Latest Progress in Pd-Based Metallenes: From Formation Mechanism, Structural Design, Optimization Strategies to Electrocatalytic Applications

Abstract

Recently, two-dimensional (2D) metallene nanomaterials have attracted widespread attention in the field of electrocatalysis, owing to their unique physical and chemical properties. Especially, palladium (Pd)-based metallenes, with their excellent conductivity, high specific surface area, unsaturated atom coordination environment, and tunable electronic structure, have emerged as promising electrocatalysts for green energy technologies such as fuel cells and hydrogen production via water electrolysis. In the review, it systematically elaborates on the recent research progress of Pd-based metallene electrocatalysts. At first, the formation mechanism of their 2D ultrathin sheet structure has been thoroughly analyzed from the perspectives of thermodynamics and kinetics. Second, the key synthesis methods, for example, wet-chemical, templating, and electrochemical synthesis, as well as the modification strategies developed for further optimizing performance, are summarized. Subsequently, based on their element composition and structural characteristics, they are classified into several categories: single-atom metallenes, intermetallic metallenes, high-entropy metallenes, and multimetallic metallenes; meanwhile, their structural advantages are discussed in detail. Finally, the review focuses on the application performance and catalytic mechanisms of Pd-based metallenes in fuel cell reactions such as the oxygen reduction reaction, formic acid oxidation reaction, ethanol oxidation reaction, hydrogen evolution reaction, and oxygen evolution reaction. This review offers a new perspective on the challenges and future development directions of 2D Pd-based metallenes for clean energy electrocatalysis.

1. Introduction

Along with the discovery of graphene in 2004, two-dimensional (2D) ultrathin sheet nanomaterials, such as nonmetallic elemental graphene, hexagonal boron nitride, transition metal sulfides, layered double hydroxides, and their derivatives, have been widely developed in recent years. , Remarkably, these 2D nonmetallic elemental nanomaterials show extraordinary potential to surpass conventional nanoparticles owing to their unique geometrical and electronic structures. , Similar to the 2D sheet materials, metallenes were constructed with several metal atomic layers, typically with a thickness less than 1 nm, along with lateral dimensions of hundreds of nanometers or even micrometers. This 2D structure with ultrathin and broadest surface provides a number of advantages of extremely high atomic exposure and surface energy, abundant unsaturated coordinated surface atoms (such as step, edge, and corner sites), strong quantum confinement effects and lattice strains, flexible and tunable electronic structure, and excellent mechanical flexibility, making metallenes highly competitive candidates for facilitating electrocatalytic intrinsic activity and designing high-performance catalysts in future advanced energy conversion devices. ,

Among many metallenes, palladium (Pd)-based metallenes are notable for their excellent performance in a variety of electrocatalytic reactions. The latest research on Pd-based metallenes has rapidly evolved from the early synthetic exploration to the in-depth investigation and rational design of the intrinsic correlation between the formation mechanism, fine structure, and electrocatalytic performance. Through optimization of synthetic strategies, the controllable construction of Pd-based metallenes in terms of their sheet thickness, lateral dimensions, crystalline surface exposure, defect density, and even complex heterostructures can be achieved. Meanwhile, with the help of advanced characterization techniques such as aberration-corrected transmission electron microscopy (TEM), X-ray absorption spectroscopy, in situ spectroscopy, etc., combined with density functional theory (DFT) calculations, in-depth probing of the atomic arrangement, electronic structure, dynamic structure evolution, and their real active sites in electrocatalytic reactions of Pd-based metallenes becomes a reality. Based on these understandings, a series of optimization strategies have been developed with the aim of further enhancing their electrocatalytic performance, involving: (1) alloying engineering introduces secondary or multiple metal atoms to induce lattice strain and ligand effects of Pd, causing the d-band center of Pd to shift downward or upward, thereby optimizing the adsorption energy of intermediates and enhancing catalytic kinetics. (2) Phase engineering regulates the crystal phase of Pd-based metallenes, altering the electronic density of states and coordination environment of Pd, thereby enabling tunable reaction pathways and activity. (3) Ordering engineering transforms disordered solid solutions into ordered intermetallic compounds to produce geometrically well-defined and electronically uniform active sites. (4) Defect engineering introduces the formation of unsaturated coordination defects such as vacancies, dislocations, or grain boundaries, and low-coordinated active centers on the Pd surface. (5) Interface engineering anchors single atoms, nanoclusters, or oxides onto the Pd metallene surface to construct heterogeneous interfaces (e.g., RuOx cluster, asymmetric W–O–Pd sites, and atom-doped), thus forming strong metal-support interaction (SMSI), and interfacial charge transfer is leveraged to modulate the local electronic structure of Pd, downshifting the d-band center and weakening intermediate adsorption. Although a series of optimization strategies have been achieved in Pd-based metallenes, the field still faces huge challenges. For example, due to the surface energy of Pd-based metallenes drastically increasing with decreasing 2D sheet thickness, the synthesis of atomically thin metallenes remains a great challenge. Therefore, systematically combining the Pd-based metallenes formation mechanism, elucidating the conformational relationship between the unique 2D sheet structure (including crystal faces, edges, defects, strains, doping, etc.) and electrocatalytic performance, summarizing effective performance optimization strategies, and looking forward to its future challenges and development directions are crucial to promote the design and preparation of next-generation high-performance electrocatalysts.

In this review, we mainly focus on the latest progress related to the formation mechanism, diversity, structural features, optimization strategies, and their applications in electrocatalysis. At first, we discuss the formation mechanism and growth kinetics of Pd-based metallenes through different synthesis pathways. Subsequently, we focus on the characterization and resolution of their fine structural features (e.g., atomic arrangement, crystalline phases, and surface states). Focused discussion and summarization of the optimization strategies for synthesizing the advanced Pd-based metallenes. Furthermore, the recent improvements in the relationship between the optimized structure and the electrocatalytic performance, especially in electrocatalytic reactions like the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and small molecule fuel (such as methanol, ethanol, or formic acid) oxidation reactions, are deeply analyzed. Finally, we provide new insights into the current challenges and future directions in the field of Pd-based metallenes. We hope that the systematic review will offer a theoretical basis and technical reference for the design and synthesis of more efficient and stable atomic-level thickness Pd-based metallene electrocatalysts.

2. Formation Mechanism of 2D Ultrathin Sheet Structures for Pd-Based Metallenes

Building upon the fundamental understanding of 2D materials, this section establishes the theoretical foundation for the whole review by elucidating why Pd-based metallenes can form as ultrathin sheets. Unlike conventional 2D materials with ultrathin sheet-like structures, such as carbon nitride, graphene, and Mxene, which can be mechanically or chemically exfoliated from the bulk, − metallenes require kinetically controlled growth due to the inherent tendency of metals to form 0D particles. Pd-based metallenes are a 2D ultrathin nanosheet material featuring a close-packed crystal structure with high symmetry, composed of one or several atomic layers with uncoordinated metal atoms. The core mechanism underlying the formation of 2D Pd-based metallenes can be attributed to the combined effects of “kinetically controlled surface-limited growth” and “thermodynamically stable 2D ultrathin morphology”.

2.1. Thermodynamically Stable 2D Ultrathin Morphology

At first, the formation reason for a stable atomic layer 2D sheet structure is analyzed through thermodynamic aspects. From a thermodynamic perspective, the key to the formation of 2D Pd-based metallenes has centered on lowering the overall surface energy to achieve a substable state. In typical synthesis environments, for example, surfactant-assisted liquid-phase methods, the reduced Pd atoms have a strong tendency to aggregate to form 0D nanoparticles, thereby minimizing the instability induced by high surface energy. However, when strongly adsorbed surfactants or templating agents (e.g., CO, halogen ions, organic macromolecules) are introduced, the thermodynamic equilibrium is altered. These ligands selectively and tightly adsorb on specific Pd crystal planes, usually the lattice plane, of the newly grown Pd crystal nucleus, thereby significantly reducing the surface energy of these planes and making the energy barrier for lateral growth of Pd atoms on these passivated faces much lower than that for vertical stacking to form new layers. Therefore, the system tends to expand laterally in 2D space rather than thickening in 3D directions from an energetic perspective, ultimately forming ultrathin 2D structures that are thermodynamically substable under the protection of ligands. In addition, certain substrates (e.g., graphene and liquid metal surfaces) provide good lattice matching and interfacial interactions, further reducing the interfacial energy for 2D growth and thermodynamically driving the formation of single- or few-layer structures.

2.2. Kinetically Controlled Surface-Limited Growth

Subsequently, the kinetics reason for how the growth process can be controlled to achieve single-layer expansion is explored. From a kinetic perspective, the key to the formation of 2D Pd-based metallenes lies in the extreme suppression of the growth rate in the 3D direction while promoting fast anisotropy growth in-plane, which can be achieved mainly through kinetic control of surface adsorbates. During the synthesis process, a high concentration of surfactant molecules is able to adsorb dynamically and rapidly on all surfaces of Pd nanoclusters with different strengths and coverage on different crystal planes. This adsorption forms a kinetic barrier, especially for the 3D direction stacking or the formation of new layer nucleation that requires high activation energy, making it very difficult for metal atoms to aggregate into nanoparticles and stabilize the nucleation. Meanwhile, after the Pd precursor in the solution is reduced, the active atoms or small nanoclusters can migrate and bond laterally along the edges of the partially passivated crystal planes (such as steps and defects) because the energy barrier of this path is relatively low. By precisely controlling the reduction rate, such as slow reduction, ligand concentration, and temperature, it can be ensured that vertical growth is almost “frozen” before lateral growth reaches the micrometer scale. This precise regulation of reaction kinetics enables atoms to be added and arranged almost exclusively on a 2D plane, ultimately resulting in ultrathin, single-atom-layer metallenes. In conclusion, thermodynamically, the addition of surfactants during the synthesis process alters the relative stability of different crystal planes, making the 2D structure a more energy-efficient approach; kinetically, the former surface adsorption layer can slow down the reduction rate of atoms, guiding the growth direction at the atomic scale and ensuring the realization of a 2D structure. Both of them work synergistically and are indispensable. In summary, the synergistic effect of thermodynamic surface energy reduction and kinetic anisotropic growth control provides the fundamental rationale for constructing 2D Pd-based metallenes. Understanding the formation mechanism is a prerequisite for rational design of synthesis methods; the next section will discuss how to put these principles into practical synthetic routes, followed by optimization strategies to further enhance electrocatalytic performance (Figure ).

1.

Schematic illustration of engineering strategies of Pd-based metallenes (doping-type Pd metallenes, Pd alloy metallenes, high entropy alloy Pd metallenes, intermediate Pd metallenes, single atom Pd metallenes, and amorphous Pd metallenes), structural effect, and their application for energy-related electrocatalytic applications.

3. General Synthetic Methods and Optimization Strategies for Pd-Based Metallenes

Controlling the synthesis of metallenes with an ultrathin atom layer thickness, composition, and physical crystal phase is a crucial prerequisite for exploring their physicochemical properties and electrocatalytic performance. , This section briefly introduces and summarizes the common synthesis strategies for preparing Pd-based metallenes, along with modification strategies employed to further enhance the electrocatalytic performance, as shown in Table . The synthesis methods for Pd-based metallenes have become increasingly diverse, evolving from the initial liquid-phase chemical reduction approach (using mild reducing agents such as ascorbic acid and methylamine borane) to CO-assisted methods (utilizing colloidal templates or CO gas bubbles as soft templates to guide 2D growth) and electrochemical deposition (directly depositing metallene films onto electrode surfaces). However, there is still a lot of room for improvement in the electrocatalytic performance of Pd-based metallenes, and their structural stability is challenging. Therefore, various optimization synthesis methods have been developed for further enhancing their performance. , These optimization strategies include (1) alloying effect, which involves doping Pd with other metal atoms to form alloy structures, thereby optimizing the electronic structure of Pd atoms. (2) Phase engineering introduces lattice mismatch into the Pd lattice by constructing core–shell structures or heterojunctions, generating tensile or compressive strain to regulate its d-band centers. (3) Ordering effect: the core objective is to regulate the arrangement of Pd atoms and other dopant/alloy atoms to form a long-range ordered crystal structure with well-defined crystal planes and controllable defects to facilitate its activity and stability. (4) Defect engineering, which deliberately creates defects such as vacancies, steps, and grain boundaries in metallene through plasma treatment, chemical etching, or doping. These defect sites often serve as highly active centers. (5) Interface engineering involves grafting organic molecules or polymers onto the metallene surface. This approach can regulate its hydrophilicity/hydrophobicity and mass transfer of reactants/products while also modulating the electronic states of the metal center through electronic interactions.

1. Summary of Reported Pd-Based Metallenes Electrocatalysts Referring to Composition, 2D Sheet Thickness, Synthetic Methods, and Electrocatalytic Applications.

Pd-based metallenes		2D sheet thickness (nm)	synthetic methods	applications	refs
single Pd metallenes	N-doped Pd	∼2	direct bubbling with CO gas	ORR
	FA-Pdene	∼4.7	two-step wet-chemical process	alcohol oxidation
	defect-rich Pd	∼0.9	a simple wet-chemical method	ORR
	defect-rich porous Pd	∼0.9	a one-pot wet-chemical method	ORR
	Pd@PANI metallene	∼1.3	a two-step wet-chemical approach	ORR
	porous Pd metallene	0.8–0.9	a one-pot wet chemical method	-
	β-PdHene	∼1.26	direct bubbling with CO gas	HER
binary Pd metallenes	Mo-doped PtBi	∼1.8	a one-pot wet chemical method	EGOR
	PdAuHene	∼1.95	a wet-chemical method + hydrogenation with ethanol	urea electrosynthesis
	PdSn metallene	∼1.9	a simple one-pot solvothermal method	EGOR
	PdLa bimetallene	∼1.92	a one-step wet chemical method	EOR + HER
	PdLa bimetallenes	∼3	a two-step solvothermal method + La doping	isopropyl alcohol OR
	PdCo bimetallene	∼1.6	a one-step wet chemical method	EOR
	PdMo MNVs	∼2	a one-step wet chemical method	ORR
	Co-Pdene	∼1.5	direct bubbling with CO gas	ORR
	Pd-M (M = Fe, Co, Ni, Cu)	2–3	a facile wet-chemical synthesis strategy	NO3RR
	PdRh	∼1.34	a one-step thermosolvent method	MOR and HER
	PdIr	∼1.7	a CO-free solvothermal approach	HER
	Fe doping Pd metallene	∼1.24	a one-pot wetchemical strategy + acid leaching	ORR
	RhPdH	∼1.8	a one-pot solvothermal method	HER
	PdMo	∼0.88	a one-pot wet-chemical approach	ORR
	H–PdZn bimetallene	∼1.2	a one-step hydrothermal method	urea electro-synthesis
ternary Pd metallenes	PdRhCu metallene	∼3.5	a straightforward wet-chemical method	ORR
	PdBiRh metallene	∼3.51	one-step solvothermal process	EGOR-coupled CO2RR
	PdBiIn metallene	-	one-step solvothermal process	CO2RR
	Pd70Cu12Bi18	∼1.27	one-step solvothermal process	glycerol oxidation coupled CO2–NO3-reduction reaction
	PdWNb	∼0.9	one-pot wet-chemical method	EOR
	PdMoSb	∼2	two-step wet-chemical method	AOR
single atom alloy metallenes	dual-single Mo–Fe/Pd	∼1.8	a one-pot wet-chemical approach	NO3RR
	single Cu/Pd	∼1.55	a one-pot wet chemical method	MOR
	single Cu/Pd	∼1.15	direct bubbling with CO gas	CO2RR
	single Ga/Pd	∼0.86	a two-step wet chemistry strategy	EOR
	single W/Pd	∼1.21	direct bubbling CO-induced gelation strategy	EOR
	single Fe/Pd	0.8–0.9	a wet-chemistry strategy	N2RR
	single p-block atoms	∼1.15	direct bubbling with CO gas	ORR
	single Bi/Pd	∼0.92	a one-pot wet chemistry route	NO3RR
	single La1Pdene	∼1.98	a wet-chemistry strategy	alkynol semi-hydrogenation
medium and high entropy alloy metallenes	Pd59W8Rh19Bi14	∼1.9	one-pot wet chemistry method	EOR
	PdCuCoZn	∼2.23	one-pot wet chemistry method	urea electrosynthesis
	PdCuNiCoZn	∼1.98	a wet-chemical approach	NO3RR
	PdPtCuNiAg	∼2	a one-pot solvothermal method	ammonia electrosynthesis
	PdPtFeCoNiMo	∼1.3	one-pot wet chemical approach	ORR
	PdCuSnCoNi	∼1.38	direct bubbling with CO gas + wet chemical method	NO3RR
	PdCuCo	∼2.43	one-pot wet chemical approach	NO3RR
	PdRhFeCoMo	∼1	one-pot wet chemical approach	EOR
	PdCuAgBiIn	∼1.05	one-pot solvothermal process	cyclohexanone oxime electrosynthesis
	PdRhMoFeMn	∼1	one-pot wet-chemical strategy	HER
	PdPtRhIrCu	∼1.31	one-step solvothermal method	HER
intermetallic Pd metallenes	Pd0.25Cu0.25Co2	-	two-step reduction solution + galvanic replacement reaction	EOR + EGOR + GOR + GLUOR
	PdIn	∼1.24	a hydrothermal method	urea electrosynthesis
	Pd3Bi	∼1.5	a hydrothermal method	cyclohexanone oxime electrosynthesis
	B-doped PdIn	∼1.75	a solvothermal method + B doping	urea electrosynthesis
	Hcp-PMene-Ru0.18	∼0.9	one-pot wet chemistry method	HOR
amorphous Pd metallene	amorphous–crystalline PdRu	∼1.1	a wet-chemical method	HER
	phosphorus-doped PdMo	∼1.2	a wet-chemical + P doping method	AOR + ORR
	P–PdCr metallene	∼2.3	a sequential wet-chemical synthesis method	MOR + EOR + GOR
loading-type Pd metallenes	Sn–O–Pd metallene	∼3.58	two-step wet-chemical approach	H2O2 activation
	C-doped PdMo	∼2.3	direct bubbling with CO gas + one-pot wet-chemical approach	ORR
	Cr-doped Pd	∼1.34	a one-pot solvothermal approach	-
	N-Pdene	∼1.8	lattice expansion by N atom	MOR + HER
	S-Pdene	∼2.2	amorphization induced by sulfurization
	atomic scale cavities (c-Pd metallene)	∼1	two-step wet-chemical approach	ORR
	Pd@RuO2 core-skin	∼2.39	a seed mediated growth method + a selective oxidation process	OER
	RuOx/Pd	∼1.12	wet chemistry strategy + a microwave treatment	NO3RR
	PdHx@Ru	∼2.36	direct bubbling with CO gas + one-pot wet-chemical approach	HER

3.1. General Synthetic Methods

The preparation of ultrathin 2D nanostructures is thermodynamically unfavorable owing to their high surface energies. Recently, Pd-based metallenes have been synthesized by using a wet chemical strategy assisted with polymer surfactants, for example, polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB), dopamine, and diethylenetriamine (DETA), which are used as surfactant/ligand to adsorb selectively on the spatial site of Pd crystals. A CO-free wet chemistry approach, as a common method, is usually performed in a liquid-phase environment such as water or organic solvents using metal precursors, reducing agents (e.g., ascorbic acid, ethylene glycol, NaBH₄, etc.), and polymer surfactant templates. The key point lies in making use of the differential adsorption ability of polymer surfactants on different crystalline surfaces of Pd metal to regulate the anisotropy of crystal growth, creating a highly anisotropic environment in growth kinetics and forcing the crystals to choose the 2D direct growth mode. Meanwhile, the control of the thickness and size of Pd-based metallenes can be realized by precisely regulating the reaction temperature, pH, reagent concentration, and addition sequence. For example, Yang prepared atomically thin PdIn bimetallene catalysts with different geometric configurations of surface atoms via a steric hindrance effect with the use of DETA, and the schematic diagram is shown in Figure a. During this preparation process, the dimethylamine formed by the synergistic interaction of N,N-dimethylformamide (DMF) and KOH can facilitate the growth of the 2D ultrathin structure via facet control, while the aliphatic terminal polyamine ligand DETA can chelate with Pd²⁺ to form Pd­[DETA]²⁺ to diminish the reduction rate, and ethylene glycol acts as a reducing agent to reduce the Pd ions into Pd atoms.

2.

Schematic diagram of the general synthetic strategy for Pd-based metallenes: (a) wet chemical CO-free strategy. Reproduced with permission. Copyright 2025, American Chemical Society. (b) CO-assisted synthesis strategy. Reproduced with permission. Copyright 2022, American Chemical Society.

Although Pd-based metallenes with subnanometer thicknesses have been prepared using wet-chemical approaches, the use of polymer surfactants will hinder the exposure of the active site. Therefore, a CO-assisted synthesis method emerged accordingly. In 2019, a well-known study in Nature succeeded in producing self-supporting PdMo bimetallene with a thickness of 0.88 nm and a distinctively curved geometry, through carefully controlling the anisotropic growth kinetics via the low-temperature decomposition of the metal carbonyl precursor and the use of a moderately reducing environment. Until now, most of the current study adopts CO-assisted synthesis strategies, including direct bubbling with CO gas or thermal decomposition of CO gas from metal carbonyl precursors. The core principles are based on the “metal–carbonyl coordination chemistry” and “spatially confined growth kinetics”. The schematic diagram is presented in Figure b; at first, the CO molecule, with a unique small size and extremely strong directional coordination ability and tunable adsorption/desorption kinetics, plays the key dual roles of “structure-directing agent” and “surface-capping agent” during the synthesis process. The carbon atoms in the CO molecule provide the lone pair of electrons to fill in the empty d orbitals of Pd atoms to form strong coordination bonds, and the partially reduced Pd atoms or tiny clusters will coordinate with a large amount of CO to form soluble carbonyl cluster anions or neutral coordination compounds. The CO ligand wraps around the surfaces of the metal clusters, stabilizing the nanoscale metal cores of these growth units and preventing their aggregation into bulk particles. Subsequently, CO tends to adsorb on specific Pd crystal planes with carbon ends. Under common solvents (e.g., DMF and methanol) and a certain pressure, CO molecules adsorb densely and almost void-free on the upper and lower two specific crystal planes of the growing Pd core (usually crystal or planes with low surface energy). This dense adsorption forms a dynamic, molecular-level “protective layer” that allows selective exposure of Pd atoms at the edge of the ultrathin 2D nanosheets, and as the growth occurs in the 2D direction, the protective layer of adsorbed CO on the upper and lower surfaces expands, ultimately forming 2D structures with atomic-level thickness and plan view size at a micrometer scale. While the solvothermal and wet-chemical methods described above have proven highly successful for laboratory-scale synthesis (typically 10–100 mg per batch), their scalability for industrial production remains questionable. The key challenges to scalable production of Pd-based metallenes were surfactant dependence, batch-to-batch reproducibility, and high cost of Pd precursors. To be more specific, general synthetic methods require surfactants (PVP, CTAB, and oleylamine) or CO gas protection to control anisotropic growth and prevent agglomeration. However, complete surfactant removal is essential for catalytic activity yet difficult to achieve at scale. Small variations in temperature, precursor concentration, or reaction time can significantly affect metallene thickness (ranging from 0.9 to 2.5 nm), lateral size (from 100 nm to 5 μm), and defect density. Establishing robust, statistically validated synthesis protocols is critical for scale-up. In addition, palladium salts (Pd­(acac)₂, K₂PdCl₄, and Na₂PdCl₄) account for >80% of total synthesis cost. While alloying with earth-abundant metals (Cu, Ni, and Fe) reduces Pd loading, the synthesis process itself remains expensive due to high-temperature, high-pressure conditions and long reaction times.

3.2. Optimization Preparation Strategy

In order to further elevate the electrocatalytic performance of Pd-based metallene catalysts, scientific researchers have developed various optimization synthesis strategies, including alloying effect, atomic structure ordering, phase engineering, defect engineering, and interface engineering, which can improve performance from multiple dimensions such as electronic structure, geometric structure, chemical environment, and synergistic effect, and a schematic illustration of the design principle for optimizing electrocatalysis performance of Pd-based metallenes is shown in Figure . In this subsection, we focus on the principles and methodologies for optimizing the electrocatalytic performance of the Pd-based metallenes. For each strategy, we explain the underlying mechanisms (e.g., strain effect, ligand effect, and d-band center regulation) and clarify how these effects regulate the d-band center and adsorption energy of intermediates for electrocatalysis. The resulting material types derived from these strategies will be systematically classified and exemplified in Section .

3.

Schematic illustration of the design principle for optimizing electrocatalysis performance of Pd-based metallenes. The optimization strategies involve three aspects of electronic structure, geometric structure, and synergistic effect through five pathways of alloying, phase, ordering, defect, and interface engineering.

3.2.1. Alloying Engineering

Alloying strategy is one of the most direct and effective strategies for enhancing the electrocatalytic performance of Pd-based metallenes. By introducing secondary or multiple metal atoms (e.g., Pt, Ag, Cu, Ru, etc.) into the Pd lattice to form binary, ternary, or high-entropy alloy structures, the d-band center of Pd can be precisely modulated through the synergistic interplay of electronic effects (ligand effect) and geometric effects (strain effect), thereby optimizing the adsorption/desorption behavior of reaction intermediates. , Depending on the synthesis conditions and target product structures, the common alloying strategies for Pd-based metallenes can be classified into two main categories: a one-step solvothermal method and a seed-mediated growth strategy, which correspond to the binary/ternary alloy and high-entropy alloy, respectively. The one-step solvothermal method, as one of the most general alloying strategies, is designed to achieve homogeneous mixing of metal atoms through a simple coreduction process with the controllable molar feed ratio of precursors. To suppress phase separation during mixing of multiple elements, high-entropy alloy metallenes require stronger kinetic control. A seed-mediated growth strategy involves the following sequential steps: synthesis of monometallic or binary seed metallene and then introducing metal precursors (e.g., Cu, Sn, Co, Ni, Pt, and Zn) into the reaction system, which offers several advantages of overcoming nucleation disparities, ensuring single-phase homogeneity, preserving ultrathin 2D morphology, and enabling universal synthesis. As a typical example, Liu et al. have reported PdCuSnCoNi high-entropy metallene aerogels (HEMAs) through the use of Pd MAs as a template, along with the deposition and diffusion of Cu, Sn, Co, and Ni elements, and the fabrication schematic illustration is displayed in Figure a. The scanning electron microscopy (SEM) image displays an aerogel constructed by ultrathin 2D sheets (Figure b). As shown in Figure c, the TEM image presents an ultrathin and uniform thickness 2D structure with the high-entropy atomic components Pd/Cu/Sn/Co/Ni of 38%/16%/14%/17%/15%. The diffraction peaks of the powder X-ray diffraction (PXRD) pattern on PdCuSnCoNi HEMAs have revealed a typical face-centered cubic (fcc) single-phase structure (Figure d). Meanwhile, PdCuSnCoNi HEMAs displayed a high surface area of 125 m²/g (Figure e). Furthermore, the strain and ligand effects of PdCuSnCoNi HEMAs induced the downshift of the d-band center from −2.45 eV to −2.97 eV observed in the valence band spectra, which weakens the adsorption of intermediates on Pd, prevents the active sites from being overly occupied, and facilitates the subsequent hydrogenation and desorption of intermediates. It is noteworthy that active metals can leach out, reducing stability, for example, PdCu metallene. It is best suited for electrocatalytic reactions that need moderate tuning of adsorption energy (like *OH adsorption in ORR, H* adsorption in HER) and is not suitable for scenarios requiring super long-term stability under high oxidation potentials (like OER).

4.

Alloying effect and phase engineering: (a) schematic illustration for fabrication of PdCuSnCoNi HEMAs. (b) SEM image, (c) TEM image, and the inset of the atomic ratio for various elements, (d) PXRD pattern and (e) adsorption/desorption isotherms. Reproduced with permission. Copyright 2025, Wiley. (f) Schematic illustration of the synthesis of P–PdMo bimetallene. (g) HAADF-STEM micrographs of P–PdMo bimetallene. (h) AFM image and corresponding height profiles of a P–PdMo bimetallene nanosheet. (n) HRTEM image. Reproduced with permission. Copyright 2024, Elsevier.

3.2.2. Phase Engineering

Phase engineering is an effective strategy for modifying the electrocatalytic performance of Pd-based metallenes by regulating crystal phases (e.g., fcc, close-packed hexagonal (hcp), amorphous, or crystalline/amorphous heterostructures) or exposed crystal planes. Typical phase engineering strategies include amorphization strategy, crystalline/amorphous heterostructure strategy, crystal phase transformation strategy, and dopant-induced phase regulation. Different crystal phases possess different electronic energy band structures and surface atomic configurations, which in turn affect the reaction activity and path selectivity at the catalyst surface. Phase engineering involves modulating the atomic surface properties by regulating the crystal phases (e.g., fcc, hcp, amorphous, etc.) or exposed crystal planes of the nanomaterials and utilizing the intrinsic physicochemical characteristics of different phases to form thermodynamically substable or amorphous phases, thereby generating highly active and high-density unsaturated active sites. For example, Zhang et al. synthesized porous ultrathin Pd metallene nanosheets with a crystalline/amorphous heterostructure, which exhibited high activity toward FAOR. It was found that the porous ultrathin Pd metallene nanosheets had a more negative onset potential and adsorbed OH_ad compared to Pd/C and black Pd due to the heterogeneous interface between the amorphous and crystal structures. Cabot groups prepared P element-doped PdMo (P–PdMo) bimetallene through a two-step wet-chemical approach, as illustrated in Figure f. High-angle annular dark field scanning TEM (HAADF-STEM) images show a curved 2D ultrathin structure with lateral dimensions (Figure g) and a partial amorphization of the P–PdMo bimetallene (Figure n). As shown in Figure h, atomic force microscopy (AFM) analysis revealed the ultrathin nanosheet structure with a thickness of 1.2 nm corresponding to a few atomic layers. The novel P–PdMo bimetallene with a partially amorphous structure demonstrated improved activity and durability toward the anodic alcohol oxidation reaction. Therefore, phase engineering is also an effective strategy for modifying the electrocatalytic performance of Pd-based metallenes, and the enhancement of their electrocatalytic activity can be attributed to (i) providing new active crystal planes: the atomic arrangement and the number of coordination sites can provide more optimized adsorption configurations for specific reaction molecules. (ii) Altering the electronic structure: the crystal phase essentially determines the electronic structure, which in turn affects the density of states around the Fermi energy level, and thus the electrocatalytic activity. (iii) Exploiting the high activity of substable states: substable phases, such as amorphous structures, usually have higher surface energy and stronger reaction tendency. The defect of the phase engineering strategy is that synthesizing metastable phases is difficult; phase transitions may occur during electrochemical cycles, and Pd-based systems have limited accessible crystal phases; mainly fcc and hcp require high-entropy alloying or special templates. It is best suited for cases where d-band center modulation cannot be met by alloying effects, but not suitable for high-temperature (>80 °C) operations, and especially good for exploratory research in unconventional active site discovery.

3.2.3. Ordering Engineering

Ordering refers to regulating the arrangement of metal atoms within the crystal to transform from a disordered solid solution into an ordered intermetallic phase (e.g., L1₀ and L1₂ structures). Different from disordered alloying, the ordering phase creates well-defined, geometrically consistent active sites with predictable electronic properties. A common preparation process involves first forming a disordered Pd–M alloy metallenes precursor and then extending the thermal annealing time in an inert or reducing atmosphere at a lower temperature (typically 200–500 °C). The thermal treatment temperature requires being high enough to drive diffusive rearrangement of atoms and will not lead to the sintering or structure collapse of the nanosheets. For example, Wang et al. reported a well-defined defective intermetallic Pd₃Bi metallene (i-Pd₃Biene) nanocatalyst through a one-step solvothermal strategy, and the schematic illustration of the growth process of i-Pd₃Biene is shown in Figure a. The ordered atomic structure obtained can significantly improve electrocatalytic activity, and the reason for this can be attributed to the following three aspects: (i) the defined active sites: the ordering induces a highly homogeneous atomic arrangement and environment. For example, in Pd₃M ordered structures, the number of coordination sites of each Pd atom and the number of surrounding M atoms are fixed, creating homogeneous and precisely designable active sites and avoiding the diversity of active sites in disordered alloys. (ii) Enhanced thermal and chemical stability: the ordered structure typically has lower enthalpies of formation and stronger bonding of adjacent atoms, resulting in significant resistance to atomic migration, agglomeration, and catalyst deactivation at high temperatures, as well as enhanced resistance to solvation in harsh electrochemical environments. (iii) Stronger strain and ligand effects: the ordered structures tend to maximize and immobilize the strain and electronic effects of alloying. However, the drawbacks include the risk of nanoparticle sintering and loss of metal nanostructure during high-temperature annealing (>500 °C) and the limitation to narrow stoichiometric ratios (such as PdM, Pd₃M, or PdM₃). The ordering strategy is best suited for electrocatalytic reactions that require ensemble effects and well-defined active site geometry, such as the Pd(111) facet, which favors the direct pathway in formic acid oxidation and is especially ideal for fundamental mechanistic studies needing clear structure–activity relationships.

5.

Ordering and defect engineering: (a) schematic illustration of the growth process of i-Pd₃Biene. (b–e) TEM images of the products obtained after different reaction times: (b) 0.5, (c) 1, (d) 3, and (e) 6 h. (f) XRD patterns and (g) ICP–MS results of the products obtained after different reaction periods. Reproduced with permission. Copyright 2024, Elsevier.

3.2.4. Defect Engineering

Defect engineering involves the introduction of point defects (vacancies, interstitial atoms, and dopant atoms), line defects (dislocations), or facet defects (grain boundaries and stacked layer dislocations) into the 2D lattice of Pd-based metallenes by post-treatments. The post-treatment strategies include plasma etching, electrochemical dealloying, thermal annealing, wet-chemical etching, or modulation of the synthesis conditions. As an example, Streb et al. prepared an atomically engineered defect-rich Pd metallene via PMo₁₂ as the reducing agent instead of citric acid that produced nondefect Pd metallene. As another example, the wet-chemical oxidative etching method also induced the formation of an irregular defective structure. As shown in Figure b–g, time-dependent experiments were conducted to study the formation and evolution of defect structure and the composition of the i-Pd₃Biene at different reaction times. Time-dependent TEM, XRD, and inductively coupled plasma (ICP) tests were performed to elucidate the growth mechanism of i-Pd₃Biene (Figure b–g) as the reaction time was extended to 3 h; the morphological curvature and defective structures appeared more and more, as shown in TEM images of Figure b–e, and the intensity of intermetallic phase diffraction peaks strengthens, as observed from the XRD pattern shown in Figure f. Meanwhile, the doping atomic fraction of the Bi element increased (Figure g). Both ordering and defect effects of i-Pd₃Biene have demonstrated the high electrocatalytic performance. The main reason for enhancing electrocatalytic activity is attributed to the following: (i) constructing unsaturated coordination sites: atoms in defective sites (e.g., vacancies and grain boundaries) usually have low coordination numbers and dangling bonds, which are highly active centers for adsorption and activation of reaction molecules. (ii) Modulation of the local electronic structure: defects disrupt the periodic lattice potential field, leading to electron localization, altering the charge density at specific sites, thereby forming “electron traps” or “electron sources”. (iii) Facilitating mass transport: vacancies and grain boundaries can act as channels for the rapid diffusion of atoms or ions (e.g., H⁺ and O^2–), increasing the rate of reactions involving mass transport processes. However, the disadvantages are that defect sites are thermodynamically metastable, are prone to annealing and reconstruction under operating conditions, are difficult to quantify in terms of defect density, and have poor reproducibility between batches. The defect strategy is well-suited for reactions where the rate-limiting step involves breaking strong bonds, like O–O bond breaking in ORR or C–H activation in EOR, and is not suitable for applications requiring ultralong stability operation, making it especially good for enhancing activity on inherently stable metal carbides.

3.2.5. Interface Engineering

Interface engineering involves constructing heterogeneous interfaces with strong interactions that are constructed at the contact interface of two different materials (e.g., metal-carriers, metal-oxides, and heterojunctions), which usually exhibit unique physicochemical properties at the interface. Major synthetic strategies for interface engineering involve single-atom anchoring, nanocluster loading, and nanoparticle decoration. Anchoring single atoms or clusters on 2D ultrathin Pd metallene leverages the SMSI between the foreign atom and the Pd substrate. Pd metallene can provide abundant coordination sites (e.g., defect sites and edge sites) that stabilize individual atoms to prevent their aggregation into clusters or nanoparticles. The resulting single-atom configuration maximizes atom utilization and creates well-defined active centers with unique electronic structures. For example, atomically dispersed tungsten (W) sites on Pd metallene (W–O–Pdene) were synthesized successfully by a simple solvent-thermal treatment to construct an artificial three-dimensional catalytic center; the structure is schematically shown in Figure a. As presented in Figure b,c, the heavy element W appears as dense bright contrast dots within the red circles. Another example: B-doped Pd NPs uniformly anchored to Pd metallenes (Pd–B/Pd HMs) were prepared using a two-step strategy, and the schematic illustration shows the synthesis of the Pd–B/Pd HMs. Pd–B NPs with an average particle size of 3.9 nm were uniformly distributed on the surface of PdMs. The loading structure optimized the interface regulation and electronic structure for enhanced ORR. Depositing subnanometer metal oxide clusters (typically 0.5–2 nm) consisting of only a few atoms onto Pd metallene creates intimate heterointerfaces with maximized contact area. The clusters with ultrasmall size result in a high density of unsaturated coordination sites and strong electronic coupling with the Pd substrate, whose configuration often enables synergistic mechanisms such as hydrogen spillover or interfacial charge transfer that are inaccessible in larger nanoparticles or single atoms alone. For example, subnm RuO _x clusters anchored on a Pd metallene (RuO _x /Pd) were prepared through a two-step procedure of a wet chemistry strategy and a microwave treatment for synergistically enhanced nitrate electroreduction to ammonia, as illustrated in Figure d. It was found that RuO _x /Pd inherits the morphology of the Pd metallene with an ultrathin 2D convoluted structure, as shown in Figure e, and displays numerous subnm clusters (∼0.92 nm) RuO _x nanoclusters, as shown in the HRTEM image of Figure f. The corresponding inverse fast Fourier transform (IFFT) analysis further highlights the subnm cluster structure of RuO _x consisting of six to nine Ru atoms (Figure g). Operando spectroscopic characterizations combined with theoretical computations unveil the synergy of RuO _x and Pd to enhance the NO₃RR energetics through a mechanism of hydrogen spillover and hydrogen-bond interactions. The main purpose of the above optimization strategies is to tailor the electronic structure (d-band centers and charge densities) and geometric structure (coordination number and atomic spacing) of Pd-based metallenes to form a synergistic effect, thereby precisely adapting to the target applications to push the limits of their intrinsic properties. The interface preparation strategy has obvious advantages, such as breaking traditional scaling relation limits, for example, the linear relationship between ΔG _OOH and ΔG _OH in ORR, increasing selectivity and improving resistance to poisoning. However, the downsides are that the modification layer might block active sites, organic modifiers can degrade at high potentials, and it may introduce extra mass transfer resistance. It is best suited for reactions where scaling relations fundamentally cap catalytic efficiency (e.g., ORR) and is not suitable for high temperatures (>100 °C) or strong oxidation conditions. It is especially good for pushing beyond intrinsic activity limits by changing reaction pathways for switching ORR from an associative to a dissociative path.

6.

Interface engineering: (a) W–O–Pdene structural model diagram. (b) AC-HAADF-STEM image of W–O–Pdene. (c) 3d intensity profile of W–O–Pdene. Reproduced with permission. Copyright 2024, Royal Society of Chemistry. (d) Schematic of the synthesis process of RuOx/Pd. (e) High-angle annular dark-field scanning TEM (HAADF) image, (f) aberration-corrected HAADF-STEM image (inset: cluster size distribution), and (g) IFFT. Reproduced with permission. Copyright 2022, American Chemical Society.

This section has systematically reviewed two interconnected aspects: general synthetic methods and five optimization strategies. Notably, each optimization strategy produces distinct structural characteristicsalloying introduces lattice strain, ordering creates homogeneous active sites, defect engineering generates unsaturated coordination centers, and interface engineering establishes synergistic heterojunctions. These divergent structural outcomes naturally lead to different material classifications. Therefore, the following section organizes Pd-based metallenes into distinct categories based on their compositional and atomic arrangements, directly linking each optimization strategy to its corresponding material type. Five optimization strategies include alloying effects, atomic structure ordering, phase engineering, defect engineering, and interface engineering. They have significant differences in mechanisms, advantages, limitations, and suitable scenarios, so you need to choose rationally based on the specific reaction and application requirements. It is worth noting that these five strategies are not mutually exclusive, and the most successful catalyst designs often achieve a synergy of multiple strategies: combining alloying with atomic ordering to create ordered intermetallic, combining alloying with interface engineering to achieve synergy in electronic regulation and reaction pathway selectively, combining defect engineering with alloying to create atomic-scale cavities with unique coordination environments, and combining phase engineering with atomic ordering to achieve hcp-structured intermetallic with unprecedented d-band center downshifts. Therefore, the choice of strategy should be based on specific application requirements.

4. Component Diversification of Pd-Based Metallenes

On the basis of the above optimization strategies, Pd-based metallenes have been divided into diversification types, presenting a wide variety of material systems that can be classified according to their components and atomic arrangements, including Pd/Pd-based alloy metallene, medium/high-entropy alloy metallene, single-atom metallene, amorphous metallene, doping-type metallene, and intermediate metallene, as presented in Figure , each type of Pd-based metallene corresponding to unique structural advantages and electrocatalytic mechanisms (Table ). In this section, we will provide a detailed description of their main synthesis methods, structural characteristics, and reasons for the enhancement of electrocatalytic performance.

7.

Component diversification of Pd-based metallenes: Pd/Pd-based alloy, medium/high entropy alloy, single atom metallene, amorphous metallenes, doping-type, and intermediate metallene.

4.1. Binary/Ternary Pd-Based Metallenes

Binary and ternary Pd-based metallenes mainly adopted a wet chemical reduction method, in which Pd is coreduced with one or two other metals (e.g., Mo, Pt, Ag, and Cu) and assembled into 2D ultrathin wrinkled nanosheets under the guidance of surfactants. , For Pd-based alloy systems, their lattice constants are changed by alloying Pd atoms with other elements (e.g., Rh, Cu, Au, and Ru) to produce lattice strain. The strain effect leads to a shift in the d-band center and alters binding energies toward adsorbates. For instance, Guo’s group systematically reviewed that strain and ligand engineering in alloy structures can shift the d-band center by up to 0.3–0.5 eV, thereby optimizing the binding strength of reaction intermediates for enhanced electrocatalysis. Indisputably, the strain effect originates from the geometric distortion of the metal lattice, typically induced by lattice mismatch in core–shell structures, alloying, or defect engineering, and the ligand effect arises from electronic interactions between different atoms, involving charge transfer due to electronegativity differences, and orbital overlap between heteroatoms that modifies the d-bandwidth. Therefore, the ligand effect functions in combination with the strain effect in Pd-based alloy metallenes, which alters the d-band center and local electronic environment of Pd active sites to optimize the adsorption strength of the reaction intermediates. In all, for binary and ternary Pd-based metallenes, the enhancement of electrocatalytic activity is attributed to (i) the electronic/ligand effect: when metal atoms with higher electronegativity (e.g., Au and Pt) are introduced into the Pd crystal lattice, they will attract the electrons of Pd and lower the d-band center of Pd. This can moderately weaken the adsorption energy of Pd with intermediates (e.g., oxygen-containing species and hydrogen atoms), thus optimizing the energy barrier of the catalytic reaction and enhancing the activity and selectivity. (ii) Geometrical/strain effects: Co-lattices of metals with different atomic radii induce changes in lattice constants. For example, the introduction of smaller atoms (e.g., Cu) causes the shrinkage of the Pd lattice, resulting in compressive strain; the introduction of larger atoms (e.g., Au) produces tensile strain, which can change the surface atomic spacing and electron cloud overlap, thereby modulating the binding strength of the adsorbate in the active site. (iii) Synergistic effect: for those reactions that contain a complex series of steps, different metal sites can catalyze different steps of the reaction to achieve a synergistic effect, thereby breaking through the limitation of a single active site.

4.2. Medium/High Entropy Alloy Pd-Based Metallenes

Medium/high entropy alloys as advanced electrocatalysts usually consist of four or more metal elements (such as Pd, Pt, Ni, Co, and Fe) uniformly distributed in the same crystal lattice to form solid solutions, resulting in strong lattice distortion and slow diffusion effects, thus showing desirable electrocatalytic performance. , However, the preparation inevitably faces challenges in suppressing phase separation and promoting homogeneous miscibility of multicomponents. High-temperature liquid-phase synthesis methods are commonly used by mixing four or more equimolar or nearly equimolar metal salt precursors with a strong reducing agent (e.g., sodium borohydride) and rapidly reducing in a high-temperature oil phase (e.g., oleylamine). The high temperature condition offers an atomic diffusion energy barrier, the rapid reduction principle can freeze the high entropy state, and the surfactant guides the 2D morphology formation. Extremely high conformational entropy stabilizes the single-phase structure, creating a highly distorted and stress-complex chemical environment inside. The modulation mechanism for the active site is that the unique “cocktail effect” stems from the synergy of multiple elements and the extreme diversity of local environments. On the catalyst surface, any active site may be surrounded by different types and numbers of neighboring atoms, forming abundant unique local atomic configurations. This continuous and adjustable energy distribution of adsorption sites makes it possible to have “adaptive” catalytic ability for complex multistep reactions (e.g., alcohol oxidation reaction), which is able to break the limitations of the linear relationship of adsorption energy of a single metal or a simple alloy electrocatalyst.

4.3. Single-Atom Pd-Based Metallenes

Single-atom Pd-based metallenes refer to the immobilization of isolated individual atoms (usually metals with specific functions) on a Pd-metal substrate. The common methods are adsorption, reduction, or in situ atomic substitution reaction. , For example, pure Pd metallenes are first synthesized, and then other metal ions are adsorbed using their surface defects or functional groups and subsequently weakly reduced; or stable, isolated single-atom sites of different metals are formed in situ on the palladium surface by underpotential deposition and electrochemical substitution to form a well-defined M₁/Pd interface. Each foreign atom is coordinated to several surrounding Pd atoms to form a well-defined active center. At the same time, the active center of the material is regulated by a well-defined mechanism, i.e., the construction of atomic-level interfaces with unsaturated coordination. The electronic structure of a single atom as the main active site is strongly regulated by the layer of Pd atoms below it. This SMSI precisely modulates the adsorption/desorption behavior of the individual atoms on the reactants. Meanwhile, the Pd substrate provides a charge conduction pathway. This structure maximizes the use of the noble metal Pd and functional single atoms, achieves extreme homogenization of the active site, maximizes atom utilization, and is commonly used where highly selective catalytic reactions are required.

4.4. Amorphous Pd-Based Metallenes

The preparation process of amorphous Pd-based metallenes is to inhibit the orderly growth of nuclei during synthesis. The common strategies include ultrafast reduction and low-temperature synthesis. By introducing strong complexing agents or using very strong reducing agents (e.g., NaBH₄ in the ice bath reaction), the metal atoms are reduced and aggregated faster than the atoms are rearranged and crystallized. The addition of metal-like elements (e.g., phosphorus and boron) can also promote the formation of amorphous phases. The resulting structure is characterized by the absence of long-range periodic lattice arrangements but the retention of short-range arrangements. The atomic arrangement is chaotic, forming a large number of topologically disordered stackings. The surface is rich in unsaturated sites, distorted atomic coordination environments, and isotropic chemical environments, with no clear crystallographic orientation. The mechanism of active site regulation in amorphous atomic structures: their high activity stems from the intrinsic anisotropy caused by long-range disorder as well as the abundance of low-coordination sites. Unlike crystalline materials, there are no obvious crystallographic differences on its surface, and the electronic structures of all sites are statistically similar but not identical, which enables it to achieve moderate adsorption of a wide range of reactants. More importantly, its disordered structure contains a large number of uncoordinated saturated dangling bonds and distorted strain sites, which are usually highly intrinsically active. This structural property allows it to outperform crystalline catalysts in certain structurally sensitive reactions.

4.5. Heterogeneous Interface Pd-Based Metallenes

Heterogeneous interface Pd-based metallenes are commonly prepared through an in situ growth and postprocessing strategy to induce stronger interfacial interactions between the Pd metallenes substrate and functional components (e.g., nanoparticles and nanoclusters). − The structural features of Pd-based metallenes nanosheets are in close contact with the high specific surface area and high conductivity supporters in a face-to-face, edge-to-supporter, or pleated wrapping manner to form a heterostructure. The supporters not only provide physical support, but also interact electronically with metallenes. The modulation mechanism for the active site is a superposition of interfacial synergistic effects and spatially restricted domain effects. First, charge transfer channels may be formed at the metal–supporter interface (e.g., transfer of electrons from the supporter to Pd), regulating the electron density of Pd. Second, the spatial domain-limiting effect of the supporter can effectively inhibit the agglomeration and migration of metallenes during the reaction process and enhance stability. − Further, a porous supporter can optimize the mass transfer of reactants/products. Certain catalytically active supporters (e.g., doped carbon and metal oxides) can also generate bifunctional catalytic synergies with metallenes to cocatalyze different steps of complex reactions.

4.6. Intermetallic Pd-Based Metallenes

The preparation conditions are more demanding than the previous ones and require the formation of 2D morphologies while achieving long-range ordering at the atomic level. A two-step process is commonly used: first, the precursor alloying clusters are synthesized (mainly disordered solid solutions), and then they are subjected to a precise low-temperature annealing process (e.g., 300–600 °C) in an inert or reducing atmosphere. The annealing process induces slow diffusion and rearrangement of atoms in the 2D plane, resulting in the formation of more thermodynamically stable ordered intermetallic phases (e.g., PdZn, PdPb, PtPd, etc.). Temperature and time should be strictly controlled to prevent sintering of the nanosheets. The resulting structural features are mainly characterized by the arrangement of atoms in a 2D lattice according to specific ratios and in a strictly periodic order, resulting in a crystal structure with well-defined stoichiometric ratios (e.g., L1₀, B2, etc.). Such ordered structures have specific symmetries and superlattices, and their bonding modes are a mixture of metallic and partially covalent/ionic bonds. The regulation mechanism for the active site is that its excellent stability and specificity stem from the precise electronic locking created by the ordered lattice, which results in an ordered atomic arrangement. The well-defined and homogeneous atomic arrangement results in highly consistent surface-active site geometries and electronic states, which leads to excellent performance in selective catalysis as well as precise recognition and adsorption of specific molecules. The strong d-orbital hybridization and the charge-ordered distribution lead to broader d-bands, and the adsorption strengths of the reaction intermediates are intermediate between those of the pure metals, which breaks the linear relationship. In addition, the strong and well-ordered interatomic bonding energies effectively inhibit atomic migration and compositional loss, providing unrivaled structural stability that is particularly suitable for long-term operation under harsh conditions.

The above six types of Pd-based metallenes represent distinct manifestations of optimization strategies. Each type possesses unique structural advantages: binary alloys offer tunable strain effects, high-entropy alloys provide cocktail synergy, single-atom structures maximize atom utilization, amorphous phases supply abundant unsaturated sites, loading-type constructs enable interfacial charge transfer, and intermetallic phases deliver exceptional stability and selectivity. These structure-specific characteristics are not merely taxonomic distinctions; they directly determine the electrocatalytic performance in different reaction environments.

5. Electrocatalytic Applications of Pd-Based Metallenes

The above discussion about extensive type Pd-based metallenes has distinct structural features (e.g., lattice strain, atomic ordering, unsaturated sites, or interfacial synergy). These structural attributes are not merely descriptive, but directly govern electrocatalytic performance. Sustainable and fossil-free energy conversion technologies, such as fuel cells and hydrogen produced by water splitting, have been rapidly developed because of their ability to deal with worsening environmental and energy problems. One of the most important solutions was to study functional and advanced electrocatalysts with desirable activities and durability. Pd-based metallenes were considered a prospective electrocatalyst for a variety of electrocatalyst reactions because of their unique 2D extensive structural and adjustable electronic properties. In this section, we review the latest developments of Pd-based metallenes with optimized structures in electrocatalysis, including ORR, small fuel molecule oxidation reactions, HER, and OER, explicitly linking catalytic outcomes to the structural characteristics established in Section .

5.1. Cathodic ORR of a Fuel Cell

It is widely known that, in the cathodic ORR for proton exchange membrane fuel cells (PEMFCs), the sluggish kinetics of ORR severely hamper PEMFC commercialization. The ORR in alkaline media proceeds via two possible pathways: the direct 4-electron associative pathway (O₂ → *OOH → *O → *OH → H₂O) or the dissociative pathway (O₂ → *O + *O). For the associative pathway, the rate-determining step (RDS) is either the first proton–electron transfer step (O₂ + H₂O + e^– → *OOH + OH^–) or the final *OH desorption step (*OH + e^– → OH^–). Computational studies indicate that the *OH desorption step is the primary RDS for most Pd-based catalysts due to the strong *OH binding affinity on Pd. However, recent breakthrough work by Cui et al. demonstrated that coconfining interstitial H atoms and single p-block atoms (In, Sn, Pb) in Pd metallenes activates the oxygen dissociative pathway (O₂ → *O + *O), bypassing the *OOH intermediate entirely. In this case, the RDS shifts to the initial O–O bond cleavage step. Pd-based catalysts are highly promising alternatives and research hotspots, which is mainly due to the abundant natural resources and lower cost compared to Pt. For long-term and scale-up applications, the development of efficient Pd-based catalysts can diminish electrocatalyst costs for fuel cell devices. Moreover, both Pd and Pt belong to the same Pt group metals and have similar lattice parameters and electronic structures, making it possible for Pd to partially or completely replace Pt in various electrocatalytic applications, offering a possibility for the design of low-Pt or Pt-free electrocatalysts. Therefore, developing Pd-based metallenes is expected to enhance their ORR performance. The active sites for ORR on Pd-based metallenes originate from strain- and ligand effect-induced d-band center modulation. In Pd-based metallenes, the ligand effect has been successfully harnessed to modulate adsorption properties in a PdRhCu ternary metallene; the lattice spacings faintly expanded from the HRTEM image and distinct diffraction peaks negative shift from the XRD pattern compared to those of pure Pd demonstrated the strong interaction between Pd, Rh, and Cu species. DFT calculations revealed that the introduction of Cu into the PdRu configuration downshifts the d-band center of Pd from PdRh of −1.585 eV to PdRhCu of −1.593 eV, presenting an improved occupation of antibonding orbitals by Cu atom incorporation. Meanwhile, ternary PdRhCu alloy metallenes demonstrated an increased occupancy of antibonding orbitals, thereby weakening the adsorption of oxygen intermediates and enhancing ORR activity. For another example of Pd–Pt alloy metallenes, the synergistic effect of Pd and Pt and the lattice strain introduced by the 2D structure together optimize the adsorption of O₂ and the O–O bond breaking process. Guo and co-workers reported the preparation of self-supporting PdMo bimetallene through the low-temperature decomposition of the metal carbonyl precursor and the use of a moderately reducing environment. The electrochemically active surface area (ECSA) of PdMo bimetallene catalysts determined from the Cuupd method is 138.7 m²/g_Pd. The apparent improvement of half-wave potential (E _1/2) of 0.95 V in 0.1 M KOH is far beyond those of the commercial Pt/C of 0.85 V and Pd/C of 0.84 V. The as-prepared PdMo bimetallene shows a 77.9-fold and 327.4-fold enhancement of mass activity compared to commercial Pt/C and Pd/C, respectively. Furthermore, the stability of PdMo bimetallene showed a little decay of ∼30% after 30,000 potential cycles. Atomic vacancy-doped N (V–N–Pd metallene) retains 84.6% of its initial ORR mass activity after 20,000 accelerated degradation test cycles in 0.1 M KOH electrolyte, with post-mortem TEM revealing that its porous structure has no significant agglomeration. The ICP–MS test displayed that the N/Pd atomic ratios of V–N–Pd metallene decrease from 11.3/88.7 to 7.7/92.3 after ADTs. Controlling the morphology and structure of metallene, such as defect-rich structure, local-coordination atomic-scale cavities, polyaniline-functionalized ones, interstitial carbon-doped ones, can offer an excellent opportunity for improving their electrocatalytic performance. Guo and their group prepared Pd metallene with atomic-scale cavities, named c-Pd M. As shown in Figure a, the half-wave potential (E _1/2) of c-Pd metallene obtained from linear sweep voltammetry curves is 0.947 V, almost 100 mV higher than that of commercial Pt/C of 0.843 V. The mass activity of c-Pd metallene is 0.37 A/mg, achieving a first-class level compared to other reported world-class studies (Figure b). As illustrated in Figure c, both remarkably elevated E _1/2 and mass activity revealed that the accelerated electron transfer in c-Pd metallene through coordination regulation enables effective adjustment of adsorption strength with oxygenated intermediates. Moreover, surface distortion engineering for the PdMo bimetallene through phosphorus doping is proposed as an effective strategy to further boost their ORR performance. As a verification, Cabot and their group synthesized phosphorus doping PdMo (P–PdMo) bimetallene with a partially amorphous structure via a wet-chemical approach. The P–PdMo catalysts displayed significantly enhanced activity and durability in ORR, attributed to their ultrathin and curved morphology of 2D nanosheets, along with electronic regulation by phosphorus atoms, thereby facilitating *OH adsorption on the surface of P–PdMo bimetallenes. In all, Pd-based metallenes offer high specific surface area and tunable electronic structure to optimize the d-band center of Pd through strain and ligand effects to weaken the adsorption free energy of oxygen-containing intermediates (e.g., *OH), thus enabling its ORR activity to even exceed that of commercial Pd/C and Pt/C catalysts (Table ).

8.

Electrochemical performance of ultrathin Pd metallene with atomic-scale cavities (c-Pd metallene). (a) ORR polarization curves. (b) The comparison of specific activity and mass activity at 0.95 V versus RHE. (c) Schematic illustration of the coordination effect in ORR electrocatalysis based on Pd metallene samples with different active site structures. Reproduced with permission. Copyright 2022, Wiley.

2. Summary of Reported Pd-Based Metallenes Electrocatalysts for the Electrocatalytic Performance of ORR Applications.

metallenes catalysts	electrolyte	half-wave potential (E1/2, VRHE)	ECSA, m2/g	durability (decay %/shift mV)	refs
PdRhCu ternary	0.1 M HClO4	0.93	63.6	21.39%@10,000 CV cycles
PtPdFeCoNiMo high entropy	0.1 M KOH	0.902	-	21%@20,000 cycles
mesoporous PdMo nanoveins	0.1 M KOH	0.94	49.7	7%@5000 cycles
cobalt-doped Pd	0.1 M KOH	0.93	35.6	20 mV negative shift@30,000 cycles
defect-rich Pd	0.1 M KOH	0.93	79	19.2%@10,000 cycles
interstitial site N-doped Pd	0.1 M KOH	0.936	81.2	12.7%@20,000 cycles
Bi p-block atom-doped Pd	0.1 M KOH	0.927	126.76	17%@5000 cycles
Pd2+-rich Pd aerogels	0.1 M KOH	0.91	44.1	29 mV negative shift@10,000 cycles
Zr-doped Pd	0.1 M KOH	0.87	63.4	12 mV negative shift@5000 cycles
Au@Pd metallene	0.1 M KOH	0.921	110.6	5.7%@10,000 cycles
amorphous/crystalline phase coexistence Pd	0.1 M KOH	0.944	80.74	7 mV negative shift@10,000 cycles
curved porous PdCu	0.1 M KOH	0.943	73.76	14 mV negative shift@10,000 cycles
porous PdZn	0.1 M KOH	1.05	51.4	7.2 mV negative shift@30,000 cycles
carbon-doped PdMo	0.1 M KOH	-	73.25	39.72%@30,000 cycles
atomic-scale cavities Pd	0.1 M KOH	0.947	-	∼30%@5000 cycles

5.2. Anodic Fuel Oxidation Reaction of Fuel Cells

As is well-known, the performance of direct liquid fuel cells is largely determined by the activity of anodic fuel (such as methanol, ethanol, and formic acid) oxidation reactions. − The anodic fuel oxidation reaction is a very central electrocatalytic process, which is a catalyst-dependent and mechanistically complex process. From methanol oxidation reaction, it is severely limited by CO poisoning. For the ethanol oxidation reaction (EOR), the complete oxidation to CO₂ (12e^–) involving C–C bond cleavage is difficult on Pd in alkaline media, which involves a 12-electron transfer process with multiple C1 and C2 intermediates; thus the main product is typically acetate on the Pd surface. The RDS in EOR is generally the initial dehydrogenation step (CH₃CH₂OH + OH^– → CH₃CH₂O* + H₂O + e^–), followed by the cleavage of the C–H bond in the adsorbed ethoxy species and the oxidation of adsorbed *CH₃CO to acetate. In alkaline media, EOR activity correlates strongly with the ability to form adsorbed OH species (Pd–OH), which act as oxygen donors to facilitate the oxidation of adsorbed intermediates. Previous studies have revealed that the onset potential for Pd–OH formation is critical for EOR activity. For formic acid oxidation reaction (FAOR), the pathway selectivity is dependent on the surface structure of catalysts. FAOR on Pd proceeds via two parallel pathways: direct (dehydrogenation) pathway (HCOOH → *COOH/HCOO* → CO₂ + 2H⁺ + 2e^–) and indirect (dehydration) pathway (HCOOH → *CO + H₂O → CO₂). For the direct pathway, the RDS is the initial C–H bond cleavage step (HCOOH → *COOH/HCOO* + H⁺ + e^–). DFT calculations revealed that the activation energy for the direct pathway was significantly lower on Pd than on Pt, which is why Pd is preferred for FAOR. For the indirect pathway, the RDS is the dehydration step that forms strongly adsorbed *CO, which then blocks active sites and requires higher overpotentials to oxidize to CO₂. Pd surface atomic arrangement can favor the direct pathway with minimal CO formation, contributing to enhanced FAOR performance. In addition, introducing smaller Cu atoms into the Pd lattice generates compressive strain (0.85–1.5%), which can optimize the electronic structure of Pd and facilitate surface adsorption and dissociation of formic acid. Notably, Pd-based metallenes are demonstrated to be among the best electrocatalysts for those reactions. Among them, Pd-based metallenes are quite effective for FAOR because of their excellent activity and antipoisoning ability. Benefiting from the high surface area, Wang et al. synthesized the freestanding ultrathin PdCuH metallene/C, which presented an improved mass activity of 2.59 A/mg_Pd in 1 M KOH with 1 M HCOOH. In addition, the stability of the PdCuH metallene demonstrated by chronoamperometry measurements for 3600 s had a considerable current density left and was much higher than that of PdCu metallene/C and commercial Pd/C. By introducing elements such as Cu and subsequently doping H atoms to form alloy, the oxidation path of formic acid can be altered to prefer the direct dehydrogenation path for generating CO₂ to the indirect dehydration path for generating the poisoning intermediate CO*, thus significantly enhancing the catalyst’s ability to resist CO poisoning. When focusing on alcohol oxidation reactions, Pd-based metallenes are regarded as the most efficient electrocatalysts in alkaline solution. For instance, Jia et al. reported preparation of Pd _x Cu _y Co₂ metallenes through a two-step reduction process and subsequently the galvanic replacement reaction. The as-made Pd _x Cu _y Co₂ metallenes deliver enhancement high electrocatalytic performance for ethylene glycol oxidation and glycerol oxidation reactions in alkaline electrolytes. The study revealed that the high activity is attributed to both the structural advantages and the synergistic effects of multiple metals, which enhance the surface electronics, fostering the creation of exclusive active sites, thereby facilitating the C–C bond cleavage for alcohol oxidation reactions. Meanwhile, Yu et al. also reported various nonmetallic atom-intercalated Pd metallenes (M-Pdene, M = S, H, N, C), S-doped Pd metallene (S-Pdene) has an amorphous structure, and H-, N-, and C-doped Pd metallenes have lattice expansion. As shown in Figure a, the lattice expansion and amorphization of M-doped Pdene were demonstrated by XRD patterns. Among them, the N-doped Pdene catalysts displayed Pd-mass-normalized peak current densities of 7.96 A/mg, which surpass those of S-, H-, and C-doped catalysts (Figure b). For the EOR, the reaction process involves two pathways, where the C₁ pathway completely oxidizes ethanol to carbon dioxide through a 12-electron process, whereas the C₂ pathway incompletely oxidizes ethanol through a 4-electron process, thereby leading to the formation of acetate. Li et al. prepared C₆₀ fullerenolamine (FA)-modified Pd metallene (FA-Pdene) and revealed that the FA molecules can facilitate the active site availabilityl meanwhile, they induce the electronic effect to facilitate the EOR electrocatalytic performance. As shown in Figure c,d, in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR–FTIR) of EOR on the FA-Pdene and Pdene electrodes was conducted, and the results demonstrated that FA-Pdene displayed advantages in efficiently catalyzing the cleavage of C–C bonds during the EOR process, thereby promoting the C₁ pathway. The electrocatalytic performance of Pd-based metallenes for fuel oxidation reactions, including methanol, ethanol, and formic acid oxidation, is systematically summarized in Table . The key metrics compiled include the ECSA, specific activity, and mass activity. As can be seen from Table , Pd-based metallenes generally exhibit a significantly large active surface area and enhanced specific/mass activity compared to commercial Pd/C or Pd black, primarily due to the ultrathin 2D sheet structure and thus abundant exposed active sites on the Pd-based metallene.

9.

Anodic small molecule oxidation: (a) XRD patterns recorded from the samples of Pdene, H-Pdene, N-Pdene, C-Pdene, and S-Pdene, respectively. (b) Pd/Pt mass-normalized CV curves for MOR. Reproduced with permission. Copyright 2022, American Chemical Society. In situ ATR–FTIR spectra for ethanol decomposition on (c) FA-Pdene and (d) Pdene in 1 M KOH + 1.0 M ethanol solution measured at different potentials varying from −0.8 to 0.2 V versus SCE at an interval of 0.1 V. Reproduced with permission. Copyright 2025, Wiley.

3. Summary of Reported Pd-Based Metallenes Electrocatalysts for the Electrocatalytic Performance of Fuel Oxidation Reaction.

catalysts	electrolyte	ECSA (m2/g)	specific activity (mA/cm2)	mass activity (A/mg)	refs
W-doped Pd	1 M KOH + 1 M ethanol	48.4	10.9	5.29
porous PdWNb	1 M KOH + 1 M ethanol	75.2	20.8	15.6
FA-modified Pd	1 M KOH + 1 M ethanol	34.57	12.0	4.123
RhPb bimetallene	1 M KOH + 1 M ethanol	34.4	4.7	∼1.6
PdCo bimetallene nanoribbons	1 M KOH + 1 M ethanol	38.62	3.80	1.47
PdGa single-atom alloy	1 M KOH + 1 M ethanol	68.4	15.13	10.34
high-entropy PdRhFeCoMo	1 M KOH + 1 M ethanol	29.3	25.5	7.47
phosphorus-doped PdMo	1 M KOH + 1 M ethanol	∼59.8	8.28	4.95
PdWRhBi	1 M KOH + 1 M ethanol	107.4	15.54	16.7
PdCuCo	1 M KOH + 1 M glycerol	269.2	∼0.4	10.9
PdCu single-atom alloys	1 M KOH + 1 M methanol	160.39	∼6.6	10.64
P–PdCr	1 M KOH + 1 M methanol	45.38	5.81	2.64
CrOX@Pd	1 M KOH + 1 M methanol	49.66	5.30	2.05
PdBiRh trimetallene	1 M KOH + 0.5 M ethylene glycol	27.99	∼9.28	2.6
Mo-doped PtBi intermetallic	1 M KOH + 1 M ethylene glycol	39.3	61.1	24.0

5.3. Electrocatalytic HER and OER of Water Splitting

Electrolytic water splitting, as a promising and sustainable solution, is regarded as producing clean and green energy sources, offering a valuable alternative to nonrenewable and environmentally hazardous fossil fuels. − However, large-scale electrolytic water splitting techniques have been hindered by the absence of highly efficient electrocatalysts involving cost and performance. − HER in alkaline media proceeds through three elementary steps: (1) Volmer step (water dissociation): H₂O + e^– → *H + OH^–; (2) Heyrovsky step (electrochemical desorption): *H + H₂O + e^– → H₂ + OH^–; (3) Tafel step (recombination): 2*H → H₂. In alkaline media, the Volmer step with a high energy barrier is universally recognized as the RDS for most Pd-based catalysts. The optimal HER catalyst follows the Sabatier principle, binding H neither too strongly nor too weakly. Pure Pd binds H slightly too strongly (ΔG _H is about −0.4 eV), which hinders H₂ desorption. According to the d-band center theory, modulating the Pd d-band center is critical for optimizing ΔG _H. In this section, Pd-based metallenes, due to their 2D unique morphology and structure, have the potential to improve electrocatalytic water splitting. At first, the ultrathin 2D structure at the atomic level extremely highlights the active sites to facilitate the utilization of Pd atoms and promote the mass transfer of reactants/products. Subsequently, the optimized electronic structure can modulate the adsorption energy of intermediates (H* and OH*) via strain effects or alloying with other metals (such as Ru and Pt), thereby accelerating the kinetics of HER and OER. Moreover, the high conductivity ensures rapid charge transfer, while the abundant defects or vacancies can further activate water molecules and stabilize reaction intermediates. Finally, the unique flexibility and structural stability can maintain the integrity of the active phase during long-term electrocatalytic processes, inhibiting the aggregation or loss of Pd. These structural characteristics collectively enhance the water splitting performance of Pd-based metal–organic frameworks in the entire pH range and in complex environments such as seawater. For example, atom-intercalated Pd metallenes (M-Pdene, M = S, H, N, C) catalysts also displayed a promising performance for HER due to to their lattice expansion and amorphization. As shown in Figure a,b, the amorphous S-Pdene/C presented the highest HER activity and the lowest Tafel slope of 48.3 mV/dec. A negligible positive shift in the polarization curve of S-Pdene/C was observed after 10,000 cycles, revealing its outstanding stability. After the stability test, TEM, EDS, and HAADF-STEM characterizations show that the morphology, composition, and amorphous structure of S-Pdene/C were retained. The improved HER activity and stability can be attributed to a longer Pd–Pd bond length and increased d-band downshift in the amorphous structure compared to that of the crystalline counterparts, thereby weakening the hydrogen bonding and facilitating the HER activity. Zhu and group synthesized β-Pd hydride metallene (β-PdHene) aerogels as advanced HER catalysts. They found that β-PdHene catalysts have the most desirable ΔG _H* of −0.54 eV relative to that of Pd of −0.67 eV. In addition, the lattice hydrogen species is systematically studied during the HER process (Figure c), and H* on site 1 experiences an H-transfer to adjacent site 2; therefore, H₂ can be facilely generated with lower energy barriers (0.02 eV) compared to the traditional pathway, revealing that the interstitial lattice hydrogen is essential to change the selectivity of the reaction path. A lattice hydrogen-involved mechanism catalyzed by β-PdHene aerogels following the experimental and theoretical results is proposed, and the schematic illustration is shown in Figure d.

10.

Electrocatalytic water splitting. (a) HER performance of Pdene/C, M-Pdene/C, S-Pdene/C, commercial Pd/C, and commercial Pt/C catalysts: (a) HER polarization curves and (b) corresponding Tafel plots. Reproduced with permission. Copyright 2022, American Chemical Society. (c) Gibbs free energy of HER on the Pd(111) and β-PdH (111) via different reaction pathways. (d) Schematic illustration of the proposed lattice hydrogen-involved mechanism on β-PdHene aerogels. Reproduced with permission. Copyright 2024, Springer Nature.

Electrocatalysis OER, as a key limiting factor for energy consumption, involves a typical four-electron transfer of multiple reaction intermediates and a high overpotential. − The conventional mechanism involves three intermediates (*OH, *O, and *OOH). According to the scaling relationship between *OH and *OOH binding energies, the third step (O → *OOH) is often the RDS for many metal surfaces, including Pd. The strong *O binding on Pd makes the formation of *OOH (which requires weakening the *O bond) energetically unfavorable. − For the OER, which is usually dominated by noble metal oxides (e.g., IrO₂ and RuO₂), − some Pd-based high-entropy metallenes or oxide-modified metallenes are beginning to show challenges to the conventional simultaneous OER due to their unique electronic structure and stability, and at the same time, 2D structures favor the release of oxygen during the reaction process, resulting in high-performance OER catalysts. , For example, Cui and group constructed a unique Pd@RuO₂ metallene with a Pd core and RuO₂ skin structure. Pd@RuO₂ metallenes presented an excellent acidic OER activity with a low overpotential of 189 mV at 10 mA/cm², surpassing most reported RuO₂-based electrocatalysts. Both spectroscopic analyses and theoretical calculations revealed that the Pd metallene core was able to act as an electron donor to enhance the migration energy of subsurface oxygen atoms and optimize the adsorption energy of intermediates on the active Ru sites, enabling a switch in the reaction mechanism. A dedicated summary of Pd-based metallenes for HER or HER-coupled anodic oxidation reactions is presented in Table . The HER overpotential on Pd-based metallenes remains at a relatively low level. For example, high-entropy PdPtRhIrCu metallene and high-entropy PdRhMoFeMn presented ultralow overpotentials of 15 mV and 6 mV under alkaline and acidic solutions at 10 mA/cm², respectively. Unlike traditional HER catalysts that operate alongside the OER, the Pd-based metallenes described here enable a more energy-efficient approach by coupling HER with thermodynamically more favorable AORs (such as hydrazine or glycerol oxidation). Typically, HER coupled with the hydrazine oxidation reaction on a Ru/Pd metallenes electrode achieves an ultralow overpotential of 0.19 V_RHE@100 mA/cm² on the two-electrode water electrolyzer system. These comparative data, as detailed in Table , underscore the potential of Pd-based metallenes for energy-saving hydrogen production.

4. Summary of Reported Pd-Based Metallenes Electrocatalysts for the Electrocatalytic Performance of HER or Coupling AOR Applications.

metallene catalysts	electrolyte	HER activity (overpotential, V)	ECSA (m2/g)	two-electrode water electrolyzer activity (cell voltage V)	durability (decay %/shift mV)	refs
PdLa bimetallene	1 M KOH + 1 M ethanol	67.4 mVRHE@10 mA/cm2	69.05	1.61 VRHE@100 mA/cm2	no degradation@2000 cycles at 10 mA/cm2
PdSb bimetallene	1 M KOH + 0.1 M ethylene glycol	-	39.3	0.71 VRHE@10 mA/cm2	∼15%@1 h at 0.75 V
PtNi nanobowls	0.5 M H2SO4	14.8 mVRHE@10 mA/cm2	1.65 mF/cm2	-	no degradation@12 h at 10 mA/cm2
heterophase Pd4S	1 M KOH + 4 M Na2S solution	79 mVRHE@10 mA/cm2	2.89 mF/cm2	0.776 VRHE@100 mA/cm2	no degradation@20 h at 10 mA/cm2
β-PdHene aerogels	0.5 M H2SO4	20 mVRHE@10 mA/cm2	-	-	3 mV@5000 potential cycles at 10 mA/cm2
PdRhMoFeMn high-entropy	0.5 M H2SO4	6 mVRHE@10 mA/cm2	14.8 mF/cm2	-	3 mV@10,000 cycles at 10 mA/cm2
Pd/Ir heterometallene	0.5 M H2SO4	69 mVRHE@500 mA/cm2	60.00	-	no decay@5000 cycles at 10 mA/cm2
c-PdMo	1 M KOH + 1 M methanol	-	223.7	0.362 VRHE@10 mA/cm2	14%@55,000 s at 10 mA/cm2
single Co atoms@Pd	0.5 M H2SO4	24.7 mVRHE@10 mA/cm2	25.4 mF/cm2	-	2.3%@40 h at 35.6 mV
Rh/Pd metallene	1 M KOH + 0.1 M hydrazine	59 mVRHE@10 mA/cm2	62.7	0.19 VRHE@100 mA/cm2	no apparent decay@20 h at 10 mA/cm2
S,P-codoped Pd metallene	1 M KOH + 1 M ethanol	124 mVRHE@10 mA/cm2	56.06	0.88 VRHE@10 mA/cm2	no decay@1000 cycles at 10 mA/cm2
HEA-PdPtRhIrCu metallene	1 M KOH	15 mVRHE@10 mA/cm2	89.24	-	no decay@10,000 cycles at 10 mA/cm2
PdPt bimetallenes	1 M KOH + 0.5 M HCOOK	27.1 mVRHE@10 mA/cm2	21.89	0.31 VRHE@10 mA/cm2	a slow current attenuation@10,000 s at 0.31 V
defect-rich PdIr bimetallene	1 M KOH + 1 M isopropanol	62 mVRHE@10 mA/cm2	79.2	0.38 VRHE@10 mA/cm2	no decay@15 h at 10 mA/cm2
PdRh bimetallene nanoribbons	1 M KOH	32 mVRHE@10 mA/cm2	72.1	-	no decay@10,000 cycles at 10 mA/cm2

6. Conclusion and Perspective

In summary, Pd-based metallenes, as an emerging 2D electrocatalyst, show an extensive application prospect in the field of clean energy conversion technology owing to their unique ultrathin and larger nanosheet structure, high atomic utilization, and flexibly tunable electronic structures. In this review, we systematically summarize the 2D ultrathin structure formation mechanism and structural characteristics, synthesis methods, optimization and preparation strategies, the diversity of element composition of Pd-based metallenes, and their excellent performance in fuel cells and electrolysis water splitting. Despite the remarkable progress, the study of Pd-based metallenes still faces many challenges; for example, the synthesis of high-quality Pd-based metallenes on a large scale, with high reproducibility and low cost, is still impeded; the in situ characterization of the dynamic structural evolution process of Pd-based metallenes under real reaction conditions still needs to be strengthened; and high-throughput computation and screening based on machine learning for guiding the design of novel multicomponent and surface morphology metallenes is still in its infancy. Future research should be devoted to the development of more accurate synthetic methodology, combined with advanced in situ characterization techniques and theoretical simulations, to deeply reveal the intrinsic relationship between structure and electrocatalysis performance, thereby promoting the Pd-based metallenes electrocatalysts from the laboratory to the actual commercial device applications, so as to inject a strong driving force for the innovation of sustainable and green clean energy technology (Figure ).

• (1)Precise synthesis methods: Although a variety of wet-chemical synthesis methods have been developed, achieving the precise customization of Pd-based metallenes at the atomic scale is still a fundamental challenge. The control of the number of atomic layers, edge structure, crystal orientation, and defect structures (e.g., grain boundaries and vacancies) of Pd-based metallenes by present synthesis methods is mostly in the coarse regulation stage, making it difficult to realize the precise control required for specific morphology and structure for the elevated electrocatalysis applications. For example, anodic oxidation reactions require a high density of step sites for a multipath reaction of intermediates, while ORR reactions may be more sensitive to specific lattice strains (e.g., Pd(111) lattice plane) on the catalyst surface. Therefore, future synthetic strategies should shift from experience-driven approaches to mechanism-guided design, enabling precise control over 2D morphology through a deep understanding of the kinetic and thermodynamic processes underlying nucleation, growth, and anisotropic extension. This requires the development of advanced in situ characterization techniques to monitor the whole synthetic process from metal precursor reduction to 2D ultrathin and large nanosheets in real time and to capture the key intermediate states, so as to guide the precise tuning of synthesis parameters. Only by basing the synthesis strategy on a deep understanding of the growth mechanism can we truly realize the structure-on-demand design of materials. Recently, machine learning screening, as a systematic optimization tool for accelerating Pd-based metallene design, can efficiently explore multidimensional parameter spaces and realize the prediction of electrocatalytic performance and the reverse design of Pd-based metallenes, promoting the transformation of material design from being experience-driven to data- and theory-driven.
• (2)Structural features and atomic-level characterization: The 2D ultrathin and extensive nanosheet structure provides extremely high surface energy, while bringing inherent thermodynamic instability. Under harsh electrocatalytic environments (e.g., high oxidation potentials, strong adsorption intermediates, and drastic changes in proton/electron fluxes) or high-temperature reaction conditions, Pd atoms tend to migrate, leading to nanosheet agglomeration, thickening, Ostwald ripening, and even structural dissolution, eventually resulting in irreversible decay of activity and durability. Future enhanced durability strategies must go beyond simple physical support or mixing to constructing stable systems with strong interactions. The main research should focus on the following: (i) constructing intermetallic compounds (e.g., PdM, M = Zn, Pb, Bi, etc.), whose ordered lattice structure and stronger interatomic bonding can significantly inhibit atomic migration and component segregation. (ii) Designing carbon-restricted or encapsulated structures, e.g., encapsulating Pd-based metallenes in graphene shells or porous carbon matrices and utilizing the dual roles of carbon layers for both physical protection and electronic modulation to isolate the reactants from harsh environments while allowing contact with the reactants. (iii) Development of a self-supporting 3D porous network structure (e.g., aerogel) to reduce high-energy surface exposure and enhance the overall mechanical strength of the structure by building a continuous metallene skeleton. The above methods aim to endow Pd-based metallenes with the ability to resist structural degradation under dynamic operating conditions at both the atomic-scale levels.
• (3)In-depth exploration of the electrocatalytic mechanism: At present, the mechanism explanation of the superior electrocatalytic performance of Pd-based metallenes (e.g., strain effect, electronic effect, and high exposure active site) remains at the macro or semiquantitative level. In order to realize the rational design of Pd-based metallene electrocatalysts, it is necessary to elucidate the relationship between specific structure and desirable performance on the atomic/molecular scale. The challenge is that the active sites of Pd-based metallenes are highly complex and dynamic, including but not limited to lattice strain regions, edge atoms, grain boundary defects, single-atom alloying sites, and charge transfer regions at the surface interface. Future research needs to deeply integrate multiscale theoretical simulations with high-resolution in situ characterization of spectra and electrochemistry. For example, in situ scanning tunneling microscopy or electrochemical mass spectrometry techniques are used to directly observe the adsorption, activation, and transformation behavior of reactive molecules on specific surface sites. At the same time, the dynamic reconfiguration process on the Pd-based metallene electrocatalyst surface under realistic reaction conditions (including potential, temperature, and solvent) is investigated through molecular dynamics simulations augmented by machine learning potential functions, whose goal is to construct models that can predict the effects of different microstructures on the adsorption energy of key reaction intermediates, thus guiding synthesis in reverse and precisely constructing active centers with optimal adsorption strength for the target reaction.
• (4)Large-scale preparation and practical application: The cost and scarcity of the precious metal Pd are among the most hampering factors limiting Pd-based metallenes toward large-scale commercial applications. Future research must focus on cost efficiency. The primary study is to maximize the Pd atomic utilization through exploring single-atom Pd catalysts anchored on metallene supports and reduce the dosage of precious metals via high-entropy metallenes with multiple earth-abundant elements (e.g., PdCuNiFeCo). On the one hand, researchers are committed to synthesizing subnanometer-thick or even single-atom-layer structures, so that each Pd atom becomes a surface-active atom. On the other hand, researchers are vigorously developing high-activity multicomponent alloy metallenes with ultralow Pd content, or even Pd-free, such as using abundant and inexpensive 3d transition metals (e.g., Co, Ni, and Fe). Developing efficient Pd recovery protocols from spent catalysts is also an efficient strategy. Furthermore, the engineering challenges of large-scale preparation face huge challenges. Current laboratory synthesis methods are difficult to meet the industrial demand in terms of yield, energy consumption, reagent cost, safety, and environmental protection. In the future, we need to energetically explore continuous flow chemical synthesis, electrochemical deposition, gas-phase physical methods, and other scalable processes, optimize the reactor design, and obtain products from milligrams to grams and even kilograms of high-quality synthesis. Only through the pathway of excellent performance and cost-controllable, scalable production can these excellent 2D Pd-based metallene electrocatalysts really go from the laboratory to practical application.

11.

Pd-based metallenes opportunities and challenges in four aspects of synthesis method, structural features, electrocatalytic mechanism, and practical application.

Haiquan Wang: Conceptualization and writingoriginal draft. Hongyan Zhang: Conceptualization. Kaili Wang: Writingreview and editing. Qian Su: Writingreview and editing, supervision, and project administration.

The authors declare no competing financial interest.