Introduction

The escalating global energy demand, coupled with the imperative to mitigate climate change, has catalyzed an accelerated development of sustainable and clean energy technologies, positioning them as viable alternatives to fossil fuels1,2,3. Water electrolysis stands out as a compelling strategy for synthesizing hydrogen, harnessing low-carbon electricity to generate clean energy vectors from water. This approach presents a viable pathway to decarbonizing a broad spectrum of global industries, encompassing energy, transportation, manufacturing, agriculture, and more1,2,3,4,5. Proton-exchange membrane water electrolyzers (PEMWE) offers significant advantages in water electrolysis, such as high productivity via high-current density operation, exceptional energy efficiency, remarkable stability, and competitive levelized costs for hydrogen production2,3,4,5. Nevertheless, challenges remain, primarily the sluggish OER kinetics and reliance on scarce materials like Ir, which underscores the imperative for developing sustainable alternative catalysts5,6,7,8,9,10,11,12,13. Ru-based catalysts in PEMWE exhibit favorable binding energy for oxygenated intermediates, which enhances their OER activity. However, they typically activate lattice oxygen via the lattice-oxygen-mediated (LOM) pathway, resulting in the formation of oxygen vacancies and the exposure of surface Ru species14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32. These species over-oxidize to form soluble RuO4 species at high potentials, which can erode the crystal structure and lead to a decrease in durability for acidic OER15,16,17,18,19,20. Therefore, it is essential to strike a well-balance between controlling lattice oxygen involvement and activating more reactive oxygen species, all while maintaining the stable crystal structure20,21,22,23.

To address this issue, substantial research has been reported on enhancing the intrinsic activity of catalysts to achieve high current density while minimizing catalyst degradation. Recent findings suggest that strategies such as foreign element doping11,22,23, morphology design24,25, stress engineering26,27, and defect construction28,29 can enhance the availability of reactive sites and modulate the adsorption energies of oxygen-containing intermediates, thereby improving the catalytic performance of RuO2. Among these strategies14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39, doping foreign elements into the main crystal structure of RuO2 is one of the most representative approaches. Nevertheless, most doping elements tend to activate the LOM pathway on RuO2 without self-replenishing oxygen vacancies30,39, thereby enhancing activity but simultaneously sacrificing stability. Other doping elements, such as La and Re, in RuO2 have been presented that can strengthen the Ru-O bond covalency or construct a bridging oxygen mediation mechanism11,14. These mechanisms prevent the formation of additional oxygen vacancies, thereby enhancing the stability of catalysts, but they also limit the further improvement of activity due to low structural flexibility. Moreover, excessive doping with metal atoms can readily diminish the number of surface Ru atoms, which are widely recognized as the active sites for the acidic OER14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39. Alternatively, heterogeneous nonmetallic doping can modulate the coordination environment and rearrange the electron density of RuO2, which benefits the improvement of activity33,38. However, these dopants are often considered inert and are prone to dissolution during the reaction process. Consequently, it is urgent to break the activity-stability trade-off of RuO2 for acidic OER, which remains a challenging task.

In this study, we precisely incorporated tantalum (Ta) and boron (B) into RuO2 matrix to form the heteroatom co-doped nanoporous RuO2 electrocatalyst with a Ru-O-Ta framework and Ru-O-B active sites using an electrochemical etching and annealing strategy. The Ta and B co-doped RuO2 exhibits notable acidic OER performance with an overpotential as low as 170 mV at 10 mA cm−2 and a small Tafel slope of 44 mV dec-1. More importantly, The PEMWE assembled using the Ta/B-RuO2 catalyst shows notable stability during acidic water electrolysis. Direct spectroscopic analysis confirms that the electronic interaction within the Ru-O-Ta framework can trigger an adsorbate evolution mechanism (AEM) through bridging oxygen mediation and self-replenishment under severe operating conditions, thereby inhibiting the dissolution of Ru. Density functional theory calculations reveal that the introduction of Ta and B heteroatoms significantly alters its electronic structure of RuO2, thereby optimizing the rate-determining step and reducing the energy barriers in the OER process. This work offers a fundamental and thorough strategy for activating and stabilizing Ru-based OER catalysts, which could make PEMWE more economically viable in a hydrogen economy.

Results

Synthesis and characterization of Ta/B-RuO2

We synthesized the Ta- and B-doped RuO2 (denoted as Ta/B-RuO2) catalysts via melt spinning and selective electrochemical corrosion strategy (Fig. 1a and Supplementary Figs. 1–3). Specifically, the precursor Ru20-xTaxB80 (1 < x < 2) ribbons were prepared by the melt spinning method and has two crystalline phases identified as RuB2 and B through X-ray diffraction analysis (XRD) (Supplementary Fig. 4). Then, the precursor ribbons underwent electrochemical corrosion through cyclic voltammetry (CV) for etching out the B phase (Supplementary Fig. 5). Further electrochemical corrosion can effectively remove the majority of B and transform RuB2 into RuO2. Finally, they were annealed in a muffle furnace to obtain a catalyst with a core-shell structure, which comprised an outer shell of Ta- and B-doped RuO2, encapsulating a core of Ru-Ta alloy. For comparative purposes, we additionally synthesized B-doped RuO2 (denoted as B-RuO2) under analogous synthesis conditions. Scanning electron microscopy (SEM) image illustrates the annealed architecture of Ta/B-RuO2, clearly highlighting a nanoporous structure with large pore sizes, which is advantageous for accelerating mass transfer (Supplementary Fig. 6)34. Meanwhile, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the ligament in Ta/B-RuO2 demonstrated the existence of an ultrafine nanoporous structure, which is advantageous for exposing more active sites (Supplementary Figs. 7–9).

Fig. 1: Synthesis and characterization of the Ta/B-RuO2.
Fig. 1: Synthesis and characterization of the Ta/B-RuO2.
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a Structural advantages of Ta/B-RuO2. b Interface and grain boundary analyses of Ta/B-RuO2 through HAADF-STEM. c HAADF-STEM image and corresponding line intensity profiles. d STEM-EDX mappings and EELS spectrum. e Ru K-edge XANES and f FT-EXAFS spectra of Ta/B-RuO2. g Ta L3-edge XANES and FT-EXAFS spectra of Ta/B-RuO2. h EPR spectra of RuO2, B-RuO2, and Ta/B-RuO2. Source data are provided as a Source Data file.

HAADF-STEM was further utilized to visualize the crystal structure of Ta/B-RuO2 (Fig. 1b and Supplementary Fig. 10). Notably, the left-side image of Fig. 1b revealed well-defined lattice fringes with d-spacings of 2.14 Å and 3.20 Å, which correspond to the (002) and (110) planes of the Ru and RuO2 phases, respectively. The corresponding selected area electron diffraction (SAED) image also proved that Ta/B-RuO2 exhibits crystallographic facets characteristic of both Ru and RuO2 (Supplementary Fig. 11). STEM-energy dispersive X-ray spectroscopy (EDX) analysis reveals a line profile across a ligament, indicating the presence of both Ru and O in the interior region. However, in the exterior region, while the O signal persists, the Ru signal is present but relatively weak. This indicates that Ta/B-RuO2 possesses a core-shell structure, with the RuO2 shell encapsulating the Ru core on the exterior (Supplementary Fig. 12)40. Moreover, the right-hand portion of Fig. 1b exhibits a rich tapestry of grain and twin boundaries, woven together in a complex pattern. The intricate boundary network is crucial for augmenting the catalytic performance35. As depicted in Fig. 1c and Supplementary Fig. 10, atomic Ta, manifesting as luminous spots, is observed to be uniformly dispersed within the lattice of Ta/B-RuO2. This observation confirms the successful incorporation of Ta atoms. STEM-EDX mappings revealed that the Ru, Ta, and O are uniformly distributed throughout the Ta/B-RuO2 (Fig. 1d). Besides, the electron energy loss spectroscopy (EELS) spectrum indicates that a trace amount of B persists within the catalysts.

X-ray photoelectron spectroscopy (XPS) analysis was performed to ascertain the chemical composition and oxidation states of the synthesized catalysts (Supplementary Table 1). In the Ru 3p XPS spectrum, a discernible pair of doublet peaks can be ascribed to the characteristic peaks of Ru 3p3/2 and 3p1/2 (Supplementary Fig. 13)20,36,41. The binding energies of Ru 3p3/2 and 3p1/2 in the Ta/B-RuO2 show a slight shift to lower energies compared with B-RuO2. This shift suggests that the oxidation state of Ru within the Ta/B-RuO2 is marginally lower compared to its counterpart in B-RuO2. XPS analysis of the Ta 4 f spectrum in Ta/B-RuO2 reveals a spectrum of Ta oxidation states37, as detailed in Supplementary Fig. 13. The XPS peak at 192.2 eV, which is intermediate between the positions of metaborate and borate species, confirmed the successful incorporation of B into both Ta/B-RuO2 and B-RuO238. The O1s XPS spectra exhibit three distinct peaks, which are well-fitted and correspond to lattice oxygen at 530.0 eV, oxygen vacancies at 531.8 eV, and adsorbed water at 533.0 eV19,42. It is evident that the ratio of surface oxygen vacancy species to lattice oxygen has increased upon Ta doping, proving that Ta doping can promote the generation of surface oxygen vacancies.

X-ray absorption spectroscopy (XAS) was employed to further investigate the electronic structure and the local coordination environment. Figure 1e displayed the Ru K-edge X-ray absorption near-edge structure (XANES) spectra of Ta/B-RuO2, Ru foil, and commercial RuO2. The absorption threshold of Ta/B-RuO2 is positioned above that of the Ru foil but below and closely aligned with that of commercial RuO2, which suggested that the oxidation state of Ru in Ta/B-RuO2 is intermediate between 0 and +4. The corresponding Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectrum of Ta/B-RuO2 indicates an elongation of the Ru-O bond length compared to that in RuO2, primarily attributed to the co-doping of Ta and B (Fig. 1f). Additionally, the XANES spectrum at the Ta L3-edge confirmed that the valence state of Ta is close to +5 (Fig. 1g and Supplementary Fig. 14), as inferred from the intensity of the white lines43. Furthermore, the corresponding FT-EXAFS result indicates that the Ta atom in Ta/B-RuO2 is surrounded by O atoms in the first shell and metal atoms in the farther shell. Electron paramagnetic resonance (EPR), a reliable technique for detecting unpaired electrons, was employed to investigate the formation and variation of oxygen vacancy. The EPR spectrum demonstrated that Ta/B-RuO2 exhibits the most intense EPR signal at g = 2.003, surpassing that of B-RuO2 and RuO2 (Fig. 1h)19. This indicates that doping with Ta and B significantly increased the surface abundance of oxygen vacancies on the catalyst.

The OER and PEMWE performance

The electrochemical OER performance of Ta/B-RuO2 was evaluated in an Ar-saturated 0.5 M H2SO4 aqueous electrolyte, along with B-RuO2 and commercial RuO2 as comparisons (Supplementary Fig. 15, 16). Figure 2a presents a linear sweep voltammetry (LSV) curve, normalized for geometric area, revealing that the incorporation of Ta and B into RuO2 enhances OER activity (Supplementary Fig. 17). The optimal Ta doping level was determined through controlled experiments, culminating in the selection of the Ta/B-RuO2 sample (Supplementary Fig. 18). Ta/B-RuO2 demonstrated a low overpotential of 170 mV at 10 mA cm−2, outperforming B-RuO2 and RuO2. Moreover, the Tafel slope of Ta/B-RuO2 was calculated to be 44 mV dec−1, which is notably lower than that of B-RuO2 (61 mV dec−1) and commercial RuO2 (69 mV dec−1)44, indicating rapid reaction kinetics (Fig. 2b and Supplementary Fig. 19). Besides, the Ta/B-RuO2 exhibits the highest electrochemical double-layer capacitance (Cdl) and ESCA, suggesting that the incorporation of Ta significantly improved the density of active site (Supplementary Fig. 20). The OER performance of Ta/B-RuO2 is notable and even comparable to latest noble-metal-based catalysts designed for acidic OER (Supplementary Table 2)12,29,33,45,46. The durability of catalysts was assessed via chronopotentiometry at a constant current of 10 mA cm−2 in a three-electrode system. As depicted in Fig. 2c, Ta/B-RuO2 maintained a stable potential of approximately 1.4 V vs. RHE for nearly 80 h without significant degradation. After stability testing, no significant changes were observed in the electronic structure of Ru (Supplementary Fig. 21). Conversely, a notable increase in the high-valence state of Ta was detected, indicating that the presence of Ta may inhibit the peroxidation of Ru by undergoing self-oxidation. This mechanism will be further elaborated upon later in the discussion. Furthermore, although B will continue to dissolve over the long term, it remains present on the surface of the catalyst and modulates its electronic structure.

Fig. 2: Electrochemical performance.
Fig. 2: Electrochemical performance.
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The OER polarization curves (a) and Tafel plots (b) of Ta/B-RuO2, B-RuO2, and RuO2. c Chronopotentiogram and dissolution curves of Ta/B-RuO2 at 10 mA cm−2. d Schematic of the PEMWE. e Polarization curves of the PEMWEs using Ta/B-RuO2, B-RuO2, and RuO2 as the anode catalysts. f Chronopotentiogram of Ta/B-RuO2-based PEMWE operated at 200 mA cm−2. Source data are provided as a Source Data file.

Subsequently, the performance of the synthesized Ta/B-RuO2 in a PEMWE system was further evaluated (Fig. 2d and Supplementary Fig. 22). As shown in Fig. 2e, the polarization curves indicates that the Ta/B-RuO2 PEMWE required a cell voltage of 1.60 V to achieve 1.0 A cm−2, significantly lower than that of B-RuO2 and commercial RuO2 (Supplementary Table 3). Furthermore, the Ta/B-RuO2-based PEMWE demonstrated notable stability. It maintained a consistent voltage of around 1.45 V during 120 h of operation at 200 mA cm−2 with negligible voltage decay (Fig. 2f). In contrast, the potentials of B-RuO2 and RuO2 rapidly increased to above 3.0 V after approximately 40 and 6 h, respectively. A comparative summary of PEMWE performance between Ta/B-RuO2 with selected typical anodic catalysts is presented as Supplementary Table 3. These combined findings confirmed that Ta/B-RuO2 is practical for use in PEMWE.

Insights into the catalytic mechanism

We investigated the influence of Ta cations on the electronic configuration and local atomic environment of Ru using operando XAS spectroscopy. As the applied potential increases, the absorption-edge in the Ru K-edge XANES spectra exhibited a shift to higher energy levels, indicating a concomitant rise in the oxidation state of Ru in B-RuO2 (Fig. 3a)29,41,47. The analogous phenomenon is likewise observable in the Ru K-edge XANES spectra of the Ta/B-RuO2 (Fig. 3b). Notably, the valence state of Ru in the Ta/B-RuO2 consistently remains lower than that observed in the B-RuO2. This demonstrated that Ta doping can effectively mitigate the escalation in the valence state of RuO2, thereby circumventing Ru dissolution during operation and preventing catalyst deactivation23. The FT-EXAFS spectra of the B-RuO2 exposed significant structural changes within the local atomic environment, evident as a reduction in the intensity of the Ru-M shell and alterations in the FT-EXAFS signal within the high R range (Fig. 3c). The intensity of the Ru-M shell is intricately correlated with both the coordination number and the degree of structural disorder. Given that the Ru core does not directly interact with the electrolyte and is not actively involved in the OER, the observed variations in intensity should be ascribed to the structural disorder that is induced by the oscillations occurring during the OER process. In stark contrast, the local atomic environment of the Ta/B-RuO2 demonstrated notable stability, as evidenced by the consistent FT-EXAFS spectra throughout the operational OER conditions (Fig. 3d). Analyzing the changes in the electronic structure of the Ta/B-RuO2 by using Ta L3-edge XANES spectra, we observed an elevation in the white-line peak intensity under operating conditions (Fig. 3e). This behavior suggests that Ta atoms donate electron to bridging O atoms, thereby avoiding peroxidation and dissolution with Ru under high overpotentials (Inset of Fig. 3e)14,48. Examining the corresponding Ta L3-edge FT-EXAFS spectra, we observed variations in the Ta-O and Ta-M scattering peak intensities under operating conditions, which suggest that the coordination environment of Ta atoms is dynamically adjusting to preserve the stability of Ru (Fig. 3f). To sum up, surface Ta undergoes significant structural transformations during the OER. Specifically, Ta has been shown to effectively mitigate the over-oxidation and dissolution of Ru by modulating its electronic and atomic structures (Fig. 3g). However, in the Ta/B-RuO2 composite, the atomic ratio of Ru to Ta is considerably higher. Only a minor fraction of Ru’s overall coordination environment is influenced by the presence of Ta. Consequently, in the Ta/B-RuO2 system, the extent of changes observed in Ru-M is significantly smaller than that in Ta-M.

Fig. 3: In-situ spectroscopic observation.
Fig. 3: In-situ spectroscopic observation.
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a, b Operando Ru K-edge XANES analyses of B-RuO2 and Ta/B-RuO2 at various biases. c, d The corresponding Ru K-edge FT-EXAFS spectra. e Operando Ta L3-edge XANES analyses of Ta/B-RuO2 at various biases. f The corresponding Ta L3-edge FT-EXAFS spectra. g The changes in oxidation states of different sites under operating conditions, as revealed by operando XANES analyses. h In-situ Raman spectra of B-RuO2 and Ta/B-RuO2 at various biases. i In-situ ATR-SEIRAS spectra of B-RuO2 and Ta/B-RuO2 at various biases. Source data are provided as a Source Data file.

To delve deeper into the OER mechanisms, we employed in-situ Raman spectroscopy and in-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) to examine the B-RuO2 and Ta/B-RuO2 under operational conditions (Fig. 3h, i and Supplementary Fig. 23). A thorough examination of the in-situ Raman spectra of B-RuO2 revealed that the intensities of the Eg, A1g, and B2g peaks decreased significantly with increasing voltage, indicating the destruction of surface Ru sites (Fig. 3h)19,28. In contrast, the Raman signals of Ta/B-RuO2 exhibit relatively small variations in the three characteristic peaks associated with the crystal structure. This observation suggests a higher structural stability of Ta/B-RuO2. Furthermore, Raman spectroscopy detected higher concentrations of oxygen vacancies (OVs) in the Ta/B-RuO2 than in B-RuO215. The filling of these oxygen vacancies under high voltage conditions confirmed that the Ta/B-RuO2 operates through the adsorbate evolution mechanism (AEM), where the reaction proceeds without the direct participation of lattice oxygen. Significantly, in-situ Raman spectroscopy and ATR-SEIRAS analyses confirmed the emergence of *OOH species on the Ta/B-RuO2 with increasing applied potential (Fig. 3h, i)7,25. This observation further substantiated that the Ta/B-RuO2 adheres to the AEM pathway during the OER.

To elucidate the involvement of lattice oxygen in various catalysts more clearly, in-situ 18O isotope labeling differential electrochemical mass spectrometry (DEMS) measurements were performed (Fig. 4a). Figure 4b depicts the mass signals for 36O2, 34O2, and 32O2 in the case of B-RuO2, which was not subjected to 18O labeling, during three cycles of CV in an H218O aqueous H2SO4 electrolyte. Notably, approximately 88% of the 36O2 product was detected, suggesting that the AEM pathway predominantly governs the electrochemical behavior of B-RuO2. However, the presence of a minor fraction of 34O2 indicated that a subset of active sites participates in OER via the LOM pathway19. The introduction of Ta atoms to the surface resulted in a significant reduction in the proportion of 34O2 product to approximately 7%. This decrease further suggests that the Ta atoms can suppress the LOM pathway through the Ru-O-Ta framework, thereby reducing the peroxidation and dissolution of Ru. In further confirmatory testing, the catalyst surfaces were labeled with 18O for conducting in-situ DEMS measurements in an H216O aqueous H2SO4 electrolyte (Fig. 4c)12. The majority of the O2 evolution signals from both B-RuO2 and Ta/B-RuO2 are attributed to 32O2, which suggested that the predominant reaction pathway for these catalysts is via the AEM. Nevertheless, B-RuO2 exhibited more pronounced 34O2 signals than Ta/B-RuO2, highlighting the pivotal role of Ta in curbing the involvement of lattice oxygen (Fig. 4d). This suppression is instrumental in enhancing the long-term stability of Ta/B-RuO2 during acidic OER.

Fig. 4: OER mechanism of Ta/B-RuO2.
Fig. 4: OER mechanism of Ta/B-RuO2.
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a A diagrammatic representation of the in-situ DEMS setup. b DEMS signals of 36O2, 34O2, and 32O2 for B-RuO2 and Ta/B-RuO2 in H218O aqueous H2SO4 electrolyte. c DEMS signals of 36O2, 34O2, and 32O2 for 18O-labeled B-RuO2 and Ta/B-RuO2 in H216O aqueous H2SO4 electrolyte. d The peak area ratio of 34O2/32O2 and 34O2/36O2 of B-RuO2 and Ta/B-RuO2. e The OER mechanisms of Ta/B-RuO2 behind high activity and stability. Error bars correspond to the standard deviation from three independent measurements. Source data are provided as a Source Data file.

Substantially, the presence of B species leads to the formation of unconventional B-O covalent in RuO2, resulting in more exposed active sites and modified electronic structure38. However, concerns regarding the catalyst’s stability persist, as there are still some active sites that catalyze OER through the LOM pathway. The synergistic co-doping of Ta and B can further bolster the stability and longevity of catalysts (Fig. 4e). Under low potential conditions, the incorporation of Ta atoms results in the creation of numerous oxygen vacancies, thereby contributing to the formation of highly active catalytic Ru centers. Under high potential conditions, Ta atoms inhibit the peroxidation and subsequent dissolution of Ru sites by facilitating electron transfer to bridging O atoms. Besides, the introduction of Ta atoms promotes the self-filling of oxygen vacancies under working conditions. These combined effects endow the catalyst surface of Ta/B-RuO2 with the ability to efficiently and reliably catalyze the OER through the AEM pathway.

DFT calculations

Density functional theory calculations were further conducted to elucidate the effects of Ru and B dopants on the OER activity49,50,51,52,53. Ta and B atoms were introduced into the optimized (110) facets of RuO2, as these surfaces were identified as both active and stable for the OER (Supplementary Figs. 24, 25 and Supplementary Data 1). We first investigated the changes in the electronic structure induced by doping, as revealed by charge density difference analyses and the density of states (DOS). The results indicates that B doping promotes electron transfer to O, leading to a redistribution of charges within the Ru-O bonds and thereby enhancing the catalytic activity at the Ru sites (Fig. 5a). The introduction of Ta can further induce charge rearrangement, thereby forming the Ru-O-Ta framework (Fig. 5b). DOS and projected density of states (PDOS) analysis revealed that the introduction of Ta and B dopants generates novel hybridized electronic states (Fig. 5c, d). Specifically, B doping significantly enhances the hybridization between O 2p orbitals and Ru 4 d orbitals near the Fermi level, which increases the density of occupied Ru 4 d states critical for OER charge transfer. Upon co-doping with Ta and B, the density of occupied Ru 4 d states shows further enhancement. Crucially, Ta doping induces direct O 2p-Ta 5 d orbital interactions that activate Ta atoms by increasing the density of accessible Ta 5 d states near the Fermi level. The synergistic coupling between B-mediated Ru site activation and Ta 5 d state modulation collectively enhances the catalytic activity beyond individual doping effects. Meanwhile, the doping of B and Ta downshifts the d-band center of catalysts, thereby modulating the adsorption energy of oxygen intermediates, which accounts for the improved catalytic activity relative to both B-RuO2 and RuO2 (Fig. 5e). Moreover, the widening of the gap between Ru d orbitals and O p orbitals in Ta/B-RuO2 can be attributed to the dual functionality of Ta as an electron donor and a structural modulator. Specifically, Ta doping induces electron transfer to Ru, which in turn lowers the Ru d-band center and concurrently polarizes the O p-band. This electronic decoupling suppresses LOM by elevating the energy barrier for O diffusion (Fig. 5f), thereby redirecting the OER pathway toward the AEM. Concurrently, the larger atomic radius of Ta introduces lattice strain, which restricts O mobility and stabilizes the RuO2 framework. These synergistic effects collectively enhance the overall durability of the material.

Fig. 5: DFT calculations.
Fig. 5: DFT calculations.
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a Charge density difference on RuO2, B-RuO2, and Ta/B-RuO2. b The Ru-O-Ta framework. c, d DOS and PDOS of RuO2, B-RuO2, and Ta/B-RuO2. e The d-band center of RuO2, B-RuO2, and Ta/B-RuO2. f Stabilization mechanism by incorporating large-sized Ta atoms into RuO2. g The reaction paths on RuO2, B-RuO2, and Ta/B-RuO2 at 1.23 V. h The adsorption of OER intermediates on B-RuO2 and Ta/B-RuO2. Source data is provided as a Source Data file.

Figure 5g shows the complete reaction energy diagrams of oxygen intermediates following the AEM mechanisms upon catalysis by RuO2, B-RuO2, and Ta/B-RuO2. Notably, the rate-determining step for both RuO2 and B-RuO2 involves the conversion from adsorbed oxygen (*O) to hydroxyl (*OOH), with energy barriers of 0.53 eV and 0.48 eV, respectively. In contrast, for Ta/B-RuO2, the rate-determining step is the subsequent transformation from hydroxyl (*OOH) to molecular oxygen (O2), featuring a lower energy barrier of 0.45 eV. Therefore, B doping effectively lowers the energy barrier associated with the rate-determining step, whereas Ta doping not only alters this step but also further diminishes its energy barrier (Fig. 5h). Overall, the introduction of Ta and B dopants into RuO2 significantly modulates its electronic structure, which in turn optimizes the rate-determining step of the OER process. Meanwhile, Ta can flexibly adjust the electronic structure of Ru within the Ru-O-Ta framework according to operating OER conditions, thereby avoiding the peroxidation and dissolution of Ru.

Discussion

In summary, we demonstrated a class of quite stable Ta/B-RuO2 catalysts with Ru-O-Ta framework and Ru-O-B active site to address the activity-stability trade-off dilemma on acidic OER catalysts. The Ta/B-RuO2 displayed a notable acidic OER performance with a low overpotential of 170 mV and an advantageous Tafel slope of 44 mV dec−1 in 0.5 M H2SO4. A PEMWE employing Ta/B-RuO2 as the anode catalyst requires only 1.6 V to reach 1.0 A cm−2 and exhibits long-term stability at 200 mA cm−2 for 120 h. A variety of operando spectroscopies have revealed that the Ru-O-Ta framework effectively mitigates peroxidation, prevents the dissolution of Ru sites, and maintains a continuous supply of oxygen vacancies, thereby enhancing catalyst stability under severe operating conditions. This confers upon the Ta/B-RuO2 the capability to efficiently and reliably facilitate the OER through the AEM pathway. Further theoretical analysis revealed that co-doping with Ta and B not only creates active Ru-O-B sites but also induces a significant charge redistribution in RuO2. This, in turn, alters the rate-determining step and reduces the energy barriers in the OER process. This work offers insights into multi-element doping strategy for simultaneously boosting activity and stability of acidic OER catalysts, which can inform the development of for practical use in PEMWE systems.

Methods

Chemicals

Ruthenium oxide powder (RuO2, 99.95%) was purchased from Adamas. Ruthenium block (Ru, 99.95%), Tantalum block (Ta, 99.95%), and Boron particles (B, 99.99%) were purchased from Beijing Jiaming Platinum Co., Ltd. Sulfuric acid (H2SO4, 96%), Hydrochloric acid (HCl, 37%), Nitric acid (HNO3, 68%), and Hydrogen peroxide (H2O2, 30%) were purchased from Sinopharm. Perchloric acid (HClO4, 70%) was purchased from GENERAL-REAGENT. Ultrapure water (18.2 MΩ·cm) was used throughout all experimental procedures. The electrolyte was prepared by direct dilution of the concentrated solution, and all solutions were freshly prepared prior to use.

Preparation of nanoporous Ta/B-RuO2

The Ru20-xTaxB80 alloy ingot was initially prepared through arc melting, utilizing a mixture of high-purity Ru, Ta, and B under an Ar atmosphere. To ensure a uniform distribution of the metal elements throughout the alloy, the ingot was subjected to multiple remelting, with a minimum of three cycles performed to achieve the desired homogeneity. Following the remelting process, the alloy ribbons were further prepared using a single roller melt-spinning technique. The nanoporous Ta/B-RuO2 catalysts were synthesized through a cyclic voltammetry (CV) selective electrical dealloying process, targeting the Ru20-xTaxB80 alloy ribbons. This process involved applying voltages from 0.5 to 1.65 V vs. RHE for 2000 cycles in a 0.1 M HClO4 solution at room temperature. After the dealloying process, the materials were meticulously rinsed with deionized water to ensure the complete removal of any residual corrosive solution. In the final step, the materials were subjected to annealing in a muffle furnace at 450 °C to achieve the desired properties.

Characterization

XRD analysis was performed using a Rigaku MiniFlex 600 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The microstructure and chemical composition were examined using a JEOL JSM-7610FPlus SEM integrated with an Oxford ULTIM MAX 40 EDX system. TEM, HAADF-STEM, and elemental mappings were conducted on a Thermo Scientific Themis Z (3.2) microscope, equipped with dual spherical aberration (Cs) correctors. XPS, utilizing a monochromatic Al Kα source on a PHI VersaProbe 4 instrument, was employed to investigate the chemical states and compositions of the samples. XAS, comprising XANES and EXAFS, was recorded at the Ru K- and Ta L3-edge at the Beamline BL11B station of the Shanghai Synchrotron Radiation Facility (SSRF). The as-obtained spectra data were analyzed utilizing Athena software. For Ru K-edge analysis: The Ru K-edge position is governed by the effective nuclear charge experienced by the 1 s core electrons. As the oxidation state of Ru increases, the loss of valence electrons (e.g., 4 d/5 s electrons) reduces electron shielding, increasing the effective nuclear charge. This strengthens the 1 s electron binding energy, thereby raising the energy required for 1 s → 4p transitions and shifting the absorption edge to higher energies. For Ta L3-edge analysis: The white line intensity at the Ta L3-edge arises from 2p3/2 → 5 d electronic transitions. Higher oxidation states of Ta correlate with increased unoccupied 5 d states due to electron depletion. This enhanced density of empty 5 d states amplifies the transition probability, resulting in greater white line intensity.

Electrochemical measurements

The OER performance was evaluated using an Ivium CompactStat.h workstation in a three-electrode cell with 0.5 M H2SO4 (ref. 54,55). In this setup, a standard Ag/AgCl electrode served as the reference, a graphite rod functioned as the counter electrode, and the nanoporous Ta/B-RuO2 catalyst acted as the working electrode. The working electrode was fabricated by depositing catalyst ink onto a piece of carbon paper (1 cm × 1 cm), resulting in a catalyst loading of 0.5 mg cm-2. This loading was calculated based on the mass of the catalyst adhered to the carbon paper and the geometric area of the electrode. The catalyst ink was prepared by dispersing 5 mg of catalyst into a 1 mL mixture composed of 970 μL isopropanol and 30 μL Nafion D520. All potentials were referenced to the RHE, calculated using the Nernst equation: ERHE = EAg/AgCl + 0.0591 pH + E0Ag/AgCl, and corrected by iR compensation. The PEMWE were constructed using a custom-built cell. The synthesized catalyst, with a mass loading of 2 mg cm−2, was applied as a coating on a Ti fiber serving as the anode. Conversely, the cathode was made by depositing a commercial Pt/C (20 wt%) catalyst onto carbon paper at a mass loading of 1 mg cm-2. The anode and cathode components were integrated with an N117 membrane (thickness: 183 µm) and then hot-pressed at a temperature of 363 K for a duration of 15 mins. All PEMWE evaluations were conducted using a 0.5 M H2SO4 solution as the electrolyte.

In-situ Raman experiments

In-situ Raman spectroscopy was performed using a Renishaw inVia Qontor Raman spectrometer. A near-infrared laser, with a wavelength of 532 nm, was utilized as the excitation source for the acquisition of Raman spectra, which were recorded across the spectral range from 100 to 1100 cm−1. The experimental setup incorporated an electrochemical cell, which featured a carbon rod serving as the counter electrode and an Ag/AgCl electrode acting as the reference electrode. The working electrode was meticulously prepared by depositing the catalyst onto a glassy carbon substrate. Subsequently, the electrochemical measurements were performed in a 0.5 M H2SO4 solution. Prior to the acquisition of Raman spectra, each potential was meticulously stabilized for no less than 5 minutes to ensure consistent and reliable data collection.

In-situ ATR-SEIRAS experiments

ATR-SEIRAS measurements were performed using a Nicolet iS50 FT-IR spectrometer equipped with a liquid nitrogen-cooled MCT detector. The conductive substrate, which served as the foundation for both the catalysts and the infrared reflection component, was meticulously crafted from an Au-coated silicon semi-cylindrical prism, featuring a diameter of 20 mm. Catalyst suspensions were carefully deposited onto the Au/Si surface to form the working electrode with a catalyst mass loading of 1 mg cm−2, and the electrolyte used was 0.5 M H2SO4. The Ag/AgCl electrode and Pt wire were utilized as the reference and counter electrodes, respectively, while a 0.5 M H2SO4 solution served as the electrolyte.

In-situ DEMS measurements

A customized electrochemical cell was integrated with a Hiden HPR40 spectrometer for the purpose of performing in-situ DEMS measurements. In this research, H218O served as the solvent for the preparation of a 0.05 M H2SO4 solution acting as the electrolyte. To incorporate the isotope ¹⁸O into the catalyst, CV was applied to the working electrode, with the process being repeated 5 times using an H218O aqueous H2SO4 electrolyte. Following the procedure, the electrodes that had been prepared and the cell were extensively washed using an H216O-based H2SO4 electrolyte, performing this rinsing operation a total of 8 times to ensure the complete removal of any remaining H218O. Ultimately, the electrodes were submerged in an H216O-containing H2SO4 electrolyte, followed by the execution of CV. At the same time, mass spectrometry was utilized to track the ongoing production of O2 throughout the OER process.

DFT calculations

The computations were conducted utilizing the DFT (refs. 49,50), as realized in the Vienna ab initio simulation package (VASP). The exchange-correlation potential was characterized through the generalized gradient approximation (GGA), with parameters specified by the Perdew-Burke-Ernzerhof (PBE) formulation (refs. 51,52,53). The RuO2 (110) surface was modeled using a 3 × 3 × 2 supercell comprising 72 O and 36 Ru atoms. In the case of B-RuO2, three Ru atoms in the RuO2 (110) surface model were substituted with B atoms. For Ta/B-RuO2, the B-RuO2 model was further modified by replacing an additional three Ru atoms with Ta atoms. A 20 Å vacuum layer was introduced along the z-axis to avoid interactions between the slab and its periodic motif. The free energies for the reaction steps were determined using the formula: ΔGads = ΔEads + ΔEZPE - TΔS, where ΔEads is the adsorption energy of the intermediates, T is the temperature, and ΔEZPE and ΔS are the changes in zero-point energy and entropy, respectively. The charge density difference is visualized using an isosurface threshold of 0.01 e/Å3. Following the standard convention for charge redistribution plots, yellow regions indicate electron density accumulatio, while blue regions correspond to electron depletion.