Coordination Engineering of Ir─Mo Atomic Pair Sites to Break Scaling Limitations for Acidic Oxygen Evolution

ABSTRACT

Coordination engineering of single‐atom catalysts (SACs) is a powerful strategy to address durability and activity challenges in the acidic oxygen evolution reaction (OER). Here, we obtain two distinct Ir single‐atom configurations on MoO₃ support by regulating the second‐shell coordination environment. Compared with the weakly interacting Ir─O─Mo structure, atomic pair sites formed through direct Ir─Mo coordination exhibit strong electronic coupling with the support, thereby enhancing atomic dispersion and structural stability. In situ experimental and theoretical studies reveal that the Ir─Mo pair sites trigger a new oxide‐mediated pathway, in which dynamic hydroxyl spillover from Mo to Ir site effectively facilitates *OOH formation. This process breaks the linear scaling relationship between *OH and *OOH adsorption, lowering the energy barrier of the rate‐limiting step and enabling superior OER kinetics. As a result, the Ir_O+Mo/MoO₃ catalyst achieves outstanding stability for over 1500 h at 10 mA cm⁻² in acidic electrolyte and sustains continuous operation for 300 h at 1.0 A cm⁻² in the proton exchange membrane water electrolyzer. This work provides novel insights into the coordination engineering of SACs and opens a promising avenue for overcoming scaling limitations in acidic OER catalysis.

Engineering the local coordination of single atoms to construct metal–metal pair sites drives the spontaneous migration of oxygenated intermediates, enabling an optimized electron‐transfer pathway that breaks the scaling relation.

1. Introduction

Proton exchange membrane water electrolysis (PEMWE) is considered a promising technology for converting renewable energy into sustainable fuels under high pressure, yet simultaneously achieving an optimal balance among activity, durability, and cost in catalyst development remains a key challenge [1, 2, 3, 4]. Compared with other efficient catalysts, the single‐atom catalysts (SACs) enable the maximization of atomic utilization and significantly reduce precious metal loading, thereby offering a novel strategy to elucidate the structure–performance relationship of the oxygen evolution reaction (OER) at the atomic scale [5, 6, 7, 8, 9]. Among them, Ir single‐atom catalysts exhibit superior durability in harsh acidic conditions, but their scalability for industrial PEMWE remains limited, necessitating further breakthroughs in understanding and optimizing the structure–performance relationship [10, 11, 12]. Recent studies have revealed that the electrocatalytic performance of single‐atom catalysts is not solely determined by the intrinsic activity of the metal centers but is also strongly governed by the local chemical composition [13, 14, 15, 16]. In the rational design of Ir single‐atom catalysts, conventional strategies, such as modifying the support composition or introducing heteroatoms, can modulate the electronic structure. However, these approaches are accompanied by multiple effects (e.g., phase transitions), thereby increasing the complexity of elucidating structure–performance relationships [11, 17, 18, 19]. Therefore, an in‐depth investigation into the electronic structure modulation of single‐atom sites at the atomic level, the rational design of coordination environments, and the interactions between metal atoms and supports represent challenging yet promising strategies.

Regulating the local coordination environment directly targets the microscopic configuration of the Ir single‐atom centers, providing new insights and theoretical guidance for the development of PEMWE electrocatalysts with both high activity and long‐term durability [20, 21, 22]. By engineering the first coordination shell (e.g., the type, number, bond length, and bond angle of coordinating atoms) as well as the second coordination shell (e.g., adjacent metal centers of the coordinating atoms), the electronic structure of Ir atoms can be selectively modulated. For instance, adjusting the oxygen coordination number allows precise tuning of the d‐band center position and orbital occupancy, thereby optimizing the adsorption energy of the key reaction intermediates [12, 23, 24]. Consequently, the local coordination environment plays a pivotal role in modulating the reaction pathway and achieving a synergistic optimization of catalytic activity and durability. For Ir single‐atom catalysts dispersed on the oxide supports, the Ir atoms are typically anchored on the surface via coordination with oxygen atoms, which are further connected to neighboring metal atoms in the lattice, forming a metal–oxygen–metal (M₁–O–M₂) configuration that modulates the OER pathway by tailoring the electronic and geometric structure of the active sites [19, 25]. However, such individual active atoms determine the single end‐on adsorption mode, typically following the conventional adsorption evolution mechanism (AEM), which intrinsically limits catalytic activity through linear scaling relationships [26, 27]. Consequently, tailoring synthetic strategies to engineer the direct coordination between Ir single atoms and adjacent metal atoms on oxide supports can generate the metal–metal (M₁–M₂) dual‐site configuration, promoting efficient dual‐site catalysis through enhanced interatomic synergy [28, 29]. Although engineering the local coordination environment of Ir single atoms to generate metal–metal dual‐site configurations holds great promise for advancing acidic OER performance, such strategies remain rarely explored.

In this work, we develop a simple strategy to regulate the local coordination environment of Ir single atoms by altering the sintering atmosphere, thereby obtaining two catalysts with distinct configurations. Specifically, Ir atoms are selectively dispersed on a MoO₃ support through Ir–O–Mo coordination (Ir_O/MoO₃) and Ir–Mo direct coordination (Ir_O+Mo/MoO₃), respectively. In the Ir_O/MoO₃ system, the conventional metal–oxygen–metal configuration limits electronic coupling between the Ir and the MoO₃ support, while the single active sites limit OER kinetics. In contrast, Ir_O+Mo/MoO₃ features direct Ir–Mo coordination, which not only promotes more efficient electronic coupling and continuous charge‐transport pathways, but also enables Ir–Mo pair sites to drive a dual‐site cooperative adsorption evolution mechanism (dual‐site co‐AEM). The affinity difference of Ir and Mo sites toward oxygenated intermediates facilitates the spontaneous migration of *OH species from oxophilic sites to neighboring sites with moderate oxygen binding strength for subsequent oxidation, which is conducive to accelerating the dynamic adsorption/desorption kinetics of reaction intermediates. As a result, Ir_O+Mo/MoO₃ exhibits superior electrochemical performance, with an overpotential as low as 228 mV at 10 mA cm⁻² and maintains stable operation at 1.0 A cm⁻² for 300 h in the assembled proton exchange membrane electrolyzer.

2. Results and Discussion

2.1. Theoretical Predictions on the Structure–Performance Relationship

To explore the design of dual‐metal active sites for acidic OER, density functional theory (DFT) calculations were performed to assess the feasibility of dual‐site co‐AEM. Considering the (010) surface of α‐MoO₃ exhibits lower surface energy and higher chemical stability, we dispersed the Ir single atoms on the MoO₃ (010) support in two different coordination forms (Figure 1a). In the Ir–O–Mo coordinated structure model (Ir_O/MoO₃), the Ir single atoms are directly coordinated with adjacent oxygen atoms and further connected to Mo atoms via bridging oxygen atoms. Conversely, in the Ir–Mo coordinated structure model (Ir_O+Mo/MoO₃), the absence of some oxygen atoms enables direct Ir–Mo coordination. Variations in coordination environments may influence the adsorption/desorption dynamics of key intermediates (e.g., *OH and *OOH) at active centers, thereby modulating the OER pathway. Based on the models, we constructed surface Pourbaix diagrams to illustrate the relationship between the terminal potential for intermediate deprotonation and solution pH at Ir and Mo sites (Figure 1b–e). Specifically, as the electrode potential increases, water and hydroxyl species are sequentially deprotonated at the active sites [30]. The significant deprotonation potential difference at Ir and Mo sites of Ir_O+Mo/MoO₃ across the oxidation potential range enhances the potential for coupling between different types of terminal groups, providing the fundamental condition for hydroxyl spillover behavior at the metal‐to‐metal sites (Figure S1).

FIGURE 1.

Mechanistic prediction of intermediates on the catalyst surface. (a) Schematic structure models of Ir_O/MoO₃ and Ir_O+Mo/MoO₃. Surface Pourbaix diagrams of (b) Ir sites and (c) Mo sites of Ir_O/MoO₃, (d) Ir sites and (e) Mo sites of Ir_O+Mo/MoO₃. (f) Energy levels of theoretical redox potential (U_R) and theoretical coupling potential (U_L). The U_R is defined as the electrode potential of H₂O* deprotonation, and the U_L is defined as the limiting electrode potential needed to remove the surface *OH species for the theoretical formation of intramolecular O─O bonds. (g) Computed H₂O*, *OH, and *O adsorption free energies on catalyst surfaces. (h) Advanced dual sites to promote hydroxyl spillover for the OER on the surface of catalysts.

To further quantify the deprotonation behavior of the surface sites, we evaluated the difference between the theoretical redox potential (U_R) and the theoretical coupling potential (U_L), along with the adsorption free energies of key reaction intermediates on the catalyst surface (Figure 1f,g). The computational results indicate that in the Ir_O+Mo/MoO₃ system, the Ir and Mo sites exhibit comparable U_R, suggesting similar deprotonation capabilities [31]. Whereas the pronounced difference in U_L confirms that direct O–O intermediate coupling for O₂ generation via dual‐site cooperative catalysis is difficult to achieve. Specifically, the Mo sites exhibit a larger U_R–U_L value than the Ir sites, thereby increasing the likelihood of coexisting surface termination states at different sites and promoting coupling between different types of terminal groups. All these results suggest that *OH species adsorbed on the Mo sites can migrate to adjacent Ir sites for further deprotonation, thereby facilitating the accumulation of *OOH intermediates at the Ir sites (Figure 1h) [32]. In contrast, the deprotonation behavior of Ir and Mo sites in the Ir_O/MoO₃ system is not well synchronized under operating conditions. Therefore, constructing an Ir–Mo pair‐site configuration proves advantageous for modulating the surface coverage of reaction intermediates and activating the dual‐site co‐AEM pathway.

2.2. Morphological and Structural Characterizations

Based on the above computational analysis, we rationally tuned the local coordination environment of Ir single atoms dispersed on the surface of MoO₃ by varying oxygen‐rich and oxygen‐deficient calcination atmospheres, thereby synthesizing Ir_O/MoO₃ with Ir–O–Mo coordination in air and Ir_O+Mo/MoO₃ with direct Ir─Mo coordination in argon. XRD analysis reveals that primary diffraction peaks of both Ir_O/MoO₃ and Ir_O+Mo/MoO₃ are predominantly indexed to MoO₃ (JCPDS No. 35–0609), with no additional signals corresponding to Ir‐based compounds (Figure S2). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show uniformly dispersed morphologies for the two catalysts, and the primary exposed crystal planes of MoO₃ in both materials are (040) and (021) (Figures S3–S4). Additionally, as shown in the aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM), the brighter atomic columns are identified as Ir single atoms (Figure 2a,d) [33, 34]. The result is further confirmed by the three‐dimensional Gaussian‐fitted atomic mapping (Figure 2b,e) [35, 36]. As further revealed by the corresponding intensity profile along the selected region, the Ir_O+Mo/MoO₃ exhibits a shortened Ir─Mo interatomic distance, indicative of a more closely coupled local Ir–Mo arrangement arising from oxygen deficiency [28, 37, 38]. Additionally, elemental mapping confirms the uniform distribution of Ir, Mo, and O (Figure 2c,f).

FIGURE 2.

Structural characterizations of Ir_O+Mo/MoO₃ and Ir_O/MoO₃. (a) Magnified aberration‐corrected HAADF STEM image of Ir_O/MoO₃. (b) Intensity line profile of the atom column extracted and line‐scanning intensity profile obtained along line 1 in (a). (c) HAADF‐STEM image and corresponding elemental mapping of Ir_O/MoO₃. (d) Magnified aberration‐corrected HAADF STEM image of Ir_O+Mo/MoO₃. (e) Intensity line profile of the atom column extracted and line‐scanning intensity profile obtained along line 2 in (d). (f) HAADF‐STEM image and corresponding EDS mapping of Ir_O+Mo/MoO₃. (g) Normalized Ir L₃‐edge XANES spectra. (h) Ir L₃‐edge Fourier transforms‐EXAFS spectra. (i) Wavelet transform EXAFS plots of Ir_O+Mo/MoO₃ and Ir_O/MoO₃.

To further validate the unique advantages of the atomic‐level dispersion of Ir on the MoO₃ surface, we constructed an embedded structural model where Ir atoms substitute Mo atoms (L‐Ir_O+Mo/MoO₃). The results indicate that the incorporation of Ir single atoms into the MoO₃ lattice leads to a relatively large atomic distance (4.7 Å) between the Ir and Mo atoms, owing to the distinctive structural features of MoO₃ (Figure S5). This increased distance prevents strong interactions between the metal‐pair sites, contradicting the experimental data. Furthermore, the calculated formation energy indicates that the Ir_O+Mo/MoO₃ system is thermodynamically more stable (Figure S6). The above results collectively confirm the unique advantage of dispersing Ir single atoms on the MoO₃ support in promoting the activation of dual metal sites. The inductively coupled plasma optical emission spectroscopy (ICP‐OES) analysis reveals that Ir_O+Mo/MoO₃ and Ir_O/MoO₃ exhibit low Ir loading of 6.49 and 6.57 wt%, respectively (Table S1).

The chemical environments of the two catalysts were further elucidated by X‐ray photoelectron spectroscopy (XPS) and x‐ray absorption near‐edge structure (XANES). The Ir L₃‐edge absorption position in both Ir_O/MoO₃ and Ir_O+Mo/MoO₃ is situated between those of Ir foil (Ir⁰) and IrO₂ (Ir⁴⁺), indicating the intermediate Ir oxidation states (Figures 2g and S7) [39]. This is consistent with the negative shift in the Ir 4f XPS peak (Figure S8). From the extended x‐ray absorption fine structure (EXAFS) of Ir L₃‐edge, the prominent peak of Ir_O+Mo/MoO₃ is assigned to the scattering path of the nearest Ir–O shell, while an additional shell at longer scattering distance corresponds to the Ir–Mo contribution. Based on the fitting results, the average Ir–O first shells coordination number in the Ir_O+Mo/MoO₃ system is 3.2, with a bond length of 2.07 Å. Notably, in the Ir_O+Mo/MoO₃, the Ir–Mo distance is shortened to 2.83 Å (Figures 2h, S9, and Table S2). Wavelet transform (WT) analysis provides compelling evidence for the successful construction of Ir–Mo dual sites (Figure 2i, S10, and S13). The introduction of Ir single atoms exerts a negligible effect on the coordination structure of the MoO₃ framework in the Ir_O/MoO₃ (Figures S11–S12). All these results demonstrate the successful synthesis of two Ir single‐atom catalysts with distinct coordination environments.

2.3. Electrocatalytic Oxygen Evolution Measurements

The electrocatalytic performance of the catalysts in acidic media was evaluated using a standard three‐electrode configuration in 0.5 M H₂SO₄, aiming to elucidate the correlation between the modulation of the local coordination environment of Ir single atoms and OER activity. Anchoring single Ir atoms on oxide supports significantly enhances OER performance, demonstrating great potential (Figure 3a). Notably, Ir_O+Mo/MoO₃ requires only 228 and 337 mV to reach current densities of 10 and 200 mA cm⁻², respectively, which are lower than those of Ir_O/MoO₃ (280 mV @ 10 mA cm⁻²; 404 mV @ 200 mA cm⁻²). Among samples annealed at various temperatures, Ir_O+Mo/MoO₃ achieves optimal activity at 500°C (Figures S14–S15). Tafel slope analysis and electrochemical impedance spectroscopy (EIS) were further employed to probe the reaction kinetics. Ir_O+Mo/MoO₃ exhibits the lowest charge‐transfer resistance and the fastest OER kinetics in an acidic environment (Figures 3b and S16). The Ir_O+Mo/MoO₃ demonstrates a double‐layer capacitance (C_dl) of 100.9 mF cm⁻², which is 1.12, 1.14, and 13.10 times higher than those of Ir_O/MoO₃, IrO₂, and MoO₃, respectively (Figures 3c and S17). At 1.53 V vs. RHE, Ir_O+Mo/MoO₃ delivers mass activity 4.6 times higher than that of Ir_O/MoO₃ (Figures 3d and S18). Importantly, the mass activity trend is further validated by turnover frequency (TOF) calculations, where Ir_O+Mo/MoO₃ reaches a TOF of 1.425 s⁻¹ at an overpotential of 300 mV (Figure S19, Table S3). The results collectively demonstrate that Ir_O+Mo/MoO₃ exhibits superior catalytic activity for acidic OER, underscoring its promising prospects for practical applications.

FIGURE 3.

Electrocatalytic performance toward acidic OER. (a) Oxygen evolution reaction polarization curves at a scan rate of 5 mV s⁻¹ for Ir_O+Mo/MoO₃, Ir_O/MoO₃, IrO₂, and MoO₃. (b) Tafel plots calculated from the iR‐corrected polarization curves. (c) C_dl plots derived from the fitting CV curves at difference scan rates of Ir_O+Mo/MoO₃, Ir_O/MoO₃, IrO₂, and MoO₃. (d) Normalized LSV curves of mass activity for Ir_O+Mo/MoO₃, Ir_O/MoO₃, and IrO₂. (e) Chronopotentiometry test for Ir_O+Mo/MoO₃, Ir_O/MoO₃ at 10 mA cm⁻². (f) The polarization curves for PEMWE using Ir_O+Mo/MoO₃ and IrO₂ as the anodic catalyst, respectively. (g) Chronopotentiometry test at 1.0 A cm⁻² in a PEMWE electrolyzer.

Electrochemical stability is another critical criterion for evaluating catalyst performance. Specifically, Ir_O/MoO₃ operates continuously for over 900 h at a current density of 10 mA cm⁻², while Ir_O+Mo/MoO₃ exhibits only a slight increase in overpotential after 1500 h of continuous operation (Figure 3e). Compared to recently reported Ir‐based catalysts, Ir_O+Mo/MoO₃ achieves a significant breakthrough, effectively overcoming the conventional trade‐off between activity and stability (Table S4). SEM and XPS analyses conducted after stability of electrolysis confirm the phase and structural integrity of the Ir_O+Mo/MoO₃ catalyst following long‐term operation (Figures S20–S21). Comprehensively, Ir_O+Mo/MoO₃ demonstrates exceptional OER performance, highlighting the critical role of local coordination environment modulation on the catalytic behavior of single‐atom systems. To further validate the industrial applicability, a PEMWE electrolyzer was assembled to evaluate electrochemical performance under realistic water‐splitting conditions. The PEMWE electrolyzer was constructed using Ir_O+Mo/MoO₃ and IrO₂ as the anode catalysts, respectively, with commercial Pt/C as the cathode catalyst, and was tested at 80°C. As shown in the polarization curves, the Ir_O+Mo/MoO₃ || Pt/C electrolyzer achieves 1 A cm⁻² at a low cell voltage of only 1.69 V, outperforming the IrO₂ || Pt/C system, which requires 1.76 V to reach the same current density (Figure 3f). Furthermore, the Ir_O+Mo/MoO₃ || Pt/C electrolyzer maintains stable operation for 300 h at 1 A cm⁻², indicating that the direct coordination between Ir and Mo contributes to the stability of the catalyst structure and reduces the dissolution loss of Ir. (Figure 3g, Table S5). The superior PEMWE performance of the Ir_O+Mo/MoO₃ || Pt/C system compared to the IrO₂ || Pt/C validates the effectiveness of tailoring the local coordination environment of single atoms, providing new insights into the design of high‐performance single‐atom catalysts beyond conventional paradigms.

2.4. In Situ Characterizations of Local Coordination Structures

In addition to the structural stabilization provided by metal–metal dual sites, the potential of dynamic oxygen spillover plays a critical role in promoting the electrochemical performance. As the applied voltage gradually increases from OCV to 1.65 V, the white line peak of the Ir L₃‐edge in Ir_O+Mo/MoO₃ shifts to a higher binding energy (Figures 4a and S22). Upon returning to the initial state, the white line peak position of the Ir_O+Mo/MoO₃ Ir L₃‐edge shows a slight shift compared to the original position, indicating the oxidation process is reversible, with the increase in the oxidation state of Ir being not only related to its own oxidation but also closely linked to the adsorption strength of oxygen intermediates during the OER (Figure S23). The dynamic shift in the K‐edge energy absorption of Mo is associated with the oxidative surface under high anodic potentials (Figures 4e and S24). To gain deeper insights into the local coordination environment of the metal sites during the reaction, fitting analysis of the FT‐EXAFS spectra was performed (Figures S25–S26). As the applied potential increases from OCV to 1.65 V, the average bond lengths of Ir–O, Ir–Mo, and Mo–O in Ir_O+Mo/MoO₃ show no significant changes (Figure 4b,f). The local coordination structure of the Ir single atom in Ir_O+Mo/MoO₃ and the Mo atoms in the support remains essentially unchanged across the range of reaction potentials.

FIGURE 4.

In‐situ x‐ray absorption spectroscopy (XAS) analysis. (a) In situ XANES spectra of the Ir L₃‐edge for Ir_O+Mo/MoO₃ at different voltages. (b) The Ir L₃‐edge FT‐EXAFS spectra of Ir_O+Mo/MoO₃ with applied potential from OCV to OCV after. (c) Summary of the Ir−O bond length and coordination number of Ir_O+Mo/MoO₃ at various potentials. (d) Summary of the Ir─Mo bond length and coordination number of Ir_O+Mo/MoO₃ at various potentials. (e) In situ XANES spectra of the Mo K‐edge for Ir_O+Mo/MoO₃ at different voltages. (f) The Mo K‐edge FT‐EXAFS spectra of Ir_O+Mo/MoO₃ with applied potential from OCV to OCV after. (g) Summary of the Mo−O bond length and coordination number of Ir_O+Mo/MoO₃ at various potentials. (h) In situ Raman spectra of Ir−O and Mo−OH bands for Ir_O+Mo/MoO₃ under applied potentials from OCP to OCV after. Bode plots at the potential range of (i) Ir_O+Mo/MoO₃, and (j) Ir_O/MoO₃ from 1.10 to 1.40 V (vs. RHE). (k) Illustration of the formation of a directional spillover path on the interface of Ir_O+Mo/MoO₃.

Since Ir single atoms are dispersed on the MoO3 surface and directly coordinate to Mo atoms in the surface phase, changes in the coordination number of the Ir single atom can provide insights into the properties of the intermediate species. Inspired by the relationship that the orbital interaction strength between metal and oxygen is approximately proportional to the metal surface *OH coverage, we further investigated the changes in the coordination numbers of Ir–O, Ir–Mo, and Mo–O in the Ir_O+Mo/MoO₃ catalyst during the OER [40]. From OCV to 1.65 V, the coordination number of Ir–O in Ir_O+Mo/MoO₃ gradually increases (from 3.4 to 4.3) (Figure 4c,d). Additionally, the coordination number of the Ir–Mo pair sites remains relatively stable throughout the potential increase (Tables S6–S7). Consequently, the increase in the oxidation state of the Ir single atom can be attributed to the adsorption of oxygen intermediates at the Ir active sites, thereby increasing the coordination number. Notably, as the potential rises from OCV to 1.45 V, the coordination number of Mo–O shows a slight increase (from 4.7 to 5.2) (Figure 4g). Subsequently, the coordination number of Mo–O exhibits a decrease, and this dynamic change implies a trend of oxygen intermediate migration from the Mo site to the Ir site, which is consistent with the dynamic increase in the coordination number of Ir–O.

In situ Raman spectroscopy was employed to explore the hydroxyl adsorption behavior of Ir and Mo sites in Ir_O+Mo/MoO₃. As shown in Figure 4h, the vibrational peak at ∼900 cm⁻¹ in Ir_O+Mo/MoO₃, attributed to Mo–OH species, decreases in intensity with increasing potential, confirming that Mo sites serve as active centers in the acidic OER process. Simultaneously, the gradual increase in the peak width of the Ir–O vibrational band at ∼503 cm⁻¹ is attributed to the emergence of a new Raman peak (Figure S27). The new peak corresponds to the gradual accumulation of *OOH intermediates at the Ir sites, reflecting a dynamic migration of oxygenated species between the dual active sites, consistent with the results from FT‐EXAFS [41]. When the potential returns to the OCV after the condition, the Raman features of Mo–OH and Ir–O both recover to a state close to the initial one, indicating that the intermediates are consumed or dissociated, which allows the vibration peaks of Mo–OH and Ir–O to revert to their original positions. This phenomenon confirms that the changes in the Raman peaks of Mo–OH and Ir–O are closely associated with the involvement of reaction intermediates and further validates the dynamic nature of oxygen intermediate migration at the Ir–Mo pair sites during the OER process.

To further elucidate the reaction kinetics and the charge/mass transport behavior, in situ EIS was conducted under various applied potentials (Figures 4i,j, and S28–S29). Compared to Ir_O/MoO₃, Ir_O+Mo/MoO₃ exhibits a more pronounced decline in the low‐frequency phase angle with increasing bias, indicating a faster charge‐transfer process at the catalyst–electrolyte interface and a significantly reduced activation energy barrier [42, 43]. During the OER process, the Ir–Mo pair‐site configuration significantly enhances reaction kinetics through a dynamic hydroxyl spillover mechanism. Specifically, *OH species initially adsorbed on Mo sites migrate to directly coordinated Ir sites, where they are further oxidized to *OOH, thereby increasing the transformation of the reaction intermediate. Such a continuous reaction pathway driven by intermediate migration effectively circumvents the intrinsic activity limitations imposed by adsorption energy scaling relationships in conventional heterogeneous electrocatalysis, particularly in complex systems involving multiple reaction intermediates (Figure 4k) [32, 44].

2.5. Dynamics and Mechanism Studies

Subsequently, methanol was used as a molecular probe to evaluate the *OH adsorption characteristics on the catalyst surface. During the methanol oxidation reaction (MOR), the binding of methanol with electrophilic *OH facilitates the oxidation of CO* intermediates, making the surface *OH coverage an effective indicator of deprotonation kinetics (Figure S30) [45]. We determined the MOR reaction orders of two catalysts with distinct coordination structures and found that both Ir_O+Mo/MoO₃ and Ir_O/MoO₃ exhibited low MOR reaction orders, suggesting limited *OH accumulation (Figure 5a). Notably, Ir_O+Mo/MoO₃ exhibits a lower *OH accumulation, providing evidence of a faster deprotonation during the acidic OER. To validate the intermediate species and gain deeper insights into the reaction mechanism, in situ attenuated total reflectance surface‐enhanced infrared absorption spectroscopy (ATR‐SEIRAS) was conducted under varying applied potentials (Figures 5b,c, and S31). The Ir_O/MoO₃ system exhibits an evident absorption band at ∼1030 cm⁻¹ as the potential increases, attributed to the bending vibrational of *OOH species, confirming that the Ir_O/MoO₃ follows an AEM pathway, where kinetics are limited by the adsorption–oxidation rate of intermediates at single active sites [46]. Conversely, when the potential exceeds 1.4 V, the Ir_O+Mo/MoO₃ system exhibits not only the gradual emergence of the *OOH band but also a distinct stretching vibration band appears around ∼1100 cm⁻¹, corresponding to the *OO* intermediate [47]. The emergence of the *OO* signal deviates from a traditional AEM pathway and instead indicates a dual‐site mechanism. Upon return to the OCV, the intermediates on the catalyst surface either desorb or transform into O₂, thereby restoring the catalyst surface to the stable configuration. To further validate this mechanism, in situ differential electrochemical mass spectrometry (DEMS) with ¹⁸O isotopic labeling was employed to detect the evolved oxygen. As shown in Figure 5d,e, only ³²O₂ and ³⁴O₂ signals are observed for Ir_O/MoO₃, whereas Ir_O+Mo/MoO₃ exhibits ³⁶O₂ signals arising from the further oxidation of *¹⁸O¹⁸OH intermediate, confirming the involvement of the typical *OO* intermediates in the reaction process (Figure S32) [48]. Collectively, the ATR‐SEIRAS and DEMS results all demonstrate that Ir_O+Mo/MoO₃ follows an internal intermediates‐coupling process at the Ir–Mo pair‐site, consistent with the dual‐site co‐AEM pathway.

FIGURE 5.

OER mechanism analysis. (a) Logarithmic plot of MOR current density versus methanol concentration on different catalysts. In situ ATR‐SEIRAS spectra of (b) Ir_O+Mo/MoO₃, and (c) Ir_O/MoO₃ under different OER potentials vs RHE. The color scale represents variations in infrared signal intensity. DEMS signals of ³²O₂, ³⁴O₂, and ³⁶O₂ from the gaseous products for ¹⁸O₂‐labeled (d) Ir_O+Mo/MoO₃ and (e) Ir_O/MoO₃. (f) The demetallation energy at the Ir sites for Ir_O+Mo/MoO₃ and Ir_O/MoO₃. (g) Free energy profiles of Ir_O+Mo/MoO₃ and Ir_O/MoO₃ with different OER pathways. (h) The transition‐state free‐energy barrier comparison of the *OH spillover and non‐spillover pathways for Ir_O+Mo/MoO₃.

DFT calculations were carried out to gain a deeper theoretical understanding of the reaction mechanism and the origin of the enhanced OER performance of the Ir_O+Mo/MoO₃. We calculated the projected density of states (PDOS) to elucidate differences in electronic interactions between the Ir 5d and adjacent Mo atoms in IrO+Mo/MoO3 and IrO/MoO3, and their impact on the adsorption of oxygen intermediates. In the Ir_O+Mo/MoO₃ system, the Ir 5d orbitals and 3d orbitals of the nearest Mo exhibit significantly enhanced orbital overlap in the −3 to 0 eV region, reflecting a stronger Ir−Mo electronic coupling (Figures S33–S34). This enhanced electronic coupling facilitates the dominance of d‐electrons near the Fermi level, providing active sites for the adsorption and activation of oxygen intermediates. We quantified the interactions between Ir−*OH and Mo−*OH using COHP to elucidate the feasibility of *OH migration. In the Ir_O+Mo/MoO₃ system, the integrated crystal orbital Hamiltonian population (ICOHP) value for Ir−*OH (6.079) is significantly higher than that for Mo−*OH (4.594) (Figures S35–S36). This adsorption asymmetry, with stronger binding at the Ir site and weaker binding at the Mo site, establishes the thermodynamic and kinetic driving forces for *OH spillover [49, 50]. As revealed by the electron localization function (ELF), the Ir and Mo atoms in the Ir_O+Mo/MoO₃ system exhibit electronic delocalization, with the Ir−Mo pair sites forming a strong electronic coupling through metal–metal synergistic interactions, displaying behavior akin to covalent bonding (Figure S37). Modifying the local coordination environment of Ir single atoms by introducing adjacent Mo atoms establishes a dual‐site metal–metal configuration, facilitating charge redistribution and electronic coupling between the Ir atom and the support. Such structural and electronic modulation plays a critical role in stabilizing Ir single atoms and improving the catalyst's durability during acidic OER.

As illustrated in Figure 5f, we evaluated the demetallation energies of Ir single‐atom sites in different coordination environments to assess the thermodynamic stability. In Ir_O+Mo/MoO₃, the demetallation energy (ΔE₁) was calculated to be 2.36 eV, higher than that of the Ir sites in Ir_O/MoO₃ (ΔE₂ = 2.07 eV), which indicates that metal–metal coordination in Ir_O+Mo/MoO₃ contributes to enhanced thermal stability of the overall structure. To further elucidate the superior OER pathway and verify the catalytic advantages of Ir_O+Mo/MoO₃, the Gibbs free energy profiles of the elementary steps were systematically evaluated (Figure 5g). First, we evaluate the conventional AEM for both catalysts (Figures S38–S39). The potential determining step (PDS) within the AEM pathway for both systems involves the *O intermediate combining with adsorbed water to protonate and form *OOH (*O + H₂O→*OOH + H⁺ + e⁻). Notably, the limiting free energy barrier for this step in Ir_O+Mo/MoO₃ is only 1.87 eV, significantly lower than that of Ir_O/MoO₃ (2.73 eV), highlighting the absolute predominance of direct Ir–Mo coordination in modulating the reaction energetics. When taking the potential hydroxyl spillover between the dual‐metal sites into consideration, the free energy barrier of the PDS in Ir_O+Mo/MoO₃ further drops to 1.45 eV, indicating that the dynamic migration of hydroxyl intermediates between coordinated Ir and Mo atoms can markedly accelerate *OOH formation and thus enhance overall catalytic efficiency (Figure S40) [41]. Importantly, the transition‐state energy barrier for the direct deprotonation of *OH at the Mo site is significantly higher than that for *OH spillover to the Ir site (Figure 5h). Comparative analysis reveals that hydroxyl species exhibit a thermodynamic preference to migrate from Mo sites to directly coordinated Ir sites for subsequent oxidation rather than to undergo further oxidation at the Mo sites [51]. This cooperative effect between Ir−Mo dual sites significantly lowers the energy barrier and facilitates efficient OER catalysis for Ir single‐atom catalysts.

3. Conclusion

In this work, we precisely regulated the local coordination environment of Ir single atoms dispersed on the MoO₃ surface and successfully constructed an Ir_O+Mo/MoO₃ single‐atom catalyst featuring a directly coordinated Ir–Mo configuration. Compared with conventional oxygen‐bridge‐linked configurations, the Ir–Mo pair‐site structure enables strong electronic coupling and partial orbital overlap, facilitating charge transport and enhancing structural stability. Furthermore, the different adsorption behavior of oxygenated intermediates at the Ir–Mo pair‐site activates a unique and efficient dual‐site co‐adsorption evolution mechanism, as confirmed by in situ ATR‐SEIRAS and DEMS measurements. This mechanism involves dynamic hydroxyl spillover from Mo sites to directly coordinated Ir sites, thereby breaking the conventional scaling relationship and lowering the reaction energy barrier. Consequently, Ir_O+Mo/MoO₃ delivers 10 mA cm⁻² at a low overpotential of only 228 mV and maintains operational stability for over 1500 h. Remarkably, when applied in a proton exchange membrane electrolyzer, the system demonstrates continuous operation at 1 A cm⁻² for more than 300 h. This study highlights the crucial role of modulation of the coordination environment in enhancing single‐atom catalyst performance and offers valuable insights into the rational design of highly efficient OER pathways.

Author Contributions

Hongjun Chen: writing ‐ review and editing, writing ‐ original draft, methodology, validation, data curation. Liming Deng: methodology, investigation. Luqi Wang: software, methodology. Sung‐Fu Hung: investigation, methodology. Gengyu Xing: methodology. Yu‐Cheng Liu: investigation, methodology. Ming‐Hsuan Li: investigation, validation. Ying Zhang: validation, methodology. Tao Wang: investigation, conceptualization. Linlin Li: conceptualization, validation. Renli Fu: methodology, validation. Yuping Wu: methodology, investigation. Shengjie Peng: data curation, methodology, validation, resources.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: anie73109‐sup‐0001‐SuppMat.docx.

Data Availability Statement

Research data are not shared.