Dynamic chromium dopant promotes interfacial water activation on cobalt spinel oxide for efficient oxygen evolution in acid

Domain: neuroscience
Published: 2026-02-10
Editorial outcome: major_revision

Abstract
Although cobalt spinel oxide-based catalysts have demonstrated promising performance for the oxygen evolution reaction in acid, strategies reported have largely focused on manipulating the binding energies of reaction intermediates through tailoring the inherent electronic structures. The role of interfacial water structures on the electrode surface in promoting the acidic oxygen evolution reaction performance through controlled water activation has been largely overlooked. Here we show that introducing chromium dopant into cobalt spinel oxide can simultaneously increase the activity and stability by promoting interfacial water dissociation and dynamic electron transfer. The chromium dopant can not only dynamically regulate the electronic structure of cobalt and mitigate the over-oxidation of active cobalt sites at high overpotential, but also enhance the coverage of hydroxyl species on the catalyst surface, thereby optimizing the interfacial water structure, accelerating the reactant free-water supply and promoting subsequent interfacial water dissociation. Consequently, the obtained catalyst demonstrates enhanced performance in both solution electrocatalysis and device electrocatalysis. This work provides a comprehensive understanding of enhancing the acidic oxygen evolution reaction performance by promoting interfacial water dissociation.

Introduction
The proton exchange membrane water electrolyzer (PEMWE) can effectively harness the intermittency and instability of renewable energy sources, making its widespread application vital for the development of green hydrogen production1–4. Nevertheless, the complex proton-coupled electron transfer process and harsh reaction environment in the anodic oxygen evolution reaction (OER) lead to the strong dependence on precious metal catalysts (IrO2), whose active component (Ir) is regarded as one of the scarcest elements on earth5,6. Consequently, developing high-performance non-precious metal based electrocatalysts for acidic OER is urgently needed to expand the application of PEMWE. In this regard, cobalt oxides (Co3O4)-based electrocatalysts, which have demonstrated activity for the OER in acidic electrolytes, have attracted considerable attention recently7–9. However, the inevitable dissolution of Co ions from Co3O4 in acidic electrolytes, which limits its catalytic stability, poses a considerable challenge for designing high-performance acidic OER catalysts10,11. More critically, Co3O4 will undergo significant surface reconstruction during the OER process, leading to the formation of dissolvable and deactivated high-valence Co4+ species, thereby causing structural collapse and decreased reaction kinetics12–15. Consequently, inhibiting the formation of Co4+ species and stabilizing the Co3+ species through dynamic electron transfer presents a promising strategy to improve the acidic OER performance of Co3O4, but has been rarely investigated.

The acidic OER, a process where two water molecules are deprotonated to form one oxygen molecule, is influenced not only by the electronic structure of the electrocatalyst but also critically by the environment at the electrode-electrolyte interface16–18. Unlike OER in base with abundant hydroxyl species, sufficient supply and rapid dissociation of interfacial water molecules are crucial for promoting the performance of acidic OER but have often been overlooked19–21. Studies on hydrogen electrode reaction have depicted that the interfacial water molecules with weaker hydrogen bond strength possess lower activation energy, making them easier to dissociate22–24. In this regard, manipulating the hydrogen-bond network in the electrical double-layer (EDL) region displays a great significance in the interfacial water adsorption/dissociation, which can affect the subsequent proton-coupled electron transfer processes25,26. Consequently, precise regulation of the interfacial water structure to accelerate water dissociation and enhance the connectivity of the hydrogen-bond network remains a formidable challenge for efficient acidic OER.

In this work, we reported a dynamic chromium (Cr) dopant with multiple oxidation states boosted the catalytic activity and stability of Co3O4 towards the OER in an acidic electrolyte. The Cr-O-Co coordination environment could significantly regulate the electronic structure of active Co site, simultaneously improving the activity and stability of Cr-Co3O4. In addition, operando Raman and in situ X-ray absorption spectroscopy (XAS) were performed to confirm that the Cr-O-Co local environment can facilitate electron donation to stabilize the activated Co site during the acidic OER process. Such dynamical electron interaction contributes to simultaneously enhancing the catalytic activity and stability. Moreover, based on the operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and in situ X-ray photoelectron spectroscopy (XPS), we found that the increased Cr3+-OH species coverage on the Cr-Co3O4 surface regulates the behavior of interfacial water molecules. This leads to an environment enriched with free water molecules and enhances water dissociation, thereby boosting the acidic OER performance. Consequently, Cr-Co3O4 requires an overpotential of only 358 mV at 10 mA cm−2 and operates steadily for 160 h, better than that of commercial RuO2 (C-RuO2) and commercial IrO2 (C-IrO2). As an anode catalyst in a PEMWE device, Cr-Co3O4 exhibits over 750 h of stability at 1 A cm-2. This work poses a promising strategy to design non-precious metal-based electrocatalysts with both high activity and stability toward acidic OER.

Results
Structural investigation of Cr-Co3O4
The Cr-Co3O4 was prepared via a simple two-step method. In brief, a CoCr bimetal precursor was fabricated by stirring the mixed solution containing Co(NO3)2·6H2O, Cr(NO3)3·9H2O, and organic ligands at room temperature. Then the obtained CoCr bimetallic precursor was pyrolyzed and oxidized at 325°C for 5 h to form the catalyst (Cr-Co3O4). Co3O4 was also prepared by a similar process without the addition of Cr(NO3)3·9H2O (details in the Supporting Information). The X-ray diffraction (XRD) characterization was used to determine the crystal structure of the catalysts. As shown in Fig. S1, the XRD pattern of Cr-Co3O4 is identical to that of Co3O4 catalyst and can be well indexed to the standard spinel Co3O4 phase (PDF#42–1467)27. Transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images (Figs. 1a and S2) show that Cr-Co3O4 is composed of uniform nanoparticles, with an average nanoparticle size of 6.17 nm, which is smaller than that of pristine Co3O4 (Fig. S3). In addition, as demonstrated in Fig. 1b, Cr-Co3O4 displays lattice spacings of 0.268 nm, 0.282 nm, and 0.454 nm, belonging to (311), (220), and (111) lattice planes of Co3O4, respectively28,29. Combined with the analysis of the energy dispersive X-ray spectroscopy (EDX) of Cr-Co3O4, the uniform distributions of Cr, Co, and O elements in Cr-Co3O4 confirm the successful introduction of Cr into the Co3O4 lattice (Fig. 1c). The metal contents in Cr-Co3O4 and Co3O4 were further determined by the inductively coupled plasma atomic emission spectroscopy (ICP-AES) tests (Table S1). XPS measurement was performed to investigate the electronic structure and valence change of surface elements. As shown in Fig. 1d, the high-resolution Co 2p XPS spectra reveal a negative shift in the 2p3/2 peak for Cr-Co3O4 compared to pristine Co3O4, indicating a lower average oxidation state of Co in the Cr-Co3O4 catalyst30. In addition, the corresponding peak position, peak area, and full width at half maximum (FWHM) of the Co 2p XPS spectra for Cr-Co3O4 and Co3O4 are listed in Tables S2 and S3, respectively. Furthermore, XPS analysis confirms that Cr on the surface of Cr-Co3O4 exists primarily as Cr3+-O and Cr3+-OH (Fig. S4)31,32. The temperature-programmed desorption-mass spectrometry (TPD-MS) was performed on Cr-Co3O4 to directly quantify the surface Cr3+-OH species. As shown in Fig. S5, Cr-Co3O4 exhibits two mass loss stages: one from 100°C to 200°C and the other from 200°C to 500°C. These are assigned to the desorption of adsorbed water and the loss of surface Cr3+-OH species, respectively33. To identify the substitution sites of Cr in the Co3O4 lattice, Raman spectroscopy was conducted on both Cr-Co3O4 and Co3O4. As demonstrated in Fig. S6, the spectrum of pristine Co3O4 exhibits the characteristic Raman modes of the spinel structure, including Eg, F2g, and A1g modes. Specifically, the F2g mode at 524 cm−1 is attributed to the stretching vibration of CoO4 (Cotet-O), whereas the A1g mode (692 cm-1) and Eg mode (484 cm-1) correspond to the vibrations of CoO6 (Cooct-O)34–37. Compared to Co3O4, the A1g mode and Eg mode in the Cr-Co3O4 catalyst remain unchanged, while the F2g mode shifts. This confirms that Cr substitutes preferentially at the Cotet sites in the Co3O4 lattice.

To further probe the electron transfer and local coordination environment, XAS characterization was conducted. As depicted in the normalized Co K-edge X-ray absorption near-edge structure (XANES) spectra of Co foil, Co3O4, and Cr-Co3O4 (Fig. 1e), the near-edge absorption energy of Cr-Co3O4 is lower than that of Co3O4, implying a significant decrease in the Co valence states accompanied by Cr doping38, in good agreement with the XPS results. To further elucidate the changes in the Co coordination environment induced by Cr doping, we analyzed the Fourier-transformed EXAFS (FT-EXAFS) spectra (Figs. 1f and S7). In the FT-EXAFS spectrum, Co foil exhibits a dominant peak at a radial distance of 2.17 Å, corresponding to the Co-Co metallic coordination. In contrast, Co3O4 shows three main peaks at radial distances of 1.49 Å, 2.45 Å, and 2.97 Å, which are attributed to the Co-O, Cooct-Cooct (octahedral site), and Cotet-Cotet (tetrahedral site) coordination shells, respectively. The Co K-edge FT-EXAFS spectrum of Cr-Co3O4 is nearly identical to that of Co3O4, except for the decreased intensity of Cotet-Cotet (Fig. 1g). This indicates the Cr elements occupy the Cotet sites in the lattice of Cr-Co3O4 39,40. Since the Cooct site has been demonstrated to be more active than Cotet site in Co3O4 for OER, the coordination environment of Cr-O-Cooct can significantly regulate the active Co site, without reducing the number of active sites, thereby simultaneously improving the activity and stability of Cr-Co3O4 41. The wavelet transform (WT) analysis at Co K-edge confirms that Cr-Co3O4 retains the well-defined Co3O4 structure after incorporation of Cr (Fig. S8). Meanwhile, the Cr K-edge XANES and EXAFS spectra of Cr-Co3O4 reveal an average Cr oxidation state of approximately +3.6 (Figs. 1h and S9), higher than that in Cr2O3. Additionally, the Cr K-edge FT-EXAFS spectrum (Figs. 1i and S10) shows peaks at 1.53 Å (Cr-O) and 2.51 Å (Cr-O-Co), but lacks the characteristic Cr-Cr and Cr-O-Cr signals found in Cr foil and Cr2O3. This spectral fingerprint provides direct evidence for the atomic dispersion of Cr in the catalyst. The absence of Cr-Cr and Cr-O-Cr scattering paths is further corroborated by WT analysis at the Cr K-edge (Fig. 1j–l). Unlike the distinct features in Cr foil and Cr2O3, the WT spectrum of Cr-Co3O4 shows no such signals, confirming the atomic-level dispersion of Cr within the Co3O4 lattice.

Performance evaluation of acidic OER and PEMWE
The acidic OER performances of Cr-Co3O4 and Co3O4 catalysts were investigated in 0.5 M H2SO4 electrolyte through a standard three-electrode system. C-RuO2 and C-IrO2 were also tested as references. The iR-corrected linear sweep voltammetry (LSV) curves measured with a sweep speed at 5 mV s−1 (Fig. 2a, b) reveal the intrinsic activities of the catalysts. The overpotential to reach 10 mA cm−2 is 358 mV for Cr-Co3O4, 438 mV for Co3O4, 279 mV for C-RuO2, and 347 mV for C-IrO2, respectively42. The corresponding uncompensated LSV curves are provided in Fig. S11. Particularly, C-RuO2 exhibits decreased current densities with the increase of potential, because of the electrochemical dissolution of RuO43. In addition, at the condition of high current density (50 mA cm−2), Cr-Co3O4 only needs 436 mV, lower than that of Co3O4 (556 mV) and C-IrO2 (438 mV), confirming the good catalytic activity of Cr-Co3O4 in acidic OER. Meanwhile, Tafel slope analysis reveals enhanced kinetics for Cr-Co3O4, which exhibits a smaller slope than Co3O4 and C-IrO2 at high current densities (Fig. 2c)44. Given that surface reconstruction may occur, we prepared the corresponding hydroxyoxide catalysts (Cr-CoOOH and CoOOH) to probe the true active phase. As shown in Figs. S12 and S13, both Cr-CoOOH and CoOOH exhibit minimal activity, indicating that the catalytic performance of Cr-Co3O4 is intrinsic to the oxide structure45. The inactivity of Cr2O3 (Fig. S14) further corroborates this conclusion. To further evaluate the intrinsic activity, we determined the electrochemically active surface area (ECSA) via double-layer capacitance (Cdl) measurements46. The Cdl values for Cr-Co3O4 and Co3O4 is 5.67 mF cm−2 and 4.17 mF cm−2, respectively (Fig. S15). Normalizing the catalytic current by the corresponding ECSA (Fig. S16) reveals that Cr-Co3O4 exhibits a higher specific activity than Co3O4, confirming that its performance stems from an enhancement in intrinsic activity rather than simply a larger surface area. Besides, operando electrochemical impedance spectroscopy (EIS) was performed to uncover the charge transfer processes (Fig. S17)47. As shown in Fig. S18, Cr-Co3O4 possesses much lower charge transfer resistance in contrast to Co3O4 at 1.5 V vs. RHE, implying the faster charge transfer property at the catalyst-electrolyte interface. Analysis of the Bode phase plots (Fig. S19) shows that the phase peak for Cr-Co3O4 shifts to higher frequencies more rapidly with increasing potential than that for Co3O4. This indicates an earlier onset of the OER and faster reaction kinetics for Cr-Co3O4 48. Furthermore, the long-term stability of the catalysts was evaluated by chronopotentiometry at 10 mA cm−2 (Fig. 2d). The Cr-Co3O4 can operate stably over 160 h, much higher than that of Co3O4, C-RuO2, and C-IrO2, indicating the promising electrocatalytic performance of Cr-Co3O4 toward acidic OER. The dissolution of Co and Cr in Cr-Co3O4 was investigated after 100 h of operation under acidic OER conditions. As shown in Fig. S20, after the stability test of 100 h in 0.5 M H2SO4, the dissolution concentration of Cr and Co for Cr-Co3O4 was determined to be approximately 4.51 ppm and 25.85 ppm, respectively. In addition, as demonstrated in Fig. S21 and Table S4, the electrocatalytic performance of Cr-Co3O4 is comparable to the reported non-precious metal-based acidic OER catalysts. To further verify the stability of Cr-Co3O4 under operating conditions, a PEMWE device was assembled in pure water electrolyte at 60°C, using Cr-Co3O4 as the anode catalyst, commercial Pt/C as the cathode catalyst, and Nafion 115 as the PEM. As shown in Fig. 2f, the homemade Cr-Co3O4||Pt/C membrane electrode assembly (MEA) requires 2.11 V, 2.29 V, and 2.40 V to reach current density of 1 A cm−2, 2 A cm−2, and 3 A cm−2, respectively. In addition, as shown in Fig. 2g and Table S5, the Cr-Co3O4||Pt/C MEA demonstrates good electrocatalytic performance, maintaining stable operation for 750 h at 1 A cm−2.

Insights into the electronic interaction during acidic OER
To monitor the structural evolution under operating conditions, we performed operando Raman spectroscopy. For Cr-Co3O4, the characteristic peak located at 694 cm−1 is attributed to the A1g mode (Figs. 3a and S22), which shows a slight decrease, suggesting a dynamic yet stable coordination environment for the active Cooct sites. In stark contrast, under the same conditions, the Raman peaks for pristine Co3O4 diminish substantially (Fig. 3b), a signature of catalyst over-oxidation and dissolution. To gain further insight into the enhanced activity and stability, we investigated the operando electronic structure of Cr-Co3O4 during acidic OER. In situ XAS characterizations of Cr-Co3O4 and Co3O4 were carried out to investigate the role of Cr in manipulating the electronic properties of Co active sites. For Co3O4, as depicted in Fig. 3c, d, the near-edge absorption energy at 1.7 V vs. RHE is obviously higher than that at open circuit potential (ocp), suggesting significantly increased Co valence states. This oxidation may account for the subsequent collapse and dissolution of Co3O4, in line with the operando Raman results11. In addition, after the potential is removed, the Co valence state in Co3O4 remains much higher than that in its pristine state. For Cr-Co3O4, however, the normalized XANES spectra at different stages reveal no significant change in the absorption edge (Fig. 3e, f). This suggests that the formation of the Cr-O-Cotet local environment through Cr doping stabilizes the Co sites by electron donation, which in turn enhances the catalytic stability. To further assess the structural stability, the spent catalysts were characterized. As shown in Figs. S23 and S24, the crystal structure and morphology of Cr-Co3O4 are maintained well after the stability test. Furthermore, XPS characterization reveals that the valence state of Co in Cr-Co3O4 after the stability test (a-Cr-Co3O4) undergoes minimal change (Fig. S25). In contrast, the Co 2p3/2 peak of the post-test Co3O4 (a-Co3O4) shifts to a higher binding energy relative to its pristine state, indicating oxidation of the Co active sites (Fig. S26). This finding is consistent with the in situ XAS results. Notably, the chemical state of Cr undergoes a significant transformation in a-Cr-Co3O4. As shown in Fig. S27, the proportion of Cr3+-OH species is markedly higher compared to the pristine catalyst. We propose that these surface hydroxyl groups can modify the local reaction environment, thereby enhancing the catalytic performance. In summary, the incorporation of Cr into Co3O4 cannot only regulate the electronic structure of the active Co site, but also donate electrons to stabilize the activated Co sites during acidic OER, thereby improving the overall catalytic performance (Fig. 3g).

Investigation of the electrode-electrolyte interface
To understand how the hydroxyl-rich surface induced by Cr doping enhances acidic OER, we were inspired by studies showing that surface hydroxyl groups can accelerate reaction kinetics by modifying interfacial water behavior23,49,50. Accordingly, we employed methanol oxidation reaction (MOR) experiments to investigate the surface hydroxyl coverage on Cr-Co3O4 and Co3O4 51,52. As demonstrated in Fig. S28, the potential ratio of VOER/VMOR for Cr-Co3O4 is higher than that of Co3O4, suggesting that the introduction of Cr into the lattice of Co3O4 can modulate the surface catalytic environment, leading to a higher surface coverage of OH species. To gain further insight into the vital role of Cr doping in the Cr-Co3O4 catalyst, in situ XPS measurements were performed to investigate the valence state changes of Co active sites and the evolution of Cr3+-OH surface species during the acidic OER process17,32. As depicted in Fig. 4a, b, the valence states of Co on the surface of Cr-Co3O4 display negligible changes under different applied potentials. In contrast, the Cr 2p3/2 binding energy increases from 576.2 eV at 1.0 V to 576.6 eV at 1.8 V. This positive shift in Cr binding energy indicates the progressive oxidation of Cr species with increasing potential. Furthermore, the O 1s spectrum (Fig. S29) provides complementary insight into the surface oxygen evolution during acidic OER. The valence states of lattice oxygens show no significant changes with increasing applied potential, which rules out the influence of lattice oxygen activation on the changes of the valence states of Cr. Based on the above analysis, the incorporation of Cr can be in favor of stabilizing the oxidation state of Co through dynamic donating electrons during the acidic OER process, thereby enhancing catalytic performance, which is in good agreement with the previous in situ XAS results. Besides, as depicted in Fig. 4c, the surface mass fractions of Co and Cr in Cr-Co3O4 remain constant during the whole operation process, which suggests enhanced catalytic stability. The surface coverage of Cr3+-OH species on Cr-Co3O4 increases progressively with applied potential, confirming that Cr doping generates a hydroxyl-enriched local environment under operating conditions. This hydroxyl-rich surface favors a more reactive interfacial water structure, which accounts for the enhanced OER activity.

Operando ATR-SEIRAS measurements in the range of 2800 cm−1 to 3800 cm−1 were carried out to investigate the interfacial water structure changes of Cr-Co3O4 and Co3O4 (Fig. S30). To elucidate the role of interfacial water in OER, we deconvoluted the operando ATR-SEIRAS spectra via Gaussian fitting (Figs. 4d, S31, and S32). The spectra can be decomposed into three distinct components, which correspond to 4-coordinated hydrogen-bonded water (4-HB-H2O), 2-coordinated hydrogen-bonded water (2-HB-H2O), and free-H2O with weak hydrogen-bond interactions53–55. The detailed fitting parameters are provided in Tables S6 and S7. The observed spectral features allow us to probe the hydrogen-bonding strength of the interfacial water, which is directly linked to the activation energy for water dissociation. As shown in Fig. 4e, the O-H stretching vibration on Cr-Co3O4 exhibits a blue shift at 1.5 V compared to Co3O4. This indicates a weakened hydrogen-bond network at the interface, which facilitates the dissociation of interfacial water molecules. Compared to Co3O4, Cr-Co3O4 exhibits a markedly higher free water content at the interface during OER (Figs. 4f and S33). To probe the interfacial water structure in greater detail, operando ATR-SEIRAS was also performed in D2SO4 53,56. The broad band extending from 2200 cm−1 to 2700 cm−1 can be assigned to the O-H stretching mode of D2O. The operando ATR-SEIRAS spectra of Cr-Co3O4 and Co3O4 under different applied potentials in 0.5 M D2SO4 are shown in Figs. S34 and S35. The O-D stretching bands were also deconvoluted into 4-HB-D2O, 2-HB-D2O, and free D2O components (detailed parameters in Tables S8 and S9), enabling a comparative analysis of the interfacial D2O structure between the two catalysts. The proportions of 4-HB-D2O, 2-HB-D2O, and free-D2O at different potentials were analyzed, as demonstrated in Fig. S36. The free-D2O proportion on Cr-Co3O4 surface is obviously much higher than that on Co3O4 surface during the acidic OER process. The formation of a reactant (free-D2O) enriched interface on Cr-Co3O4 during the acidic OER process, driven by the dynamic evolution from 4-HB-D2O and 2-HB-D2O to free-D2O, accelerates the overall catalytic reaction (Fig. 4g).

DFT calculations
Density functional theory (DFT) calculations were performed to elucidate the origin of the enhanced activity and stability of Cr-Co3O4. We evaluated the relative stability of Cr doping at the Cotet and Cooct sites of Co3O4. The calculated formation energy for Cr at the Cotet site (−2.00 eV) is significantly lower than that at the Cooct site (−1.12 eV) (Figs. S38 and S39). This confirms a strong thermodynamic driving force for Cr to preferentially occupy the tetrahedral positions, in good agreement with our experimental analysis. To model the catalytically relevant state, we constructed a Cr-Co3O4 surface model with a pre-adsorbed OH* species on the Cr site (Fig. S40), consistent with the observed Cr3+-OH surface species. Mulliken charge analysis shows that the average charge on Cooct sites in Cr-Co3O4 is lower than that in pure Co3O4 (Fig. 5a). This decrease indicates electron transfer from Cr to Co, a conclusion further supported by complementary Bader charge analysis (Fig. S41). In addition, as shown in Fig. 5b, the calculated water dissociation energy for Cr-Co3O4 is significantly lower than that for Co3O4, indicating that Cr doping facilitates the water activation step57. The critical role of the Cr-OH* is further underscored by comparing the dissociation barrier on the hydroxyl-covered surface versus the clean surface. The significantly lower energy for the former (Fig. S42) confirms that the adsorbed OH group facilitates water dissociation. To investigate the reaction mechanism, in situ 18O-labeled differential electrochemical mass spectrometry (DEMS) measurements were carried out (Fig. S43)33,58,59. As shown in Fig. S44, after the Cr-Co3O4 catalyst is labeled with the 18O isotope and then tested in H216O, the signal of 32O2 (16O16O) increases and the signals corresponding to 18O-containing species (34O2 and 36O2) decrease significantly. This result clearly indicates that the Cr-Co3O4 catalyst follows adsorbate evolution mechanism. In addition, to decipher the enhanced stability from the theoretical view, the energy consumption of surface Co demetallation on the surface of Co3O4 and Cr-Co3O4 were calculated (Fig. 5c). The energy required for Co dissolution on the Cr-Co3O4 surface was higher than that on Co3O4 surface, suggesting the improvement of stability60. Besides, associated with the projected density of states (PDOS) analysis (Fig. 5d, e), it can be calculated that the d-band center of Co in Cr-Co3O4 is −1.88 eV, closer to the Fermi level compared with Co3O4 (−1.83 eV), suggesting the optimized adsorption ability with reaction intermediates61. Moreover, in conjunction with the calculations of the reaction coordinates of Co3O4 and Cr-Co3O4 (Fig. 5f), it is calculated that the energy barriers of rate-determining step (RDS) for Cr-Co3O4 is 1.95 eV, lower than that of Co3O4 (2.17 eV). Consequently, the introduction of Cr can not only dynamically regulate the electronic structure of Co during the acidic process, but also optimize the interfacial water molecules environment through the increased surface OH* coverage, in favor of accelerating the dissociation of interfacial water, thereby boosting the kinetics of water oxidation in acid (Fig. 5g).

Methods
Chemicals
All reagents are of analytical grade and used without purification. Cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O, Sinopharm Chemical Reagent, ~98.5%), chromium (Ⅲ) nitrate, nonahydrate (Cr(NO3)3·9H2O, Sinopharm Chemical Reagent, ~99.0%), 2,5-dihydroxyterephthalic acid (C8H6O6, Shanghai Macklin Biochemical Technology, ~98.0%), N,N-dimethylformamide (DMF, Sinopharm Chemical Reagent), absolute ethanol (C2H5OH, AR, Sinopharm Chemical Reagent), triethylamine (C6H15N, Sinopharm Chemical Reagent, ~99.0%), urea (CO(NH2)2, Sinopharm Chemical Reagent, AR), ammonium fluoride (NH4F, Sinopharm Chemical Reagent, AR), sulfuric acid (H2SO4, Sinopharm Chemical Reagent, 95.0–98.0%), isopropanol (Sinopharm Chemical Reagent, ≥99.7%), Nafion® 117 solution (Sigma-Aldrich, ~5 wt% in a mixture of lower aliphatic alcohols and water), chromium oxide (Cr2O3, Shanghai Macklin Biochemical Technology Co., Ltd. 99.95%), and commercial RuO2 (Changcheng Chemical, >99.0%).

Synthesis of Cr-Co3O4 and pristine Co3O4
In a typical synthesis, 1.50 mmol Co(NO3)2·6H2O, a certain amount of Cr(NO3)3·9H2O, and 120.00 mg C8H6O6 were dissolved in a mixed solution, containing of 50.00 mL DMF, 3.34 mL C2H5OH, 3.34 mL U H2O, and 0.37 mL C6H15N. After stirring 24 h at 30°C, the mixture was collected using C2H5OH through centrifugation. Finally, it was dried in a vacuum drier to obtain the precursor. Then the obtained precursor was annealed at 325°C for 5 h in air to obtain Cr-Co3O4. For comparison, pristine Co3O4 was synthesized using a similar protocol without the addition of Cr(NO3)3·9H2O.

Synthesis of Cr-CoOOH and pristine CoOOH
Specifically, 1.50 mmol Co(NO3)2·6H2O, a certain amount of Cr(NO3)3·9H2O, 5 mmol CO(NH2)2, and 5 mmol NH4F were completely dissolved in 10 mL of deionized water. Subsequently, the above solution was transferred to a 20 mL Teflon-lined stainless-steel autoclave. Then the autoclave was sealed increased temperature to 120°C with a rate of 2°C min-1, and maintained the temperature for 6 h. After cooling down to room temperature, the obtained precursor was thoroughly washed with deionized water and dried in a vacuum-drying oven. Then the precursor and 5 mL NaClO (5.2 wt%) were added in the 20 mL Teflon-lined stainless-steel autoclave and adjust the pH to 12. The autoclave was heated at 90°C with a heating rate of 2°C min−1 and allowed to react for 10 h. After cooling down to room temperature, the obtained Cr-CoOOH was thoroughly washed with deionized water and dried in a vacuum drying oven. The CoOOH was synthesized using a similar protocol without the addition of Cr(NO3)3·9H2O.

Physical characterizations
The XRD patterns were obtained on a Rigaku Miniflex 600 X-ray powder diffractometer equipped with a Cu Kα radiation source (λ=0.154178 nm). All of the diffraction data were collected in a 2θ range from 10° to 80° at a scanning rate of 10° min−1. The TEM images were performed with JEM-2100 Plus operated at 200 kV. HAADF-STEM was conducted by JEM-ARM200CF. STEM imaging and EDX mapping were acquired on a JEOL JEM-ARM200CF microscope operated at 200 kV with a Schottky cold-field emission gun in Wuhan University. The EDX elemental mapping was carried using the JEOL SDD-detector with two 100 mm2 X-ray sensor. ICP-AES was conducted on a Thermo IRIS Intrepid II XSP atomic emission spectrometer. XPS measurement was carried out with a Thermo Fischer ESCALAB 250Xi spectrophotometer.

In situ/operando characterizations
For the in situ XPS measurements62, the OER tests were conducted in a custom three-electrode cell within an N2-purged glovebox. The self-supported catalyst samples (1 cm × 1 cm) served as the working electrode, with an Ag/AgCl reference electrode and a graphite foil counter electrode. Each sample was subjected to a constant anodic potential (ranging from 1.0 to 1.8 V vs. RHE) for 20 minutes. After testing, the samples were dried and transferred to the XPS analysis chamber without air exposure. Spectra were recorded at 30 eV pass energy, 50 ms dwell time, and chamber pressure <4.3×10−7 Torr. XAFS spectra were collected in transmission mode using a RapidXAFS 2M spectrometer (Anhui Absorption Spectroscopy Analysis Instrument Co., Ltd.) operating at 20 kV and 40 mA, with a Si (533) spherically bent crystal analyzer (radius of curvature: 500 mm) for Co K-edge measurements. ATR-SEIRAS was performed on a Bruker Invenio R spectrometer equipped with a liquid nitrogen-cooled detector, utilizing a custom-built IR cell with a polished Si prism. In situ DEMS measurements were conducted using a QAS100 system. The working electrode was prepared by depositing the catalyst ink onto a gold-plated PTFE membrane (loading: 0.3 mg cm−2), with an Ag/AgCl reference electrode and a Pt wire counter electrode. Polarization curves were recorded in 0.5 M H2SO4 (prepared with H218O) by scanning the potential from 1.3 to 1.75 V vs. RHE, while the generated O2 was monitored in real time by mass spectrometry.

Electrochemical experiments and PEMWE tests
All electrochemical measurements were conducted at 30°C using a CHI760E workstation with a three‑electrode configuration. The working electrode was a rotating disk electrode (RDE, 5 mm glassy carbon, geometric area = 0.19625 cm2) coated with the electrocatalyst, rotating at 1600 rpm. A carbon rod and a saturated Hg/Hg2Cl2 electrode served as the counter and reference electrodes, respectively. Potentials were converted to the reversible hydrogen electrode (RHE) scale using E (vs. RHE) = E(vs. Hg/Hg2Cl2) + 0.059 × pH + 0.24 V. Catalyst ink was prepared with 4 mg of the pre‑synthesized catalyst, 1 mg of XC‑72R carbon, and 1 mL of isopropyl alcohol containing 0.1 wt% Nafion. An 18‑µL aliquot of the ink (total loading ≈ 0.09 mg) was drop‑cast onto the glassy carbon surface. For comparison, C‑RuO2 and C‑IrO2 inks were prepared identically. LSV was performed in O2‑saturated 0.5 M H2SO4 (pH ≈ 0.20 ± 0.03) from 1.0 to 1.6 V (vs. SCE) at 5 mV s−1. Cyclic voltammetry (CV) was recorded in non‑Faradaic regions at scan rates of 10–50 mV s−1. ECSA was derived from the Cdl obtained by plotting Δj/2 (Δj = ja − jc, where ja is the anodic current and jc is the cathodic current at the middle voltage) against scan rate, using a specific capacitance (Cs) of 0.035 mF cm−2. EIS was measured from 105 to 10−2 kHz with a 5 mV perturbation. The uncompensated resistance (Ru) taken at 1 kHz was used for full iR‑correction: EiR‑free = E − iRu. Chronopotentiometry was carried out at 10 mA cm−2. The PEM electrolyzer tests employed a Nafion 115 membrane (thickness of 135 µm) with pure water circulated at 40 mL min−1. The electrode active area was 1 cm2. Catalyst loadings for Cr-Co3O4 and commercial 40% Pt/C were 4 and 3 mg cm−2, respectively, operated at 60°C.

DFT calculation method
All DFT calculations were performed using the CASTEP code within the Materials Studio package (Accelrys Inc.). The exchange-correlation interactions were treated with the generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) functional. A plane-wave cutoff energy of 400 eV and an SCF convergence tolerance of 1×10−6 eV were applied63. The (311) surface was selected to model both Cr-doped and pristine Co3O4. Geometry optimizations employed the Methfessel-Paxton (MP) smearing scheme with a width of 0.1 eV. The Brillouin zone was sampled using Monkhorst-Pack k-point meshes, with a 4×4×1 grid for all structural relaxations. A 15 Å vacuum layer was added along the z-direction to eliminate periodic interactions. During relaxation, atoms in the bottom layer were fixed at their bulk positions while all other atoms were fully relaxed. The adsorption free energy (ΔG) of reaction intermediates was calculated as ΔG = ΔE + ΔZPE − TΔS, where ΔE is the DFT-computed adsorption energy, and ΔZPE and TΔS are the zero-point energy and entropy corrections, respectively.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors report a Cr-doped Co₃O₄ (Cr-Co₃O₄) electrocatalyst for acidic oxygen evolution reaction (OER), claiming that Cr dopants simultaneously enhance activity (358 mV overpotential at 10 mA cm⁻²) and stability (160 h for half-cell, 750 h for proton exchange membrane water electrolyzer (PEMWE)) by dynamically regulating Co’s electronic structure and optimizing interfacial water dissociation. The work combines multi-technique operando characterizations (Raman, XAS, XPS, ATR-SEIRAS) and density functional theory (DFT) calculations, addressing a critical need for non-precious catalysts in acidic OER. However, several fundamental and experimental issues undermine the rigor and novelty of the conclusions, preventing recommendation for publication in Nature Communications in its current form, major revisions are required. Please find the specific comments/suggestions below:\n1. Novelty and Motivation: Insufficient Distinction from Prior Work\nThe authors frame “dynamic Cr doping for interfacial water activation” as a key innovation, but this narrative overlooks closely related literature.\n(1) Cr-mediated electronic modulation of Co-based catalysts for acidic OER has been reported (e.g., Nat. Commun. 2024, 15, 7861, which shows Cr-doped RuO₂ optimizing hydroxyl spillover). The manuscript fails to clarify how the “dynamic electron transfer” between Cr and Co differs from existing dopant-induced electronic effects.\n(2) Interfacial water structure regulation via catalyst surface engineering (e.g., hydroxyl enrichment) for acidic OER was previously demonstrated (e.g., Nat. Catal. 2022, 5, 900–911; J. Am. Chem. Soc. 2024, 146, 35438–35448). The authors do not establish whether their water activation mechanism (reduced H-bond connectivity) is a unique contribution or a replication of known effects.\n(3) The PEMWE stability (750 h at 1 A cm⁻²) is compared to low-current or Ir-based benchmarks (Table S3), but it is outperformed by recent non-precious catalysts (e.g., Science 2024, 384, 1373–1380, which reports 608 h at 1 A cm⁻² for CoWO₄). The manuscript’s benchmarking is incomplete, overstating the catalyst’s practical advantage.\n2. Structural and Electronic Characterization: Unresolved Ambiguities\n(1) The authors claim Cr occupies tetrahedral (Coₜₑₜ) sites in Co₃O₄ (Fig. 1g) based on reduced Coₜₑₜ-Coₜₑₜ intensity in EXAFS (Fig. 1f). However, no direct evidence (e.g., aberration-corrected STEM imaging of atomic positions or EXAFS wavelet transform (WT) analysis of Cr-Co correlations) confirms Cr’s tetrahedral occupancy. The WT of Cr K-edge (Fig. 1j–l) only rules out Cr-Cr clusters but provides no insight into Cr-Co bonding.\n(2) XPS shows Cr exists as Cr³⁺-O and Cr³⁺-OH (Fig. S4), but the manuscript does not clarify if surface Cr species (OH-terminated) differ from bulk lattice Cr. This distinction is critical, as surface Cr might dominate interfacial water effects, while lattice Cr drives electronic modulation—yet the authors treat them as a single “Cr dopant” effect.\n(3) Operando XAS (Fig. 3c–f) shows stable Co valence in Cr-Co₃O₄ vs. irreversible Co⁴⁺ formation in Co₃O₄, attributed to “dynamic electron transfer from Cr to Co.” However, no time-resolved or potential-dependent data (e.g., in-situ XPS valence shifts of Cr during OER) directly confirms electron transfer. The positive shift of Cr 2p peaks with potential (Fig. 4b) is interpreted as Cr oxidation, but this could also arise from surface hydroxyl desorption or lattice oxygen activation—alternative explanations are not ruled out.\n(4) Post-OER XPS (Fig. S19) shows increased Cr³⁺-OH content, but the manuscript does not link this to “dynamic” behavior (e.g., reversible Cr valence changes during potential cycling). Without evidence of reversibility, the “dynamic” label is unsupported.\n3. Interfacial Water Mechanism: Overinterpreted and Inconsistent Data\n(1) The decomposition of O-H stretching bands into “4-HB-H₂O,” “2-HB-H₂O,” and “free H₂O” (Fig. 4d, S21–S23) relies on Gaussian fitting, but the fitting parameters (peak positions, full-width at half-maximum) are not provided. Prior work (e.g., Angew. Chem. Int. Ed. 2020, 59, 22397–22402) shows that such deconvolution is highly sensitive to baseline correction—without validation (e.g., using D₂O as a control), the claim of “increased free H₂O” is speculative.\n(2) The authors use VOER/VMOR ratios to infer higher OH coverage on Cr-Co₃O₄ (Fig. S20), but MOR activity depends on both OH availability and metal surface reactivity. Cr may suppress MOR via electronic effects (not just OH coverage), making this proxy unreliable. Direct quantification of surface OH (e.g., in-situ titration or TPD-MS) is missing.\n4. The DFT work supports two key claims: electron transfer from Cr to Co, and reduced water dissociation energy. The calculated average charge of Coₒ(cid:1)ₜ (0.81 eV for Cr-Co₃O₄ vs. 0.85 eV for Co₃O₄) is used to infer electron transfer, but Mulliken charges are highly basis-set dependent. Bader charge analysis (a more robust method for electronic structure) is not provided to confirm this effect. The rate-determining step (RDS) energy barrier for Cr-Co₃O₄ (1.60 eV) is lower than Co₃O₄ (2.17 eV) (Fig. 5f), but the model assumes a single OER mechanism (adsorbate evolution mechanism, AEM). The authors do not rule out lattice oxygen-mediated mechanism (LOM), which is common in Co-based catalysts under high overpotentials. Without validating the mechanism via isotope labeling (e.g., ¹⁸O water), the DFT-derived barrier is irrelevant to the actual reaction pathway.\n5. Post-OER characterizations (Raman, XPS) suggest partial conversion of Cr-Co₃O₄ to oxyhydroxides, but the manuscript does not isolate the activity of the original oxide vs. in-situ formed oxyhydroxides. A control experiment (e.g., pre-synthesized Cr-Co oxyhydroxide tested for OER) is needed to clarify whether the oxide or its reconstructed phase is the true active species.", "reviewer_2": "The authors fabricate chromium-doped cobalt spinel oxide (Cr-Co3O4) as an efficient OER electrocatalyst. The operando measurements demonstrate the Cr doping effect on interfacial H2O dissociation kinetics. The increased hydroxyl group coverage on the Cr site optimizes the interfacial water structure. In addition, Cr doping effectively enhances long-term durability in acidic electrolyte system. However, the manuscript needs some additional clarifications. Therefore, publication in the Nature Communications is recommended after major revisions.\nComment 1 : In Figure 1i, why do the two peaks after the Cr-O around 2.5Å and 3.3Å in the EXAFS differ markedly in intensity between Cr-Co3O4 and Co3O4? Also, aside from the decrease in Cotet-Cotet intensity, is there additional evidence that Cr substitutes into the tetrahedral sites?\nComment 2 : In the HAADF-STEM images (Figures S2 and S3), Cr-Co3O4 particle shows smaller size than Co3O4. Furthermore, Cdl value of Cr-Co3O4 is smaller than that of Co3O4 (Figure S10). Please investigate the reasons for these differences.\nComment 3 : In the manuscript page 4 (line 113-114), authors state that ‘As shown in Figure 1d, the high resolution XPS of Co 2p for Cr-Co3O4 and Co3O4 catalysts reveal that with the introduction of Cr, the ratio of Co2+/Co3+ is reduced, implying the decreased valance state of Co sites in Cr-Co3O4’. If the Co2+/Co3+ is reduced, the average Co valence should increase (a larger Co3+ fraction). Moreover, the XPS Co 2p spectra appear to indicate a higher Co3+ ratio in Cr-Co3O4 compared to Co3O4, which seems to contradict the paper's assertion. In addition, please add a table summarizing quantitative Co2+ and Co3+ fraction with numerical values.\nComment 4 : In this manuscript, the long-term stability test of PEMWE device was operated with an ultrapure water electrolyte. However, most PEMWE devices operate under acidic electrolytes, and many of the comparators listed in Table S3 appear to evaluated under acidic conditions. Please validate the PEMWE device with Cr-Co3O4 under acidic electrolyte condition, and update Table S3 to explicitly include the electrolyte identity and concentration.\nComment 5 : To definitively investigate the origin of Cr-Co3O4 long-term stability, it is recommended to confirm the metal leaching amount following long-term operation evaluation. Furthermore, add ex-situ XPS data to verify the peroxidation and irreversible Co4+ species in Co3O4 samples after OER behaviour.\nComment 6 : Generally, in operando XANES, a larger increase in oxidation state upon voltage application indicates a more efficient oxidation reaction. However, further clarification is required regarding the fact that the increase in oxidation state of cobalt, the main activation site (octahedral site) in Cr-Co3O4, remains almost unchanged. Rather, the behaviour of OH- attaching to the Cr surface may indicate a change in the active site. Furthermore, a clear explanation is required as to whether the observed increase in oxidation state is sufficient to be termed overoxidation.\nComment 7 : In Figure 4b, XPS Cr 2p shows negative shifts upon potential increase, contrary to the statement in line 251 of the manuscript: “the Cr 2p XPS spectra reveal a positive binding energy shift of the main peak for Cr-Co3O4 with increasing applied potential.”.\nComment 8 : The authors consistently argue that the enhancement in water dissociation in catalyst-electrolyte interface are originated from OH* on Cr site. However, the DFT calculation presented for Cr-Co3O4 model without OH* already show a decrease in the Gibbs free energy for the H2O dissociation step. To substantiate the claim thar Cr- OH* is the origin of the enhancement of water dissociation, please include DFT calculations that explicitly incorporate pre-adsorbed OH* on Cr, and compare the full reaction profiles with vs without OH*."}, "author_response": "Response: We sincerely express our appreciation to the reviewer for constructive comments and suggestions, which are very valuable and helpful for revising and improving our manuscript. We have revised our manuscript carefully and supplemented the relevant analysis and discussion, and the specific responses are as follows.\n1. Novelty and Motivation: Insufficient Distinction from Prior Work\nThe authors frame “dynamic Cr doping for interfacial water activation” as a key innovation, but this narrative overlooks closely related literature.\n(1) Cr-mediated electronic modulation of Co-based catalysts for acidic OER has been reported (e.g., Nat. Commun. 2024, 15, 7861, which shows Cr-doped RuO optimizing hydroxyl spillover). The manuscript fails to clarify how the “dynamic electron transfer” between Cr and Co differs from existing dopant-induced electronic effects.\nResponse: Thanks for your valuable comment. It has been reported that Cr doping can optimize the electronic structure of noble metal oxides and enhance their catalytic performance (Nat. Commun., 2024, 15, 7861; Nat. Commun., 2019, 10, 162; Nat. Commun., 2023, 1, 843). However, these studies have predominantly focused on modulating the electronic structure of the catalysts to regulate the adsorption energy of reaction intermediates, thereby enhancing catalytic performance, while the relationship between the catalytic structure and the microenvironment at the catalytic interface has been largely overlooked.\nFor the referenced paper of Cr-doped RuO (Nat. Commun., 2024, 15, 7861), it revealed that the dopant-host interaction enhances the OER kinetics via a dynamic doping-mediated hydroxyl spillover mechanism. Specifically, OH* species adsorbed on Cr dopants can migrate to adjacent Ru sites to form OOH*, thereby increasing the OOH* coverage on the catalyst surface. Although the work provides valuable insights, it focuses on a precious metal-based system, which is often associated with high cost. In contrast, our study presents a noble metal-free catalyst, Cr-doped Co O , which demonstrates comparable or even superior performance for the acidic OER. Prior research focuses on the relationship between OH* on Cr, and OOH* on Ru sites to elucidate the mechanism behind the improved catalytic performance. In contrast, our investigation targets the electrode-electrolyte interface microenvironment and its relationship with the oxide structure, a significant yet underexplored factor in catalytic performance that has not been well addressed in the reported literatures.\nIn this work, we found the introduction of Cr into Co O lattice can create Cr-O-Co local environment and Cr3+-OH species modification on the surface. Operando Raman and in situ XAS were performed to confirm that the Cr-O-Co local environment can regulate the electronic structure of Co active sites through dynamic electron interaction during the acidic OER process. Before the acidic OER, the unique coordination environment of Cr -O-Co can significantly regulate the active Co site, without reducing the number of active sites, thereby simultaneously improving the activity and stability of Cr-Co O , since the Co site has been demonstrated to be more active than Co site in Co O for OER. At high overpotential, the Cr dopants can donate electrons to stabilize activate Co site, thereby sustaining catalytic performance. In addition, Operando ATR-SEIRAS and in situ XPS were carried out to found that the dynamic increased Cr3+-OH species coverage on the Cr-Co O surface can manipulate the behavior of interfacial water molecules during the acidic OER process, leading to the enriched free water molecules environment and promoted dissociation of water, thereby boosting the acidic OER performance.\nThe related information has been updated in the Revised Manuscript as follow:\n“The unique Cr -O-Co coordination environment can significantly regulate the electronic structure of active Co site, simultaneously improving the activity and stability of Cr-Co O . In addition, operando Raman and in situ X-ray absorption spectroscopy (XAS) were performed to confirm that the Cr -O-Co local environment can also donate electrons to stabilize activate Co sites during the acidic OER process. Such a dynamical electron interaction can be in favor of simultaneously enhancing the catalytic activity and stability. Moreover, associated with the operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and in situ X-ray photoelectron spectroscopy (XPS), we found that the increased Cr3+-OH species coverage on the Cr-Co O surface can manipulate the behavior of interfacial water molecules, leading to the enriched free water molecules environment and promoted dissociation of water, thereby boosting the acidic OER performance.” (Page 2 in the revised manuscript)\n“While for Cr-Co O , based on the normalized XANES spectra at different stages during the acidic OER process (Figures 3e and 3f), it can be found that the near-edge absorption energies under varied applied potential exhibit no obvious changes, suggesting the introducing of Cr to form the Cr -O-Co local environment can stabilize the Co active sites during the acidic OER process by donating electrons, thereby enhancing the catalytic stability.” (Page 9 in the revised manuscript)\n“Based on the above analysis, it can be found that the introduction of Cr into the lattice of Co O can not only regulate the electronic structure of active Co site, but also donate electrons to stabilize activate Co sites during the acidic OER process to improve the catalytic activity and stability (Figure 3g).” (Page 9 in the revised manuscript)\n“Operando electrochemical experiments and spectral characterization monitoring reveal that the introduction of Cr to form Cr -O-Co local environment can realize the dynamic regulation of the electronic structure of Co active site, suppressing its further oxidation, thereby leading to a significant enhancement in both activity and stability.” (Page 13 in the revised manuscript)\n(2) Interfacial water structure regulation via catalyst surface engineering (e.g., hydroxyl enrichment) for acidic OER was previously demonstrated (e.g., Nat. Catal., 2022, 5, 900-911; J. Am. Chem. Soc., 2024, 146, 35438-35448). The authors do not establish whether their water activation mechanism (reduced H-bond connectivity) is a unique contribution or a replication of known effects.\nResponse: Thanks for your valuable comment. The structure of interfacial water has been demonstrated critically determined the energy and chemical conversion processes, thereby significantly influencing the kinetics of electrocatalytic reactions, including hydrogen evolution/oxidation reactions (HER/HOR), and CO electroreduction.\nFor HER/HOR, the alkaline conditions typically cause a significant reduction in reaction kinetics by ∼2-3 orders of magnitude compared to acidic conditions, even for state-of-the-art platinum (Pt)-based catalysts. The referenced paper (Nat. Catal., 2022, 5, 900-911) investigated the kinetic pH effect in HER/HOR. The authors proposed that the significant difference in the connectivity of hydrogen bond network within the interfacial electrical double layer is the origin of the substantial kinetic pH effect. Using a Pt-Ru alloy as a model catalyst, they further revealed the essential role of the OH adsorption intermediate in improving the kinetics of alkaline HER/HOR by enhancing the connectivity of the hydrogen bond network in the electrical double layer, rather than merely affecting the surface reaction pathway and its energetics as previously believed. This is a pioneering work that advances our understanding of the origin of the pH effect. In fact, factors affecting interfacial water structure, thus improve the electrochemical reaction kinetics, have been studied in the hydrogen evolution reaction. For instance, Li and colleagues proposed that Ru surfaces exist in different valence states in the reaction potential range, dissociating interfacial water differently and generating two distinct H*, resulting in different activities. The local cation tuning effect and the large work function of high-valence Ru surfaces promote interfacial water dissociation (Nat. Commun., 2023, 14, 5289). Koper and coworkers highlighted that interfacial water reorganization is closely related to the potential of zero (free) charge (pzfc). The strength of the interfacial electric field, as determined by the distance of the pzfc from the actual electrode potential, impacts on the rate of hydrogen adsorption by influencing the energetic barrier associated with the reorganization of interfacial water during charge transfer (Nat. Energy, 2017, 2, 17031). Recently, Zhai and colleagues identified the catalyst-water Coulomb interaction can affect the water wall at the catalyst/water interface and thus influence the ORR kinetics (Energy Environ. Sci., 2024, 17, 6046-6057). In comparison, the acidic OER has received considerably less attention, and considerable development is still required in this field.\nHowever, for the OER, which involves a series of four coupled proton-electron transfers and complex reaction mechanisms, research into the critical role of interfacial water is still in its early stages and lacks systematic investigation. The other referenced paper (J. Am. Chem. Soc., 2024, 146, 35438-35448) is one of the few studies that investigate the critical role of hydrogen bond network during the acidic OER. The authors designed a series of RuO catalysts incorporating Lewis acid (LA) species (LA-RuO , where LA = TiO , ZrO , HfO ). In these catalysts, the LA acts as a hydrophilic unit that optimizes the hydrogen bond network, while RuO drives the activation of water molecules into oxygen. However, their conclusion is primarily based on studies of noble metal-based catalysts. In this study, we present a noble metal-free catalyst (Cr-Co O ) that demonstrates remarkable acidic OER performance. Cr doping not only modulates electron redistribution at Co active sites during the OER process, but also introduces Cr3+-OH species to tailor the interfacial water structure, which creates an environment enriched with free water molecules, thereby facilitating water dissociation and enhancing acidic OER efficiency. Our work provides a comprehensive investigation into the relationship among oxide structure, catalytic performance, and interfacial microenvironment, offering a promising strategy for designing non-precious metal-based electrocatalysts with high activity and stability for acidic OER.\nThe related information has been updated in the Revised Manuscript as follow:\n“Moreover, the increased Cr3+-OH species coverage on Cr-Co O surface can further optimize the interfacial water structure in the electrode-electrolyte interface, leading to accelerated reactant free-water supply and the follow-up interfacial water dissociation.” (Page 1 in the revised manuscript)\n(3) The PEMWE stability (750 h at 1 A cm-2) is compared to low-current or Ir-based benchmarks (Table S3), but it is outperformed by recent non-precious catalysts (e.g., Science 2024, 384, 1373–1380, which reports 608 h at 1 A cm-2 for CoWO ). The manuscript’s benchmarking is incomplete, overstating the catalyst’s practical advantage.\nResponse: Thanks for your valuable comment. In the context of previously reported non-noble metal acidic OER catalysts, we have re-assessed the electrocatalytic performance of Cr-Co O in PEMWE. As shown in Table R1, the Cr-Co O || Pt/C MEA still demonstrates good PEMWE performance.\nTable R1. Comparison of PEMWE performance employed in this study with other electrocatalysts.\nCatalyst temperature Cell voltage Stability Reference\nCr-Co O 60 ℃ 2 A cm-2/2.29 V 1 A cm-2/750 h This work\nMn-Co O @CN 80 ℃ ~ 0.2 A cm-2/50 h [2]\nCo O F 50 ℃ ~ 0.1 A cm-2/120 h [14]\nVo-Co O HNCs 80 ℃ 1 A cm-2/1.82 V 1 A cm-2/14 h [15]\nCWO-del-48 80 ℃ 1.8 A cm-2/2 V 1 A cm-2/608 h [16]\nCWO-del-48 80 ℃ 1 A cm-2/1.77 V 0.2 A cm-2/278 h [16]\nW-Co O 50 ℃ 1 A cm-2/1.83 V 1 A cm-2/240 h [17]\nW-Co O 50 ℃ 2 A cm-2/2.04 V 2 A cm-2/50 h [17]\nLaMn@Co-ZIF 60 ℃ 2 A cm-2/2.47 V 0.21 A cm-2/100 h [18]\nLaMn@Co-ZIF 60 ℃ 4 A cm-2/3.00 V 0.1 A cm-2/60 h [18]\nNi(5%)Co MnO 80 ℃ 1 A cm-2/2.3 V 0.5 A cm-2/110 h [19]\nLCM 80 ℃ 2 A cm-2/2.32 V 2 A cm-2/1200 h [20]\nCoFeNiMoWTe 70 ℃ 1 A cm-2/1.81 V 1 A cm-2/100 h [21]\nN-HECGs\nAccordingly, the Table R1 has been updated in the Revised Supporting Information as Table S5. The related information has been updated in the Revised Manuscript as follow:\n“In addition, as shown in Figure 2g and Table S5, the Cr-Co O || Pt/C MEA demonstrates superior electrocatalytic performance, maintaining stable operation for 750 h at 1 A cm-2.” (Page 7 in the revised manuscript)\nThe related information has been updated in the Revised Supporting Information as follow:\n“19. Lee, C. W. et al. Facet engineering of cobalt manganese oxide for highly stable acidic oxygen evolution reaction. Adv. Energy Mater. 15, 2402786, (2025).\n20. Lin, M. et al. A self-healing non-precious metal oxide anode in proton exchange membrane electrolysis beyond 1000 h stability at 2 A cm-2. Energy Environ. Sci. 18, 9183, (2025).\n21. Sohn, J. I. et al. Nonprecious high-Entropy chalcogenide glasses-based electrocatalysts for efficient and stable acidic oxygen evolution reaction in proton exchange membrane water electrolysis. Adv. Energy Mater. 13, 2301420, (2023).”\n2. Structural and Electronic Characterization: Unresolved Ambiguities\n(1) The authors claim Cr occupies tetrahedral (Coₜₑₜ) sites in Co O (Fig. 1g) based on reduced Coₜₑₜ-Coₜₑₜ intensity in EXAFS (Fig. 1f). However, no direct evidence (e.g., aberration-corrected STEM imaging of atomic positions or EXAFS wavelet transform (WT) analysis of Cr-Co correlations) confirms Cr’s tetrahedral occupancy. The WT of Cr K-edge (Fig. 1j–l) only rules out Cr-Cr clusters but provides no insight into Cr-Co bonding.\nResponse: Thank you very much for your valuable comment and it is useful to improve the quality of this work. Based on the analysis of the inductively coupled plasma atomic emission spectroscopy (ICP-AES), the atomic ratio of Cr to Co was determined to be 0.07. Such a low doping level of Cr in Cr-Co O presents considerable difficulty in identifying its specific form and configuration within the crystal lattice. XAS is a useful characterization technique for analyzing elemental structural information. Specifically, the XANES can provide electronic structure information of elements, including oxidation state and geometric configuration, while the EXAFS yields atomic structural information of elements. Through mathematical processing such as Fourier transform, structural parameters of the coordination shells can be directly obtained. For example, Zhang and coworkers developed a Ru and Zn co-doped Co O spinel catalyst (RuZn-Co O ) for the alkaline OER. Through XAS characterization, they confirmed that Ru and Zn atoms occupy the Co and Co oct tet sites, respectively (Angew. Chem. Int. Ed., 2024, 63, e202407509). In this manuscript, the FT-EXAFS spectrum of Cr-Co O at the Co K-edge reveals a decrease in the intensity of the Co -Co tet tet peak compared to that of pure Co O (Figure R1). This suggests that Cr atoms occupy the Co sites in the Cr-Co O lattice. The WT analysis at Co K-edge of Co foil, Co O , and Cr-Co O was shown in Figure R2. The resolving power of WT analysis primarily hinges on its ability to discriminate between backscattering atoms based on their atomic numbers, with its principal advantage lying in distinguishing light from heavy elements. In this work, however, WT analysis cannot effectively differentiate between the Co -Co and Co -Cr coordination environments. This limitation arises because the similar atomic numbers of Cr and Co result nearly identical backscattering signatures, rendering their signals indistinguishable in the wavelet transform contours.\nAlthough aberration-corrected scanning transmission electron microscopy (STEM) is unquestionably an efficient technique for identifying atomic occupancy, its capacity to resolve individual sites is limited by atomic number contrast. The proximity in atomic number between Cr and Co leads to negligible contrast, which prevents their reliable discrimination. Raman spectroscopy can probe the vibrational modes of a crystal lattice, which are associated with specific atomic bond vibrations. Consequently, the positions of Raman peaks can be directly correlated with different coordination environments. The Co O spinel presents distinctive Raman modes, including E , F , and A modes, as demonstrated in Figure R3. Specially, the F mode (524 cm-1) is in connection with the stretching of CoO (Co -O), while the A mode (692 cm-1) and E mode (484 cm-1) are in connection with the stretching of CoO (Co -O) (J. Am. Chem. Soc., 2021, 143, 5201-5211; J. Am. Chem. Soc., 2023, 145, 17995-18006; Phys. Status Solidi (b), 2016, 253, 2270-2282; Nat. Commun., 2021, 12, 3036). Compared with Co O , the A mode and E mode in the Cr-Co O catalyst remain unchanged, while the F mode shifts. This further confirms that Cr substitutes for the Co position in the Co O lattice.\nFigure R1. FT-EXAFS spectra of Co foil, Co O , and Cr-Co O .\nFigure R2. WT of the Co K-edge for Co foil (a), Co O (b), and Cr-Co O (c).\nFigure R3. Raman spectra of Cr-Co O and Co O catalysts.\nWe also performed DFT calculations to gain further insight into the formation energy of the different substitution sites of Cr into Co O . As shown in Figures R4 and R5, the calculated formation energy of Cr -Co O and Cr -Co O is -2.00 eV and -1.12 eV, respectively. This indicates the structure of Cr in the Co position of Co O lattice possesses much lower energy, suggesting the Cr elements preferentially occupy the Co position in the lattice of Co O .\nFigure R4. (a) and (b) are the top view and side view of Co O (311) surface with Cr atom substitutes the Co site (Cr -Co O ). The blue balls, grey ball, and red balls represent Co atoms, Cr atom, and O atoms, respectively.\nFigure R5. (a) and (b) are the top view and side view of Co O (311) surface with Cr atom substitutes the Co site (Cr -Co O ). The blue balls, grey ball, and red balls represent Co atoms, Cr atom, and O atoms, respectively.\nFurthermore, the EXAFS and WT spectra of Cr-Co O at the Cr K-edge were also investigated. As demonstrated in Figure R6, the peak at 1.53 Å corresponds to Cr-O coordination environment, while the peak at 2.51 Å is attributed to Cr-O-Co coordination environment. In contrast to the spectra of Cr foil and Cr O , no characteristic signals for Cr-Cr (2.23 Å) or Cr-O-Cr (2.46 Å and 3.27 Å) coordination environments are observed, suggesting the high monodispersed Cr atoms in Cr-Co O .\nAs depicted in Figure R7, the distinct WT spectrum at Cr K-edge of Cr-Co O also displays no obvious signals of Cr-Cr coordination (as in Cr foil) and Cr-O-Cr coordination (as in Cr O ), suggesting that Cr dopants are uniformly distributed in the Co O lattice.\nFigure R6. FT-EXAFS spectra of Cr foil, Cr O , and Cr-Co O .\nFigure R7. WT of the Cr K-edge for Co foil (a), Cr O (b), and Cr-Co O (c).\nAccordingly, the Figures R3, R4, and R5 have been added in the Revised Supporting Information as Figures S6, S35, and S36. The related information has been updated in the Revised Manuscript as follow:\n“To investigate the substitution sites of Cr into Co O , Raman characterizations were conducted on. The Co O spinel presents distinctive Raman modes, including E , F , and A modes, as demonstrated in Figure S6. Specially, the F mode (524 cm-1) is in connection with the stretching of CoO (Co -O), while the A mode (692 cm-1) and E mode (484 cm-1) are in connection with the stretching of CoO (Co -O).[34-37] Compared with Co O , the A mode and E mode in the Cr-Co O catalyst remain unchanged, while the F mode shifts. This confirms that Cr substitutes for the Co position in the Co O lattice.” (Page 3 in the revised manuscript)\n“The FT-EXAFS spectrum of Cr-Co O at Co K-edge is almost identical to that of Co O , except the decreased intensity of Co -Co , which further indicates the Cr elements occupy the Co position in the lattice of Cr-Co O (Figure 1g).[39,40]” (Page 5 in the revised manuscript)\n“Additionally, associated with the FT-EXAFS analysis (Figures 1i and S10), the peak at 1.53 Å can be assigned to the Cr-O coordination environment, while the peak at 2.51 Å is attributed to Cr-O-Co coordination environment. In contrast to the spectra of Cr foil and Cr O , no characteristic signals for Cr-Cr (2.23 Å) or Cr-O-Cr (2.46 Å and 3.27 Å) coordination environments are observed, suggesting the high monodispersed Cr atoms in Cr-Co O . The decreased Cr-O in Cr-Co O suggests stronger adsorption ability with oxygen species.[32] Furthermore, the distinct WT spectrum at Cr K-edge of Cr-Co O also displays no obvious signals of Cr-Cr coordination (as in Cr foil) and Cr-O-Cr coordination (as in Cr O ) (Figures 1j-l), suggesting that Cr dopants are uniformly distributed in the Co O lattice.” (Page 5 in the revised manuscript)\n“The calculated model of Co O was shown in Figure S34. The different substitution sites of Cr into Co O were considered. As shown in Figures S35 and S36, the calculated formation energy of Cr -Co O and Cr -Co O is -2.00 eV and -1.12 eV, respectively. This indicates the structure of Cr in the Co position of Co O lattice possesses much lower energy, suggesting the Cr elements preferentially occupy the Co position in the lattice of Co O , in good agreement with the previous analysis. In conjunction with the fact that Cr3+-OH surface species exists under realistic catalytic conditions, the structure depicted in Figure S37 was chosen for modeling the Cr-Co O catalyst with OH* pre-adsorbed on the Cr site (denoted as Cr-Co O ).” (Page 11 in the revised manuscript)\n“34. Shan, J. et al. Short range ordered iridium single atoms integrated into cobalt oxide spinel structure for highly efficient electrocatalytic water oxidation. J. Am. Chem. Soc. 143, 5201-5211, (2021).\n35. Nayak, S. et al. Spectroscopic studies of Co TiO and Co O two-phase composites. Phys. Status Solidi (b) 253, 2270-2282, (2016).\n36. Zhu, W. et al. Direct dioxygen radical coupling driven by octahedral ruthenium-oxygen-cobalt collaborative coordination for acidic oxygen evolution reaction. J. Am. Chem. Soc. 145, 17995-18006, (2023).\n37. Huang, J. et al. Modifying redox properties and local bonding of Co O by CeO enhances oxygen evolution catalysis in acid. Nat. Commun. 12, 3036, (2021).”\n(2) XPS shows Cr exists as Cr3+-O and Cr3+-OH (Fig. S4), but the manuscript does not clarify if surface Cr species (OH-terminated) differ from bulk lattice Cr. This distinction is critical, as surface Cr might dominate interfacial water effects, while lattice Cr drives electronic modulation—yet the authors treat them as a single “Cr dopant” effect.\nResponse: Thank you very much for your valuable comment and it is useful to improve the quality of this work. In the manuscript, we have fabricated Cr dopant Co O catalyst for acidic OER. The introduction of Cr into Co O lattice can create Cr-O-Co local environment and Cr3+-OH species modification on the surface. Operando Raman and in situ XAS were performed to confirm that the Cr-O-Co local environment can regulate the electronic structure of Co active sites through dynamic electron interaction during the acidic OER process. Before the acidic OER, the unique coordination environment of Cr -O-Co can significantly regulate the active Co site, without reducing the number of active sites, thereby simultaneously improving the activity and stability of Cr-Co O , since the Co site has been demonstrated to be more active than Co site in Co O for OER. At high overpotential, the Cr dopants can donate electrons to stabilize activate Co site, thereby sustaining catalytic performance. In addition, Operando ATR-SEIRAS and in situ XPS were carried out to found that the dynamic increased Cr3+-OH species coverage on the Cr-Co O surface can manipulate the behavior of interfacial water molecules during the acidic OER process, leading to the enriched free water molecules environment and promoted dissociation of water, thereby boosting the acidic OER performance.\nThe related information has been updated in the Revised Manuscript as follow:\n“The unique Cr -O-Co coordination environment can significantly regulate the electronic structure of active Co site, simultaneously improving the activity and stability of Cr-Co O . In addition, operando Raman and in situ X-ray absorption spectroscopy (XAS) were performed to confirm that the Cr -O-Co local environment can also donate electrons to stabilize activate Co site during the acidic OER process. Such a dynamical electron interaction can be in favor of simultaneously enhancing the catalytic activity and stability. Moreover, associated with the operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and in situ X-ray photoelectron spectroscopy (XPS), we found that the increased Cr3+-OH species coverage on the Cr-Co O surface can manipulate the behavior of interfacial water molecules, leading to the enriched free water molecules environment and promoted dissociation of water, thereby boosting the acidic OER performance.” (Page 2 in the revised manuscript)\n“While for Cr-Co O , based on the normalized XANES spectra at different stages during the acidic OER process (Figures 3e and 3f), it can be found that the near-edge absorption energies under varied applied potential exhibit no obvious changes, suggesting the introducing of Cr to form the Cr -O-Co local environment can stabilize the Co active sites during the acidic OER process by donating electrons, thereby enhancing the catalytic stability.” (Page 9 in the revised manuscript)\n“Based on the above analysis, it can be found that the introduction of Cr into the lattice of Co O can not only regulate the electronic structure of active Co site, but also donate electrons to stabilize activate Co sites during the acidic OER process to improve the catalytic activity and stability (Figure 3g).” (Page 9 in the revised manuscript)\n“Operando electrochemical experiments and spectral characterization monitoring reveal that the introduction of Cr to form Cr -O-Co local environment can realize the dynamic regulation of the electronic structure of Co active site, suppressing its further oxidation, thereby leading to a significant enhancement in both activity and stability.” (Page 13 in the revised manuscript)\n(3) Operando XAS (Fig. 3c-f) shows stable Co valence in Cr-Co O vs. irreversible Co4+ formation in Co O , attributed to “dynamic electron transfer from Cr to Co.” However, no time-resolved or potential-dependent data (e.g., in-situ XPS valence shifts of Cr during OER) directly confirms electron transfer. The positive shift of Cr 2p peaks with potential (Fig. 4b) is interpreted as Cr oxidation, but this could also arise from surface hydroxyl desorption or lattice oxygen activation-alternative explanations are not ruled out.\nResponse: Thanks for your valuable comment. In situ XPS measurements were performed to investigate the valence state changes of Co active sites and the evolution of Cr3+-OH surface species of Cr-Co O during the acidic OER process. As depicted in Figure R8, the valence states of Co on the surface of Cr-Co O display negligible changes under different applied potentials. In contrast, the Cr 2p binding energy increases from 576.2 eV at 1.0 V to 576.6 eV at 1.8 V. This positive shift indicates that the main Cr 2p XPS peak for Cr-Co O shifts to higher binding energy with increasing applied potential. Besides, the surface concentration of Cr3+-OH species and lattice oxygen in Cr-Co O were also investigated. As shown in Figure R9, the surface concentration of Cr3+-OH species in Cr-Co O increases progressively with applied potential, demonstrating that Cr doping creates a hydroxyl species enriched local environment. The in situ XPS spectra of O 1s for Cr-Co O in the potential range of 1.0 V to 1.8 V were depicted in Figure R10. It can be observed that the valance states of the lattice oxygen show no significant changes with the increased applied potential, ruling out the lattice oxygen activation during the catalytic process. Therefore, it can be concluded that the incorporation of Cr can be in favor of stabilizing the oxidation state of Co through dynamic donating electrons during the acidic OER process, thereby enhancing catalytic performance.\nFigure R8. In situ XPS measurements of Co 2p (a) and Cr 2p (b) for Cr-Co O in the potential range of 1.0 V to 1.8 V.\nFigure R9. The proportion of Cr3+-OH species on the surface of Cr-Co O under different applied potentials.\nFigure R10. The in situ XPS spectra of O 1s for Cr-Co O in the potential range of 1.0 V to 1.8 V.\nAccordingly, the Figure R10 has been updated in the Revised Supporting Information as Figure S27. The related information has been updated in the Revised Manuscript as follow:\n“In contrast, the Cr 2p binding energy increases from 576.2 eV at 1.0 V to 576.6 eV at 1.8 V. This positive shift indicates that the main Cr 2p XPS peak for Cr-Co O shifts to higher binding energy with increasing applied potential. In addition, the in situ XPS spectra of O 1s for Cr-Co O in the potential range of 1.0 V to 1.8 V were depicted in Figure S27. The valence states of lattice oxygen show no significant changes with increasing applied potential, which rules out the influence of lattice oxygen activation on the changes of the valence states of Cr. Based on the above analysis, the incorporation of Cr can be in favor of stabilizing the oxidation state of Co through dynamic donating electrons during the acidic OER process, thereby enhancing catalytic performance, which is in good agreement with the previous in situ XAS results. ” (Page 10 in the revised manuscript)\n(4) Post-OER XPS (Fig. S19) shows increased Cr3+-OH content, but the manuscript does not link this to “dynamic” behavior (e.g., reversible Cr valence changes during potential cycling). Without evidence of reversibility, the “dynamic” label is unsupported.\nResponse: Thanks for your valuable comment. The term “dynamic” describes a state that is non-static and evolves over time or with changing conditions, without prescribing the final direction of this change or whether it is reversible. With the rapid advancement of in situ/operando characterization techniques, a growing number of researchers are focusing on the dynamic evolution of catalyst structures and the electrode-electrolyte interface during electrochemical processes (J. Am. Chem. Soc., 2025, 147, 33770-33779; J. Am. Chem. Soc., 2025, 147, 37808-37818; Angew. Chem. Int. Ed., 2025, e202513246; Nat. Commun., 2025, 16, 3502; Angew. Chem. Int. Ed., 2025, 64, e202510743; App. Catal. B: Environ., 2026, 384, 126158). For instance, Peng and coworkers proposed a stable dynamic oxygen replenishment mechanism to enhance the performance and durability of spinel oxide electrocatalysts in acidic OER. By incorporating high-valent element into the NiCo O spinel lattice, strongly oxophilic W sites are introduced to modulate the local electronic environment, activate lattice oxygen, and facilitate vacancy-mediated oxygen exchange. W substitution induces localized charge redistribution and promotes interfacial water dissociation, enabling real-time lattice oxygen replenishment. Yang and coworkers presented direct experimental evidence of dynamic evolution from single-atom catalysts to Cu nano structures rich in active nano grains, based on a conductive metal-organic framework-based Cu single-atom catalyst through operando synchrotron-based high-energy-resolution X-ray spectroscopy and IR absorption spectroscopy quantitatively tracked the structural and molecular finger prints during single-atom-to-nanograin evolution.\nIn this manuscript, we have fabricated Cr-Co O catalyst with the formation of Cr -O-Co coordination environment and Cr3+-OH species modification on surface. The unique Cr -O-Co coordination environment can significantly regulate the electronic structure of active Co site, thereby simultaneously improving the activity and stability of Cr-Co O . In addition, operando Raman and in situ XAS were performed to confirm that the Cr -O-Co local environment can also donate electrons to stabilize activate Co sites during the acidic OER process. Such a dynamical electron interaction can be in favor of simultaneously enhancing the catalytic activity and stability. Moreover, associated with the operando ATR-SEIRAS and in situ XPS, we found that the increased Cr3+-OH species coverage on the Cr-Co O surface can manipulate the behavior of interfacial water molecules, leading to the enriched free water molecules environment and promoted dissociation of water, thereby boosting the acidic OER performance. In this work, we investigate the role of Cr in Co O throughout the entire catalytic process of the acidic OER, encompassing its modulation of both the electronic structure of the oxide and the electrode-electrolyte interface environment. Given this comprehensive scope of investigation, the process is appropriately described as dynamic.\n3. Interfacial Water Mechanism: Overinterpreted and Inconsistent Data\n(1) The decomposition of O-H stretching bands into “4-HB-H O,” “2-HB-H O,” and “free H O” (Fig. 4d, S21-S23) relies on Gaussian fitting, but the fitting parameters (peak positions, full-width at half-maximum) are not provided. Prior work (e.g., Angew. Chem. Int. Ed. 2020, 59, 22397-22402) shows that such deconvolution is highly sensitive to baseline correction-without validation (e.g., using D O as a control), the claim of “increased free H O” is speculative.\nResponse: Thank you very much for your valuable comment and it is useful to improve the quality of this work. The operando ATR-SEIRAS spectra of Cr-Co O and Co O under different applied potentials in 0.5 M H SO without baseline correction were demonstrated in Figures R11 and R12. The broad band extending from 3000 cm-1 to 3800 cm-1 was belonged to the O-H stretching mode of H O. To figure out the critical role of interfacial water on the OER performance, Gaussian fitting was performed to decompose the interfacial water into three distinct components, assigned to the 4-coordinated hydrogen-bonded water (4-HB-H O) and 2-coordinated hydrogen-bonded water (2-HB-H O), and free-H O with weak hydrogen-bond interactions, respectively. The peak positions and full-width at half-maximum (FWHM) of the 4-HB-H O, 2-HB-H O, and free H O for Cr-Co O and Co O are shown in Tables R2 and R3. The operando ATR-SEIRAS spectra reveal that the main peak position does not shift significantly with increasing potential, while the intensity of the high-wavenumber peak gradually decreases. As depicted in Figure R13, there is a blue shift of the OH stretching vibration at 1.5 V on the surface of Cr-Co O , in comparison with Co O , revealing the decreased hydrogen bond connectivity, which is in favor of accelerating interfacial water dissociation. To gain deeper insight into the role of interfacial water molecules structure on catalyzing acidic OER, the proportions of 4-HB-H O, 2-HB-H O, and free-H O at different potentials were analyzed (Figure R14). In conjunction with the changes of the free-H O proportions on the Cr-Co O and Co O surfaces, it can be found that the free-H O proportion on Cr-Co O surface is much higher than that on Co O surface during the acidic OER process, which is responsible for the accelerated dissociation of interfacial water, thereby boosting the kinetics of water oxidation in acid.\nFigure R11. (a) Operando ATR-SEIRAS spectra of Cr-Co O without baseline correction in 0.5 M H SO . (b) Deconvolution of the O-H stretching bands into 4-HB-H O, 2-HB-H O, and free H O for Cr-Co O .\nFigure R12. (a) Operando ATR-SEIRAS spectra of Co O without baseline correction in 0.5 M H SO . (b) Deconvolution of the O-H stretching bands into 4-HB-H O, 2-HB-H O, and free H O for Co O .\nFigure R13. Operando ATR-SEIRAS spectra of Cr-Co O and Co O at 1.5 V vs. RHE to investigate the interfacial water structure.\nFigure R14. The proportions of different types of water for Cr-Co O (a) and Co O (b) tested in 0.5 M H SO . (c) The proportions of free-H O at varied potentials of Cr-Co O and Co O .\nTo further investigate the electrode-electrolyte interface mincroenvironment, the operando ATR-SEIRAS measurements in D SO were also carried out. The broad band extending from 2200 cm-1 to 2700 cm-1 was belonged to the O-H stretching mode of D O. The operando ATR-SEIRAS spectra of Cr-Co O and Co O under different applied potentials in 0.5 M D SO , without baseline correction, are demonstrated in Figures R15 and R16. The corresponding deconvolution of the O-H stretching bands into 4-HB-D O, 2-HB-D O, and free D O is also shown. The peak positions and FWHM of the 4-HB-D O, 2-HB-D O, and free D O for Cr-Co O and Co O are shown in Tables R4 and R5. The proportions of 4-HB-D O, 2-HB-D O, and free-D O at different potentials were analyzed, as demonstrated in Figure R17. The free-D O proportion on Cr-Co O surface is obviously much higher than that on Co O surface during the acidic OER process. The formation of a reactant (free-D O) enriched interface on Cr-Co₃O₄ during the acidic OER process, driven by the dynamic evolution from 4-HB-D O and 2-HB-D O to free-D O, accelerates the overall catalytic reaction.\nFigure R15. (a) Operando ATR-SEIRAS spectra of Cr-Co O without baseline correction in 0.5 M D SO . (b) Deconvolution of the O-H stretching bands into 4-HB-D O, 2-HB-D O, and free D O for Cr-Co O .\nFigure R16. (a) Operando ATR-SEIRAS spectra of Co O without baseline correction in 0.5 M D SO . (b) Deconvolution of the O-H stretching bands into 4-HB-D O, 2-HB-D O, and free D O for Co O .\nFigure R17. The proportions of different types of water for Cr-Co O (a) and Co O (b) tested in 0.5 M D SO . (c) The proportions of free-D O at varied potentials of Cr-Co O and Co O .\nTable R2. The peak positions and full-width at half-maximum (FWHM) of the 4-HB-H O, 2-HB-H O, and free H O under different potentials for Cr-Co O .\n4-HB-H O 2-HB-H O free H O\npeak position FWHM peak position FWHM peak position FWHM\n1.1 V 3150 cm-1 230 3356 cm-1 280 3546 cm-1 180\n1.2 V 3147 cm-1 230 3353 cm-1 280 3544 cm-1 180\n1.3 V 3152 cm-1 230 3351 cm-1 280 3545 cm-1 180\n1.4 V 3153 cm-1 230 3355 cm-1 280 3548 cm-1 180\n1.5 V 3153 cm-1 230 3351 cm-1 280 3546 cm-1 180\n1.6 V 3153 cm-1 230 3353 cm-1 280 3544 cm-1 180\n1.7 V 3151 cm-1 230 3352 cm-1 280 3543 cm-1 180\n1.8 V 3148 cm-1 230 3351 cm-1 280 3540 cm-1 180\nTable R3. The peak positions and FWHM of the 4-HB-H O, 2-HB-H O, and free H O under different potentials for Co O .\n4-HB-H O 2-HB-H O free H O\npeak position FWHM peak position FWHM peak position FWHM\n1.1 V 3142 cm-1 230 3336 cm-1 280 3523 cm-1 180\n1.2 V 3143 cm-1 230 3334 cm-1 280 3526 cm-1 180\n1.3 V 3140 cm-1 230 3332 cm-1 280 3521 cm-1 180\n1.4 V 3137 cm-1 230 3332 cm-1 280 3524 cm-1 180\n1.5 V 3138 cm-1 230 3332 cm-1 280 3521 cm-1 180\n1.6 V 3137 cm-1 230 3332 cm-1 280 3526 cm-1 180\n1.7 V 3137 cm-1 230 3330 cm-1 280 3523 cm-1 180\n1.8 V 3140 cm-1 230 3331 cm-1 280 3524 cm-1 180\nTable R4. The peak positions and FWHM of the 4-HB-D O, 2-HB-D O, and free D O under different potentials for Cr-Co O .\n4-HB-D O 2-HB-D O free D O\npeak position FWHM peak position FWHM peak position FWHM\n1.1 V 2392 cm-1 170 2556 cm-1 207 2654 cm-1 130\n1.2 V 2390 cm-1 170 2558 cm-1 207 2657 cm-1 130\n1.3 V 2390 cm-1 170 2552 cm-1 207 2654 cm-1 130\n1.4 V 2390 cm-1 170 2555 cm-1 207 2652 cm-1 130\n1.5 V 2390 cm-1 170 2558 cm-1 207 2654 cm-1 130\n1.6 V 2395 cm-1 170 2558 cm-1 207 2654 cm-1 130\n1.7 V 2392 cm-1 170 2556 cm-1 207 2659 cm-1 130\n1.8 V 2390 cm-1 170 2560 cm-1 207 2655 cm-1 130\nTable R5. The peak positions and FWHM of the 4-HB-D O, 2-HB-D O, and free D O under different potentials for Co O .\n4-HB-D O 2-HB-D O free D O\npeak position FWHM peak position FWHM peak position FWHM\n1.1 V 2350 cm-1 170 2476 cm-1 207 2628 cm-1 130\n1.2 V 2350 cm-1 170 2472 cm-1 207 2625 cm-1 130\n1.3 V 2355 cm-1 170 2480 cm-1 207 2630 cm-1 130\n1.4 V 2354 cm-1 170 2480 cm-1 207 2628 cm-1 130\n1.5 V 2353 cm-1 170 2480 cm-1 207 2627 cm-1 130\n1.6 V 2353 cm-1 170 2477 cm-1 207 2625 cm-1 130\n1.7 V 2352 cm-1 170 2474 cm-1 207 2625 cm-1 130\n1.8 V 2355 cm-1 170 2475 cm-1 207 2624 cm-1 130\nAccordingly, the Figures R11, R12, and R14 have been updated in the Revised Supporting Information as Figure S28-S30. The Figures R15-R17 have been updated in the Revised Supporting Information as Figure S31-S33. The Tables R2-R5 have been updated in the Revised Supporting Information as Tables S6-S9.\nThe related information has been updated in the Revised Manuscript as follow:\n“To investigate the critical role of interfacial water on the OER performance, Gaussian fitting was performed to decompose the interfacial water into three distinct components, assigned to the 4-coordinated hydrogen-bonded water (4-HB-H O) and 2-coordinated hydrogen-bonded water (2-HB-H O), and free-H O with weak hydrogen-bond interactions, respectively (Figures 4d, S28, and S29).[53-55] The peak positions and full-width at half-maximum (FWHM) of the 4-HB-H O, 2-HB-H O, and free H O for Cr-Co O and Co O are shown in Tables S6 and S7. ” (Page 11 in the revised manuscript)\n“To further investigate the electrode-electrolyte interface mincroenvironment, operando ATR-SEIRAS measurements in D SO were also carried out.[53,56] The broad band extending from 2200 cm-1 to 2700 cm-1 can be assigned to the O-H stretching mode of D O. The operando ATR-SEIRAS spectra of Cr-Co O and Co O under different applied potentials in 0.5 M D SO are shown in Figures S31 and S32. The corresponding deconvolution of the O-H stretching bands into 4-HB-D O, 2-HB-D O, and free D O is studied. The peak positions and FWHM of the 4-HB-D O, 2-HB-D O, and free D O for Cr-Co O and Co O are shown in Tables S8 and S9. The proportions of 4-HB-D O, 2-HB-D O, and free-D O at different potentials were analyzed, as demonstrated in Figure S33. The free-D O proportion on Cr-Co O surface is obviously much higher than that on Co O surface during the acidic OER process. The formation of a reactant (free-D O) enriched interface on Cr-Co O during the acidic OER process, driven by the dynamic evolution from 4-HB-D O and 2-HB-D O to free-D O, accelerates the overall catalytic reaction (Figure 4g). ” (Page 11 in the revised manuscript)\n“53. Shen, L. et al. Interfacial structure of water as a new descriptor of the hydrogen evolution reaction. Angew. Chem. Int. Ed. 59, 22397-22402, (2020).”\n“56. Zhu, S. et al. Reconstructing hydrogen-bond network for efficient acidic oxygen evolution. Angew. Chem. Int. Ed. 63, e202319462, (2024).”\n(2) The authors use V /V ratios to infer higher OH coverage on Cr-Co O (Fig. S20), but MOR activity depends on both OH availability and metal surface reactivity. Cr may suppress MOR via electronic effects (not just OH coverage), making this proxy unreliable. Direct quantification of surface OH (e.g., in-situ titration or TPD-MS) is missing.\nResponse: Thanks for the reviewer’s valuable comment. According to your suggestion, the temperature-programmed desorption-mass spectrometry (TPD-MS) was performed on Cr-Co O to directly quantify the surface Cr3+-OH species. As shown in Figure R18, Cr-Co O exhibits two mass loss stages: one from 100 °C to 200 °C and the other from 200 °C to 500 °C. These are assigned to the desorption of adsorbed water and the loss of surface Cr3+-OH species, respectively (Sci. Adv., 2023, 9, eadh1718).\nFigure R18. TPD-MS curves of Cr-Co O .\nAccordingly, the Figure R18 has been added in the Revised Supporting Information as Figure S5. The related information has been updated in the Revised Manuscript as follow:\n“The temperature-programmed desorption-mass spectrometry (TPD-MS) was performed on Cr-Co O to directly quantify the surface Cr3+-OH species. As shown in Figure S5, Cr-Co O exhibits two mass loss stages: one from 100 °C to 200 °C and the other from 200 °C to 500 °C. These are assigned to the desorption of adsorbed water and the loss of surface Cr3+-OH species, respectively.[33] ” (Page 3 in the revised manuscript)\n“33. Xu, J. et al. IrO ·nH O with lattice water-assisted oxygen exchange for high-performance proton exchange membrane water electrolyzers. Sci. Adv. 9, eadh1718, (2023). ”\n4. The DFT work supports two key claims: electron transfer from Cr to Co, and reduced water dissociation energy. The calculated average charge of Coₒ𝑐ₜ (0.81 eV for Cr-Co O vs. 0.85 eV for Co O ) is used to infer electron transfer, but Mulliken charges are highly basis-set dependent. Bader charge analysis (a more robust method for electronic structure) is not provided to confirm this effect. The rate-determining step (RDS) energy barrier for Cr-Co O (1.60 eV) is lower than Co O (2.17 eV) (Fig. 5f), but the model assumes a single OER mechanism (adsorbate evolution mechanism, AEM). The authors do not rule out lattice oxygen-mediated mechanism (LOM), which is common in Co-based catalysts under high overpotentials. Without validating the mechanism via isotope labeling (e.g., ¹⁸O water), the DFT-derived barrier is irrelevant to the actual reaction pathway.\nResponse: We appreciate the reviewer for the thoughtful comments on this work. The reviewer’s suggestions help us to substantially improve the quality of manuscript. The negative and positive Bader charge values are corresponding to the loss and gain of electrons. As depicted in Figure R19, the Bader charge of Co in Co O is more negative, meaning the decreased electrons density. And the calculated results of Bader charge are in good agreement with the Mulliken charge results.\nFigure R19. The Bader charges of different Co sites in Co O , and Co site and Cr site in Cr-Co O .\nTo investigate the reaction mechanism, in situ 18O-labeled differential electrochemical mass spectrometry (DEMS) measurements were carried out. As shown in Figure R20, when tested in the electrolyte using H 18O as the solvent, the evolved O from Cr-Co O consists of 90.1% of 36O (18O18O) and 9.9% of 34O (18O16O). The signal of 32O (16O16O) appears as an inverted peak because the concentration is falling below the detection limit. However, after the Cr-Co O catalyst is labeled with the 18O isotope and then tested in the electrolyte using H 16O as the solvent, the signal of 32O (16O16O) increases dramatically to 98.4%, while the signals corresponding to 18O-containing species (34O and 36O ) decrease significantly. This result clearly indicates that the Cr-Co O catalyst follows adsorbate evolution mechanism (AEM).\nFigure R20. (a) DEMS signals of O products for Cr-Co O in the electrolyte using H 18O as the solvent. (b) DEMS signals of O products for 18O isotope labeled Cr-Co O in the electrolyte using H 16O as the solvent.\nAccordingly, the Figures R19 and R20 have been added in the Revised Supporting Information as Figures S38 and S40.\nThe related information has been updated in the Revised Manuscript as follow:\n“The Bader charge of Cr-Co O and Co O were also calculated (Figure S38), which is in good agreement with the Mulliken charge results.” (Page 12 in the revised manuscript)\n“To investigate the reaction mechanism, in situ 18O-labeled differential electrochemical mass spectrometry (DEMS) measurements were carried out.[33,58,59] As shown in Figure S40, after the Cr-Co O catalyst is labeled with the 18O isotope and then tested in H 16O, the signal of 32O (16O16O) increases and the signals corresponding to 18O-containing species (34O and 36O ) decrease significantly. This result clearly indicates that the Cr-Co O catalyst follows adsorbate evolution mechanism.” (Page 12 in the revised manuscript)\n“33. Xu, J. et al. IrO ·nH O with lattice water-assisted oxygen exchange for high-performance proton exchange membrane water electrolyzers. Sci. Adv. 9, eadh1718, (2023).”\n“58. Lin, C. et al. In-situ reconstructed Ru atom array on α-MnO with enhanced performance for acidic water oxidation. Nat. Catal. 4, 1012-1023, (2021).\n59. Shi, Z. et al. Enhanced acidic water oxidation by dynamic migration of oxygen species at the Ir/Nb O catalyst/support interfaces. Angew. Chem. Int. Ed. 61, e202212341 (2022).”\nThe related information has been updated in the Revised Supporting Information as follow:\n“In situ differential electrochemical mass spectrometry (DEMS) measurements were performed with a QAS 100 device. The catalysts were dripped onto a gold-plated PTFE membrane electrode with a catalyst loading of 0.3 mg cm-2. The gold-plated PTFE membrane electrode with catalysts, Ag/AgCl electrode and pure Pt wire were used as the working electrode, reference electrode, and counter electrode, respectively. Polarization curves were tested in 0.5 M H SO (prepared with H 18O) at voltages from 1.30 to 1.75 V vs. RHE. Meanwhile, mass spectrometry was used to detect O generated during the OER process in real time.”\n“When tested in the electrolyte using H 18O as the solvent, the evolved O from Cr-Co O consists of 90.1% of 36O (18O18O) and 9.9% of 34O (18O16O). The signal of 32O (16O16O) appears as an inverted peak because the concentration is falling below the detection limit. However, after the Cr-Co O catalyst is labeled with the 18O isotope and then tested in the electrolyte using H 16O as the solvent, the signal of 32O (16O16O) increases dramatically to 98.4%, while the signals corresponding to 18O-containing species (34O and 36O ) decrease significantly. This result clearly indicates that the Cr-Co O catalyst follows adsorbate evolution mechanism (AEM)”\n5. Post-OER characterizations (Raman, XPS) suggest partial conversion of Cr-Co O to oxyhydroxides, but the manuscript does not isolate the activity of the original oxide vs. in-situ formed oxyhydroxides. A control experiment (e.g., pre-synthesized Cr-Co oxyhydroxide tested for OER) is needed to clarify whether the oxide or its reconstructed phase is the true active species.\nResponse: Thanks for your constructive question and it is extremely useful to improve the manuscript. The Cr-CoOOH and CoOOH catalyst samples were fabricated based on the method described in the prior literature (Angew. Chem. Int. Ed., 2024, 63, e202408005; Chin. J. Chem., 2024, 42, 343-350). As shown in Figure R21, the powder X-ray diffraction (XRD) patterns of the prepared Cr-CoOOH and CoOOH catalysts match well with the standard CoOOH (PDF#07-0169).\nThe acidic OER performances of Cr-CoOOH and CoOOH catalysts were investigated in 0.5 M H SO electrolyte through a standard three-electrode system. The electrocatalyst decorated RDE with glassy carbon was used as the working electrode. The diameter of glassy carbon was 5 mm and the disk area was 0.19625 cm2. The carbon rod and Hg/Hg Cl electrode (saturated KCl solution) were applied as the counter electrode and reference electrode, respectively. As shown in Figure R22, both Cr-CoOOH and CoOOH catalysts exhibit very poor OER activity in acidic medium. This result indicates that the high activity of Cr-Co O and Co O originates primarily from the oxide itself, with minimal contribution from any reconstructed phase.\nFigure R21. The XRD patterns of Cr-CoOOH and CoOOH.\nFigure R22. The LSV curves of Cr-Co O , Co O , Cr-CoOOH, and CoOOH.\nAccordingly, the Figures R21 and R22 have been added in the Revised Supporting Information as Figures S11 and S12. The related information has been updated in the Revised Manuscript as follow:\n“Given that transition metal oxides may undergo reconstruction during catalysis, the corresponding hydroxyoxide catalysts (Cr-CoOOH and CoOOH) were prepared to investigate the true active species.[45] As shown in Figures S11 and S12, both Cr-CoOOH and CoOOH catalysts exhibit very poor OER activity in acidic medium. This result indicates that the high activity of Cr-Co O and Co O originates primarily from the oxide itself.” (Page 7 in the revised manuscript)\n“45. Yao, N., Zhu, J., Jia, H., Cong, H., & Luo, W. Identification of in situ generated iron-vacancy induced oxygen evolution reaction kinetics on cobalt iron oxyhydroxide. Chin. J. Chem. 42, 343-350, (2024).”\nThe related information has been updated in the Revised Supporting Information as follow:\n“urea (CO(NH ) , Sinopharm Chemical Reagent, AR), ammonium fluoride (NH F, Sinopharm Chemical Reagent, AR)”\n“Synthesis of Cr-CoOOH and pristine CoOOH\nSpecifically, 1.50 mmol Co(NO ) ·6H O, a certain amount of Cr(NO ) ∙9H O, 5 mmol CO(NH ) , and 5 mmol NH F were completely dissolved in 10 mL of deionized water by using the ultrasonic treatment. Subsequently, the above solution was transferred to a 20 mL Teflon-lined stainless-steel autoclave. Then the autoclave was sealed increased temperature to 120 °C with a rate of 2 °C min-1 and maintained the temperature for 6 h. After cooling down to room temperature, the obtained precursor was thoroughly washed with deionized water and dried in a vacuum-drying oven. Then the precursor and 5 mL NaClO (5.2 wt%) were added in the 20 mL Teflon-lined stainless-steel autoclave and adjust the pH to 12. The autoclave was heated at 90 °C with a heating rate of 2 °C min-1 and allowed to react for 10 h. After cooling down to room temperature, the obtained Cr-CoOOH was thoroughly washed with deionized water and dried in a vacuum drying oven. The CoOOH was synthesized using a similar protocol without the addition of Cr(NO ) ∙9H O.”\nReviewer #2 (Remarks to the Author):\nThe authors fabricate chromium-doped cobalt spinel oxide (Cr-Co O ) as an efficient OER electrocatalyst. The operando measurements demonstrate the Cr doping effect on interfacial H O dissociation kinetics. The increased hydroxyl group coverage on the Cr site optimizes the interfacial water structure. In addition, Cr doping effectively enhances long-term durability in acidic electrolyte system. However, the manuscript needs some additional clarifications. Therefore, publication in the Nature Communications is recommended after major revisions.\nResponse: Thanks for the reviewer for the valuable comments and suggestions, which have been duly incorporated into the revised manuscript. Our detailed responses are herein provided:\nComment 1: In Figure 1i, why do the two peaks after the Cr-O around 2.5Å and 3.3Å in the EXAFS differ markedly in intensity between Cr-Co O and Co O ? Also, aside from the decrease in Co -Co intensity, is there additional evidence that Cr substitutes into the tetrahedral sites?\nResponse: Thanks for the reviewer’s valuable comment. The Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectrum at Cr K-edge of Cr-Co O is demonstrated in Figure R1 (Figure 1i in the manuscript). The peak at 1.53 Å corresponds to Cr-O coordination environment, while the peak at 2.51 Å is attributed to Cr-O-Co coordination environment. In contrast to the spectra of Cr foil and Cr O , no characteristic signals for Cr-Cr (2.23 Å) or Cr-O-Cr (2.46 Å and 3.27 Å) coordination environments are observed, suggesting the high monodispersed Cr atoms in Cr-Co O .\nFigure R1. FT-EXAFS spectra of Cr foil, Cr O , and Cr-Co O .\nFigure R2. FT-EXAFS spectra of Co foil, Co O , and Cr-Co O .\nFigure R3. Raman spectra of Cr-Co O and Co O catalysts.\nTo investigate the substitution sites of Cr into Co O , the X-ray absorption spectroscopy (XAS) characterizations at Co K-edge were carried out. As shown in Figure R2, the FT-EXAFS spectrum of Cr-Co O at Co K-edge is almost identical to that of Co O , except the decreased intensity of Co -Co (tetrahedral site), which indicates the Cr elements occupy the Co position in the lattice of Cr-Co O .\nBesides, Raman spectroscopy can probe the vibrational modes of a crystal lattice, which are associated with specific atomic bond vibrations. Consequently, the positions of Raman peaks can be directly correlated with different coordination environments. The Co O spinel presents distinctive Raman modes, including E , F , and A modes, as demonstrated in Figure R3. Specially, the F mode (524 cm-1) is in connection with the stretching of CoO (Co -O), while the A mode (692 cm-1) and E mode (484 cm-1) are in connection with the stretching of CoO (Co -O) (J. Am. Chem. Soc., 2021, 143, 5201-5211; J. Am. Chem. Soc., 2023, 145, 17995-18006; Phys. Status Solidi (b), 2016, 253, 2270-2282; Nat. Commun., 2021, 12, 3036). Compared with Co O , the A mode and E mode in the Cr-Co O catalyst remain unchanged, while the F mode shifts. This further confirms that Cr substitutes for the Co position in the Co O lattice.\nFigure R4. (a) and (b) are the top view and side view of Co O (311) surface with Cr atom substitutes the Co site (Cr -Co O ). The blue balls, grey ball, and red balls represent Co atoms, Cr atom, and O atoms, respectively.\nFigure R5. (a) and (b) are the top view and side view of Co O (311) surface with Cr atom substitutes the Co site (Cr -Co O ). The blue balls, grey ball, and red balls represent Co atoms, Cr atom, and O atoms, respectively.\nWe also performed DFT calculations to gain further insight into the formation energy of the different substitution sites of Cr into Co O . As shown in Figures R4 and R5, the calculated formation energies of Cr -Co O and Cr -Co O are -2.00 eV and -1.12 eV, respectively. This indicates the structure of Cr in the Co position of Co O lattice possesses much lower energy, suggesting the Cr elements preferentially occupy the Co position in the lattice of Co O .\nAccordingly, the Figures R3, R4, and R5 have been added in the Revised Supporting Information as Figures S5, S36, and S37. The related information has been updated in the Revised Manuscript as follow:\n“To investigate the substitution sites of Cr into Co O , Raman characterizations were conducted on. The Co O spinel presents distinctive Raman modes, including E , F , and A modes, as demonstrated in Figure S6. Specially, the F mode (524 cm-1) is in connection with the stretching of CoO (Co -O), while the A mode (692 cm-1) and E mode (484 cm-1) are in connection with the stretching of CoO (Co -O).[34-37] Compared with Co O , the A mode and E mode in the Cr-Co O catalyst remain unchanged, while the F mode shifts. This confirms that Cr substitutes for the Co position in the Co O lattice.” (Page 3 in the revised manuscript)\n“The FT-EXAFS spectrum of Cr-Co O at Co K-edge is almost identical to that of Co O , except the decreased intensity of Co -Co , which further indicates the Cr elements occupy the Co position in the lattice of Cr-Co O (Figure 1g).[39,40]” (Page 5 in the revised manuscript)\n“Additionally, associated with the FT-EXAFS analysis (Figures 1i and S10), the peak at 1.53 Å can be assigned to the Cr-O coordination environment, while the peak at 2.51 Å is attributed to Cr-O-Co coordination environment. In contrast to the spectra of Cr foil and Cr O , no characteristic signals for Cr-Cr (2.23 Å) or Cr-O-Cr (2.46 Å and 3.27 Å) coordination environments are observed, suggesting the high monodispersed Cr atoms in Cr-Co O . The decreased Cr-O in Cr-Co O suggests stronger adsorption ability with oxygen species.[32] Furthermore, the distinct WT spectrum at Cr K-edge of Cr-Co O also displays no obvious signals of Cr-Cr coordination (as in Cr foil) and Cr-O-Cr coordination (as in Cr O ) (Figures 1j-l), suggesting that Cr dopants are uniformly distributed in the Co O lattice.” (Page 5 in the revised manuscript)\n“The calculated model of Co O was shown in Figure S34. The different substitution sites of Cr into Co O were considered. As shown in Figures S35 and S36, the calculated formation energy of Cr -Co O and Cr -Co O is -2.00 eV and -1.12 eV, respectively. This indicates the structure of Cr in the Co position of Co O lattice possesses much lower energy, suggesting the Cr elements preferentially occupy the Co position in the lattice of Co O , in good agreement with the previous analysis. In conjunction with the fact that Cr3+-OH surface species exists under realistic catalytic conditions, the structure depicted in Figure S37 was chosen for modeling the Cr-Co O catalyst with OH* pre-adsorbed on the Cr site (denoted as Cr-Co O ).” (Page 11 in the revised manuscript)\n“34. Shan, J. et al. Short range ordered iridium single atoms integrated into cobalt oxide spinel structure for highly efficient electrocatalytic water oxidation. J. Am. Chem. Soc. 143, 5201-5211, (2021).\n35. Nayak, S. et al. Spectroscopic studies of Co TiO and Co O two-phase composites. Phys. Status Solidi (b) 253, 2270-2282, (2016).\n36. Zhu, W. et al. Direct dioxygen radical coupling driven by octahedral ruthenium-oxygen-cobalt collaborative coordination for acidic oxygen evolution reaction. J. Am. Chem. Soc. 145, 17995-18006, (2023).\n37. Huang, J. et al. Modifying redox properties and local bonding of Co O by CeO enhances oxygen evolution catalysis in acid. Nat. Commun. 12, 3036, (2021).”\nComment 2: In the HAADF-STEM images (Figures S2 and S3), Cr-Co O particle shows smaller size than Co O . Furthermore, C value of Cr-Co O is smaller than that of Co O (Figure S10). Please investigate the reasons for these differences.\nResponse: Thanks for the reviewer’s valuable comment and it is useful to improve the quality of this work. Indeed, it is a well-recognized principle that smaller catalyst particles generally result in a greater electrochemical active surface area. We have retested the electrochemical data and implemented the corrections accordingly, as demonstrated in Figure R6.\nFigure R6. CV curves with different sweep speeds of Cr-Co O (a) and Co O (b). The corresponding electrochemical double-layer capacity (C ) values of Cr-Co O and Co O (c).\nFigure R7. LSV curves normalized by the ECSA of Cr-Co O and Co O .\nAccordingly, the Figures R6 and R7 have been updated in the Revised Supporting Information as Figures S14 and S15. The related information has been updated in the Revised Manuscript as follow:\n“It can be calculated that the C value of Cr-Co O and Co O is 5.67 mF cm-2 and 4.17 mF cm-2, respectively (Figure S14). As shown in Figure S15, the current density normalized by ECSA of Cr-Co O is also higher than that of Co O , suggesting the enhancement of intrinsic activity.” (Page 7 in the revised manuscript)\nComment 3: In the manuscript page 4 (line 113-114), authors state that ‘As shown in Figure 1d, the high resolution XPS of Co 2p for Cr-Co O and Co O catalysts reveal that with the introduction of Cr, the ratio of Co2+/Co3+ is reduced, implying the decreased valance state of Co sites in Cr-Co O ’. If the Co2+/Co3+ is reduced, the average Co valence should increase (a larger Co3+ fraction). Moreover, the XPS Co 2p spectra appear to indicate a higher Co3+ ratio in Cr-Co O compared to Co O , which seems to contradict the paper's assertion. In addition, please add a table summarizing quantitative Co2+ and Co3+ fraction with numerical values.\nResponse: We appreciate the reviewer for the thoughtful comments on this work. The reviewer’s suggestions help us to substantially improve the quality of manuscript. We sincerely apologize for the confusion caused by our inadvertent oversight. In response, we have carefully reprocessed the relevant experimental data to ensure its accuracy. As depicted in Figure R8, in contrast to Co O , the binding energy of the main peak of Co 2p for Cr-Co O shifts to lower binding energy, indicating the lower valence state of Co on the Cr-Co O surface. In addition, the corresponding peak position, peak area, and full widths at half maximum (FWHM) of the Co 2p XPS spectra for Cr-Co O and Co O are summarized in Tables R1 and R2, respectively.\nFigure R8. The XPS spectra of Co 2p for Cr-Co O and Co O .\nTable R1. The corresponding values of peak position, peak height, peak area and FWHM of Co 2p XPS spectrum for Cr-Co O .\nCr-Co O Co 2p Co 2p\nCo3+ Co2+ Co3+ Co2+\npeak position 779.7 eV 781.5 eV 794.7 eV 796.5 eV\nPeak area 391302.7 427205.8 195131.6 213648.7\nFWHM 2.3 3.6 3.6 2.3\nTable R2. The corresponding values of peak position, peak area and FWHM of Co 2p XPS spectrum for Co O .\nCo O Co 2p Co 2p\nCo3+ Co2+ Co3+ Co2+\npeak position 780.0 eV 781.8 eV 795.1 eV 796.8 eV\nPeak area 458924.0 440138.1 228321.6 220363.3\nFWHM 2.3 3.6 3.6 2.3\nAccordingly, the Figure R8 has been updated in the Revised Manuscript as Figure1d and the Tables R1 and R2 have been updated in the Revised Supporting Information as Tables S2 and S3. The related information has been updated in the Revised Manuscript as follow:\n“As shown in Figure 1d, the high-resolution XPS spectra of Co 2p for the Cr-Co O and Co O catalysts reveal that the introduction of Cr causes Co 2p peak shifting to a lower binding energy compared to that of pristine Co O . This shift suggests a decreased average valence state of Co in the Cr-Co O catalyst.[30] In addition, the corresponding peak position, peak area, and full widths at half maximum (FWHM) of the Co 2p XPS spectra for Cr-Co O and Co O are summarized in Tables S2 and S3, respectively.” (Page 3 in the revised manuscript)\nComment 4: In this manuscript, the long-term stability test of PEMWE device was operated with an ultrapure water electrolyte. However, most PEMWE devices operate under acidic electrolytes, and many of the comparators listed in Table S3 appear to evaluated under acidic conditions. Please validate the PEMWE device with Cr-Co O under acidic electrolyte condition, and update Table S3 to explicitly include the electrolyte identity and concentration.\nResponse: Thanks for your constructive suggestion. From the perspective of current development trends, the fundamental aim for both proton exchange membrane water electrolysis (PEMWE) and anion exchange membrane water electrolysis (AEMWE) are to achieve operation using pure water as the electrolyte. This approach makes the overall system simpler, more reliable, and more cost-effective, thereby expanding its potential for commercial application. Currently, with the mature development of proton exchange membrane technology, PEMWE can now achieve pure water electrolysis (Figure R9 shows the components of a PEMWE device). However, due to the lower OH- conductivity and poorer cationic group stability of the anion exchange membranes used, AEMWE is currently insufficient to independently support efficient and stable water electrolysis without relying on external alkaline electrolytes.\n[FIGURE REDACTED]\nFigure R9. Components of a PEMWE. BPP, bipolar plate; GDL, gas diffusion layer; CL, catalyst layer; PTL, porous transport layer (Nat. Nanotechnol., 2024, 19, 1074-1076).\nConsequently, researchers have directed substantial efforts toward developing high-performance catalysts, a pursuit critical for enabling PEMWE to meet or surpass the U.S. Department of Energy (DOE)’s targets (Science, 2025, 387, 791-796; Science, 2024, 384, 1373-1380; Science, 2025, 387, 48-55; Nat. Commun., 2025, 1, 801; Nat. Commun., 2025, 16, 3976; Nat. Commun., 2024, 1, 10315; Nat. Nanotechnol., 2024, 19, 1074-1076). Furthermore, it is essential to emphasize that the performance benchmarks for PEMWE are specifically evaluated under pure water operating conditions.\nIn this manuscript, we have fabricated noble metal-free catalyst (Cr-Co O ) that demonstrates outstanding acidic OER performance both in standard three-electrode testing system and in operating PEMWE devices. We also have reassessed the electrocatalytic performance of Cr-Co O in PEMWE, as shown in Table R3.\nTable R3. Comparison of PEMWE performance employed in this study with other electrocatalysts.\nCatalyst temperature Cell voltage Stability Reference\nCr-Co O 60 ℃ 2 A cm-2/2.29 V 1 A cm-2/750 h This work\nMn-Co O @CN 80 ℃ ~ 0.2 A cm-2/50 h [2]\nCo O F 50 ℃ ~ 0.1 A cm-2/120 h [14]\nVo-Co O HNCs 80 ℃ 1 A cm-2/1.82 V 1 A cm-2/14 h [15]\nCWO-del-48 80 ℃ 1.8 A cm-2/2 V 1 A cm-2/608 h [16]\nCWO-del-48 80 ℃ 1 A cm-2/1.77 V 0.2 A cm-2/278 h [16]\nW-Co O 50 ℃ 1 A cm-2/1.83 V 1 A cm-2/240 h [17]\nW-Co O 50 ℃ 2 A cm-2/2.04 V 2 A cm-2/50 h [17]\nLaMn@Co-ZIF 60 ℃ 2 A cm-2/2.47 V 0.21 A cm-2/100 h [18]\nLaMn@Co-ZIF 60 ℃ 4 A cm-2/3.00 V 0.1 A cm-2/60 h [18]\nNi(5%)Co MnO 80 ℃ 1 A cm-2/2.3 V 0.5 A cm-2/110 h [19]\nLCM 80 ℃ 2 A cm-2/2.32 V 2 A cm-2/1200 h [20]\nCoFeNiMoWTe 70 ℃ 1 A cm-2/1.81 V 1 A cm-2/100 h [21]\nN-HECGs\nAccordingly, the Table R3 has been updated in the Revised Supporting Information as Table S5.\nComment 5: To definitively investigate the origin of Cr-Co O long-term stability, it is recommended to confirm the metal leaching amount following long-term operation evaluation. Furthermore, add ex-situ XPS data to verify the peroxidation and irreversible Co4+ species in Co O samples after OER behaviour.\nResponse: Thanks for the reviewer’s valuable comment. The dissolution of Co and Cr in Cr-Co O was investigated after 100 hours of operation under acidic OER conditions. As shown in Figure R10, after the stability test of 100 hours in 0.5 M H SO electrolyte, the dissolution concentrations of Cr and Co for Cr-Co O were determined to be approximately 4.51 ppm and 25.85 ppm, respectively.\nFigure R10. The dissolution concentrations of Cr and Co for Cr-Co O after the stability test of 100 hours in 0.5 M H SO electrolyte.\nWe also performed the XPS characterization of Co O after the long-term stability test (a-Co O ) to probe the valance state changes of Co active sites. As demonstrated in Figure R11, the Co 2p peak of a-Co O shift to a higher binding energy compared to that of Co O , indicating the further oxidation of Co active sites.\nFigure R11. The XPS spectra of Co 2p for Co O and a-Co O .\nAccordingly, the Figures R10 and R11 have been added in the Revised Supporting Information as Figures S19 and S24. The related information has been updated in the Revised Manuscript as follow:\n“The dissolution of Co and Cr in Cr-Co O was investigated after 100 h of operation under acidic OER conditions. As shown in Figure S19, after the stability test of 100 h in 0.5 M H SO , the dissolution concentration of Cr and Co for Cr-Co O was determined to be approximately 4.51 ppm and 25.85 ppm, respectively.” (Page 7 in the revised manuscript)\n“In addition, when remove the applied potential, the valance state of Co is still much higher than the pristine state.” (Page 9 in the revised manuscript)\n“Furthermore, XPS characterization reveals that the valence state of Co in Cr-Co O after the stability test (a-Cr-Co O ) undergoes minimal change (Figure S23). In contrast, the Co 2p peak of the post-test Co O (a-Co O ) shifts to a higher binding energy relative to its pristine state, indicating oxidation of the Co active sites (Figure S24). This finding is consistent with the in situ XAS results.” (Page 9 in the revised manuscript)\nComment 6: Generally, in operando XANES, a larger increase in oxidation state upon voltage application indicates a more efficient oxidation reaction. However, further clarification is required regarding the fact that the increase in oxidation state of cobalt, the main activation site (octahedral site) in Cr-Co O , remains almost unchanged. Rather, the behavior of OH- attaching to the Cr surface may indicate a change in the active site. Furthermore, a clear explanation is required as to whether the observed increase in oxidation state is sufficient to be termed overoxidation.\nResponse: Thanks for the reviewer’s valuable comment. Currently, among non-precious metal-based catalysts, only cobalt- and manganese-based systems have demonstrated practical capability for the acidic OER in PEMWE. In this context, Co O -based electrocatalysts have shown promising activity for the OER in acidic electrolytes and have thus garnered considerable recent attention. However, the widespread application of Co O is hampered by its limited stability due to the inevitable dissolution of cobalt ions under acidic conditions. More critically, Co O undergoes significant surface reconstruction during OER, leading to the formation of soluble and deactivated high-valence Co4+ species. This process is a primary cause of structural collapse. Therefore, stabilizing the lattice and electronic structures of Co O presents a promising strategy for enhancing its acidic OER performance.\nIn this manuscript, we have fabricated Cr-Co O catalyst with the formation of Cr -O-Co coordination environment and Cr3+-OH species modification on surface. It is crucial to note that the active sites in Cr-Co O remain centered on Co, given that Cr O itself is inactive toward the acidic OER, as corroborated by Figure R12. In addition, the characteristic binding energies for Cr3+ and Cr6+ species are assigned to approximately 576 eV and 580 eV, respectively. The in situ XPS spectra of the Cr 2p core level in Cr-Co O show no discernible signal at 580 eV, ruling out the over-oxidation of Cr3+ to Cr6+ under operating conditions (Figure R13).\nFigure R12. The LSV curve of Cr O .\nFigure R13. In situ XPS measurements of Cr 2p for Cr-Co O in the potential range of 1.0 V to 1.8 V.\nAccordingly, the Figure R12 has been added in the Revised Supporting Information as Figure S13. The related information has been updated in the Revised Manuscript as follow:\n“In addition, the activity of Cr O was evaluated, indicating that Cr O itself is inactive toward the acidic OER (Figure S13).” (Page 7 in the revised manuscript)\nThe related information has been updated in the Revised Supporting Information as follow:\n“chromium oxide (Cr O , Shanghai Macklin Biochemical Technology Co., Ltd. 99.95%)”\nComment 7: In Figure 4b, XPS Cr 2p shows negative shifts upon potential increase, contrary to the statement in line 251 of the manuscript: “the Cr 2p XPS spectra reveal a positive binding energy shift of the main peak for Cr-Co O with increasing applied potential.”.\nResponse: Thanks for the reviewer’s valuable comment. As depicted in Figure R14, the Cr 2p binding energy increases from 576.2 eV at 1.0 V to 576.6 eV at 1.8 V. This positive shift indicates that the main Cr 2p XPS peak for Cr-Co O shifts to higher binding energy with increasing applied potential.\nFigure R14. In situ XPS measurements of Cr 2p for Cr-Co O in the potential range of 1.0 V to 1.8 V.\nThe related information has been updated in the Revised Manuscript as follow:\n“In contrast, the Cr 2p binding energy increases from 576.2 eV at 1.0 V to 576.6 eV at 1.8 V. This positive shift indicates that the main Cr 2p XPS peak for Cr-Co O shifts to higher binding energy with increasing applied potential.” (Page 9 in the revised manuscript)\nComment 8: The authors consistently argue that the enhancement in water dissociation in catalyst-electrolyte interface are originated from OH* on Cr site. However, the DFT calculation presented for Cr-Co O model without OH* already show a decrease in the Gibbs free energy for the H O dissociation step. To substantiate the claim thar Cr-OH* is the origin of the enhancement of water dissociation, please include DFT calculations that explicitly incorporate pre-adsorbed OH* on Cr, and compare the full reaction profiles with vs without OH*.\nResponse: Thanks for the reviewer’s valuable comment. We apologize for our unclear statements, which may have caused a misunderstanding for the reviewer. The DFT calculations presented in this manuscript were performed on the models of Cr-Co O with OH* adsorbed on the Cr site (denoted as Cr-Co O ) and Co O , respectively. To substantiate the claim thar Cr-OH* is the origin of the enhancement of water dissociation, we have provided the DFT calculations on the model of Cr-Co O without the OH* adsorbed on the Cr site (Cr -Co O ) (Figure R15). As shown in Figure R16, the water dissociation energy of Cr-Co O with the OH* adsorbed on the Cr site is much lower than that of Cr-Co O without the OH* adsorbed on the Cr site, suggesting the introduction of Cr-OH* can be favorable for the water dissociation process.\nFigure R15. (a) and (b) are the top view and side view of Co O (311) surface with Cr atom substitutes the Co site (Cr -Co O ). The blue balls, grey ball, and red balls represent Co atoms, Cr atom, and O atoms, respectively.\nFigure R16. DFT calculated the energy barrier of water dissociation for Cr-Co O with OH* pre-adsorbed on the Cr site (denoted as Cr-Co O ) and Cr-Co O without OH* pre-adsorbed on the Cr site (denoted as P-Cr-Co O ).\nAccordingly, the Figures R15 and R16 have been added in the Revised Supporting Information as Figures S35 and S39. The related information has been updated in the Revised Manuscript as follow:\n“The calculated model of Co O was shown in Figure S34. The different substitution sites of Cr into Co O were considered. As shown in Figures S35 and S36, the calculated formation energy of Cr -Co O and Cr -Co O is -2.00 eV and -1.12 eV, respectively. This indicates the structure of Cr in the Co position of Co O lattice possesses much lower energy, suggesting the Cr elements preferentially occupy the Co position in the lattice of Co O , in good agreement with the previous analysis. In conjunction with the fact that Cr3+-OH surface species exists under realistic catalytic conditions, the structure depicted in Figure S37 was chosen for modeling the Cr-Co O catalyst with OH* pre-adsorbed on the Cr site (denoted as Cr-Co O ).” (Page 12 in the revised manuscript)\n“Furthermore, DFT calculations on the model of Cr -Co O were also provided. As shown in Figure S39, the water dissociation energy of Cr-Co O is much lower than that of Cr -Co O , suggesting the introduction of Cr-OH* can be favorable for the water dissociation process.” (Page 12 in the revised manuscript)"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have fully revised the manuscript based on the detailed comments. The quality and novelty of the work are high which can be considered in Nature Commun. In light of that, I am happy to accept the work.", "reviewer_2": "The authors have satisfactorily addressed all concerns raised during the review process; therefore, I recommend acceptance of the manuscript."}, "author_response": ""}]

Decision letter