Facet-selected oxygen-deficient ruthenium oxide for acidic oxygen evolution catalysis

Ru-O π ligand strengthening strategy

To determine the feasibility of Ru-based catalysts being rationally engineered to achieve both high stability and high activity for the OER, we first investigated their intrinsic electronic properties through theoretical analysis. According to the fundamental principles of the AEM, the low covalency of the metal–oxygen (M–O) bond, to ensure removal of electrons from Ru 4d orbitals rather than O 2p orbitals, is an essential condition for AEM realization²⁶. However, conventional RuO₂ represents a typical high-covalency case, characterized by a small energy gap between the Ru 4d and O 2p states (a small charge transfer energy Δ, as shown in Fig. 1a). As a result, the t_2g^* band retains substantial O 2p character near the Fermi level (E_F)^27,28, which is unfavorable for the AEM because it increases lattice oxygen participation in redox processes. In addition, compared to the σ-type overlap with the Ru e_g orbitals (\(d_{z^2}\) and \(d_{x^2-y^2}\), which point directly towards the ligands along the bond axes, as shown in Fig. 1b), the π-type overlap with the t_2g orbitals corresponds to a smaller splitting and is weaker (d_xy, d_xz, and d_yz, which are oriented between the oxygen ligands and thus overlap with the O 2p π orbitals sideways relative to the Ru–O bond axes, as shown in Fig. 1c). Consequently, the hybridization of the t_2g and O_2p orbitals is incomplete. These two factors make the lattice oxygen more redox-active, enabling direct participation in the O–O bond formation step (as the LOM). Therefore, to suppress this undesirable pathway and promote the AEM pathway, a strategy to strengthen the hybridization between Ru t_2g orbitals and π-donor oxygen ligands by designing intrinsic structurally engineered RuO₂ catalysts is proposed.

Theoretically, π-donor oxygen ligands can be strengthened by introducing coordinatively unsaturated Ru atoms^29,30, which promote stronger Ru–O π interactions and facilitate charge redistribution near the Fermi level (a relatively lower Ru valance and higher t_2g^* occupation, as shown in Fig. 1d). In addition, when oxygen vacancies, which are ubiquitous in metal oxides³¹, become sufficiently abundant in sufficiently small (nanoscale) particles, the exposed surfaces naturally feature abundant surface oxygen vacancies. These surface oxygen vacancies, corresponding to surface oxygen-deficient Ru atoms, are precisely the targeted active sites, which can provide stronger adsorbate binding than normal oxide sites and assist in adsorbate dissociation at the surface³². This stronger adsorbate binding stabilizes key intermediates (OH^*, O^*, and OOH^*), while promoted adsorbate dissociation helps sustain the AEM pathway by preventing lattice oxygen from directly participating in O–O bond formation. Guided by this insight, to identify a favourable facet for surface oxygen-deficient Ru atoms, theoretical density functional theory (DFT) calculations were conducted. The DFT calculations were performed using idealized models under vacuum conditions. The results showed that the (101) plane of RuO₂ possesses both high surface stability and a favorable electronic configuration for AEM. As the AEM pathway benefits from minimal occupation of O 2p orbitals near the Fermi level³³, projected density of states (PDOS) calculations across all the planes reveal that the (101) surface displays a positive energy band separation (ΔE = +0.29 eV, as the energy difference between the lowest Ru 4d state and the highest O 2p state near the Fermi level), significantly distinct from the negative ΔE of other low-index planes (Fig. 1e and Supplementary Fig. 1). This pronounced band separation effectively reduces Ru–O covalency and suppresses O 2p participation, thereby promoting electron transfer from Ru 4d orbitals rather than lattice oxygen. Such an electronic configuration provides a clear basis for the AEM preference on the (101) plane, accounting for its stability under acidic OER conditions. Moreover, comparative analysis of saturated and unsaturated (101) surfaces indicates that the introduction of surface oxygen vacancies on (101) plane can markedly increase the Ru t_2g and O π hybridization by broadening and upshifting the Ru 4d t_2g band occupation (band center ε_d from –3.09 eV to –2.76 eV) and reducing the electron density of O 2p orbitals (band center ε_p from –2.61 eV to –2.77 eV) near the Fermi level, thereby promoting preferential adsorption of OH⁻ at Ru active sites (Fig. 1f). Overall, oxygen-vacancy formation lowers the Ru valence and increases the Ru-centered 4d electron density, while the designed oxygen-deficient (101) facet induces an upshifted and broadened Ru 4d distribution, increasing the Ru 4d contribution in the frontier-energy region near the Fermi level. Together with the downward shift of O 2p states, this electronic redistribution promotes Ru-centered adsorbate evolution and suppresses lattice oxygen redox participation. Concurrently, the pre-existing surface vacancies on the (101) surface not only enable the OER to efficiently proceed at surface sites but also impart strong resistance to new internal lattice oxygen loss and minimize structural degradation, as evidenced by its highest calculated oxygen vacancy formation energy (E_Ovac = 5.91 eV) among the commonly observed low-index RuO₂ surfaces (Supplementary Fig. 2 and 3).

In general, the orbital hybridization calculations and oxygen vacancy formation energy calculations complement each other and establish the intrinsic electronic and structural basis for AEM-favorable od-RuO₂. Therefore, unlike previous approaches that relied solely on dopant modification or external strain to adjust electronic structures, this method achieves intrinsic modulation of Ru–O orbital interactions by rational structural design. The overall strategy for redox center modulation—combining π-donor ligand strengthening through oxygen vacancy engineering with facet-selective (101) exposure—is schematically illustrated in Fig. 1g.

Defect-inheritance synthesis strategy

To realize the above described surface oxygen-deficient configuration, perovskites are preferentially considered precursors. Typically, in SrCoO_3-δ, the cobalt atoms inherently exhibit highly unsaturated coordination (3-δ ≈ 2.52)³⁴, making SrCoO_3−δ a particularly suitable host for incorporating Ru atoms and initiating controlled lattice reconstruction. Therefore, a tailored defect-inheritance synthesis route involving a straightforward and scalable sol-gel method with SrCoO_3-δ as a precursor, followed by a hydrothermal process, was employed. First, metal nitrates were gelled to form a liquogel, which was air annealed to transform it into a Ru-doped perovskite precursor, SrCo_1-xRu_xO_3-δ (Fig. 2a and Supplementary Fig. 4). Within this perovskite lattice, Ru and Co atoms are strongly bonded within oxygen octahedra, whereas Sr atoms occupy weakly bonded cuboctahedral sites and readily leach under acidic hydrothermal conditions (Fig. 2b). This selective Sr dissolution triggers complete structural reconstruction, during which the confinement effect of the cobalt spinel oxide promotes nanoscale growth of RuO₂. Most residual cobalt oxide species, which exist as independent phases, can be removed during the subsequent rinsing process, as confirmed with inductively coupled plasma mass spectrometry (ICP–MS) results (Supplementary Fig. 5). As a result, the reconstructed RuO₂ inherits the oxygen-deficient environment of the perovskite, yielding the final od-RuO₂ catalyst (Fig. 2c). The aforementioned (101) lattice plane is particularly suited for stabilizing surface oxygen vacancies, as the (101) plane has the lowest surface energy (E_surf) among the commonly observed low-index RuO₂ surfaces when vacancies are primarily distributed at the surface (Supplementary Fig. 6), indicating its thermodynamic stability that favors its preferential formation, verifying the thermodynamic feasibility of forming the oxygen-deficient (101) surface during synthesis reconstruction. By leveraging a rational perovskite-to-oxide transformation, this defect-inheritance synthesis strategy enables precise control over the spatial distribution of surface defects and electronic structure, which ensures the synthesis of od-RuO₂ nanoclusters with a high density of AEM-favorable active sites, integrating structural stability, electronic optimization, and synthetic feasibility into a single high-performance OER catalyst.

To determine if the developed design strategy could provide an od-RuO₂ nanocatalyst with high stability and high activity for the OER, the nanocatalyst was synthesized, and its structural properties were examined by morphological characteristics. X-ray diffraction (XRD) confirms the precursor with a crystalline phase corresponding to the perovskite SrCoO_2.52 (Supplementary Fig. 7). Whereas, no distinct RuO₂ diffraction peaks were observed in the XRD patterns of od-RuO₂ (only the baseline could be detected, shown as shown in Supplementary Fig. 8), indicating that this special RuO₂ consists of ultrasmall nanoclusters. Previous studies have confirmed that particles having crystalline domain sizes below 5 nm become difficult to analyze, due to both broad peaks and low signal-to-noise ratios³⁵. Next, to further determine the composition of the catalyst, energy-dispersive X-ray spectroscopy (EDS, Supplementary Fig. 9) and electron energy loss spectroscopy (EELS, Fig. 2f) were performed. Although both Ru and Co signals were present, the cobalt oxide only served as original sites for the defect-inheritance of Ru, and most of cobalt also dissolved during the acidic hydrothermal process, as only some residual signals remained that were not completely washed away. The EELS analysis of randomly selected regions revealed that Ru predominantly covered the cobalt oxide, thereby suggesting that the residual cobalt oxide just serves as a constraint or a support for od-RuO₂ recombination and growth, rather than serving as the exposed active sites. The O signal is reduced in regions with the highest Ru intensity, indicating the possible presence of substantial surface oxygen vacancies.

To ensure high exposure of a specific lattice plane, the particle size of od-RuO₂ should be in the nanoscale range. High-resolution transmission electron microscopy (HR-TEM) images confirmed the morphology of od-RuO₂ as dispersed nanoparticles with uniform sizes, averaging 2 – 4 nm in diameter (Fig. 2d and Supplementary Fig. 10). High-angle annular dark-field scanning TEM (HAADF-STEM) images (Fig. 2e) revealed Ru as brighter atomic columns, corroborating the presence of nanoscale RuO₂ clusters, while only negligible Co₃O₄ was observed. The statistical result of fast Fourier transform (FFT) diffraction pattern analysis of identifiable regions across all STEM images (Supplementary Figs. 11 and 12) shows that the majority of these nanoparticles displayed well-defined lattice fringes corresponding to the (101) plane of rutile RuO₂, with a lattice spacing of 0.25–0.26 nm. Notably, this orientation significantly dominated over the common (110) plane typically observed in commercial RuO₂. As designed by theoretical calculations, this preferential exposure of the (101) facet is significant because the (101) surface possesses distinct orbital hybridization characteristics and higher oxygen vacancy formation energy that stabilize surface-deficient configurations and favor the AEM over the LOM. Therefore, given the ultrasmall nanocluster size (possessing a high surface-to-volume ratio) and the statistically and theoretically predominant exposure of the (101) facet, a substantial fraction of oxygen-deficient Ru sites is expected to be enriched at or near the (101) surface. These results suggest that the hydrothermal process not only confines RuO₂ growth to nanoclusters, avoiding overgrowth into larger crystallites, but also favors specific lattice plane exposure, potentially endowing the material with observed catalytic properties. These morphological results preliminarily indicate that a targeted nanocatalyst with the desired surface oxygen-deficient configuration has been synthesized.

Characterization of od-RuO₂

To experimentally identify the oxygen-deficient configuration of od-RuO₂ in depth, its microstructure and electronic properties were further investigated. Electron paramagnetic resonance (EPR) was utilized for preliminary detection of the overall oxygen vacancy state of the catalyst. EPR signals around g ≈ 2 are commonly associated with unpaired electrons trapped at oxygen-vacancy-related defect sites in metal oxides^36,37,38. More notable peaks were observed for both the perovskite precursor and od-RuO₂ than for commercial RuO₂ (Fig. 3a), indicating that significant oxygen vacancies were introduced into od-RuO₂ from the perovskite, consistent with the design strategy.

X-ray absorption fine structure (XAFS) analysis was further employed to elucidate the local chemical environment and oxidation state of Ru in od-RuO₂. The Ru K-edge X-ray absorption near-edge structure (XANES) spectra (Fig. 3b) were compared with those of references, including Ru foil, commercial RuCl₃, and commercial RuO₂, which correspond to typical oxidation states. The white-line intensity confirmed that the predominant substance was RuO₂, whose spectrum was clearly distinct from that of RuCl₃. The half-peak K-edge energy revealed that the oxidation state of Ru in od-RuO₂ lay between those in commercial RuO₂ and commercial RuCl₃, indicating an intermediate valence between +3 and +4. The first derivative of the Ru K-edge XANES spectra, the Ru L₃-edge X-ray absorption spectra, and also the X-ray photoelectron spectroscopy (XPS) further corroborated this electronic structure (Supplementary Figs. 13, 14, and 15). Notably, as shown in the L₂-edge spectra (Fig. 3i), the t_2g peak, which was prominent in commercial RuO₂, was absent in od-RuO₂, indicating substantial filling of the t_2g orbitals and thereby a lower Ru valence state in od-RuO₂ than in commercial RuO₂, corroborating the K-edge XAS results. This reduced oxidation state of Ru is significant, as it serves as supplementary evidence for the abundant pre-existing oxygen vacancies and an upshift in the Ru 4d band. This reduced Ru oxidation state in od-RuO₂ indicates a markedly higher t_2g^* occupation near the Fermi level, thereby facilitating the targeted stronger Ru–O π interactions and contributing to it following the AEM pathway.

To gain insights into the coordination environment and verify the presence of unsaturated Ru sites implied by the morphological analysis, the Fourier transform extended X-ray absorption fine-structure (FT-EXAFS) spectrum (Fig. 3c) of od-RuO₂ in R space (the radial distance from a central absorbing atom to its neighboring atoms) exhibited a prominent peak at approximately 1.50 Å, corresponding to the Ru–O coordination shell. Additional peaks at ~2.70 Å, ~3.2 Å, and ~4.1 Å were attributed to different Ru–Ru coordination shells^39,40,41. These Ru–Ru peaks had reduced intensities compared with the commercial RuO₂, consistent with short-range ordering in nanoclusters and limited long-range crystallinity. The wavelet transform of EXAFS (WT-EXAFS) analysis more intuitively substantiates this conclusion (Fig. 3d–f). In contrast to the commercial RuO₂, od-RuO₂ exhibited substantially weaker Ru-Ru scattering signals in Ru K-edge WT-EXAFS, indicating the formation of these nanoscale clusters rather than extended crystalline domains. The resulting high degree of dispersion leads to increased exposed surface area. Therefore, the observed unsaturated coordination Ru atoms could have a greater tendency to emerge on the surface. In addition, for more in-depth illustrating the bonding state, the WT-EXAFS plots of both Ru K-edge and Co K-edge for od-RuO₂, standard RuO₂, and standard Co₃O₄ are demonstrated (Supplementary Fig. 16). As reported previously⁴², the bonding of Ru and Co will induce a significant and characteristic shift of local maximum to a lower k region for Ru K-edge plots or a higher k region for Co K-edge plots relative to the corresponding Ru–Ru and Co–Co scattering features. Notably, the local maximum of od-RuO₂ at the Ru K-edge (even though the Ru-Ru bonding is weak) remains consistent with that of standard RuO₂. Accordingly, no local maximum of the residual Co species in od-RuO₂ emerges at the higher k region than the standard Co-Co region in the Co K-edge spectra. These results collectively indicate that no Ru-Co bonding is observed in od-RuO₂.

Quantitative analysis through FT-EXAFS fitting (Fig. 3g) further confirmed unsaturated coordination of the Ru atoms in od-RuO₂. The coordination number for Ru in od-RuO₂ was determined to be approximately 5.5, with an average Ru–O apparent radial position in the first shell of 1.5 Å. This coordination number was notably lower than the ideal coordination number of 6²⁷ (Supplementary Fig. 17), providing further evidence for the effectiveness of the defect-inheritance strategy, whereby the oxygen-deficient environment of ruthenium and cobalt in the Ru-doped perovskite is successfully transferred to the reconstructed RuO₂ (Supplementary Fig. 18). As the reduced coordination environments in Ru-based catalysts are associated to enhanced adsorption dynamics⁴³, these surface oxygen-deficient Ru sites serve as nucleophilic centers for OH⁻ adsorption, which plays a crucial role in increasing the OER activity. However, unlike other reports that rely on dopant elements, the od-RuO₂ accesses both high reactivity and improved stability by intrinsic structure design with specified tuned local coordination on target surfaces—not merely creating more vacancies.

Moreover, the O K-edge spectra (Fig. 3h) were investigated to confirm the presence of oxygen vacancies and the preference for the AEM pathway. Compared with commercial RuO₂, the od-RuO₂ spectrum exhibited a clear inversion in the relative intensities of the t_2g (530.7 eV) and e_g (533.7 eV) peaks, with the t_2g peak becoming weaker than the e_g peak, in contrast to the trend observed in the commercial RuO₂. This inversion indicates the increased electron occupation of the od-RuO₂ t_2g band (consistent with the Ru L₂-edge results, Fig. 3i), supporting a reduction in the Ru valence. In addition, the intensities of both the t_2g and e_g peaks were considerably decreased in the od-RuO₂ spectrum, suggesting a markedly weakened Ru–O covalency and providing additional evidence for AEM pathway promotion. Furthermore, the narrowed energy gap between the t_2g and e_g peaks also implied an upshift of the t_2g band, which would facilitate more rapid electron transfer.

These findings provide compelling evidence that the hydrothermal reconstruction process effectively transforms the perovskite precursor into surface-distributed RuO₂ nanoclusters. In this synthesis strategy, the reconstruction process spatially confines the growth of od-RuO₂ from the cobalt spinel oxide, thereby preventing overgrowth into larger crystalline domains. The resulting od-RuO₂ (compared with commercial RuO₂) exhibits a relatively low Ru valence state, a weakened Ru–O covalency, and surface unsaturated coordination of Ru. Collectively, these features endow od-RuO₂ with an optimized electronic structure favorable for the AEM-dominated OER pathway, bridging the long-standing trade-off between activity and stability in acidic environments. This design concept not only advances the understanding of structure–activity relationships in Ru-based catalysts but also offers a generalizable strategy for developing next-generation acid-stable electrocatalysts for scalable hydrogen production and related electrochemical energy technologies.

Electrocatalytic performance

Having determined the structural and electronic properties of od-RuO₂, its electrocatalytic performance for the OER was next evaluated to verify the capability advantages imparted by the designed surface structure. The OER performance of the catalysts was systematically assessed in a 0.5 M H₂SO₄ electrolyte at 25°C using linear sweep voltammetry (LSV), with commercial RuO₂ and pure Co₃O₄ (to exclude any contribution from the activity of Co₃O₄) serving as references. An iR compensation of 90% was applied to all polarization curves unless stated otherwise (the non-iR corrected results are presented in Supplementary Fig. 19). The optimized sample of od-RuO₂ catalyst exhibited an overpotential to reach 10 mA cm^-2 (η₁₀) of 161 mV, significantly outperforming commercial RuO₂ (η₁₀ = 352 mV) and Co₃O₄ (η₁₀ = 543 mV, with limited OER activity when Co acted as the active site) (Fig. 4a). Correspondingly, the Tafel slope (a kinetic parameter describing the required amount of overpotential increase for every tenfold rise in current density) of od-RuO₂ was determined to be 62.4 mV dec^-1 (Fig. 4b), indicating more favorable OER kinetics compared with those of commercial RuO₂ (97.4 mV dec^-1) and Co₃O₄ (231.5 mV dec^-1). The electrochemical surface area (ECSA), which reflects truly active catalyst surface area, was evaluated to further understand the intrinsic activity of the catalysts. According to the non-faradaic method, ECSA is proportional to the electrochemical double-layer capacitance (C_dl), which can be obtained from cyclic voltammetry (CV) in a non-faradaic potential region. The od-RuO₂ exhibited a C_dl value of 89.22 mF, markedly higher than those of commercial RuO₂ (27.80 mF) and Co₃O₄ (3.45 mF) (Fig. 4c). The LSV curves normalized to the ECSA (Fig. 4d) indicated that od-RuO₂ demonstrated not only a substantially larger number of electrochemically active sites but also higher intrinsic activity per unit area, which indicates that od-RuO₂ possesses improved intrinsic catalytic activity, not merely a larger surface area, thus demonstrating the effectiveness of the rational surface engineering strategy. In addition, to further clarify that the role of Co₃O₄ in this work is structural rather than catalytic, the OER performance of parallel samples with varying rinsing cycles was evaluated (Supplementary Fig. 20). The samples with lower Co content after different rinsing times not only do not show performance loss, but even show a certain degree of improvement. Overall, the samples with varying rinsing times exhibited essentially consistent high performance, as Ru species themselves are the real active sites.

To optimize the synthesis process for achieving the highest catalytic activity, a series of od-RuO₂ samples were prepared with the same method by varying the Ru:Co molar ratio and the hydrothermal pH, temperature, and reaction time. Among these samples, the sample derived from the perovskite with an initial composition of SrCo_0.8Ru_0.2O₃ (20% Ru) exhibited the highest catalytic activity (Supplementary Fig. 21). Consequently, a Ru:Co molar ratio of 20% was identified as optimal and selected as the standard for further characterization. The hydrothermal pH, which is a critical parameter modulating the extent of the reconstruction process, was also investigated. The pH was systematically controlled by varying the amount of acid added, with the tested pH values ranging from 0.1 to 1.6. Both excessive and insufficient acid negatively affected the performance (Supplementary Fig. 22). Insufficient acid likely impeded extraction of Co and Ru from the perovskite, resulting in incomplete recombination. Conversely, excessive acid could have caused overdissolution of Co, leading to insufficient confinement of Ru growth and a subsequent reduction in nanocluster formation. Additionally, highly acidic conditions with strong oxidizing HNO₃ were considered to be associated with overoxidation and loss of Ru (as Ru oxidation state over +4, e.g., RuO₄, will be soluble^19,44), thereby impairing nanocluster formation and further decreasing the catalytic activity. Other synthesis parameters, including the hydrothermal temperature and reaction time, were similarly optimized (Supplementary Fig. 23). While a too low temperature or a too short reaction time resulted in incomplete transformation of the perovskite precursor to the final catalyst and thus reduced activity, extending the reaction time or increasing the temperature to beyond a certain threshold did not result in further performance enhancements. Under optimal conditions (150 °C for 12 h), the reconstruction process was complete, and the resulting od-RuO₂ samples displayed consistent activity (used for all aforementioned characterization). In addition, unless otherwise specified, a standard catalyst loading of 5 mg cm^-2 (with Ru atom content of ~50%, Supplementary Fig. 5) was appropriately set for all electrochemical measurements (Supplementary Fig. 24).

Durability tests in acidic media, which are a key benchmark for practical applications, were performed to determine the long-term stability of od-RuO₂. The chronopotentiometric (CP) response, an electrochemical test that measures the potential change of a catalyst under a constant applied current density, indicates that the catalyst maintains its activity over time, thus serving as a direct measure of durability. Under a constant OER current density of 10 mA cm^-2 in a standard three-electrode water electrolysis cell, od-RuO₂ maintained a nearly constant overpotential during more than 3000 h of continuous operation, with an increase of only 4 mV observed during the first 500 h (Fig. 4e). In contrast, commercial RuO₂ underwent rapid performance decay ( + 100 mV in 10 h) under the same conditions, underscoring the durability of od-RuO₂. Additionally, ICP–MS was used to quantify dissolved Ru species in the filtered electrolyte after the stability tests. The measured concentrations were negligible compared with the theoretical value expected for complete catalyst dissolution ( ~ 20 ppm) as shown in Supplementary Fig. 25, indicating minimal Ru leaching during operation. To further assess structural stability during prolonged operation, post-OER HR-TEM images of the catalyst after long-term acidic OER operation were also statistically analysed (Supplementary Fig. 26 and 27). The catalyst largely retains its nanocluster morphology after extended electrolysis, without obvious degradation or agglomeration. Importantly, the post-OER images also confirmed that the catalyst continues to exhibit a predominantly RuO₂ (101)-oriented structure, consistent with the designed facet-selective structure of original od-RuO₂. The prolonged acidic OER stability of od-RuO₂ was also verified by accelerated stress tests (AST) via fast-scan CV, which revealed only a 5 mV overpotential increase in 0.5 M H₂SO₄ after 200,000 cycles over a voltage range of 1.2 to 1.4 V vs. reversible hydrogen electrode (RHE) (Supplementary Fig. 28), and only an 18 mV increase after 65,000 cycles over a wider potential window of 1.2 to 1.7 V vs. RHE (Fig. 4f and Supplementary Fig. 29). In addition, supplementary electrochemical measurements were also performed on a glassy carbon (GC) electrode to further validate the catalytic performance of od-RuO₂ (Supplementary Fig. 30). As presented in Supplementary Table 1, od-RuO₂ demonstrates competitive catalytic activity and stability compared with state-of-the-art Ru-based catalysts for the acidic OER.

Insights into the OER process

To elucidate the OER mechanism of the od-RuO₂ catalyst, differential electrochemical mass spectrometry (DEMS) with isotope labeling was employed to identify the source of oxygen evolved during the OER process. The theoretical OER pathways and corresponding expected signals are illustrated in Fig. 5a, b. The typical oxygen products expected for both the AEM and LOM are summarized in Supplementary Table 2.

Preliminary ex-situ DEMS results revealed that od-RuO₂ likely followed the AEM pathway, as evidenced by the significantly lower ³⁴O₂ signal than that for commercial RuO₂, as shown in Supplementary Fig. 31. However, ex-situ DEMS still requires supplementation from operando DEMS to definitively confirm the AEM pathway. The key evidence for distinguishing the AEM pathway from the LOM pathway lies in detecting ³⁶O₂ formation.

To further substantiate the AEM pathway, operando DEMS was performed using the setup illustrated in Supplementary Fig. 32a. In this integrated system, gaseous products generated at the electrode surface are instantaneously extracted by the vacuum chamber and directly introduced into the mass spectrometer, enabling real-time detection with high sensitivity and accuracy. The experiments were conducted in two stages (Supplementary Fig. 32b), in which H₂¹⁸O and H₂¹⁶O were utilized as the electrolyte in 0.5 M H₂SO₄. The catalyst was deposited on a porous gas-permeable Au working electrode and subjected to CV between 1.165 V and 2.015 V vs. RHE.

In the first stage, the catalyst underwent 12 CV cycles in the H₂¹⁸O electrolyte. For the AEM pathway, at least one ¹⁸O atom derived from the electrolyte is involved in the O₂ products. As shown in Fig. 5c, mass spectrometry revealed a minor amount of ³⁴O₂ (¹⁶O + ¹⁸O) initially, arising from the original residual surface ¹⁶O coupling with ¹⁸OH. The predominant product was ³⁶O₂ (¹⁸O + ¹⁸O), originating entirely from adsorbed ¹⁸O. Notably, no ³²O₂ (¹⁶O + ¹⁶O) was detected, confirming the minimal involvement of lattice oxygen and supporting the AEM pathway. The catalyst subsequently underwent several additional CV cycles to fully label the surface with ¹⁸O.

In the second stage, the catalyst was washed with deionized (DI) water, dried at 25 °C, and then subjected to CV cycling in the H₂¹⁶O electrolyte. If the AEM mechanism is active, only the remaining residual surface adsorbates (containing ¹⁸O) would combine with ¹⁶OH from the electrolyte, yielding ³⁴O₂ as the product initially. With continued cycling, the predominant product should be ³²O₂ (¹⁶O + ¹⁶O), originating entirely from adsorbed ¹⁶O, while no ³⁶O₂ should be observed. The experimental data confirmed this prediction: ³⁴O₂ signals were detected because of partial ¹⁸O exchange, whereas the ³⁶O₂ signal, which would indicate lattice oxygen coupling, was absent (Fig. 5d, e). The combined evidence from operando and ex-situ DEMS measurements unequivocally demonstrates that the OER proceeds predominantly via the AEM mechanism in od-RuO₂.

To theoretically investigate the OER mechanism and origins of od-RuO₂, the Gibbs free-energy for the AEM and LOM were calculated. The calculations consider adsorption on surface models under vacuum within the computational hydrogen electrode (CHE) framework developed by Nørskov and co-workers⁴⁵. Although this approach does not fully reproduce the realistic electrochemical environment at the catalyst surface under an applied potential, it can still provide a useful basis for comparing the relative energetic trends of the related reaction pathways. The Gibbs free-energy profiles can directly indicate the lowest-energy OER pathway. On the basis of the aforementioned design strategy, a RuO₂ model was constructed with oxygen vacancies on the predominant RuO₂ (101) surface. The effect of surface unsaturated coordination Ru atoms on the OER kinetics was investigated. The Gibbs free energies for both the AEM and LOM pathways were calculated to identify the more energetically preferred OER mechanism. For the AEM pathway, the rate-determining step (RDS) was deprotonation of *OH to form *O, with a calculated free-energy change of 1.76 eV, as shown in Fig. 5f. For the LOM pathway, as at least one lattice oxygen is involved in oxygen coupling, two conditions were considered: 1) O–O coupling between two lattice oxygen atoms (O_lat) and 2) O–O coupling between a lattice oxygen atom and an adsorbed oxygen atom (O_ads). Under the two LOM pathway conditions, the RDS was the formation of O₂ from O*O, with substantially higher free-energy changes of 6.07 eV and 2.23 eV, respectively, as shown in Fig. 5g, h. These results confirmed that the AEM pathway is energetically more favorable than the LOM pathway for the oxygen-deficient (101) surface. Additionally, these calculations confirmed that surface oxygen vacancies facilitate adsorption of oxygenated intermediates, effectively reducing the energy barrier for the OER. These mechanistic insights explain the increased OER stability and activity of the designed (101) plane.

In conclusion, the DEMS and DFT results collectively and mechanistically demonstrate that this od-RuO₂ catalyst, driven by the design strategy of oxygen-deficient Ru sites, predominantly follows the AEM pathway. This mechanistic insight not only elucidates the origin of the catalyst’s activity and durability but also validates the effectiveness of the oxygen-vacancy–engineered surface design strategy.

PEMWE performance

To assess the practicality of od-RuO₂ for water electrolysis under industrial conditions, a PEMWE device was assembled by employing od-RuO₂ and commercial RuO₂ successively as the anode catalysts, commercial Pt/C as the cathode catalyst, Nafion 115 as the proton-exchange membrane (PEM), porous transport layer (PTL), and gas diffusion layer (GDL), as shown in Fig. 6a. The synthesis of the nanoparticulate od-RuO₂ via a scalable modified sol–gel method that is amenable to mass production. To evaluate the electrochemical performance of this PEMWE, the current–voltage (I–V) polarization curves were recorded to determine cell efficiency under different operating current densities, while stability tests were performed to assess long-term durability under constant current operation. The I–V polarization curves demonstrated that the PEMWE device equipped with od-RuO₂ achieved a current density of 1.0 A cm^–2 at a voltage of only 1.5126 V, compared with 2.1483 V for the device using commercial RuO₂ (Fig. 6b), indicating its efficiency. Notably, this od-RuO₂ employed PEMWE device even exhibited a voltage of 1.8535 V for a high current density of 3.0 A cm^-2, which approached the 2026 PEMWE technical targets (Supplementary Fig. 33) proposed by the U.S. Department of Energy (DOE). Furthermore, continuous operation at 1.0 A cm^-2 and 80 °C (conditions representative of typical industrial proton exchange membrane electrolyser operating environments) proceeded for more than 1200 h without observable performance degradation (Fig. 6c). In addition, after a temporary interruption caused by equipment relocation, the original membrane electrode assembly (MEA) was reactivated and subjected to extended durability testing, maintaining stable operation for more than 3000 h (Supplementary Fig. 34). In contrast, the commercial RuO₂-based PEMWE exhibited rapid voltage escalation and deactivation under the same conditions, emphasizing the competitive stability and practical applicability of od-RuO₂ for long-term acidic water electrolysis and hydrogen production. As presented in Supplementary Table 3, od-RuO₂ demonstrates competitive PEMWE performance compared with state-of-the-art Ru-based catalysts.

To further evaluate the structural robustness and degradation behavior of the developed catalyst under realistic operation, post-operation analyses were conducted on the MEA following prolonged PEMWE testing. The cross-sectional scanning electron microscopy (SEM) and EDS analyses of the MEA before and after operation (Fig. 6d–i) confirmed a stable ruthenium distribution at the anode. Although the catalyst layer becomes less morphologically uniform compared with its pristine state, a substantial amount of Ru remains firmly anchored on the membrane surface, indicating minimal catalyst degradation or loss during PEMWE operation. Although further industrial-scale evaluations are ongoing to assess the harsh condition long-term stability, Ru utilization efficiency, and performance under dynamic load conditions, these findings position od-RuO₂ as a promising anode catalyst for efficient and durable PEMWE. Future efforts could extend this rational reconstruction approach to other noble or transition metal based systems, offering potential routes to design cost-effective and durable catalysts for large-scale hydrogen production.