a–d). Due to asymmetric charge distribution induced by p‐d orbital hybridization between Fe and Se atoms, the adsorption and desorption of reaction intermediates are better balanced. As a result, Fe‐Se/NC demonstrated outstanding bifunctional oxygen electrocatalytic performance among known Fe‐based single‐atom systems. Lv et al. successfully regulated the electronic structure and oxidation state by introducing Fe and P elements to form a Fe/P‐CoO₂ core‐shell structure.^{[ 97 ]} Fe doping in the shell induces a transformation of the oxidation state of Co from Co²⁺ to Co³⁺, simultaneously, while dynamic regulation of Fe³⁺ to Fe⁴⁺ enabled multivalent electronic channels, improving the catalyst's oxidative capacity. Fe doping changes the spin state of Co, thereby optimizing e_g orbital filling at Co centers and improving the adsorption‐desorption balance of oxygen species. In this catalyst, Co provides a stable coordination environment, and Fe provides the ability to regulate electrons, while the synergistic effect of the two breaks is through the activity limitations of traditional SACs. Sun et al. developed Pt‐Ru dual‐atom active sites anchored on N‐doped carbon nanotubes (Pt‐Ru dimer/NCNTs) using a sequential ALD method.^{[ 98 ]} To control the precise pairing of metal atoms, a Pt precursor (MeCpPtMe₃) was first deposited on NCNTs to form isolated Pt single atoms. Then, a second ALD step introduced the Ru precursor (bis(ethylcyclopentadienyl)ruthenium(II)), which selectively bonded to the pre‐anchored Pt atoms—thus ensuring the formation of Pt–Ru dimeric structures (Figure 10e). Although the mixed pyrolysis strategy offers simplicity and low cost, it can result in a heterogeneous mixture of isolated and dual‐atom configurations. Conversely, while the ALD method avoids the generation of single‐atom sites, the precursor preparation is complicated and costly.

Figure 10.

Heteronuclear DASCs. a–d) Synthesis and characterization of Fe‐Se/NC. Reproduced with permission.^{[ 96 ]} Copyright 2023, Wiley‐VCH. e) ALD‐based synthesis of Pt‐Ru dimers on N‐doped carbon nanotubes. Reproduced with permission.^{[ 98 ]} Copyright 2019, Springer Nature. f) Illustration of metal ion recognition (MIR) strategy for M₁M₁’‐DASCs.^{[ 100 ]} Reproduced with permission. Copyright 2024, American Chemical Society.

In summary, the limited “precision engineering” often leads to structural heterogeneity. The resulting materials typically contain a mixture of isolated atoms and various combinations of dual‐metal sites (a–a, a–b, b–b), complicating efforts to distinguish the intended dual‐atom pairs from dense atomic ensembles and potentially leading to unwanted side reactions.^{[ 99 ]} Recently, Liu et al. introduced a general metal ion recognition (MIR) approach for constructing a series of DASCs (Figure 10f).^{[ 100 ]} Heteronuclear DASCs synthesized by this strategy not only have a well‐defined coordination structure but also exhibit high reactivity and stability. Therefore, this research group utilized the MIR strategy to synthesize DASCs (M₁M₁′‐DASC). This methodology involves the thermal transformation of heterometallic dual‐complex ion pairs ([ML_x]ⁿ⁺[M′L′_y]^n‐), which are obtained by sequentially adsorbing the target cation [ML_x]ⁿ⁺ and anion [M′R′_y]^n‐ onto an NC substrate. In contrast to physically pre‐mixed metal salts, the sequentially adsorbed heterometallic dual‐complex metal‐ion pairs display a defined stoichiometry and proximity between the two metals through electrostatic interactions, ensuring the maximal likelihood of generating the desired dual‐atom sites during pyrolysis. Taking the synthesis of Fe₁Sn₁‐DASC as an example, the [Fe(bpy)₃]²⁺[SnCl₆]^2‐ (bpy = 2,2′‐bipyridine) metal ion pair was obtained by sequential [Fe(bpy)₃]²⁺ and [SnCl₆]^2‐ adsorption onto an NC support. Subsequent pyrolysis yielded Fe₁Sn₁‐DASC. The MIR strategy was further validated to be capable of synthesizing a diverse range of DASCs (M₁ = Fe, Co, Ni, Cu, Mn, and M'₁ = Fe, Co, Ni, Cu, Sn) by selecting appropriate inorganic metal cation and anion salts. The high performance of these catalysts is attributed to cooperative effects arising from neighboring metal centers. Theoretical simulations and in situ Raman spectroscopic analysis further substantiated that the outstanding catalytic activity and stability stem from the interplay between adjacent Fe and Sn atoms. The Sn site was found to reduce electron localization at Fe, thereby facilitating OH^* desorption and lowering the energy barrier for the RDS of the ORR. Concurrently, compared with Fe, the Sn site expedites the thermodynamic conversion of O^* to O₂, thereby mitigating the generation and diffusion of reactive oxygen species during the catalytic process. Together, this synergistic behavior significantly improves both the activity and robustness of the Fe₁Sn₁‐DASC system. In a complementary approach, Niu et al. employed a series of binuclear macrocyclic complexes as precursors, combining MOF encapsulation with pyrolysis to obtain well‐defined DASCs (Figure 11a).^{[ 101 ]} The high tunability of the macrocyclic precursor enabled the synthesis of both homonuclear (Fe₂, Co₂, Ni₂, Cu₂, Mn₂, and Pd₂) and heteronuclear (Fe‐Cu, Fe‐Ni, Cu‐Mn, and Cu‐Co) catalysts. TG‐FTIR‐MS confirmed that the macrocyclic framework preserved the [M₁M₂N₄O₂] unit during processing. Combined with EELS, TOF‐SIMS, and XAS, the dual‐atom pair sites and the obvious electronic interaction between the dual atoms were identified (Figure 11b–e). These two strategies for precisely constructing heteronuclear DASCs utilize the spin‐charge coupling effect to fine‐tune the electronic configuration of magnetic active centers. This approach effectively reduces energy barriers in electrocatalytic reactions and continues to gain attention as a powerful tool in catalyst design.

Figure 11.

Heteronuclear DASCs. a–e) Synthesis and structure characterizations of FeCu‐DASC. Reproduced with permission.^{[ 101 ]} Copyright 2024, American Chemical Society. f–k) Synthesis and structure characterizations of FeCo‐N₃@C. Reproduced with permission.^{[ 106 ]} Copyright 2024, Springer Nature.

The electronic characteristics and spatial configurations of metal centers significantly influence both the reaction kinetics and mechanistic pathways, while coordinated anions help stabilize intermediate species and guide the catalytic route.^{[ 102 ]} However, due to the difficulty in precisely controlling the coordination environment of two distinct metal atoms, most DASCs developed in recent years feature only one type of coordinating anion and maintain relatively symmetrical coordination. This uniformity imposes constraints on performance in terms of activity, stability, and product selectivity. To fully leverage DASCs in complex, multistep catalytic systems, two important and demanding strategies. One is to construct an asymmetric coordination structure, and adjust the electronic distribution and spatial arrangement of the active sites, thus enhancing intermediate adsorption/desorption and reducing reaction‐free energy barriers.^{[ 103 ]} The second strategy centers on using different anionic ligands for the two metals, forming Janus bimetallic sites that derive their function from diverse synergistic interactions.^{[ 96 ]} Regarding asymmetric coordination structures, several forms have been reported, including low coordination numbers, heteroatom coordination, and bimetallic sites. The bridging‐bimetallic site is a novel and typical asymmetric coordination that fully utilizes the multiple coupling interactions of spin‐charge‐orbit‐coordination (lattice). It promotes the interaction between adjacent atoms and retains the inherent catalytic performance of the metal sites. For instance, a catalyst featuring an N‐bridged Co‐N‐Ni bimetallic site could adjust the charge between Co and Ni via the N‐bridge,^{[ 104 ]} modify the protonation energy barriers of intermediates, and display potential in CO₂RR. Nevertheless, the high electronegativity of the N atom may result in insufficient intermediate adsorption strength, consequently decelerating the overall reaction rate. Hence, the selection of an appropriate bridging atom is essential for maintaining the asymmetric structure and enhancing charge redistribution between metals. Chen et al. fabricated a bimetallic site catalyst with a novel Cu‐S‐Ni bridging site (Cu‐S‐Ni/SNC) using biomass wool keratin as the support material.^{[ 105 ]} Rich in disulfide bonds, keratin allows direct incorporation of sulfur atoms during synthesis, and its semi‐ordered protein structure offers regular binding sites for metal species. The coordination environment of the S‐bridged Cu‐S‐Ni bimetallic site was characterized by X‐ray absorption spectroscopy (XAFS). The asymmetric structure and unique electronic effect endow Cu‐S‐Ni/SNC with excellent CO₂RR performance in an H‐type cell. The high performance of Cu‐S‐Ni/SNC is ascribed to the electronic regulation effect between Cu and Ni via the S‐bridge, with electrons transferred from Cu to Ni. Cu, as the main adsorption site, is dually regulated by S and Ni atoms, and the Ni and S atoms enhance the CO₂ activation and formation of the key intermediate COOH^* at the Cu site. The S‐type bridging bimetallic site presents a novel bridging architecture, offering a new avenue for effectively tuning the electronic structure and tailoring the electronic effects of bimetallic atoms. For Janus bimetallic site catalysts, the scarcity of reports on Janus DASCs with precisely defined multiple ligands can be attributed to the paucity of efficient synthesis methodologies and the formidable challenges associated with discerning and determining the precise amorphous structures of complex active sites. Wang et al. successfully fabricated a Janus FeCo‐N₃O₃@C DASC containing Fe‐Co dual sites coordinated with N and O, respectively (Figure 11f–k).^{[ 106 ]} The synthesis involved a multistep protocol. First, Fe‐N₄ single atoms (FeN₄@C) were prepared using two‐dimensional MOF nanosheets as precursors. These were treated with argon plasma to induce vacancy formation, yielding defect‐rich FeN₃@C (denoted d‐FeN₃@C). Because of the elevated surface energy resulting from heteroatom (N and O) doping, vacancies tend to preferentially form in the vicinity of Fe atoms. Finally, low‐temperature pyrolysis with Cobalt acetylacetonate was conducted at 330 °C to introduce Co‐O₃ groups into the d‐FeN₃@C. The controlled thermal treatment limited atomic diffusion, minimizing metal aggregation. Furthermore, selective adsorption at metastable defect sites enabled targeted cobalt anchoring. The resulting Janus FeCo‐N₃O₃@C displayed impressive bifunctional performance in ORR and OER. Comprehensive characterization and theoretical analysis confirmed that its exceptional catalytic activity stems from the strong coupling between Fe‐N₃ and Co‐O₃ centers. This coupling optimized the occupation of Co and Fe 3d orbitals and fine‐tuned the adsorption/desorption behavior of oxidation intermediates, thereby accelerating reaction kinetics.

In conclusion, previous works demonstrate that spin effects play a pivotal role in the catalytic performance of heteronuclear DASCs, yet it remains a complex and evolving area of research. Interactions between the spin of two dissimilar metal atoms contribute to the modulation of electron transport and intermediate binding, which in turn fine‐tunes catalytic outcomes in reactions such as ORR and OER. Strategies like sequential metal ion adsorption, defect engineering, and the use of bridging atoms have emerged as promising approaches. Despite these advances, heteronuclear DASCs still face several pressing key challenges and potential solutions are outlined below: 1) Spin state matching dilemma. The difference in the number of d electrons of different metals may lead to spin state incompatibility, weakening the synergistic effect. To address this issue, 3d‐4d/5d metal combinations can be selected to stabilize specific spin states by leveraging the strong SOC effect of heavy elements, or axial coordination can be introduced to induce spin splitting. 2) Uncontrollable charge transfer between metals. Metal pairs with large electronegativity differences are prone to the “electron siphon effect”, resulting in the deactivation of active sites. To solve this problem, charge‐balancing channels can be designed. For example, heteronuclear sites can be connected through π‐bridging ligands to establish a two‐way electron buffer and achieve charge self‐balancing. 3) Risk of dynamic structure reconstruction. During cyclic voltammetry scanning, heteronuclear metals may experience selective dissolution. The metal dissolution rate can be reduced by protecting metal migration through a double‐vacancy anchoring strategy or a core‐shell structure. Furthermore, there is still a need for more fundamental insight into spin‐related mechanisms. This can be achieved through the potential integration of physical concepts like electron exchange interactions, such as double exchange and super exchange interactions, to enhance the precision of spin engineering.

3.2. Regulation of the Spin‐Related Properties of DASCs via Coordination Environments

The physicochemical properties of atomically dispersed DASCs are highly sensitive to their coordination environment, including the coordination type, number, and shell of surrounding atoms.^{[ 107 ]} This characteristic indicates that the synthesis and physicochemical property modulation of DASCs can be achieved through coordination environment engineering. Carbon‐based materials serve as optimal substrates for the anchoring of bimetallic atoms, and their electrocatalytic performance can be regulated through doping. Axial ligands or coordinating heteroatoms such as N, O, P, and S can create electron‐withdrawing or donating conditions while donating lone pair electrons. These ligands play a significant role in modulating the spin electronic structure of the central active site. Simultaneously, by exploiting the spin‐lattice coupling interaction between the ligands and the active metal atoms, a stable coordination structure can be formed, thereby precluding the sintering and aggregation of metal atoms.^{[ 108 ]} Thus fine‐tuning ligands/coordinating species in coordination shell of active site is crucial to enhance activity.

Sun et al. successfully prepared a porous carbon‐supported Fe‐Co DASC (FeCo‐DASC) by using a pre‐anchoring strategy. The electron structure analysis shows that in FeCo‐DASCs, the spin splitting of the Fe‐orbitals is suppressed, which enhances its coupling with the Co‐orbitals. Compared with Fe‐SAC and Co‐SAC, the orbitals of both Fe and Co in FeCo‐DASCs shift to lower energy states, resulting in significant orbital coupling, effectively optimizing the adsorption strength of oxygen intermediates and leading to superior oxygen electrocatalysis. DASCs with N‐coordinated metals embedded in carbon can catalyze the ORR/OER in rechargeable zinc (Zn)–air batteries. However, they are restricted by the LSR of intermediate binding energies in the adsorption evolution mechanism (AEM).^{[ 108 ]} Triggering the lattice oxygen mechanism (LOM) holds the potential to surmount this challenge;^{[ 9 ]} however, it has remained unverified due to the lack of bridging oxygen in the rigid coordination environment of the metal center.^{[ 12 ]} To address this challenge, Song et al. customized a FeCo–N–C DASC with a unique structure (Figure 12a–d).^{[ 109 ]} First, the feasibility of selectively modulating the pyrrolic‐N site in the FeCo–N–C structure was evaluated through DFT calculations, which revealed the disparity in C‐N bond strengths between pyrrolic‐ and pyridinic‐N. Compared to the pristine FeCo–N–C, the d‐band center of FeCo–N–C with pyrrolic‐N vacancies is nearer to the E _f , favoring the ORR and OER reconstruction processes. Based on this, the author team first synthesized FeCo–N–C via pyrolysis and then utilized liquid ammonia‐assisted lithiation (LAAL) to tailor the C‐N bonds. Initially, when exposed to a liquid ammonia/Li⁺ solution (comprising Li⁺ and ammonia‐coordinated electrons, denoted as e^‐ (NH₃)_n), the Li⁺ ions could penetrate the interlayers of FeCo–N–C nanosheets, forming the intercalated FeCo–N–C/Li compounds. With more Li⁺ ions bound to the pyrrolic‐N, Li₃N would gradually form and be expelled from the FeCo–N–C matrix preferentially,^{[ 112 ]} forming N vacancies (Figure 12e). The resulting defect‐induced reduction of the Fe–N bond length improved ORR kinetics. Moreover, the pyrrolic‐N defect promoted the in‐plane oxygen reconstruction at the metal site during the OER process, triggered the LOM pathway with a lower energy barrier, and broke the LSR imposed by the AEM pathway. The symmetric structure of M‐N₄ leads to excessively high adsorption energy of ORR intermediates, restricting ORR activity. Strategies for modulating the coordination atom type and coordination environment can disrupt the electronic/geometric symmetry of TM‐N₄. Adjustments that slightly displace the metal center from the M‐N_x plane or elongate the M–N bonds lead to charge polarization and changes in the spin occupancy of metal d orbitals, thereby reducing intermediate binding energy. Furthermore, the presence of nonmetallic dopants and secondary metal atoms can alter the electronic structure of M‐N_x sites. The electronic coupling between dissimilar metal orbitals enhances charge transport and optimizes the adsorption of ORR intermediates. In this context, Wang et al. employed adenine and carboxylic acid–based ligands rich in N/O donors to synthesize a metal‐organic framework (IISERP‐MOF27) using Zn²⁺, terephthalic acid (H₂BDC), and adenine (Ad) as building blocks (Figure 12f).^{[ 110 ]} The resulting framework displayed a columnar layered architecture, where Zn²⁺, Ad, and BDC^2‐ formed two‐dimensional inorganic layers, with BDC^2‐ also extending out of the plane to create a pillar‐like structure. Fe³⁺ and Cu²⁺ ions were subsequently introduced and coordinated with –NH₂ and ─COOH functional groups on IISERP‐MOF27 to form MOF‐FeCu through a simple coordination capture method. Following pyrolysis at 900 °C, the framework was converted in situ into N‐doped carbon–anchored Fe/Cu dual atoms, referred to as FeCu‐NC (Figure 12g‐k). This catalyst features a novel and unique asymmetric FeN₃O‐CuN₄ dual‐atom coordination geometry. Cu‐N₄ can perturb the localized electrons in the Fe‐3d orbital, expedite electron transfer. The combined effect of the Fe‐Cu dual sites introduces an asymmetric electronic distribution and improves both adsorption and desorption dynamics of oxygen intermediates.

Figure 12.

Regulation of the spin‐related properties of DASCs via coordination environments. a–d) Synthesis and structure characterizations of FeCo–N–C. e) EPR spectra of FeCo–N–C with different lithiation degrees. Reproduced with permission.^{[ 109 ]} Copyright 2024, American Chemical Society. f–k) Synthesis and characterization of FeCu‐NC. Reproduced with permission.^{[ 110 ]} Copyright 2023, Wiley‐VCH. l) Illustration of one‐step pyrolysis strategy to fabricate of Ni₂‐N_xC_y.^{[ 111 ]} Reproduced with permission. Copyright 2022, Wiley‐VCH.

To optimize the electronic properties of DASCs and enhance their CO₂ electroreduction performance, Lu et al. modified the coordination environment surrounding Ni dual‐atom sites to regulate their electronic structure.^{[ 111 ]} First, the reaction raw materials were ball‐milled and mixed uniformly. Then, the pyrolysis temperature was controlled to obtain three DASCs with different Ni coordination environments (Ni₂–N₇, Ni₂–N₅C₂, and Ni₂–N₃C₄) (Figure 12l). As the temperature increased during synthesis, the N coordination number around Ni declined, and its valence state was concurrently reduced. These changes indicate that the electronic configuration of Ni‐based DASCs can be precisely modulated through coordination environment engineering. The Ni₂–N₃C₄ structure, in particular, exhibited significantly improved CO₂RR activity due to cooperative interactions between Ni atoms and a favorable balance in COOH^* and CO^* adsorption energies. Charge‐asymmetric dual‐atom sites can not only regulate intermediate adsorption energetics and disrupt outer electron symmetry at the metal centers, enhancing charge transport. Additionally, the use of N‐doped hollow carbon spheres as support materials reduces the diffusion length for reactants, improving catalytic accessibility. Pang et al. used a dimeric Fe compound, bis(dicarbonylcyclopentadienyliron) (CDD), as a metal precursor for constructing a dual‐atom Fe catalyst.^{[ 113 ]} During dopamine polymerization, CDD and thiourea were encapsulated in situ within polydopamine (PDA), forming an S/CDD/PS@PDA core‐shell structure. Upon pyrolysis, the polystyrene (PS) template decomposed, resulting in a hollow structure, while the PDA shell was carbonized into porous N‐doped carbon (N–C–H). The pre‐selection of CDD facilitated the formation of Fe diatomic sites, while the N–C–H matrix provided anchoring points to stabilize these atoms. By optimizing pyrolysis parameters, the organic ligands from the CDD precursor were completely removed, minimizing diatom aggregation. Simultaneously, thiourea aided in modulating the coordination environment, incorporating both N and S atoms into the coordination sphere. The resulting catalyst, Fe₂–S₁N₅/SNC, featured predominantly dual‐atom Fe centers, as confirmed by HAADF‐STEM imaging. Through DFT calculations, it was found that there were four highly active reaction sites in the catalyst Fe₂–S₁N₅/SNC. Its ORR activity was enhanced because of the optimized OH^* adsorption energies at the charge‐asymmetric Fe dual‐atom sites for the ORR reaction intermediate OH^*, which was beneficial to the desorption of OH^* and improved the reaction kinetics. Recent findings also indicate that incorporating S into the coordination structure can stabilize the microenvironment of metal atoms, thereby improving catalytic stability.^{[ 107 ]}

Collectively, current research underscores that designing asymmetric DASCs through coordination engineering, leveraging spin‐charge‐orbit‐lattice interactions, is among the most effective strategies for developing robust and highly active catalysts for electrochemical energy conversion.^{[ 113} , ^{114 ]} The key lies in manipulating ligand identity and geometry to modulate spin distribution at active metal centers.^{[ 115 ]} These adjustments allow for precise control over the energetics of adsorption and activation of intermediates, which in turn influences performance metrics such as catalytic activity, selectivity, and longevity. While this approach provides a targeted and controlled route for catalyst optimization, elucidating the roles of spin–charge–orbit–lattice coupling remains a challenge. Understanding how these variables interact requires advanced experimental methods and theoretical models capable of capturing subtle variations in electronic structure. Incorporating physical principles such as double exchange and superexchange could help clarify the role of spin alignment in catalytic processes and support the design of next‐generation DASCs. Thus, while modifying spin states can enhance reactivity, overmodulation may compromise structural integrity or reduce charge mobility if not carefully controlled. Another key limitation is scalability; producing uniform coordination environments across large volumes of material remains technically demanding and limits practical implementation at an industrial scale.

3.3. Regulation of the Spin‐Related Properties of DASCs via Metal–Support Interactions

Metal‐support interaction (MSI) is recognized as a fundamental factor in determining the catalytic performance of supported metal systems. MSIs are broadly classified into four types: covalent metal–support interactions (CMSI), oxidative strong metal–support interactions (OMSI), electronic metal–support interactions (EMSI), and strong metal–support interactions (SMSI).^{[ 116 ]} SMSI, the earliest and most classic form of these, was initially postulated by Tauster et al. in the late 1970s to account for the substantial reduction in the chemisorption capacity of H₂ and CO on the Pt/TiO₂ catalyst subsequent to high‐temperature H₂ treatment under reducing conditions.^{[ 117 ]} In SMSI, the strong interaction between the reduced oxide support and the supported metal facilitates electron donation from the support to the metal, thereby reducing the chemisorption capacity of H₂. Metal‐metal bonds, core‐shell interface morphologies, interface charge transfer, and mass transport constitute the prevalent explanations for the SMSI mechanism. Fundamentally, this interaction represents an electron transfer effect. The prevailing elucidation for the SMSI posits that the quantity of “unidirectional” electron exchange between the support and metal exceeds the typical electron transfer number, endowing the “interaction” with enhanced strength.^{[ 118 ]} EMSI, which exhibits the closest association with atomically dispersed catalysts, was first introduced by Rodriguez et al. in 2012 as a novel class of strong interaction, particularly within the Pt cluster‐CeO₂ support system.^{[ 119 ]} In this context, when Pt clusters are deposited on a CeO₂ surface, the interfacial electronic redistribution (defined as EMSI) notably modifies the d‐band center of the active site. Tang et al. further reported EMSI effects in various SAC systems, demonstrating that such interactions can dynamically influence the electronic state of isolated metal atoms.^{[ 120 ]} In contrast to SMSI (requiring high temperature and reducing atmosphere conditions), EMSI can be implemented under a broad spectrum of conditions (ranging from low to moderate temperatures) and is applicable to both reducing and oxidizing atmospheres. Notably, EMSI pertains to electronic coupling without necessitating new atomic‐scale structural configurations at the metal‐support interface.

For atomically dispersed catalysts, each atomic site is requisite to expedite reactions and sustain such performance over long‐term application. Presently, the cutting‐edge methodology for augmenting the catalytic performance of these atomic sites lies in modulating EMSI.^{[ 123 ]} Moreover, atomically dispersed metal catalysts offer an optimal paradigm for probing the influence of EMSI on spin state transitions. For example, Zhang et al. affixed iron phthalocyanine (FePc) molecules onto the atomic step edges of TiO₂ nanotubes (FePc@TiO₂) via physical adsorption (Figure 13a).^{[ 121 ]} The localized magnetic field at these atomic steps prompted a spin‐flip transition in Fe 3d orbitals near the E _f . This transition altered the configuration of the Fe center from high‐spin Fe³⁺ (t_2g ³e_g ²) to intermediate‐spin Fe²⁺ (t_2g ⁵e_g ¹) and low‐spin Fe³⁺ (t_2g ⁴e_g ¹) (Figure 13b), resulting in an antiparallel alignment between Fe 3d electrons and the π‐antibonding orbital of the O₂ intermediate, reinforcing the Fe–O bond. The result was enhanced protonation from OO^* to OOH, accelerating ORR kinetics. Zou et al. deposited Pt onto the (012) crystal plane of α‐Fe₂O₃ to fabricate a high‐performance catalyst featuring Pt‐Fe atomic pair sites.^{[ 124 ]} Strong electronic coupling between Pt and Fe facilitated electron transfer from high‐spin Fe³⁺ to the dz² orbital of Pt, partially filling Pt‐Fe orbitals. This process transformed Fe³⁺ to an intermediate‐spin state, allowing cooperative O₂ adsorption and efficient O─O bond dissociation at the dual‐metal site. OH^* could subsequently desorb from the Pt site, thereby disrupting the traditional ORR scaling limitations. This catalyst exhibited remarkable performance in alkaline environments, achieving an onset potential of 1.15 V and a half‐wave potential of 1.05 V, along with a mass activity of 14.9 A mg_Pt⁻¹ at 0.95 V. After 50 000 durability cycles, performance loss was negligible. Tests conducted on Zn‐air batteries and H₂–O₂ fuel cells also evidence that this catalyst preponderates over 20% Pt/C in energy density and Pt utilization.

Figure 13.

Regulation of the spin‐related properties of DASCs via metal–support interactions. a) Schematic illustration of synthetic procedure of FePc@TiO₂. b) Orbital interactions between Fe and the O₂ during the ORR. Reproduced with permission.^{[ 121 ]} Copyright 2024, American Chemical Society. c–f) Synthesis and structure characterizations of Ti₃C₂T_x MXene. Reproduced with permission.^{[ 122 ]} Copyright 2024, American Chemical Society.

Compared with the majority of disordered porous carbon supports, ordered hierarchical porous N‐doped carbon offers a highly exposed surface area, improving the accessibility of reactants to catalytic centers and mitigating deactivation caused by product accumulation.^{[ 125 ]} This enhancement ensures optimal utilization of active sites and boosts mass transport efficiency. Employing ZIF‐8‐derived N‐doped carbon as a host matrix for single metal atoms provides distinct advantages. Nevertheless, traditional ZIF‐8‐derived carbon materials predominantly exhibit microporous architectures,^{[ 126 ]} resulting in most of the active atom sites being deeply buried in the carbon matrix, limiting both reactant accessibility and diffusion during catalysis. To improve this, increasing the proportion of large mesopores (>5 nm) in the ordered porous framework facilitates more efficient diffusion pathways, thereby accelerating overall reaction kinetics.^{[ 127 ]} Zhu et al. developed a FeNi dual‐atom catalyst with an engineered porous superstructure containing abundant and tunable macro‐ and mesopores, synthesized from ordered macroporous ZIF‐8 single crystals.^{[ 128 ]} First, Fe‐doped ordered ZIF‐8 crystals were produced through a dual‐solvent‐induced multiphase nucleation method. Subsequently, Ni was incorporated through an impregnation method and then pyrolyzed with dicyandiamide (DCDA) at an elevated temperature. The ammonia gas generated in situ from DCDA was utilized to uniformly etch the ordered macroporous cavities, constructing the FeNi DASCs/HOPSNC catalyst with an ordered hierarchical porous superstructure. The 2‐N bridged dual‐atom structure within the dual‐atom site effectively modulates the activity of the dual‐atom site. Meanwhile, the structured hierarchical porous superstructure markedly improves mass transport and the availability of dual‐atom sites. Computational studies confirmed that nearby Fe atoms induced a shift in the Ni d‐orbital energy levels closer to the E _f , reducing the rate‐determining energy barrier and facilitating COOH^* adsorption and CO^* dissociation during CO₂RR. He et al. anchored Fe₂ dimers on a range of two‐dimensional carbon‐based nanosheets,^{[ 129 ]} including defective graphene with pristine (C‐DV/G) or porphyrin‐like divacancies (N‐DV/G), graphitic carbon nitride (g‐C₃N₄), and graphdiyne (GDY), to investigate the influence of electronic metal‐support interactions on NRR through spin‐polarized DFT calculations. These studies revealed that Fe₂ dimers are stably bound at defect sites, where charge polarization around the dimers promotes N₂ adsorption and activation. Notably, the g‐C₃N₄‐supported Fe₂ cluster demonstrated a remarkably low theoretical limiting potential (−0.32 V), suggesting exceptional potential for ammonia synthesis. Therefore, by selecting an appropriate substrate, Fe₂ clusters can function as ideal electrocatalysts for activating and catalyzing inert N₂ molecules.

Typically, a variety of functional carbon nanomaterials and transition metal oxides are widely employed as supports for dual‐metal sites.^{[ 130 ]} Nevertheless, their inferior corrosion resistance and weak electrical conductivity in alkaline electrolytes curtail their overall catalytic performance. Consequently, it is imperative to search for more suitable carrier materials for tailoring the electronic microenvironment around active centers of DASCs. Andrey et al. reported a strategy involving surface modification of MXene carriers via pre‐adsorption of L‐tryptophan molecules (Figure 13c).^{[ 122 ]} This enabled Co/Ni dual‐atom electrocatalysts to bond onto Ti₃C₂T_x surfaces (CoNi‐Ti₃C₂T_x, with a loading capacity up to 5.6 wt%) through N‐Co/Ni‐O coordination (Figure 13d–f). The tryptophan modification not only improved the interlayer spacing and specific surface area of the MXene support but also created abundant anchoring sites for metal atom immobilization. Furthermore, the synergistic interaction between Ni and Co sites and the conductive MXene substrate optimized the charge distribution, enabling more efficient electronic coupling. As a result, the energy barriers of rate‐determining steps in OER and HER were lowered, improving both catalytic activity and long‐term operational durability. This exhibits significant promise as an effective bifunctional electrocatalyst for water electrolysis. Liang et al. introduced Ir into SrMnO₃ by employing a highly straightforward ion exchange method.^{[ 131 ]} The ion exchange reaction exhibits a self‐limiting characteristic, thereby facilitating the formation of Ir‐Mn dual‐atoms in IrMnO₉. Among them, the Ir atom adjusts the electronic structures of Ir and Mn, resulting in a shorter distance (2.41 Å) between the Ir‐Mn diatoms and exhibiting a strong MSI. Moreover, adjustment of the Ir‐Mn diatoms enables the spin direction of O(↑) to align with that of the adsorbed O^* intermediate, which aids in the adsorption of O^* to form O₂ through the ^*O‐O^* coupling reaction.

In conclusion, by carefully manipulating the interaction between the DASCs and their supporting substrates, the spin‐related properties and couplings can be tailored. The strength and nature of these interactions can directly affect the spin distribution of metal centers, thereby regulating their catalytic activity. An advantage of this approach is that it can regulate the spin effect without altering the metal atom's coordination number, which is particularly beneficial for the fundamental mechanism investigation, as it enables the isolation of spin effects from other structural changes, providing deeper insights into the underlying reaction pathways. However, this approach also presents a set of intrinsic limitations, along with possible remedial strategies: 1) Low efficiency of carrier‐mediated spin coupling. Traditional oxide supports, such as TiO₂, often exhibit weak orbital hybridization with the d‐orbitals of transition metals.^{[ 132 ]} To tackle this, magnetic substrates could be exploited. They enhance spin coupling through electron exchange interactions at the interface, effectively improving the coupling efficiency. Alternatively, the use of topological insulator/semiconductor heterojunctions may offer spin‐polarized charge transport via surface states, providing an efficient route for spin information relay. 2) Uncontrollable interfacial charge transfer. SMSI may cause over‐reduction or oxidation of the metal active centers. A feasible countermeasure is the introduction of gradient‐doped supports. Through bandgap engineering, these materials can mediate interfacial charge flow and maintain a favorable charge balance. Additionally, forming Schottky junctions may help control electron dynamics at the interface via internal electric fields, ensuring directional and stable electron movement. 3) Spin instability under thermal stress. When the reaction temperature exceeds 500 °C, lattice oxygen mobility in oxide supports may alter the local coordination structure surrounding metal atoms, disrupting spin states and reducing catalytic stability.^{[ 133 ]} This challenge can be mitigated by applying ALD to coat the catalyst with thin Al₂O₃ layers. These coatings act as diffusion barriers, preserving coordination integrity by limiting oxygen migration. Alternatively, carbide‐based carriers with higher thermal resilience can be employed to better maintain spin‐state integrity under elevated temperatures. 4) Interference from structural reconstruction: Under electrochemical conditions involving high potentials, such as in the OER, metal segregation within oxide supports may occur, inadvertently altering the spin configuration of neighboring sites.^{[ 134 ]} One way to address this is to pre‐introduce oxygen vacancies into the host material. These defects can stabilize the support structure, minimizing unwanted segregation. Another promising route involves the design of biphasic supports, where interfacial phase boundaries act to anchor the metal sites, thereby suppressing dynamic reconstruction effects.

In summary, throughout both the synthesis and operational phases of DASCs, the stability of spin structures is subject to various constraints. During synthesis, challenges arise from the difficulty in precisely controlling atom distribution and interatomic distances, as well as from the complexity of interatomic forces.^{[ 135 ]} Poor control over these parameters can result in atypical overlap of electron clouds, which affects electron delocalization and weakens spin coupling effect. Moreover, undefined interatomic interactions hinder the accurate modulation of their strength and character, potentially displacing the spin state from its ideal configuration and rendering it susceptible to environmental disturbances.

In catalytic reactions, both operating conditions and the behavior of reaction intermediates influence spin stability.^{[ 15a ]} High temperatures or elevated pressures increase atomic vibrations, which can destabilize the structure of dual‐atom sites and distort the electron cloud distribution. This leads to fluctuations in spin configuration. Additionally, harsh environments such as strong acidic or alkaline conditions may chemically alter or corrode the active sites, further impacting integrity. When molecules interact with the catalyst surface and form intermediate species, charge redistribution and orbital interactions with the dual‐atom sites can alter the electronic configuration and affect the spin state.^{[ 136 ]} If the intermediates bind too strongly or fail to desorb efficiently, these prolonged interactions may alter the temporary spin configuration of active sites. To preserve stability and improve the durability of DASCs, catalyst structures with intrinsic robustness must be rationally designed. Among the coordination environment modification strategies, element doping proves effective. It introduces additional charge carriers, either electrons or holes, that can redistribute the local electron cloud at the dual‐atom sites. For example, doping with highly electronegative elements draws electrons from the surrounding atoms, possibly increasing spin polarization and contributing to spin state stabilization.^{[ 137 ]} Doping also modifies the local crystal field environment, which alters the energy level splitting of the metal centers. Optimizing crystal field strength and symmetry facilitates orbital reconfiguration, enhancing spin coupling and promoting stability.^{[ 138 ]} Defect engineering offers another promising route by generating localized spin features near the dual‐metal centers. Vacancies, interstitials, or substitutional defects can interact with the metal center spins and help establish favorable spin arrangements. For example, oxygen vacancies modulate nearby electron density, introducing unpaired electrons that can couple with the spins of dual‐atom sites and reinforce spin stability.^{[ 10 ]} However, excessive or uncontrolled defect introduction may induce disruptive perturbations, thereby reducing overall stability. The role of MSI is also crucial. MSI contributes structural confinement to dual‐atom configurations, restricting migration and aggregation.^{[ 139 ]} This structural stabilization is critical for preserving defined interatomic distances and spin arrangements. Moreover, charge exchange between metal centers and the support mediates electronic equilibrium and optimizes spin configurations. For example, supports with high electron‐donating properties can increase electron density at dual‐atom centers and stabilize the spin state.^{[ 140 ]} However, overly strong MSI can limit the electronic adaptability of active sites, hindering the dynamic tuning of spin states during catalysis, which may impair stability. Therefore, ensuring spin structure robustness in DASCs requires a comprehensive strategy that incorporates coordination environment design, controlled doping, defect engineering, and appropriate MSI tuning. These interventions should be tailored to the specific catalytic application and reaction conditions to realize both high stability and optimal performance.

Compared with methods such as morphology modulation, heteroatom incorporation, or defect tuning, which require structural adjustments to the catalytic center, the application of external magnetic fields represents an alternative, structure‐preserving method for modulating spin dynamics and improving catalytic outcomes. This strategy does not necessitate changes to the catalyst's geometry or chemical composition, nor does it involve elaborate synthesis steps.^{[ 141 ]} Through mechanisms including Lorentz‐force‐induced charge movement, spin polarization, and magnetohydrodynamic phenomena, the application of magnetic fields has been shown to enhance mass transport and reaction kinetics during electrocatalysis. However, implementing external magnetic fields requires specialized magnetic field generation apparatus (e.g., permanent magnets or electromagnets), which adds to system complexity and cost. Additionally, for industrial‐scale applications, ensuring uniform magnetic field distribution and precise intensity control presents substantial engineering challenges.^{[ 142 ]} Thus, despite its theoretical appeal, magnetic field‐assisted electrocatalysis has yet to see widespread commercial deployment due to constraints related to feasibility, economic factors, and safety. Consequently, ongoing efforts in spin‐related catalysis have shifted towards exploiting built‐in magnetic interactions intrinsic to the catalyst design.

4. Spin Engineering of DASCs for Efficient Electrochemical Energy Conversion

Electrochemical catalytic reactions, with favorable selectivity and mild reaction conditions, represent an appealing approach for sustainable energy conversion. Recent studies have demonstrated that the spin‐related properties of DASCs are highly effective in modulating electrochemical catalytic activity and stability (Table 1 ). These catalysts have been employed across a broad range of reactions, including oxygen‐involved electrochemical processes in fuel cells and electrolyzers (e.g., ORR, HOR, OER, HER),^{[ 143 ]} as well as other electrocatalytic energy conversion reactions such as CO₂RR,^{[ 144 ]} NRR,^{[ 145 ]} and NO₃ ^‐RR.^{[ 46b ]} In this section, a detailed exposition of the applications of spin engineering of DASCs in diverse catalytic reactions will be provided.

Table 1.

Summary of the improvement of various catalytic reaction performances by spin engineering of DASCs.

Sample	Catalytic reaction	Catalytic mechanism	Electrolyte	Activity	Stability	Refs.
Fe, Mn/N‐C	ORR	The Mn‐N moiety triggers Fe3+ spin‐state transitions, facilitating penetration of O anti‐bonding π orbitals.	0.1 m HClO4	E1/2, 0.804 V	Stable operation for 40 000 s	[18]
NCAG/Fe‐Cu	ORR	The adjacent Cu atomic sites induce a decrease in the spin magnetic moment of Fe, and reduce the energy barrier of the RDS	0.1 m KOH	E1/2, 0.94 V	Stable cycle 8000 cycles	[146]
Eu2O3‐O/NC	ORR	The gradient orbital coupling of the Co 3d‐O 2p‐Eu 4f unit sites weakens the O = O bond	0.1 m KOH	E1/2, 0.887 V	Stable operation for 10 h	[147]
stereo‐Fe‐Co DASC	OER	The electron orbital interaction between the two metals realizes the OPM	1.0 m KOH	Overpotential, 190 mV@10 mA cm−2	Stable operation for 160 h	[4a]
CFS‐ACs/CNT	OER	The built‐in magnetic field induces the parallel alignment of the spins of O‐O electrons, accelerating the release of O2	1.0 m KOH	Overpotential, 270 mV@20 mA cm−2	Stable operation for 180 h	[1b]
Co3‐xFexO4	OER	The spin‐pinning effect promotes the spin polarization of oxygen free radicals and reduces the subsequent O‐O coupling potential barrier	1.0 m KOH	Overpotential, 350 mV@10 mA cm−2	Stable operation for 72 h	[2a]
Co2‐N‐HCS‐900	HER	The Co2N5 dual‐atom catalyst with a lower spin state achieves an ideal adsorption/desorption equilibrium of intermediates.	1.0 m KOH	Overpotential, 166 mV@10 mA cm−2	Stable operation for 1000 h	[148]
FeCo‐NC	CO2RR	The dual‐atom centers provide large spin polarization and multi‐electron transfer capabilities, enabling the CO intermediate to serve as an effective electronic and geometric regulator in the CO2RR	0.1 m KHCO3	limiting potentials, ‐0.64 V for both CH3OH and CH4	–	[149]
Mn1Ox(OH)y@Ru/C	HOR	Ru excites Mn in the cluster to a high‐spin state, which slows down the adsorption of O species on Ru	0.1 m KOH	Peak power density, 1.731 W cm−2	Stable operation for 180 min	[7b]
FeMoPPc	NRR	MoN4 induces the FeN4 sites to transform from a high‐spin state to a intermediate‐spin state, effectively activating the N≡N bond	0.1 m KOH	NH3 yield, 36.33 µg h−1 mgcat −1; FE, 20.62% at ‐0.3 V	Operate stably for six cycles	[13a]
SP‐Fe1‐Ti	NO3 ‐RR	The spin electrons in the 3d orbitals of the spin‐polarized Fe and Ti atoms are effectively injected into the key intermediates, promoting the deoxygenation of NO3 ‐ and the hydrogenation process of NO*	1.0 m NO3 ‐	NH3 yield, NH3 yield, 272000 µg h−1 mgcat −1; FE, 95.2% at ‐0.4 V	Stable operation for 20 h	[10]

4.1. Electrocatalytic Oxygen Reduction Reaction

4.1.1. Spin‐Related Core Catalytic Mechanisms

When considering a spin effect, the core catalytic mechanism of electrocatalytic ORR mainly lies in the dynamic matching between the spin state of the catalyst and O₂ as well as the intermediates. Modulating spin‐polarized electron transfer, orbital overlap, and adherence to spin conservation rules can lower activation barriers and refine pathway selectivity. The specific mechanism can be analyzed from the following two aspects:^{[ 53} , ^{150 ]}

1. O₂ spin state and activation via adsorption. O₂ in its ground state adopts a triplet configuration (S = 1), possessing two unpaired electrons aligned in parallel. This configuration makes direct adsorption and activation on non‐magnetic catalytic surfaces energetically unfavorable. By engineering the spin properties of catalytic surfaces, for instance, using catalysts that are intrinsically magnetic or that generate spin‐polarized electrons, the spin of O₂ can be effectively aligned, thereby facilitating its adsorption and reducing the associated energy barrier. High‐spin transition metals (e.g., Fe³⁺, Co³⁺) are often effective in enhancing this coupling.

2. Spin‐dependent behavior of reaction intermediates. The intermediates involved in ORR, such as OOH^*, O^*, and OH^*, also exhibit unique spin states. The interaction between these intermediates and the catalytic surface is influenced by the spin compatibility, which affects their free energy of adsorption and ultimately dictates the dominant reaction pathway. Furthermore, conservation of total spin angular momentum throughout the catalytic cycle may impose restrictions on reaction energetics. For example, the conversion of O₂ from a triplet to a singlet‐state intermediate may require a spin‐flip mechanism, which the catalyst must be capable of facilitating.

4.1.2. Regulation of Activity

ORR serves as the cathode reaction and a major bottleneck in fuel cells and metal‐air batteries due to sluggish reaction kinetics. Fundamentally, the kinetic mechanisms underlying the ORR are intrinsically associated with a series of sequential proton‐coupling and electron‐transfer steps.^{[ 151 ]} In detail, the interaction between O₂/oxygen intermediates and the metal active center typically involves the spin electron evolution from paramagnetic O₂ to diamagnetic oxygen intermediates. More critically, the varying orbital interactions between oxygen intermediates and metal sites will significantly impact the adsorption model, bond dissociation, and adsorption behavior of intermediates, thereby regulating the reaction pathways (associative or dissociative pathway) and kinetics.^{[ 8a ]} These orbital interactions are closely linked to the spin configuration of catalytic active sites. Therefore, the modulation of spin states is anticipated to contribute to the enhancement of ORR performance. Theoretical investigations have demonstrated that the 3d unoccupied orbitals of transition metals (such as Mn, Fe, Co, Ni, etc.) can accept incoming electrons, weakening the bonding interaction of intermediates such as OOH^* and O^*/OH^*, and thereby facilitating the reduction of O₂.^{[ 152 ]} Although transition metal SACs manifest remarkable activity and selectivity in ORR, they are invariably constrained by the scaling relationship between adsorption and desorption, rendering it arduous for their activity and durability to fulfill practical requisites. The FeN₄ site, a representative SAC configuration, exhibits strong oxygen binding, which can impede the O^* → OH^* protonation step during ORR. To overcome this, Zhang et al. developed a bimetallic Fe, Mn atom‐dispersed Fe, Mn/N‐C electrocatalyst using a pre‐polymerization and pyrolysis route (Figure  14a).^{[ 18 ]} The integration of Mn–N moieties facilitated the delocalization of Fe³⁺ electrons and induced a spin transition from a low‐spin (t_2g ⁵e_g ⁰) to an intermediate‐spin (t_2g ⁴e_g ¹) configuration, thereby improving orbital overlap with the antibonding π orbital of O₂ (Figure 14b,c). DFT simulations further clarified the origin of improved catalytic behavior. On Fe/N‐C, the Fe–O₂ bond measures 1.884 Å, with overly strong binding limiting subsequent proton‐electron transfer, resulting in a higher yield of peroxide species and lower catalytic efficiency. By contrast, Fe, Mn/N‐C features optimized adsorption at the bimetallic sites, decreasing the energy barrier for bond dissociation and facilitating intermediate adsorption, leading to accelerated ORR kinetics. Experimentally, this catalyst exhibited notable performance in both alkaline and acidic conditions, with half‐wave potentials of 0.928 V in 0.1 m KOH and 0.804 V in 0.1 m HClO₄, respectively. The catalyst demonstrated strong durability and outperformed commercial Pt/C in alkaline media while delivering comparable results in acidic environments. Furthermore, it also achieved superior power output and sustained performance in rechargeable Zn‐air battery systems.

Figure 14.

Research on ORR catalyst design and performance. a–c) ORR performances and room‐temperature ⁵⁷Fe Mössbauer spectroscopy of Fe, Mn/N‐C. Reproduced with permission.^{[ 18 ]} Copyright 2021, Springer Nature. d–g) ORR performance of NCAG/Fe‐Cu, NCAG/Fe‐Fe, and commercial Pt/C in 0.1 m KOH, 0.1 m HClO₄, and 1.0 m PBS. h) Illustration of how magnetic moment affects ΔG _OH* for the bimetallic sites located at nanopores. Reproduced with permission.^{[ 146 ]} Copyright 2020, Wiley‐VCH.

Previous studies have shown that the catalytic performance of MN₄ (M = Fe, Co, Ni) single‐atom active centers in ORR tends to diminish as the magnetic moment of the metal center decreases. Nonetheless, the exact relationship between magnetic moment and catalytic efficiency remains unclear,^{[ 68 ]} impeding the systematic development of high‐activity ORR catalysts. In the ORR pathway catalyzed by Fe‐based single‐atom sites, the RDS is the final stage involving OH^* desorption. Therefore, the desorption‐free energy of OH^* (ΔG_OH*) is often used as a key descriptor of catalytic performance. Studies have examined whether the ΔG_OH* in the ORR reaction can be modulated by varying the magnetic moment of the metal site. To address this challenge, Chen et al. found through theoretical calculations that FeN₄ sites located at different positions (e.g., bulk, pore edge) on graphite carbon exhibit different ΔG_OH* and magnetic moments,^{[ 146 ]} and are nearly linearly correlated. It can be inferred that the modulation of ORR activity can be achieved by changing the spin magnetic moment of the metal active center. Further theoretical and experimental results confirmed that the magnetic moment of Fe can be regulated by introducing an adjacent Cu single‐atom site at the single‐atom Fe site, thereby achieving the modulation of ORR catalytic activity. This regulatory effect is much stronger for FeN₄ sites located at the nanopore edge than for FeN₄ in the bulk site. Based on this, a bimetallic single‐atom‐doped carbon aerogel catalyst (NCAG/Fe‐Cu) was developed, which exhibits excellent ORR activity in a wide pH range (0–14). The half‐wave potentials in alkaline and neutral media reach 0.94 and 0.84 V, respectively (Figure 14d–h). The flexible/neutral aluminum‐air battery and flexible/alkaline Zn‐air battery assembled with NCAG/Fe‐Cu as the electrode catalyst both show extremely high open‐circuit potentials (2.00 and 1.51 V, respectively) and power densities (130 and 186 mA cm⁻², respectively), as well as excellent mechanical flexibility, all superior to commercial Pt/C or Pt/C‐RuO₂ catalysts. Guo et al. further confirmed that the atomically dispersed bimetallic Fe, Cu/N‐C catalyst involved electron spin state modulation and rapid tandem reaction kinetics.^{[ 153 ]} By facilitating localized charge/spin transition and enabling cascade O‐species transfer at the dual active site, the reaction barrier was reduced, and orbital interactions optimized. Compared with catalysts based solely on Fe or Cu, the Fe, Cu/N‐C system demonstrated superior ORR activity under various pH conditions, outperforming benchmark Pt/C catalysts. These insights underscore the vital role of spin‐state engineering in governing ORR efficiency and offer valuable direction for the design of next‐generation electrocatalysts for energy applications.

4.1.3. Regulation of Stability

However, in acidic conditions, the intermediate valence state Fe²⁺ is prone to cause various side reactions, and the performance decays severely during long‐term ORR. Therefore, it is of great significance to explore strategies to effectively improve the durability of Fe‐based catalysts. Li et al. explored combinations of Fe and third‐period elements by evaluating their protonation‐free energy (Figure 15a).^{[ 54 ]} The prepared Fe‐Zn dual‐atom exhibited the highest protonation‐free energy. Based on theoretical predictions, a Fe/Zn‐N‐C DASC featuring a paired dual‐atom arrangement was synthesized. Chitosan, serving as both a N‐rich carbon source and a robust metal chelator due to its rigid backbone and coordination capability, was pyrolyzed in admixture with a bimetallic precursor at an elevated temperature. Subsequent acid leaching was then performed to eliminate particles. The presence of non‐magnetic Zn²⁺ modified the electronic structure of the Fe─N─C system, inducing a shift from semiconducting to semi‐metallic behavior, which facilitated spontaneous spin‐polarized electron accumulation at the E _f . Notably, while the 3d band of Fe─N─C is inherently narrow, it broadened upon Zn incorporation. Additionally, positively charged migrating electrons enhanced interfacial charge transfer between the catalyst surface and reactive intermediates. In the spin‐polarized environment of the catalyst, the two unpaired electrons in the π ^* orbital of paramagnetic O₂ tended to align parallel, thereby facilitating stronger binding through spin coupling. Concurrently, the system exhibited an increased protonation energy barrier (Figure 15b–d). During the ORR process, ZnN₄ was sacrificed to protect the FeN₄ site. Owing to these attributes, Fe/ZnN‐C catalyst delivered outstanding ORR performance in both alkaline and acidic environments, achieving half‐wave potentials of 0.906 and 0.806 V, respectively. Significantly, it exhibited a mere 12 mV stability loss after 5000 cycles. These findings suggest a viable pathway for designing robust electrocatalysts with high activity and longevity, propelling the practical advancement of fuel cells and metal–air batteries. Recently, with the increasing attention paid to RE elements in the energy field, the SOC induced by the unique and abundant 4f energy level in the valence shell has aroused extensive research interest in the electronic structure modulation strategy leveraging 4f bridging. Nevertheless, from the perspective of the valence shell structure, the 4f energy level is too arduous to directly exploit. This is due to the shielding effect of 5d and 6s on the 4f level, which typically exhibits inert traits in chemical reactions and consequently results in low covalency in the bonds between REs and O₂. Recently, Fu et al. put forward a practicable gradient orbital coupling strategy (Figure 15e).^{[ 147 ]} They fabricated a Co 3d‐O 2p‐Eu 4f unit site on a Eu₂O₃‐Co composite, where the Co‐O‐Eu linkage enabled an increase in covalent character via orbital hybridization. This structure enhanced Co 3d e_g orbital modulation, particularly suppressing $d_{x^2-y^2}$ electron density, indicating the role of Co as an electron donor and Eu as an electron acceptor. The formation of Eu²⁺ generated an asymmetric surface charge distribution, which facilitated O₂ intermediate binding and accelerated conversion to OH^*, thereby weakening the O–O bond. Furthermore, the approach disrupted the linear scaling constraint between ΔG_(OOH*) and ΔG_(OH*) on the Co‐O‐Eu site, approaching an ideal thermodynamic value of 2.46 eV (Figure 15f). Owing to this orbital gradient design, the Eu₂O₃‐Co catalyst achieved a half‐wave potential of 0.887 V in 0.1 m KOH, comparable to commercial Pt/C and outperforming many reported Co‐based materials (Figure 15g). It also exhibited superior performance in Zn‐air batteries across multiple metrics: peak power density, rate capability, cycling life, and capacity. This study highlights the potential of utilizing RE‐4f orbital modulation and holds the promise of inaugurating a novel and efficient pathway for achieving electronic structure, that is, activating the RE‐4f state for electrocatalytic reactions through gradient orbital coupling modulation.

Figure 15.

Research on ORR catalyst design and performance. Fe/Zn‐N‐C DASC of a) Calculated free energies for Fe/M‐N‐C and schematic. b) Theoretical ORR overpotential. c) Charge density difference plot. d) Schematic electronic structure. Reproduced with permission.^{[ 54 ]} Copyright 2022, Springer Nature. e) Illustration of metal‐oxygen‐metal interaction via gradient 3d‐2p‐4f orbital coupling. f) Scaling relation analysis between ΔG _OOH*–ΔG _OH*. g) Comparative assessment of catalytic activity and stability. Reproduced with permission.^{[ 147 ]} Copyright 2022, Wiley‐VCH.

Spin effect plays a crucial role in ORR, and its core theory involves complex interactions in multiple aspects. In the catalyst system, the spin‐orbit‐charge coupling effect is of great significance, collaborating with spin‐related effects to optimize the interaction between active sites and O₂, as well as reaction intermediates. On one hand, spin polarization reshapes electron density near the active sites, promoting O₂ activation and lowering energy barriers. On the other hand, the spin‐orbit‐charge coupling effect leads to a strong inter‐relation among electron orbits, spins, and charge distributions, influencing the electronic structure and chemical reactivity of the active sites. This coupling effect not only facilitates electron transition between different orbits but also precisely regulates the electron transfer process in key steps, enabling electrons to participate in the reaction more efficiently. This interplay contributes to improved adsorption, conversion, and desorption of species such as OOH^* and OH^*, accelerating ORR kinetics from various angles and significantly elevating overall performance.

4.2. Electrocatalytic Oxygen Evolution Reaction

4.2.1. Spin‐Related Core Catalytic Mechanisms

The spin‐dependent catalytic mechanism underlying the OER is primarily governed by the modulation of O–O bond formation via spin‐polarized electron transfer and orbital interactions. This mechanism can be understood from two key perspectives:^{[ 77} , ^{154 ]}

1. Matching between the spin state of the catalyst and the spin state of the intermediate. The OER typically proceeds through a four‐electron transfer process, with three principal mechanistic pathways: the adsorbate evolution mechanism (AEM), the lattice oxygen‐mediated mechanism (LOM), and the oxide path mechanism (OPM). In AEM, O₂ is released through the sequential formation of surface‐bound intermediates on a single metal site via the pathway OH^* → O^* → OOH^* → O₂ ^*. In contrast, LOM circumvents the formation of OOH^* by facilitating direct O–O coupling through lattice oxygen, thereby reducing the reaction energy barrier. The OPM involves the direct coupling of adsorbed oxygen species (O–O) by optimizing both adsorption geometry and spatial proximity of oxygen‐containing intermediates. Across all three mechanisms, effective spin‐state matching between the catalyst surface and the reaction intermediates is essential to lower the activation energy. In cases of spin‐state mismatch, spin transitions must be facilitated, often via spin flipping (such as SOC), which can increase the activation barrier of the reaction.

2. Regulation of the formation of the O–O bond by the spin state of the catalyst. O–O bond formation constitutes the rate‐determining step (RDS) in OER and requires overcoming a substantial energy barrier. Catalysts in a high‐spin state (e.g., Fe³⁺, Co³⁺) possess d electrons occupying high‐energy e_g orbitals, which enhances hybridization with the p orbitals of oxygen‐containing intermediates, thereby facilitating O–O bond formation. For example, the high‐spin state of Fe³⁺ in FeOOH has been shown to strengthen OOH^* adsorption. Conversely, low‐spin‐state catalysts (e.g., Ni³⁺) exhibit lower‐energy d orbitals that may favor the stable adsorption of O^*, but generally exhibit reduced efficiency in promoting O–O coupling. Furthermore, spin‐polarized electrons on the surface of magnetic catalysts (e.g., Co₃O₄) can be directionally injected into the antibonding orbitals of oxygen intermediates, effectively weakening the O–H bond and facilitating O–O bond formation.

4.2.2. Overcoming the Scaling Relationship to Enable the OPM

The OER represents a fundamental step in water electrolysis for hydrogen production and remains one of the primary kinetic limitations in this process. Conventional catalysts typically operate via the AEM, where a well‐established scaling relationship exists between key oxygen intermediates, particularly OOH^* and OH^*. Alternatively, the LOM leverages lattice oxygen involvement, which can lead to structural changes and instability, thereby compromising the balance between catalytic activity and durability. By contrast, the OPM, observed in homogeneous catalytic systems, facilitates direct O–O bond formation without requiring oxygen vacancies or OOH^* intermediates, offering a pathway to further enhance OER efficiency.^{[ 143b ]} However, OPM has been rarely utilized under alkaline conditions, and the identification of catalysts capable of facilitating this mechanism under such environments continues to be a significant challenge.^{[ 1a ]} DASCs, by enabling direct O–O bond coupling rather than proceeding through OOH^* intermediates, present a viable strategy for overcoming the theoretical overpotential limitation of AEM (370 mV). The spatial configuration of metal atoms, specifically their relative positions and interatomic distances, plays a pivotal role in modulating orbital interactions, thereby tuning the binding strengths of catalytic intermediates. However, achieving precise spatial control over dual‐metal configurations remains a key obstacle in the rational design of highly efficient catalysts. Tian et al. developed two structurally distinct Fe–Co DASCs: one exhibiting a stereoscopic geometry (stereo‐Fe‐Co DASC) and the other featuring a planar arrangement (planar‐Fe‐Co DASC)^{[ 4a ]} (Figure 16a). Using isotopic labeling in conjunction with in situ differential electrochemical mass spectrometry (DEMS), TMA⁺ ion blocking studies, and infrared spectroscopy, the researchers confirmed that both catalyst types follow the OPM route rather than the AEM pathway (Figure 16b). Their findings demonstrated that optimized Fe‐Co distances and spatial orientations in the planar configuration enhance the transformation of adsorbed intermediates into O^*, which subsequently undergoes direct O–O coupling without OOH^* involvement (Figure 16c–e). This bypasses the energy‐intensive four‐electron transfer characteristic of AEM. As a result, the planar‐Fe‐Co DASC achieved superior OER performance, requiring only 190 mV overpotential at 10 mA cm⁻² and maintaining excellent operational stability for over 160 h. This outperformed the stereo‐Fe–Co configuration, positioning the planar form as a promising candidate for low‐cost, non‐precious metal electrocatalysts. Zhang et al. prepared a new dual‐site ferromagnetic cluster catalyst (CFS‐ACs/CNT) using a simple adsorption‐reduction‐hydrothermal method and proposed an unconventional CoFe dual‐site segment synergy mechanism (DSSM) (Figure 16f),^{[ 1b ]} which can effectively break the AEM LSR without compromising stability. In this mechanism, multi‐site cooperation occurs where Co³⁺ (low‐spin, t_2g ⁶e_g ⁰) with strong OH^* adsorption, along with Fe³⁺ (intermediate‐spin, t_2g ⁴e_g ¹) having better O^* affinity (Figure 16h), accelerates the formation of the key ^*O‐O^* intermediate and exploits the ferromagnetic coupling induced by the intrinsic magnetic effect of the single‐domain cluster catalyst to prompt the rapid spin‐parallel alignment of ^*O‐O^* and release triplet O₂ (↑O═O↑), ultimately effectuating the efficient conversion of singlet H₂O into O₂, transcending the spin‐forbidden barrier. Performance test results indicate that CFS‐ACs/CNT delivers excellent OER activity at 20 mA cm⁻², with overpotentials 270 mV lower than commercial IrO₂ and 100–80 mV lower than FeS_x/CNT. A Tafel slope of just 77.6 mV dec⁻¹ was achieved, indicating rapid reaction kinetics. Moreover, the catalyst maintained long‐term electrochemical stability for 633 h, with minimal potential drift. In contrast, FeS_x/CNT showed significant degradation after approximately 180 h, with a 38% increase in overpotential (Figure 16g). This work not only illustrates the concurrent formation of ^*O‐O^* intermediates and O₂ liberation while providing a manipulable approach based on distinctive understanding of how spin state variations affect oxygen intermediates in the OER mechanism.

Figure 16.

Research on the design and catalytic performance of catalysts for electrocatalytic OER. a) Schematic illustration of synthetic procedure of Fe‐Co DASC. b) Scheme of OER mechanism for stereo‐Fe‐Co DASC. c) The Gibbs free energy diagram of the OER process. d) Projected density of states of surface Co atoms in planar‐Fe‐Co DASC. e) Projected density of states of surface Fe atoms in planar‐Fe‐Co DASC. Reproduced with permission.^{[ 4a ]} Copyright 2023, National Academy of Sciences. f) The schematic illustration of dual‐site adsorbate evolution mechanisms during the OER. g) The chronopotentiometry curve of FeS_x/CNT and CFS‐ACs/CNT. h) Schematic illustration of the transfer of spin state of the high OER activity for CFS‐ACs/CNT. Reproduced with permission.^{[ 1b ]} Copyright 2024, Springer Nature.

4.2.3. Regulation of Catalytic Performance

In the OER process, reactants, such as OH^‐ and H₂O, exist in a singlet state, whereas the molecular oxygen (O₂) product exists predominantly in a triplet ground state. The energy difference between triplet and singlet is approximately 1.0 eV, with the singlet form being energetically less favorable.^{[ 155 ]} Therefore, spin‐selective electron transfer occurs during the formation of triplet O₂ from singlet OH⁻ or H₂O. Thus, strategies that exploit spin polarization to influence electron transfer kinetics are highly beneficial in catalyst design. One promising approach involves utilizing ferromagnetic materials and manipulating their magnetic ordering to reduce the kinetic energy barrier associated with triplet O₂ formation. Xu et al. implemented the spin‐pinning effect.^{[ 2a ]} Through simple magnetization, the spin electrons of the paramagnetic (hydroxide) hydroxide layer were more regularly arranged, thereby attaining higher intrinsic OER activity (Figure 17a). Specifically, Co_3‐xFe_xO₄ (x = 0–2.0) was prepared via the sol‐gel method. Co_3‐xFe_xO₄ powder was blended with a small quantity of sulfur and sulfided at 300 °C. The sulfidation treatment promotes the surface reconstruction of Co_3‐xFe_xO₄ under OER: the M–S covalency is stronger than M–O, so the reactivity of lattice sulfur is greater than that of lattice oxygen in OER. When lattice S resides on the oxide surface, it can facilitate the oxide surface reconstruction. To investigate magnetic properties, the spin behavior of both the pre‐treated and reconstructed catalysts was evaluated under field cooling (FC) and zero‐field cooling (ZFC) conditions. Research findings indicate that the inherent spin‐pinning effect caused by the strong magnetic anisotropy at the interface originates from the existence of local ferromagnetic domains. The spins within the reconstructed Co(Fe)O_xH_y are governed by robust interfacial magnetic anisotropy and align with the spin electron alignment direction in local magnetic domain. After magnetization, the magnetic domains are oriented to form a long‐range ferromagnetic order, resulting in the spin electrons in Co(Fe)O_xH_y orderly arranged. That is, the spin‐pinning effect is introduced into the interface of the stable Co_3‐xFe_xO₄/Co(Fe)O_xH_y configuration formed after the reconstruction of the precatalytic Co_3‐xFe_xO₄(s) under alkaline OER conditions, promoting the polarization of the spin electrons of the catalyst during the OER process. Consequently, this facilitates the generation of triplet O₂ (↑O═O↑). DFT studies have verified that the spin‐ordered Co_3‐xFe_xO₄/Co(Fe)O_xH_y is more prone to follow Path 1 to generate triplet O₂, effectively decreasing the kinetic barrier of ^*O‐O^* coupling (Figure 17b,c). Additionally, this paper proposes that the oxygen radicals generated in the reconstructed hydroxide are crucial for the spin polarization of O ligands in the OER process. Under high pH values and the spin‐pinning effect, the spin polarization of oxygen radicals is more readily achieved, thereby effectively reducing the ^*O–O^* coupling barrier (Figure 17d). In summary, the spin‐pinning effect holds significant promise in promoting spin‐related kinetics and further enhancing OER activity.

Figure 17.

Research on OER catalyst design and performance. a–c) Spin‐pinning effect for triplet oxygen evolution on the oxyhydroxide. d) Linear sweep voltammetry results of the reconstructed Co_2.75Fe_0.25O₄(s) before and after magnetization. Reproduced with permission.^{[ 2a ]} Copyright 2021, Springer Nature. e) Charge density difference of Fe/Ni‐N‐C. f) The free energy diagrams at different electrode potentials. g) LSV curves for OER. Reproduced with permission.^{[ 42a ]} Copyright 2021, Elsevier.

Gao et al. discovered that employing a ferromagnetic ordered catalyst to induce spin polarization under a constant magnetic field can enhance OER.^{[ 42b ]} Notably, this strategy is not applicable to non‐ferromagnetic catalysts. Based on the principle of spin angular momentum conservation, spin polarization initiates during the first electron transfer step during OER, involving rapid spin electron exchange between the ferromagnetic catalyst and adsorbed oxygen. In the subsequent three electron transfer steps, as the adsorbed oxygen species assume a fixed spin direction, OER electrons must spontaneously undergo spin polarization conforming to Hund's rule and the Pauli exclusion principle, leading to the formation of triplet O₂. The spin–spin coupling between two metals is critical for determining the electronic structure. However, obtaining the dual‐atom heteronuclear active center in specific experimental studies remains a highly challenging task. Giving full play to the predictability of theoretical research and deeply understanding the dual‐atom heteronuclear catalytic mechanism through the study of theoretical models has significant guidance for the targeted synthesis of high‐performance catalytic systems. Zhao et al. first predicted through DFT theoretical calculations that the FeNi dual‐atom active center had softened spin‐polarized conducting electrons.^{[ 42a ]} Comparative studies revealed that in Fe─N─C (Figure 17e), the active center Fe 3d is highly spin‐polarized with a spin magnetic moment of 1.88 µ _B, and charge localization is observed. Consequently, the highly spin‐polarized Fe 3d has a strong adsorption effect on the key intermediate OH^*, leading to high overpotentials for ORR and OER (Figure 17f). In Ni–N–C, the metal exhibits paramagnetism with approximately zero spin magnetic moment value. Hence, the non‐spin‐polarized Ni 3d electrons have weak adsorption on the key intermediates OOH^* and O^*, failing to achieve a good bifunctional catalytic effect. Through the construction of heteronuclear Fe‐Ni dual‐atom pair, the spin coupling between the two metals lessens the degree of spin polarization of the Fe 3d electrons, reducing their spin magnetic moment to 1.48 µ _B. Moreover, hydroxyl modification enhances charge delocalization at the active center, improving charge transfer efficiency. Drawing from the theoretical research findings, the authors devised a straightforward one‐step wet chemical method to synthesize the Fe/Ni–N–C catalyst. In this process, the pretreatment of chitosan aids in improving the complexation of transition metal ions and releasing ‐NH₂ functional groups, which is essential for obtaining the Fe‐Ni bimetallic active center. The Fe/Ni‐N‐C catalyst demonstrates an onset potential of 1.005 V and a half‐wave potential of 0.861 V in ORR. In the OER process, E_{J = 10} is 1.552 V. Moreover, the potential difference (ΔE) from ORR to OER is 0.691 V, notably smaller than that of the benchmark commercial Pt/C (1.025 V) and IrO₂ (1.05 V) (Figure 17g). The long‐term stability test of ORR was conducted in an O₂‐saturated 0.1 M KOH solution at a potential of 0.6 V and a rotation speed of 1600 rpm using the chronoamperometry method. After 20,000 s, the current could be maintained at over 97%, far superior to Pt/C (<83%), indicating its excellent stability. Previous studies have demonstrated that designing OER catalysts with spin‐polarized electrons is conducive to the formation of spin‐parallel arranged oxygen intermediate and further promotes the formation of triplet O₂. However, due to the instability of materials in acidic conditions, spin catalysts for acidic OER have been rarely studied.^{[ 156 ]} Recently, Lu et al. constructed a Co‐doped Ir catalyst via spin engineering for the first time,^{[ 3a ]} which exhibits excellent activity as a unique magnetic field‐enhanced OER catalyst in acidic electrolytes. Experimental results reveal that under a constant magnetic field, the CoIr nanocluster (NC) demonstrates an OER overpotential of 220 mV at a current density of 10 mA cm⁻², a turnover frequency (TOF) as high as 7.4 s⁻¹ at 1.5 V, and an activity retention rate of 70% after 120 h of continuous electrolysis at 10 mA cm⁻². These metrics exceed those of the CoIr NC catalyst without magnetic field application (260 mV, 2.5 s⁻¹, and 20 h). Moreover, compared with Mn‐ and Ni‐doped Ir catalysts, the CoIr NC exhibits a higher magnetic field enhancement factor because of the lowest d‐band center of CoIr NC that effectively mitigates the adsorption of reaction intermediates, thereby diminishing the OER reaction energy barrier. DFT studies suggest that electron transfer in the acidic OER reaction is a spin‐dependent process (O^* + H₂O → OOH^* + H⁺). The construction of ordered magnetic domains enables the oxygen termination formed in the first reaction step to have a fixed spin orientation. That is, spin polarization occurs in the first electron transfer step, forming a triplet intermediate O(↓)O(↓)H. Therefore, applying a constant magnetic field and increasing the macroscopic antiparallel magnetic moment near the Ir atom provides more spin‐polarized surfaces for spin‐dependent OER. This work demonstrates the feasibility of enhancing acidic OER activity through the spin polarization effect and provides a strategy for developing highly efficient spin‐polarized catalysts to drive reactions efficiently under harsh conditions.

The spin effect, particularly the interaction mechanisms of spin with orbit, charge, and lattice coupling, significantly enhances OER performance and long‐term durability by modulating the electronic configuration and reaction kinetics of catalytic sites.^{[ 157 ]} Essentially, spin effects allow fine control over the adsorption energy of reactants and intermediates, as well as the routes of electron transport, thus reducing the reaction's energy threshold. SOC enhances the electronic density of states (DOS) at active sites, promoting the activation of oxygen intermediates; spin‐charge coupling optimizes the local electronic environment of catalytic active sites by modulating spin‐polarized charge distribution; spin‐lattice coupling provides additional energy for charge transfer and chemical reactions by means of lattice vibrations, enhances structural adaptability, stabilizes the catalyst structure, inhibits the dissolution and agglomeration of active sites, reduces the structural damage and component loss of the catalyst during the reaction, and ensures its long‐term stable operation. These multi‐field coupling mechanisms work together to synergistically optimize reaction pathways, enhance charge transport, and inhibit active site deactivation of active centers, thereby providing essential theoretical support for the development of efficient OER electrocatalysts.

4.3. Electrocatalytic Carbon Dioxide Reduction Reaction

4.3.1. Spin‐Related Core Catalytic Mechanisms

The electrocatalytic CO₂RR is a complex multi‐electron transfer process involving the formation and transformation of multiple intermediates, with its reaction pathways and product selectivity highly dependent on the electronic structure of catalytic active centers. In recent years, the regulation of spin‐related properties at catalytic active centers has emerged as a novel strategy, demonstrating unique advantages in enhancing the selectivity and activity of CO₂RR.

The core of the spin‐related mechanisms in CO₂RR mainly includes the adsorption and activation of CO₂, as well as the regulation of product selectivity through spin‐state control, which can lead to the formation of different reaction pathways and final products. The spin state can act as a “molecular switch” to precisely control the pathways and products of CO₂RR through the chain of “electron spin‐orbital occupation‐intermediate interaction.” By modulating the electronic spin states of active sites within catalysts, some pivotal processes, including intermediate adsorption, activation, and C‐C coupling will be influenced, ultimately steering the formation of specific high‐value‐added products. The specific core mechanisms can be understood from the following two dimensions:

1. Spin state regulation of intermediate adsorption and activation.^{[ 19} , ²¹ , ^{46 ]} The initial activation of CO₂ requires significant structural and electronic rearrangement, from a linear, diamagnetic molecule (singlet ground state) to a bent, spin‐polarized adsorbed intermediate (e.g., COOH^*). This process of orbital hybridization between the metal center and the adsorbate is also governed by the spin state of the catalytic active site. For instance, high‐spin transition metal sites (e.g., Fe³⁺, S = 5/2) with partially filled e_g orbitals that can effectively overlap with the π^* anti‐bonding orbitals of CO₂, enhance hybridization with the anti‐bonding orbitals (π) of CO₂ through d‐orbital electron filling (e_g↑), thereby facilitating the electron transfer and cleavage of C = O bonds. Moreover, the spin polarization of the catalyst can stabilize specific intermediates, thereby steering reaction pathways. For example, the spin‐polarized FeN₄ catalyst, preferentially stabilizing CO^* over OCHO^* due to differential spin‐matching, can achieve high‐selectivity ethylene production by regulating the relative stability of OCHO^* (the formic acid pathway) and CO^* (the C₂₊ pathway).

2. Spin‐dependent C–C coupling mechanism.^{[ 158 ]} Beyond single‐molecule activation, the formation of multi‐carbon products involves the coupling of CO^* intermediates, which is strongly influenced by spin selection rules. Spin conservation dictates that radical pairs must exhibit matching total spin angular momentum, with antiparallel alignment facilitating σ‐bond formation. Consequently, C‐C coupling necessitates antiparallel spin alignment (↑↓) of adjacent CO^* intermediates to reduce the kinetic barrier. For instance, in Fe‐Ni dual‐atom catalysts, the local spin states can be modulated to facilitate this alignment. Sulfur coordination balances the d‐band centers of high‐spin Fe and low‐spin Ni, enabling spin flipping of CO^* between Fe to Ni sites, which reduces the spin‐restriction barrier during coupling and promotes desorption and enhancing CO selectivity.

4.3.2. Regulation of Catalytic Performance

To date, on SACs, the product of CO₂ reduction is predominantly CO, with a selectivity that can exceed 90%. However, SACs generally lack the capacity to facilitate deeper CO₂ reduction into more valuable multi‐carbon products.^{[ 4b ]} For a catalyst to facilitate the deep reduction of CO₂, three requisites must be satisfied: 1) abundant adsorption sites; 2) efficient catalysis of CO₂ to CO^*; and 3) propulsion of further reactions of CO^*. DASCs can not only provide on‐top adsorption sites but also enable bridging configurations, favoring the stabilization of more complex intermediates. These dual‐atom structures retain the CO₂‐to‐CO^* conversion efficiency typical of SACs while potentially offering enhanced reactivity through electronic synergism, facilitating subsequent CO^* transformation. As a result, DASCs are promising candidates for CO₂RR. Lu et al. investigated the electronic behavior of a FeCo dual‐atom catalyst (FeCo‐NC) in comparison with its single‐atom counterparts (Fe‐NC and Co‐NC).^{[ 149 ]} The findings demonstrated that the cooperative effect between Fe and Co centers significantly enhances CO₂RR performance, enabling the production of CH₃OH and CH₄ (Figure 18a). The limiting potentials for CH₃OH/CH₄ production via CO₂RR are reduced from −1.67 V (Fe‐NC) and −1.12 V (Co‐NC) to −0.64 V (FeCo‐NC), outperforming that of Cu SAC. The superior performance of the DASC mainly stems from: 1) the structural and electronic synergy between the two metal sites, which ensures both stability and reactivity; 2) pronounced spin polarization and multi‐electron transfer capabilities, allowing CO^* to adsorb more stably and adjusting the d‐band center to promote electronic and geometric rearrangements; and 3) superior performance of FeCo‐NC under applied potential conditions, compared to Fe‐NC and Co‐NC in catalyzing CO₂RR to produce CH₃OH and CH₄ and suppressing hydrogen evolution. In summary, precise regulation of the spin states of active sites in catalysts can directly influence the activation of reactants, the configuration of intermediates, and product selectivity, making the catalytic process more precise and controllable. However, the application of spin in electrocatalytic CO₂RR is still in the early stages of research. The spin interactions between catalysts and reactants, as well as those during the catalytic process, remain unclear. Therefore, understanding the structure‐performance relationship between spin‐related properties and catalytic performance is of great significance for designing efficient CO₂RR catalysts.

Figure 18.

Research on CO₂RR, NRR, and NO₃ ^‐RR catalysts design and performance. a) CO promotion mechanism during CO₂RR on FeCo‐NC catalysts. Reproduced with permission.^{[ 149 ]} Copyright 2023, Wiley‐VCH. b) Simplified schematic of N₂ bonding to transition metals. c) The free‐energy diagrams for the HER. Reproduced with permission.^{[ 13a ]} Copyright 2023, Wiley‐VCH. Illustration of d) spin‐polarized Fe₁‐Ti and spin‐depressed Fe₁‐Ti. e) Calculated free energy diagrams for NO₃ ^‐RR. f) Major orbital interactions between Fe and NO. g–i) NO₃ ^‐RR performances of different electrodes. Reproduced with permission.^{[ 10 ]} Copyright 2024, Springer Nature.

4.4. Electrocatalytic Nitrogen/Nitrate Reduction Reaction

4.4.1. Spin‐Related Core Catalytic Mechanisms

Electrochemical NRR for NH₃ synthesis presents a viable approach to lowering the energy demands associated with conventional industrial processes for NH₃ production.^{[ 11 ]} However, the electrocatalytic NRR involves a multi‐electron‐proton transfer process (N₂ + 6H⁺ + 6e⁻ → 2NH₃), whose efficiency is limited by the chemical inertness of N₂ molecules and the competitive HER. In recent years, spin engineering, as an emerging strategy, has become a key approach to enhancing NRR performance by regulating the spin states of active sites in catalysts, which can significantly influence the adsorption and activation of N₂, the stability of intermediates, and the selection of reaction pathways. Electrocatalytic NO₃ ^‐ RR serves as a promising approach for ammonia synthesis and water denitrification. Similar to NRR, NO₃ ^‐ RR involves a complex multi‐electron transfer pathway with multiple nitrogenous intermediates (e.g., NO₂ ^‐, NO, NH₂OH), and faces challenges in product selectivity and reaction efficiency. Recent studies have also highlighted that spin‐state regulation of catalytic active sites can play a decisive role in enhancing the adsorption and transformation of NO₃ ^‐ derived intermediates (NO). The spin‐related core catalytic mechanisms in electrocatalytic NRR and NO₃ ^‐ RR involve the matching between catalyst spin states and N₂/NO₃ ^‐ as well as intermediates, and the promotion of antibonding orbital filling via spin‐polarized charge transfer at magnetic active sites, thereby facilitating adsorption, activation, and hydrogenation of intermediates:

1. NRR.^{[ 159 ]} Spin‐related mechanisms in NRR pertain to both N≡N bond activation and spin conservation along the reaction pathway. N₂, with high bond dissociation energy (≈941 kJ mol⁻¹), making their activation extremely difficult. To activate the N≡N bond, the catalyst must inject electrons into the π antibonding orbitals of the N₂ molecule. This electron injection process requires both orbital energy level alignment and spin compatibility, meaning the transferred electrons must enter the molecular orbitals in a spin‐allowed manner. High‐spin Fe³⁺ centers (S = 5/2) can populate antibonding orbitals, effectively injecting electrons into the π antibonding orbitals of the N₂ molecule and weakening the N≡N bond. Additionally, as the reaction proceeds, certain key intermediates such as NH₂ ^* exist in a singlet state. The continuation of the reaction requires spin‐state matching or spin‐flipping mechanisms—such as spin–orbit coupling (SOC)—to prevent spin‐forbidden transitions that would otherwise hinder the reaction pathway.

2. NO₃ ^‐ RR.^{[ 10} , ^{46 ]} In NO₃RR, spin‐related effects influence both nitrate adsorption and electron transfer, as well as ammonia selectivity. Initially, the planar geometry of NO₃ ^‐ must be converted into a bent adsorbed configuration (^*NO₃ ^‐) via spin‐polarized orbital hybridization. On magnetic catalyst surfaces such as Co₃O₄, spin‐polarized electrons can be selectively injected into the N–O antibonding orbitals, thereby weakening the N–O bonds and promoting efficient cleavage. Additionally, stabilizing the NH₂ ^* intermediate (a singlet state) helps suppress the formation of N₂O byproducts and steers the reaction pathway toward selective NH₃ formation.

4.4.2. Regulation of Catalytic Performance

Owing to their well‐defined atomic and electronic configurations, atomically dispersed catalysts provide a promising platform for mechanistic elucidation. In this context, Zhang et al. examined the influence of spin states on the NRR activity of FeN₄ by integrating atomically dispersed Fe and Mo centers—coordinated in FeN₄ and MoN₄ configurations—within a polyphthalocyanine (PPc) framework.^{[ 13a ]} Their study revealed that the neighboring MoN₄ moiety modulates the spin configuration of the Fe site, inducing a shift from a high‐spin $({d_{xy}}^2{d_{yz}}^1{d_{xz}}^1{d_{z^2}}^1{d_{x^2-y^2}}^1)$ to intermediate‐spin $({d_{xy}}^2{d_{yz}}^2{d_{xz}}^1{d_{z^2}}^1)$. This adjustment enhances the orbital overlap between Fe 3d and N 2p orbitals, improving N≡N bond activation (Figure 18b,c). Computational results indicated that NRR activity is primarily localized on the FeN₄ unit rather than MoN₄, and that the spin transition substantially decreases the energy barrier for the potential‐determining step, thereby facilitating the initial protonation of N₂. Experimentally, FeMoPPc with intermediate‐spin FeN₄ demonstrated a Faradaic efficiency for NRR that was 2.0 and 9.0 times greater than FePPc and MoPPc, respectively, along with 2.0‐ and 17.2‐fold increases in NH₃ yield. These results underscore the potential of spin‐state engineering at metal centers as a strategy for developing next‐generation NRR electrocatalysts. Although spin regulation has demonstrated significant advantages in electrocatalytic NRR, it still faces the following challenges: 1) The precise correlation between spin states and reaction intermediates has not been fully elucidated, requiring further analysis through in situ characterization and theoretical calculations. 2) Most spin catalysts exhibit insufficient stability in strong acid/alkali electrolytes. Future research should focus on dynamic monitoring and regulation of spin states, develop spin catalyst systems with high activity, high selectivity, and high stability, and promote their application in practical electrolysis devices.

The electrocatalytic NO₃ ^‐ RR to NH₃ reaction not only serves as a solution for mitigating environmental contamination and carbon emissions but also emerges as a sustainable pathway for NH₃ synthesis.^{[ 2c ]} Nevertheless, the spin state transition leads to a sluggish NO^* hydrogenation process. Zhang et al. designed spin‐polarized Fe₁‐Ti dual‐atom configurations on a monolithic Ti electrode by introducing oxygen vacancies (Figure 18d,e).^{[ 10 ]} These engineered catalysts achieved an NH₃ yield rate of 272 000 µg h⁻¹ mg_Fe ⁻¹ and a Faradaic efficiency of 95.2% at −0.4 V, substantially outperforming their spin‐depressed analogs (51 000 µg h⁻¹ mg_Fe ⁻¹) and most previously reported NO₃ ^‐RR catalysts (Figure 18f–i). The unpaired spin electrons on both Fe and Ti sites enable effective orbital interactions with key reaction intermediates, thereby facilitating NO^* hydrogenation. Furthermore, by integrating a flow electrolyzer with a membrane‐based NH₃ capture system, the process simultaneously achieves NO₃ ^‐RR and NH₃ separation. This work introduces a forward‐looking approach for manipulating spin polarization in dual‐atom electrocatalysts during nitrate remediation applications. Despite significant progress has been made in spin regulation for electrocatalytic NO₃ ^‐ RR, the following challenges remain to be addressed: 1) Lack of long‐term stability data for spin catalysts under high‐concentration NO₃ ^‐ conditions. 2) Incomplete clarification of the correlation between spin states and complex reaction networks (involving multi‐step conversions such as NO₃ ^‐→NO₂ ^‐→NO→N₂O→N₂/NH₃). Future research should prioritize in‐depth analysis of structure‐activity relationships in spin catalysts, develop in situ dynamic spin detection technologies, and advance the interdisciplinary integration of spin engineering with cutting‐edge technologies (e.g., membrane separation and photoelectrocatalysis), aiming to achieve efficient and energy‐efficient nitrate wastewater treatment and resource utilization.

4.5. Electrocatalytic Hydrogen Evolution and Oxidation Reaction

4.5.1. Spin‐Related Core Catalytic Mechanisms

The electrocatalytic HER and HOR are core processes in hydrogen energy conversion and utilization, being critical to the development of renewable energy storage and fuel cell technologies. The HER involves the reduction of protons (H⁺) to molecular hydrogen (H₂) and is typically represented as 2H⁺ + 2e^‐ → H₂. Conversely, the HOR is the reverse process, where H₂ is oxidized to protons and electrons. Both HER and HOR are two‐electron processes and are fundamentally governed by the adsorption and desorption behavior of hydrogen intermediates (*H) on catalyst surfaces. Despite their apparent simplicity, their kinetics and efficiency are significantly affected by factors such as adsorption energy, electronic structure, and recently, spin‐related interactions at the catalytic interface. The spin‐related mechanisms in electrocatalysis critically influence both activation energy and pathway selectivity by modulating interactions between the electronic spin states of catalysts and those of reactants or intermediates. Below summarizes the main spin‐related mechanisms for two key electrochemical reactions, HER and HOR:

1. HER.^{[ 160 ]} The spin effects in the hydrogen evolution reaction (HER) mainly originate from the close correlation between the adsorption of H atoms and the breaking of O–H bonds with the electronic structure of metal active centers. Magnetic catalysts, such as those based on Co and Ni, leverage spin‐polarized electrons to modulate the adsorption free energy of H and enhance the efficiency of the Volmer step (H⁺ + e^‐ → H^*). High‐spin‐state centers (e.g., Ni²⁺) can facilitate the Tafel step (2H^* → H₂↑) by enhancing d–p orbital hybridization between the metal and ligands, which lowers the energy barrier for H–H bond formation. Moreover, the transfer of spin‐polarized electrons from isolated metal centers into hydrogen antibonding orbitals can lower the energy barrier for H₂ desorption.

2. HOR.^{[ 161 ]} In the HOR, which inherently involves electron transfer processes, spin engineering can modulate both the efficiency of electron shuttling between the catalyst and reaction intermediates and the adsorption/desorption dynamics of H₂ molecules and oxidative intermediates at active sites. Spin‐polarized surface states enable heterolytic cleavage of H₂ and concurrently adjust OH^* adsorption strength via the d‐band center, mitigating catalyst poisoning through excessive OH^* accumulation.

In electrochemical energy conversion reactions, spin‐related catalytic mechanisms primarily enhance performance through three fundamental pathways: 1) Spin matching and charge redistribution—optimizing reactant adsorption and intermediate stabilization; 2) Spin‐polarized orbital hybridization—promoting directional weaken/form chemical bonds; 3) Spin conservation constraints—controlling reaction pathways and minimizing energy barriers by facilitating or restricting specific spin transitions.

4.5.2. Regulation of Catalytic Performance

Alkaline water splitting offers a scalable, eco‐friendly solution for hydrogen production. The HER, as the cathodic half‐reaction in this context, still primarily relies on Pt/C materials. Thus, researchers are increasingly focusing on developing cost‐effective alternatives capable of achieving similarly low energy barriers for H^* and OH^* intermediate adsorption/desorption through engineered spin interactions at the atomic scale. Huang et al. employed anatomization and sintering strategies to achieve controllable modulation from Co nanoparticles (Co_NP/HCS‐900) to single Co atoms (Co_SA‐N‐HCS‐900), and subsequently to dual‐atom Co₂N₅ configurations (Co₂‐N‐HCS‐900), via N exfoliation and thermally induced migration (Figure 19a).^{[ 148 ]} In detail, a simple dual‐solvent impregnation approach was utilized to synthesize the initial Co‐hollow polymer sphere (Co‐HPS) precursor. Direct pyrolysis of this precursor yielded Co_NP/HCS‐900, whereas pyrolysis with melamine (acting as an N source) at 300 °C produced Co_NP/N‐HCS‐300. Additionally, melamine during the heating process encouraged N incorporation and coordination with Co atoms, forming Co‐N_x sites and facilitating Co atom dispersion. Through precise adjustment of heat treatment time, individual Co atoms could combine into Co₂N₅ dimers through a thermal migration process (Figure 19c,d). This methodology supports the scalable production of 22 distinct DASCs. During synthesis, the spin states of Co atoms were dynamically modulated as they transitioned from nanoparticle to single atom and ultimately to dual‐atom structures. The Co₂N₅ dimer, with its relatively low‐spin configuration, successfully circumvented the conventional LSR between ΔG_OOH* and ΔG_OH*, enabling ideal adsorption of O^* and moderate interaction with H^*, thus achieving a balanced H^*/O^* binding profile (Figure 19b). Electrocatalytic testing demonstrated that Co₂‐N‐HCS‐900 possessed high HER activity, with an overpotential of 166 mV at 10 mA cm⁻² and 252 mV at 100 mA cm⁻², along with a Tafel slope of 83.9 mV dec⁻¹. The catalyst remained operational for over 1000 h, evidencing strong stability during extended water‐splitting operation.

Figure 19.

Research on HER and HOR catalysts design and performance. a) Synthesis of Fe₂‐S₁N₅/SNC. b) Linear correlation between the magnetic moment and ΔG_H*. c,d) Structure characterizations of Fe₂‐S₁N₅/SNC. Reproduced with permission.^{[ 148 ]} Copyright 2023, Springer Nature. e,f) Mechanism for improving the HOR performance of Mn₁O_x(OH)_y@Ru/C. Reproduced with permission.^{[ 7b ]} Copyright 2024, American Chemical Society.

With advancements in anion exchange membranes and Pt‐free catalysts for cathode ORR under alkaline conditions, anion exchange membrane fuel cells (AEMFCs) exhibit great application prospects.^{[ 2} , ^{4 ]} Electrocatalysts possessing an optimal (apparent) adsorption/dissociation equilibrium of H₂ and hydroxyl (OH_ad) can effectively augment alkaline HOR activity.^{[ 6 ]} The activity of Ru‐based alkaline HOR electrocatalysts typically decreases rapidly with an increment in catalytic potential, owing to their stronger oxygen affinity compared to Pt. Even the highly active Ru₇Ni₃ cannot evade this predicament. Additionally, during the startup and shutdown of hydrogen fuel cells, cell polarity reversal customarily occurs unavoidably, leading to an elevation in anode potential. When a single anion exchange membrane fuel cell stack is employed, the anode potential also augments with an increase in internal resistance. Therefore, a viable HOR catalyst must sustain high performance at potentials reaching 0.3 V to obtain considerable power output. Ru tends to combine with oxygen species in solution to form OH_ad–Mn⁺(H₂O) at low potentials (<0.4 V), thereby improving the kinetic process. However, Ru‐based materials have a stronger adsorption capacity for oxygen species at higher potentials (>0.4 V), and the nearly stable Ru–O species across the Helmholtz layer, at a distance of 3.5 Å from the surface, thereby inhibiting the adsorption of active H^* and oxygen species during HOR and accelerating the corrosion of Ru‐based electrocatalysts. To resolve this challenge, Chen et al. postulated that the utilization of excess electrons in highly energetic transfer‐charged Ru nanoparticles can curtail the adsorption of oxygen species,^{[ 7b ]} thereby considerably enhancing the stability of the electrocatalyst (Figure 19e,f). Mn, being a redox‐sensitive metal with variable oxidation states, serves as a valuable electron acceptor. For example, the d orbitals of manganese are split in an octahedral field (e_g and t_2g), with (most) electrons in the t_2g shell and the e_g state empty, enabling it to accommodate foreign electrons. If manganese clusters are constructed on Ru nanoparticles, and the Ru nanoparticles are overcharged in the electric field applied during the HOR reaction, electrons may be transferred to manganese, and the low‐spin state manganese sites within the cluster may be excited to a high‐spin state. Therefore, the authors constructed atomically dispersed Mn₁O_x(OH)_y clusters on the Ru surface, enabling the Mn₁O_x(OH)_y@Ru/C catalyst to exhibit ultra‐high activity and durability across a broad potential window (0.0–1.0 V).

In summary, the electrocatalytic HER and HOR, as core half‐reactions in hydrogen energy technologies, directly determine the performance of water electrolysis for hydrogen production and fuel cells. As a novel strategy, spin engineering significantly influences the adsorption energy of hydrogen intermediates, proton‐coupled electron transfer processes, and interfacial water molecule structures by regulating the spin structure of active sites in catalysts, emerging as a key approach to enhance HER/HOR performance. Despite notable advancements in spin regulation for HER and HOR, several challenges remain to be addressed. For HER: 1) Lack of stability data for spin catalysts under high current densities (>500 mA cm⁻²). 2) Incomplete clarification of the correlation between spin states and complex interfacial electric double‐layer structures. For HOR: 1) Unclear stability mechanisms of spin catalysts under high‐pH conditions. 2) Absence of in‐situ evidence for the correlation between spin states and interfacial water molecule structures. 3) Virtual absence of research on spin effects in actual membrane electrode assembly (MEA) fabrication. Future directions should focus on developing in situ magnetic characterization techniques, designing adaptive spin‐regulating materials, exploring integrated innovation of spin engineering with membrane electrode technology, and exploring the integrated application of spin engineering in anion exchange membrane fuel cells.

5. Characterization Techniques and High‐Throughput Screening DASCs Model

5.1. Characterization Techniques

To elucidate the mechanism of spin‐dependent electrocatalytic reactions of electrocatalysts, spin configuration characterization techniques are required to bridge the gap between spin‐related theory and catalytic applications. Principal techniques include electron paramagnetic resonance (EPR), temperature‐dependent magnetization, Mössbauer spectroscopy, K‐ and L‐edge X‐ray absorption spectroscopy (XAS), and scanning tunneling microscopy (STM).

5.1.1. Electron Paramagnetic Resonance

EPR, also known as electron spin resonance (ESR), is a spectroscopic technique that provides insight into the electronic and structural dynamics of systems with unpaired electrons.^{[ 162 ]} EPR operates on the principle of Zeeman splitting, wherein unpaired electron spin states become energetically distinct in the presence of an external magnetic field. Upon exposure to microwave radiation with energy corresponding to this splitting, resonant absorption occurs, facilitating electron transitions between spin states. The resulting spectral features, line position, shape, and intensity, yield critical information regarding the concentration, spin state, and magnetic interactions of the unpaired electrons present in the sample.

EPR has proven instrumental in investigating the spin behavior of catalytic materials. Deligiannakis et al. used EPR to monitor redox transitions and spin state changes in FePc during catalytic oxidation processes.^{[ 163 ]} The axial coordination of the oxidant leads to a complex multi‐path redox/spin conformation of FePc. Among them, the high‐spin conformation of FePc/imidazole evolves more slowly than the low‐spin one, which helps to distinguish between the catalytically active and inactive conformations and is beneficial for revealing the catalytic reaction mechanism. Building on this research, Sun et al. constructed an axially coordinated Fe‐N₄ catalyst using poly(iron phthalocyanine) (PFePc) and determined the spin states of the Fe centers in the catalyst using EPR characterization technology (Figure 20a).^{[ 50 ]} PFePc‐I and PFePc‐OH exhibited major EPR signals at g‐values of 5.91 and 2.09, indicative of high‐spin Fe³⁺ (S = 5/2) species with axial symmetry. A minor signal at g = 4.21 was attributed to intermediate‐spin Fe (S = 3/2). In contrast, high‐spin Fe³⁺ signals were absent in PFePc‐NCS and PFePc, implying that Fe³⁺ in these samples primarily exists in the intermediate‐spin state. The correlation of EPR data with computational studies and other spectroscopic analyses revealed that catalysts coordinated with weak‐field ligands (e.g., PFePc‐I) exhibited enhanced ORR activity, associated with the stabilization of lower Fe $3d_{z^2}$ orbital energy levels. In another study, Guo et al. developed Fe–Cu dual‐atom catalysts to enhance ORR via spin‐state modulation and a tandem reaction mechanism.^{[ 153 ]} EPR spectroscopy revealed substantial alterations in the spectral line shape and g‐factor values relative to those of single‐atom catalysts, confirming that the spin configuration of the FeN₄ active sites had undergone significant transformation. In summary, EPR spectroscopy offers a highly sensitive and selective platform for elucidating the spin configurations of DASCs. It plays a pivotal role in advancing understanding of catalytic mechanisms, guiding rational catalyst design, and enabling the optimization of catalytic performance.

Figure 20.

Characterization techniques. a) X‐band EPR spectra.^{[ 50 ]} Copyright 2023, Wiley‐VCH. b) M‐T and χ⁻¹‐T curves.^{[ 166 ]} Copyright 2024, Wiley‐VCH. c) ⁵⁷Fe Mössbauer spectrum.^{[ 170 ]} Copyright 2023, Wiley‐VCH. d) XANES spectra at K‐edge. e) FT‐EXAFS spectra.^{[ 172 ]} Copyright 2024, Wiley‐VCH. f,g) XMCD spectra.^{[ 173 ]} Copyright 2024, Wiley‐VCH. h) Real‐space SP‐STM imaging of spin density waves.^{[ 177 ]} Copyright 2022, Springer Nature. i) Schematic of STM measurements. j) STM‐IETS measurements.^{[ 178 ]} Copyright 2019, The American Association for the Advancement of Science.

5.1.2. Temperature‐Dependent Magnetization

Temperature‐dependent magnetization (M‐T) is a fundamental characterization technique for probing the evolution of a material's magnetic properties as a function of temperature. This method provides critical insights into magnetic phase transitions, spin states, and the underlying microscopic magnetic interactions within materials. It is typically implemented using either a vibrating sample magnetometer (VSM) or a superconducting quantum interference device (SQUID).^{[ 164 ]} Changes in temperature influence magnetic interactions and ordering, leading to variations in the overall magnetization of the material. In ferromagnetic systems, a phase transition from ferromagnetism to paramagnetism occurs upon reaching the Curie temperature (T_C ), typically accompanied by a marked and abrupt drop in magnetization. In the paramagnetic regime (T > T_C ), the magnetic susceptibility (χ) generally adheres to the Curie–Weiss law: (χ = C/(T – θ)) (where C is the Curie constant), which is correlated with the effective magnetic moment (µ _eff), and (θ) is the Weiss temperature, indicative of magnetic interaction strength (θ > 0 for ferromagnetic, and θ < 0 for antiferromagnetic behavior).^{[ 75} , ^{165 ]} Once the value of C is determined, the effective magnetic moment can be calculated using the relation ${\mu }_{\mathrm{eff}}=\sqrt{8C}{\mu }_{\mathrm{B}}$, where µ _B is the Bohr magneton. The spin state of transition metals can be inferred from the number of unpaired electrons or the spin quantum number. The number of unpaired electrons, n, is calculated using $({\mu }_{\mathrm{eff}}=\sqrt{n(n+2)}{\mu }_{\mathrm{B}})$, where n is the number of unpaired electrons. The spin quantum number can be derived using µ _eff = gS _eff µ _B, where g is the Landé factor (typically approximated as 2.0). These parameters provide valuable information for evaluating the spin configurations in transition metal centers. For example, Qu et al. covalently linked FePc to CuO nanosheets via click confinement.^{[ 166 ]} M‐T measurements (Figure 20b) and DFT calculations revealed that the spin state of Fe transitioned from intermediate‐spin to low‐spin, thereby enhancing the d‐p orbital hybridization between Fe and lithium polysulfides. Lan et al. synthesized a Hofmann‐type framework using pyrazine ligands and tetracyanoplatinate ions to study the effect of Fe²⁺ spin states on the photocatalytic production of H₂O₂.^{[ 167 ]} M‐T and in situ characterization revealed that only the low‐spin Fe²⁺ state (S = 0) could complete the entire catalytic cycle, while the high‐spin Fe²⁺ (S = 2) was only capable of driving half of the reaction. Similarly, Lum et al. employed SQUID measurements to demonstrate that phosphorus doping induced a spin transition in Fe–N₄ from a low‐spin configuration $({d_{x^2-y^2}}^0,{d_{z^2}}^0,{d_{xz}}^2,{d_{yz}}^1,{d_{xy}}^2)$ to a high‐spin state (d_x2‐y2 ⁰, d_xz ¹, d_yz ¹, d_z2 ¹, d_xy ²) in Fe‐N₃‐P.^{[ 46a ]} In the context of DASCs, Zhang et al. used temperature‐dependent susceptibility (χ‐T) characterization to demonstrate that in the Fe,Mn/N‐C catalyst, neighboring atomically dispersed Mn‐N sites effectively activated Fe³⁺ sites, leading to a t_2g ⁴e_g ¹ configuration at the FeN₄ sites. This configuration facilitates electron transfer into the antibonding orbitals of oxygen, thereby enhancing O₂ activation. Overall, temperature‐dependent magnetization techniques, by evaluating magnetization across temperature ranges and computing effective magnetic moments, provide essential information for determining spin states, identifying magnetic phase transitions, and investigating spin‐lattice coupling. These insights contribute significantly to understanding the relationship between spin configurations and catalytic performance in dual‐atom site catalysts.

5.1.3. Mössbauer Spectroscopy

Mössbauer spectroscopy is a highly sensitive technique based on the recoilless resonant absorption of γ‐rays by atomic nuclei. It is particularly well‐suited for probing the electronic structures, spin states, and local coordination environments of materials containing specific isotopes, such as ⁵⁷Fe and ¹¹⁹Sn.^{[ 168 ]} The Mössbauer effect refers to the phenomenon wherein certain atomic nuclei emit and absorb γ‐rays without energy loss due to nuclear recoil, thus enabling resonant γ‐ray absorption under well‐defined conditions. Atomic nuclei situated in distinct chemical environments exhibit characteristic Mössbauer spectral features, including variations in isomer shifts, quadrupole splitting, and magnetic hyperfine interactions. These spectral signatures provide valuable insights into the oxidation state, electronic configuration, and magnetic properties of the nucleus and its surrounding environment. For example, in the case of ⁵⁷Fe, different oxidation states and coordination geometries yield distinct hyperfine parameters, which can be used to analyze the structure and reactivity of iron‐containing materials. In the context of DASCs, Mössbauer spectroscopy serves as a powerful diagnostic tool for confirming atomic dispersion and identifying the electronic and magnetic environments of metal sites. Wu et al. utilized ⁵⁷Fe Mössbauer spectroscopy to verify the atomic‐level dispersion of Fe in the Pd₁@Fe₁ catalyst system.^{[ 169 ]} Moreover, Mössbauer spectroscopy allows for the precise determination of oxidation states, coordination environments, and spin states of Fe atoms in DASCs. Fu et al. applied Mössbauer spectroscopy (Figure 20c) to reveal that isolated Fe atoms predominantly existed in the low‐spin state of Fe²⁺ (denoted as D1).^{[ 170 ]} Upon the introduction of Cu atoms, perturbations in the crystal field surrounding Fe led to an increase in the population of intermediate‐spin Fe³⁺ species (D2), accompanied by a noticeable decrease in the D1 signal, indicating a crystal‐field‐induced spin‐state transition from low‐spin Fe²⁺ to intermediate‐spin Fe³⁺. Under an applied magnetic field, both D1 and D2 peak intensities decreased, while the D3 peak, corresponding to high‐spin Fe³⁺, increased significantly, demonstrating a further transition to the high‐spin state. Owing to its exceptionally high energy resolution, Mössbauer spectroscopy is uniquely capable of detecting subtle electronic structure changes and elucidating the origin of catalytic activity in Fe‐based active sites. Looking forward, the integration of time‐resolved Mössbauer spectroscopy (on the femtosecond scale) with artificial intelligence‐assisted spectral interpretation is anticipated to significantly advance mechanistic understanding in spin‐catalysis and accelerate the rational design of next‐generation spin‐engineered catalysts.

5.1.4. X‐Ray Absorption Spectroscopy

XAS, a synchrotron radiation‐based technique, is widely employed to investigate the local electronic structure, coordination environment, and oxidation states of elements by measuring the energy‐dependent absorption of X‐rays by a sample.^{[ 171 ]} Depending on the energy range and specific analytical objectives, XAS is generally divided into three major categories: 1) X‐ray absorption near‐edge structure (XANES). This region, extending ≈50–100 eV around the absorption edge, is highly sensitive to the oxidation state of the absorbing atom (e.g., the edge position shift between Fe²⁺ and Fe³⁺), the electronic configuration (such as the occupancy and splitting of d‐orbitals, e.g., e_g and t_2g orbitals), and coordination geometry (e.g., octahedral, tetrahedral, or square planar). XANES provides critical information on the local electronic and geometric environment surrounding the absorbing atom. (2) Extended X‐ray absorption fine structure (EXAFS). Located in the energy region starting about 100 eV above the absorption edge and extending to several thousand eV, EXAFS arises from the interference of outgoing photoelectrons with electrons scattered by neighboring atoms. Analysis of EXAFS enables the determination of interatomic distances, coordination numbers, the identity of surrounding atoms, and the degree of local structural disorder, thereby offering a detailed view of the atom's local structure. 3) L‐edge XAS. Specifically suitable for transition metal elements such as Fe, Co, and Ni, this technique detects the transitions of 2p electrons to unoccupied d‐orbitals. It provides valuable insights into spin states, indicated by the L₃/L₂ peak intensity ratio (≈2:1 for high‐spin and around 4:1 for low‐spin states), as well as oxidation states (e.g., energy shifts of the L₃ edge between Co²⁺ and Co³⁺), and coordination field symmetry (inferred from changes in spectral peak shapes due to crystal field splitting). In the context of spin‐catalysis involving DASCs, XAS plays a pivotal role in elucidating the spin states of metal centers, identifying local coordination symmetries, and revealing synergistic effects between dual sites, thus guiding the rational design of high‐performance catalysts. For example, Song et al. utilized XANES and EXAFS to investigate the local coordination chemistry of Mn‐Zn dual‐atom sites.^{[ 172 ]} The Mn K‐edge XANES (Figure 20d) revealed that Mn exhibited oxidation states between +2 and +3, while Zn ranged from 0 to +2. In the Mn EXAFS spectrum (Figure 20e), a peak at 1.44 Å was attributed to first‐shell Mn–N coordination, and a peak at 2.23 Å corresponded to Mn–Zn bonding. The Zn EXAFS spectrum showed a main peak at 1.5 Å, assigned to Zn–N scattering. Notably, no other metal–metal coordination peaks were observed, confirming the atomic‐level dispersion of Mn and Zn. By integrating quantitative EXAFS fitting, simulated spectra, and theoretical modeling, the MnZnN₆ configuration was identified, where both Mn and Zn atoms are coordinated with four N atoms. Zhang et al. employed L‐edge XANES to assess the structural stability of a FeCo dual‐site catalyst before and after OER testing.^{[ 1b ]} The consistent strength ratio of Fe L‐edge features indicated that Fe³⁺ retained a valence configuration corresponding to an orbital occupancy near unity. This suggested that, despite valence fluctuations during the catalytic cycle, the spin state remained in the favorable intermediate‐spin configuration (t_2g ⁵e_g ¹). Based on these observations, a CoFe dual‐site OPM mechanism was proposed, linking spin‐orbit interactions at the metal‐oxygen interface with the observed redox behavior of the metal sites.

X‐ray magnetic circular dichroism (XMCD) is a synchrotron‐based spectroscopic technique that enables the element‐specific probing of magnetic properties in materials (Figure 20f,g).^{[ 173 ]} When magnetic samples are irradiated with circularly polarized X‐rays, the asymmetry in the interaction of the spin magnetic moment (m_s) and the orbital magnetic moment (m_l) of the electrons in the magnetic atoms with left‐ and right‐circularly polarized light leads to a differential absorption intensity, known as the XMCD signal. This differential signal directly reflects the magnitude and orientation of the magnetic moments of the probed atoms and reveals detailed information regarding their electronic and magnetic configurations. Using XMCD, Fan et al. identified an anomalously large m_l/m_s ratio (>0.1) in a single‐atom cobalt catalyst,^{[ 174 ]} which was indicative of pronounced SOC effects that favored OER kinetics. In another study, Shao et al. conducted element‐resolved XMCD measurements to uncover ferromagnetic interactions between ruthenium and manganese centers in a Mn‐RuO₂ dual‐atom catalyst,^{[ 175 ]} which adhered to the Goodenough–Kanamori exchange rule. This ferromagnetic coupling contributed to significantly reduced OER overpotentials in acidic media. The synergistic integration of XMCD with complementary techniques such as XAS and temperature‐dependent magnetometry (M–T) is increasingly recognized as indispensable for the rational development of spin‐regulated electrocatalysts. Such combined approaches advance the design of catalytic materials at the intersection of atomic‐level precision and spin state modulation.

5.1.5. Scanning Tunneling Microscopy

STM is a surface‐sensitive technique capable of resolving atomic‐scale features of material surfaces. Its extension into spin‐resolved measurements, known as spin‐polarized STM (SP‐STM), enables the mapping of local magnetic states by detecting spin polarization within the tunneling current through the use of a magnetized probe.^{[ 176 ]} SP‐STM provides direct visualization of magnetic configurations at the atomic level, including isolated atoms and clusters. For example, Zhang et al. utilized low‐temperature SP‐STM to capture real‐space evidence of itinerant spin density wave (SDW) behavior on the surface of a chromium single crystal (Figure 20h).^{[ 177 ]} Simultaneously, the corresponding charge density waves were observed, allowing insights into the phase relationship and domain structure of these coexisting quantum orders. Inelastic electron tunneling spectroscopy (IETS), when integrated with STM, permits the detection of magnetic excitation spectra. Through electron scattering processes, IETS reveals critical parameters such as spin excitation energies and magnetic anisotropies of surface‐anchored species. Wu et al., for instance, studied spin interactions at the sub‐ångström scale by employing magnetic molecules on the STM tip to interrogate single‐molecule magnets adsorbed on surfaces (Figure 20i,j).^{[ 178 ]} Their findings exposed a strong hybridization of quantum states between adjacent magnetic molecules. Although SP‐STM and STM‐IETS have not yet seen widespread adoption in the field of electrocatalysis, their potential to unravel spin‐structure‐performance relationships at the atomic level holds substantial promise. Mastery of these techniques will offer powerful analytical routes to decode the fundamental mechanisms underlying spin‐regulated behavior in DASCs.

5.2. High‐Throughput Screening DASCs Model

Owing to the proximity of coordination‐unsaturated metal sites and the cooperative interaction between adjacent atoms, atomically dispersed DASCs exhibit remarkable catalytic performance in certain catalytic reactions, with particular emphasis on electrocatalytic reactions.^{[ 179 ]} The spin state at the active center, whether low, intermediate, or high, or its magnetic alignment significantly influences reaction energy barriers by modulating the interaction strength with adsorbed intermediates.^{[ 180 ]} Notwithstanding extensive research endeavors, the precise elucidation of the correlation between spin dynamics and catalytic efficacy, alongside the precise and reproducible synthesis of highly active DASCs, continues to pose major challenges. By tailoring the catalyst surface's electronic characteristics, considering both charge and spin properties, the link between surface electronic structure and catalytic efficiency can be better understood, aiding in property prediction. First‐principles calculations based on DFT have occupied a pivotal position in the realm of catalysis. However, their computational cost, particularly for accurate energy barrier calculations, remains considerable.^{[ 181 ]} High‐throughput DFT and ML have emerged as significant means for the discovery and optimization of spin‐sensitive catalytic materials, presenting novel opportunities for theoretical and experimental research in electrochemistry and materials science.^{[ 182 ]}

To efficiently bridge computational design and experimental validation, the selection of meaningful descriptors is crucial. Although the traditional d‐band model facilitates the understanding of adsorption trends, its accuracy in predicting performance within specific systems still needs to be improved.^{[ 183 ]} Lee et al. proposed using the total DOS profile as a descriptor for high‐throughput catalyst screening.^{[ 184 ]} Employing DFT calculations, they examined 4350 bimetallic alloy structures, focusing on their closest‐packed surfaces, to evaluate thermodynamic viability and shortlisted eight candidates exhibiting catalytic properties on par with Pd, which were subsequently confirmed through experimental validation. This strategy highlights the potential to identify novel alternatives to Pt group metals in catalysis.

High‐throughput DFT methodologies have significantly contributed to the theoretical modeling and rational design of SACs. Through DFT and other theoretical tools, the reaction mechanisms of several catalysts/supports can be probed at the molecular level, facilitating the identification and prediction of prospective catalysts. The principal obstacle, however, is encountered in formulating universally applicable descriptors that accurately reflect the complex interplay between structural geometry and electronic configuration. Zeng et al., using ML, introduced a structural descriptor φ′ for evaluating SAC performance in reactions such as ORR, OER, and HER.^{[ 182 ]} Unlike its predecessor φ, the φ′ descriptor accounts not only for the local electronic environment and coordination number of the metal center but also for its intrinsic valence electron characteristics. Applied across 112 graphene‐based SACs with varied local coordination, φ′ showed a robust correlation with catalytic behavior. The descriptor has proven effective in rationalizing the observed experimental activity trends and offers theoretical guidance for future experiments. Meanwhile, the universality of φ′ is not only limited to graphene‐based SACs but also applicable to SACs embedded in macrocyclic complexes of varied sizes, as long as the metal center maintains the same local coordination environment, demonstrating its wide applicability on carbon‐based supports with different structural variations. However, researchers observe that the current descriptor φ′ does not encompass all contemplated eigenvectors. This accounts for its precision deficiency in correlating adsorption energies of all metal atoms and the consequent prediction errors at specific starting potential values. The pursuit of comprehensive and multipurpose descriptors (φ″, φ″′…) with requisite accuracy mandates more exhaustive research in the future. DASCs often exhibit excellent electrocatalytic activity due to their flexible combinations and synergetic effects. However, neglecting the dynamic axial adsorption of active sites and reactant adsorption as the RDE underworking potential has hindered the establishment of accurate high‐throughput screening strategies. Cao et al. systematically investigated the ORR of 42 kinds of 3d‐3d metal DASCs through DFT calculations.^{[ 183 ]} They discovered that both the proton–electron transfer step and ORR kinetics are limited by O₂ ^* adsorption, and the active centers of DASCs could be reconstructed through the axial pre‐adsorption of intermediates under working potential. Accordingly, an ORR volcano plot was devised, with O₂ ^* and OH^* adsorption serving as activity descriptors. Subsequently, the researchers devised a high‐throughput screening approach and identified 38 prospective ORR DASCs from 267 DASCs incorporating 3d, 4d, or 5d metals, which was experimentally validated. This work not only proposed a high‐throughput screening method based on the volcano plot but also provided proof of concept for experimentally validating theoretical predictions, which can inspire the heuristic design of electrocatalysts for other reactions.

To address the lack of universal descriptors, research has emphasized combining physical insights to construct a streamlined feature space for identifying the key factors influencing catalytic performance. Interpretable ML and symbolic regression algorithms enable the development of descriptor models that are effective in high‐dimensional systems.^{[ 102} , ^{185 ]} Gong et al. proposed a general mathematical descriptor, termed ARSC, representing atomic characteristics (A), reactant properties (R), synergistic effects (S), and coordination interactions (C).^{[ 186 ]} This descriptor is tailored for the comprehensive integration of various electrocatalytic reactions and provides in‐depth insight into the nature of dual‐atom sites. The ARSC model is grounded in the physically meaningful feature engineering and feature selection/sparsification (PFESS) approach, which draws from d‐band theory and frontier molecular orbital theory. This framework enables simultaneous prediction of activity and selectivity across multiple electrocatalytic pathways and highlights the decisive influence of d‐orbital overlap on dual‐atom site behavior. By utilizing less than 4,500 DFT calculations, the ARSC model can rapidly predict highly active dual‐atom sites, significantly reducing the reliance on traditional high‐throughput calculations (>50 000 times). Up to now, most studies have mainly focused on the catalytic activity of DASCs in catalytic reactions,^{[ 187 ]} while lacking theoretical research on the stability of DASCs in different environments. Therefore, screening thermodynamically stable DASCs is of significant guiding importance for the experimental synthesis of novel materials. Cao et al. constructed 335 dual‐atom models (M₁M₂‐NC) using N‐doped graphene as the substrate through high‐throughput DFT.^{[ 179 ]} When DASCs serve as catalysts for electrocatalytic reactions, the presence of an applied potential may induce the dissolution of metal atoms in DASCs, thereby resulting in the destruction of active sites. Consequently, the authors evaluated the electrochemical stability of DASCs in ORR, OER, and HER by computing the dissolution potential of DASCs based on the theoretical working potentials of commercial catalysts in these reactions. Furthermore, they developed a predictive model linking various intrinsic metal properties, such as valence electron count and electronegativity, to the thermodynamic stability of DASC structures. This model efficiently estimates resistance to single‐atom isolation, metal aggregation, and electrochemical degradation. The design space of DASCs is extensive, and it is not feasible to screen all potential catalysts cost‐effectively and expeditiously through experiments or theoretical simulations. Therefore, high‐throughput computing deserves broader attention, as it enables rapid screening and calculation of vast material systems, precisely identifying element combinations suitable for constructing dual‐atom sites with specific spin‐related properties.^{[ 188 ]} With the aid of quantum mechanical methods, high‐throughput calculations can simulate interactions such as spin‐orbit, charge, and lattice under different atomic arrangements and electronic structures and effectively predict the spin states of different dual‐atom sites and their impacts on catalytic activity. Thus, the geometric and electronic structures of dual‐atom sites can be optimized to enhance their adsorption and activation capabilities for reactants. Meanwhile, high‐throughput calculation can assist in analyzing the interactions between the catalyst and the support, ensuring the stability of dual‐atom sites. Additionally, it can also evaluate the performance of the catalyst in various electrocatalytic reactions in advance, shortening the development cycle and reducing experimental costs, providing theoretical support for the development of high‐efficiency DASCs.

The steps for screening novel and highly efficient DASCs by integrating spin engineering with machine learning are summarized in Figure 21 . The first step is data generation: 1) Experimental data collection. A large amount of experimental data related to spin regulation is collected, covering the spin state data of DASCs under various conditions, including spin configurations, spin‐related physical properties (such as magnetic moments, spin polarizability), as well as corresponding performance metrics in electrocatalysis, magnetism, etc. 2) Theoretical calculation data generation. With the help of quantum mechanical calculation methods, such as DFT, are employed to simulate the spin structures, properties, and catalytic performances of materials. This includes parameters like d‐band centers, spin densities, binding energies, overpotentials, etc. The relationships between atomic structures, electron cloud distributions, spin states, and catalytic performances are derived through these calculations, providing supplementary data to the experimental results. 3) Spin feature extraction. Key features reflecting spin‐related properties are extracted from the data. This may include quantitative indicators such as spin quantum numbers, SOC strengths, and spin density distributions. For complex systems, spin‐based descriptors, such as local environment descriptors of spin centers, are constructed to characterize the effects of atomic types, distances, and arrangements around spin‐active sites on spin properties.

Figure 21.

Machine learning steps for high‐throughput screening of DASCs.

The second step is model training and obtaining the optimal model: 1) Model selection. For supervised learning tasks, where clear relationships exist between inputs (such as material structures or external conditions) and outputs (such as spin states or catalytic performances), appropriate machine learning models are selected. These may include decision trees, support vector machines, or neural networks. 2) Model training and optimization. a) Designing the loss function: A suitable loss function is chosen based on the specific task. For regression tasks predicting spin states, a mean squared error (MSE) loss function is applied, while for classification tasks (e.g., distinguishing between different spin phases), a cross‐entropy loss function is employed. Spin‐related physical constraints may be incorporated into the loss function to improve the model's predictive accuracy. b) Hyperparameter adjustment: The model's hyperparameters (e.g., the number of layers in a neural network, the number of neurons, learning rate) are optimized through methods such as grid search, random search, or Bayesian optimization to achieve the best performance on the training data. c) Data augmentation and regularization: To prevent overfitting, spin‐related data undergo augmentation (e.g., rotating and scaling spin images). Techniques such as L1 or L2 regularization and dropout are used to limit model complexity and enhance generalization capabilities.

The third step is the evaluation and application of the model, that is, screening DASCs with specific spin configurations and excellent catalytic performances based on ML: 1) Selection of model evaluation indicators. Multiple evaluation metrics are employed to assess the model's performance in tasks related to spin properties and other relevant outcomes. 2) Applications in spin engineering. a) Material design optimization: The trained machine learning model predicts spin states and material performances under various design parameters. This enables the efficient design of new spin‐regulated materials. For instance, when designing new DASCs, the model can predict how spin states influence catalytic activity based on atomic configurations and structures, facilitating rapid screening of high‐performance material structures and reducing experimental trial‐and‐error costs. b) Process optimization and control: In spin‐regulation processes (e.g., electrocatalytic reactions), real‐time monitored reaction conditions (such as temperature, pressure, and electric field) and spin‐related signals are used as inputs. The machine learning model predicts the progress and final performance of the reaction, thereby optimizing and precisely controlling the reaction process.

6. Challenges and Opportunities

In summary, the main methods for effectively modulating the spin structure of DASCs to enhance electrocatalytic performance can be systematically categorized as follows: 1) Coordination environment regulation of each individual 3d metal centers (e.g., ligand type, coordination number, or coordination symmetry), which alters the crystal field splitting energy to influence spin states of these metal centers, thereby affecting the spin‐dependent behaviors. 2) Metal–metal interaction regulation, including interatomic electron transfer and spin/electron exchange interactions between two adjacent metals), which modulate the spin multiplicity of metal centers and overall spin polarization through spatial arrangement, electronic coupling, or heteronuclear pairing. 3) External magnetic field regulation, which induces spin polarization by aligning unpaired spins or breaking spin degeneracy. Compared to traditional bulk materials with strong magnetic ordering, DASCs feature spatially separated and magnetically flexible dual‐atom sites, making them more susceptible to magnetic perturbations. This enables more efficient spin manipulation under external fields, thereby enhancing spin‐selective reaction pathways. Despite the promising potential of DASCs in spin‐regulated electrocatalysis, the precise regulation and understanding of spin states in such systems remain highly challenging. One of the most fundamental obstacles lies in the difficulty of detecting spin‐related signals at the low atomic concentrations typically associated with dual‐atom sites. Conventional magnetic characterization techniques, such as XMCD or EPR, often lack the spatial or sensitivity resolution to distinguish spin states at such dilute levels, especially under operando conditions. This severely limits our ability to track the real‐time evolution of spin configurations during the catalytic process. Moreover, the intrinsic structural instability of DASCs under reaction conditions often leads to dynamic changes in spin states. Since dual‐atom sites may undergo migration, aggregation, or coordination transformation, the initially designed spin configuration can evolve into an unanticipated state during catalysis. This dynamic nature poses a significant challenge for correlating the initial spin structure with the actual catalytic performance. Another layer of complexity arises from the fact that the coupling between dual atoms is not solely limited to spin‐spin interactions. Orbital hybridization, charge transfer, and ligand field effects can all co‐exist, making it difficult to decouple the specific contribution of spin exchange from other electronic effects. Furthermore, the exchange interaction between dual atoms, central to spin alignment, is highly sensitive to the local atomic configuration, including bond angles, interatomic distances, and the nature of the surrounding coordination environment. As a result, even small structural perturbations can drastically alter the spin coupling mode (e.g., ferromagnetic vs. antiferromagnetic), leading to significant variations in catalytic behavior.

To address these challenges, future research must focus on developing ultra‐sensitive, site‐specific magnetic characterization techniques capable of resolving spin states at the atomic scale under reaction conditions. In parallel, the design of structurally stable DASC systems that can preserve their spin identity during operation is crucial. Theoretical efforts should also aim to isolate the spin contributions from other electronic effects through advanced modeling of spin‐dependent exchange mechanisms. Additionally, high‐throughput screening combined with machine learning may help map the structure–spin–activity relationship by capturing how local coordination modulates spin interactions. The main challenges faced, the corresponding solutions (Figure  22 ), and the future development opportunities are elaborated in detail as follows.

1. Effects of temperature, pressure, and reactant concentration on spin‐dependent electrocatalysis. In actual electrochemical energy conversion processes, reaction temperatures often exceed ambient levels. For example, the operating temperature of low‐temperature hydrogen fuel cells ranges from 60 to 90 °C. Magnetic materials possess a characteristic temperature for magnetic transition, namely the Curie temperature. Below the Curie temperature, they exhibit ferromagnetism; above it, paramagnetism prevails. Nevertheless, the Curie temperature of most ferromagnetic catalysts approximates or even lies below the ambient temperature. Therefore, at high operating temperatures, the ability of spin regulation to influence catalytic performance may decline. In addition to temperature, pressure and reactant concentration can significantly influence the spin configuration and catalytic performance of catalysts. Variations in pressure can modulate the adsorption strength and surface coverage of reactants, thereby facilitating electron transfer from the catalyst to the adsorbates and altering the distribution of spin‐polarized electrons. High‐pressure environments may also induce lattice distortion or phase transitions in the catalyst, further modifying the spin configuration of active sites and consequently affecting catalytic activity. Reactant concentration, such as H⁺ or OH^‐ can alter local pH, influencing the surface charge state of the catalyst and the strength of SOC. Additionally, the synergistic effects of high pressure and high reactant concentration are expected to markedly enhance mass transport efficiency while intensifying the interfacial electric field strength. However, research on these factors remains limited. Understanding the effects of temperature, pressure, and reactant concentration on spin configurations is crucial for practical spin‐dependent electrocatalytic applications. To elucidate the regulatory mechanisms of these factors, three approaches can be pursued: 1) Combining synchrotron radiation with property measurement. In situ XMCD can be paired with a high‐pressure electrochemical cell to directly observe the evolution of spin states under varying conditions. 2) Designing a controllable experimental environment. Microfluidic reactors can be used to precisely regulate local pressure and concentration gradients, isolating the influence of these variables. Alternatively, isotope labeling methods can trace how reactant concentration influences the spin pathway. 3) Utilizing machine learning for modeling. Machine learning can assist in developing models to predict the spin phase diagram under multiple parameters by training neural networks with experimental data.

2. Distinguish the spin states of the bulk phase and the surface phase, as well as those of the active and surface phases. Catalytic reactions take place on the catalyst surface, where the properties deviate from those of the bulk. Different metal cations exhibit diverse spin states under varying conditions and locations. Hence, during spin state characterization, the disparity between bulk and surface spin states must be accentuated. In catalytic reactions, the surface phase may undergo reconstruction, accompanied by potential alterations in chemical composition, electronic structure, and spin state, which remain inadequately explored. Thus, to comprehensively discuss the role of the spin state in electrocatalysts, a systematic examination of the catalytic behaviors of spin states within the surface‐active phase, surface, and bulk is essential. Achieving this goal necessitates the integration of advanced experimental methods with theoretical calculations. A critical component of this approach is the use of experimental characterization techniques. For instance, angle‐resolved photoelectron spectroscopy (ARPES) allows for adjustments to the detection depth by varying the emission angle of photoelectrons, enabling the acquisition of electronic structure information from both the surface and bulk regions. Scanning probe SP‐STM can detect the spin orientations and spin densities of surface atoms, effectively distinguishing the spin states of the surface phase. Another effective method involves selecting chemical probe molecules with specific reactivity that selectively interact with the active phase on the catalyst surface. By monitoring the spin‐related signals of these probe molecules before and after the reaction, and correlating these signals with surface‐phase characterization via surface analysis techniques, it is possible to differentiate between the spin states of the active and surface phases.

3. Development of operando characterization techniques for accurate spin structure analysis of dual‐atom sites and their dynamic changes during the catalytic process. Accurately characterizing the spin structures of dual‐atom sites and their dynamic changes during electrocatalytic processes presents significant technical challenges. Current characterization techniques are limited in their ability to obtain spin information at the atomic scale. While techniques such as EPR can provide spin‐related data, their application in complex electrocatalytic environments is constrained. High‐resolution spin characterization methods suited for in‐situ electrocatalytic conditions must be developed. For example, combining femtosecond XMCD with STM, and employing operando SP‐STM, would allow for real‐time monitoring of spin dynamics during catalytic reactions.

4. Large‐scale preparation of high‐efficiency dual‐atom site catalytic materials. Dual‐atom site catalytic materials are recognized for their exceptional activity and selectivity owing to atomic‐level coordination and synergistic electronic interactions. Nonetheless, achieving precise dual‐atom sites requires fine control over the arrangement and combination of atoms. Current fabrication strategies include atomic layer deposition and chemical vapor deposition, which can achieve precise control at the atomic level, but they have complex processes and excessive costs, thus hindering large‐scale deployment. Thus, the development of simplified, economical, and scalable synthesis methodologies is imperative. Key challenges in scale‐up include ensuring the anchoring stability of dual‐atom sites and maintaining consistent product quality across production batches. To address these issues, two strategic approaches can be adopted. On one hand, it is necessary to design suitable support materials and utilize the strong interaction between the support and dual‐atom sites to enhance stability. On the other hand, by establishing a mathematical model, precisely control and optimize the key parameters in the preparation process to ensure the stability and consistency of the quality of each batch of products.

Figure 22.

Prospects for the future development of research related to spin‐dependent electrocatalysis.

Advancing the understanding of spin configuration dynamics within DASCs relies on the development of well‐defined spin‐active architectures, sophisticated regulation methodologies, state‐of‐the‐art characterization techniques, and scalable synthesis protocols. Such advancements will inform the rational design of high‐efficiency DASCs tailored for green energy storage and conversion systems, contributing to solutions for global energy challenges. Moreover, as a frontier in catalysis research, spin engineering also has other promising directions beyond DASCs, such as integration with topological quantum materials, spintronic devices, and quantum technologies. These emerging fields offer exciting opportunities to harness spin‐related phenomena like topological edge states, magneto‐electric (ME) coupling, or other effects for advanced applications in catalysis. For instance, topological edge states, with non‐trivial spin‐polarized states existing at the boundaries of certain materials, can conduct electrons with minimal energy loss, even under harsh conditions. This merit could offer several advantages, such as efficient and selective spin‐polarized electron transfer, which is beneficial for spin‐sensitive reactions. Besides, their robust nature also helps stabilize active sites and reaction intermediates, potentially enhancing both the activity and durability of the catalyst. Moreover, when combined with chiral or asymmetric environments, these edge states may further amplify spin selectivity, opening up new pathways for designing high‐performance catalysts. ME coupling effect refers to the interconversion between magnetic and electric polarization, which could enable electric‐field controlled spin reconfiguration, offering a powerful manipulation of the spin‐related factors for enhanced catalytic activity. Especially in ferroelectric–ferromagnetic (multiferroic) catalysts, ME coupling can be exploited to control spin alignment at active sites, modulate electron density, and optimize reaction kinetics without changing the catalyst composition.

Conflict of Interest

The authors declare no conflict of interest.

Biographies

Dongping Xue is a lecturer in the College of Chemistry and Chemical Engineering, Henan Polytechnic University, China. She received her Ph.D. in materials science and engineering from Zhengzhou University, China, in 2024. Currently, her work mainly focuses on spin‐related catalysis and electrocatalysis mechanisms, along with applications in energy conversion.

Zhao Yu is a master's candidate at Henan Polytechnic University. She obtained her B.S. from Henan Polytechnic University, China, in 2024. Her main research direction is electrocatalytic energy conversion reactions.

Jianliang Cao is a Henan Province Distinguished Professor in the College of Chemistry and Chemical Engineering, Henan Polytechnic University, China. He is the Vice Dean of the Zhengzhou Institute for Advanced Research of Henan Polytechnic University, the Vice Dean of Graduate College for Elite Engineers. He is also the Leader of the Science & Technology Innovative Research Team in the University of Henan Province for the Detection, Treatment, and Utilization of Industrial Waste with Environmental Hazards. His research interests focus on the research and development of nanostructure materials for catalysis and gas sensing application.

Yan Wang is a professor in the College of Safety Science and Engineering, Henan Polytechnic University, China. She is the Vice Dean of the School of Safety Science and Engineering, Director of the Office of Safety and Energy Engineering Department, and Director of the Henan Province Engineering Technology Research Center of Explosion Dynamic Disaster Early Warning and Emergency. Her research interests focus on chemical engineering safety and energy storage safety, and won the National Coal Youth Science and Technology Award, the Excellent Young Science and Technology Talent Award in Mining, Petroleum, and Safety Engineering of National Universities, and the Green Mining Youth Science and Technology Award.

Xiaoning Li is a Vice‐Chancellor's Research Fellow at RMIT University. With over 30 papers as the first or corresponding author, her research has garnered more than 2400 citations, and she holds an h‐index of 27. Her expertise spans interdisciplinary studies at the intersections of catalysis, ferroelectricity, and magnetism. Currently, her work focuses on spin‐related catalysis, electrocatalytic, photocatalytic, and piezocatalytic mechanisms, along with applications in energy conversion and storage.

Tianyi Ma is a RMIT University Distinguished Professor, Fellow of Royal Society of Chemistry, and Clarivate's Global Highly Cited Researcher. He is Director of ARC Industrial Transformation Hub for Intelligent Energy Efficiency in Future Protected Cropping (E2Crop), and Research Director of Centre for Atomaterials and Nanomanufacturing (CAN). His ground‐breaking research has been acknowledged by internationally recognised experts and authorities via 2024 Prime Minister's Prize for Science‐the Malcolm McIntosh Prize for Physical Scientist of the Year, AAS Le Févre Medal, Young Tall Poppy Science Award, and Horizon Prize of Royal Society of Chemistry.

Xue D., Zhao Y., Cao J., Wang Y., Li X., and Ma T., “Spin Engineering of Dual‐Atom Site Catalysts for Efficient Electrochemical Energy Conversion.” Adv. Mater. 37, no. 35 (2025): 37, 2504213. 10.1002/adma.202504213