Engineering Co-ion vacancy in dynamically reconstructed Co-based catalysts for practical anion-exchange membrane electrolysis

Domain: chemistry
Published: 2026-02-17
Editorial outcome: minor_revision

Abstract
Precisely engineering metal-ion vacancy in dynamically reconstructed electrocatalysts provides an attractive symmetry-breaking tool to develop robust electrocatalysts under operation conditions. Herein, we demonstrate a convenient Sr-ion-mediated reconstruction strategy to fabricate Co-vacancy-enriched Sr-CoOOH nanosheets under oxygen evolution reaction conditions with balanced activation and refilling of lattice oxygen for practical anion-exchange membrane water electrolysis. The Sr-ion substitution in pre-catalysts can weaken bond strength for facilitated Co-ion vacancies formation in reconstructed oxyhydroxides. These Co-ion vacancies not only strengthen Co-O covalency for lattice oxygen activation, but also improve hydroxyl affinity for lattice oxygen refilling. The corresponding water electrolyzer exhibits a cutting-edge current of 3.3 A cm-2 at 2.0 V with a lowered energy consumption of 43.5 kWh kg-1 H2, and negligible degradation of 0.10 mV h-1 for 1000 h. This work paves an enlightening guideline to finely engineer metal-ion vacancy in dynamically reconstructed electrocatalysts, demonstrating the feasibility to develop targeted catalysts under practical conditions.

Introduction
The anion-exchange membrane water electrolysis (AEMWE) is becoming highly attractive for industrial hydrogen production due to its facility of combining advantages of alkaline water electrolysis (AWE) and proton-exchange membrane water electrolysis (PEMWE), such as employing earth-abundant catalysts and affording high operating currents1–3. Nevertheless, the industry-scale application of AEMWE is largely limited by the lack of non-noble metal-based catalysts with both high activity and stability for sluggish oxygen evolution reaction (OER) at anode4,5. Although several non-noble 3d metal-based materials have been reported to be active relative to commercial Ir- and Ru-based oxides, most of them still suffer from dynamic reconstruction with severe degradation under long-term OER conditions6–8. Specially, cobalt oxyhydroxide (CoOOH), known as the real catalytic phase reconstructed from pristine Co-based materials under OER conditions9–11, has been recently identified as one of the most promising OER catalysts for practical utilization with a relatively slow degradation rate and adjustable activity12,13. To overcome the theoretically limited OER overpotential of 370 mV in traditional adsorbate evolution mechanism (AEM)14–16, activation of lattice oxygen by improving metal-oxygen covalency has been revealed to trigger lattice-oxygen oxidation mechanism (LOM) for activity enhancement17–19. However, the generated oxygen deficiencies due to the participation of lattice oxygen always render the dissolution of adjacent lattice metal with structure collapse, leading to the degraded performance of catalysts under LOM route for long-term utilization20–22. Accordingly, it is challenging and fundamentally important to develop well-reconstructed Co-based catalysts under OER conditions with balanced activation and refilling of lattice oxygen for practical AEMWE.

As a typical symmetry-breaking structure, the construction of metal-ion vacancies has been recently recognized to make an adjustment in the local atomic configuration of metal-based catalysts with altered charge density gradient, thus resulting in distinct catalytic behaviors23–25. For instance, Wang and co-workers demonstrated that the La-ion vacancy in LaFeO3 can greatly enhance the mobility of lattice oxygen due to the decreased electron density, subsequently reducing the barrier for C-H cleavage and oxidation of methane to generate syngas26. Of note, almost all the current efforts have been made to develop strategies to fabricate metal-ion vacancies in as-synthesized nanomaterials, while the inevitably dynamic reconstruction of metal-based catalysts under operation conditions could rebuild the local atomic structures with vanished vacancy sites27–29. For example, Magnussen et al. observed the dynamic reconstruction of Cu catalysts from low-coordinated Cu nanoclusters into isolated Cu atoms with increased potentials during electrochemical reduction of CO2 (CO2RR)30. Yang et al. proved the electrochemical reconstruction of Cu single atoms in C3N4 to Cu nanoparticles as the active phase for C2+ products during CO2RR31. As such, the precise design of metal-ion vacancies in dynamically reconstructed metal-based catalysts deserves to be investigated, which could serve as a versatile tool to develop high-performance catalysts under practical conditions.

Here, we demonstrate a facile Sr-ion-mediated reconstruction strategy to synthesize Co-vacancy-enriched Sr-CoOOH nanosheets under OER conditions with balanced activation and refilling of lattice oxygen as efficient and stable anodic catalysts for practical AEMWE. Mechanistic investigations unveiled that Sr-ion substitution in selenide pre-catalysts could largely weaken the bond strength for accelerated reconstruction under OER conditions, contributing to the generation of abundant Co-ion vacancy in reconstructed oxyhydroxides. These Co-ion vacancies not only strengthened valence-electron sharing between Co and O sites with enhanced covalency for lattice oxygen activation with triggered LOM route, but also promoted valence-electron interaction between Co sites and OH adsorbates with enhanced hydroxyl affinity for lattice oxygen refilling during LOM route. Accordingly, these catalysts served as promising anodic OER catalysts for AEMWE. Relative to CoOOH and commercial RuO2, the Co-vacancy-enriched Sr-CoOOH nanosheets showed an 18.6-fold and 25.0-fold increase in OER currents, respectively, as well as a significantly promoted OER stability with only 2% degradation in overpotentials after 10000 accelerated potential cycles. Practical testing in AEMWE presented a cutting-edge cell voltage of 2.0 V to achieve 3.3 A cm−2 with a lowered energy consumption of 43.5 kWh kg−1 H2, and a good operation stability with a negligible degradation rate of 0.10 mV h−1 for 1000 h, outperforming commercial RuO2. The competitive AEMWE performance further identified the compatibility of Co-vacancy-enriched Sr-CoOOH for practical utilization.

Results
To begin with, Sr-ion-substituted CoSe2 nanobelts with weakened bond strength (denoted as Sr-CoSe2-w) were synthesized and serve as the pre-catalysts before electrochemical reconstruction (Fig. 1a). As theoretically calculated in the well-established Sr-ion-substituted CoSe2 and pure CoSe2 models (Supplementary Fig. 1), the Co-Se bond length around Sr sites exhibited an obvious increase (Supplementary Fig. 2a), together with the decreased Se vacancy formation energy with Sr substitution (Supplementary Fig. 2b), suggesting the weakened Co-Se bonding by Sr substitution. These selenides possessed smooth and belt-like morphology with uniform distribution of Co, Se, and Sr, as identified by transmission electron microscopy (TEM, Fig. 1b), and high-angle annular dark-field scanning TEM (HAADF-STEM) with elemental mapping (Fig. 1c). The Sr/Co molar ratio was determined as 1.0% by inductively coupled plasma-optical emission spectrometry (ICP-OES). The aberration-corrected HAADF-STEM image clearly suggested the well-maintained cubic CoSe2 structure of 1.0%Sr-CoSe2-w32, where the Sr-ion substitution can be confirmed by the intensity profiles of selected region (Fig. 1d). Similarly, by varying added amounts of precursors, 0.6%Sr-CoSe2-w, 0.2%Sr-CoSe2-w and pure CoSe2 pre-catalysts with uniform morphology and cubic phase were obtained (Supplementary Fig. 3). The Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra and fitting results demonstrated the largely reduced Co-Se coordination from 3.9 for pure CoSe2 to 2.3 for 1.0%Sr-CoSe2-w, as well as the obviously increased Co-Se bond length after Sr substitution (Fig. 1e and Supplementary Table 1). These results clearly suggested the largely weakened bond strength in these selenides through Sr-ion substitution. To deeply understand the weakened Co-Se bond caused by Sr-ion substitution, the valence-electron states were analyzed. The Co 2p X-ray photoelectron spectroscopy (XPS) spectrum for 1.0%Sr-CoSe2-w exhibited two typical peaks at 778.5 eV and 793.4 eV that assigned to Co-Se bond in cubic CoSe233. Obviously, a negative shift of Co 2p peaks to low energy was observed on going from pure CoSe2 to 1.0%Sr-CoSe2-w (Fig. 1f), indicating the enriched valence-electron density of Co sites with Sr-ion substitution. Meanwhile, a positive shift of Sr 3d XPS peaks of Sr-Se bond was also found with increased Sr/Co ratio from 0.2% to 1.0% (Supplementary Fig. 4)34. These results suggested the substituted Sr atoms in selenide pre-catalysts could supply electrons more efficiently than Co atoms to bond Se sites, in accordance with the difference of work functions, thus accounting for the weakened Co-Se bond strength.

The weakened bond strength in Sr-CoSe2-w pre-catalysts endowed a lowered dissociation energy of Co-Se bond relative to that in pure CoSe235,36, which could accelerate the bond dissociation under the attack of OH- ions from alkaline electrolyte with forming low-coordinated Co sites during reconstruction, reasonably facilitating the formation of abundant Co-ion vacancies in the reconstructed oxyhydroxide catalysts (Fig. 2a). To clarify the reconstruction process, in situ Raman analysis was conducted under a series of potential cycles (Supplementary Fig. 5). As shown in Fig. 2b, the Raman spectra of pristine 1.0%Sr-CoSe2-w pre-catalysts showcased three typical peaks at 476, 517 and 683 cm−1 assigned to cubic CoSe2 phase37. As the potential cycles proceeded, these peaks tended to vanish, together with the formation of another two peaks at 498 and 594 cm−1 assigned to Eg and A1g vibrational modes of Co-O in CoOOH (Supplementary Fig. 6)38, as well as the gradually elevated Co oxidation state to ~3 (Fig. 2c), manifesting the complete reconstruction from selenide to oxyhydroxide phase. Relative to the pre-catalysts without Sr-ion substitution, the Sr-ion-mediated reconstruction can be notably accelerated, as proved by the obviously reduced potential cycles for reconstruction. The morphology of reconstructed oxyhydroxides was characterized by TEM (Fig. 2d), where interconnected nanosheets with massive boundaries were observed. Such boundaries could facilitate the generation of low-coordination vacancies in these Sr-ion-substituted oxyhydroxide nanosheets39. For simplicity, we designated them as Sr-CoOOH nanosheets. The STEM image and element mapping exhibited homogeneous distribution of Co and Sr in these nanosheets (Fig. 2e), where the Sr/Co ratio was maintained as 1.0% by ICP-OES. The atomic structure was further visualized by an aberration-corrected HAADF-STEM image (Fig. 2f). The bright spots marked by yellow circles clearly suggested the substituted Sr atoms, while the diffraction rings in the corresponding selected area electron diffraction (SAED) pattern suggested the amorphous structure. The atomic dispersion of these Sr atoms could also be verified by the absence of Sr-Sr coordination in Sr K-edge EXAFS spectra (Fig. 2g). Similar morphology and amorphous structures were also observed in other reconstructed Sr-CoOOH nanosheets with maintained Sr/Co ratios of 0.2% and 0.6%, as evidenced by the TEM images, HRTEM images, XRD patterns and ICP-OES analysis (Supplementary Figs. 7, 8).

To verify the local coordination environment in these Sr-CoOOH nanosheets, Co K-edge EXAFS measurements were conducted (Fig. 2h). All the Sr-CoOOH nanosheets exhibited two main peaks at around 1.5 Å and 2.5 Å, attributing to Co-O and Co-Co bonds for CoOOH, respectively40. Notably, with increased Sr contents, a shrinkage of Co-O bond length was observed, suggesting the enhanced Co-O covalency. The strengthened Co-O covalency could efficiently activate lattice oxygen to trigger the LOM pathway for bypassing the theoretically limited OER overpotential41. Meanwhile, least-squares fittings were performed to quantitatively understand the coordination structure (Supplementary Table 2). As collected in Fig. 2i, the Co-Co coordination numbers decreased from 3.0 for CoOOH to 2.7 for 0.2%Sr-CoOOH, 2.6 for 0.6%Sr-CoOOH, and 2.4 for 1.0%Sr-CoOOH, together with the well-maintained Co-O coordination numbers, clearly demonstrating the formation of abundant Co vacancies by Sr-ion-mediated reconstruction. In addition, such the construction of metal-ion vacancies by Sr-ion-mediated reconstruction could also be extended to other reconstructed metal-based materials (Supplementary Figs. 9, 10 and Table 3), suggesting the Sr-ion-mediated reconstruction method as a useful tool to realize metal-ion vacancies in dynamically reconstructed catalysts.

The valence electron properties of the Sr-CoOOH nanosheets were characterized by XPS analysis. As shown in Fig. 2j, the Co 2p3/2 XPS peaks at 779.9 eV and 781.1 eV corresponded to the Co3+ cations in CoOOH42, while a positive shift of Co 2p peaks to high energy was observed from pure CoOOH to 1.0%Sr-CoOOH. Meanwhile, the Sr 3d XPS spectra, which were divided into two peaks at 133.4 and 135.1 eV for Sr cations43 and in accordance with the cationic nature from Sr K-edge XANES spectra (Supplementary Fig. 11a), showed a negative shift to low energy from 0.2%Sr-CoOOH to 1.0%Sr-CoOOH (Supplementary Fig. 11b). The decreased valence-electron density of Co sites and increased valence-electron density of Sr sites implied enhanced valence-electron interaction of Co-O bonds in Sr-CoOOH (Supplementary Table 4), in accordance with the elevated Co-O covalency. The electron state of O sites was studied by O 1s XPS spectra (Supplementary Fig. 12), which could be fitted into four contributions from O-Co bonds (529.7 eV), OH-Co bonds (530.8 eV), low-coordination O (OL, 531.5 eV) and adsorbed water (H2Oad, 532.1 eV)44. The corresponding contents were collected in Fig. 2k. Of note, an obvious increase of O-Co bonds can be observed from pure CoOOH to 1.0%Sr-CoOOH, which could be attributed to the enhanced Co-O covalency and elevated structure stabilization due to the higher bond dissociation energy of Sr-O bond (454 kJ mol−1) than Co-O bond (368 kJ mol−1)45. The decreased OH-Co bonds and retained low-coordination O consisted with the increased Co vacancies and retained Co-O coordination from CoOOH to 1.0%Sr-CoOOH. Moreover, the increased O contents for water adsorption indicated the elevated affinity of oxygenous species of Sr-CoOOH, which could accelerate the refilling process of lattice oxygen during OER.

The electrocatalytic OER performance of all the Sr-CoOOH catalysts were investigated in O2-saturated 1 M KOH electrolyte. As illustrated by the polarization curves in Fig. 3a, which were all corrected for the ohmic loss (Supplementary Fig. 13), 1.0%Sr-CoOOH exhibited the earliest onset potential and an overpotential as low as 255 mV at 10 mA cm−2, significantly outperforming that of 312 mV for pure CoOOH and 327 mV for commercial RuO2. Notably, 1.0%Sr-CoOOH can achieve the current density of 100 mA cm−2 with a low overpotential of 293 mV (Fig. 3b), which was 5.0, 8.8, 18.6, and 25.0 times higher than that for 0.6%Sr-CoOOH, 0.2%Sr-CoOOH, pure CoOOH, and commercial RuO2, respectively. Meanwhile, the electrochemical active surface area (ECSA) for 1.0%Sr-CoOOH, achieved by the double-layer capacitance (Cdl) measurements (Supplementary Figs. 14, 15), showed only the 1.1, 1.8, and 2.7-fold increase relative to that for 0.6%Sr-CoOOH, 0.2%Sr-CoOOH, and pure CoOOH, respectively. Obviously, the current increase for these samples was much larger than the variation of ECSA, suggesting the remarkably enhanced intrinsic activity. The activity elevation was further verified by the Tafel plot and turnover frequency (TOF) analysis. Figure 3c displays a much lowered Tafel slope of 38 mV dec−1 for 1.0%Sr-CoOOH relative to 0.6%Sr-CoOOH (46 mV dec−1), 0.2%Sr-CoOOH (65 mV dec−1), CoOOH (70 mV dec−1) and commercial RuO2 (117 mV dec−1), indicating the accelerated reaction kinetics. The TOF values for 1.0%Sr-CoOOH also remained higher than those for all the tested samples throughout applied potentials, further implying the intrinsically boosted OER activity (Fig. 3d). Of note, the OER activity of 1.0%Sr-CoOOH was competitive (Supplementary Table 5), and could further afford a more competitive target on the gas diffusion electrode with a significantly lowered overpotential of 198 mV at 10 mA cm−2 and a Tafel slope of 32 mV dec−1 as the efficient Co-based OER catalyst (Supplementary Fig. 16).

The stability of these Sr-CoOOH catalysts under OER conditions were systematically elucidated. To understand the operating stability, we conducted galvanostatic start-stop measurements at 10 mA cm−2 (Fig. 3e). For 1.0%Sr-CoOOH, the voltages at 10 mA cm−2 showed neglectable variation during repeated start-stop tests, implying the attractive stability for practical operation46. The long-term stability was assessed by means of accelerated voltammetry cycling tests (Fig. 3f). In comparison with the significantly degraded activities of pure CoOOH under long-term cycling, the activity of 1.0%Sr-CoOOH showed negligible variation after 10000 potential cycles. As quantitatively summarized in Fig. 3g, the overpotentials for 1.0%Sr-CoOOH to achieve 10 mA cm−2 only exhibited a 2% increase after 10000 scan cycles, much lower than the 35% increase in overpotentials for pure CoOOH and thus indicating the high stability of Sr-CoOOH for long-term utilization. The dissolution of metal ions under different scan cycles was also measured by ICP-OES to further evaluate the stability (Fig. 3h). For pure CoOOH, Co components dissolved obviously after 2000 cycles and showed a 49% decrease in content after 10000 cycles. For 1.0%Sr-CoOOH, these Co components were basically retained during the long-term cycling with only 4% decrease in content after 10000 cycles, clearly suggesting the improved stability of Sr-CoOOH. The structural stability was further demonstrated by the well-maintained Sr/Co molar ratios in 1.0%Sr-CoOOH during long-term cycling, as well as the well-maintained morphology from TEM image (Supplementary Fig. 17a), amorphous phase from HRTEM image (Supplementary Fig. 17b) and XRD patterns (Supplementary Fig. 17c), and stable valence states of Co, Sr, O sites from XPS spectra (Supplementary Fig. 17d–f). The high stability of 1.0%Sr-CoOOH also ensured sufficient exposure of active sites for long-term OER catalysis, as verified by the well-maintained ECSAs during long-term cycling (Supplementary Figs. 18, 19 and Fig. 3i).

Considering the enhanced Co-O covalency can trigger the switch of OER route from AEM to LOM for activity improvement, the reaction mechanism for these Sr-CoOOH catalysts were experimentally evaluated. Figure 4a shows the in situ Raman spectra for 1.0%Sr-CoOOH during the real-time OER process with applied potentials. At the open-circuit potential (OCP), two typical peaks at 498 and 594 cm−1 were visible, in accordance with Co-O stretching of amorphous CoOOH. When the applied potential increased, these two peaks shifted to 477 and 574 cm−1 as the Co-O stretching of CoO247, together with the generation of a new peak at 1064 cm−1 for superoxide O-O bands with the participation of lattice oxygen48, suggesting the occurrence of the LOM route for 1.0%Sr-CoOOH containing the deprotonation and immediate coupling with lattice oxygen. For comparison, potential-dependent Raman spectra for pure CoOOH were also detected (Fig. 4b). The absence of peaks at around 1064 cm−1 indicated the different OER pathway for pure CoOOH from the LOM route for 1.0%Sr-CoOOH. Besides, the variation of Co sites could also be verified by XANES spectra (Supplementary Fig. 20), where the increased oxidation states of Co with applied potentials were clearly observed. Of note, with applied potential back to OCP, both of these samples exhibited a reversed Co oxidation state to ~3, together with the well-maintained coordination structure around Co sites (Supplementary Fig. 21 and Table 6, 7), suggesting the stable Co active sites in these oxyhydroxide catalysts for the OER process. To further identify the OER route, the pH-dependent activities were tested. With pH values decreased from 14.0 to 12.5, the OER activities for 1.0%Sr-CoOOH exhibited a significant decrease (Fig. 4c), while pure CoOOH only showed a tiny variation (Fig. 4d). To precisely clarify the correlation between pH and activity, the proton reaction order on RHE scale (ρRHE) was calculated (Fig. 4e), where the concerted proton-electron-transfer process tends to be zeroth-order49. Notably, the ρRHE value of 0.84 for 1.0%Sr-CoOOH was much larger than that of 0.04 for pure CoOOH, suggesting the triggered LOM route for 1.0%Sr-CoOOH from the AEM route for CoOOH50–52. Furthermore, to track the O-O coupling species generated with a negative charge in a non-concerted proton-electron-transfer LOM route, tetramethylammonium cation (TMA+) was introduced into the electrolyte as a probe due to its specific interaction with negative oxygenated species for hindered OER kinetics53. As shown in Fig. 4f, the 1.0%Sr-CoOOH showed obviously reduced OER activity in 1 M TMAOH relative to 1 M KOH, together with the increased Tafel slope from 38 to 65 mV dec−1, confirming the LOM route occurred with strong binding of TMA+ on O2- species. For pure CoOOH, only a slight change in activities was observed (Fig. 4g), indicating the AEM route. The binding behaviors of TMA+ were also detected by Raman spectra (Fig. 4h). Besides the two peaks at 498 and 594 cm–1 referring to CoOOH phase for both samples, two new peaks at 752 and 954 cm−1 corresponding to TMA+ were only observed on 1.0%Sr-CoOOH under 1.50 V in TMAOH54, further indicating the switched LOM route for 1.0%Sr-CoOOH from AEM route for pure CoOOH (Fig. 4i).

To gain an in-depth understanding on the intrinsic reason for switching pathway with activity enhancement as well as the improvement of stability, we conducted first-principles calculations using DFT. The models of Sr-CoOOH and pure CoOOH were well-established in Supplementary Data 1, Figs. 22, 23, and Table 8, where another Sr-CoOOH flat surface without Co vacancy was also constructed for comparison, and denoted as Sr-CoOOHf. Figure 5a shows the change of valence-electron density of Co sites in these structures. The inhomogeneity of electron loss at each Co sites in pure CoOOH could be attributed to the Jahn-Teller distortion. Relative to pure CoOOH, the Sr substitution in Sr-CoOOHf induced increased electron loss from adjacent Co sites, which could be attributed to the limited valence-electron providing ability of Sr to bond O and thus the increased valence-electron supply from adjacent Co sites. As such, further introduction of Co vacancy would induce extra valence-electron supply from adjacent Co sites to bond O, which was also suggested by the extra electron loss of Co sites in Sr-CoOOHv. The increased valence-electron sharing between Co and O could reasonably shorten the adjacent Co-O bonds (Supplementary Fig. 24) with enhanced covalency, in accordance with the XPS and EXAFS results. The enhanced Co-O covalency was further verified by the projected density of states (pDOS) with averaging the potential Co and O sites (Supplementary Fig. 25). As presented in Fig. 5b, relative to pure CoOOH, the Sr substitution in Sr-CoOOHf could diminish the difference between Co 3d band center (εd) and O 2p band center (εp) from 0.335 eV to 0.167 eV, indicating the partially promoted covalency. Meanwhile, with further introduction of Co vacancy in Sr-CoOOHv, the d band of Co and p band of O exhibited a further shortened band-center difference of 0.127 eV, indicating that the Co vacancy could enhance the Co-O covalency and thus lead to the efficient lattice oxygen activation for triggering the LOM pathway. The reaction sites for LOM could be further verified by calculating the energy difference of the potential-determining step (PDS, Supplementary Fig. 26). In view of the obvious difference of step I between AEM and LOM pathways (Fig. 5c and Supplementary Fig. 27), the activation barriers from the AEM intermediate of adsorbed O after step I to the LOM intermediate after step I for O-O coupling were then assessed to clarify the switch difficulty of OER pathways. As shown in Fig. 5d, the energy barrier from AEM intermediate to the isomeric LOM intermediate after step I was 0.51 eV for Sr-CoOOHv, obviously lower than that of 1.52 eV for Sr-CoOOHf and 1.68 eV for pure CoOOH (Fig. 5e and Supplementary Fig. 28), and thus indicating the facilitated switch of OER route from AEM to LOM for Sr-CoOOHv with rich Co vacancies. The switched route from AEM to LOM because of the lattice oxygen activation could boost the theoretically plagued OER kinetics by overcoming the linear relation of OH* and OOH* with direct O-O coupling55. To clarify the role of the altered OER pathway in activity, the relevant Gibbs free energies for reaction steps on Sr-CoOOHv and CoOOH were further depicted. For both structures in Fig. 5f, g, the deprotonation of OH* (step I) in the AEM route or the deprotonation of OH* with immediate O-O coupling (step I) in the LOM route could serve as the potential-determining step. For CoOOH, the energy barrier of 2.01 eV of PDS in the AEM route was lower than that of 2.63 eV in the LOM route, suggesting the OER process follows the AEM route. A similar AEM route could also be observed for Sr-CoOOHf (Supplementary Fig. 29), identifying the deficient activation of lattice oxygen only by Sr substitution. In contrast, for Sr-CoOOHv, the PDS energy in the AEM route was higher than that in the LOM pathway, even under varied U values (Supplementary Fig. 30), reflecting the switched LOM route for Sr-CoOOHv. Moreover, the well-established Sr-CoOOHv exhibited an obviously lowered PDS energy barrier of 1.58 eV than that of 2.01 eV for pure CoOOH, as well as those for the other configurations with varied position of Co vacancy sites (Supplementary Figs. 31, 32), indicating the intrinsically accelerated OER process for Sr-CoOOHv with switched LOM route.

Considering the release of lattice oxygen involving in the LOM route would dynamically generate oxygen deficiencies in catalysts and result in structural collapse, it is of significant importance to efficiently refill these lattice oxygen deficiencies during the LOM route for long-term stability56. Accordingly, the behaviors for lattice oxygen refilling in the LOM route on Sr-CoOOHv and CoOOH were analyzed. Relative to pure CoOOH, the Co-Co coordination missing in Sr-CoOOHv with abundant Co vacancies and Sr substitution with limited valence-electron providing ability could narrow the d band of adjacent Co sites due to the reduced Co d-band overlap, as verified by the upshifted d-band center (Fig. 5b) and the calculated d-band width57, and thus leading to the strengthened OH adsorption affinity for Sr-CoOOHv. To clearly understand the behaviors for lattice oxygen refilling in the LOM route, the electron density differences of intermediate structures of Sr-CoOOHv and CoOOH for lattice oxygen refilling with OH adsorbates were analyzed. As illustrated in Fig. 5h, relative to pure CoOOH, the Co vacancy in Sr-CoOOHv could trigger an enhanced valence-electron interaction between adjacent Co sites and OH adsorbates to refill the lattice oxygen deficiency generated during the LOM route, indicating the promoted lattice oxygen refilling ability of Sr-CoOOHv with Co vacancies. The efficient lattice oxygen refilling was further verified by the energy barrier calculation (Fig. 5i). Notably, the Sr-CoOOHv still exhibited a lower energy barrier of 0.79 eV for incorporating O into the lattice than that of 1.20 eV for pure CoOOH, suggesting the efficient refilling ability of lattice oxygen of Sr-CoOOHv and thus the strengthened OER stability. The structure stability under OER conditions could be further proved by the electrochemical phase diagram58–60, where Sr-CoOOHv could exhibit obviously enhanced stability with much-lowered formation free energy with respect to pure CoOOH (Supplementary Fig. 33).

The boosted activity with efficient lattice oxygen activation and strengthened stability with efficient lattice oxygen refilling of Sr-CoOOHv suggest its attractive potential for realistic utilization as anodic OER catalysts. To this end, we conducted the tests in an anion-exchange membrane electrolyzer at 80 °C with 1 M KOH as feed and commercial Pt/C as cathodic catalysts (Supplementary Figs. 34, 35). Polarization curves were collected in Fig. 6a. Notably, the AEMWE with 1.0%Sr-CoOOHv anode achieved a remarkably high current density of 3.3 A cm−2 with a much-lowered voltage of 2.0 V, clearly outperforming the pure CoOOH anode with a current density of 1.9 A cm−2 at 2.0 V, as well as the commercial RuO2 anode with a current density of 0.63 A cm−2 at 2.0 V. Moreover, at 1.0 A cm−2, the AEMWE with 1.0%Sr-CoOOHv anode produced 1.78 L of H2 at the running electrolyzer for 1 h. The energy consumption of the AEMWE device was deduced to be 43.5 kWh kg−1 H2 (Fig. 6b), obviously lower than that of CoOOH and the goal of 47.9 kWh kg−1 H2 from the Office of Energy Efficiency & Renewable Energy (EERE) in the US Department of Energy (DOE). The Faradaic efficiency of O2 product in the AEMWE device with 1.0%Sr-CoOOHv anode was quantified as a near unity value of 99.3% (Fig. 6c), suggesting the entire consumption of charge without parasitic reactions. To further confirm the applicability of the electrolyzer assembled with 1.0%Sr-CoOOHv, the operating stability measurements were performed in AEMWE at a constant industrial-level current density of 500 mA cm−2 under 80 °C. As shown in Fig. 6d, the AEMWE with 1.0%Sr-CoOOHv anode presented a well-maintained performance during 1000 h electrolysis with a negligible degradation rate of 0.10 mV h−1, significantly lower than that of 1.33 mV h−1 during 95 h electrolysis on a commercial RuO2 anode and implying the improved stability. The overall performance of the AEMWE device with 1.0%Sr-CoOOHv anode, including current density at 2.0 V, operating time and degradation rate, were competitive with using a commercial anion-exchange membrane and a commercial Pt/C cathode (Supplementary Table 9).

Methods
Chemicals
Cobalt(II) acetate tetrahydrate (Co(AC)2·4H2O, ≥99.5%), strontium acetate (Sr(AC)2, 99%), sodium selenite (Na2SeO3, ≥97.0%), isopropanol (C3H8O, ≥99.7%), ethanol (C2H6OH, ≥99.7%), diethylenetriamine (DETA, ≥98.5%), and potassium hydroxide (KOH, ≥85.0%) were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Ruthenium dioxide (RuO2, 99.9%) and Tetramethylammonium hydroxide solution (TMAOH, 25 wt% in H2O) were obtained from Sigma-Aldrich. Ultrapure water (18.2 MΩ) was used in all the experiments.

Materials preparation
The Sr-CoOOHv nanosheets rich in Co vacancies were prepared in two steps. To begin with, the Sr-CoSe2-w nanobelts were synthesized as pre-catalysts by a facile hydrothermal method. In brief, 247.1 mg of Co(AC)2·4H2O, 1.62 mg of Sr(AC)2, and 173 mg of Na2SeO3 were dispersed in 40 mL of mixed solution containing DETA and DI water with a volume ratio of 2:1. The mixture was heated to 180 °C for 16 h to obtain the 1.0%Sr-CoSe2-w samples. Sr/Co atomic ratios were controlled by varying the amount of two precursors without changing other conditions. Specifically, we added 249 mg of Co(AC)2·4H2O for pure CoSe2, 248.7 mg of Co(AC)2·4H2O and 0.32 mg of Sr(AC)2 for 0.2%Sr-CoSe2-w, as well as 247.8 mg of Co(AC)2·4H2O and 0.97 mg of Sr(AC)2 for 0.6%Sr-CoSe2-w, respectively. Then, for the preparation of Sr-CoOOHv nanosheets, we performed the electrochemical reconstruction of selenide pre-catalysts in O2-saturated 1 M KOH solution via a typical cyclic voltammetry cycling method between 1.02 and 1.5 V (vs. RHE) with a sweep rate of 50 mV s−1 and forty potential cycles. After being washed with DI water and ethanol, Sr-CoOOHv nanosheets were obtained.

XAS characterization
The X-ray absorption spectroscopy (XAS) was collected from the TLS 13B2 beamline of the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. The storage of TLS worked at 1.5 GeV with a beam current of 360 mA. The X-ray pre- and post-edge absorptions were normalized to 0 and 1, respectively.

DFT calculations
The first-principles calculations were carried out using the spin-polarized density functional theory (DFT) within the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA), implemented in the Vienna Ab initio Simulation Package (VASP). The projected augmented wave (PAW) method was employed with a plane-wave cutoff energy of 520 eV. To prevent interactions between periodic images, a vacuum layer of approximately 15 Å was introduced along the z-axis in all slab models. The lattice constants and atomic positions were fully relaxed until the forces on each atom were below 0.02 eV/Å, with the total energy convergence criterion of 1×10−5 eV.

For selenide raw structures, the models were constructed by periodically repeating the CoSe2 (210) slab with three layers containing 72 atoms. The Brillouin zone was sampled using a 3×3×1 k-point grid. For pure CoOOH, the CoOOH(001) surface was simulated, which comprises two cobalt oxide layers slab with each layer containing 32 Co, 64 O and 32 H atoms. To account for van der Waals interactions between CoOOH layers, Grimme’s DFT-D3 semi-empirical method was adopted. In the geometric optimization, all atoms except those in the bottom layer were fully relaxed. For improved accuracy in describing Co 3d electrons, a Hubbard U term (Ueff = 3.3 eV) was corrected. For Sr-substituted CoOOH, we simulated the substitution of one Co atom with a Sr atom on the surface. For Sr-substituted CoOOH rich in Co vacancies, different Co vacancy configurations were considered, including nearest neighbor, next-nearest neighbor, and third-nearest neighbor positions relative to the Sr site. Due to the large unit cell and computational limitations, the Brillouin zone was sampled only at the Γ point.

The Gibbs free energy changes for water oxidation steps following the AEM mechanism were calculated as ref. 56
ΔG1 = ΔG(O*) − ΔG(OH*) (1)
ΔG2 = ΔG(OOH*) − ΔG(O*) (2)
ΔG3 = ΔG(OO*) − ΔG(OOH*) (3)
ΔG4 = 4.92 eV − ΔG(OO*) (4)
ΔG5 = ΔG(OH*) (5)

The Gibbs free energy changes for water oxidation steps following the LOM mechanism were calculated as:
ΔG1 = ΔG(OO*) − ΔG(OH*) (6)
ΔG2 = ΔG([OO+HO]*) − ΔG(OO*) (7)
ΔG3 = 4.92 eV + ΔG([Ov+HO]*) − ΔG([OO+HO]*) (8)
ΔG4 = ΔG(OHv*) − ΔG([Ov+HO]*) (9)
ΔG5 = ΔG(OH*) − ΔG(OHv*) (10)

The Gibbs free energy differences of these intermediates include zero-point energy (ZPE) and entropy corrections were calculated as:
ΔG = ΔE + ΔZPE − TΔS (11)

where the energy differences ΔE were calculated with respect to H2O and H2, and the vibrational entropy changes ΔS were calculated at 298.15 K. The detailed correction values were listed in Supplementary Table 10–12. The climbing image nudged elastic band method was used to search the transition state (TS).

Electrochemical measurements
The electrochemical measurements were taken on a standard three-electrode system in 175 mL of 1 M KOH (pH = 14.0 ± 0.01) using a CHI 760E electrochemical workstation by using a graphite rod and Hg/HgO electrode as the counter and reference electrode at room temperature (25 ± 1 °C), respectively. The 1 M KOH electrolyte was prepared freshly with dissolving 56.11 g of KOH in 1 L of DI water. The potentials in this work were referenced to the reversible hydrogen electrode (RHE), and in 1 M KOH, ERHE = EHg/HgO + 0.903 V. The reference electrode calibration was conducted by using a typical cyclic voltammetry (CV) method within a three-electrode system in 1 M KOH saturated with high-purity hydrogen, where two Pt meshes served as the working and counter electrode, respectively. The CV curve was recorded at the sweep rate of 1 mV s−1, and the average value of the two potentials where the current crossed zero was taken as the calibration value.

For the preparation of the working electrode, 5 mg of catalyst powders and 1 mg of Ketjen black (KB) were dispersed in 1 mL mixed solution consisted of isopropanol and 5 wt% Nafion solution. Then 39.2 μL of the ink was dropped onto the glass carbon rotating disk electrode (RDE, 5 mm in diameter) with a mass loading of catalyst of 1 mg cm−2.

Before OER tests, the electrolyte was degassed for at least 30 min by oxygen bubbling. The polarization curves were collected with a sweep rate of 5 mV s−1 at a rotating speed of 1600 rpm, which were all corrected for the ohmic loss. The R values measured for 1.0%Sr-CoOOHv, 0.6%Sr-CoOOHv, 0.2%Sr-CoOOHv, and CoOOH were 7.2 ± 0.5, 6.8 ± 0.2, 6.0 ± 0.3, and 7.0 ± 0.2 Ω, respectively. The accelerated stability tests were performed by potential cycling between 1.2 and 1.48 V (vs. RHE) at a sweep rate of 100 mV s−1. The electrochemical active surface area (ECSA) was measured by using cyclic voltammetry within the potential region of 1.205-1.305 V (vs. RHE) at varied scan rates from 35 to 55 mV s−1. The linear slope was obtained by plotting the charging current density differences (Δj = ja − jc) against scan rates, which is twice of the double-layer capacitance (Cdl) and used to represent ECSA.

The turnover frequency (TOF) was calculated from the equation below:
TOF = 4 × j × F / A × n (12)
where j is the measured current density, A is the surface area of the electrode, F is the Faraday constant, and n is the moles of metal atoms on the electrode calculated from XPS results.

AEMWE measurements
The anion exchange membrane water electrolyzer (AEMWE) was assembled with the anode (Sr-CoOOHv, CoOOH, or RuO2), cathode (commercial Pt/C), and anion exchange membrane (PiperION, 40 μm in thickness). To prepare the anodic catalyst ink, the catalysts were homogeneously dispersed in a mixed solution containing DI water, isopropanol and ionomer under ultrasonication. Then the well-dispersed ink was sprayed on nickel foam with a loading mass of 5 mg cm−2. The cathodic catalyst (Pt/C, 40 wt% loading, Johnson Matthey) ink was prepared in the same method and sprayed on the carbon paper with a loading mass of Pt/C catalysts of 1 mg cm−2. Subsequently, these cathode and anode were sandwiched with the anion exchange membrane in a fixture with the applied torque of 3 N·m. The performance of the AEM electrolyzer was recorded in a continuously circulated 1 M KOH solution at an operation temperature of 80 °C. The stability test was recorded using galvanostatic electrolysis at a constant current density of 500 mA cm−2.

The energy consumption of the AEMWE is calculated by the following:
Energy consumption = Ucell × Icell × t / m (13)
where Icell is the current delivered (A), t is the operation time (h), and m is the mass of hydrogen produced in the operation process. Three independent measurements were conducted to achieve the standard deviation of the energy consumption.

The Faradaic efficiency was calculated from the amount of charge passed through the electrolyzer and the amount of oxygen produced as follows:
Faradaic efficiency = n × F × m / Q (14)
where F is the Faraday constant, n is 4 for the electron transfer in OER process, m is the moles of O2 gas produced at 1 A cm−2, and Q is the amount of charge passed. Three independent measurements were conducted to obtain the standard deviation of the Faradaic efficiency.

In situ Raman measurements
For detecting the dynamic reconstruction process, the in situ Raman spectra were conducted by Raman microscope (RENISHAW inVia Qontor) with 50xL (NA = 0.5) objective and 532 nm laser with 1800 l/mm optical grating. The samples were used as working electrodes, together with Pt wire and Hg/HgO as counter electrode and reference electrode, respectively. The in situ Raman spectra of different potential cycles were recorded 3 times for 10 s to study the reconstructed process. For detecting the reaction route, the in situ Raman spectra of different applied voltages from 1.20 V to 1.50 V were recorded 10 times for 30 s to study the OER process, which was conducted by using the 785 nm laser and 1200 l/mm optical grating.

Characterization techniques
The Talos L120C G2 transmission electron microscope with an acceleration voltage of 120 kV was used to characterize the morphology of samples. The field-emission transmission electron microscope (Talos F200x) with 200 kV acceleration voltage was used to collect HRTEM images, HAADF-STEM images, and elemental mapping images. The AC HAADF-STEM images were achieved by using the spherical aberration corrected JEM-ARM300F2 microscope with 300 kV accelerating voltage. The X-ray diffractometer (XRD-6100, SHIMADZU) with a Cu-Kα radiation source was used to collect XRD patterns of samples. XAS spectra under varied conditions were collected on the XAS spectrometer (Table XAFS-500A, Specreation Instruments Co. Ltd.). XPS spectra were collected by using the X-ray photoelectron spectrometer (K-alpha, Thermo Scientific) with a monochromatic Al Kα X-ray source. ICP-OES (Agilent 5110) was used to detect the content of Co and Sr elements.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a very interesting and highly important work to precisely engineer the metal-ion vacancy in dynamically reconstructed electrocatalysts under practical conditions for high-performance anion-exchange membrane water electrolysis. The designing strategy in this work is technically sound and convenient, and the enhancement in current density and long-term operating stability as AEMWE anode catalysts is impressive. More importantly, the clear understanding of the intrinsic mechanism for metal-ion vacancy control in dynamically reconstructed electrocatalysts as well as its critical role in tuning lattice-oxygen activation and refilling for practical utilization in AEMWE provides a broad guideline for rational design of dynamically reconstructed catalysts under realistic condition with high practical performance. I would like to recommend its publication after addressing the following issues.\n1) To clearly exhibit the variation of valence-electron density of Co and Sr sites in these Sr-CoOOHv nanosheets, it is better to provide a comparison of detailed locations of main peaks in Co 2p and Sr 3d XPS spectra of all the samples.\n2) In view of the important role of in-situ Raman tests in detecting dynamic reconstruction and understanding reaction route, the detailed methods for conducting these in-situ Raman measurements should be provided.\n3) The authors use TMA+ to track the O-O coupling species generated in LOM route, while the M-OO intermediates could also form in AEM route. More details about how it works for identifying LOM route should be presented.\n4) To visually demonstrate the uniform spraying areas of catalyst layer for AEMWE testing, the authors should further provide the photographs of the sprayed catalyst layer for all the tested samples.\n5) A few unit typos should be corrected, like the non-uniform superscript glyph in “mA cm-2” and “mA cm−2”.", "reviewer_2": "In this manuscript, the authors introduced a convenient Sr-ion-mediated reconstruction strategy to fabricate Co-vacancy-enriched Sr-CoOOH nanosheets under oxygen evolution reaction (OER) conditions, which exhibits good catalytic activity for the OER, thus showing great potential for practical anion-exchange membrane water electrolysis (AEMWE). They performed many characterizations, and proposed lattice-oxygen oxidation mechanism for the OER. I think this manuscript is very ordinary topic. The catalytic performance is not so good, and the preparation method is very simple. Especially, no interesting new points have been reported in this work. Therefore, I don't recommend its acceptance in this high-level journal. Before submission, I would suggest the authors considering the following comments.\n1. What's the role of Sr in the oxygen evolution? How did this metal site affect the reconstruction process?\n2. What is the final active site for OER after reconstruction?\n3. The OER performance is so ordinary, not to mention the AEMWE activity.\n4. How did they determine that the catalyst is made of CoOOH, not CoOx or Co(OH)2?\n5. The in situ Raman analysis seem not correct. Especially, the Raman signals are too weak to be used for determining the chemical compounds. So additional structural characterizations are necessary.\n6. The peak of Co-O-Sr in EXAFS spectra is so weak to be detected, which is not so convinced.\n7. The OER performance is very poor, and the method for catalyst preparation is not new. Can they provide some new points on their manuscript?", "reviewer_3": "This paper reports a very interesting approach to nicely engineer Co-ion vacancies in dynamically reconstructed Co-based electrocatalysts under OER conditions for practical AEMWE. The experimental results documenting the control of Co-ion vacancies during reconstruction are very nice, and convincing. The mechanism of these confined Co-ion vacancies in efficiently and stably catalyzing anodic OER for AEMWE is soundly revealed. These results are fundamentally important, which can also help to develop high-performance catalysts under practical conditions for other applications. I strongly recommend publication of this work after minor revision.\n1.Please provide more details about how pH-dependent activities indicate the reaction route of lattice oxygen participation.\n2.In AEMWE section, the authors should clarify what kind of Pt/C is used and the loading mass of Pt on carbon support.\n3.The assembly process for AEMWE tests should be described in more detail, including the sprayed catalyst layer, the applied torque, etc.\n4.In Supplementary Table 3, the last cited reference shows 0.1 M KOH in the “Electrolyte” column, which does not allow for direct comparison among the other listed references with 1 M KOH in the “Electrolyte” column and should be replaced.\n5.Please provide more details on how the in-situ Raman tests are performed, for both the reconstruction process to form oxyhydroxide catalysts and the reaction process with applied OER potentials.\n6.The mechanism of Sr-mediated Co vacancy formation during electrochemical reconstruction requires deeper exploration.\n7.The discussion of LOM activation lacks a direct comparison to recent works on lattice oxygen participation in Co-based catalysts (e.g., refs. 17, 19), which would better situate this study within the field.", "reviewer_4": "In this manuscript the authors propose that Sr incorporation with simultaneous formation of Co vacancies enhances the OER activity of CoOOH formed from CoSe2 under reaction conditions. Concerning the dynamic reconstruction to an oxyhydroxide taking place under operation conditions, the authors cite literature on copper nanocatalysts, but relevant literature that has considered the effect of surface transformation at oxide surfaces into an oxyhydroxide is not cited: e.g. ACS Catal. 2019, 9, 5, 3811–3821; ibid 2022, 12, 6, 3256; Nat. Mater. 20, 674–682, 2021; Nat. Commun. 14, 8284, 2023\nOverall, the manuscript combines experimental and theoretical evidence form several techniques, but a number of aspects need to be considered/clarified:\nAn enhanced covalency is proposed to trigger a switch from AEM to LOM mechanism – but the proof of such enhanced Co-O covalency is only based on the reduction of Co-O bonds, which does not seem to be sufficient.\nIn the following I will concentrate on the DFT results: The authors start by stating that the “DFTmodels of Sr-CoOOHv and pure CoOOH were well-established”, but it is not clear what exactly is well established – eg. what polymorph of CoOOH is used for the calculations? In particular, the data presented in Fig. 5 is not always sufficient and only after consulting the SI some of the statements become clearer. In particular it is not clear what is shown in Fig 5a: what is described as an “electron loss” is presumably modulation of the charge state of the Co sites, but why is this modulation /inhomogeneity stronger for the pristine CoOOH and the least for the Sr-CoOOHv? This seems counterintuitive. Moreover, this figure does not give enough information of the simulation cell as it shows merely the Co positions. The position of oxygen and defects and, most importantly, the variation in bond lengths are missing. These can be found only in Fig 12 in SI, which I find much more suitable to support the results in the main text in contrast to fig. 5a.\nThe authors state that they have considered different Co vacancy configuration but it remains unclear on what grounds the one shown in Fig. 5 is selected, the authrors should provide inforamtion on the relative stability. Moreover, which reaction sites for OER were probed and how do the calculated overpotentials for those vary. Also, a comparison to previous literature on OER at CoOOH surfaces should be added.\nFig. 5b shows pDOS of Co and O sites, but it is unclear whether these are averages over all Co/O sites or specific sites.\nMoreover, only a small energy range of the pDOS of the valence band is displayed, thus e.g. the figure does not provide any information on the charge or spin state of Co. To this end, were any spin-dependent effects considered? Despite the shift of the band centers between the three different structures, the band overlap shown in Fig. 5b does not seem to change, thus it is questionable if the presented data actually indicate a difference in covalency.\n5d is named “step of the coupling of O-O bond” – unclear what process is meant here.\nFigs 5g,h show Gibbs free energies as a function of the reaction coordinate for the LOM and AEM, but it is unclear what each step corresponds to, the specific steps need to be explicitly given on the x-axis of the figures.\nNo references are provided for the AEM and OER mechanisms used in the theoretical part and described in the methods part. Moreover, it seems there is some confusion in using the terms AEM and LOM, as the one shown as AEM specifically involves a lattice oxygen site and corresponds rather to what is considered as LOM in the literature. Overall, it is difficult to see the reaction intermediates from the top views in Fig 5c and Fig 13 from SI, a better view of the intermediates is strongly recommended.\nSince here no barriers are calculated, the term potential determining step should be used instead of rate limiting step.\nFurthermore, in line 276 the authors write activation barriers from the AEM intermediate to LOM intermediate were assessed – can the authors clarify what exactly is meant here?\nFig. 5b is used later on p 11 concerning the affinity to refill oxygen vacancies and it is argued that the d-band of adjacent Co is narrower (hardly visible from the figure) and thus a higher OH adsorption affinity is expected, which is a very handwaving statement, considering the shortcomings of Fig 5b discussed above. Beyond showing electron density (here again it is unclear what view, i.e. which crystallographic orientation is shown in the inset and what are the reference systems and the electron density isolevel), such statements should be based on energetics for incorporating O or OH.\nSome further technical issues, typos to be double checked:\n270 can minish the gap – pls check wording.\nBand-center gap – is here band-center difference meant?\nLine 128: Eg 127 and A1g vibrational models of Co-O in CoOOH – perhaps some typo – I guess vibrational modes are meant"}, "author_response": "Point-by-point response to reviewers’ comments\nManuscript Number: NCOMMS-25-24017\nMS Type: Article\nTitle: \" Engineering Co-ion vacancy in dynamically reconstructed Co-based catalysts for practical\nanion-exchange membrane electrolysis\"\nReviewer #1 (Remarks to the Author):\n“This manuscript presents a very interesting and highly important work to precisely engineer the\nmetal-ion vacancy in dynamically reconstructed electrocatalysts under practical conditions for\nhigh-performance anion-exchange membrane water electrolysis. The designing strategy in this\nwork is technically sound and convenient, and the enhancement in current density and long-term\noperating stability as AEMWE anode catalysts is impressive. More importantly, the clear\nunderstanding of the intrinsic mechanism for metal-ion vacancy control in dynamically\nreconstructed electrocatalysts as well as its critical role in tuning lattice-oxygen activation and\nrefilling for practical utilization in AEMWE provides a broad guideline for rational design of\ndynamically reconstructed catalysts under realistic condition with high practical performance. I\nwould like to recommend its publication after addressing the following issues.”\nWe genuinely thank the reviewer for his/her valuable comments and positive recommendation.\nWe have addressed the reviewer’s comments as follows.\n“1) To clearly exhibit the variation of valence-electron density of Co and Sr sites in these Sr-\nCoOOH nanosheets, it is better to provide a comparison of detailed locations of main peaks in\nv\nCo 2p and Sr 3d XPS spectra of all the samples.”\nAs suggested, we have added the comparison of detailed locations of main peaks in Co 2p and Sr\n3d XPS spectra of all the Sr-CoOOH samples. As shown in Table R1, a positive shift of Co 2p\nv\nXPS peaks to higher energy side was clearly observed from pure CoOOH to 1.0%Sr-CoOOH ,\nv\ntogether with a negative shift of Sr 3d XPS peaks to lower energy side from 0.2%Sr-CoOOH to\nv\n1.0%Sr-CoOOH . Such the decreased valence-electron density of Co sites and increased valence-\nv\nelectron density of Sr sites implied enhanced valence-electron interaction of Co-O bonds in Sr-\nCoOOH , in accordance with the elevated Co-O covalency.\nv\nTable R1. Comparison of main peaks in Co 2p and Sr 3d XPS spectra of all the Sr-CoOOH\nv\nsamples.\nMaterials Co 2p Co 2p Sr 3d Sr 3d\n3/2 1/2 5/2 3/2\nCoOOH 779.9 781.1 782.8 794.9 796.1 797.8 / /\n0.2%Sr-\n780.1 781.3 783.0 795.1 796.3 798.0 133.4 135.1\nCoOOH\nv\n0.6%Sr-\n780.3 781.5 783.2 795.3 796.5 798.2 133.3 135.0\nCoOOH\nv\n1.0%Sr-\n780.5 781.7 783.4 795.5 796.7 798.4 133.2 134.9\nCoOOH\nv\nWe have added the relevant description and the table of XPS fitting results in the revised\nmanuscript and supporting information (please see page 7, line 16-17 & page S35, Supplementary\nTable 4).\n“2) In view of the important role of in-situ Raman tests in detecting dynamic reconstruction and\nunderstanding reaction route, the detailed methods for conducting these in-situ Raman\nmeasurements should be provided.”\nWe thank the reviewer for this suggestion. The relevant description of in-situ Raman tests has been\nadded in the Methods section. For detecting the dynamic reconstruction process, the in-situ Raman\nspectra were conducted by Raman microscope (RENISHAW InVia Qontor) with 50xL (N =0.5)\nA\nobjective and 532 nm laser with 1800 l/mm optical grating. The samples were used as working\nelectrodes, together with Pt wire and Hg/HgO as counter electrode and reference electrode,\nrespectively. The in-situ Raman spectra of different potential cycles were recorded 3 times for 10\ns to study the reconstruction process. For detecting the reaction route, considering the difficulty in\ndetecting these oxygenous intermediates due to their short lifetime and low coverage during\nelectrocatalytic process (Nat. Energy 4, 60-67 (2019)), we have changed a lower energy laser for\nmuch more time to detect the O-O intermediate. The in-situ Raman spectra of different applied\nvoltages from 1.20 V to 1.50 V were recorded 10 times for 30 s to study the OER process, which\nwere conducted by 785 nm laser and 1200 l/mm optical grating.\nWe have added the detailed description in the revised manuscript (please see page 17, line 23-29\n& page 18, line 1-2).\n“3) The authors use TMA+ to track the O-O coupling species generated in LOM route, while the\nM-OO intermediates could also form in AEM route. More details about how it works for\nidentifying LOM route should be presented.”\nAs suggested, we have added the relevant details about the mechanism of using\ntetramethylammonium cation (TMA+) to identify LOM route. Although the O-O coupling species\ngenerated in LOM route seem similar with the M-OO intermediates formed in AEM route at first\nglance, but are completely different. In detail, for the conventional AEM route, the oxygen\nevolution process generally contains a series of concerted proton-electron-transfer steps, where the\nM-OO intermediates were generated with the concerted deprotonation of M-OOH intermediates\nand electron transfer step, thus leading to the non-charged M-OO intermediates. In contrast, due\nto the non-concerted and decoupled proton-electron-transfer process during LOM route (Nat.\nChem. 9, 457-465 (2017)), the formed O-O coupling species from lattice oxygen were in negative\ncharge. As a typical chemical probe, the TMA+ has been well-known to interact with negative\noxygenated species with hindered OER kinetics (Nat. Energy 4, 329-338, (2019)). As such, the\nTMA+ can be used to track the O-O coupling species in negative charge generated in LOM route\nand thus identify the LOM route.\nTo make it more readable, we have supplemented the relevant discussion in the revised manuscript\n(please see page 10, line 8-11).\n“4) To visually demonstrate the uniform spraying areas of catalyst layer for AEMWE testing, the\nauthors should further provide the photographs of the sprayed catalyst layer for all the tested\nsamples.”\nWe thank this reviewer for his/her suggestion. As suggested, we have taken the photographs of the\nsprayed catalyst layer for the tested samples in AEMWE. To prepare the sprayed catalyst layer,\nthe catalysts were firstly dispersed in a mixed solution containing DI water, isopropanol and\nionomer under ultrasonication. Then the well-dispersed ink was sprayed on nickel foam with the\nloading mass of 5 mg cm-2. As shown in Fig. R1, the photographs of the sprayed catalyst layer for\nthe tested catalysts in AEMWE exhibited the uniform layer with a spraying area of 4 cm2.\nFig. R1. The photographs of the sprayed catalyst layer of 1.0%Sr-CoOOH catalysts, pure CoOOH\nv\ncatalysts, and commercial RuO catalysts for AEMWE tests.\nsprayed catalyst layer of samples in the revised manuscript and supporting information (please see\npage 12, line 29 & page S31, Supplementary Fig. 30).\n“5) A few unit typos should be corrected, like the non-uniform superscript glyph in “mA cm-2” and\n“mA cm−2”.”\nWe thank the reviewer for this detailed suggestion. As suggested, we have corrected the few unit\ntypos of superscript glyph in the revised manuscript (please see page 8, line 2,3 & page 12, line 30\n& page 13, line 1,2). Besides, we have also carefully checked the whole contents to avoid any\nother typos. All of the revisions are highlighted in yellow color.\nReviewer #2 (Remarks to the Author):\n“In this manuscript, the authors introduced a convenient Sr-ion-mediated reconstruction strategy\nto fabricate Co-vacancy-enriched Sr-CoOOH nanosheets under oxygen evolution reaction (OER)\nconditions, which exhibits good catalytic activity for the OER, thus showing great potential for\npractical anion-exchange membrane water electrolysis (AEMWE). They performed many\ncharacterizations, and proposed lattice-oxygen oxidation mechanism for the OER. I think this\nmanuscript is very ordinary topic. The catalytic performance is not so good, and the preparation\nmethod is very simple. Especially, no interesting new points have been reported in this work.\nTherefore, I don't recommend its acceptance in this high-level journal. Before submission, I would\nsuggest the authors considering the following comments.”\nWe genuinely thank this reviewer for his/her careful reading of our manuscript and valuable\ncomments to help us improve the quality of our manuscript. The interesting and new points in our\nwork are as follows:\ni) Our work focuses on the topic of precise design of metal-ion vacancies in dynamically\nreconstructed non-noble metal-based electrocatalysts under OER conditions to serve as active and\nstable OER catalysts for AEMWE utilization, which is challenging and fundamentally important\nin the field of developing high-performance catalysts under practical conditions. As a typical\nsymmetry-breaking structure, constructing metal-ion vacancies has been recently recognized to\nmake an adjustment in local atomic configuration of metal-based catalysts with altered charge\ndensity gradient for distinct catalytic behaviors. However, almost all the current efforts have been\nmade to develop strategies to fabricate metal-ion vacancies in as-synthesized nanomaterials, while\nthe inevitably dynamic reconstruction for metal-based catalysts under operation conditions,\nespecially for the non-noble 3d metal-based materials under the highly corrosive alkaline OER\nconditions, could rebuild the local atomic structures with vanished metal-ion vacancies, leading to\nthe uncontrollable catalytic performance. Moreover, the generated oxygen deficiencies due to the\nparticipation of activated lattice oxygen during LOM route of OER could further render the\ndissolution of adjacent lattice metal with structure collapse, leading to the degraded stability under\nLOM route. Accordingly, it is challenging and fundamentally important to finely engineer metal-\nion vacancies in dynamically reconstructed non-noble metal-based catalysts under OER conditions\nwith balanced OER activity and stability for AEMWE utilization.\nii) Our work demonstrates a new Sr-ion-mediated reconstruction method to finely engineer metal-\nion vacancy in dynamically reconstructed oxyhydroxide electrocatalysts under OER conditions.\nWe have indeed searched the relevant researches of fabricating metal-ion vacancies in dynamically\nreconstructed electrocatalysts under practical conditions, while rare work could be found. Our\nwork proved that the Sr-ion substitution in cobalt selenide pre-catalysts could supply electrons\nmore efficiently than Co atoms to bond Se sites to weaken Co-Se bond strength, resulting in the\nmuch-lowered Co-Se bond dissociation energy to accelerate leaching of lattice Se during OER\nreconstruction. Such the accelerated Se leaching could induce the much-lowered coordination of\nadjacent Co sites and thus accelerate the leaching of lattice Co to form abundant Co-ion vacancy\nin reconstructed CoOOH catalysts. In addition, we have also proved in the revised manuscript that\nthis Sr-ion-mediated reconstruction method could be extended to create metal-ion vacancies in\nother dynamically reconstructed metal-based electrocatalysts.\niii) Our work demonstrates the critical role of the well-defined Co-ion vacancy of dynamically\nreconstructed CoOOH catalysts in altering the OER pathway from AEM to LOM route for activity\nenhancement and efficiently refilling lattice oxygen for stability strengthening. As mentioned by\nthe reviewer, we have conducted comprehensive investigations by using experimental\ncharacterizations and DFT calculation to make the above mechanism clear. These compelling\nevidences in our manuscript clearly proved that the well-defined Co-ion vacancy in dynamically\nreconstructed CoOOH catalysts can not only strengthen valence-electron sharing between Co and\nO sites with enhanced Co-O covalency for lattice oxygen activation with triggered LOM route, but\nalso promoted valence-electron interaction between Co sites and OH adsorbates with enhanced\nhydroxyl affinity for lattice oxygen refilling during LOM route, thus leading to the highly active\nand stable OER performance.\niv) Our work demonstrates an outstanding AEMWE performance by using the developed Co-\nvacancy-enriched Sr-CoOOH as anodic OER catalysts. The AEMWE with Co-vacancy-enriched\nSr-CoOOH anode achieved a remarkably high current density of 3.3 A cm−2 with a much-lowered\nvoltage of 2.0 V, outperforming the pure CoOOH anode with 2.1 A cm−2 at 2.0 V and the\ncommercial RuO anode with 0.63 A cm−2 at 2.0 V. Moreover, the AEMWE with Co-vacancy-\n2\nenriched Sr-CoOOH anode presented an outstanding stability during 1000 h electrolysis at an\noperation temperature of 80 °C with a negligible degradation rate of 0.10 mV h-1, significantly\nlower than that of 1.33 mV h-1 during 93 h on commercial RuO anode. Of note, in comparison\n2\nwith the recently reported AEMWE performance with using unmodified commercial anion-\nexchange membrane and commercial Pt/C cathode, the remarkable currents and lowered\ndegradation rate of AEMWE in our work outperformed most of the recently reported AEMWE\nwith low-cost anode catalysts.\nFrom the perspective of the outstanding AEMWE performance, these Co-vacancy-enriched Sr-\nCoOOH should possess excellent OER performance as the anodic catalysts. Relative to CoOOH\nand commercial RuO , the Co-vacancy-enriched Sr-CoOOH catalysts showed 18.6-fold and 25.2-\n2\nfold increase in OER currents, respectively. Meanwhile, an outstanding OER stability for these\ncatalysts was verified with only 2% degradation in overpotentials after 10000 ADT cycles, much\nlower than the 35% degradation for pure CoOOH. Such the order-of-magnitude improvement in\nactivity and stability verified the significantly enhanced OER performance by engineering metal-\nion vacancy in reconstructed electrocatalysts. In addition, as suggested by the reviewer, we have\nalso double-checked the OER activity. Considering the high mass loading of catalysts in RDE tests\nwould restrict the mass transfer during OER process due to the thick and compact catalyst layer as\nwell as residual O bubble, we have tested the catalysts onto gas diffusion electrode (i.e., Ni foam)\n2\nto accelerate mass transfer and O bubble release. Notably, these Co-vacancy-enriched Sr-CoOOH\n2\ncatalysts on Ni foam exhibited an overpotential as low as 198 mV at 10 mA cm-2, together with a\nsmall Tafel slope of 32 mV dec-1, far exceeding the other recently reported Co-based OER catalysts\nand further verifying the excellent OER performance.\nAs mentioned above, we believe this work paves an enlightening guideline to finely engineer\nmetal-ion vacancy in dynamically reconstructed electrocatalysts for developing targeted\nelectrocatalysts under realistic conditions, which fits well with the scope of Nature\nCommunications. We have paid close attention to the comments, observations and\nrecommendations made by the reviewer, and carefully revised the manuscript with the intent of\nmaking a stronger, more complete and compelling case.\n“1. What's the role of Sr in the oxygen evolution? How did this metal site affect the reconstruction\nprocess?”\nWe thank the reviewer for this suggestion. In this work, we have systematically discussed the roles\nof Sr-ion substitution in the reconstruction process and the oxygen evolution process as follows:\ni) For the reconstruction process, mechanistic investigations unveiled that Sr-ion substitution in\nselenide pre-catalysts could largely weaken the Co-Se bond strength for accelerated reconstruction\nunder OER conditions, contributing to the generation of abundant Co-ion vacancy in reconstructed\nCoOOH catalysts. In detail, due to the much-lowered work function of Sr (2.59 eV) relative to Co\n(5 eV), the substituted Sr atoms in CoSe pre-catalysts could supply electrons more efficiently than\n(denoted as Sr-CoSe -w). As theoretically verified by the supplemented first-principles\n2\ncalculations (Fig. R2a), relative to pure CoSe model, the Co-Se bond length around Sr sites in Sr-\n2\nin these selenides by Sr substitution. To further illustrate the weakened Co-Se bonding by Sr\nsubstitution, we calculated the Se vacancy formation energy in these two selenide models with or\nwithout Sr substitution (Fig. R2c), which showed an obvious decrease in Sr-CoSe -w that that in\n2\npure CoSe , clearly suggesting the weakened Co-Se bonding in these selenides by Sr substitution.\nEXAFS analysis (please see Supplementary Table 1), the increase of Co-Se bond length in 1.0%Sr-\nCoSe -w than that in pure CoSe was in accordance with the weakened bond strength. Moreover,\n2 2\nthe negative shift of Co 2p XPS peaks (please see Fig. 1f) and the positive shift of Sr 3d XPS peaks\n(please see Supplementary Fig. 4) identified the substituted Sr atoms in selenides could supply\nelectrons more efficiently than Co atoms to bond Se sites, leading to the weakened Co-Se bond\nstrength.\nSuch the weakened bond strength in selenide pre-catalysts by Sr-ion substitution can reasonably\nfacilitate the structural reconstruction under OER conditions to generate abundant Co-ion vacancy\nin reconstructed oxyhydroxide catalysts. As illustrated in the supplemented illustration in Fig. R2d,\nunder OER conditions, these selenide pre-catalysts tended to be attacked by OH- ions from alkaline\nelectrolytes, together with the leaching of lattice Se due to the smaller bond dissociation energy of\n286.6 kJ mol-1 of Co-Se bonding (J. Chem. Phys. 152, 124305 (2020)) than that of 365.2 kJ mol-1\nof Co-O bonding (Comprehensive Handbook of Chemical Bond Energies, CRC Press, Boca Raton,\n(2007)), leading to the generation of reconstructed CoOOH catalysts. With the existence of Sr ions,\nthe weakened Co-Se bonding in Sr-CoSe -w pre-catalysts could result in an even lowered Co-Se\n2\nbond dissociation energy relative to that in pure CoSe , together with the intrinsically weak Sr-Se\n2\nbonding with a small bond dissociation energy of 251 kJ mol-1 relative to Co-Se bonding, thus\nleading to the facilitated structural reconstruction. Meanwhile, such the accelerated leaching of\nlattice Se during OER reconstruction could induce the much-lowered coordination of adjacent Co\nsites and thus accelerate the leaching of lattice Co to form abundant Co-ion vacancy in\nreconstructed CoOOH catalysts (denoted as Sr-CoOOH ). These results could also be verified by\nv\nthe experimental characterization. The variation of Raman spectra clearly suggested the\nreconstruction process from selenide pre-catalysts to reconstructed oxyhydroxide catalysts under\nOER conditions (please see Fig. 2b). From the EXAFS analysis (please see Fig. 2h,i), the Co-Co\ncoordination numbers decreased from 3.0 for CoOOH to 2.4 for 1.0%Sr-CoOOH , together with\nv\nthe well-maintained Co-O coordination numbers, clearly demonstrating the formation of abundant\nCo vacancies in reconstructed oxyhydroxide catalysts by Sr-mediated reconstruction.\nFig. R2. a, Established CoSe models with and without Sr-ion substitution. b, Comparison of the\nmediated reconstruction method to fabricate reconstructed CoOOH rich in Co-ion vacancy.\nii) To gain an in-depth understanding on the intrinsic role of Sr in the oxygen evolution process,\nwe have conducted first-principles calculations with establishing models of Sr-CoOOH with Co\nv\nvacancy, Sr-CoOOH without Co vacancy and pure CoOOH (please see Supplementary Fig. 20).\nf\nRelative to pure CoOOH, the Sr substitution in Sr-CoOOH induced increased electron loss from\nf\nadjacent Co sites (please see Fig. 5a), owing to the limited valence-electron providing ability of Sr\nto bond O and thus the increased valence-electron supply from adjacent Co sites. As such, further\nintroduction of Co vacancy could induce extra valence-electron supply from adjacent Co sites to\nbond O, which was also verified by the extra electron loss of Co sites in Sr-CoOOH .\nThe increased\nv\nvalence-electron sharing between Co and O could reasonably shorten the adjacent Co-O bonds\nwith enhanced covalency (please see Supplementary Fig. 21). The enhanced Co-O covalency was\nfurther verified by the pDOS with reduced difference between Co 3d and O 2p band center (please\nsee Fig. 5b) and thus led to the efficient lattice oxygen activation for triggering LOM pathway.\nThe triggered LOM route could also be demonstrated by the significantly lowered switch energy\nbarrier from AEM route to LOM route by calculating the activation barriers of lattice oxygen\ncoupling (please see Fig. 5d,e). Such the switched route from AEM to LOM because of the lattice\noxygen activation could boost the OER kinetics by overcoming the linear relation of OH* and\nOOH* with direct O-O coupling. As supplemented in Fig. R3a,d, the Sr-CoOOH , with a preferred\nv\nLOM route, exhibited an obviously lowered potential-determining-step (PDS) energy barrier than\nthat for pure CoOOH with a preferred AEM route (Fig. R3c,f), indicating the intrinsically\naccelerated OER process for Sr-CoOOH with switched LOM route. Notably, we have also added\nv\nthe calculations of Gibbs free energies for reaction steps on Sr-CoOOH. As supplemented in Fig.\nf\nR3b,e, similar AEM route with pure CoOOH could be observed for Sr-CoOOH with a still high\nf\nPDS energy barrier, identifying the deficient activation of lattice oxygen only by Sr substitution\nwith theoretically poor OER activity.\nIn addition, relative to pure CoOOH, the abundant Co vacancies in Sr-CoOOH , together with the\nv\nSr substitution with limited valence-electron providing ability, could narrow the d band of adjacent\nCo sites, as verified by the upshifted d-band center (please see Fig. 5b) and smaller d band width\ncalculated with using the reported method (please see Ref. 57), and thus leading to the strengthened\nOH adsorption affinity of Sr-CoOOH for efficient lattice oxygen refilling to realize stable LOM.\nv\nAs illustrated by electron density differences of intermediate structures for lattice oxygen refilling\nwith OH adsorbates (please see Fig. 5h), relative to CoOOH, the Co vacancy in Sr-CoOOH could\nv\ntrigger an enhanced valence-electron interaction between adjacent Co and OH adsorbates to refill\nthe lattice oxygen deficiency generated during LOM route, validating the promoted lattice oxygen\nrefilling ability. This result was further verified by the much-lowered energy barrier for lattice\noxygen refilling of Sr-CoOOH (please see Fig. 5i), in accordance with the experimental stability\nv\nenhancement (please see Fig. 3e-i).\nFig. R3. Calculation models and Gibbs free energies of AEM and LOM routes of OER for a,d,\nCoOOH, b,e, Sr-CoOOH without Co vacancy, and c,f, Sr-CoOOH with Co vacancy.\nf v\nTo make it clearer, we have added the relevant discussion, three figures, and two references in the\nrevised manuscript (please see page 4, line 29-31 & page 5, line 1-2, 22-25, 27-31 & page 11, line\n28-30 & page 12, line 10-11 & page 21, ref. 35-36 & page 25, Fig. 2a), as well as three figures in\nthe revised supporting information (please see page S2-S3, Supplementary Fig. 1-2 & page S27,\nSupplementary Fig. 26).\n“2. What is the final active site for OER after reconstruction?”\nThe reviewer raised a good point. For these 3d transition metal-based materials, previous studies\nhave widely proved that transition metal oxyhydroxides (MOOH) are always the real catalytically\nactive species generated from irreversible electrochemical reconstruction under OER conditions\n(Nat. Mater. 16, 925-931 (2017); Adv. Mater. 35, 2209307 (2022)). In our work, systematic\ncharacterizations have suggested the generation of CoOOH active phase after electrochemical\nreconstruction under OER conditions, including the typical peaks attributing to E and A\ng 1g\nvibrational modes of CoOOH from in-situ Raman (please see Fig. 2b), the typical peaks attributing\nto Co-O and Co-Co coordination of CoOOH from EXAFS (please see Fig. 2h), as well as the\ntypical peaks attributing to Co3+ cations in CoOOH from XPS (please see Fig. 2j).\nFurthermore, we have also conducted the XANES measurements during the reconstruction process.\nAs shown in Fig. R4, the valence state of Co sites changed from Co2+ to Co3+ after reconstruction,\nin accordance with the formation of CoOOH active phase after reconstruction under OER\nconditions. In addition, as observed from the XANES measurements (Fig. R5), these Co active\nsites could undergo the reversable change in valence state from Co3+ at open-circuit potential (OCP)\nto transient higher valence states with increased OER potentials, and then back to Co3+ when the\nreaction potential back to OCP. Considering the definition of catalysts as the substance that affects\nthe reaction rate but emerges from the process unchanged (Elements of Chemical Reaction\nEngineering, Pearson Education, (2021)), it could be clarified that the Co sites in these\nreconstructed CoOOH served as the active sites.\nTo clearly understand the active sites during OER process, we have also conducted the EXAFS\nmeasurements to detect the local structures of 1.0%Sr-CoOOH and pure CoOOH catalysts under\nv\napplied OER potentials. As shown in Fig. R6a,b, the EXAFS spectra revealed two stable peaks at\naround 1.5 Å and 2.5 Å assigned to Co-O and Co-Co bonds of CoOOH phase at the voltage from\nOCP to 1.50 V and then back to OCP, together with the well-maintained coordination numbers\n(Fig. R6c,d), clearly suggesting the stable Co active sites within the typical oxyhydroxide\ncoordination configuration for OER catalysis, even with the variation of Co valence states during\nOER process.\nFig. R4. a, Co K-edge XANES spectra during reconstruction with Sr-ion mediation. b,\nCorresponding Co oxidation states during reconstruction.\nFig. R5. a, c, Co K-edge XANES spectra with applied OER potentials for 1.0%Sr-CoOOH and\nv\npure CoOOH, respectively. b, d, Corresponding Co oxidation states during OER process.\nFig. R6. a, b, Co K-edge EXAFS spectra with applied OER potentials for 1.0%Sr-CoOOH and\nv\npure CoOOH catalysts, respectively. c, d, Corresponding Co-O and Co-Co coordination numbers\nfor 1.0%Sr-CoOOH and pure CoOOH catalysts, respectively.\nv\nTo make it clearer, we have added the relevant discussion and a figure in the revised manuscript\n(please see page 6, line 6 & page 9, line 26-31 & page 25, Fig. 2c), as well as two figures and two\ntables in the revised supporting information (please see page S19, Supplementary Fig. 18 & page\nS20, Supplementary Fig. 19 & page S37-38, Supplementary Table 6-7).\n“3. The OER performance is so ordinary, not to mention the AEMWE activity.”\nWe thank this reviewer for his/her suggestion. In our work, we have demonstrated a facile strategy\nto finely engineer the Co-ion vacancy in dynamically reconstructed Co-based oxyhydroxides as\nactive and stable OER catalysts for AEMWE. The abundant Co vacancies in the reconstructed Sr-\nCoOOH catalysts not only strengthened valence-electron sharing between Co and O sites with\nv\nenhanced covalency for lattice oxygen activation with triggered LOM route, but also promoted\nvalence-electron interaction between Co sites and OH adsorbates with enhanced hydroxyl affinity\nfor lattice oxygen refilling during LOM route. Accordingly, relative to the pure CoOOH catalysts\nwithout Co vacancies, the Sr-CoOOH catalysts rich in Co vacancies should exhibit notably\nv\nenhanced OER performance. As shown in Fig. 3, Relative to pure CoOOH and commercial RuO ,\n2\nthe Co-vacancy-enriched Sr-CoOOH catalysts showed an 18.6-fold and 25.2-fold increase in\nv\nOER currents, respectively. Of note, the activity of Sr-CoOOH catalysts was also higher than\nv\nthose of recently reported Co-based oxyhydroxide catalysts (please see Supplementary Table 5).\nMeanwhile, for these Co-vacancy-enriched Sr-CoOOH catalysts, a significantly promoted OER\nv\nstability with only 2% increase in overpotentials after 10000 accelerated potential cycles was\npresent, much smaller than the 35% increase in overpotentials for pure CoOOH catalysts. Such the\norder-of-magnitude improvement in activity and stability verified the significantly enhanced OER\nperformance, making it feasible to realize high-performance AEMWE. As shown in Fig. 6, the\nAEMWE with Sr-CoOOH anode achieved a remarkably high current density of 3.3 A cm−2 with\nv\na much-lowered voltage of 2.0 V, clearly outperforming the pure CoOOH anode with 2.1 A cm−2\nat 2.0 V and the commercial RuO anode with 0.63 A cm−2 at 2.0 V. Moreover, the AEMWE with\n2\n1.0%Sr-CoOOH anode presented an outstanding stability during 1000 h electrolysis with a\nv\nnegligible degradation rate of 0.10 mV h-1, significantly lower than that of 1.33 mV h-1 during 95\nh on commercial RuO anode. The remarkable currents and lowered degradation rate of AEMWE\n2\nwith Sr-CoOOH anode also outperformed most of the recently reported low-cost anode\nv\nelectrocatalysts in AEMWE with using unmodified commercial anion-exchange membrane and\ncommercial Pt/C cathode (please see Supplementary Table 8 and some selected examples as Fig.\nR7 here).\nFig. R7. Comparison of AEMWE performance with reported anodic catalysts with using\nunmodified commercial anion-exchange membrane and commercial Pt/C cathode.\nIn addition, as suggested by the reviewer, we have also double-checked the OER activity.\nConsidering the relatively high mass loading of 1 mg cm-2 of catalysts in RDE tests would restrict\nthe mass transfer during OER process due to the thick and compact catalyst layer as well as residual\nO bubble, we have tested the catalysts onto gas diffusion electrode (i.e., Ni foam) to accelerate\n2\nmass transfer and O bubble release to further understand the remarkable OER performance of\n2\nthese developed Co-vacancy-enriched Sr-CoOOH catalysts. The mass loading on the gas\nv\ndiffusion electrode was maintained as 1 mg cm-2, as the same with that on RDE. As illustrated by\nthe polarization curves in Fig. R8a, the Co-vacancy-enriched Sr-CoOOH catalysts on Ni foam\nv\nwith a working area of 0.5 cm2 could exhibit an overpotential as low as 198 mV at 10 mA cm-2,\ntogether with a small Tafel slope of 32 mV dec-1, far exceeding the other recently reported Co-\nbased OER catalysts (Fig. R8b, Supplementary Ref. 4-12) and thus demonstrating the excellent\nOER performance of our catalysts developed by engineering metal-ion vacancy in dynamically\nreconstructed electrocatalysts.\nFig. R8. a, OER polarization curve and Tafel slope of Sr-CoOOH catalysts on gas diffusion\nv\nelectrode. b, Comparison of the OER performance with recently reported Co-based catalysts.\nTo make it clearer, we have added the relevant discussion, a figure, eleven references as well as\nupdated the performance table in the revised manuscript and supporting information (please see\npage 8, line 18-21 & page 13, line 13-14, 17-18 & page S15, Supplementary Fig. 14 & page S39,\nSupplementary Table 8 & page S40-S43, Supplementary Ref. 6-14, 34, 39).\n“4. How did they determine that the catalyst is made of CoOOH, not CoO or Co(OH) ?”\nx 2\nThe reviewer raised a good point. Actually, we have conducted a series of characterizations to\nidentify the phase of catalysts. After the electrochemical reconstruction from selenide pre-catalysts\nunder OER conditions, these obtained catalysts were demonstrated as amorphous phase by the\ndisappeared XRD patterns and the diffraction rings in SAED patterns (please see Fig. 2f &\nSupplementary Fig. 6-7). The corresponding Co 2p XPS spectra exhibited two typical peaks at\n779.9 eV and 781.1 eV corresponded to the Co3+ cations in CoOOH but not CoO or Co(OH)\nx 2\n(please see Fig. 2j), as also verified by the Co valence states of ~3 of these reconstructed catalysts\nby XANES measurements (Fig. R9). Meanwhile, the co-existed O-Co bonds (529.7 eV) and OH-\nCo bonds (530.8 eV) in O 1s XPS spectra were also in accordance with the CoOOH coordination\n(please see Supplementary Fig. 11). Moreover, the Raman spectra clearly exhibited two typical\npeaks at 498 and 594 cm-1 assigned to E and A vibrational modes of Co-O in CoOOH, verifying\ng 1g\nthe reconstructed catalysts were made of CoOOH phase (please see Fig. 2b). The broadened\nRaman peaks were consistent with the amorphous phase of these catalysts.\nFig. R9. a, Co K-edge XANES spectra during reconstruction with Sr-ion mediation. b,\nCorresponding Co oxidation states during reconstruction.\nTo clearly understand the Raman evidence of CoOOH and exclude CoO or Co(OH) , we further\nx 2\nconducted the Raman spectra of amorphous CoOOH, Co O and Co(OH) samples for reference.\n3 4 2\nThese reference samples were synthesized by using the previously reported methods (please see\nSupplementary Ref. 1-3). As shown in Fig. R10, different form the Raman peaks of amorphous\nCo O or Co(OH) references, only the Raman peaks of amorphous CoOOH reference matched\n3 4 2\nwell with our reconstructed samples, clearly suggesting the reconstructed catalysts were made of\nCoOOH phase.\nFig. R10. Raman spectra of a, amorphous CoOOH, b, amorphous Co O and c, amorphous\n3 4\nCo(OH) references as obtained by using previously reported methods.\nR11), these CoOOH catalysts could undergo the reversable change in valence state from Co3+ of\nCoOOH phase at open-circuit potential (OCP) to transient higher valence states with increased\npotentials and then back to Co3+ when the reaction potential back to OCP. Besides, the EXAFS\nspectra also verified the same Co-O and Co-Co coordination structures with those in CoOOH\nphase even with the variation of Co valence states during OER process. Considering the definition\nof catalysts as the substance that affects the reaction rate but emerges from the process unchanged\n(Elements of Chemical Reaction Engineering, Pearson Education, 2021), it could be clarified that\nthe catalysts were made of CoOOH phase.\nFig. R11. a, Co K-edge XANES spectra during OER process for 1.0%Sr-CoOOH catalysts. b,\nv\nCorresponding Co valence states during OER process. c, Co K-edge EXAFS spectra. d,\nCorresponding Co-O and Co-Co coordination numbers with applied potentials.\nTo make it more readable, we have added the relevant discussion and a figure in the revised\nmanuscript (please see page 6, line 5-6 & page 9, line 26-31 & page 25, Fig. 2c), as well as three\nfigures and three references in the revised supporting information (please see page S6,\nSupplementary Fig. 5 & page S19-20, Supplementary Fig. 18-19 & page S40, Supplementary Ref.\n1-3).\n“5. The in situ Raman analysis seem not correct. Especially, the Raman signals are too weak to\nbe used for determining the chemical compounds. So additional structural characterizations are\nnecessary.”\nAs suggested, we have double-checked the in-situ Raman analysis, where the seemingly weak\nsignals of reaction intermediates could be attributed to their short lifetime and low coverage during\nelectrocatalytic process (Nat. Energy 4, 60-67 (2019)). To make it more readable and well-\ndocumented, we have retested the in-situ Raman spectra of 1.0%Sr-CoOOH and pure CoOOH\nv\ncatalysts during real-time OER process with well-tuned Raman testing parameters. In detail, for\ndetecting the chemical compounds during real-time OER process, we have changed a lower energy\nlaser (785 nm) for much more time to detect the O-O intermediate. The in-situ Raman spectra of\ndifferent applied OER voltages from 1.20 V to 1.50 V were recorded with increased 10 times for\nincreased 30 s to study the OER process, which were conducted by Raman microscope\n(RENISHAW InVia Qontor) with 50xL (N =0.5) objective and 785 nm laser and 1200 l/mm\nA\noptical grating. As shown in Fig. R12, at OCP, two typical broadened peaks at 498 and 594 cm-1\nwere visible for both samples, in accordance with the E and A vibrational modes of Co-O\ng 1g\nstretching of amorphous CoOOH phase (Chem. Mater. 25, 1922-1926 (2013)). When the applied\npotential increased, these two peaks showed a negative shift to 477 and 574 cm-1 for Co-O\nstretching of CoO (Angew. Chem. Int. Ed. 63, e202405839 (2024)), suggesting the transient\n2\nconversion to high valence state of Co sites during OER process. Notably, for 1.0%Sr-CoOOH\nv\ncatalysts, a new peak at 1064 cm-1 appeared with applied potentials, in accordance with the\nsuperoxide O-O intermediates with the participation of lattice oxygen (J. Am. Chem. Soc. 142,\n11901-11914 (2020)), suggesting the occurrence of LOM route. Such the superoxide O-O\nintermediates have also been verified by using the TMA+ as chemical probe (please see Fig. 4f-h).\nIn contrast, the absence of Raman peaks at around 1064 cm−1 for pure CoOOH indicated the\ndifferent OER pathway.\nFig. R12. In-situ Raman spectra for a, 1.0%Sr-CoOOH and b, pure CoOOH with applied OER\nv\npotentials, respectively.\nTo gain a further insight into these bond changes during OER, we have also conducted additional\nstructural characterizations (i.e., XAS measurements) of the 1.0%Sr-CoOOH and pure CoOOH\nv\ncatalysts under OER potentials. As shown in the XANES spectra and corresponding Co valence\nstates for both samples in Fig. R13, at OCP, both of these two catalysts exhibited the Co valence\nstates to be ~3, in accordance with the Co valence states of CoOOH phase as well as the in-situ\nRaman results of the typical signals for Co-O stretching of amorphous CoOOH at OCP. When the\napplied potential increased, the Co valence states gradually intensified to ~3.6 at 1.50\nV, suggesting the transient conversion to high valence state of Co sites during OER process. This\nresult was also in accordance with the in-situ Raman results of the appearance of signals for Co-O\nstretching of CoO with the applied OER potentials. In addition, regarding the local structures of\n2\n1.0%Sr-CoOOH and pure CoOOH catalysts under OER potentials, the EXAFS spectra revealed\nv\ntwo stable peaks at around 1.5 Å and 2.5 Å assigned to Co-O and Co-Co bonds for CoOOH phase\nat the voltage from OCP to 1.50 V (Fig. R14), together with the well-maintained coordination\nnumbers (Fig. R15 and Table R2,3), clearly suggesting the stable Co active sites within the typical\noxyhydroxide coordination configuration for OER catalysis.\nFig. R13. a, c, Co K-edge XANES spectra with applied OER potentials for 1.0%Sr-CoOOH and\nv\npure CoOOH catalysts, respectively. b, d, The calculated Co oxidation states for 1.0%Sr-CoOOH\nv\nand pure CoOOH catalysts, respectively.\nFig. R14. Co K-edge EXAFS spectra with applied OER potentials for a, 1.0%Sr-CoOOH and b,\nv\npure CoOOH catalysts, respectively.\nFig. R15. Co-O and Co-Co coordination numbers with applied OER potentials for a, 1.0%Sr-\nCoOOH and b, pure CoOOH catalysts, respectively.\nv\nTable R2. EXAFS fitting results at Co K-edge for 1.0%Sr-CoOOH with applied OER potentials.\nv\nσ2\nSample Shell CN R(Å) ΔE (eV) R factor\n0\n(Å2·10-3)\nCo-O 3.9±0.8 1.88±0.01 4.9±2.5 -9.2±1.2\nOCP 0.017\nCo-Co 2.4±0.7 2.82±0.02 4.8±2.6 -9.2±1.3\nCo-O 3.9±1.1 1.88±0.04 4.1±1.9 -7.5±2.1\n1.20 V 0.02\nCo-Co 2.4±0.9 2.82±0.03 7.4±1.4 -6.3±1.7\nCo-O 3.9±0.6 1.88±0.02 1.8±1.1 3.9±0.6\n1.25 V 0.013\nCo-Co 2.4±0.7 2.82±0.01 6.9±2.6 -5.6±1.1\nCo-O 3.9±0.5 1.88±0.08 2.9±1.5 -6.8±0.8\n1.30 V 0.007\nCo-Co 2.4±0.6 2.82±0.02 5.7±1.9 -7.0±0.9\nCo-O 3.9±0.6 1.88±0.04 2.6±2.0 -3.5±1.1\n1.35 V 0.009\nCo-Co 2.4±0.8 2.82±0.03 5.5±2.8 -3.6±1.2\nCo-O 3.9±0.6 1.88±0.01 3.9±1.9 -8.7±0.9\n1.40 V 0.01\nCo-Co 2.4±0.7 2.82±0.2 6.9±2.4 -8.4±1.1\nCo-O 3.9±0.4 1.88±0.01 2.8±1.3 -4.4±0.7\n1.45 V 0.004\nCo-Co 2.4±0.5 2.82±0.02 4.3±1.7 -4.2±0.8\nCo-O 3.9±0.5 1.88±0.01 3.3±1.7 -2.9±0.8\n1.50 V 0.018\nCo-Co 2.4±0.6 2.82±0.02 6.1±2.2 -3.9±0.9\nCo-O 3.9±0.6 1.88±0.01 3.7±2.1 -5.4±1.1\nBack to OCP 0.013\nCo-Co 2.4±0.7 2.82±0.03 4.4±2.5 -5.6±1.2\nCN: coordination number; R: bond distance; σ2: Debye-Waller factors; ΔE : the inner potential\n0\ncorrection; R factor: goodness of fit.\nTable R3. EXAFS fitting results at Co K-edge for CoOOH with applied OER potentials.\nσ2\nSample Shell CN R(Å) ΔE (eV) R factor\n0\n(Å2·10-3)\nCo-O 4.0±0.4 1.92±0.01 5.1±1.6 1.5±0.6\nOCP 0.004\nCo-Co 3.0±0.6 2.83±0.02 8.7±2.1 -2.9±0.8\nCo-O 4.0±0.3 1.92±0.05 2.2±1.5 -0.37±0.1\n1.20 V 0.018\nCo-Co 3.0±0.8 2.83±0.07 4.6±2.5 -3.8±2.6\nCo-O 4.0±0.4 1.92±0.01 3.4±1.3 -2.87±1.0\n1.25 V 0.009\nCo-Co 3.0±0.5 2.83±0.05 5.8±1.6 -7.32±2.7\nCo-O 4.0±0.3 1.92±0.02 1.0±0.5 3.41±0.83\n1.30 V 0.012\nCo-Co 3.0±0.9 2.83±0.03 3.5±0.3 0.41±0.12\nCo-O 4.0±0.6 1.92±0.03 3.6±1.9 -3.1±0.71\n1.35 V 0.009\nCo-Co 3.0±0.7 2.83±0.02 6.5±2.1 -7.4±0.89\nCo-O 4.0±0.8 1.92±0.01 1.1±0.3 -0.95±0.53\n1.40 V 0.015\nCo-Co 3.0±1.1 2.83±0.02 5.3±3.2 -4.16±1.21\nCo-O 4.0±0.6 1.92±0.05 2.9±1.1 -1.98±0.91\n1.45 V 0.009\nCo-Co 3.0±0.8 2.83±0.03 8.6±2.4 -6.7±0.93\nCo-O 4.0±0.3 1.92±0.01 2.5±1.2 -2.1±0.86\n1.50 V 0.013\nCo-Co 3.0±0.7 2.83±0.02 6.2±2.3 -6.2±0.95\nCo-O 4.0±0.2 1.92±0.02 3.1±1.1 2.1±0.8\nBack to OCP 0.015\nCo-Co 3.0±0.3 2.83±0.01 2.9±1.2 -2.5±0.9\nCN: coordination number; R: bond distance; σ2: Debye-Waller factors; ΔE : the inner potential\n0\ncorrection; R factor: goodness of fit.\nWe have updated the in-situ Raman spectra with relevant discussion and two references, and\nfurther added the additional XAS characterizations with relevant discussion and fitting results in\nthe revised manuscript and supporting information (please see page 9, line 20, 26-28 & page 27,\nFig. 4a,b & page 22, Ref. 47, 48 & page S19-S20, Supplementary Fig. 18-19 & page S37-S38,\nSupplementary Table 6-7).\n“6. The peak of Co-O-Sr in EXAFS spectra is so weak to be detected, which is not so convinced.”\nWe thank the reviewer for this suggestion. Regarding the Co-O-Sr path in our previous EXAFS\nanalysis, we referred to a previously reported work in Energy Environ. Sci. 15, 3912-3922 (2022),\nwhich identified the Co-O-Sr path at a radial distance of ~3.5 Å. In our case, based on the EXAFS\nfitting, we observed a coordination shell at ~3.43 Å, which was in good agreement with the\nreference. However, the limited Sr content in these samples (1.0 at%) inherently restricted the\nnumber of observable Co-O-Sr scattering paths, leading to the weak signal and the fitted\ncoordination number as low as 0.52. Considering the concerns by the reviewer about these weak\nsignals, which actually derived from the limited Sr content in these samples, and also the complex\ncoordination environment when the radius was greater than 3 Å, we have re-fitted the EXAFS data\nwithout considering the Co-O-Sr path (please see the updated Fig. 2h,i & Supplementary Table 2).\nTo gain a compelling evidence for understanding the coordination environment of Sr atoms in\nthese oxyhydroxide samples, we have further conducted the Sr K-edge XAS analysis for 1.0%Sr-\nCoOOH . As shown in Fig. R16a, the edge position of 1.0%Sr-CoOOH were located at the energy\nv v\nbetween Sr foil and SrO, indicating the cationic nature of Sr atoms in 1.0%Sr-CoOOH with a\nv\nlower valence state than +2. This result was also in accordance with the Sr 3d XPS peaks at 133.4\nand 135.1 eV for Sr cations (please see Supplementary Fig. 10b). The EXAFS spectra of Sr foil,\nSrO, and 1.0%Sr-CoOOH were collected in Fig. R16b. Notably, for 1.0%Sr-CoOOH , two major\nv v\npeaks at 1.9 Å and 2.7 Å assigned to Sr-O and Sr-Co coordination (Adv. Funct. Mater. 8, 2307116\n(2023)), together with the absence of Sr-Sr coordination, indicating the atomic dispersion of Sr\natoms.\nFig. R16. a, Sr K-edge XANES spectra, and b, EXAFS spectra for 1.0%Sr-CoOOH .\nv\nTo make it more readable, we have added the XAS analysis and relevant discussion in the revised\nmanuscript and supporting information (please see page 6, line18-20 & page 7, line 12-13 & page\n25, Fig. 2g & page S11, Supplementary Fig. 10a).\n“7. The OER performance is very poor, and the method for catalyst preparation is not new. Can\nthey provide some new points on their manuscript?”\nWe genuinely thank the reviewer for this suggestion. The new points that have been demonstrated\nin our work are as follows:\ni) We demonstrate a new Sr-ion-mediated reconstruction method to finely engineer Co-ion vacancy\nin dynamically reconstructed CoOOH electrocatalyst under OER conditions. As a typical\nsymmetry-breaking structure, the construction of metal-ion vacancies has been recently recognized\nto make an adjustment in local atomic configuration of metal-based catalysts with altered charge\ndensity gradient, thus resulting in distinct catalytic behaviors. However, almost all the current\nefforts have been made to develop strategies to fabricate metal-ion vacancies in as-synthesized\nnanomaterials, while the inevitably dynamic reconstruction for metal-based catalysts under\noperation conditions, especially for the non-noble 3d metal-based materials under the highly\ncorrosive alkaline OER conditions, could rebuild the local atomic structures with vanished vacancy\nsites, leading to the uncontrollable catalytic performance. Accordingly, precise design of metal-\nion vacancies in dynamically reconstructed metal-based catalysts deserves to be investigated,\nwhich could serve as a versatile tool to develop high-performance catalysts under practical\nconditions. In our work, we have demonstrated a new Sr-mediated reconstruction method to realize\nthe finely design of metal-ion vacancy in dynamically reconstructed electrocatalysts under OER\nconditions. Our work proved that the Sr-ion substitution in cobalt selenide pre-catalysts could\nsupply electrons more efficiently than Co atoms to bond Se sites to weaken Co-Se bond strength,\nresulting in the much-lowered Co-Se bond dissociation energy to accelerate leaching of lattice Se\nduring OER reconstruction. Such the accelerated Se leaching could induce the much-lowered\ncoordination of adjacent Co sites and thus accelerate the leaching of lattice Co to form abundant\nCo-ion vacancy in reconstructed CoOOH catalysts. As such, we have presented a new Sr-ion-\nmediated reconstruction method to realize the finely design of metal-ion vacancy in dynamically\nreconstructed electrocatalysts under OER conditions.\nii) We demonstrate the critical role of the well-defined Co-ion vacancy of dynamically\nreconstructed CoOOH catalysts in altering the OER pathway for activity enhancement and refilling\nlattice oxygen for stability strengthening. To overcome the theoretically limited OER overpotential\nof 370 mV in traditional adsorbate evolution mechanism (AEM) for activity enhancement, it is\nhighly desired to trigger lattice-oxygen oxidation mechanism (LOM) by activating lattice oxygen.\nHowever, the generated oxygen deficiencies due to the participation of lattice oxygen during LOM\nroute always render the dissolution of adjacent lattice metal with structure collapse, leading to the\ndegraded performance of catalysts under LOM route for long-term utilization. Accordingly, it is\nchallenging and fundamentally important to develop well-reconstructed Co-based catalysts under\nOER conditions with balanced activation and refilling of lattice oxygen for active and stable OER.\nOur work proved that the well-defined Co-ion vacancy in dynamically reconstructed CoOOH\ncatalysts can not only strengthen valence-electron sharing between Co and O sites with enhanced\nCo-O covalency for lattice oxygen activation with triggered LOM route, but also promoted\nvalence-electron interaction between Co sites and OH adsorbates with enhanced hydroxyl affinity\nfor lattice oxygen refilling during LOM route, thus leading to the highly active and stable OER\nperformance. As such, we have presented the crucial role of the well-defined Co-ion vacancy of\ndynamically reconstructed CoOOH catalysts in altering the OER pathway from AEM to LOM\nroute for OER activity enhancement and refilling lattice oxygen efficiently for OER stability\nstrengthening.\niii) We demonstrate an outstanding AEMWE performance by using the developed Co-vacancy-\nenriched Sr-CoOOH as anodic OER catalysts. The AEMWE with Co-vacancy-enriched Sr-\nCoOOH anode achieved a remarkably high current density of 3.3 A cm-2 with a much-lowered\nvoltage of 2.0 V, outperforming the pure CoOOH anode with 2.1 A cm-2 at 2.0 V and the\ncommercial RuO anode with 0.63 A cm-2 at 2.0 V. The energy consumption was deduced to be\n2\n43.5 kWh kg-1 , exceeding the goal of 47.9 kWh kg-1 from the Office of Energy Efficiency &\nH2 H2\nRenewable Energy (EERE) in the US Department of Energy (DOE). Moreover, the AEMWE with\nCo-vacancy-enriched Sr-CoOOH anode presented an outstanding stability during 1000 h\nelectrolysis with a negligible degradation rate of 0.10 mV h-1, significantly lower than that of 1.33\nmV h-1 during 95 h on commercial RuO anode. Of note, we have searched the recently reported\ncommercial Pt/C cathode, the remarkable currents and lowered degradation rate of AEMWE with\nCo-vacancy-enriched Sr-CoOOH anode in our work outperformed most of the recently reported\nAEMWE with low-cost anode catalysts.\nFrom the perspective of the outstanding AEMWE performance, these Co-vacancy-enriched Sr-\nCoOOH should possess excellent OER performance as the anodic catalysts. Relative to CoOOH\nand commercial RuO , the Co-vacancy-enriched Sr-CoOOH catalysts were proved to show an\n2\n18.6-fold and 25.2-fold increase in OER currents, respectively. Meanwhile, these Co-vacancy-\nenriched Sr-CoOOH catalysts also presented a significantly promoted OER stability with only 2%\ndegradation in overpotentials after 10000 accelerated potential cycles, much lower than the 35%\nincrease in overpotentials for pure CoOOH. Such the order-of-magnitude improvement in activity\nand stability verified the significantly enhanced OER performance by engineering metal-ion\nvacancy in dynamically reconstructed electrocatalysts. In addition, as suggested by the reviewer,\nwe have also double-checked the OER activity of these catalysts. Considering the high mass\nloading of catalysts in RDE tests would restrict the mass transfer during OER process due to the\nthick and compact catalyst layer as well as residual O bubble, we have tested the catalysts onto\n2\ngas diffusion electrode (i.e., Ni foam) to accelerate mass transfer and O bubble release to further\n2\nunderstand the remarkable OER performance of these developed Co-vacancy-enriched Sr-CoOOH\ncatalysts. Notably, these Co-vacancy-enriched Sr-CoOOH catalysts on Ni foam exhibited an\noverpotential as low as 198 mV at 10 mA cm-2, together with a small Tafel slope of 32 mV dec-1,\nfar exceeding the other recently reported Co-based OER catalysts and further verifying the\nexcellent OER performance of these developed catalysts in our work.\niv) We demonstrate that the proposed Sr-ion-mediated reconstruction method could be extended\nto engineer metal-ion vacancy in other reconstructed electrocatalysts under OER conditions. To\nillustrate it, a series of other compounds, including Co S and NiFeSe with and without Sr-ion\n3 4 2\nsubstitution, have been further adopted as pre-catalysts and underwent the electrochemical\nreconstruction under OER conditions (Fig. R17,18 & Table R4). From TEM image and XRD\npatterns, all of these pre-catalysts exhibited the similar sheet-like morphology and phase structure\nafter Sr-ion substitution. After reconstruction under OER conditions, these pre-catalysts could\nconvert into oxyhydroxides as stable phase, which were evidenced by the Raman spectra with\ntypical vibrational modes for oxyhydroxide phase. From the EXAFS analysis of these\nreconstructed oxyhydroxides, the typical peaks attributing to M-O and M-M coordination bonds\nfor MOOH could be observed. Of note, from the least-squares fittings, the M-M coordination\nnumbers in all these Sr-mediated reconstructed oxyhydroxides exhibited obvious decrease relative\nto the pure oxyhydroxides, together with the well-maintained M-O coordination numbers, clearly\ndemonstrating the formation of abundant metal-ion vacancies in the other dynamically\nreconstructed catalysts by using the proposed Sr-ion-mediated reconstruction method in our work.\nFig. R17. a, b, TEM images of Co S and Sr-Co S . c, XRD patterns. The standard pattern of cubic\n3 4 3 4\nCo S (JCPDS No. 47-1738) is attached at the bottom. d, e, TEM images of reconstructed\n3 4\nCoOOH and Sr-CoOOH . The Sr/Co molar ratio after reconstruction was determined as 0.2%\n(S) (S)\nby ICP-OES. f, Raman spectra of reconstruction samples. These two peaks were assigned to the\nE and A vibrational modes of Co-O in CoOOH. g, EXAFS spectra of reconstruction samples.\ng 1g\nh, Comparison of Co-O and Co-Co coordination numbers. The decreased Co-Co coordination\nnumbers and well-retained Co-O coordination numbers clearly suggested the formation of\nabundant Co vacancies by Sr-ion-mediated reconstruction.\nFig. R18. a, b, TEM images of NiFeSe and Sr-NiFeSe . c, XRD patterns. The standard pattern of\n2 2\ncubic NiSe (JCPDS No. 88-1711) is attached at the bottom. d, e, TEM images of reconstructed\n(Se) (Se)\ndetermined as 3.0% by ICP-OES. f, Raman spectra of reconstruction samples. These two peaks\nwere assigned to the E and A vibrational modes of Ni-O in NiOOH. g, Ni-K edge EXAFS\ng 1g\nspectra of reconstruction samples with coordination numbers of Ni-O and Ni-Ni. h, Ni-K edge\nEXAFS spectra of reconstruction samples with Fe-O and Fe-Fe coordination numbers. The\ndecreased Ni-Ni and Fe-Fe coordination numbers and well-retained Ni/Fe-O coordination numbers\nclearly suggested the formation of abundant metal-ion vacancies by Sr-ion-mediated\nreconstruction.\nTable R4. EXAFS fitting results at Co, Ni, and Fe K-edge for other reconstructed oxyhydroxide\nsamples by using Sr-ion-mediated reconstruction method.\nσ2 (Å2·10-\nSample Shell CN R(Å) ΔE (eV) R factor\n0\n3)\nCo-O 4.0±1.0 1.92±0.02 4.4±3.9 4.1±1.7\nCoOOH 0.02\n(S)\nCo-Co 3.0±1.6 2.83±0.01 13.2±6.1 -2.2±1.8\nCo-O 4.0±0.3 1.90±0.04 4.5±2.5 -1.1±1.0\nSr-CoOOH 0.02\n(S)\nCo-Co 2.4±1.1 2.83±0.03 8.2±4.3 -4.1±1.7\nNi-O 5.1±0.9 2.06±0.02 5.3±1.8 -2.7±1.0\nNiFeOOH 0.009\n(Se)\nNi-Ni 3.5±1.0 3.10±0.04 10.3±2.5 -2.6±0.9\nNi-O 5.1±1.0 2.04±0.04 6.4±2.1 -6.7±1.0\nSr-NiFeOOH 0.008\n(Se)\nNi-Ni 3.2±0.9 3.10±0.03 10.7±2.8 -3.2±0.9\nFe-O 5.2±0.8 1.97±0.01 6.4±1.8 -8.7±1.0\nNiFeOOH 0.008\n(Se)\nFe-Fe 3.4±1.2 3.09±0.02 5.1±2.9 -2.2±1.2\nFe-O 5.2±2.1 1.96±0.06 3.2±1.2 -5.9±3.4\nSr-NiFeOOH 0.01\n(Se)\nFe-Fe 3.1±1.7 3.09±0.07 12.4±2.3 5.3±1.2\nCN: coordination number; R: bond distance; σ2: Debye-Waller factors; ΔE : the inner potential\n0\ncorrection; R factor: goodness of fit.\nTo make it clearer and more readable, we have added the relevant discussion and figures in the\nrevised manuscript (please see page 4, line 29-31 & page 5, line 1-2, 23-25, 27-31 & page 7, line\n4-7 & page 8, line 18-21 & page 9, line 26-31 & page 13, line 13-14, 17-18 & page 25, Fig. 2a,c,g)\nand in the revised supporting information (please see page S2-S3, Supplementary Fig. 1-2 & page\nS6, Supplementary Fig. 5 & page S9-S10, Supplementary Fig. 8-9 & page S15, Supplementary\nFig. 14 & page S19-S20, Supplementary Fig. 18-19 & page S34, Supplementary Table 3 & page\nS37-39, Supplementary Table 6-8).\nReviewer #3 (Remarks to the Author):\n“This paper reports a very interesting approach to nicely engineer Co-ion vacancies in\ndynamically reconstructed Co-based electrocatalysts under OER conditions for practical AEMWE.\nThe experimental results documenting the control of Co-ion vacancies during reconstruction are\nvery nice, and convincing. The mechanism of these confined Co-ion vacancies in efficiently and\nstably catalyzing anodic OER for AEMWE is soundly revealed. These results are fundamentally\nimportant, which can also help to develop high-performance catalysts under practical conditions\nfor other applications. I strongly recommend publication of this work after minor revision.”\nWe sincerely thank this reviewer for his/her valuable comments and positive recommendation. We\nhave addressed all of the reviewer’s suggestions as follows.\n“1.Please provide more details about how pH-dependent activities indicate the reaction route of\nlattice oxygen participation.”\nThe reviewer raised a good point. As investigated by Shao-Horn, Y., the relationship between OER\nkinetics and pH in electrolyte is akin to the following equation (Catalysis Today 262, 2-10 (2016)):\n𝜕𝐸 𝜕𝐸 𝜕𝑙𝑜𝑔(𝑖)\n( ) = −( ) ( )\n𝜕𝑝𝐻 𝜕𝑙𝑜𝑔(𝑖) 𝜕𝑝𝐻\n𝑝𝐻 𝐸\nWhere the reaction order ρ = (∂log(i)/∂pH) plays an crucial role in identifying the OER behaviors.\nE\nThe reaction order suggests how the OER kinetics depends on the proton activity. A zeroth-order\nreaction implies no dependence, while a first-order reaction implies a linear dependence. The\nreaction order would be zero if the reaction follows the concerted path, where the proton transfer\nis not decoupled from the electron transfer. In contrast, the reaction order would be non-vanishing\nif the transfer processes are decoupled. For the LOM and AEM routes of alkaline OER, it has been\nwell demonstrated the concerted proton-electron-transfer process of AEM route and the decoupled\nproton-electron-transfer process of LOM route. As such, the reaction order, as obtained by\nmeasuring the OER currents with different pH electrolyte, could be used to identify whether the\nlattice oxygen participates in the OER process. The highly pH-dependent activities indicate the\nnon-zero reaction order, in accordance with the decoupled proton-electron-transfer process of\nLOM route.\nTo make it clearer, we have added the relevant discussion and a reference in the revised manuscript\n(please see page 10, line 5-6 & page 22, Ref. 49).\n“2.In AEMWE section, the authors should clarify what kind of Pt/C is used and the loading mass\nof Pt on carbon support.”\nWe thank the reviewer for this detailed suggestion. For the preparation of AEMWE cathode with\ncommercial Pt/C catalysts, the Pt/C catalysts (40 wt% loading, Johnson Matthey) was\nultrasonically suspended in the mixture of isopropanol, water, and ionomer solution. Then the\ncatalyst ink was sprayed onto the carbon paper with a 2×2 cm2 area to obtain a loading mass of\nPt/C was 1 mg cm-2. As suggested, we have clarified the detailed information of the Pt/C catalysts\nin the Methods section of the revised manuscript (please see page 17, line 6-8).\n“3.The assembly process for AEMWE tests should be described in more detail, including the\nsprayed catalyst layer, the applied torque, etc.”\nAs suggested, we have introduced more details about the assembly process for AEMWE tests in\nMethods section. In detail, the AEMWE was assembled with the anode (Sr-CoOOH , CoOOH, or\nv\ncommercial RuO catalysts), cathode (commercial Pt/C catalysts), and anion exchange membrane\n2\n(PiperION). The catalyst ink was prepared in the mixture of isopropanol, water, and ionomer\nsolution by ultrasonication, and then sprayed on nickel foam with a 2×2 cm2 area with the mass\nloading of 5 mg cm-2 for anodic samples, as well as on carbon paper with a loading mass of Pt/C\ncatalysts of 1 mg cm-2 as cathode. Subsequently, these cathode and anode were sandwiched with\nthe anion exchange membrane in a fixture with the applied torque of 3 N·m.\nTo make it clearer, we have added the relevant description and a figure in the revised manuscript\nand supporting information (please see page 12, line 29 & page 17, line 6-9 & page S31,\nSupplementary Fig. 30).\n“4.In Supplementary Table 3, the last cited reference shows 0.1 M KOH in the “Electrolyte”\ncolumn, which does not allow for direct comparison among the other listed references with 1 M\nKOH in the “Electrolyte” column and should be replaced.”\nWe thank the reviewer for this detailed suggestion. As suggested, we have replaced the mentioned\nreference with another literature with the sample of CoOOH hollow sphere (J. Energy Chem. 71,\n89-97 (2022)), where the performance was evaluated in 1 M KOH to be 320 mV of overpotential\nat 10 mA cm-2 with a Tafel slope of 59.6 mV dec-1. These results were far below the performance\nof the developed Co-vacancy-enriched CoOOH catalysts in our work, suggesting the advantages\nof our developed strategy in elevating the OER performance.\nWe have updated the relevant description and a reference in the revised supporting information\n(please see page S36, Supplementary Table 5 & page S42, Supplementary Ref. 30).\n“5.Please provide more details on how the in-situ Raman tests are performed, for both the\nreconstruction process to form oxyhydroxide catalysts and the reaction process with applied OER\npotentials.”\nThe reviewer raised a good point. As suggested, we have supplemented the details of in-situ Raman\ntests as follows.\ni) For detecting the reconstruction process to form oxyhydroxide catalysts, the in-situ Raman\nspectra were conducted by Raman microscope (RENISHAW InVia Qontor) with 50xL (N =0.5)\nA\nobjective and 532 nm laser with 1800 l/mm optical grating. The samples were used as working\nelectrodes, together with Pt wire and Hg/HgO as counter electrode and reference electrode,\nrespectively. The in-situ Raman spectra of different potential cycles were recorded 3 times for 10\ns to study the reconstructed process.\nii) For detecting the reaction process with applied OER potentials, considering the difficulties in\ndetecting these oxygenous intermediates due to their short lifetime and low coverage during\nelectrocatalytic process (Nat. Energy 4, 60-67 (2019)), we have changed a lower energy laser for\nmuch more time to detect the O-O intermediate. The in-situ Raman spectra of different applied\nvoltages from 1.20 V to 1.50 V were recorded 10 times for 30 s to study the OER process, which\nwere conducted by using the 785 nm laser and 1200 l/mm optical grating.\nWe have added the detailed description in the Methods section of the revised manuscript (please\nsee page 17, line 23-29 & page 18, line 1-2).\n“6.The mechanism of Sr-mediated Co vacancy formation during electrochemical reconstruction\nrequires deeper exploration.”\nThe reviewer raised a good point. For the reconstruction process, mechanistic investigations\nunveiled that Sr-ion substitution in selenide pre-catalysts could largely weaken the Co-Se bond\nstrength for accelerated reconstruction under OER conditions, contributing to the generation of\nabundant Co-ion vacancy in reconstructed CoOOH catalysts. In detail, due to the much-lowered\nwork function of Sr (2.59 eV) relative to Co (5 eV), the substituted Sr atoms in CoSe pre-catalysts\n2\ncould supply electrons more efficiently than Co atoms to bond Se sites, thus leading to the\nweakened Co-Se bond strength around these Sr sites (denoted as Sr-CoSe -w). As theoretically\n2\nverified by the supplemented first-principles calculations (Fig. R19a), relative to pure CoSe ,\n2\nthe Co-Se bond length around Sr sites in Sr-CoSe -w model exhibited an obvious increase (Fig.\nillustrate the weakened Co-Se bonding by Sr substitution, we calculated the Se vacancy formation\nenergy in these two selenide models (Fig. R19c), which showed an obvious decrease in Sr-CoSe -w\n2\nthat that in pure CoSe , clearly suggesting the weakened Co-Se bonding in these selenides by Sr\n2\nsubstitution. These results could also be verified by the experimental characterization. As obtained\nfrom EXAFS analysis (please see Supplementary Table 1), the increase of Co-Se bond length in\n1.0%Sr-CoSe -w than that in pure CoSe was in accordance with the weakened bond strength.\n2 2\nMoreover, the negative shift of Co 2p XPS peaks (please see Fig. 1f) and the positive shift of Sr\n3d XPS peaks (please see Supplementary Fig. 4) identified the substituted Sr atoms in selenides\ncould supply electrons more efficiently than Co atoms to bond Se sites, leading to the weakened\nCo-Se bond strength.\nSuch the weakened bond strength in selenide pre-catalysts by Sr-ion substitution can reasonably\nfacilitate the structural reconstruction under OER conditions to generate abundant Co-ion vacancy\nin reconstructed oxyhydroxide catalysts. As illustrated in the supplemented illustration Fig. R19d,\nunder OER conditions, these selenide pre-catalysts tended to be attacked by OH- ions from alkaline\nelectrolytes, together with the leaching of lattice Se due to the smaller bond dissociation energy of\n286.6 kJ mol-1 of Co-Se bonding (J. Chem. Phys. 152, 124305 (2020)) than that of 365.2 kJ mol-1\nof Co-O bonding (Comprehensive Handbook of Chemical Bond Energies, CRC Press, Boca Raton,\n(2007)), leading to the generation of reconstructed CoOOH catalysts. With the existence of Sr ions,\nthe weakened Co-Se bonding in Sr-CoSe -w pre-catalysts could result in an even lowered Co-Se\n2\nbond dissociation energy relative to that in pure CoSe , together with the intrinsically weak Sr-Se\n2\nbonding with a small bond dissociation energy of 251 kJ mol-1 relative to Co-Se bonding, thus\nleading to the facilitated structural reconstruction. Meanwhile, such the accelerated leaching of\nlattice Se during OER reconstruction could induce the much-lowered coordination of adjacent Co\nsites and thus accelerate the leaching of lattice Co to form abundant Co-ion vacancy in\nreconstructed CoOOH catalysts (denoted as Sr-CoOOH ). These results could also be verified by\nv\nthe experimental characterization. The variation of Raman spectra clearly suggested the\nreconstruction process from selenide pre-catalysts to reconstructed oxyhydroxide catalysts under\nOER conditions (please see Fig. 2b). From the EXAFS analysis (please see Fig. 2h,i), the Co-Co\ncoordination numbers decreased from 3.0 for CoOOH to 2.4 for 1.0%Sr-CoOOH , together with\nv\nthe well-maintained Co-O coordination numbers, clearly demonstrating the formation of abundant\nCo vacancies in reconstructed oxyhydroxide catalysts by Sr-mediated reconstruction.\nFig. R19. a, Established CoSe models with and without Sr-ion substitution. b, Comparison of the\nIllustration of Sr-ion-mediated reconstruction method to fabricate reconstructed CoOOH rich in\nCo-ion vacancy.\nTo make it more readable, we have added the relevant discussion, two figures, and two references\nin the revised manuscript (please see page 4, line 29-31 & page 5, line 1-2, 23-25, 27-31 & page\n21, ref. 35-36 & page 25, Fig. 2a,c), as well as two figures in the revised supporting information\n(please see page S2-S3, Supplementary Fig. 1-2).\n“7.The discussion of LOM activation lacks a direct comparison to recent works on lattice oxygen\nparticipation in Co-based catalysts (e.g., refs. 17, 19), which would better situate this study within\nthe field.”\nWe thank the reviewer for this suggestion. As suggested, we have made the comparison of LOM\nactivation mechanism between our work and the mentioned two references. In ref. 17 (Angew.\nChem. Int. Ed. 63, e202405839 (2024)), the lattice oxygen in the Co N-derived reconstructed\n4\ncatalysts was proved to be efficiently activated by Mn doping, where the Mn dopant could induce\nelectron enrichment around connected O to enhance Co-O covalency for lattice oxygen activation.\nSimilarly, in ref. 19 (Adv. Mater. 36, 2409530 (2024)), the LOM route in the multi-metallic\noxyhydroxide catalysts was proved to be efficiently activated by Zn doping, where the introduction\nof Zn in the CoFe oxyhydroxide could cause the shift of O 2p band toward Fermi level to trigger\nlattice oxygen activation. Obviously, these reported mechanisms for LOM activation depended\ncritically on the modulated electronic structure by introducing dopants, which would restrict the\navenues of developing high-performance OER catalysts. In our work, we have demonstrated a new\nperspective to activate LOM in dynamically reconstructed Sr-CoOOH catalysts by finely\nengineering Co-ion vacancies under OER conditions. Although the introduced Sr ions could\npartially induce increased electron loss from adjacent Co sites to bond O due to the limited valence-\nelectron providing ability of Sr ions relative to Co ions, it is still deficient to activate LOM as\nevidenced by the preferred AEM route for Sr-CoOOH only with Sr substitution (Fig. R20).\nf\nOf\nnote, our work proved that the engineered Co-ion vacancies in the Sr-CoOOH catalysts could\nv\ninduce extra valence-electron supply from adjacent Co sites to bond O to efficiently enhance the\nCo-O covalency, thus leading to the LOM activation. As such, our work can provide an\nenlightening and new avenue to trigger LOM for developing highly efficient OER catalysts by\nfinely engineering the metal-ion vacancies in the reconstructed catalysts, which is actually\ndifferent with the reported mechanisms in recent works (refs.17, 19).\nFig. R20. Gibbs free energies of both OER routes of LOM and LOM for Sr-CoOOH without any\nf\nCo-ion vacancy.\nTo make it more readable, we have added the relevant discussion in the revised manuscript (please\nsee page 11, line 28-30), as well as a figure in the revised supporting information (please see page\nS27, Supplementary Fig. 26).\nReviewer #4 (Remarks to the Author):\n“In this manuscript the authors propose that Sr incorporation with simultaneous formation of Co\nvacancies enhances the OER activity of CoOOH formed from CoSe2 under reaction conditions.\nConcerning the dynamic reconstruction to an oxyhydroxide taking place under operation\nconditions, the authors cite literature on copper nanocatalysts, but relevant literature that has\nconsidered the effect of surface transformation at oxide surfaces into an oxyhydroxide is not cited:\ne.g. ACS Catal. 2019, 9, 5, 3811–3821; ibid 2022, 12, 6, 3256; Nat. Mater. 20, 674–682, 2021;\nNat. Commun. 14, 8284, 2023”\nWe thank the reviewer for his/her careful reading of our manuscript with this constructive\nsuggestion. We have carefully considered the mentioned references, in which the dynamic\nreconstruction of metal-based compounds under OER conditions to form oxyhydroxide active\nphase for OER catalysis were insightfully discussed. These works demonstrated the main concerns\nand fundamentally important research topic about the dynamically structural reconstruction and\nmechanism investigation of the non-noble metal-based catalysts under OER conditions for\neffective and stable OER catalysis, which could further imply the necessity and significance of our\nwork of finely engineering metal-ion vacancies in dynamically reconstructed non-noble metal-\nbased catalysts under OER conditions with balanced OER activity and stability for AEMWE\nutilization. As suggested, we have cited the relevant literature in the revised manuscript (please\nsee page 3, line 31 & page 20, ref. 27-29).\n“Overall, the manuscript combines experimental and theoretical evidence form several techniques,\nbut a number of aspects need to be considered/clarified:”\nWe sincerely thank the reviewer for the valuable suggestions, which have helped us to improve\nthe quality of our work. We have addressed all of the reviewer’s comments as follows.\n“An enhanced covalency is proposed to trigger a switch from AEM to LOM mechanism – but the\nproof of such enhanced Co-O covalency is only based on the reduction of Co-O bonds, which does\nnot seem to be sufficient.”\nWe thank the reviewer for this suggestion. In our work, we have conducted a series of experimental\ncharacterizations to fully demonstrate the enhanced Co-O covalency. The EXAFS measurements\nshowcased the shrinkage of Co-O bond length from CoOOH to Co-vancancy-enriched Sr-\nCoOOH (please see Fig. 2h and Supplementary Table 2), suggesting the enhanced Co-O\nv\ncovalency. Meanwhile, the Co 2p XPS peaks presented a positive shift to higher energy side from\nCoOOH to Co-vancancy-enriched Sr-CoOOH , together with the negative shift to lower energy\nv\nside of Sr 3d XPS spectra, implying the enhanced valence-electron interaction of Co-O bonds\n(please see Fig. 2j and Supplementary Fig. 10b), in accordance with the elevated Co-O covalency.\nThe Co-O covalency could also be suggested by the increased content of O-Co bonds of O 1s XPS\nspectra from pure CoOOH to Co-vancancy-enriched Sr-CoOOH (please see Fig. 2k). Moreover,\nv\nin view of the fact that enhanced Co-O covalency could activate lattice oxygen to trigger the LOM\nroute, the identification of OER pathway could also reflect the Co-O covalency. In detail, relative\nto pure CoOOH without any Raman signals at around 1064 cm-1 within an AEM route, in-situ\nRaman spectra for Co-vancancy-enriched Sr-CoOOH showed a typical peak at 1064 cm-1 for\nv\nsuperoxide O-O bands due to the coupling of lattice oxygen (please see Fig. 4a,b), suggesting the\noccurrence of LOM route by enhanced Co-O covalency. Such the switched LOM route by\nenhanced Co-O covalency could also be proved by the pH-dependent OER activity for Co-\nvancancy-enriched Sr-CoOOH relative to the pH-independent OER activity for pure CoOOH\nv\n(please see Fig. 4c-e), as well as the obviously decreased OER activity in TMAOH for Co-\nvancancy-enriched Sr-CoOOH with the bonding of O-O coupling species generated in LOM route\nv\nby TMA+ (please see Fig. 4f-i). As results, the enhanced Co-O covalency has been fully proved in\nthis manuscript.\nTo make it more readable, we have added the detailed comparison of XPS peaks, the updated in-\nsitu Raman spectra, and the relevant discussion about the mechanism of detecting LOM route by\npH-dependence experiments and TMA+ inhibition experiments in the revised manuscript and\nsupporting information (please see page 7, line 16-17 & page 10, line 5-6, 8-11 & page 27, Fig.\n4a,b & page S35, Supplementary Table 4).\n“In the following I will concentrate on the DFT results: The authors start by stating that the “DFT\nmodels of Sr-CoOOHv and pure CoOOH were well-established”, but it is not clear what exactly\nis well established – eg. what polymorph of CoOOH is used for the calculations? In particular,\nthe data presented in Fig. 5 is not always sufficient and only after consulting the SI some of the\nstatements become clearer. In particular it is not clear what is shown in Fig 5a: what is described\nas an “electron loss” is presumably modulation of the charge state of the Co sites, but why is this\nmodulation /inhomogeneity stronger for the pristine CoOOH and the least for the Sr-CoOOHv? This\nseems counterintuitive. Moreover, this figure does not give enough information of the\nsimulation cell as it shows merely the Co positions. The position of oxygen and defects and, most\nimportantly, the variation in bond lengths are missing. These can be found only in Fig 12 in SI,\nwhich I find much more suitable to support the results in the main text in contrast to fig. 5a.”\nWe thank the reviewer for this constructive suggestion. For establishing the CoOOH models, we\nhave used the β-CoOOH for the calculations. Previous investigation has demonstrated that the β-\nCoOOH is stable and highly active at high OER potentials under alkaline conditions (J. Am. Chem.\nSoc. 135, 13521-13530, (2013)). This finding has made the β-CoOOH as a common used structure\nmodel for Co-based oxyhydroxide calculations under alkaline OER conditions in recent years.\nSimilar works by using β-CoOOH as structure models to study the alkaline OER process have\nbeen widely reported (Angew. Chem. Int. Ed. 63, e202404418 (2024); ACS Catal. 14, 13759-13767\n(2024)). Besides, the other detailed parameters for establishing these CoOOH models in our work\nhave been presented in the Methods section (page 14-15, the part of DFT calculations). To make\nit clearer, we have added the relevant discussion and a reference in the revised supporting\ninformation (please see page S21, line 2-3 & page S40, Supplementary Ref. 15).\nIn Fig. 5a, the electron loss in our discussion indeed represents the charge state of Co sites, where\nthe inhomogeneity of color represents the difference of electron loss at each Co sites. As mentioned\nby the reviewer, the pure CoOOH possesses high inhomogeneity, i.e., high difference of electron\nloss from each Co sites. This could be attributed to the Jahn-Teller effect in the octahedral\ncoordination structure. In detail, the Bader charge analysis in our work has presented the electron\nloss between 1.31 and 1.46 of these Co sites in all the established models, which suggests the\npreferred t 6e 1 distribution of valence electron with unevenly occupied e orbitals and thus tend\n2g g g\nto yield a strong Jahn-Teller distortion in these CoO coordination cells. Such the random Jahn-\nCo sites in pure CoOOH. As such, when introducing Sr atoms and Co vacancies, the significantly\nlimited valence-electron providing ability of Sr cations and valence-electron missing of Co\nvacancies can reasonably result in the enhanced electron loss from Co sites, thus leading to the\nmost homogeneous electron loss in Sr-CoOOH presented in Fig. 5a. To make it clearer, we have\nv\nadded the relevant discussion in the revised manuscript (please see page 10, line 26-27).\nBesides, we totally agree with the reviewer about increasing more information of the simulation\ncell in Fig. 5a by merging with Fig. 12 in SI. As shown in Fig. R21, the left part presents the\ncoordination structure of Co, O, Sr, and Co vacancy in the simulation cell, while the right part\nexhibits the electron loss from the corresponding Co sites in left part. Moreover, we have also tried\nto add the detailed bond lengths in left part. However, too massive numbers of bond lengths within\nthe left part of this figure made it hard to be labeled and recognized. Accordingly, we have to retain\na figure in supporting information to clearly illustrate the variation of bond length between these\nmodels. To make it more readable, we have made the relevant revision in the revised manuscript\nand supporting information (please see page 28, Fig. 5a & page S22, Supplementary Fig. 21).\nFig. R21. Valence-electron loss number of Co sites in CoOOH, Sr-CoOOH and Sr-CoOOH .\nf v\n“The authors state that they have considered different Co vacancy configuration but it remains\nunclear on what grounds the one shown in Fig. 5 is selected, the authrors should provide\ninforamtion on the relative stability. Moreover, which reaction sites for OER were probed and\nhow do the calculated overpotentials for those vary. Also, a comparison to previous literature on\nOER at CoOOH surfaces should be added.”\nWe thank the reviewer for his/her suggestions. As suggested, we have considered the structural\nstability by calculating the structure energy for the different Sr-CoOOH configurations with varied\nposition of Co vacancy sites, including nearest neighbor, next-nearest neighbor, and third-nearest\nneighbor positions relative to the Sr site. For simplified, we have denoted them as Sr-CoOOH\nv\nconfiguration with the nearest neighbor position of Co vacancy site relative to Sr site (Fig. R22a),\nSr-CoOOH configuration with the next-nearest neighbor position of Co vacancy sites relative\nv(N)\nto Sr site (Fig. R22b), and Sr-CoOOH configuration with the third-nearest neighbor position of\nv(T)\nCo vacancy site relative to Sr site (Fig. R22c), respectively. The relevant structural energies were\ncalculated and collected in Fig. R22d. Notably, the selected Sr-CoOOH configuration possessed\nv\na larger energy value of -701.03 eV than that of -700.16 eV and -699.99 eV for Sr-CoOOH and\nv(N)\nSr-CoOOH , respectively. The results implied that the selected Sr-CoOOH configuration with\nv(T) v\nthe nearest neighbor positions of Co vacancy sites relative to Sr sites was the most stable\nconfiguration among these structures. To make it clearer, we have added the detailed discussion\nand a figure in the revised manuscript and supporting information (please see page 12, line 3 &\npage S28, Supplementary Fig. 27).\nFig. R22. The configurations of a, Sr-CoOOH , b, Sr-CoOOH , and c, Sr-CoOOH .\nv v(N) v(T)\nPurple,\nblue, and pink spheres represent Sr, Co, and O atoms, respectively. d, Comparison of the\ncorresponding structure energies.\nMoreover, focusing on the selected Sr-CoOOH configuration, we have also probed the reaction\nv\nsites for OER and studied the relevant OER processes. Considering the proved LOM route for the\nSr-CoOOH configuration in Fig. 5, we have paid attention to the potential reaction sites in this\nv\nconfiguration for occurring the LOM route of OER. It should be noted that the reaction center in\nLOM would contains at least two adjacent Co sites due to the participation of lattice oxygen during\nreaction. As such, we have tested the potential reaction centers adjacent to Co vacancy of Sr-\nCoOOH for OER process. The reaction processes via LOM route at these active centers were\nv\nshown in Fig. R23a-d, where the corresponding Gibbs free energies for each reaction steps were\ncalculated (Fig. R23e). As calculated in Fig. R23f, the reaction center I possessed the lowest\noverpotential among these reaction centers, which was then selected as the final reaction center\nused in our work.\nFig. R23. a-d, LOM processes at potential reaction centers adjacent to Co vacancy of Sr-CoOOH .\nv\ne, Corresponding Gibbs free energies for each reaction steps. f, Calculated overpotentials.\nIn addition, we have also supplemented the comparison between the calculated overpotential in\nour work and previous literatures on OER at CoOOH surfaces. As shown in Fig. R24, the\ncalculated overpotential of Sr-CoOOH in our work is 0.35 V, which is comparable to those at\nv\nCoOOH surfaces in the reported literatures (please see Supplementary Ref. 16-20).\nFig. R24. Comparison of reported calculated overpotentials at CoOOH surfaces.\nTo make it clearer, we have added the relevant discussion and a figure about probing the reaction\nsites in the revised manuscript and supporting information (please see page 11, line 11-13 & page\nS24, Supplementary Fig. 23).\n“Fig. 5b shows pDOS of Co and O sites, but it is unclear whether these are averages over all Co/O\nsites or specific sites. Moreover, only a small energy range of the pDOS of the valence band is\ndisplayed, thus e.g. the figure does not provide any information on the charge or spin state of Co.\nTo this end, were any spin-dependent effects considered? Despite the shift of the band centers\nbetween the three different structures, the band overlap shown in Fig. 5b does not seem to change,\nthus it is questionable if the presented data actually indicate a difference in covalency.”\nWe thank this reviewer for his/her suggestions. When we conducted the pDOS analysis, we indeed\nhave considered all the potential Co/O sites. The relevant sites were shown in Fig. R25. For pure\nCoOOH, the pDOS was analyzed by averaging all the 16 Co and 16 O sites at the surface layer\n(Fig. R25a). For Sr-CoOOH with Sr substitution but not Co vacancy, we have taken the average\nf\nof 6 Co and 6 O atoms around Sr sites to study the effect of Sr substitution (Fig. R25b). For Sr-\nCoOOH with Sr substitution and Co vacancy, we have taken the average of 8 Co and 10 O atoms\nv\naround Sr and Co vacancy to study the effect of Sr substitution and Co vacancy (Fig. R25c). To\nmake it clearer, we have added the relevant description and a figure in the revised manuscript and\nsupporting information (please see page 11, line 5 & page S23, Supplementary Fig. 22).\nFig. R26. The selected Co sites (red circles) and O sites (black circles) for pDOS analysis in a,\nCoOOH, b, Sr-CoOOH, and c, Sr-CoOOH .\nf v\nMoreover, to provide a more complete picture of the electronic structure, we have updated the Fig.\n5b to show the spin-resolved pDOS for both the Co and O sites. This change could better reflect\nour underlying methodology. Our work has based on spin-polarized density functional theory\ncalculations, and the results of the d-band center shifts (as depicted in Fig. R26) could intrinsically\naccount for spin-dependent effects. The introduction of Co vacancies and Sr dopants induces\ngeometric changes in these CoOOH structures, which in turn influences the spin magnetic\nmoments on the Co atoms. The average spin magnetic moment of the Co atoms has been detected\nto change from 1.00 μB for pure CoOOH to 0.60 μB for Sr-CoOOH, and subsequently to 1.63 μB\nf\nfor Sr-CoOOH in our work, showing no obvious trend. In view of the ultimate effect of the\nv\nchanges in geometric structure and the variation in spin magnetic moment on the shift of band\ncenter of pDOS, we have used the band-center difference as covalent character of the Co-O bonds.\nTo make it clearer, we have updated the pDOS figure in the revised manuscript (please see page\n28, Fig. 5b).\nFig. R26. pDOS of Co and O sites.\nBesides, regarding the band overlap in Fig. 5b, the seemingly small change could be attributed to\nthe average effects of the d orbitals of multiple Co atoms and the p orbitals of multiple O atoms.\nFor these Co and O bands, since there are five d degenerate orbitals of each Co site relative to three\np degenerate orbitals of each O site, the p orbital band of single O site tend to be overlapped by\nthe d orbital band of single Co site. Moreover, when we actually considered all the possible Co/O\nsites for pDOS, including 16 Co/16 O in pure CoOOH, 6 Co/6 O in Sr-CoOOH, and 8 Co/10 O\nf\nin Sr-CoOOH , and took the average, the Co/O band overlap difference could become further\nv\ndifficult to be recognized. As such, we have not taken the degree of band overlap between Co and\nO to understand the covalency in our work. Instead, we have calculated the band-center difference\nbetween Co and O bonds to evaluate the covalency. To make it more readable, we have revised\nthe description of Fig. 5b without discussing the band overlap in our models (please see page 11,\nline 9-10).\n“5d is named “step of the coupling of O-O bond” – unclear what process is meant here.”\nThe reviewer raised a good point. As shown in the reaction processes of AEM and LOM in Fig.\n5c, both of the two routes started with a unified configuration. Afterwards, the step I of AEM route\nis the deprotonation of OH* to generate adsorbed O* intermediate, while the step I of LOM route\ncontains both the deprotonation of OH* to generate adsorbed O* and immediate O-O coupling\nwith a lattice oxygen to finally generate O-O coupling species. Accordingly, the LOM step I\ncontains the same intermediate with that generated after AEM step I, i.e., the adsorbed O*. Fig. 5d\npresents the activation barrier from the AEM intermediate after step Ⅰ to LOM intermediate after\nstep Ⅰ, which means the process from adsorbed O* to generate O-O coupling with a lattice oxygen.\nAs such, it is named as the step of the coupling of O-O bond from the AEM intermediate after step\nⅠ to LOM intermediate after step Ⅰ with lattice O participation.\nTo make it more readable, we have added the relevant description in the revised manuscript (please\nsee page 28, line 4 & page 11, line 15-16).\n“Figs 5g,h show Gibbs free energies as a function of the reaction coordinate for the LOM and\nAEM, but it is unclear what each step corresponds to, the specific steps need to be explicitly given\non the x-axis of the figures.”\nAs suggested, we have added the corresponding steps on the x-axis of the mentioned figures to\nclarify the process of each reaction step for AEM and LOM routes. These steps are in accordance\nwith the reaction process illustration in Fig. 5c and Supplementary Fig. 24.\n“No references are provided for the AEM and OER mechanisms used in the theoretical part and\ndescribed in the methods part. Moreover, it seems there is some confusion in using the terms AEM\nand LOM, as the one shown as AEM specifically involves a lattice oxygen site and corresponds\nrather to what is considered as LOM in the literature. Overall, it is difficult to see the reaction\nintermediates from the top views in Fig 5c and Fig 13 from SI, a better view of the intermediates\nis strongly recommended.”\nAs suggested, we have added the reference for the AEM and LOM mechanisms used in the\ntheoretical part and described in the methods part. The mechanisms of AEM and LOM in our work\nmatched well with those in the reference (Nat. Energy, 4, 329-338 (2019)). To make it clearer, we\nhave added the reference in the Methods part in the revised manuscript (please see page 15, line\n20).\nMoreover, based on the five elementary steps of AEM and LOM route in our work and reported\nliterature, the AEM route did not involve a lattice oxygen site. To make it more readable, we have\nchanged the perspective of intermediates in reaction process for a better view (Fig. R27,28).\nFig. R27. Illustration of OER mechanisms, including AEM (left) and LOM (right) routes for Sr-\nCoOOH .\nv\nFig. R28. Illustration of OER mechanisms, including AEM (left) and LOM (right) routes for a,\nCoOOH and b, Sr-CoOOH.\nf\nWe have updated the modulated view of intermediates during AEM and LOM routes in the revised\nmanuscript and supporting information (please see page 28, Fig. 5c & page S25, Supplementary\nFig. 24).\n“Since here no barriers are calculated, the term potential determining step should be used instead\nof rate limiting step. Furthermore, in line 276 the authors write activation barriers from the AEM\nintermediate to LOM intermediate were assessed – can the authors clarify what exactly is meant\nhere?”\nWe thank the reviewer for the suggestions. As suggested, we have changed the term of rate limiting\nstep with using potential determining step (please see page 11, line 26-27, 30 & page 12, line 2 &\npage S29, line 4,6).\nFurthermore, in view of the obvious difference of step I between AEM and LOM pathways (Fig.\n5c and Supplementary Fig. 24), the activation barriers from the AEM intermediate of adsorbed O\nafter step I to LOM intermediate after step Ⅰ for O-O coupling were then assessed to clarify the\nswitch difficulty of OER pathways. In detail, as shown in the reaction processes of AEM and LOM\nin Fig. 5c, both of the two routes started with a unified configuration. Afterwards, the step I of\nAEM route is the deprotonation of OH* to generate adsorbed O* intermediate, while the step I of\nLOM route contains both the deprotonation of OH* to generate adsorbed O* and immediate O-O\ncoupling with a lattice oxygen to finally generate O-O coupling species. Accordingly, the LOM\nstep I could generate the same intermediate with that generated after AEM step I, i.e., the adsorbed\nO*. The main difference between these two routes of step I is that the generated O* could\nimmediately couple with lattice O in LOM route, but stay on the surface for further OH adsorption\nin AEM route. As such, the activation barrier from the AEM intermediate after step Ⅰ to LOM\nintermediate after step Ⅰ means the process barrier from adsorbed O* to generate O-O coupling\nwith a lattice oxygen, where the barrier could be evaluated to reasonably clarify the switch\ndifficulty of OER pathways from AEM to LOM. To make it more readable, we have added the\nrelevant description in the revised manuscript (please see page 11, line 15-16).\n“Fig. 5b is used later on p 11 concerning the affinity to refill oxygen vacancies and it is argued\nthat the d-band of adjacent Co is narrower (hardly visible from the figure) and thus a higher OH\nadsorption affinity is expected, which is a very handwaving statement, considering the\nshortcomings of Fig 5b discussed above. Beyond showing electron density (here again it is unclear\nwhat view, i.e. which crystallographic orientation is shown in the inset and what are the reference\nsystems and the electron density isolevel), such statements should be based on energetics for\nincorporating O or OH.”\nWe genuinely thank this reviewer for his/her suggestion. To further quantify the narrowed d band\nin our work, we have quantitively calculated the d band width of these different structures by using\nthe recently reported method (Angew. Chem. Int. Ed. e202512466, (2025)). In detail, the d band\nwidth of Sr-CoOOH was calculated to be 3.8 eV, which was smaller than that of pure CoOOH\nv\n(3.9 eV), clearly suggesting the Sr-CoOOH with abundant Co vacancies could narrow the d band\nv\nfor enhanced OH adsorption ability relative to pure CoOOH. To make it clearer, we have added\nthe relevant description and a reference in the revised manuscript (please see page 12, line 12 &\npage 22, Ref. 57).\nBesides, we have added the reference systems, the electron density isolevel, and the\ncrystallographic orientation in Fig. 5h. As illustrated in the electron density difference of the OH\nadsorbates on CoOOH and Sr-CoOOH along the sections marked by gray lines in Fig. R29a,\nv\nrelative to pure CoOOH, the Co vacancy in Sr-CoOOH could trigger an enhanced valence-\nv\nelectron interaction between adjacent Co sites and OH adsorbates to refill the lattice oxygen\ndeficiency generated during LOM route, validating the promoted lattice oxygen refilling ability of\nSr-CoOOH . Such the efficient lattice oxygen refilling has been further verified by the Gibbs free\nv\nenergy change calculation (Fig. R29b). Notably, the Sr-CoOOH still exhibited a lower Gibbs free\nv\nenergy change of 0.79 eV for incorporating O into lattice than that of 1.20 eV for pure CoOOH,\ndemonstrating the efficient refilling ability of lattice oxygen of Sr-CoOOH and thus the\nv\nstrengthened OER stability. To make it clearer, we have revised the relevant contents in the revised\nmanuscript (please see page 12, line 21-22).\nFig. R29. a, Electron density difference of the OH adsorbates on CoOOH and Sr-CoOOH .\nv\nb, Energies for incorporating O into CoOOH and Sr-CoOOH .\nv\n“Some further technical issues, typos to be double checked:\n270 can minish the gap – pls check wording.\nBand-center gap – is here band-center difference meant?\nLine 128: Eg 127 and A1g vibrational models of Co-O in CoOOH – perhaps some typo – I guess\nvibrational modes are meant”\nWe thank the reviewer for these detailed suggestions. As suggested, we have double-checked the\nword of minish and replaced it with the word of diminish (please see page 11, line 6). Moreover,\nthe term of band-center gap here indeed represents the band-center difference, as which we have\nchanged the term to make it more readable (please see page 11, line 6, 10). In addition, we have\ncorrected the typo of mode (please see page 6, line 5). Besides, we have also carefully checked the\nwhole contents to avoid any other typos in the revised manuscript and supporting information. All\nof the revisions are highlighted in yellow color.\nPoint-by-point response to reviewers’ comments\nManuscript Number: NCOMMS-25-24017A\nMS Type: Article\nTitle: \" Engineering Co-ion vacancy in dynamically reconstructed Co-based catalysts for practical\nanion-exchange membrane electrolysis\"\nReviewer #1 (Remarks to the Author):\n“In the revised manuscript, the authors have addressed most of the issues from the reviewers. Thus,\nthis manuscript can be accepted for publication.”\nWe sincerely thank the reviewer for his/her positive recommendation.\nReviewer #2 (Remarks to the Author):\n“Now this manuscript has been well revised, which can be accepted as it is.”\nWe genuinely thank this reviewer for his/her careful reading of our manuscript and positive\nrecommendation.\nReviewer #3 (Remarks to the Author):\n“The authors have addressed the all comments and I agree with the publication of the present\nversion.”\nWe sincerely thank the reviewer for his/her valuable comment and positive recommendation.\nReviewer #5 (Remarks to the Author):\n“I’m not the original reviewer but agree with the reviewer in general: the DFT part was pretty\nsloppy. While the authors have addressed many of original comments, I still have substantial\nconcerns. Specifically, while the author claimed the study of a CoOOH phase, CoO2 (single layer?)\nseems to be actually used in the study. It is also demonstrated through the calculated magnetic\nmoment of 1.00 μB for Co. So, it is highly confusing and thus questionable for the correlation\nbetween experiment and calculations.”\nWe genuinely thank the covering reviewer for his/her careful reading of our manuscript with\npositive comments on our original revision and valuable suggestions which help us to further\nimprove the quality of our work.\nIn our work, we have established the two-layer CoOOH models for investigation, which matches\nwell with our experimental results. As shown in the experimental HRTEM results (Fig. R1a), the\nthickness of these reconstructed CoOOH nanosheets was examined to be 1.0 nm. Accordingly, we\nhave theoretically established the CoOOH models comprising two layers slab with each layer\ncontaining 32 Co, 64 O and 32 H atoms (please see Supplementary Fig. 20). Such the thickness of\nthe calculated two-layer CoOOH model matches well with that of experimental CoOOH\nnanosheets (Fig. R1b). The seemingly single-layer models from the side view in original Fig. 5c\nand similar original sub-figures in SI actually derived from the consideration of zooming in the\nlocal bond structures of these well-established two-layer CoOOH models during reaction, so that\nthe reaction process with detailed bond variation could be clearly presented. To make it more\nreadable, we have supplemented the relevant description about the thickness of established two-\nlayer CoOOH models (please see page S21, line 1, 3-5), and updated these present two-layer\nmodels in elementary reaction steps with enlarged reaction area of active centers to clearly\nunderstand the bond variation during reaction process in the revised manuscript and supporting\ninformation (please see page 29, Fig. 5c & page S25, Supplementary Fig. 24 & page S26,\nSupplementary Fig. 25 & page S31, Supplementary Fig. 30).\n.\nFig. R1. a, The HRTEM image of the experimentally prepared CoOOH nanosheets. b, The\ntheoretically established CoOOH model.\nIn addition, the observed magnetic moment of Co sites in these established CoOOH models could\nbe attributed to the local coordination configuration of H atoms in the layer gap of these two-layer\nCoOOH models. In detail, due to the breaking of symmetry, these H atoms in the layer gap of the\ntwo-layer CoOOH models could have two possible coordination sites (Fig. R2). When these H\natoms coordinated with the O atoms of the upper layer of the CoOOH model near the layer gap\nregion (denoted as CoOOH-u, Fig. R2a), the structure energy was calculated to be -701.54 eV,\ntogether with a magnetic moment of 0 μ of Co sites. When these H atoms coordinated with the O\nB\natoms of the bottom layer of the CoOOH model near the layer gap region (denoted as CoOOH-b,\nFig. R2b), the system exhibited an obviously lowered structure energy of -711.64 eV, together\nwith a magnetic moment of ~1 μ of Co sites. Notably, these CoOOH-b models with a magnetic\nB\nmoment are much more stable than the CoOOH-u models without magnetic moment (Fig. R2c),\naccounting for the finally obtained CoOOH models with a magnetic moment of ~1 μ of Co sites,\nB\ni.e., CoOOH-b models, after the geometric optimization from initial zero-magnetic-moment\nCoOOH-u models in our actual calculations. Furthermore, to verify the rationality of our calculated\nCoOOH models, we have systematically investigated the effect of different Hubbard U values\neff\napplied to the Co atoms on the total structure energy of these CoOOH systems. As shown in Table\nR1, the structure energy of the CoOOH-b remained consistently lower than that of CoOOH-u\nacross all the applied U values, further confirming the rationality of the finally selected CoOOH\neff\nmodels, i.e., CoOOH-b models, in our work. Such the theoretical results could also match well\nwith our experimental results. Experimentally, these ultrathin CoOOH nanosheets were obtained\nby electrochemical reconstruction from selenide pre-catalysts under OER conditions (please see\nFig. 2a). The EXAFS results clearly proved the generally unsaturated coordination environments\nof these ultrathin nanosheets relative to the bulk phase (please see Fig. 2h,i). It has been recently\ndemonstrated that for the Co3+ sites in CoOOH, the unsaturated coordination environments could\nincrease the 3d splitting energy and electron pairing energy, resulting in the formation of high-spin\nstate of Co3+ sites in CoOOH accompanied by unpaired electrons with a magnetic moment of Co3+\nsites (Nat. Commun. 15, 1383 (2024)). As such, the theoretically established CoOOH models could\nmatch well with the experimentally prepared ultrathin CoOOH nanosheets. Similar CoOOH\nmodels, where the layer-gap H atoms coordinated with the bottom-layer O atoms near the layer\ngap region, have also been demonstrated as the finally stable configuration for investigation in\nrecent work (Nat. Commun. 13, 2473, (2022)). To make it clearer, we have added a figure, a table\nand the relevant discussion in the revised manuscript and supporting information (please see page\n10, line 24 & page S21, line 1,3-5 & page S22, Supplementary Fig. 21 & page S42, Table 8).\nFig. R2. The two-layer models of a, CoOOH-u where the layer-gap H atoms coordinated with the\nupper-layer O atoms near the layer gap region, and b, CoOOH-b where the layer-gap H atoms\ncoordinated with the bottom-layer O atoms near the layer gap region. Blue, pink, and white spheres\nrepresent Co, O, and H atoms, respectively. c, Comparison of structure energy between these\nmodels.\nTable R1. Comparison of Energies of the CoOOH models under different U values.\neff\nEnergy\nModel\nU = 2.3 eV U = 3.3 eV U = 4.3 eV\neff eff eff\nCoOOH-u -721.16 eV -701.54 eV -682.73 eV\nCoOOH-b (This work) -726.55 eV -711.64 eV -698.36 eV\n“Based on figure 5d and e, there is an >1.2 eV activation barrier for the chemical step in LOM.\nIf the barrier is so high, how could the reaction happen at all?”\nThe reviewer raised a good point. As suggested, we have double-checked the activation barrier\ncalculations in Fig. 5d and e for the intermediate switch during the chemical step I in LOM.\nAs illustrated in Fig. 5c and Supplementary Fig. 25, the chemical step I of LOM contains both the\ndeprotonation of OH* with OH- to generate adsorbed O* and a water molecule (also same with\nthe AEM step I), and then the immediate O-O coupling with a lattice oxygen to finally generate\nO-O coupling species. Notably, the high calculated barrier of the process from adsorbed O* to\ngenerate O-O coupling with a lattice oxygen could be attributed to the neglected role of in-situ\ngenerated water molecule on the active centers during the chemical step. To this end, we have\nrecalculated the activation barriers for the intermediate switch during the chemical step I in LOM\nfor all the samples with considering the in-situ generated water molecule at active center. As shown\nin Fig. R3, the energy barrier for the intermediate switch from adsorbed O* to O-O coupling was\n0.51 eV for Sr-CoOOH , significantly lower than that of 1.52 eV for Sr-CoOOH and 1.68 eV for\nv f\npure CoOOH. Notably, relative to the Sr-CoOOH and pure CoOOH, the dramatically lowered\nf\nbarrier than 1.2 eV for Sr-CoOOH reasonably demonstrated the facilitated switch of OER route\nv\nfrom AEM for Sr-CoOOH and pure CoOOH to LOM for Sr-CoOOH with rich Co vacancies.\nf v\nFig. R3. Activation barriers for the formation of O-O bond and corresponding transition state\nconfiguration on a, Sr-CoOOH , b, Sr-CoOOH, and c, CoOOH. Purple, blue, pink, and white\nv f\nspheres represent Sr, Co, O, and H atoms, respectively. d, Comparison of the activation barriers.\nWe have updated the activation barrier calculations with relevant discussion and figures in the\nrevised manuscript and supporting information (please see page 11, line 18-19 & page 29, Fig.\n5d,e & page S27, Supplementary Fig. 26).\n“Based on the reviewer’s comments, the authors studied the relative stability of different systems.\nIn my opinion, the authors should also study the “absolute” stability of most stable systems, i.e.,\nthe formation energy of Sr-CoOOHf and Sr-CoOOHv under OER conditions with respect to\nCoOOH and some standard references. Specifically, electrochemical phase diagrams are needed\nto judge whether the structures proposed could exist under OER conditions or not.”\nWe thank this reviewer for his/her suggestion. As suggested, we have further investigated the\n“absolute” stability of each system by calculating the formation energy under OER conditions\naccording to the reported calculation methods (J. Phys. Chem. C 119, 18177, (2015); Nat. Energy\n2, 17070, (2017); Nat. Commun. 11, 2522, (2020); Angew. Chem. Int. Ed. 60, 14446, (2021); Nat.\nCommun. 14, 997, (2023)). Specifically, to clarify the structure existence under OER conditions,\nwe have provided the electrochemical phase diagram by calculating the formation free energy\n(ΔG ) of Sr-CoOOH, Sr-CoOOH , and Co(OH) reference under OER conditions with respect\nform f v 2\nto CoOOH. As shown in Fig. R4, relative to pure CoOOH, the Sr-CoOOH with only Sr\nf\nsubstitution exhibited a finitely enhanced structural stability under OER conditions, while the Sr-\nCoOOH with rich Co vacancy can significantly elevate the structure stability with dramatically\nv\nlowered formation free energy with applied OER potentials. These results matched well with the\nexperimental OER stability measurements in Fig. 3f-i and the structural analysis in Supplementary\nFig. 15 after long-term stability tests, where the Sr-CoOOH nanosheets presented a negligible\nv\nchange in OER performance relative to pure CoOOH nanosheets and well-maintained structure\nduring long-term OER tests. Besides, relative to pure CoOOH, the reduced potential of Sr-\nCoOOH and Sr-CoOOH for the transformation from Co(II) in Co(OH) reference to Co(III) in\nf v 2\nthese CoOOH systems were also consistent with the experimental reconstruction process for\npreparing these CoOOH samples under OER conditions (please see Fig. 2a,b), where the Sr-ion\nsubstitution were well-demonstrated to accelerate the reconstruction process from Co(II) pre-\ncatalysts to form Co(III) oxyhydroxides under OER conditions.\nFig. R4. The free energy diagrams of Sr-CoOOH, Sr-CoOOH , and Co(OH) reference with\nf v 2\nrespect to CoOOH.\nThe calculation details for the free energy of formation of these CoOOH systems are as follows.\nFor Sr-CoOOH, the free energy of formation was calculated by considering the following process:\nf\nCo O H + Sr2+ + 2OH- + H O → Sr-Co O H + H++ e- + Co3+·3OH- (1)\n32 64 32 2 31 64 32\nwhere the free energy of formation can be obtained as:\nΔG = {µ(Sr-Co O H ) + [µ(H+) + µ(e-)] + µ(Co3+·3OH-)} - {µ(Co O H ) + [µ(Sr2+) + 2\nform 31 64 32 32 64 32\nµ(OH-)] + µ(H O)} (2)\nchemical potentials of Sr²⁺ and OH- were replaced with:\nµ(Sr2+) + 2µ(OH-) = µ[Sr(OH) (s)] - 0.485 eV (3)\n2\nwhere -0.485 eV was the free energy of\nSr(OH) (s) → Sr2+(aq) + 2OH-(aq) (4)\nCo3+·3OH- + H+ + e- → Co2+·2OH- + H O (5)\n2\nwhere the free energy of -0.996 eV for this process includes Eq. 6 with the free energy of -0.17 eV\n(Front. Chem. 6, 231, (2018)) and Eq. 7 with the free energy of -0.826 eV:\nCo3+·3OH- + e- → Co2+·2OH- + OH- (6)\nH+ + OH- → H O (7)\nCo3+·3OH- + 1/2H → Co2+·2OH- + H O (8)\n2 2\nThen, the value of µ(Co3+·3OH-) can be calculated as:\nµ(Co3+·3OH-) = µ(Co2+·2OH-) + µ(H O) - 1/2µ(H ) + 0.996 eV (9)\n2 2\nAccordingly, under the alkaline OER conditions (pH=14), the free energy of formation for Sr-\nCoOOH could be calculated as:\nf\nΔG = {µ(Sr-Co O H ) + 1/2µ(H ) - eU - 0.826eV + µ(Co2+·2OH-) + µ(H O) - 1/2µ(H ) +\nform 31 64 32 2 2 2\n0.996 eV]} - {µ(Co O H ) + µ[Sr(OH) (s)] - 0.458 eV + µ(H O)} (10)\n32 64 32 2 2\nThen,\nΔG = 0.585 eV - eU (11)\nform\nSimilarly, for Sr-CoOOH , the free energy of formation can be calculated by considering the\nv\nfollowing process:\nCo O H + Sr2+ + 2OH- + 4H O → Sr-Co O H + 4H++ 4e- + 2(Co3+·3OH-) (12)\n32 64 32 2 30 64 32\nUnder the alkaline OER conditions (pH=14), the free energy of formation for Sr-CoOOH could\nv\nbe calculated as:\nΔG = {µ(Sr-Co O H ) + 2µ(H ) - 4eU - 4×0.826 eV + 2µ(Co2+·2OH-) + 2µ(H O) - µ(H )\nform 30 64 32 2 2 2\n+ 1.992 eV]} - {µ(Co O H ) + µ[Sr(OH) (s)] - 0.458 eV + 4µ(H O)} (13)\n32 64 32 2 2\nThen,\nΔG = 2.277 eV - 4eU (14)\nform\nTo make it clearer, we have added the relevant discussion with a figure and three references in the\nrevised manuscript and supporting information (please see page 12, line 24-27 & page 23, Ref. 58-\n60 & page S32, Supplementary Fig. 31).\n“The authors compared the calculated overpotential of Sr-CoOOHv with some doped CoOOH in\nthe literature. In my opinion, the authors should first compare the calculated overpotential of\nCoOOH with that of CoOOH in the literature. As the authors seem to study CoO2, a comparison\nwith CoO2 is also recommended (See the DFT results in doi.org/10.1038/s41467-023-36718-3)”\nWe thank the reviewer for this suggestion. As discussed above, we have reasonably established\nand studied the CoOOH models, rather than CoO , in our work for OER process. Accordingly, we\n2\nhave supplemented the comparison between the calculated overpotential of CoOOH in our work\nand that of CoOOH in reported literatures, as suggested by the reviewer. As shown in Fig. R5,\nthe calculated overpotential of pure CoOOH in our work is 0.78 V, which is comparable to those of\npure CoOOH in the reported literatures (Adv. Energy Mater. 15, 2403744 (2025); Adv. Energy\nMater. 13, 2301162 (2023); Appl. Catal. B: Environ. 320, 121988 (2023); Chem. Eng. J. 517,\n164529 (2025)). Moreover, relative to the pure CoOOH, we could gain an in-depth comparison\nbetween the calculated overpotential of Sr-CoOOH in our work and the other reported CoOOH-\nv\nbased catalysts in literatures. Notably, the calculated overpotential of Sr-CoOOH in our work is\nv\n0.35 V, which exhibits a tremendous decrease of 55% relative to pure CoOOH and is also superior\nthan the other reported CoOOH surfaces.\nFig. R5. Comparison of reported calculated overpotentials at CoOOH surfaces with pure CoOOH\nof each system as reference.\nWe have updated the figure with relevant discussion and references in the revised supporting\ninformation (please see page S25, Supplementary Fig. 24f & page S48, Supplementary Ref. 16-\n19).\n“Figure 5c and many other similar figures are illegible. I strongly recommend the authors make\neach sub-figure an independent figure to improve the readability.”\nWe thank the reviewer for this constructive suggestion. As suggested, we have zoomed in the\nreaction region and made each sub-figure an independent figure in Fig. 5c and the other similar\nfigures, including Supplementary Fig. 24, 25, & 30, for a better readability. The updated sub-\nfigures are also shown here in Fig. R6-10 as follows.\nFig. R6. Illustration of OER pathway, including AEM and LOM routes, for Sr-CoOOH .\nv\nFig. R7. LOM processes at potential reaction centers adjacent to Co vacancy of Sr-CoOOH .\nv\nFig. R8. The schematic of OER pathway for different Sr-CoOOH configurations with varied\nv\nposition of Co vacancy sites. a, Sr-CoOOH configuration with the next-nearest neighbor\nv(N)\npositions of Co vacancy sites relative to Sr sites. b, Sr-CoOOH configuration with the third-\nv(T)\nnearest neighbor positions of Co vacancy sites relative to Sr sites.\nFig. R9. Illustration of OER pathway, including AEM and LOM routes, for CoOOH.\nFig. R10. Illustration of OER pathway, including AEM and LOM routes for Sr-CoOOH.\nf\nTo make it more readable, we have updated the modulated sub-figures of elementary intermediates\nduring AEM and LOM routes in the revised manuscript and supporting information (please see\npage 29, Fig. 5c & page S25, Supplementary Fig. 24 & page S26, Supplementary Fig. 25 & page\nS31, Supplementary Fig. 30).\n“The Hubbard correction has been adopted from previous work. The Co oxidation trends they are\nstudying could significantly shift depending on the chosen U. A short validation within a narrow\nrange seems like a good sanity check.”\nAs suggested, we have conducted the calculations by changing the U value within a narrow range\neff\n(±0.2 eV) from the current value of 3.3 eV according to previous work (Nat. Commun. 14, 997,\n(2023)). As such, different U values, varying from 3.1 eV to 3.3 eV, and then to 3.5 eV, were\neff\napplied to the Co atoms of Sr-CoOOH to further investigate the reaction process. As shown in\nv\nFig. R11, under different U values, the deprotonation of OH* (step I) in AEM route or the\neff\ndeprotonation of OH* with immediate O-O coupling (step I) in LOM route still served as the\npotential-determining step (PDS) of OER. Notably, the PDS energy in AEM route was consistently\nlarger than that in LOM pathway under different U values, further demonstrating the switched\neff\nreaction pathway of LOM route for Sr-CoOOH even with varied U values. To make it clearer,\nv eff\nwe have added the relevant discussion and a figure in the revised manuscript and supporting\ninformation (please see page 11, line 31 & page 12, line 1 & page S29, Supplementary Fig. 28).\nFig. R11. Gibbs free energies of AEM and LOM routes for Sr-CoOOH under different U values.\nv eff\n“They mention ZPE correction values at 298 K, but no actual list of correction values is shown.\nIncluding these values in the SI, can increase the reproducibility of the results.”\nWe thank the reviewer for this detailed suggestion. As suggested, we have added the lists of\ncorrection values used in the free energy calculations in Table R2-4. In detail, Table R2 shows the\ncorrection values of the zero-point energies (ZPE), integrated heat capacity (∫C dT), entropy at\nv\n298.15 K (T∆S) and the correction energies (∆G ) for the free energy calculations of H O and\ncorr 2\nH molecules. Meanwhile, Table R3-4 present the correction values of ZPE, ∫C dT, T∆S, and ∆G\n2 v corr\nfor the free energy calculations of intermediates of AEM and LOM routes, respectively.\nTable R2. Correction values used in the free energy calculations of molecules.\nMolecule ZPE (eV) ∫C dT (eV) T∆S (eV) ∆G (eV)\nv corr\nH O 0.58 0.10 0.65 0.03\nCoOOH.\nSpecies ZPE (eV) ∫C dT (eV) T∆S (eV) ∆G (eV)\nv corr\nOH∗ 0.38 0.03 0.05 0.36\nO∗ 0.08 0.02 0.03 0.07\nOOH∗ 0.46 0.08 0.14 0.40\nOO∗ 0.14 0.09 0.18 0.06\nTable R4. Correction values used in the free energy calculations of LOM intermediates on\nCoOOH.\nSpecies ZPE (eV) ∫C dT (eV) T∆S (eV) ∆G (eV)\nv corr\nOH∗ 0.47 0.06 0.09 0.44\nOO∗ 0.16 0.06 0.09 0.13\n[OO+HO]∗ 0.54 0.11 0.20 0.45\n[O +HO]∗ 0.39 0.03 0.04 0.38\nv\nOHv∗ 0.77 0.07 0.11 0.72\nTo make it clearer, we have added the relevant discussion and tables of the detailed correction\nvalues in the revised manuscript and supporting information (please see page 16, line 11-12 &\npage S44-46, Supplementary Table 10-12).\nReviewer #6 (Remarks to the Author):\n“I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is\npart of the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts.”\nWe sincerely thank this co-reviewer for his/her careful reading of our manuscript and valuable\nsuggestions which help us to further improve the quality of our work."}, {"round": 2, "reviewer_comments": {"reviewer_1": "In the revised manuscript, the authors have addressed most of the issues from the reviewers. Thus, this manuscript can be accepted for publication.", "reviewer_2": "Now this manuscript has been well revised, which can be accepted as it is.", "reviewer_3": "The authors have addressed the all comments and I agree with the publication of the present version.", "reviewer_5": "The authors have addressed my comments. I recommend it for publication now.", "reviewer_6": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Point-by-point response to reviewers’ comments\nManuscript Number: NCOMMS-25-24017A\nMS Type: Article\nTitle: \" Engineering Co-ion vacancy in dynamically reconstructed Co-based catalysts for practical\nanion-exchange membrane electrolysis\"\nReviewer #1 (Remarks to the Author):\n“In the revised manuscript, the authors have addressed most of the issues from the reviewers. Thus,\nthis manuscript can be accepted for publication.”\nWe sincerely thank the reviewer for his/her positive recommendation.\nReviewer #2 (Remarks to the Author):\n“Now this manuscript has been well revised, which can be accepted as it is.”\nWe genuinely thank this reviewer for his/her careful reading of our manuscript and positive\nrecommendation.\nReviewer #3 (Remarks to the Author):\n“The authors have addressed the all comments and I agree with the publication of the present\nversion.”\nWe sincerely thank the reviewer for his/her valuable comment and positive recommendation.\nReviewer #5 (Remarks to the Author):\n“I’m not the original reviewer but agree with the reviewer in general: the DFT part was pretty\nsloppy. While the authors have addressed many of original comments, I still have substantial\nconcerns. Specifically, while the author claimed the study of a CoOOH phase, CoO2 (single layer?)\nseems to be actually used in the study. It is also demonstrated through the calculated magnetic\nmoment of 1.00 μB for Co. So, it is highly confusing and thus questionable for the correlation\nbetween experiment and calculations.”\nWe genuinely thank the covering reviewer for his/her careful reading of our manuscript with\npositive comments on our original revision and valuable suggestions which help us to further\nimprove the quality of our work.\nIn our work, we have established the two-layer CoOOH models for investigation, which matches\nwell with our experimental results. As shown in the experimental HRTEM results (Fig. R1a), the\nthickness of these reconstructed CoOOH nanosheets was examined to be 1.0 nm. Accordingly, we\nhave theoretically established the CoOOH models comprising two layers slab with each layer\ncontaining 32 Co, 64 O and 32 H atoms (please see Supplementary Fig. 20). Such the thickness of\nthe calculated two-layer CoOOH model matches well with that of experimental CoOOH\nnanosheets (Fig. R1b). The seemingly single-layer models from the side view in original Fig. 5c\nand similar original sub-figures in SI actually derived from the consideration of zooming in the\nlocal bond structures of these well-established two-layer CoOOH models during reaction, so that\nthe reaction process with detailed bond variation could be clearly presented. To make it more\nreadable, we have supplemented the relevant description about the thickness of established two-\nlayer CoOOH models (please see page S21, line 1, 3-5), and updated these present two-layer\nmodels in elementary reaction steps with enlarged reaction area of active centers to clearly\nunderstand the bond variation during reaction process in the revised manuscript and supporting\ninformation (please see page 29, Fig. 5c & page S25, Supplementary Fig. 24 & page S26,\nSupplementary Fig. 25 & page S31, Supplementary Fig. 30).\n.\nFig. R1. a, The HRTEM image of the experimentally prepared CoOOH nanosheets. b, The\ntheoretically established CoOOH model.\nIn addition, the observed magnetic moment of Co sites in these established CoOOH models could\nbe attributed to the local coordination configuration of H atoms in the layer gap of these two-layer\nCoOOH models. In detail, due to the breaking of symmetry, these H atoms in the layer gap of the\ntwo-layer CoOOH models could have two possible coordination sites (Fig. R2). When these H\natoms coordinated with the O atoms of the upper layer of the CoOOH model near the layer gap\nregion (denoted as CoOOH-u, Fig. R2a), the structure energy was calculated to be -701.54 eV,\ntogether with a magnetic moment of 0 μ of Co sites. When these H atoms coordinated with the O\nB\natoms of the bottom layer of the CoOOH model near the layer gap region (denoted as CoOOH-b,\nFig. R2b), the system exhibited an obviously lowered structure energy of -711.64 eV, together\nwith a magnetic moment of ~1 μ of Co sites. Notably, these CoOOH-b models with a magnetic\nB\nmoment are much more stable than the CoOOH-u models without magnetic moment (Fig. R2c),\naccounting for the finally obtained CoOOH models with a magnetic moment of ~1 μ of Co sites,\nB\ni.e., CoOOH-b models, after the geometric optimization from initial zero-magnetic-moment\nCoOOH-u models in our actual calculations. Furthermore, to verify the rationality of our calculated\nCoOOH models, we have systematically investigated the effect of different Hubbard U values\neff\napplied to the Co atoms on the total structure energy of these CoOOH systems. As shown in Table\nR1, the structure energy of the CoOOH-b remained consistently lower than that of CoOOH-u\nacross all the applied U values, further confirming the rationality of the finally selected CoOOH\neff\nmodels, i.e., CoOOH-b models, in our work. Such the theoretical results could also match well\nwith our experimental results. Experimentally, these ultrathin CoOOH nanosheets were obtained\nby electrochemical reconstruction from selenide pre-catalysts under OER conditions (please see\nFig. 2a). The EXAFS results clearly proved the generally unsaturated coordination environments\nof these ultrathin nanosheets relative to the bulk phase (please see Fig. 2h,i). It has been recently\ndemonstrated that for the Co3+ sites in CoOOH, the unsaturated coordination environments could\nincrease the 3d splitting energy and electron pairing energy, resulting in the formation of high-spin\nstate of Co3+ sites in CoOOH accompanied by unpaired electrons with a magnetic moment of Co3+\nsites (Nat. Commun. 15, 1383 (2024)). As such, the theoretically established CoOOH models could\nmatch well with the experimentally prepared ultrathin CoOOH nanosheets. Similar CoOOH\nmodels, where the layer-gap H atoms coordinated with the bottom-layer O atoms near the layer\ngap region, have also been demonstrated as the finally stable configuration for investigation in\nrecent work (Nat. Commun. 13, 2473, (2022)). To make it clearer, we have added a figure, a table\nand the relevant discussion in the revised manuscript and supporting information (please see page\n10, line 24 & page S21, line 1,3-5 & page S22, Supplementary Fig. 21 & page S42, Table 8).\nFig. R2. The two-layer models of a, CoOOH-u where the layer-gap H atoms coordinated with the\nupper-layer O atoms near the layer gap region, and b, CoOOH-b where the layer-gap H atoms\ncoordinated with the bottom-layer O atoms near the layer gap region. Blue, pink, and white spheres\nrepresent Co, O, and H atoms, respectively. c, Comparison of structure energy between these\nmodels.\nTable R1. Comparison of Energies of the CoOOH models under different U values.\neff\nEnergy\nModel\nU = 2.3 eV U = 3.3 eV U = 4.3 eV\neff eff eff\nCoOOH-u -721.16 eV -701.54 eV -682.73 eV\nCoOOH-b (This work) -726.55 eV -711.64 eV -698.36 eV\n“Based on figure 5d and e, there is an >1.2 eV activation barrier for the chemical step in LOM.\nIf the barrier is so high, how could the reaction happen at all?”\nThe reviewer raised a good point. As suggested, we have double-checked the activation barrier\ncalculations in Fig. 5d and e for the intermediate switch during the chemical step I in LOM.\nAs illustrated in Fig. 5c and Supplementary Fig. 25, the chemical step I of LOM contains both the\ndeprotonation of OH* with OH- to generate adsorbed O* and a water molecule (also same with\nthe AEM step I), and then the immediate O-O coupling with a lattice oxygen to finally generate\nO-O coupling species. Notably, the high calculated barrier of the process from adsorbed O* to\ngenerate O-O coupling with a lattice oxygen could be attributed to the neglected role of in-situ\ngenerated water molecule on the active centers during the chemical step. To this end, we have\nrecalculated the activation barriers for the intermediate switch during the chemical step I in LOM\nfor all the samples with considering the in-situ generated water molecule at active center. As shown\nin Fig. R3, the energy barrier for the intermediate switch from adsorbed O* to O-O coupling was\n0.51 eV for Sr-CoOOH , significantly lower than that of 1.52 eV for Sr-CoOOH and 1.68 eV for\nv f\npure CoOOH. Notably, relative to the Sr-CoOOH and pure CoOOH, the dramatically lowered\nf\nbarrier than 1.2 eV for Sr-CoOOH reasonably demonstrated the facilitated switch of OER route\nv\nfrom AEM for Sr-CoOOH and pure CoOOH to LOM for Sr-CoOOH with rich Co vacancies.\nf v\nFig. R3. Activation barriers for the formation of O-O bond and corresponding transition state\nconfiguration on a, Sr-CoOOH , b, Sr-CoOOH, and c, CoOOH. Purple, blue, pink, and white\nv f\nspheres represent Sr, Co, O, and H atoms, respectively. d, Comparison of the activation barriers.\nWe have updated the activation barrier calculations with relevant discussion and figures in the\nrevised manuscript and supporting information (please see page 11, line 18-19 & page 29, Fig.\n5d,e & page S27, Supplementary Fig. 26).\n“Based on the reviewer’s comments, the authors studied the relative stability of different systems.\nIn my opinion, the authors should also study the “absolute” stability of most stable systems, i.e.,\nthe formation energy of Sr-CoOOHf and Sr-CoOOHv under OER conditions with respect to\nCoOOH and some standard references. Specifically, electrochemical phase diagrams are needed\nto judge whether the structures proposed could exist under OER conditions or not.”\nWe thank this reviewer for his/her suggestion. As suggested, we have further investigated the\n“absolute” stability of each system by calculating the formation energy under OER conditions\naccording to the reported calculation methods (J. Phys. Chem. C 119, 18177, (2015); Nat. Energy\n2, 17070, (2017); Nat. Commun. 11, 2522, (2020); Angew. Chem. Int. Ed. 60, 14446, (2021); Nat.\nCommun. 14, 997, (2023)). Specifically, to clarify the structure existence under OER conditions,\nwe have provided the electrochemical phase diagram by calculating the formation free energy\n(ΔG ) of Sr-CoOOH, Sr-CoOOH , and Co(OH) reference under OER conditions with respect\nform f v 2\nto CoOOH. As shown in Fig. R4, relative to pure CoOOH, the Sr-CoOOH with only Sr\nf\nsubstitution exhibited a finitely enhanced structural stability under OER conditions, while the Sr-\nCoOOH with rich Co vacancy can significantly elevate the structure stability with dramatically\nv\nlowered formation free energy with applied OER potentials. These results matched well with the\nexperimental OER stability measurements in Fig. 3f-i and the structural analysis in Supplementary\nFig. 15 after long-term stability tests, where the Sr-CoOOH nanosheets presented a negligible\nv\nchange in OER performance relative to pure CoOOH nanosheets and well-maintained structure\nduring long-term OER tests. Besides, relative to pure CoOOH, the reduced potential of Sr-\nCoOOH and Sr-CoOOH for the transformation from Co(II) in Co(OH) reference to Co(III) in\nf v 2\nthese CoOOH systems were also consistent with the experimental reconstruction process for\npreparing these CoOOH samples under OER conditions (please see Fig. 2a,b), where the Sr-ion\nsubstitution were well-demonstrated to accelerate the reconstruction process from Co(II) pre-\ncatalysts to form Co(III) oxyhydroxides under OER conditions.\nFig. R4. The free energy diagrams of Sr-CoOOH, Sr-CoOOH , and Co(OH) reference with\nf v 2\nrespect to CoOOH.\nThe calculation details for the free energy of formation of these CoOOH systems are as follows.\nFor Sr-CoOOH, the free energy of formation was calculated by considering the following process:\nf\nCo O H + Sr2+ + 2OH- + H O → Sr-Co O H + H++ e- + Co3+·3OH- (1)\n32 64 32 2 31 64 32\nwhere the free energy of formation can be obtained as:\nΔG = {µ(Sr-Co O H ) + [µ(H+) + µ(e-)] + µ(Co3+·3OH-)} - {µ(Co O H ) + [µ(Sr2+) + 2\nform 31 64 32 32 64 32\nµ(OH-)] + µ(H O)} (2)\nchemical potentials of Sr²⁺ and OH- were replaced with:\nµ(Sr2+) + 2µ(OH-) = µ[Sr(OH) (s)] - 0.485 eV (3)\n2\nwhere -0.485 eV was the free energy of\nSr(OH) (s) → Sr2+(aq) + 2OH-(aq) (4)\nCo3+·3OH- + H+ + e- → Co2+·2OH- + H O (5)\n2\nwhere the free energy of -0.996 eV for this process includes Eq. 6 with the free energy of -0.17 eV\n(Front. Chem. 6, 231, (2018)) and Eq. 7 with the free energy of -0.826 eV:\nCo3+·3OH- + e- → Co2+·2OH- + OH- (6)\nH+ + OH- → H O (7)\nCo3+·3OH- + 1/2H → Co2+·2OH- + H O (8)\n2 2\nThen, the value of µ(Co3+·3OH-) can be calculated as:\nµ(Co3+·3OH-) = µ(Co2+·2OH-) + µ(H O) - 1/2µ(H ) + 0.996 eV (9)\n2 2\nAccordingly, under the alkaline OER conditions (pH=14), the free energy of formation for Sr-\nCoOOH could be calculated as:\nf\nΔG = {µ(Sr-Co O H ) + 1/2µ(H ) - eU - 0.826eV + µ(Co2+·2OH-) + µ(H O) - 1/2µ(H ) +\nform 31 64 32 2 2 2\n0.996 eV]} - {µ(Co O H ) + µ[Sr(OH) (s)] - 0.458 eV + µ(H O)} (10)\n32 64 32 2 2\nThen,\nΔG = 0.585 eV - eU (11)\nform\nSimilarly, for Sr-CoOOH , the free energy of formation can be calculated by considering the\nv\nfollowing process:\nCo O H + Sr2+ + 2OH- + 4H O → Sr-Co O H + 4H++ 4e- + 2(Co3+·3OH-) (12)\n32 64 32 2 30 64 32\nUnder the alkaline OER conditions (pH=14), the free energy of formation for Sr-CoOOH could\nv\nbe calculated as:\nΔG = {µ(Sr-Co O H ) + 2µ(H ) - 4eU - 4×0.826 eV + 2µ(Co2+·2OH-) + 2µ(H O) - µ(H )\nform 30 64 32 2 2 2\n+ 1.992 eV]} - {µ(Co O H ) + µ[Sr(OH) (s)] - 0.458 eV + 4µ(H O)} (13)\n32 64 32 2 2\nThen,\nΔG = 2.277 eV - 4eU (14)\nform\nTo make it clearer, we have added the relevant discussion with a figure and three references in the\nrevised manuscript and supporting information (please see page 12, line 24-27 & page 23, Ref. 58-\n60 & page S32, Supplementary Fig. 31).\n“The authors compared the calculated overpotential of Sr-CoOOHv with some doped CoOOH in\nthe literature. In my opinion, the authors should first compare the calculated overpotential of\nCoOOH with that of CoOOH in the literature. As the authors seem to study CoO2, a comparison\nwith CoO2 is also recommended (See the DFT results in doi.org/10.1038/s41467-023-36718-3)”\nWe thank the reviewer for this suggestion. As discussed above, we have reasonably established\nand studied the CoOOH models, rather than CoO , in our work for OER process. Accordingly, we\n2\nhave supplemented the comparison between the calculated overpotential of CoOOH in our work\nand that of CoOOH in reported literatures, as suggested by the reviewer. As shown in Fig. R5,\nthe calculated overpotential of pure CoOOH in our work is 0.78 V, which is comparable to those of\npure CoOOH in the reported literatures (Adv. Energy Mater. 15, 2403744 (2025); Adv. Energy\nMater. 13, 2301162 (2023); Appl. Catal. B: Environ. 320, 121988 (2023); Chem. Eng. J. 517,\n164529 (2025)). Moreover, relative to the pure CoOOH, we could gain an in-depth comparison\nbetween the calculated overpotential of Sr-CoOOH in our work and the other reported CoOOH-\nv\nbased catalysts in literatures. Notably, the calculated overpotential of Sr-CoOOH in our work is\nv\n0.35 V, which exhibits a tremendous decrease of 55% relative to pure CoOOH and is also superior\nthan the other reported CoOOH surfaces.\nFig. R5. Comparison of reported calculated overpotentials at CoOOH surfaces with pure CoOOH\nof each system as reference.\nWe have updated the figure with relevant discussion and references in the revised supporting\ninformation (please see page S25, Supplementary Fig. 24f & page S48, Supplementary Ref. 16-\n19).\n“Figure 5c and many other similar figures are illegible. I strongly recommend the authors make\neach sub-figure an independent figure to improve the readability.”\nWe thank the reviewer for this constructive suggestion. As suggested, we have zoomed in the\nreaction region and made each sub-figure an independent figure in Fig. 5c and the other similar\nfigures, including Supplementary Fig. 24, 25, & 30, for a better readability. The updated sub-\nfigures are also shown here in Fig. R6-10 as follows.\nFig. R6. Illustration of OER pathway, including AEM and LOM routes, for Sr-CoOOH .\nv\nFig. R7. LOM processes at potential reaction centers adjacent to Co vacancy of Sr-CoOOH .\nv\nFig. R8. The schematic of OER pathway for different Sr-CoOOH configurations with varied\nv\nposition of Co vacancy sites. a, Sr-CoOOH configuration with the next-nearest neighbor\nv(N)\npositions of Co vacancy sites relative to Sr sites. b, Sr-CoOOH configuration with the third-\nv(T)\nnearest neighbor positions of Co vacancy sites relative to Sr sites.\nFig. R9. Illustration of OER pathway, including AEM and LOM routes, for CoOOH.\nFig. R10. Illustration of OER pathway, including AEM and LOM routes for Sr-CoOOH.\nf\nTo make it more readable, we have updated the modulated sub-figures of elementary intermediates\nduring AEM and LOM routes in the revised manuscript and supporting information (please see\npage 29, Fig. 5c & page S25, Supplementary Fig. 24 & page S26, Supplementary Fig. 25 & page\nS31, Supplementary Fig. 30).\n“The Hubbard correction has been adopted from previous work. The Co oxidation trends they are\nstudying could significantly shift depending on the chosen U. A short validation within a narrow\nrange seems like a good sanity check.”\nAs suggested, we have conducted the calculations by changing the U value within a narrow range\neff\n(±0.2 eV) from the current value of 3.3 eV according to previous work (Nat. Commun. 14, 997,\n(2023)). As such, different U values, varying from 3.1 eV to 3.3 eV, and then to 3.5 eV, were\neff\napplied to the Co atoms of Sr-CoOOH to further investigate the reaction process. As shown in\nv\nFig. R11, under different U values, the deprotonation of OH* (step I) in AEM route or the\neff\ndeprotonation of OH* with immediate O-O coupling (step I) in LOM route still served as the\npotential-determining step (PDS) of OER. Notably, the PDS energy in AEM route was consistently\nlarger than that in LOM pathway under different U values, further demonstrating the switched\neff\nreaction pathway of LOM route for Sr-CoOOH even with varied U values. To make it clearer,\nv eff\nwe have added the relevant discussion and a figure in the revised manuscript and supporting\ninformation (please see page 11, line 31 & page 12, line 1 & page S29, Supplementary Fig. 28).\nFig. R11. Gibbs free energies of AEM and LOM routes for Sr-CoOOH under different U values.\nv eff\n“They mention ZPE correction values at 298 K, but no actual list of correction values is shown.\nIncluding these values in the SI, can increase the reproducibility of the results.”\nWe thank the reviewer for this detailed suggestion. As suggested, we have added the lists of\ncorrection values used in the free energy calculations in Table R2-4. In detail, Table R2 shows the\ncorrection values of the zero-point energies (ZPE), integrated heat capacity (∫C dT), entropy at\nv\n298.15 K (T∆S) and the correction energies (∆G ) for the free energy calculations of H O and\ncorr 2\nH molecules. Meanwhile, Table R3-4 present the correction values of ZPE, ∫C dT, T∆S, and ∆G\n2 v corr\nfor the free energy calculations of intermediates of AEM and LOM routes, respectively.\nTable R2. Correction values used in the free energy calculations of molecules.\nMolecule ZPE (eV) ∫C dT (eV) T∆S (eV) ∆G (eV)\nv corr\nH O 0.58 0.10 0.65 0.03\nCoOOH.\nSpecies ZPE (eV) ∫C dT (eV) T∆S (eV) ∆G (eV)\nv corr\nOH∗ 0.38 0.03 0.05 0.36\nO∗ 0.08 0.02 0.03 0.07\nOOH∗ 0.46 0.08 0.14 0.40\nOO∗ 0.14 0.09 0.18 0.06\nTable R4. Correction values used in the free energy calculations of LOM intermediates on\nCoOOH.\nSpecies ZPE (eV) ∫C dT (eV) T∆S (eV) ∆G (eV)\nv corr\nOH∗ 0.47 0.06 0.09 0.44\nOO∗ 0.16 0.06 0.09 0.13\n[OO+HO]∗ 0.54 0.11 0.20 0.45\n[O +HO]∗ 0.39 0.03 0.04 0.38\nv\nOHv∗ 0.77 0.07 0.11 0.72\nTo make it clearer, we have added the relevant discussion and tables of the detailed correction\nvalues in the revised manuscript and supporting information (please see page 16, line 11-12 &\npage S44-46, Supplementary Table 10-12).\nReviewer #6 (Remarks to the Author):\n“I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is\npart of the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts.”\nWe sincerely thank this co-reviewer for his/her careful reading of our manuscript and valuable\nsuggestions which help us to further improve the quality of our work."}]

Decision letter