Defect-interface coupling for stable lattice-oxygen-driven oxygen evolution at industrial current densities

Domain: neuroscience
Published: 2026-01-29
Editorial outcome: major_revision

Abstract
For industrial water electrolysis, the development of active and stable catalysts for the oxygen evolution reaction remains a challenge. Here, we report a heterostructure catalyst composed of NiFe layered double hydroxide nanosheets anchored on pyramidal Fe (MoO ) to activate lattice oxygen for efficient and durable oxygen evolution. Our investigation reveals that oxygen vacancies within the NiFe layered double hydroxide and the internal electrical field at the material interface optimize the electronic states, allowing oxygen atoms within the crystal lattice to participate directly in the reaction. The resulting heterostructured NiFe LDH/FeMoO catalysts possess high oxygen evolution reaction activity in 1M KOH electrolyte with a low overpotential of 316 mV at 2 A cm−2 and maintain long-term stability over 3,000 h. Furthermore, integrating this anode into a solar-powered electrolyzer yields a high solar-to-hydrogen efficiency of 20.15%. This work provides a promising strategy for designing stable catalysts and advancing the integration of renewable energy with water electrolysis to produce clean hydrogen at scale.

Introduction
Anion exchange membrane water electrolysis (AEMWE) has emerged as a promising technology for sustainable production of hydrogen, combining the advantages of low-cost alkaline water electrolysis (AWE) and the high current density capability of proton exchange membrane water electrolysis (PEMWE)1,2. However, its overall efficiency is fundamentally constrained by the sluggish reaction kinetics of the oxygen evolution reaction (OER) at the anode, which involves a multi-step four-electron transfer process and requires high overpotentials3. Conventional OER electrocatalysts acting through the adsorbate evolution mechanism (AEM) are limited by a linear scaling relationship between *OH and *OOH intermediates, requiring a theoretical overpotential of more than 0.37 V4–6. In contrast, the lattice oxygen mechanism (LOM), wherein lattice oxygen directly participates in O–O bond formation, bypasses this scaling constraint and enables substantially enhanced intrinsic activity7–9. Electrocatalysts designed to activate the LOM thus offer a viable pathway to reduce overpotentials and unlock the full performance potential of AEMWE systems.

NiFe-based hydroxides, especially NiFe layered double hydroxide (LDH), are promising OER electrocatalysts in alkaline conditions due to their low cost, high intrinsic activity, and flexible structure10,11. Recent efforts have focused on activating the LOM in NiFe LDH catalysts in order to overcome the activity limitations imposed by the conventional AEM12,13. Strategies such as heteroatom doping and heterostructure have been shown to be effective in modulating the covalence of metal-oxygen and enabling lattice oxygen redox14–16. However, activation of the LOM often undermines long-term durability of catalysts, as repeated lattice-oxygen redox and vacancy cycling progressively destabilize the structure and degrade the catalytic performance17–19. To address this issue, strategies have been reported to either prevent the direct consumption of lattice oxygen or enable the reversible regeneration to reinforce the structural integrity of electrocatalysts20–22. For instance, the introduction of oxygen vacancies in the catalyst can further trigger the lattice oxygen-mediated mechanism-oxygen vacancy site mechanism (LOM-OVSM), enabling the cycling of reversible oxygen species20,21. Coupling NiFe LDH with Ni Mo alloy has been introduced to act as an oxygen pump, continuously replenishing lattice oxygen and mitigating vacancy-induced degradation22.

Beyond the structural instability induced by lattice-oxygen activation, the dissolution of Fe poses a severe challenge for NiFe-based catalysts in AEMWE. In alkaline OER, Fe is leached predominantly as ferrate (FeO 2−), a chemically irreversible process that extracts lattice oxygen and disrupts the redox cycle, thereby further aggravating the lattice-oxygen instability23–25. This ferrate formation is accelerated under harsh industrial conditions, including high current densities (≥2 A cm−2) and concentrated alkaline electrolytes (30 wt.% KOH), which lead to the loss of active-site and rapid degradation of the performance26. Currently, most LOM activation strategies focus on increasing redox activity, but often overlook the chemical stability of metallic components under these industrial operating conditions. Therefore, the balance between high LOM-driven activity and robust stability at high current densities remains a key challenge in the design of OER electrocatalysts.

Here, we show a heterostructure electrocatalyst consisting of Fe (MoO ) with a pyramid structure wrapped in amorphous NiFe LDH nanosheets (named NiFe LDH/FeMoO) for use as an efficient and stable OER. The NiFe LDH/FeMoO electrode produces oxygen with a low overpotential of only 316 mV at 2 A cm−2 in 1M KOH. Remarkably, the NiFe LDH/FeMoO electrode can operate stably at 2 A cm−2 for 3000 h with a low operating voltage degradation rate of only 4.5 μV h−1. Using NiFe LDH/FeMoO anode and Y-NiMo/MoO 2−x cathode in an AEMWE, an industrial-level performance of 2 A cm−2 at 2.0 V can be achieved. A photovoltaic (PV)-AEMWE system with high solar-to-hydrogen efficiency (20.15%) under practical current densities has been further demonstrated. This work provides fundamental insights into the design of robust OER catalysts and offers a practical strategy for advancing integrated solar-to-hydrogen conversion systems.

Results
The NiFe LDH/FeMoO electrode was synthesized via a facile one-step water bath process. Specifically, clean nickel foam (NF) was used as substrate, immersed in a precursor solution containing sources of Ni, Fe, and Mo for 6 h at 90°C (Supplementary Fig. 1). In this heterostructure, NiFe LDH is amorphous due to a competitive growth mechanism during the synthesis27. For comparison, NiFe LDH was synthesized by removing the source of Mo from the precursor solution under the same conditions (Supplementary Fig. 2). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) elucidate the microstructure of the NiFe LDH/FeMoO heterostructure. The NiFe LDH/FeMoO heterostructure formed by incorporation of the NiFe LDH nanosheets and FeMoO pyramid structure is evenly distributed on the NF substrate (Fig. 1a and Supplementary Fig. 3). In the heterostructure, FeMoO serves as a support with a pyramidal edge length of approximately 800 ± 25 nm, and NiFe LDH is characterized by thin nanosheets with a rough surface (Supplementary Fig. 4). The heterostructure configuration is adjustable according to the reaction time (Supplementary Fig. 5). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and elemental mapping (Fig. 1b, Supplementary Figs. 6 and 7) show a uniform distribution of Ni, Fe, and O across the nanosheets, while Fe, Mo, and O are concentrated in the pyramidal structure. These results confirm that the encapsulation of the pyramidal FeMoO with NiFe LDH nanosheets results in a well-defined heterostructure. The high-resolution TEM (HRTEM) image shows the distinct lattice fringes with a spacing of 0.207 nm, corresponding to the (121) plane of FeMoO (Fig. 1c). The adjacent amorphous regions are assigned to the NiFe LDH. As shown in Supplementary Fig. 8, the selected area electron diffraction (SAED) pattern of the nanosheet region displays broad and diffuse features. This is consistent with the amorphous nature of NiFe LDH, where weak rings are indexed to the NiFe LDH (110) and (012) planes. Figure 1d shows the X-ray diffraction (XRD) patterns of NiFe LDH and NiFe LDH/FeMoO. For the NiFe LDH/FeMoO heterostructure, three diffraction peaks are observed at 10.7°, 27.0°, and 29.4°, corresponding to the planes of (013), (028), and (051) for Fe (MoO ) crystal, respectively. The lack of detectable NiFe LDH diffraction peaks confirms its low crystallinity as observed in HRTEM and SAED. Even after a longer reaction time, only an increase in intensity of the diffraction peaks attributable to FeMoO is observed (Supplementary Fig. 9). This is probably due to the amorphous nature of NiFe LDH, which makes it susceptible to ion diffusion limitations, whereas the crystalline structure of FeMoO allows for a continuous epitaxial growth. Their structural properties were further verified with Raman spectroscopy (Supplementary Fig. 10).

The coordination and electronic structure of the NiFe LDH/FeMoO electrocatalyst were characterized by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The normalized Ni K-edge X-ray absorption near-edge structure (XANES) spectra (Fig. 1e) show that the absorption edge energy of NiFe LDH and NiFe LDH/FeMoO is higher than that of Ni foil (Ni0) and NiO (Ni2+), indicating that the average oxidation state of Ni is greater than +2. Moreover, a slight shift of the absorption edge of NiFe LDH/FeMoO in comparison with NiFe LDH toward the higher energy region is consistent with the XPS results (Supplementary Fig. 11a). This can be attributed to the transfer of charge from NiFe LDH to FeMoO in the heterostructure, which reduces the electron density around Ni and increases its oxidation state. In the normalized Fe K-edge XANES spectra (Fig. 1f), the absorption edge energy of NiFe LDH and NiFe LDH/FeMoO is significantly higher than that of Fe foil (Fe0) and Fe O (Fe3+), implying that the average oxidation state of Fe is greater than +3. Interestingly, in contrast to Ni, the Fe absorption edge of NiFe LDH/FeMoO is shifted to a lower energy relative to NiFe LDH, consistent with the negative shift in XPS Fe 2p binding energy (Supplementary Fig. 11b). The opposite shifts observed for Ni and Fe indicate a charge redistribution across the heterostructure interface, generating an intrinsic electric field directed from NiFe LDH to FeMoO. The high-resolution Mo 3d XPS spectra (Supplementary Fig. 11c) reveal two main peaks at ~235.3 eV and ~232.2 eV, which can be assigned to the Mo 3d 5/2 and Mo 3d 3/2 levels of Mo6+ ion in the MoO 2− units, confirming the presence of the Fe (MoO ) compound in the heterostructure28. The coordination structure of the NiFe LDH/FeMoO heterostructure was further investigated using Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) spectroscopy. As depicted in Fig. 1g, the Ni K-edge FT-EXAFS spectra show two prominent peaks for NiFe LDH/FeMoO at 1.65 and 2.79 Å, respectively, corresponding to the Ni–O and Ni–M (M = Ni, Fe, Mo) coordination. Similarly, the Fe K-edge FT-EXAFS analysis (Fig. 1h) reveals Fe–O and Fe–M coordination at 1.62 and 2.85 Å, respectively. Compared to NiFe LDH, NiFe LDH/FeMoO heterostructure exhibits a decreased peak intensity of Ni–M and Fe–M coordination. This observation explains the introduction of lattice disorder during the formation of the heterostructure of amorphous nature of NiFe LDH29. Meanwhile, the significant attenuation of peak intensity of Ni–O and Fe–O coordination in NiFe LDH/FeMoO likely originates from the generation of oxygen vacancy19. Oxygen vacancy characteristics were systematically investigated through high-resolution O 1s XPS. As shown in Fig. 1i, deconvolution of the O 1s XPS signal reveals four distinct components corresponding to the lattice oxygen (M–O, 529.6 eV), hydroxyl groups (M–OH, 531.0 eV), oxygen vacancies (Ov, 532.0 eV), and adsorbed water molecules (O–O, 532.8 eV)30. Quantitative analysis based on the peak areas shows that the NiFe LDH/FeMoO heterostructure possesses a remarkably high oxygen vacancy content of 27.9%, which is significantly higher than that of the well-crystallized NiFe LDH (11.8%) (Supplementary Table 1). Electron paramagnetic resonance (EPR) analysis also suggests an increase in the concentration of oxygen vacancies in the NiFe LDH/FeMoO heterostructure (Supplementary Fig. 14)31.

The electrocatalytic OER performance of the NiFe LDH/FeMoO electrode was systematically assessed in an alkaline solution (1M KOH) using a standard three-electrode configuration. The reference electrode was calibrated with a reversible hydrogen electrode (RHE). Electrocatalysts such as NiFe LDH and commercial IrO were also tested for comparison. Figure 2a displays the linear sweep voltammetry (LSV) polarization curves of different electrocatalysts (90% iR compensation). The polarization curves without iR compensation are shown in Supplementary Fig. 16a. The NiFe LDH/FeMoO electrode exhibits notable OER performance requiring a low overpotential of 289 at 1 A cm−2 and 316 mV at 2 A cm−2, respectively (Fig. 2b). This is much lower than for the NiFe LDH (335 and 399 mV) and the IrO electrode (444 and 577 mV). The NiFe LDH/FeMoO electrode requires a low overpotential of 336 mV to achieve a high current density of 3 A cm−2. Supplementary Table 2 compares the overpotentials at various current densities of NiFe LDH/FeMoO with those of recently reported NiFe LDH-based electrocatalysts.

The NiFe LDH/FeMoO electrode has a smaller Tafel slope (48.9 mV dec−1) than that of NiFe LDH (70.7 mV dec−1), suggesting that the former has a more rapid OER kinetics (Supplementary Fig. 16b). This result also points out a possible change in the OER mechanism. The performance of the catalyst at high current densities is influenced by activity and mass transfer32. The slope of the overpotential versus current density was analyzed to determine the relative contribution of these factors. As shown in Fig. 2c, the NiFe LDH/FeMoO heterostructure exhibits a moderate increase in Δη/Δlog|j| from 53.0 to 113.6 mV dec−1 over the current density range of 0.1-3 A cm−2, in stark contrast to NiFe LDH (56.9 to 386.4 mV dec−1). This indicates that the NiFe LDH/FeMoO heterostructure enhances reaction kinetics and facilitates rapid mass transfer at high current densities. The thin, rough NiFe LDH nanosheets shorten the pathways for mass transfer, while the pyramidal FeMoO protrusions prevent the formation of gas bubbles and promote the recovery of electrolytes. This is further supported by contact angle measurements (Supplementary Fig. 17).

The resistance of the interfacial charge transfer (R ) of the OER was determined by electrochemical impedance spectroscopy (EIS) (Supplementary Figs. 18 and 19), and the results are listed in Supplementary Tables 3 and 4. The NiFe LDH/FeMoO electrode shows a lower R than that of NiFe LDH at all tested potentials (Fig. 2d). This enhancement reflects the good interfacial charge‐transfer kinetics of NiFe LDH/FeMoO heterostructure. The electrochemical surface area (ECSA) of the NiFe LDH/FeMoO electrode was estimated to be 58.7 cm−2 with cyclic voltammetry (Supplementary Fig. 20), which is higher than that of the flat NiFe LDH (48.8 cm−2). The intrinsic activity of electrocatalysts was also compared through the ECSA-normalized polarization curves in Supplementary Fig. 21, showing that the NiFe LDH/FeMoO electrode maintains the lowest overpotential for the OER. In addition, the turnover frequency (TOF) of NiFe LDH and NiFe LDH/FeMoO was assessed to allow a clear comparison of their catalytic ability to the OER (Fig. 2e and Supplementary Fig. 22). At an overpotential of 300 mV, the NiFe LDH/FeMoO achieves a TOF of 0.140 s−1, which surpasses the NiFe LDH (0.056 s−1).

The NiFe LDH/FeMoO heterostructure shows extraordinary OER activity of lower overpotential and Tafel slope, suggesting a significant change in the reaction kinetics. Figure 2f shows the OER polarization curves of NiFe LDH/FeMoO in electrolytes with different pH levels, suggesting a strong pH dependence for the OER activity of NiFe LDH/FeMoO. Compared to the AEM pathway, the LOM pathway generates superoxide (O −) and peroxide (O 2−) anionic species, which serve as compelling indicators to determine whether the OER mechanism follows the LOM pathway4,33. These species were probed using tetramethylammonium cations (TMA⁺), which have a strong interaction with oxygen species. Figure 2g shows that the OER performance of the NiFe LDH/FeMoO heterostructure is significantly reduced in 1M TMAOH electrolyte, as shown by increased polarization voltage and Tafel slope, whereas NiFe LDH is less affected (Supplementary Fig. 25).

To further investigate the OER mechanisms in NiFe LDH and NiFe LDH/FeMoO, differential electrochemical mass spectrometry (DEMS) was employed to monitor the oxygen signals generated during the OER process (Supplementary Fig. 26). In traditional AEM, 18O-labeled catalysts should only produce 32O during OER in a H 16O electrolyte. Therefore, the release of 34O reflects the involvement of lattice oxygen34. The DEMS measurements showed a strong 34O signal and a weaker 36O signal, in addition to the normal 32O signal. This result indicates that the 18O-labeled lattice oxygen is activated and participates in the formation of 18O16O and 18O18O (Supplementary Fig. 27). Figure 2h shows the 34O /32O ratio over four cyclic voltammetry (CV) cycles, which was used to quantify the contribution of the LOM35. The 34O /32O ratio for NiFe LDH/FeMoO was approximately twice that for NiFe LDH, providing direct evidence that lattice oxygen in NiFe LDH/FeMoO participates more actively in the OER process via the LOM pathway. Furthermore, the gradual decrease in the 34O /32O ratio in NiFe LDH/FeMoO over multiple cycles indicates that the 18O-labeled lattice oxygen was gradually consumed and replaced by 16O from the electrolyte36.

Apart from activity, electrocatalyst durability is a critical parameter for industrial applications, particularly under large current densities. As shown in Fig. 2i, the NiFe LDH/FeMoO heterostructure exhibits long-term stability, with a degradation rate of merely ~4.5 μV h−1 for the operating voltage over 3000 h of continuous operation at 2 A cm−2. Extrapolating this degradation rate suggests an increase of only ~88 mV in overpotential after 2 years of continuous operation. In contrast, NiFe LDH can only maintain stable performance for approximately 300 h before undergoing a significant performance decline. SEM and TEM images reveal that the specific architecture of the heterostructure, consisting of thin NiFe LDH nanosheets that wrap around the FeMoO pyramids, remains intact even after prolonged testing (Supplementary Figs. 28 and 29). In contrast, the NiFe LDH structure is susceptible to nanosheet agglomeration and structural collapse. These results highlight the industrial application potential of the NiFe LDH/FeMoO heterostructure.

In-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) was used to monitor the intermediates and to elucidate the mechanism of the reaction during oxygen evolution on NiFe LDH and NiFe LDH/FeMoO. The electrocatalysts were drop-cast onto a gold-coated silicon prism and tested in an electrochemical cell (Supplementary Figs. 30 and 31). ATR-SEIRAS spectra of NiFe LDH and NiFe LDH/FeMoO electrocatalysts were collected at different applied potentials (0.8-2.0 V), and the results are shown in Fig. 3a and b. The observed absorption peaks at ~1640 cm−1 correspond to the H–O–H bending mode (δ H–O–H ) of interfacial water molecules at open circuit potential (OCP)37. With the increase in applied potential, a new absorption at ~1110 cm−1 is observed, which is assigned to the *OOH intermediate38–40. This result supports the involvement of an AEM pathway for the OER process. Meanwhile, an absorption peak at ~1230 cm−1 corresponding to the *OO intermediate is observed, indicating the LOM pathway for OER41,42. The coverage of *OO and *OOH intermediates gradually increases as the potential increases, leading to a corresponding enhancement in peak intensity. Notably, NiFe LDH exhibits a more pronounced *OOH peak, indicating that the OER process predominantly follows the AEM pathway. In contrast, the NiFe LDH/FeMoO heterostructure shows a significantly stronger *OO signal than *OOH, suggesting greater involvement of the LOM pathway30.

Density functional theory (DFT) calculations were performed to provide insights into the catalytic enhancement of the NiFe LDH/FeMoO heterostructure. A heterostructure model was constructed with a top layer of NiFe LDH rich in oxygen-vacancies and a bottom layer of Fe(MoO ) (Supplementary Fig. 33a) according to the experimental results. NiFe LDH without oxygen vacancy was also constructed and assessed for comparison (Supplementary Fig. 33b). Differential charge density of NiFe LDH/FeMoO heterostructure was evaluated. Bader charge analysis was then performed along the Z axis, which is defined as the direction perpendicular to the interface, extending from the NiFe LDH layer toward the FeMoO substrate. A pronounced charge redistribution is revealed at the interface between NiFe LDH and FeMoO via differential charge density analysis (Fig. 3c). Specifically, electrons are depleted in the NiFe LDH layer but accumulated in the FeMoO layer. This electron density gradient suggests a net charge migration from NiFe LDH to FeMoO, consistent with the XANES measurements. This electron redistribution indicates the generation of an electric field within the heterostructure interface, thereby forming a space charge region that modulates the electronic structure of the catalyst.

The projected density of states (DOS) was analyzed to investigate the changes in the electronic states of the NiFe LDH/FeMoO heterostructure. Supplementary Fig. 34 presents the DOS of NiFe LDH and NiFe LDH/FeMoO heterostructure. A significant overlap between the d-band of metal and the 2p band of oxygen for NiFe LDH/FeMoO, indicating enhanced covalency between the metal centers and oxygen ligands. This improved covalency promotes the electron delocalization in the NiFe LDH/FeMoO heterostructure, which provides the basis for the lattice oxygen participation in the OER process15. Moreover, the partial density of states (PDOS) analysis shows that the O 2p band center (ε p) shifts from −3.67 eV in NiFe LDH to −3.22 eV in the NiFe LDH/FeMoO heterostructure (Fig. 3d). This upward shift can be attributed to the abundant oxygen vacancies present in NiFe LDH, which elevate the O 2p states closer to the Fermi level and facilitate electron delocalization from oxygen centers. The increased proximity between the O 2p band and the Fermi level enhances the electronic coupling with adsorbed species under anodic potentials, thereby accelerating lattice oxygen oxidation and promoting a shift in the OER pathway toward the LOM43. According to molecular orbital theory, the hybridization between O 2p and Ni 3d orbitals leads to the formation of metal-oxygen (M–O) antibonding bands, which are split into an unoccupied upper Hubbard band (UHB) and an occupied lower Hubbard band (LHB)44,45. The calculated positions of the LHB and UHB centers, along with their corresponding energy gaps (U), for NiFe LDH and NiFe LDH/FeMoO are summarized in Fig. 3e and Supplementary Table 5. Notably, the NiFe LDH/FeMoO heterostructure exhibits a larger energy gap U value of 6.65 eV compared to 6.07 eV for NiFe LDH, which can be ascribed to the intrinsic electric field at the hetero-interface that depletes electron density within the NiFe LDH layer, thereby reducing local dielectric screening46. The increased U value induces a downward shift of the LHB, as illustrated in Fig. 3f, significantly facilitating electron removal from oxygen sites under anodic polarization47. Importantly, the LHB center in NiFe LDH/FeMoO is located below the O 2p band center (Fig. 3f). Consequently, the combined effect of the downward shift of the LHB and the upward shift of the O 2p band center reduces the overlap between the metal 3d and oxygen 2p orbitals, thereby weakening the M–O bond strength. The presence of weaker M–O bonds lowers the activation energy for lattice oxygen release12,48. Together, these insights demonstrate that the cooperative regulation of oxygen vacancies and interfacial electric fields fundamentally reprogram the electronic structure of catalyst, thereby triggering the LOM pathway and enabling comparable OER performance.

The Gibbs free energies for OER via the AEM and LOM were calculated for the NiFe LDH/FeMoO heterostructure to validate the reaction mechanism (Supplementary Figs. 35–37 and Supplementary Table 6). As illustrated in Fig. 3g, the rate-determining step (RDS) for OER via the AEM pathway on NiFe LDH/FeMoO involves the evolution of *O to *OOH with a thermodynamic barrier of 0.58 eV. In contrast, the RDS for the LOM pathway shifts to the transformation of *O–OH to *O–O, reducing the barrier to 0.46 eV. These results indicate that the NiFe LDH/FeMoO heterostructure enhances OER kinetics by favoring the LOM pathway. As illustrated in Fig. 3h, the OER process via the LOM pathway begins with the adsorption of OH− anions onto lattice oxygen through nucleophilic attack. This leads to direct O–O coupling and the formation of O , which then desorbs from the electrocatalyst surface. The generated oxygen vacancy is subsequently replenished by OH−, regenerating active sites for continuous catalysis.

According to the above reaction mechanism, strong adsorption affinity for OH− can effectively repair the oxygen vacancies generated along the LOM pathway, thereby suppressing the propagation of lattice defects and minimizing subsequent structural degradation. As shown in Supplementary Fig. 39a, b, the broad absorption band observed in the 2800–3800 cm−1 range in the in-situ ATR-SEIRAS corresponds to the O–H stretching vibrations (ν O–H ) of interfacial water molecules37. The intensity of the ν O–H vibration increases progressively with the applied potential, indicating increasing adsorption of water molecules at the charged interface. Furthermore, the variation in the intensity of this band under different applied potentials was compared between NiFe LDH and NiFe LDH/FeMoO (Supplementary Fig. 39c). The NiFe LDH/FeMoO heterostructure exhibits a significantly stronger signal, indicating a higher surface coverage of *OH intermediates that facilitates the regeneration of lattice oxygen22,49. Laviron analysis was further conducted to investigate the adsorption behavior of OH− on the electrocatalyst surfaces50,51. As shown in Supplementary Fig. 41, steady-state redox currents associated with OH− transfer exhibit a linear correlation with the square root of the scan rate, indicating diffusion-limited adsorption behavior52. The calculated rate constant (k) for NiFe LDH/FeMoO is 0.14 s−1, significantly higher than that of NiFe LDH (0.11 s−1) (Fig. 4a). This supports stronger OH− adsorption affinity on the NiFe LDH/FeMoO surface.

In-situ Raman spectroscopy was performed to further understand the evolution of the local structure of the catalyst during the OER process, which was collected within the potential range from the OCP to 1.41 V for the NiFe LDH and NiFe LDH/FeMoO electrodes (Supplementary Figs. 42 and 43). As shown in Fig. 4b, vibration peaks at 451 and 521 cm−1, which are attributed to Ni–OH and Ni–O in NiFe LDH/FeMoO, were consistently observed until the applied potential reached 1.32 V. As the potential increased to 1.35 V, two peaks appeared at 476 and 557 cm−1 corresponding to the E g and A 1g bending and stretching vibration modes of Ni–O in γ-NiOOH53,54. A distinct peak at ~1060 cm−1 was observed, which can be assigned to the vibrations of interlayer carbonate species in the NiFe LDH55. In addition, the peaks at 780 cm−1 and 904 cm−1, originating from the asymmetric and symmetric stretching modes of the Mo–O bonds in the MoO units, were also detected56. With the applied potential increased from OCP to 1.41 V, the peak at 904 cm−1 remained clearly discernible, indicating the good structural stability of the pyramidal framework. In contrast, the peak at 780 cm−1 gradually decreased in intensity and eventually disappeared. This is likely due to the intrinsically weaker signal of FeMoO located beneath the NiFe LDH overlayer, combined with signal interference caused by gas bubble formation during electrolysis57.

In addition, as the voltage increased from the OCP to 1.41 V, the peaks of vibration corresponding to Mo–O bonding in FeMoO were always clearly visible, indicating the notable structural stability of its pyramid structure. Similarly, the nearly identical transformation process has been observed for NiFe LDH (Fig. 4c). It should be noted that the phase-transition potentials for NiFe LDH and NiFe LDH/FeMoO were 1.29 and 1.35 V, respectively. These results suggest that the nickel redox process of the NiFe LDH/FeMoO heterostructure is delayed by the aid of lattice oxygen oxidation12. Although elevated metal oxidation states are often observed in LOM-active catalysts, lattice-oxygen participation does not necessarily require highly oxidized metal centers15,58. In the NiFe LDH/FeMoO heterostructure, interfacial charge redistribution and oxygen-vacancy-induced modulation of the O 2p band enhance oxygen-hole character while suppressing excessive oxidation of Ni and Fe sites. Therefore, compared to NiFe LDH, NiFe LDH/FeMoO can be more easily stabilized during the OER process.

To further elucidate the underlying stabilization mechanism, a chronopotentiometric test was conducted on NiFe LDH/FeMoO and NiFe LDH in 10M KOH under a high current density of 4 A cm−2 to accelerate the catalytic degradation. As shown in Supplementary Fig. 45, NiFe LDH/FeMoO electrode maintained its initial performance within 30 h, whereas the activity of NiFe LDH decreases to 96% of its initial value. The electrolyte was then examined every 4 h and the concentrations of Ni and Fe ions was measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES). As depicted in Fig. 4d, in NiFe LDH/FeMoO, Fe experienced slight dissolution in the early stage of the test and then remained stable at approximately 0.03 mg L−1, whereas in NiFe LDH, the leaching of Fe gradually increased to 0.29 mg L−1 along with the declined activity. Ni shows almost no dissolution in NiFe LDH/FeMoO and NiFe LDH (Fig. 4d), because Ni-based compounds are thermodynamically relatively stable under OER conditions59. To determine the form in which Fe elements are dissolved in the electrolyte from the catalyst, the electrolyte was examined with an ultraviolet-visible (UV-Vis) spectrophotometer after a 30-h chronopotentiometric test60. As shown in Fig. 4e, a signal peak at approximately 500 nm appears in the case of NiFe LDH, which corresponds to the d-d electron transition of the highly oxidized FeO 2−61. In contrast, no such signal was observed for NiFe LDH/FeMoO, indicating the suppression of Fe over-oxidation. This is attributed to the formation of fast electron transport channels by the intrinsic electric field at the heterostructure interface, which effectively prevents the excessive oxidation and dissolution of iron species.

Collectively, the good stability of NiFe LDH under industrial conditions can be primarily ascribed to three synergistic effects (Fig. 4f). Firstly, the pyramidal FeMoO framework provides robust mechanical support for NiFe LDH nanosheets, mitigating bubble-induced detachment and enhancing electrolyte diffusion kinetics. Secondly, the enhanced OH− chemisorption facilitates rapid replenishment of lattice oxygen vacancies, maintaining structural integrity during prolonged OER operation. Thirdly, the built-in electric field at the heterostructure interface accelerates charge transfer, suppressing over-oxidation and subsequent dissolution of active metal species.

To further assess the practical feasibility of the catalysts, we constructed an AEMWE cell utilizing a self-supported NiFe LDH/FeMoO anode and a Y-NiMo/MoO 2−x 62 cathode, as illustrated in Fig. 5a. A conventional AEMWE was also prepared using commercial catalysts of IrO 2 and Pt/C loaded onto NF as an anode and cathode for comparison. As shown in Fig. 5b, the electrolyzer based on NiFe LDH/FeMoO||NiMo/MoO 2−x requires 1.7 V to reach a current density of 1 A cm−2 and 2.0 V for 2 A cm−2 at 60°C, respectively, which was supplemented with a 3M KOH solution at a flow rate of 380 mL min−1. In contrast, the IrO 2||Pt/C electrolyzer requires a 1.9 V to reach a current density of 1 A cm−2. This performance, summarized in Supplementary Table 7, surpasses most state-of-the-art AEMWEs. In addition, the NiFe LDH/FeMoO||Y-NiMo/MoO 2−x electrolyzer demonstrates durability over a period of 100 h at a current density of 1 A cm−2 with minimal voltage fluctuations (Fig. 5c). In contrast, the IrO 2||Pt/C electrolyzer exhibits a rapid decline in performance under the same testing conditions.

The integration of solar cell with electrolyzer has emerged as a promising strategy for achieving solar-water-splitting and cost-effective production of renewable hydrogen. As illustrated in Fig. 5d, a solar-water-splitting system was constructed using a monolithic perovskite/Si tandem solar cell (TSC) with high output voltage as the power source for an electrolyzer. The device exhibited notable photovoltaic characteristics with a V of ~1.82 V and a PCE exceeding 23.6% (Supplementary Figs. 46 and 47, and Supplementary Table 8)63. The predicted operating point of the integrated solar water splitting system, defined as the intersection of the j-V curves of the tandem solar cell and the electrolysis cell, yields a current density of 16.6 mA cm−2 at 1.43 V, corresponding to a solar-to-hydrogen (STH) efficiency of 20.41% (Fig. 5e). More importantly, this intersection point is very close to the maximum power point of the perovskite/Si tandem solar cell (16.19 mA cm−2 at 1.486 V), indicating that the energy loss during the conversion of electrical energy to chemical energy in this system is relatively small. In addition, the gas evolution of the PV-AEMWE system was measured by online gas chromatography (Supplementary Fig. 47a). As shown in Supplementary Fig. 47b, the yields of H 2 and O 2 in 30 h are close to a ratio of 2:1, being 9.16 and 4.64 mmol, respectively. Figure 5f presents the water splitting current generated by the PV-AEMWE system and the corresponding STH efficiency under continuous AM1.5G illumination. Encouragingly, the PV-AEMWE system exhibited an comparable STH efficiency of 20.15% under practical operation and maintained 97.1% of its initial current density after continuous operation for over 140 h, confirming its good durability (Supplementary Fig. 47d). Compared with recently reported photovoltaic-electrochemical (PV-EC) water-splitting devices, the present system exhibits notable performance in terms of both STH efficiency and operational stability (Supplementary Table 9). Additionally, the PV-EC systems based on perovskite/Si tandem cells consistently outperform single-junction Si or perovskite devices in terms of STH efficiency, highlighting the great potential of tandem perovskite photovoltaics to further increase photocurrent output and push the efficiency of integrated solar water splitting towards its theoretical limit.

Methods
Materials
Nickel nitrate (Ni(NO ) ·6H O, ≥99%), chloroauric acid (HAuCl ·xH O, Au ≥47.5%), and iridium oxide (IrO, Ir ≥84.5%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Ferric nitrate (Fe(NO ) ·9H O, 98%), yttrium nitrate (Y(NO ) , 99.99%), ammonium fluoride (NH F, 99%), and potassium hydroxide (KOH, 95%) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Ammonium molybdate tetrahydrate ((NH ) Mo O ·4H O, 99%), platinum on carbon (Pt/C, 20%), urea (CH N O, 99%), Nafion 117 (5%), and nickel foam (NF, 99.9%) were purchased from Beijing InnoChem Science & Technology Co., Ltd. Sodium sulfite (Na SO , ≥97%), sodium thiosulfate (Na S O , 98.5%), hydrochloric acid (HCl, 36–38%) were purchased from Sinopharm Chemical Reagent Co. Ltd. All chemicals were used as received without purification.

Synthesis of NiFe LDH/FeMoO and NiFe LDH
Firstly, 500 mL precursor solutions were prepared with deionized water as solvent, containing nickel nitrate (2.9079 g), ferric nitrate, ammonium molybdate, urea (9.0090 g), and ammonium fluoride (1.3890 g). To regulate the Fe content, ferric nitrate concentrations were set to 2, 4, 6, and 8 mM while keeping other solute concentrations constant. Similarly, ammonium molybdate concentrations were adjusted to 13, 14, 15, and 16 mM to tune the Mo content under the same conditions. Nickel foam (NF, thickness: 1.5 mm, 5×5 cm2) was pre-treated by ultrasonic cleaning in 3 M hydrochloric acid for 15 min to remove surface impurities, followed by sequential ultrasonic rinsing in deionized water and absolute ethanol (15 min each). Subsequently, 150 mL of the precursor solution was transferred into a polytetrafluoroethylene beaker containing the cleaned NF, and the hydrothermal reaction was conducted at 90°C for 6 h. After reaction, the as-synthesized NiFe LDH/FeMoO were collected, thoroughly rinsed with deionized water and absolute ethanol, and dried in a vacuum oven at 60°C for 12 h. For comparison, NiFe LDH catalysts were synthesized via the same protocol but with the molybdenum source omitted from the precursor solution.

Preparation of Y-NiMo/MoO 2−x
The preparation of Y-NiMo/MoO 2−x was adapted from our group’s previous work62. Specifically, 0.6979 g of Ni(NO ) ·6H O, 0.0184 g of Y(NO ) ·6H O, and 0.7415 g of (NH ) Mo O ·4H O were dissolved in deionized water and stirred until homogeneous. The solution was then transferred to a Teflon-lined stainless-steel autoclave, along with a piece of cleaned NF, followed by hydrothermal reaction at 150°C for 6 h to form Y-NiMoO . Subsequently, the as-prepared Y-NiMoO precursor was subjected to reduction in a tube furnace under an Ar/H 2 atmosphere at 500°C for 30 min, yielding Y-NiMo/MoO 2−x .

Preparation of IrO and Pt/C catalysts on NF
The mass loading of NiFe LDH/FeMoO on NF was approximately 2.2 mg cm−2, calculated from the weight increase after the hydrothermal reaction. For reference, IrO and Pt/C catalysts were deposited on NF via the drop-casting method. Briefly, 22 mg commercial IrO catalyst was dispersed in a mixture of 90 μL 5 wt% Nafion, 260 μL ethanol, and 650 μL deionized water to form a homogeneous slurry. A 100 μL aliquot of this slurry was drop-cast onto cleaned NF and air-dried at room temperature. The commercial Pt/C catalyst was loaded on NF following the same procedure as IrO.

Characterization
Scanning electron microscopy (SEM) was conducted on a JEOL-JSM IT800. Transmission electron microscopy (TEM) was performed on a JEOL-JEM 2100F coupled with energy-dispersive spectroscopy (EDS). X-ray diffraction (XRD) was performed using a PANalytical X’pert PRO with Cu Kα radiation. Raman spectra acquired using a Horiba iHR320 spectrometer with a 532 nm He-Ne laser. X-ray absorption fine structure (XAFS) spectra were acquired at the Shanghai Synchrotron Radiation Facility (beamline BL14W1, China), with transmission mode employed for Ni, Fe, and Mo K-edges. X-ray photoelectron spectroscopy (XPS) was analyzed by a Thermo Fisher-ESCALAB 250Xi. Electron paramagnetic resonance (EPR) spectroscopy was recorded on a Bruker A300 instrument at −150°C under a microwave frequency of 9.83 GHz.

Electrochemical measurements
All electrochemical measurements were conducted using a three-electrode configuration on an Autolab PGSTA302N workstation coupled with a BOOSTER 10A current amplifier. The working electrode was the catalyst to be tested with a testing area of 1 cm2, the reference electrode was a Hg/HgO (1 M KOH, pH = 13.9 ± 0.1) electrode, and the counter electrode was a platinum plate. Calibration of the Hg/HgO reference electrode against the Reversible Hydrogen Electrode (RHE) was performed using a platinum wire in an electrolyte saturated with high-purity hydrogen. 56.11 g of KOH was weighed and transferred into a 1000 mL volumetric flask, followed by dilution to the graduation mark with deionized water. The obtained electrolyte (1 M KOH) was then transferred to a polytetrafluoroethylene (PTFE) bottle for storage. The experiments were performed under ambient conditions with the temperature maintained at 25°C (room temperature). Before the linear sweep voltammetry (LSV) test, the working electrode was subjected to 30 cycles of CV activation treatment. The scanning range was from 0.8 to 1.1 V at a scan rate of 50 mV s−1. The polarization curves were measured using a slow-scan mode at 5 mV s−1. The obtained potential data were corrected by 90% iR compensation and calibrated to the RHE scale in accordance with the Nernst equation, as expressed by: E = E + 0.0591 × pH + 0.098. Electrochemical impedance spectroscopy (EIS) analysis was carried out at different potentials ranging from 1.48 to 1.58 V vs. RHE. The frequency scanning range covered from 0.1 Hz to 100 kHz with an AC amplitude of 10 mV. The solution resistance is determinable through EIS analysis. The electrochemically active surface area (ECSA) was estimated via the double-layer capacitance (C ) method. Specifically, CV scans were recorded in the non-Faradaic region (0.924-1.024 V vs. RHE) at various rates, where C was determined from the slope of the charging current vs. scan rate. ECSA was calculated as ECSA = C /Cs, assuming a specific capacitance of Cs = 60 μF cm−2. The turnover frequency (TOF) using the formula: TOF = (j × A)/(4 × F × n), where j is the anodic current density from polarization curves, A is the electrode area, F is the Faraday constant, and n is the number of active sites. The n value was determined by integrating the charge from CV curves in 1 M PBS (pH = 7 ± 0.1) at 50 mV s−1. Faradaic efficiency (η ) was evaluated by chronoamperometry at a constant current density of 16.5 mA cm−2 in 1 M KOH for 30 h, with H and O products quantified by gas chromatography. The Faradaic efficiency of gaseous products is calculated according to the following equation: η = α × n × F/Q × 100%, where α is the electron transfer number, n represents the mole number of the product, F is the Faraday constant (96485 C mol−1), and Q denotes the total accumulated charge (C) during the testing process. TOF and Faradaic efficiency measurements were only performed once.

Fabrication and testing of AEMWE and PV-AEMWE systems
The anion exchange membrane water electrolysis (AEMWE) system is mainly composed of bipolar plate current collectors, catalytic electrodes, and an anion exchange membrane assembly. Specifically, the anion exchange membrane (FAA-3-30, 6×6 cm2) is immersed in a 1 M KOH solution for 24 h to activate the functional groups. This process enables the quaternary ammonium groups within the membrane to form stable coordination with hydroxide ions, significantly enhancing its ion-conduction efficiency. For the catalytic electrodes, NiFe LDH/FeMoO grown on an NF substrate and Y-NiMo/MoO 2−x electrocatalysts are used as the cathode and anode, respectively. Without the need for a hot-pressing assembly process, the activated membrane material and the catalytic electrodes are physically stacked in a sandwich structure to construct a NiFe LDH/FeMoO||Y-NiMo/MoO 2−x composite electrolysis system. For comparison, an IrO||Pt/C electrolytic cell is also assembled using noble-metal catalysts. The system stabilization treatment is carried out in a constant-current mode, with a direct current of 10 mA cm−2 applied for a certain period. The electrochemical performance test is conducted in a 3 M KOH electrolyte system maintained at 60 ± 0.5°C. The polarization curves were recorded using a potentiostat (Keysight E36154A) without iR compensation.

The fabrication of monolithic perovskite/silicon tandem solar cell is referenced to our group’s previous work63. Then, the perovskite/silicon tandem solar cell was connected as a power source to the NiFe LDH anode and the Y-NiMo/MoO 2−x cathode to construct a photovoltaic-electrochemical (PV-EC) electrolysis system. The active area of the tandem solar cell was 4.0 cm2, which matched the effective electrode area of the AEMWE (~4.0 cm2). The performance of the PV-EC system was measured under an irradiance of 100 mW cm−2 on the surface of the solar cell, which was provided by a xenon light source solar simulator (Oriel, 9119) equipped with an AM1.5G filter. The solar-to-hydrogen (STH) conversion efficiency of the photovoltaic-AEMWE system is determined by the following equation: STH = 1.23 × j op × η / P in , where j op is the operating current, η is the Faradaic efficiency, A sc is the solar cell area, and P in is the incident irradiation power.

Operando DEMS measurement
The DEMS experiment was conducted using an isotope labeling method in a typical three-electrode electrochemical cell, utilizing a QAS100 system (Shanghai, Linglu Instruments). The electrolyte and the vacuum system were isolated from each other by a porous polytetrafluoroethylene (PTFE) membrane featuring a maximum pore size of 20 nm. Firstly, the Ni(OH) , NiFe LDH, and NiFe LD/FeMoO catalysts were labeled with the 18O isotope. Specifically, CV was performed in a 1 M KOH electrolyte prepared with H 18O, within the voltage range of 1.1–1.7 V vs. RHE at a scan rate of 5 mV s−1. The mass signals of gas products with different molecular weights were recorded in real time. Then, the electrode was washed with H 16O to completely remove any residual H 18O. Finally, four consecutive CV cycles were performed in a 1 M KOH electrolyte prepared with H 16O, within the voltage range of 1.1–1.8 V vs. RHE at a scan rate of 5 mV s−1, and the mass signals of the gas products were recorded again.

In-situ ATR-SEIRAS measurement
All spectral measurements were acquired using a Fourier-transform infrared (FTIR) spectrometer (Nicolet iS50R, Thermo Fisher Scientific) equipped with a liquid-nitrogen-cooled MCT detector, with a spectral resolution of 4 cm−1. Gold-coated silicon prisms were prepared via a wet-chemical method: 0.045 g of HAuCl was dissolved in 10.5 mL of Milli-Q water, followed by the addition of 0.015 g of NaOH. Separately, 0.02 g of NH Cl, 0.141 g of Na SO , and 0.093 g of Na S O ·5H O were dissolved in 4.5 mL of deionized water. The two solutions were then mixed to form the gold-plating solution. Cleaned silicon prisms were immersed in a 40% ammonium fluoride solution for 90 s, followed by immersion in a mixture of 4 mL gold-plating solution and 0.034 mL hydrofluoric acid at 60°C for 5 min to yield gold-coated prisms. The catalyst inks were prepared by dispersing 1 mg of catalyst in a mixture of 800 µL of deionized water, 190 µL of isopropanol, and 10 µL of Nafion. The mixture was sonicated for 30 min to obtain a uniformly dispersed ink solution. The ink was then dropped onto gold-coated prisms, dried under an infrared lamp, and the catalyst loading was determined to be 200 μg. The gold-coated prisms were mounted as working electrodes in a three-electrode electrochemical cell with an Ag/AgCl electrode serving as the reference electrode and a platinum wire serving as the counter electrode. The electrochemical cell was operated in chronoamperometric mode using a workstation, and Fourier-transform infrared (FTIR) spectra were recorded in real time at open-circuit potential, as well as under applied potentials ranging from 0.8 to 2.0 V versus RHE. Spectra were collected at 0.1 V intervals with an acquisition time of 45 s for each measurement.

In-situ Raman spectroscopy measurement
In-situ Raman spectroscopy for monitoring dynamic surface reconstruction was performed using a confocal Raman spectrometer (Horiba iHR320) with a 532 nm laser wavelength. The laser power was set at 25 mW, and the acquisition range covered 200–1000 cm−1. The as-prepared catalyst served as the working electrode, a platinum wire as the counter electrode, and an Ag/AgCl electrode as the reference electrode, with 1 M KOH solution used as the electrolyte. Raman signals were recorded at open-circuit potential (OCP) and applied potentials ranging from 1.02 to 1.41 V vs. RHE. All spectral data were subjected to baseline correction and background subtraction.

DFT calculation
All density functional theory (DFT) calculations were performed using the Vienna Ab Initio Package (VASP) within the generalized gradient approximation (GGA) with the PBE functional64–66. The projected augmented wave (PAW) method was employed to describe the ionic cores, and valence electrons were treated using a plane wave basis set in a kinetic energy cutoff of 500 eV67. Partial occupancies of the Kohn-Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10−5 eV. A geometry optimization was considered convergent when the force change was smaller than 0.05 eV Å−1. Grimme’s DFT-D3 methodology was used to describe the dispersion interactions68. The Hubbard U had been considered in structures. And A non-site Hubbard term U-J was added to address the open-shell d-electrons (4.7 eV, 5 eV, and 4.6 eV) for Fe, Ni, and Mo. Ab initio molecular dynamics (AIMD) simulation was carried out to study disorder structures. During geometry optimization, the cut-off energy was set as 450 eV for structures, respectively. The PBE-D3 dispersion term was introduced to correct the van der Waals interactions69. AIMD simulations were run for 10 ps as equilibration with time steps of 1 fs, performing a constant temperature of 400 K in the Nosé-Hoover isokinetic ensemble. Surface slabs for NiFe LDH and the NiFe LDH/FeMoO heterostructure were built with a 20 Å vacuum. The NiFe LDH slab was cleaved along the (001) plane, and Fe (MoO ) was cleaved along its (013) plane. The heterostructure was formed by placing the NiFe LDH slab atop the Fe (MoO ) surface, aligning lattice vectors and relaxing the interfacial spacing to 2.5 Å. Full atomic relaxation was applied to the top NiFe LDH layers and the bottom Fe (MoO ) layers. The Brillouin zone was sampled by a Monkhorst-Pack (MP) k-point grid of 3×3×1 for structure optimizations. The VESTA 3 software was used to visualize optimized structures70. The free energy of a gas-phase molecule or an adsorbate on the surface was calculated by the equation G = E + ZPE − TS, where E is the total energy, ZPE is the zero-point energy, T is the temperature in kelvin (298.15 K is set here), and S is the entropy. The OER via the AEM pathway involves four fundamental steps: formation of *OH, transformation to *O, subsequent oxidation to *OOH, and finally desorption of O 71. Free energy of each step can be calculated by considering the standard states of H O and H , in order to describe the transfer of OH− and e−. The theoretical overpotential (η) can be determined by the potential-limiting step, which is identified as the maximum free energy increment among the four intermediate transitions. Alternatively, the LOM pathway considers the direct participation of lattice oxygen (Ol) in the oxygen evolution process. This five-stage mechanism involves the oxidation of lattice sites to *OlH, *Ol, *OlOH, and *OlO, leading to the subsequent coupling and evolution of O 71. The energy barrier for these transitions can be evaluated by comparing the total energy of the respective surface state, in which the overpotential is defined by the step exhibiting the highest thermodynamic barrier.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript by Liu and co-workers proposes a NiFe LDH/FeMoO heterostructure catalyst for efficient and stable OER in alkaline electrolysis. The catalyst exhibits a low overpotential of 316 mV at 2 A cm−2 and exceptional stability, driven by LOM activation via oxygen vacancies and interfacial electric fields. The integrated system using the catalyst with a PV-AEMWE system achieves solar to hydrogen efficiency of 20.15%. These findings will contribute significantly to the development of OER catalysts for sustainable hydrogen production. While the work is innovative and methodologically robust, several aspects require clarification to strengthen the scientific rigor and contextualize its impact.\n1. The authors have analyzed the valence states and coordination environments of Ni and Fe elements in the NiFe LDH/FeMoO catalyst using XAF and XPS. However, no detailed analysis of the Mo element is provided. Given the importance of FeMoO to the heterostructure, it is critical to understand the contribution of Mo to the catalyst. Such analysis (chemical state and coordination of Mo) would be helpful in clarifying role of Mo in enhancing the catalytic performance.\n2. The one-step water bath synthesis method for the catalyst is ingenious. However, the manuscript does not mention whether the proportions of raw materials in the precursor solution were adjusted to optimize the catalytic performance. Different ratios of Ni, Fe, or Mo sources may cause a change in the catalyst’s structure and composition, which may affect its activity and stability. Additionally, the reaction temperature plays an important role in the growth of the catalyst.\n3. The authors have explained the excellent stability of the NiFe LDH/FeMoO heterostructure catalyst very well. However, it remains unclear whether the catalyst morphology changes after long-term stability testing. Given the importance of the heterostructure in determining catalytic performance, it is necessary to investigate whether the heterostructure can still be maintained. Characterization techniques such as SEM or TEM after long-term testing could provide valuable insights into the morphological stability of the catalyst.\n4. The authors constructed a PV-AEMWE system to explore the application of the catalyst in hydrogen production through renewable energy - powered water electrolysis, achieving a practical STH efficiency of 20.15%. Since this performance is not currently being studied much, it would be more persuasive to compare it with other relevant studies. A table or figure summarizing the key performance metrics of different PV-AEMWE systems, such as STH efficiency and current density, would clearly show the advantages of the proposed catalyst system.", "reviewer_2": "In this study, Liu et al. developed NiFe layered double hydroxide nanosheets on pyramid FeMoO for efficient and durable oxygen evolution reactions. They demonstrated that the heterostructure catalyst achieves a lower overpotential at various current densities compared to previously reported electrocatalysts. Through comprehensive analysis using in-situ spectroscopy and DFT calculations, they provided mechanistic insights into the enhanced catalytic reactivity and structural stability of the system. Furthermore, they demonstrated an anion exchange membrane water electrolyzer using NiFe LDH/FeMoO anode and Y-NiMo/MoO2−x cathode, achieving a PV-EC system with a 20.1% STH efficiency when coupled with a perovskite/Si tandem solar cell. In my opinion, the work aligns with the readership of Nature Communications and should be considered for publication after minor revisions.\n1. The authors employed in-situ Raman spectroscopy to show the structural stability of the pyramid structure of the catalyst. They are encouraged to provide further morphological and crystallographic data following prolonged OER operation to illustrate the structural integrity.\n2. The authors should include complete JV and EQE data, showing at least up to the VOC and JSC for both sweep directions, and detailed PV parameters of the tandem solar cells. What are the areas of the PV and electrolysis cells?\n3. The STH efficiency of the integrated PV-AEMWE system appears to be limited by the current of the PV cell, when compared to some state-of-the-art perovskite-on-silicon tandems. The authors are encouraged to discuss the potential for improving the STH efficiency of water splitting systems using perovskite-based PV technologies.\n4. The NiFe LDH/FeMoO||Y-NiMo/MoO2−x electrolyzer demonstrated durability for 100 hours. However, for the integrated PV-AEMWE system, only a 30-hour measurement was performed. The authors are encouraged to present results from extended durability tests and compare these with reported PV-EC systems utilizing perovskite-based solar cells. Even if the system is not yet stable, they should analyze the components that limit stability and potential solutions to enhance its development for practical applications.", "reviewer_3": "General Comment\nThis study introduces pyramidal Fe2(MoO4)3 into the benchmark alkaline OER catalyst NiFe LDH to promote the lattice oxygen-mediated mechanism (LOM), thereby enhancing OER activity. The catalyst achieves an impressive current density of 2 A cm⁻² at an overpotential of 316 mV and demonstrates a solar-to-hydrogen (STH) efficiency of 20.15% when integrated with a perovskite/Si tandem solar cell. While the catalytic performance and durability are excellent, the manuscript lacks sufficient discussion on the fundamental aspects of the catalyst and does not provide enough experimental detail for reproducibility. In particular, substantial improvements are needed in the experimental data and interpretation related to in-situ SEIRAS and Raman analyses. Without major revisions in these areas, I cannot support the publication of this manuscript.\nSpecific Comments\n1. The authors use RuO2 as a reference, but IrO2 is generally considered the commercial benchmark noble metal catalyst for alkaline OER, especially due to its superior stability compared to RuO2. Please clarify this choice and, ideally, include experimental data for IrO2.\n2. The manuscript primarily provides indirect evidence for LOM and AEM mechanisms (except for the theoretical calculation data). Direct validation using isotope-labeling experiments (e.g., oxygen isotope mass spectroscopy, SEIRAS, or Raman with H218O or D2O) is strongly recommended.\n3. The performance of in-situ SEIRAS is highly sensitive to catalyst film thickness and uniformity. Details such as ink formulation, loading amount, and deposition method (e.g., whether to use spin coater, rpm settings) should be clearly described to ensure reproducibility.\n4. There is a lack of essential information on potential step intervals and residence time during SEIRAS measurements. Furthermore, Figures 3a and 3b do not clearly indicate at what potentials peaks emerge or whether their intensities increase progressively. The authors must clarify and update this section with detailed discussion.\n5. Assignment of OO and OOH peaks: No experimental evidence is provided to support the assignment of SEIRAS peaks to *OO or *OOH species. The authors should consider isotope-labeling experiments (e.g., with H218O and D2O-KOD) to validate these peak assignments.\n6. *Ambiguity of OO origin: The manuscript claims that a higher *OO coverage correlates with LOM preference, but only theoretical DFT evidence is provided. It remains unclear whether the observed *OO signal reflects a true lattice oxygen-involved intermediate or a dual-site *OO species also seen in AEM. Further clarification and data are required.\n7. The interfacial H2O band at ~3280 cm-1 and bending H2O band at ~1640 cm-1 shown in Figures 3a and 3b lack interpretation and do not appear to be critical to the current study. For clarity, the spectra should be rescaled to highlight only the *OO and *OOH peaks.\n8. The authors attribute this O-H vibrational band (~3280 cm-1) to *OH adsorption, but such peaks are widely recognized as arising from H2O species (e.g., 2HB-H2O, 4HB-H2O, and free H2O). If the authors intend to argue a link between water adsorption and LOM preference, they must perform deconvolution and revise the interpretation. Otherwise, the current explanation associating this band with *OH adsorption is not convincing.\n9. The authors mentioned that Mo–O species remain after OER (Figure 4b), which is not well supported. Mo is known to leach under alkaline conditions even without applied potential, and even more so under OER conditions. The authors should provide additional evidence (e.g., post-OER ICP analysis or ex-situ/OCP Raman data) to demonstrate that Mo–O peaks persist with significant intensity.\n10. Typo in Figure 4f   The label “Fas telectron transport” should be corrected to “Fast electron transport.”\n11. Figure 4d presents Rct comparisons, but the corresponding description in the manuscript discusses ICP-OES results. Please revise and clarify.\n12. A distinct peak near 760 cm-1 is observed in NiFe LDH/FeMoO but not in NiFe LDH. This feature should be identified, especially since it disappears during OER.\n13. LDH-based samples often exhibit broad bands in the 900–1200 cm⁻¹ range, associated with O–O vibrations and linked to LOM, which is namely “active oxygen”. The authors should provide high-frequency Raman data to discuss the potential-dependent evolution of these species.\n14. Catalysts favoring LOM typically exhibit higher oxidation states in metal sites. However, the authors report delayed Ni redox and suppressed Fe oxidation, which appears to contradict prior studies. A discussion reconciling this discrepancy is needed."}, "author_response": "We thank the reviewers for their detailed comments and constructive suggestions for this manuscript. We attach our point-by-point responses to the comments; additional characterisation and tests have been done accordingly. All textual modifications are given in the responses, and they are highlighted in the revised text. The paper format, including references, has been adapted to the journal style.\nComment: The manuscript by Liu and co-workers proposes a NiFe LDH/FeMoO heterostructure catalyst for efficient and stable OER in alkaline electrolysis. The catalyst exhibits a low overpotential of 316 mV at 2 A cm−2 and exceptional stability, driven by LOM activation via oxygen vacancies and interfacial electric fields. The integrated system using the catalyst with a PV-AEMWE system achieves solar to hydrogen efficiency of 20.15%. These findings will contribute significantly to the development of OER catalysts for sustainable hydrogen production. While the work is innovative and methodologically robust, several aspects require clarification to strengthen the scientific rigor and contextualize its impact.\nResponse: We are grateful to the reviewer for the recommendation. Below you will find our responses to the respective comments.\nC1.1 The authors have analyzed the valence states and coordination environments of Ni and Fe elements in the NiFe LDH/FeMoO catalyst using XAF and XPS. However, no detailed analysis of the Mo element is provided. Given the importance of FeMoO to the heterostructure, it is critical to understand the contribution of Mo to the catalyst. Such analysis (chemical state and coordination of Mo) would be helpful in clarifying role of Mo in enhancing the catalytic performance.\nResponse: We sincerely thank the reviewer for this valuable comment highlighting the importance of clarifying the role of Mo in the NiFe LDH/FeMoO heterostructure.\nIn response to this suggestion, we have performed a detailed analysis of the chemical state and coordination environment of Mo using both XPS and XAS at the Mo K-edge.\nThe high-resolution Mo 3d XPS spectra (Supplementary Fig. 11c) reveal two main peaks at ~235.3 eV and ~232.2 eV. These can be assigned to the Mo 3d and Mo 3d 5/2 3/2 of Mo6+ in MoO 2− units, respectively, confirming the presence of Fe (MoO ) in the heterostructure.27\n2 4 3\nAs suggested, the following sentences were added on page 19, lines 5-9 in the Supplementary Information: ‘As shown in Supplementary Fig. 12a, the absorption edge position of NiFe LDH/FeMoO is close to that of MoO , indicating that the oxidation 3 state of the Mo species in NiFe LDH/FeMoO is approximately +6. This is consistent with the XPS results.17 The corresponding Fourier-transformed EXAFS spectra (Supplementary Fig. 12b) show a prominent peak at ~1.26 Å, indicative of Mo–O bonding within MoO tetrahedra.’\n28. Zhu, C., et al. Fe-Ni-Mo Nitride Porous Nanotubes for Full Water Splitting and Zn-Air Batteries. Adv. Energy Mater. 8, (2018).\nNew reference was added in the Supplementary Information:\nS17. Yang, Y., et al. Tuning Binding Strength of Multiple Intermediates towards Efficient pH-universal Electrocatalytic Hydrogen Evolution by Mo O -NbN O 8 26 x y Heterocatalysts. Angew. Chem. Int. Ed. 62, e202306896 (2023).\nSupplementary Fig. 12. (a) Normalized Mo K-edge XANES spectra of Mo foil, MoO , 2 and NiFe LDH/FeMoO. (b) Mo K-edge FT-EXAFS spectra in R-space for Mo foil, MoO , 2 and NiFe LDH/FeMoO.\nC1.2 The one-step water bath synthesis method for the catalyst is ingenious. However, the manuscript does not mention whether the proportions of raw materials in the precursor solution were adjusted to optimize the catalytic performance. Different ratios of Ni, Fe, or Mo sources may cause a change in the catalyst’s structure and composition, which may affect its activity and stability. Additionally, the reaction temperature plays an important role in the growth of the catalyst.\nResponse: We thank the reviewer for the constructive comments on the optimization of the synthesis parameters.\nFollowing the suggestion, we have systematically investigated the influence of 4 ).u.a( noitprosbA 1 9 9 8 0 N M M M iF e L D H o fo il o O 2 o O 3 2 0 /F e M o O 0 0 0 E n e rg y (e 2V 0 0) 2 0 M o K e d g e 2 0 0 4 0 )3−Å( |))k(c3k(TF| 6 4 2 0 0 N M M M iF e L D o fo il o O 2 o O 3 M o 1 H -O /F M e M o o -O R O 2 (Å M ) o -M o 3 M o K e d g e 4 c Mo 3d ).u Mo 3d 5/2 .a ( y Mo 3d 3/2 tis n e tn I 238 236 234 232 230 Binding energy (eV)\nprecursor composition and reaction temperature on the catalytic performance of NiFe LDH/FeMoO. First, we varied the concentrations of the Fe and Mo sources in the precursor solution, while keeping the Ni source concentration constant. As shown in Supplementary Figs. 15a and 15b, the content of both Fe and Mo has a pronounced effect on the OER activity. The optimal concentrations were determined to be 15 mM Fe and 6 mM Mo, at which the catalyst exhibited the lowest overpotential, indicating superior intrinsic activity. Excess or deficiency of either element led to structural or compositional deviations from the ideal heterojunction, resulting in reduced performance. Secondly, we examined the effect of reaction temperature on catalyst growth. As illustrated in Supplementary Fig. 15c, the optimal growth temperature for the NiFe LDH/FeMoO heterostructure was identified as 90 °C. Polarization curves obtained at different growth temperatures further confirmed that the catalyst synthesized at 90 °C achieved the best OER activity (Supplementary Fig. 15d).\nAs suggested, the following sentences were added on page 22, lines 7-19 in the Supplementary Information: ‘To optimize the catalytic performance of the NiFe LDH/FeMoO heterostructure, the influence of the composition of the precursors and the reaction temperature was systematically investigated. First, the concentrations of the Fe and Mo sources in the precursor solution were varied while keeping the Ni source constant. As shown in Supplementary Figs. 15a and 15b, the OER activity is significantly affected by the content of both Fe and Mo. The optimal concentrations were found to be 15 mM of Fe and 6 mM of Mo. At these concentrations, the catalyst exhibited the lowest overpotential, indicating superior intrinsic activity. Excess or deficiency of either element resulted in structural or compositional deviations from the ideal heterojunction, leading to reduced performance. Secondly, the effect of reaction temperature on catalyst growth was examined. As shown in Supplementary Fig. 15c, 90 °C was identified as the optimal growth temperature for the NiFe LDH/FeMoO heterostructure. Polarization curves obtained at different growth temperatures further confirmed that the catalyst synthesized at 90 °C exhibited the best OER activity (Supplementary Fig. 15d). The catalyst barely grows on NF at 130 °C, thus exhibiting 5 extremely poor OER activity.’\nSupplementary Fig. 15. (a) Polarization curves of NiFe LDH/FeMoO prepared with different concentrations of ammonium molybdate (b) Polarization curves of NiFe LDH/FeMoO prepared with different concentrations of ferric nitrate. (c) Photograph of the NiFe LDH/FeMoO prepared with different water bath temperatures. (d) Polarization curves of NiFe LDH/FeMoO prepared with different water bath temperatures.\nC1.3 The authors have explained the excellent stability of the NiFe LDH/FeMoO heterostructure catalyst very well. However, it remains unclear whether the catalyst morphology changes after long-term stability testing. Given the importance of the heterostructure in determining catalytic performance, it is necessary to investigate whether the heterostructure can still be maintained. Characterization techniques such as SEM or TEM after long-term testing could provide valuable insights into the morphological stability of the catalyst.\nResponse:\n6 )2−mc A( ytisned tnerruC c 3 2 1 0 0 .0 1 3 m M 1 4 m M 1 5 m M 1 6 m M 0 .1 O v 0 .2 e rp o te n 0 .3 tia l (V ) 0 .4 0 .5 )2−mc A( ytisned tnerruC )2−mc A( ytisned tnerruC 3 2 1 0 0 3 2 1 0 0 .0 .0 2 m M 4 m M 6 m M 8 m M 0 .1 7 0 ℃ 9 0 ℃ 1 1 0 ℃ 1 3 0 ℃ 0 .1 O O v v 0 .2 e rp 0 .2 e rp o o te te n n 0 .3 tia l (V 0 .3 tia l (V ) ) 0 0 .4 .4 0 0 .5 .5\nFollowing the suggestion, we characterized the NiFe LDH/FeMoO catalyst again after the long-term stability tests. SEM images (Supplementary Fig. 28) clearly show that the ultrathin NiFe LDH nanosheets remain conformally wrapped on the FeMoO pyramids, maintaining the distinctive heterostructure without significant collapse or separation. In contrast, the NiFe LDH suffers from nanosheet agglomeration and structural collapse.\nFurther TEM analysis confirms that the interface between FeMoO and NiFe LDH remains well-defined even after extended operation (Supplementary Fig. 29a-c). High-resolution TEM images show that the interface between FeMoO and NiFe LDH remains well-defined, with a lattice spacing of 0.21 nm corresponding to the (012) plane of FeMoO (Supplementary Fig. 29c). The main structure of the NiFe LDH nanosheets remains amorphous. Notably, at the edges of the NiFe LDH nanosheets, a lattice spacing of 0.24 nm is observed, corresponding to NiOOH, which is the electrochemically derived active phase of NiFe LDH under OER conditions (Supplementary Fig. 29d). These results unambiguously demonstrate that the NiFe LDH/FeMoO heterostructure maintains its structural and crystal integrity after long-term durability testing, which is critical to its excellent catalytic performance and stability.\nAs suggested, the following sentences were added on page 8, lines 22-25 in the revised manuscript: ‘SEM and TEM images reveal that the unique architecture of the heterostructure, consisting of ultrathin NiFe LDH nanosheets that wrap around the FeMoO pyramids, remains intact even after prolonged testing (Supplementary Figs. 28 and 29). In contrast, the NiFe LDH structure is susceptible to nanosheet agglomeration and structural collapse.’\nAs suggested, the following sentences were added on page 36, lines 4-13 in the Supplementary Information: ‘As shown in Supplementary Fig. 29a–c, the intimate interface between the pyramidal core of FeMoO and the conformally coated NiFe LDH nanosheets remains clearly discernible. This indicates that the heterointerface is well-preserved under long-term electrochemical conditions. High-resolution TEM images reveal lattice fringes with a spacing of 0.21 nm, which can be indexed to the (012) plane of FeMoO, while the main structure of the NiFe LDH nanosheets remains amorphous (Supplementary Fig. 29c). Notably, at the edges of the NiFe LDH nanosheets, a lattice spacing of 0.24 nm is observed, corresponding to NiOOH, which is the electrochemically derived active phase of NiFe LDH under OER conditions.18 These results clearly demonstrate that the NiFe LDH/FeMoO heterostructure maintains both its structural integrity and crystal structure after long-term durability testing, which is essential for its exceptional catalytic performance and stability.’\nNew reference was added in the Supplementary Information:\nS18. Li, X., et al. Amorphous NiFe Oxide-based Nanoreactors for Efficient Electrocatalytic Water Oxidation. Angew. Chem. Int. Ed. 62, e202300478 (2023)\nSupplementary Fig. 28. SEM images of NiFe LDH/FeMoO (a) before and (b) after stability test. SEM images of NiFe LDH (c) before and (d) after stability test.\nC1.4 The authors constructed a PV-AEMWE system to explore the application of the catalyst in hydrogen production through renewable energy - powered water electrolysis, achieving a practical STH efficiency of 20.15%. Since this performance is not currently being studied much, it would be more persuasive to compare it with other relevant studies. A table or figure summarizing the key performance metrics of different PV-AEMWE systems, such as STH efficiency and current density, would clearly show the advantages of the proposed catalyst system.\nResponse: We thank the reviewer for this constructive suggestion.\nAs suggested, we have compiled a comprehensive comparison of representative PV-EC systems reported in recent high-level publications. As summarized in Supplementary Table 9, the comparison includes the types of photovoltaic cells, the catalysts in the cathode and anode, as well as the STH efficiency and stability. This analysis highlights that the perovskite photovoltaic-AEMWE system in this system, 9 featuring a NiFe LDH/FeMoO anode and a Y-NiMo/MoO cathode, achieves an STH 2₋x efficiency of 20.15%. This is one of the highest values reported to date for integrated PV-EC systems. Moreover, the system demonstrates excellent operational stability under practical conditions, further underlining its potential for large-scale solar-to-hydrogen conversion.\nAs suggested, the following sentences were added on page 14, lines 4-6 in the manuscript: ‘Compared with recently reported photovoltaic-electrochemical (PV-EC) water-splitting devices, the present system exhibits superior performance in terms of both STH efficiency and operational stability (Supplementary Table 9).\nSupplementary Table 9. Performance comparison in the STH conversion efficiency and stability of PV-EC devices for water splitting.\nη STH Solar cells Anode Cathode Stability Reference (%) GaAs Co P/Mo C@NC Co P/Mo C@NC 18.1 - 65 2 2 2 2 Si Ni–Gr–CNTs–Sn P Ni–Gr–CNTs–Sn P 10.82 90 h 66 4 3 4 3 Si CoFe O /CoNi LDH CoFe O 12.7 - 67 2 4 2 4 Si RuO Pt2/Ni(OH) 13.5 - 68 2 2 Si Co-OSP-0.2 Co-OSP-0.2 9.8 - 69 Si FeMoNi FeMoNi 12.47 700 s 70 Si p-S-Ni(Fe,Co)OOH p-S-Ni(Fe,Co)OOH 14.2 30 h 71 Perovskite NiFe MoS 12.67 17 h 72 2 Perovskite Co S @MoS Pt/C 13.6 2500 s 73 9 8 2 Perovskite Ir Co:WP/W 10.3 300 s 74 Perovskite NiFe/ITO Pt 13.75 500 s 75 10 Perovskite tandem Fe/Ni Pt/C 13.0 2 h 76 Perovskite tandem RuO Pt/C 17.8 184 h 4 2 Hetero-tandem NiFeO (OH) Pt 10 600 s 77 organic x y Perovskite/organic Ni-TFBDC Pt/C 16.75 3 h 78 tandem Perovskite/Si Fe (NiCo) Cr /NiMo Fe (NiCo) Cr /NiMo 20.6 375 s 79 tandem 60 30 10 60 30 10 Perovskite/Si s-NiFeCrMo–OH r-NiCrMo 20.63 50 h 80 tandem Perovskite/Si NiFe NiMo 20.01 15 h 3 tandem Perovskite/Si This NiFe LDH/FeMoO Y-NiMo/MoO 20.15 140 h tandem 2−x work\nNew references were added in the Supplementary Information:\nS62. Sun, P., et al. Round-the-clock bifunctional honeycomb-like nitrogen-doped carbon-decorated Co P/Mo C-heterojunction electrocatalyst for direct water 2 2 splitting with 18.1% STH efficiency. Appl. Catal. B-Environ. 310, 121354 (2022).\nS63. Riyajuddin, S., et al. Super-Hydrophilic Leaflike Sn P on the Porous Seamless 4 3 Graphene–Carbon Nanotube Heterostructure as an Efficient Electrocatalyst for Solar-Driven Overall Water Splitting. ACS Nano 16, 4861-4875 (2022).\nS64.Si, F., et al. Natural light driven photovoltaic-electrolysis water splitting with 12.7% solar-to-hydrogen conversion efficiency using a two-electrode system grown with metal foam. J. Power Sources 538, 231536 (2022).\nS65. Sun, J., Zhang, Z., Meng, X. Low-Pt supported on MOF-derived Ni(OH) with 2 highly-efficiently electrocatalytic seawater splitting at high current density. Appl. Catal. B-Environ. 331, 122703 (2023).\nS66. Chong, B., et al. Hierarchical phosphorus-oxygen incorporated cobalt sulfide hollow micro/nano-reactor for highly-efficient electrocatalytic overall water splitting. Chem. Eng. J. 465, 142853 (2023).\nS67. Zhao, Z., Sun, J., Li, X., Zhang, Z., Meng, X. Joule heating synthesis of NiFe alloy/MoO and in-situ transformed (Ni,Fe)OOH/MoO heterostructure as 2 2 11 effective complementary electrocatalysts for overall splitting in alkaline seawater. Appl. Catal. B-Environ. 340, 123277 (2024).\nS68. Lin, J.-Y., et al. Photo-assisted synthesis of ternary metal (oxy)hydroxide electrode for enhanced seawater splitting and solar-to-hydrogen conversion. J. Colloid Interface Sci. 691, 137410 (2025).\nS69. Asiri, A. M., et al. Solar Water Splitting Using Earth-Abundant Electrocatalysts Driven by High-Efficiency Perovskite Solar Cells. ChemSusChem 15, e202102471 (2022).\nS70. Wang, M., et al. Heterostructured Co/Mo-sulfide catalyst enables unbiased solar water splitting by integration with perovskite solar cells. Appl. Catal. B-Environ. 309, 121272 (2022).\nS71. Kim, D., Kim, D., Oh, S., Park, T., Yong, K. Ex situ flame vapor-doped oxophilic metals on WP/WO nanowires for enhanced alkaline hydrogen evolution activity. x Appl. Surf. Sci. 608, 155044 (2023).\nS72. Li, Y., et al. Transition metals-based electrocatalysts on super-flat substrate for perovskite photovoltaic hydrogen production with 13.75% solar to hydrogen efficiency. J. Colloid Interface Sci. 677, 599-609 (2025).\nS73. Sun, Q., et al. Integrated 4-Terminal All-Inorganic Perovskite Tandem Solar Cell with Open-Circuit Voltage Exceeding 2.1 V for Water Splitting. ACS Energy Lett. 7, 4215-4223 (2022).\nS74. Kim, Y. K., et al. Hetero-tandem organic solar cells drive water electrolysis with a solar-to-hydrogen conversion efficiency up to 10%. Appl. Catal. B-Environ. 309, 121237 (2022).\nS75. Li, X., et al. Efficient Solar-Driven Water Splitting Enabled by Perovskite Photovoltaics and a Halogen-Modulated Metal–Organic Framework Electrocatalyst. ACS Nano 17, 23478-23487 (2023).\nS76. Park, H., et al. Surface-Tailored Medium Entropy Alloys as Radically Low Overpotential Oxygen Evolution Electrocatalysts. Small 18, 2105611 (2022).\nS77. Pan, S., et al. Floating Seawater Splitting Device Based on NiFeCrMo Metal 12 Hydroxide Electrocatalyst and Perovskite/Silicon Tandem Solar Cells. ACS Nano 17, 4539-4550 (2023).\nComment: In this study, Liu et al. developed NiFe layered double hydroxide nanosheets on pyramid FeMoO for efficient and durable oxygen evolution reactions. They demonstrated that the heterostructure catalyst achieves a lower overpotential at various current densities compared to previously reported electrocatalysts. Through comprehensive analysis using in-situ spectroscopy and DFT calculations, they provided mechanistic insights into the enhanced catalytic reactivity and structural stability of the system. Furthermore, they demonstrated an anion exchange membrane water electrolyzer using NiFe LDH/FeMoO anode and Y-NiMo/MoO cathode, achieving a 2−x PV-EC system with a 20.1% STH efficiency when coupled with a perovskite/Si tandem solar cell. In my opinion, the work aligns with the readership of Nature Communications and should be considered for publication after minor revisions.\nResponse: We thank the reviewer for taking the time to review our manuscript. We have carefully considered the comments and made the necessary revisions accordingly. Below you will find our response to the respective comments.\nC2.1 The authors employed in-situ Raman spectroscopy to show the structural stability of the pyramid structure of the catalyst. They are encouraged to provide further morphological and crystallographic data following prolonged OER operation to illustrate the structural integrity.\nResponse: We would like to thank the reviewer for the insightful suggestion.\nFollowing the suggestion, we characterized the NiFe LDH/FeMoO catalyst again after the long-term stability tests. SEM images (Supplementary Fig. 28) clearly show that the ultrathin NiFe LDH nanosheets remain conformally wrapped on the FeMoO pyramids, preserving the distinctive heterostructure without significant collapse or separation. In contrast, the NiFe LDH suffers from nanosheet agglomeration and structural collapse.\nAs suggested, the following sentences were added on page 8, lines 22-25 in the revised manuscript: ‘SEM and TEM images reveal that the unique architecture of the heterostructure, consisting of ultrathin NiFe LDH nanosheets that wrap around the FeMoO pyramids, remains intact even after prolonged testing (Supplementary Figs. 28 and 29). In contrast, the NiFe LDH structure is susceptible to nanosheet agglomeration and structural collapse.’\nAs suggested, the following sentences were added on page 36, lines 4-13 in the Supplementary Information: ‘As shown in Supplementary Fig. 29a–c, the intimate interface between the pyramidal core of FeMoO and the conformally coated NiFe LDH nanosheets remains clearly discernible. This indicates that the heterointerface is well-preserved under long-term electrochemical conditions. High-resolution TEM images reveal lattice fringes with a spacing of 0.21 nm, which can be indexed to the (012) plane of FeMoO, while the main structure of the NiFe LDH nanosheets remains amorphous (Supplementary Fig. 29c). Notably, at the edges of the NiFe LDH nanosheets, a lattice spacing of 0.24 nm is observed, corresponding to NiOOH, which is the electrochemically derived active phase of NiFe LDH under OER conditions.18 These 15 results clearly demonstrate that the NiFe LDH/FeMoO heterostructure maintains both its structural integrity and crystal structure after long-term durability testing, which is essential for its exceptional catalytic performance and stability.’\nc\nSupplementary Fig. 28. SEM images of NiFe LDH/FeMoO (a) before and (b) after stability test. SEM images of NiFe LDH (c) before and (d) after stability test.\nSupplementary Fig. 29. (a, b) TEM images of NiFe LDH/FeMoO heterostructure after 16 stability test. (c, d) HR-TEM images of NiFe LDH/FeMoO heterostructure after stability test.\nA new reference was added in the Supplementary Information:\nS18. Li, X., et al. Amorphous NiFe Oxide-based Nanoreactors for Efficient Electrocatalytic Water Oxidation. Angew. Chem. Int. Ed. 62, e202300478 (2023).\nC2.2 The authors should include complete JV and EQE data, showing at least up to the VOC and JSC for both sweep directions, and detailed PV parameters of the tandem solar cells. What are the areas of the PV and electrolysis cells?\nResponse: We appreciate the reviewer's valuable suggestion.\nAs suggested, we have included the complete J-V characteristics of the tandem perovskite/silicon solar cells in both the forward and reverse sweep directions. This clearly shows the open-circuit voltage (V ) and short-circuit current density (J ) OC SC (Supplementary Fig. 46a). We have summarized the detailed PV parameters in Supplementary Table 8, including V , j , fill factor (FF), and power conversion OC SC efficiency (PCE). The tandem solar cells exhibited a V of ~1.82 V, a J of ~17.47 OC SC mA cm−2, an FF of ~74.22%, and a PCE of ~23.61%. In addition, we have included the corresponding external quantum efficiency (EQE) spectra, and the integrated J values SC from the EQE are consistent with those obtained from the J-V measurements, confirming the reliability of the photovoltaic performance data (Supplementary Fig. 46b).\nIn terms of the device dimensions, the active area of the tandem solar cell used in this study was 4.0 cm2, while the effective electrode area of the AEM electrolyzer was ~4.0 cm2. The current matching between the PV and electrolysis cells was carefully optimized to ensure that the electrolyzer operated near the maximum power point of the tandem solar cell.\nAs suggested, the following sentences were added on page 13, lines 19-23 in the revised 17 manuscript: ‘As illustrated in Fig. 5d, a solar-water-splitting system was constructed using a monolithic perovskite/Si tandem solar cell with high output voltage as the power source for an electrolyzer. The device exhibited excellent photovoltaic characteristics with a V of ~1.82 V and a PCE exceeding 23.6% (Supplementary Figs. 45 and 46, OC and Supplementary Table 8).62’\nAs suggested, the following sentences were added on page 4, lines 28-29 in the Supplementary Information: ‘The active area of the tandem solar cell was 4.0 cm2, which matched the effective electrode area of the AEMWE (~4.0 cm2).’\nSupplementary Fig. 46. (a) J-V curves of the perovskite/Si tandem solar cell with forward and reverse scans. (b) The external quantum efficiency (EQE) spectroscopy of the perovskite/Si tandem solar cell with an effective area of 4 cm2.\nSupplementary Table 8. The detailed PV parameters of the tandem perovskite/Si solar cells.\nV J FF PCE OC SC (V) (mA cm−2) (%) (%) Forward 1.821 17.47 74.22 23.61 Reverse 1.822 17.44 75.72 24.06\nC2.3 The STH efficiency of the integrated PV-AEMWE system appears to be limited by the current of the PV cell, when compared to some state-of-the-art perovskite-on-silicon tandems. The authors are encouraged to discuss the potential for improving the 18 )2−mc Am( ytisned tnerruC 2 1 1 0 5 0 5 00 .0 0 .5 V o 1 .0 lta g e (V ) 1 .5 R F e ve rse o rw a rd 2 .0 )%( EQE 1 0 8 6 4 2 0 0 0 0 0 03 0 0 P erovskite top cell 6 0 0 W a v e le n S g i bottom 9 th (n cell 0 0m ) S um E Q E 1 2 0 0\nSTH efficiency of water splitting systems using perovskite-based PV technologies.\nResponse: We thank the reviewer for this insightful comment.\nIn line with the reviewer’s recommendation, we have summarized representative PV-EC systems based on different types of photovoltaic cells, including Si, perovskite, and perovskite/Si tandem architectures (Supplementary Table 9). The comparison clearly shows that PV-EC systems based on perovskite/Si tandem cells deliver significantly higher STH efficiencies than those constructed from single-junction Si or perovskite cells. In line with these reports, we have added a discussion to the revised manuscript, which highlights the potential of perovskite/Si tandem photovoltaics to further enhance the current output and improve the overall STH efficiency of integrated water-splitting systems further.\nAs suggested, the following sentences were added on page 14 (lines 1-10) in the revised manuscript: ‘Encouragingly, in practical operation, the PV-AEMWE system can also exhibit an excellent STH efficiency of 20.15% and demonstrate ultra-stable and highly efficient operation over a measurement period exceeding 140 hours (Supplementary Fig. 47d). Compared with recently reported photovoltaic-electrochemical (PV-EC) water-splitting devices, the present system exhibits superior performance in terms of both STH efficiency and operational stability (Supplementary Table 9). Additionally, the PV-EC systems based on perovskite/Si tandem cells consistently outperform single-junction Si or perovskite devices in terms of STH efficiency, highlighting the great potential of tandem perovskite photovoltaics to further increase photocurrent output and push the efficiency of integrated solar water splitting towards its theoretical limit.’\nC2.4 The NiFe LDH/FeMoO||Y-NiMo/MoO electrolyzer demonstrated durability for 2−x 100 hours. However, for the integrated PV-AEMWE system, only a 30-hour measurement was performed. The authors are encouraged to present results from extended durability tests and compare these with reported PV-EC systems utilizing perovskite-based solar cells. Even if the system is not yet stable, they should analyze 19 the components that limit stability and potential solutions to enhance its development for practical applications.\nResponse: We thank the reviewer for this valuable suggestion emphasizing the importance of extended stability evaluation for the integrated PV-AEMWE system.\nFollowing the reviewer’s recommendation, the stability measurement of the PV-AEMWE device under continuous illumination and electrolysis operation was extended from 30 hours to 140 hours.\nAs shown in Supplementary Fig. 47d, the system maintained 97.1% of its initial current after 140 hours, confirming its outstanding operational durability. In comparison, most reported PV-EC systems exhibit continuous operation times typically below 50 hours (Supplementary Table 9). These results demonstrate that our system represents a significant step forward in enhancing the durability of integrated PV-AEMWE devices.\nIt is important to note that the catalytic electrodes in the electrolyzer are intrinsically capable of stable operation for thousands of hours at industrial-level current densities. Therefore, the slight current attenuation observed during the long-term PV-AEMWE test is primarily attributed to the gradual degradation of the perovskite/Si tandem photovoltaic component rather than the electrolyzer itself. Such degradation likely originates from interfacial instability and environmental sensitivity of the perovskite absorber under continuous illumination. Future optimization strategies, including advanced encapsulation, interface passivation, and the development of more stable perovskite compositions, are expected to further enhance the long-term durability of integrated PV-AEMWE systems.\nAs suggested, the following sentences were added on page 14 (lines 1-4) in the revised manuscript: ‘Encouragingly, the PV-AEMWE system exhibited an excellent STH efficiency of 20.15% under practical operation and maintained 97.1% of its initial current density after continuous operation for over 140 hours, confirming its outstanding durability (Supplementary Fig. 47d).’\n21\n)%( tnerruc dezilamroN 1 2 9 6 0 0 0 0 4 0 T im e 8 0 ( h ) 1 2 9 0 7 .1 %\nReviewer #3\nComment: The NiFe LDH/FeMoO||Y-NiMo/MoO electrolyzer demonstrated 2−x durability for 100 hours. However, for the integrated PV-AEMWE system, only a 30- hour measurement was performed. The authors are encouraged to present results from extended durability tests and compare these with reported PV-EC systems utilizing perovskite-based solar cells. Even if the system is not yet stable, they should analyze the components that limit stability and potential solutions to enhance its development for practical applications.\nResponse: We thank the reviewer for his valuable time on reviewing our manuscript. The proposals were carefully considered and any necessary revisions were made accordingly. Below you will find our response to the respective comments.\nC3.1 The authors use RuO as a reference, but IrO is generally considered the 2 2 commercial benchmark noble metal catalyst for alkaline OER, especially due to its superior stability compared to RuO . Please clarify this choice and, ideally, include 2 experimental data for IrO .\nFollowing the reviewer’s recommendation, we have now performed additional experiments using commercial IrO as the reference under the same conditions. As 2 shown in Figs. 2a and 2b, IrO requires overpotentials of ~444 mV at 1 A cm−2 and 2 ~577 mV at 2 A cm−2, which are significantly higher than those of our NiFe LDH/FeMoO catalyst (289 and 316 mV at the same current densities, respectively). We also analyzed the Δη/Δlog|j| value at high current densities (Fig. 2c). Within the current density range of 0.1 to 2 A cm−2, IrO exhibited a rapid increase in Δη/Δlog|j|, rising 2 from 112.6 mV dec−1 to 445.6 mV dec−1. This indicates considerable reaction kinetics 22 and mass transport limitations at high current densities. In sharp contrast, the increase for the NiFe LDH/FeMoO heterostructure was much smaller (from 53.0 mV dec−1 to 113.6 mV dec−1).\nFurthermore, in the AEMWE configuration, we tested IrO as the anode coupled with 2 Pt/C as the cathode. As presented in Fig. 5b, the IrO ||Pt/C electrolyzer requires a cell 2 voltage of ~1.90 V to reach 1 A cm−2, which is notably higher than that required by our NiFe LDH/FeMoO||Y-NiMo/MoO system. With regard to durability, IrO indeed 2-x 2 shows slightly better stability than RuO , but still exhibits a gradual degradation trend 2 over time (Fig. 5c). In contrast, the NiFe LDH/FeMoO catalyst maintains nearly consistent activity under the same testing conditions, highlighting its superior stability.\nAs suggested, the following sentences were revised on page 6, lines 29-30 in the revised manuscript: ‘This is much lower than for the NiFe LDH (335 and 399 mV) and the IrO 2 electrode (444 and 577 mV).’\nAs suggested, the following sentences were revised on page 13 (lines 7-9, 12-13, and 16-17) in the revised manuscript: ‘A conventional AEMWE was also prepared using commercial catalysts of IrO and Pt/C loaded onto NF as anode and cathode for 2 comparison.’ ‘In contrast, the IrO ||Pt/C electrolyzer requires a 1.9 V to reach a current 2 density of 1 A cm−2.’ ‘In contrast, the IrO ||Pt/C electrolyzer exhibits a rapid decline in 2 performance under the same testing conditions.’\nc\n3 750 600\nNiFe LDH 1.0 A cm−2 NiFe LDH\n2−\ny tis n e d tn e m rr c u C A ( ) 0 1 2 N IrO iF 2 e LDH/FeMoO )V m ( la itn e to p re v O 6 4 3 0 5 0 0 0 0 335 2 3 3 . . 9 0 0 9 A A 4 c c 6 m m 7 − − 2 2 289 316 336 444 577 j η ||g o lΔ / Δ 4 2 0 0 0 0 N IrO iF 2 e LDH/FeMoO 1.2 Po 1 te .4 ntial (V vs 1 . . R 6 HE) 1.8 NiFe LDH NiFe LDH/FeMoO IrO2 0 0.1-0.2 Cur 0 r . e 2- n 0 t .5 de 0 n . s 5- i 1 ty .0 (A 1 c . m 0-2 − . 2 0 ) 2.0-3.0\nFig. 2. (a) Polarization curves with 90% iR compensation in 1 M KOH. (b) Corresponding overpotentials at 1, 2 and 3 A cm−2 of NiFe LDH/FeMoO, NiFe LDH, and IrO . (c) Δη/Δlog|j| values for different current density ranges.\n2\nC3.2 The manuscript primarily provides indirect evidence for LOM and AEM mechanisms (except for the theoretical calculation data). Direct validation using isotope-labeling experiments (e.g., oxygen isotope mass spectroscopy, SEIRAS, or Raman with H 18O or D O) is strongly recommended.\n2 2\nResponse: We thank the reviewer for the valuable suggestion.\nAs suggested, we have conducted differential electrochemical mass spectrometry (DEMS) experiments to directly validate the direct involvement of lattice oxygen in the OER process and to differentiate between the reaction mechanisms of NiFe LDH and NiFe LDH/FeMoO (Supplementary Fig. 26). First, the catalysts were labeled with 18O by conducting cyclic voltammetry (CV) scans in a solution of 1 M KOH containing H 18O. After labeling, the catalysts were thoroughly washed with H ¹⁶O to remove any 2 2 residual 18O species. The labeled catalysts were then subjected to CV scans in a solution of 1 M KOH containing H 16O, with real-time oxygen signals were monitored using 2 mass spectrometry throughout the OER process.\nAccording to the traditional adsorbate evolution mechanism (AEM), when 18O-labeled catalysts undergo OER in the solution containing H 16O, the oxygen produced should 2 only come from the 16O in the electrolyte, thus generating 32O . The amount of 34O 2 2 released reflects the involvement of lattice oxygen in the OER process. As shown in 24 )V( egatlov lleC 2 1 1 1 .1 .8 .5 .2 0 .0 C N iF e L D H /F e M IrO ||P t/C 2 0 .5 1 .0 1 .5 u rre n t d e n s ity o O (A 2 .0 c m − 2 ) iM o /M o 2 .5 2 ) O 2−x 3 .0 )V( egatlov lleC 2 2 1 1 .5 .0 .5 .0 0 2 0 N iF e L IrO ||P 2 4 0T im D H t/C e /F e M o 6 0 (h ) O ||Y -N iM 8 o /M o 0 2−x 1 0 0 c\nSupplementary Fig. 27, our real-time DEMS measurements revealed a strong 34O 2 signal and a weaker 36O signal, in addition to the normal 32O signal. This indicates 2 2 that 18O-labeled lattice oxygen was activated and participated in the formation of both 18O16O and 18O18O.\nAs shown in Fig. 2h, we quantified the extent to which the lattice oxygen-mediated (LOM) mechanism was involved by calculating the ratio of 34O /32O over four CV 2 2 cycles. In the first CV cycle, the 34O /32O ratio for NiFe LDH/FeMoO was found to be 2 2 7.8%, which is significantly higher than the value of 3.9% for NiFe LDH. This result provides direct experimental evidence that the lattice oxygen in the NiFe LDH/FeMoO compound is more actively involved in the OER process via the LOM pathway.\nFurthermore, after several cycles, we observed a gradual decrease in the 34O /32O ratio 2 2 in the NiFe LDH/FeMoO sample, indicating that the 18O-labeled lattice oxygen was being consumed and replaced by 16O from the electrolyte. This further confirms that that lattice oxygen in NiFe LDH/FeMoO actively participates in the OER process via the LOM pathway.\nAs suggested, the following sentences were added on pages 8 lines 3-15 in the revised manuscript: ‘To further investigate the OER mechanisms in NiFe LDH and NiFe LDH/FeMoO, differential electrochemical mass spectrometry (DEMS) was employed to monitor the oxygen signals generated during the OER process (Supplementary Fig. 26). In traditional AEM, 18O-labeled catalysts should only produce 32O during OER in 2 a H 16O electrolyte. Therefore, the release of 34O reflects the involvement of lattice 2 2 oxygen.34 The DEMS measurements showed a strong 34O signal and a weaker 36O 2 2 signal, in addition to the normal 32O signal. This result indicates that the 18O-labeled 2 lattice oxygen is activated and participates in the formation of 18O16O and 18O18O (Supplementary Fig. 27). Fig. 2h shows the 34O /32O ratio over four cyclic voltammetry 2 2 (CV) cycles, which was used to quantify the contribution of the LOM.35 The 34O /32O 2 2 ratio for NiFe LDH/FeMoO was approximately twice that for NiFe LDH, providing direct evidence that lattice oxygen in NiFe LDH/FeMoO participates more actively in 25 the OER process via the LOM pathway. Furthermore, the gradual decrease in the 34O /32O ratio in NiFe LDH/FeMoO over multiple cycles indicates that the 18O-labeled 2 2 lattice oxygen was gradually consumed and replaced by 16O from the electrolyte.36’\nNew references were added in the manuscript:\nS34. Ju, M., et al. Fe(III) Docking-Activated Sites in Layered Birnessite for Efficient Water Oxidation. J. Am. Chem. Soc. 145, 11215-11226 (2023).\nS35. Hu, Y., et al. Understanding the sulphur-oxygen exchange process of metal sulphides prior to oxygen evolution reaction. Nat. Commun. 14, 1949 (2023).\nS36. Wang, C., et al. Identification of the Origin for Reconstructed Active Sites on Oxyhydroxide for Oxygen Evolution Reaction. Adv. Mater. 35, 2209307 (2023).\nSupplementary Fig. 26. Photograph of the in-situ differential electrochemical mass spectrometry (DEMS) test equipment.\nSupplementary Fig. 27. DEMS signals of 32O , 34O and 36O of (a) NiFe LDH and (b) 2 2 2\n26\n0 5 0 0 1 0 0 0 1 5 0 32 O 0 2 1E-9 T im e (h ) 34 O 2 ).u .a( la n g is s s a M 1E-10 0 5 0 0 1 0 0 0 1 5 0 32 O 0 2 1E-11 36 O 2 1E-9 T im e (h ) 34 O 2 ).u .a( la n g is s s a M 1E-10 1E-11 36 O 2\nNiFe LDH/FeMoO from the reaction products cycled in H 16O aqueous KOH 2 electrolyte.\nFig. 2. (h) The 34O /32O ratio over four cyclic voltammetry (CV) cycles. 2 2\nC3.3 The performance of in-situ SEIRAS is highly sensitive to catalyst film thickness and uniformity. Details such as ink formulation, loading amount, and deposition method (e.g., whether to use spin coater, rpm settings) should be clearly described to ensure reproducibility.\nResponse: We thank the reviewer for the valuable comments.\nIn the revised manuscript, we have included detailed information regarding the catalyst film preparation process in the experimental section of the supplementary information.\nAs suggested, the following sentences were revised on page 5, lines 22-31 in the revised Supplementary Information: ‘The catalyst inks were prepared by dispersing 1 mg of catalyst in a mixture of 800 µL of deionized water, 190 µL of isopropanol and 10 µL of Nafion. The mixture was sonicated for 30 minutes to obtain a uniformly dispersed ink solution. The ink was then dropped onto gold-coated prisms, dried under an infrared lamp and the catalyst loading was determined to be 200 μg. The gold-coated prisms were mounted as working electrodes in a three-electrode electrochemical cell with an Ag/AgCl electrode serving as the reference electrode and a platinum wire serving as the counter electrode. The electrochemical cell was operated in chronoamperometric 27 )%( ) O23(A/) O43(A 2 2 1 1 5 2 9 6 3 0 3 .9 N N 8 1 iFiF .2 ee L L DD HH 3 /F e .8 e M 7 o O .8 C y c 4 le .0 3 7 .6 3 .9 4 7 .5\nmode using a workstation, and Fourier-transform infrared (FTIR) spectra were recorded in real time at open-circuit potential, as well as under applied potentials ranging from 0.8 to 2.0 V versus RHE. Spectra were collected at 0.1 V intervals with an acquisition time of 45 s for each measurement.’\nC3.4 There is a lack of essential information on potential step intervals and residence time during SEIRAS measurements. Furthermore, Figures 3a and 3b do not clearly indicate at what potentials peaks emerge or whether their intensities increase progressively. The authors must clarify and update this section with detailed discussion.\nResponse: We thank the reviewer for the valuable suggestion.\nFollowing the reviewer’s recommendation, we have added the missing experimental details in the Supplementary Information. We have specified that the potential step interval was 0.1 V with a residence time of 45 s. In addition, Fig. 3a and Fig. 3b have been redrawn as contour plots, which provide a clearer visualization of the potential-dependent evolution of the SEIRAS signals and allow the progressive emergence and growth of the characteristic peaks to be readily identified.\nAs suggested, the following sentences were revised on page 9, lines 8-12 in the revised manuscript: ‘The coverage of *OO and *OOH intermediates gradually increases as the potential increases, leading to a corresponding enhancement in peak intensity. Notably, NiFe LDH exhibits a more pronounced *OOH peak, indicating that the OER process predominantly follows the AEM pathway. In contrast, the NiFe LDH/FeMoO heterostructure shows a significantly stronger *OO signal than *OOH, suggesting greater involvement of the LOM pathway.29’\nAs suggested, the following sentences were revised on page 5, lines 28-31 in the revised Supplementary Information: ‘The electrochemical cell was operated in 28 chronoamperometric mode using a workstation, and Fourier-transform infrared (FTIR) spectra were recorded in real time at open-circuit potential, as well as under applied potentials ranging from 0.8 to 2.0 V versus RHE. Spectra were collected at 0.1 V intervals with an acquisition time of 45 s for each measurement.’\nFig. 3. In-situ ATR-SEIRAS spectra at open-circuit voltage and various applied potentials (from 0.8 V to 2.0 V vs. RHE) for (a) NiFe LDH and (b) NiFe LDH/FeMoO.\nC3.5 Assignment of OO and OOH peaks: No experimental evidence is provided to support the assignment of SEIRAS peaks to *OO or *OOH species. The authors should consider isotope-labeling experiments (e.g., with H 18O and D O-KOD) to validate 2 2 these peak assignments.\nResponse: We thank the reviewer for the valuable suggestion. Although isotope-labelling ATR-SEIRAS is conceptually a powerful validation approach, the expected isotopic shifts for oxygen-related vibrations are typically small and can be easily obscured by broad, overlapping O–O and O–H vibrational features in aqueous electrochemical environments. Consequently, clear isotopic differentiation is often experimentally inaccessible with current SEIRAS sensitivity, which has been recognized as a general limitation in this field rather than an issue specific to our study.\nTo overcome this limitation and verify the spectral assignments, we adopted an alternative strategy combining control-catalyst analysis and DEMS evidence. Ni(OH)₂, a well-known AEM-type OER catalyst, was selected as a reference. As shown in Supplementary Fig. 32a, Ni(OH) required overpotentials of 588 mV and 665 mV at 2 current densities of 1 A cm−2 and 2 A cm−2, respectively. We also evaluated the OER performance of Ni(OH) in KOH electrolytes with different pH values. As the pH 2 29 )EHR .sv V( laitnetoP O 2 .0 1 .6 1 .2 0 .8 C P1 8 0 0 ~ 1 H O : 2 6 4 0 c m W a −1 1 v e 5 0n 0u m b e *O O : −1 ~ 1 2 3 0 c m *O O ~ 1 1 1 0 1 2 0 0 − 1 r (c m ) H c : −1 m 0.036 0.030 0.022 0.014 0.006 -0.002 Intensity (a.u.) )EHR .sv V( laitnetoP O 2 .0 1 .6 1 .2 0 .8 C P1 8 0 0 ~ 1 H O : 2 6 4 0 c m W a −1 1 v e 5 0n 0u m b e *O O : −1 ~ 1 2 3 0 c m *O O ~ 1 1 1 0 1 2 0 0 − 1 r (c m ) H c : −1 m 0.045 0.037 0.028 0.018 0.008 -0.001 Intensity (a.u.)\nincreased from 12.5 to 14.0, Ni(OH) displayed weak pH dependence (Supplementary 2 Fig. 32b). The log of the current density at 1.6 V showed a significantly lower slope of 0.43 with respect to pH than the slope of 0.85 for NiFe LDH/FeMoO (Supplementary Fig. 32c). Additionally, TMA+ had almost no effect on the OER activity of Ni(OH) 2 (Supplementary Fig. 32d). Most importantly, during DEMS testing, no significant 34O 2 or 36O signals were observed from Ni(OH) , indicating that the majority of its intrinsic 2 2 lattice oxygen does not participate in O–O bond formation (Supplementary Fig. 32e). These results suggest that Ni(OH) undergoes OER via the AEM pathway. To further 2 support the assignment of the SEIRAS peaks, we performed attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) on Ni(OH) at 2 different applied potentials (from 0.8 V to 2.0 V vs. RHE). As shown in Supplementary Fig. 32f, at open-circuit potential, the spectra exhibited characteristic peaks at 1640 cm−1 and 3280 cm−1, corresponding to the bending (δ H–O–H ) and stretching (ν O–H ) vibrations of water molecules. Upon applying higher potentials, Ni(OH) displayed an 2 additional peak at 1100 cm−1, but did not show the absorption peak at 1230 cm−1, which was observed in NiFe LDH and NiFe LDH/FeMoO. Based on these findings, we conclude that the peak at 1100 cm−1 corresponds to the *OOH intermediate involved in the AEM pathway, while the peak at 1230 cm−1 is specific to the *OO intermediate, characteristic of the LOM.18, 19, 20, 21, 22\nAs suggested, the following sentences were added on pages 39-40 in the revised Supplementary Information: ‘As shown in Supplementary Fig. 32a, Ni(OH) required 2 overpotentials of 588 mV and 665 mV at current densities of 1 A cm−2 and 2 A cm−2, respectively. The OER performance of Ni(OH) was also evaluated in KOH electrolytes 2 with varying pH values. As the pH increased from 12.5 to 14.0, Ni(OH) displayed 2 weak pH dependence (Supplementary Fig. 32b). The log of the current density at 1.6 V showed a pH-dependent slope of 0.43, which is significantly lower than the slope of 0.85 observed for NiFe LDH/FeMoO (Supplementary Fig. 32c). Additionally, TMA+ had almost no effect on the OER activity of Ni(OH) (Supplementary Fig. 32d). Most 2 importantly, during DEMS testing, no significant 34O or 36O signals were observed 2 2 30 from Ni(OH) , indicating that the majority of its intrinsic lattice oxygen does not 2 participate in O–O bond formation (Supplementary Fig. 32e). These results suggest that Ni(OH) undergoes OER via the AEM pathway. As shown in Supplementary Fig. 30f, 2 at open-circuit potential, characteristic peaks were observed at 1640 cm−1 and 3280 cm−1, corresponding to the bending (δ ) and stretching (ν ) vibrations of water H–O–H O–H molecules. Upon applying higher potentials, Ni(OH) displayed an additional peak at 2 1100 cm⁻¹, but did not show the absorption peak at 1230 cm−1, which was observed in NiFe LDH and NiFe LDH/FeMoO. Based on these findings and previous literature reports, the peak at 1100 cm−1 is assigned to the OOH intermediate involved in the AEM pathway, while the peak at 1230 cm−1 is specific to the OO intermediate, characteristic of the LOM.19, 20, 21, 22, 23’\nSupplementary Fig. 32. (a) Polarization curves with 90% iR compensation in 1.0 M KOH. (b) Polarization curve of Ni(OH) in alkaline electrolytes with different pH. (c) 2 The relationship between the logarithm of current density and pH for Ni(OH) at a 2 potential of 1.6 V vs. RHE. (d) Polarization curves of Ni(OH) in 1 M KOH and 1 M 2 TMAOH, respectively. The inset shows corresponding Tafel plot. (e) DEMS signals of 32O , 34O and 36O of Ni(OH) . (f) In-situ ATR-SEIRAS spectra at various applied 2 2 2 2 potentials for Ni(OH) .\n31\na ) 2-mc Am( ytisned tnerruC ksid 400 300 200 100 0 1.2 1E-12 ).u.a( la 1E-11 ngis ssaM 1E-10 0 12.5 13.0 13.5 14.0 O v e 500 1.4 rp o te n tia l (V 1000 T im e (h ) ) N i(O H )2 1.6 36 O 2 34 O 2 32 O 2 1500 2 10 1 10 0 10 12.5 13.0 13.5 14.0 k=0.43 ) 2-mc Am( ytisned tnerruC ksid N i(O H )2 p H k=0.43 )2-mc A( ytisned tnerruC 1.5 1.0 0.5 0.0 0.0 )V( laitnetoprevO 1 M K O H 1 M TM A O H 0.56 0.48 0.40 -1.2 -0.8 -0.4 -2 j log || m A cm ) ( 0.1 0.2 0.3 O v e rp o te n tia l (V ) 0.4 0.5 b c 143 1 .4 4 3 m .3 V m de V c d -1 ec-1 d )2−mc A( ytisned tnerruC 3 2 1 0 0.0 N N N i(O H )2 iFe LD H iFe LD H /FeM oO 0.2O v e rp o 0.4 te n tia l (V ) 0.6 e f ).u.a( noitprosbA 0.01 O C P 0.8 V 2.0 V 1800 O2H 1600 W a v e n u 1400 m b e r (c m −1) N i(O HOO* H )2 1000\nS19. Lv, X., et al. Cobalt and Yttrium Doping to Activate Dual-Site Mechanism of Amorphous NiFeOOH for Large-Current Water Electrooxidation. Adv. Sci. n/a, e12638 (2025).\nS20. Song, H., et al. RuO –CeO Lattice Matching Strategy Enables Robust Water 2 2 Oxidation Electrocatalysis in Acidic Media via Two Distinct Oxygen Evolution Mechanisms. ACS Catal. 14, 3298-3307 (2024).\nS21. Thakur, N., et al. Identifying Active Sites of IrOx Catalysts for OER: A Combined Operando XAS, SEIRAS, and Theoretical Study. J. Am. Chem. Soc. 147, 30613-30625 (2025).\nS22. Luo, X., et al. Fe-S dually modulated adsorbate evolution and lattice oxygen compatible mechanism for water oxidation. Nat. Commun. 15, 8293 (2024).\nS23. Li, J., Yao, M., Yuan, Z., Ma, J., Geng, S., Liu, F. Phase engineering boosting heterogeneous interface effect in RuO /MnO catalysts for acidic oxygen 2 2 evolution reaction. Chem. Eng. J. 496, 153921 (2024).\nC3.6 *Ambiguity of OO origin: The manuscript claims that a higher *OO coverage correlates with LOM preference, but only theoretical DFT evidence is provided. It remains unclear whether the observed *OO signal reflects a true lattice oxygen-involved intermediate or a dual-site *OO species also seen in AEM. Further clarification and data are required.\nResponse: We thank the reviewer for this important comment.\nPrevious studies (Nat. Energy 4, 329 (2019); Energy Environ. Sci. 14, 4647 (2021); J. Am. Chem. Soc. 145, 23659 (2023); Nat. Commun. 15, 8293 (2024)) have reported that the LOM pathway involves the direct participation of lattice oxygen in forming the *OO intermediate. In contrast, the AEM pathway relies on the stepwise transformation of surface adsorbates, with *OH, *O, and *OOH acting as the typical intermediates.\nTo further clarify this point, we conducted oxygen isotope DEMS experiments alongside comparative ATR-SEIRAS measurements. As shown in Supplementary Fig. 32 27, DEMS measurements with 18O-labeled NiFe LDH/FeMoO catalysts revealed clear 34O and 36O signals during OER. This provides direct experimental evidence that 2 2 lattice oxygen is involved in O–O bond formation. In contrast, Ni(OH) , which follows 2 the AEM pathway, generated only 32O under identical conditions (Supplementary Fig. 2 32e). Furthermore, ATR-SEIRAS analysis demonstrated that Ni(OH) exhibits a peak 2 at 1100 cm−1, which is assigned to the OOH intermediate in the AEM pathway. However, it shows no feature at 1230 cm−1 (Supplementary Fig. 32f). By comparison, NiFe LDH/FeMoO displayed the related peak of *OOH at 1100 cm−1 as well as an additional peak at 1230 cm−1 that is characteristic of lattice oxygen activation. This is consistent with the DEMS results.\nTaken together, these findings confirm that the *OO signal observed in this study originates from intermediates generated through the LOM pathway rather than from AEM pathway. Please refer to the reply above regarding comments 3.2, 3.3, and 3.4.\nC3.7 The interfacial H O band at ~3280 cm-1 and bending H O band at ~1640 cm-1 2 2 shown in Figures 3a and 3b lack interpretation and do not appear to be critical to the current study. For clarity, the spectra should be rescaled to highlight only the *OO and *OOH peaks.\nResponse: We thank the reviewer for this constructive suggestion.\nAs suggested, we have redrawn Fig. 3a and Fig. 3b as contour plots, which reduce the x-axis range to 1800-1000 cm−1. This effectively eliminates the broad H O-related 2 bands, highlighting only the characteristic *OO and *OOH peaks. These modifications improve the clarity of the spectra and emphasizes the relevant intermediates for the OER mechanism. 33 Fig. 3. In-situ ATR-SEIRAS spectra at open-circuit voltage and various applied potentials (from 0.8 V to 2.0 V vs. RHE) for (a) NiFe LDH and (b) NiFe LDH/FeMoO.\nC3.8 The authors attribute this O-H vibrational band (~3280 cm-1) to *OH adsorption, but such peaks are widely recognized as arising from H O species (e.g., 2HB-H O, 2 2 4HB-H O, and free H O). If the authors intend to argue a link between water adsorption 2 2 and LOM preference, they must perform deconvolution and revise the interpretation. Otherwise, the current explanation associating this band with *OH adsorption is not convincing.\nResponse: We thank the reviewer for the valuable suggestion.\nFollowing the reviewer’s suggestion, a more detailed analysis of the broad absorption band in the range of 2800-3800 cm−1 was conducted in the in-situ ATR-SEIRAS. As shown in Supplementary Figs. 39a and 39b, the broad band observed at 2800-3800 cm−1 corresponds to the O–H stretching vibrations (ν ) of interfacial water molecules. The O–H intensity of the ν vibration increases progressively with the applied bias potential, O–H indicating increased adsorption of water molecules at the charged interface.\nFurthermore, the variation of the ν with applied potential was compared between O–H NiFe LDH and NiFe LDH/FeMoO (Supplementary Figs. 39c). This is consistent with recent reports on (Angew. Chem. Int. Ed. 64, e202413250, 2025; J. Power Sources 614, 235017, 2024). The NiFe LDH/FeMoO heterostructure exhibited a markedly stronger ν upon increasing potential and the onset of OER, suggesting a higher surface O–H coverage of *OH intermediates that facilitates the regeneration of lattice oxygen.22, 48 34 )EHR .sv V( laitnetoP O 2 .0 1 .6 1 .2 0 .8 C P1 8 0 0 ~ 1 H O : 2 6 4 0 c m W a −1 1 v e 5 0n 0u m b e *O O : −1 ~ 1 2 3 0 c m *O O ~ 1 1 1 0 1 2 0 0 − 1 r (c m ) H c : −1 m 0.036 0.030 0.022 0.014 0.006 -0.002 Intensity (a.u.) )EHR .sv V( laitnetoP O 2 .0 1 .6 1 .2 0 .8 C P1 8 0 0 ~ 1 H O : 2 6 4 0 c m W a −1 1 v e 5 0n 0u m b e *O O : −1 ~ 1 2 3 0 c m *O O ~ 1 1 1 0 1 2 0 0 − 1 r (c m ) H c : −1 m 0.045 0.037 0.028 0.018 0.008 -0.001 Intensity (a.u.)\nIn addition, the broad ν bands of NiFe LDH and NiFe LDH/FeMoO recorded at 2.0 O–H V vs. RHE were deconvoluted using Gaussian fitting (Supplementary Figs. 39d and 39e). These spectra can be resolved into three components centered at approximately 3279, 3438, and 3570 cm−1, which are attributed to four-hydrogen-bonded water (4- HB·H O), two-hydrogen-bonded water (2-HB·H O), and weakly hydrogen-bonded 2 2 water (free H O), respectively.24 Compared with NiFe LDH, the proportion of weakly 2 hydrogen-bonded water in NiFe LDH/FeMoO increased by more than twofold, indicating that interfacial water tends to adopt a less hydrogen-bonded configuration. This weakened hydrogen-bond network may facilitate the adsorption and dissociation of water molecules at active sites, thereby promoting the OER process.25\nTo further investigate the behavior of interfacial water molecules, in-situ Raman spectroscopy was performed in the 2800 to 4000 cm⁻¹ range at different applied potentials. As shown in Supplementary Figs. 40a and 40b, the broad peak can be attributed to the O–H stretching vibration mode of interfacial water, which can be further deconvoluted into three distinct peaks: four-hydrogen-bonded water (4- HB·H O), two-hydrogen-bonded water (2-HB·H O), and weakly hydrogen-bonded 2 2 free water, consistent with the results from in-situ ATR-SEIRAS.26 At open-circuit potential, the proportion of free water on the surface of NiFe LDH/FeMoO was 14.5%, which is higher than the 13.9% observed on NiFe LDH (Supplementary Fig. 40c), suggesting that it has a higher affinity for water molecules that can be directly oxidized.27 As the voltage increased from 1.2 V to 1.4 V, free water on the surface of NiFe LDH/FeMoO remained stable, whereas NiFe LDH showed a sharp decrease in free water. Generally, interfacial water with fewer hydrogen bonds is more easily dissociated, which facilitates the initiation of the water oxidation process.28 Therefore, the higher enrichment of free water on the NiFe LDH/FeMoO surface during OER leads to better catalytic activity for the OER.\nAs suggested, the following sentences were revised on page 11, lines 4-12 in the revised manuscript: ‘As shown in Supplementary Figs. 39a and 39b, the broad absorption band 35 observed in the 2800-3800 cm−1 range in the in-situ ATR-SEIRAS corresponds to the O–H stretching vibrations (ν ) of interfacial water molecules.36 The intensity of the O–H ν vibration increases progressively with the applied potential, indicating increasing O–H adsorption of water molecules at the charged interface. Furthermore, the variation in the intensity of this band under different applied potentials was compared between NiFe LDH and NiFe LDH/FeMoO (Supplementary Fig. 39c). The NiFe LDH/FeMoO heterostructure exhibits a significantly stronger signal, indicating a higher surface coverage of *OH intermediates that facilitates the regeneration of lattice oxygen.22, 49’\nAs suggested, the following sentences were revised on page 47 (lines 8-16) and page 48 (5-16) in the revised Supplementary Information: ‘As shown in Supplementary Figs. 39d and 39e, the broad ν bands of NiFe LDH and NiFe LDH/FeMoO recorded at O–H 2.0 V (vs. RHE) were deconvoluted with the Gaussian fitting model. These spectra can be resolved into three components centered at approximately 3279, 3438, and 3570 cm−1, which are attributed to four-hydrogen-bonded water (4-HB·H O), two-hydrogen- 2 bonded water (2-HB·H O), and weakly hydrogen-bonded water (free H O), 2 2 respectively.24 Compared with NiFe LDH, the proportion of weakly hydrogen-bonded water in NiFe LDH/FeMoO increased by more than twofold, indicating that interfacial water tends to adopt a less hydrogen-bonded configuration. This weakened hydrogen-bond network may facilitate the adsorption and dissociation of water molecules at active sites, thereby promoting the OER process.25’ ‘As shown in Supplementary Figs. 40a and 40b, the broad peak can be attributed to the O–H stretching vibration mode of interfacial water, which can be further deconvoluted into three distinct peaks: four-hydrogen-bonded water (4-HB·H O), two-hydrogen-bonded water (2-HB·H O), and 2 2 weakly hydrogen-bonded free water, consistent with the results from in-situ ATR-SEIRAS. At open-circuit potential, the proportion of free water on the surface of NiFe LDH/FeMoO was 14.5%, which is higher than the 13.9% observed on NiFe LDH (Supplementary Fig. 40c), suggesting that it has a higher affinity for water molecules that can be directly oxidized. As the voltage increased from 1.2 V to 1.4 V, free water on the surface of NiFe LDH/FeMoO remained stable, whereas NiFe LDH showed a 36 sharp decrease in free water. Generally, interfacial water with fewer hydrogen bonds is more easily dissociated, which facilitates the initiation of the water oxidation process. Therefore, the higher enrichment of free water on the NiFe LDH/FeMoO surface during OER leads to better catalytic activity for the OER.’\nA new reference was added in the manuscript:\n49. Wang, X., et al. Electrocatalytic selective oxygen evolution of FeOOH-modified perovskite for alkaline seawater electrolysis. J. Power Sources 614, 235017 (2024).\nNew references were added in the Supplementary Information:\nS24. Li, P., et al. Hydrogen bond network connectivity in the electric double layer dominates the kinetic pH effect in hydrogen electrocatalysis on Pt. Nat. Catal. 5, 900-911 (2022).\nS25. Sun, K., et al. Interfacial water engineering boosts neutral water reduction. Nat. Commun. 13, 6260 (2022).\nS26. Wang, Y.-H., et al. In situ Raman spectroscopy reveals the structure and dissociation of interfacial water. Nature 600, 81-85 (2021).\nS27. Zhu, S., et al. Reconstructing Hydrogen-Bond Network for Efficient Acidic Oxygen Evolution. Angew. Chem. Int. Ed. 63, e202319462 (2024).\nS28. Wu, L., et al. Unveiling the Structure and Dissociation of Interfacial Water on RuO for Efficient Acidic Oxygen Evolution Reaction. Angew. Chem. Int. Ed. 64, 2 e202413334 (2025).\nSupplementary Fig. 40. In-situ Raman spectra at open-circuit voltage various applied potentials (from 1.2 V to 1.4 V vs. RHE) for (a) NiFe LDH/FeMoO and (b) NiFe LDH. (c) The potential-dependent content of free water for NiFe LDH and NiFe LDH/FeMoO obtained from (a) and (b).\nC3.9 The authors mentioned that Mo–O species remain after OER (Figure 4b), which is not well supported. Mo is known to leach under alkaline conditions even without applied potential, and even more so under OER conditions. The authors should provide 38 0.01 ).u.a( 2.0 V noitprosbA 0.8 V O C P 4000 W ). 2-H B·H O u 2 .a( no Free H O i 2 tprosbA 3600 W 3600 a v e n 3400 a v e n u u N iFe LD H 3200 2800 −1 m b e r (c m ) N iFe LD H 4-H B·H O2 3200 3000 2800 −1 m b e r (c m ) ).u.a( noitprosbA ).u.a( noitprosbA 0.01 2.0 V 0.8 V O C P 4000 W 2-H B·H O2 Free H O2 3600 W 3600 a v e n 3400 a v e n u u N iFe LD H /FeM 3200−1 m b e r (c m ) N iFe LD H /FeM oO 4-H B·H O2 3200 3000 −1 m b e r (c m ) oO 2800 oO 2800 c ).u.a( ytisnetnI 100 )% 80 ( ytisnetnI ev 20 italeR 0 4 N iFe LD H N iFe LD H /FeM oO 0.005 1.2 1.5 1.8 2.1 P o te n tia l (V v s . R H E ) N iFe LD H N iFe LD H /FeM oO 70.0 67.2 26.0 24.0 8.8 4.0 -H B ·H O 2 -H B ·H O F re e H O 2 2 2 ).u .a( y tis n e tn I a NiFe LDH F /F re e e M H o 2 O O 2-HB H2O 4-HB H2O 1 1 1 1 1 1 1 1 1 1 1 1 1 1 O .4 .3 .3 .3 .2 .2 .2 .2 .1 .1 .1 .0 .0 .0 C 1 V 8 V 5 V 2 V 9 V 6 V 3 V 0 V 7 V 4 V 1 V 8 V 5 V 2 V P δ N (N i-O ) N i-O N i-O H R a m a n s h N M o -O iF e L D H /F e M o -O C e O M o O 2− 3 × 1 5 × 1 0 × 5 3 0 0 6 0 0 9 0 0 1 2 0 0 ).u .a( y tis n e tn I c 1 .4 1 V 1 .3 8 V 1 .3 5 V 1 .3 2 V 1 .2 9 V 1 .2 5 V 1 .2 3 V 1 .2 0 V 1 .1 7 V 1 .1 4 V 1 .1 1 V 1 .0 8 V 1 .0 5 V 1 .0 2 V O C P δ (N i-O N i-O H R a v(N i-O N i-O m a n ) s h ift (c m − 1 ) N iF e L D 2− C O 3 H\nadditional evidence (e.g., post-OER ICP analysis or ex-situ/OCP Raman data) to demonstrate that Mo–O peaks persist with significant intensity.\nResponse: We thank the reviewer for the valuable suggestion.\nFollowing the reviewer’s suggestion, ex-situ Raman spectroscopy was conducted to further verify the structural stability of the Mo–O species in NiFe LDH/FeMoO after OER. As shown in Supplementary Fig. 43, the pristine Raman spectrum was first collected under open-circuit potential (OCP). After applying an oxidative potential of 1.4 V (vs. RHE) for 5 minutes, the Raman spectrum exhibited a clear transformation of the Ni–OH and Ni–O vibration modes of the NiFe LDH into the E bending and A g 1g stretching modes that are characteristic of γ-NiOOH. Additionally, distinct peaks at 780 cm−1 and 904 cm−1, corresponding to the asymmetric and symmetric stretching vibrations of the Mo–O bonds in the MoO units of the FeMoO component, were 4 observed, consistent with the in-situ Raman results.\nSubsequently, Raman spectra were recorded again after maintaining OCP for 10, 20, and 30 minutes. Notably, the Mo–O vibrational modes associated with the MoO units 4 remained clearly visible even after prolonged relaxation, confirming the remarkable structural stability of FeMoO during OER. This observation is further supported by post-stability SEM and TEM images (Supplementary Figs. 28 and 29), which reveal that the FeMoO pyramids remain conformally wrapped by ultrathin NiFe LDH nanosheets after long-term electrolysis. Moreover, the reappearance of Ni–OH and Ni–O vibration bands upon removal of the applied potential indicates that a small fraction of NiOOH formed during OER is electrochemically reversible, converting back to Ni(OH) under open-circuit conditions. 2\nAs suggested, the following sentences were revised on page 52, lines 3-13 in the revised Supplementary Information: ‘Ex-situ Raman spectroscopy was conducted to verify the structural stability of NiFe LDH/FeMoO after OER (Supplementary Fig. 43). The pristine spectrum recorded at open-circuit potential (OCP) exhibits the characteristic 39 Ni–OH and Ni–O vibration modes. After applying a potential of 1.4 V vs. RHE for 5 minutes, these bands transformed into the E bending and A stretching modes of γ- g 1g NiOOH. In addition, distinct peaks corresponding to the asymmetric and symmetric stretching vibrations of the Mo–O bonds within the MoO units of the FeMoO 4 component were detected, which is consistent with the in-situ Raman results. After resting at OCP for 10, 20, and 30 minutes, the Mo–O vibrational features of the MoO 4 units remained clearly visible, confirming the remarkable stability of the FeMoO pyramids during OER. The reappearance of the Ni–OH and Ni–O modes when the applied potential is removed suggests that some of the electrochemically generated NiOOH species can convert back to Ni(OH) under open-circuit conditions.26’ 2\nNew reference was added in the Supplementary Information:\nS26. Gallenberger, J., et al. Stability and decomposition pathways of the NiOOH OER active phase of NiO electrocatalysts at open circuit potential traced by ex situ x and in situ spectroscopies. Catal. Sci. Technol. 13, 4693-4700 (2023).\nSupplementary Fig. 43. Ex-situ Raman spectra of NiFe LDH/FeMoO.\nC3.10 Typo in Figure 4f  The label “Fas telectron transport” should be corrected to “Fast electron transport.”\nResponse: We thank the reviewer for pointing out this typographical error.\nAccording to the suggestion, we have thoroughly revised the article and corrected the typos for better readability. All changes in the text are highlighted in yellow.\n40\n).u.a( ytisnetnI 4 0 N i-O (N d 0 H i-O N ) i-O (N i-O  6 R a ) 0m 0 a n s h M ift ( o -O 8 0 c m 0− 1 ) M o -O 3 2 1 0 m in 0 m in 0 m in 1 .4 V O C P1 0 0 0\nC3.11 Figure 4d presents R comparisons, but the corresponding description in the ct manuscript discusses ICP-OES results. Please revise and clarify.\nResponse: We thank the reviewer for carefully checking the manuscript.\nIn the revised manuscript, the R comparisons are presented in Fig. 2d, and the ICP- ct OES results are shown in Fig. 4d. We have carefully rechecked the figure citations to ensure consistency with the correct figures in the main text.\nC3.12 A distinct peak near 760 cm-1 is observed in NiFe LDH/FeMoO but not in NiFe LDH. This feature should be identified, especially since it disappears during OER.\nResponse: We thank the reviewer for the valuable suggestion.\nFollowing the reviewer’s recommendation, we have revised the manuscript to clarify the origin of this peak. The peaks at ~780 cm−1 and ~904 cm−1 can be attributed to the asymmetric and symmetric stretching modes of the Mo–O bonds in the MoO unit.55 4 With an increasing potential from OCP to 1.41 V, the peak at 904 cm−1 remained stable, highlighting the structural robustness of the pyramidal framework. In contrast, the 780 cm−1 peak gradually decreased in intensity and eventually disappeared. This behavior may be due to the relatively weak intrinsic signal of FeMoO beneath the NiFe LDH overlayer, together with interference from gas bubble formation during electrolysis.56\nAs suggested, the following sentences were revised on page 11, lines 24-32 in the revised manuscript: ‘A distinct peak at ~1060 cm−1 was observed, which can be assigned to the vibrations of interlayer carbonate species in the NiFe LDH.55 In addition, the peaks at 780 cm−1 and 904 cm−1, originating from the asymmetric and symmetric stretching modes of the Mo–O bonds in the MoO units, were also detected.56 With the 4 applied potential increased from OCP to 1.41 V, the peak at 904 cm−1 remained clearly discernible, indicating the excellent structural stability of the pyramidal framework. In contrast, the peak at 780 cm⁻¹ gradually decreased in intensity and eventually disappeared. This is likely due to the intrinsically weaker signal of FeMoO located 41 beneath the NiFe LDH overlayer, combined with signal interference caused by gas bubble formation during electrolysis.57’\nNew references were added in the manuscript:\n55. Deng, P.-J., et al. Layered Double Hydroxides with Carbonate Intercalation as Ultra-Stable Anodes for Seawater Splitting at Ampere-Level Current Density. Adv. Energy Mater. 14, 2400053 (2024).\n56. Tian, H., Wachs, I. E., Briand, L. E. Comparison of UV and Visible Raman Spectroscopy of Bulk Metal Molybdate and Metal Vanadate Catalysts. The Journal of Physical Chemistry B 109, 23491-23499 (2005).\n57. Liu, M., Min, K.-A., Han, B., Lee, L. Y. S. Interfacing or Doping? Role of Ce in Highly Promoted Water Oxidation of NiFe-Layered Double Hydroxide. Adv. Energy Mater. 11, 2101281 (2021).\nFig. 4. In-situ Raman spectra of (b) NiFe LDH/FeMoO, and (c) NiFe LDH.\nC3.13 LDH-based samples often exhibit broad bands in the 900–1200 cm⁻¹ range, 42 3 0 0 ν(N i-O 6 0 0 ) 9 0 0 1 2 0 0 ).u .a( y tis n e tn I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 O .4 .3 .3 .3 .2 .2 .2 .2 .1 .1 .1 .0 .0 .0 C 1 V 8 V 5 V 2 V 9 V 6 V 3 V 0 V 7 V 4 V 1 V 8 V 5 V 2 V P δ N (N i-O ) N i-O N i-O H R a m a n s h N M o -O iF e L D H /F e M o -O C e O M o O 2− 3 × 1 5 × 1 0 × 5 3 0 0 6 0 0 9 0 0 1 2 0 0 ).u .a( y tis n e tn I c 1 .4 1 V 1 .3 8 V 1 .3 5 V 1 .3 2 V 1 .2 9 V 1 .2 5 V 1 .2 3 V 1 .2 0 V 1 .1 7 V 1 .1 4 V 1 .1 1 V 1 .0 8 V 1 .0 5 V 1 .0 2 V O C P δ (N i-O N i-O H R a v(N i-O N i-O m a n ) s h ift (c m − 1 ) N iF e L D 2− C O 3 H\nassociated with O–O vibrations and linked to LOM, which is namely “active oxygen”. The authors should provide high-frequency Raman data to discuss the potential-dependent evolution of these species.\nResponse: We thank the reviewer for this insightful suggestion.\nFollowing the reviewer’s suggestion, the in-situ Raman spectra have been replotted with an extended x-axis range of 200-1200 cm−1. As shown in Fig. 4b and Fig. 4c, a distinct peak at ~1060 cm−1 was observed, which can be assigned to the vibrations of interlayer carbonate species in NiFe LDH.52 No obvious peaks corresponding to O–O vibrations were detected in this range. Since the roles of lattice oxygen and the *OO intermediate have already been thoroughly examined using DEMS and ATR-SEIRAS measurements in the earlier sections of this manuscript, this part now emphasizes the carbonate-related feature while avoiding redundancy in the mechanistic discussion.\nAs suggested, the following sentences were revised on page 11, lines 24-26 in the revised manuscript: ‘A distinct peak at ~1060 cm−1 was observed, which can be assigned to the vibrations of interlayer carbonate species in the NiFe LDH.55’\nC3.14 Catalysts favoring LOM typically exhibit higher oxidation states in metal sites. However, the authors report delayed Ni redox and suppressed Fe oxidation, which appears to contradict prior studies. A discussion reconciling this discrepancy is needed.\nResponse: We thank the reviewer for this important point.\nWe agree that many LOM-active systems exhibit signatures of high metal oxidation states during OER (Nat. Commun. 11, 4066 (2020); Nat. Commun. 14, 1873 (2023); J. Am. Chem. Soc. 147, 20716 (2025)). For example, Zhang et al. reported that dynamically generated Ni4+ species in surface-reconstructed multimetal (oxy)hydroxides can inject holes into oxygen ligands, thereby facilitating intramolecular O–O coupling and activating lattice oxygen. Similarly, Sha et al. demonstrated that higher Ni oxidation states enhance the covalency of metal–oxygen 43 (M–O) bonds, leading to an upward shift of the O 2p band center towards the Fermi level, which promotes the redox participation of lattice oxygen adjacent to Ni sites.\nAlthough the enhancement of M–O covalency via elevated oxidation states can facilitate the involvement of lattice oxygen in O–O bond formation, there are also inherent drawbacks. First, excessive consumption of lattice oxygen can result in the uncontrolled accumulation of oxygen vacancies and disruption of the crystalline framework. Second, over-oxidation of Fe can lead to the formation of soluble high-valent ferrate species (e.g., FeO 2−), which causes irreversible dissolution, depletion of 4 lattice oxygen, interruption of the LOM cycle, and eventual loss of active sites. These issues highlight the well-recognized challenge of balancing high activity with long-term stability in LOM catalysts. Additionally, recent studies by Shao et al. (Nat. Energy 4, 329 (2019); Nat. Commun. 12, 3992 (2021)) have revealed that the oxygen species involved in O–O coupling is not necessarily derived from the direct oxidation of transition-metal centers. Instead, electrophilic oxygen intermediates such as peroxo-like (O 2−) or superoxo-like (O −) species can be generated through ligand-to-metal 2 2 charge transfer, while the metal centers maintain relatively moderate oxidation states. These findings suggest that lattice oxygen participation can proceed without a pronounced increase in the formal valence of the metal cations, further supporting that high metal oxidation states are not an essential prerequisite for the LOM pathway.\nIn the NiFe LDH/FeMoO heterostructure designed in this study, both experimental results and theoretical calculations reveal that interfacial interactions cause the O 2p band centre to shift upwards. Furthermore, oxygen-vacancy-assisted electronic redistribution promotes the participation of lattice oxygen without triggering excessive Fe oxidation. This regulation arises from the synergistic effects of oxygen vacancies that tune the O 2p orbitals, and of interfacial built-in electric fields that modulate the Ni 3d states. Consequently, the oxygen ligands accommodate a greater fraction of the oxidative charge, thereby lowering the barrier to lattice oxygen redox participation. Meanwhile, the oxidation of Ni and Fe cations is delayed rather than being forced into extremely high formal valence states under low overpotentials. In other words, it is 44 lattice oxygen rather than the transition-metal cations that serves as the primary oxidative reservoir. This mechanism has recently been corroborated by independent LOM studies (Nat. Commun. 13, 2191 (2022)).\nTherefore, we emphasize that an increase in metal valence is neither a necessary nor a sufficient condition for LOM activity. Instead, contemporary theoretical and experimental studies (Nat. Energy 4, 329 (2019); Nat. Commun. 12, 3992 (2021); Nat. Commun. 14, 6019 (2023); Adv. Funct. Mater. 34, 2401610 (2024)) have consistently demonstrated that the tendency of lattice oxygen to participate in the OER is more directly governed by (i) the degree of metal–oxygen covalency and orbital overlap, (ii) the relative position of the O 2p band center with respect to the Fermi level and metal d states, and (iii) local structural factors, such as oxygen vacancies and interface-induced band bending. When these conditions favor oxygen-hole formation, LOM becomes thermodynamically and kinetically accessible, even without a significant elevation of the formal oxidation states of the transition metals.\nAs suggested, the following sentences were revised on page 12, lines 6-10 in the revised manuscript: ‘Although elevated metal oxidation states are often observed in LOM-active catalysts, lattice-oxygen participation does not necessarily require highly oxidized metal centers.15, 58 In the NiFe LDH/FeMoO heterostructure, interfacial charge redistribution and oxygen-vacancy-induced modulation of the O 2p band enhance oxygen-hole character while suppressing excessive oxidation of Ni and Fe sites.’\nA new reference was added in the manuscript:\n58. Sha, Q., et al. Lattice Oxygen Mechanism Induced on Nickel Sites by Cl– Adsorption for Efficient Seawater Oxidation Reaction. J. Am. Chem. Soc. 147, 20716-20724 (2025).\n45"}, {"round": 2, "reviewer_comments": {"reviewer_1": "All of my concerns have been satisfactorily addressed.", "reviewer_2": "The authors have addressed my concerns. I am pleased to recommend publishing this work in Nature Communications.", "reviewer_3": "I believe the comments to the authors have been sufficiently clarified. The manuscript appears to be significantly improved and is suitable for publication."}, "author_response": ""}]

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