Cr and Nd co-doped cobalt oxide for stable proton exchange membrane water electrolysis

Domain: chemistry
Published: 2026-04-13
Editorial outcome: major_revision

Abstract
Developing noble-metal-free electrocatalyst for oxygen evolution reaction in a proton exchange membrane water electrolyzer is a key to sustainable and economical hydrogen production. Herein, we rationally design and develop a chromium and neodymium co-doped cobalt oxide (CrNd-Co3O4) electrocatalyst that exhibits high activity and durability in the acidic oxygen evolution reaction condition. Furthermore, an in-situ acid circulation strategy is proposed to tackle the ubiquitous issue of membrane poisoning by leached cations in proton exchange membrane water electrolyzers. Consequently, the proton exchange membrane water electrolyzer with CrNd-Co3O4 anode achieves a stable operating current density of 2 amperes per square centimeter at 2.27 volts and 4 amperes per square centimeter at 2.54 volts for 1,000 hours.

Introduction
Proton exchange membrane (PEM) water electrolysis is regarded as the most promising technology for green hydrogen production. However, the overall energy efficiency of PEM water electrolyzer (WE) is constrained by the sluggish oxygen evolution reaction (OER) at anode, calling for development of highly active and stable OER electrocatalyst1-4. Iridium oxide (IrOx) is the state-of-the-art OER electrocatalyst, which exhibits good activity and stability under acidic and oxidative PEMWE operating conditions. However, the scarcity and high cost of iridium prevent the large-scale implementation of PEMWEs5-7.

Cobalt oxide (Co3O4) has recently been investigated as a prospective candidate in PEMWE because of its comparable activity to IrOx8-11. However, its stability is problematic due to the loss of lattice oxygen and formation of cobalt vacancies under acidic OER conditions12,13, which cause substantial Co2+ leaching from the catalyst14. Several strategies like surface engineering10, interfacial water structure modulation11 have been proposed to improve the stability of Co3O4, while Co2+ leaching could not be eliminated. The leached cations unavoidably poison PEM, significantly impairing the PEMWE performance15,16, which was universally observed in many noble-metal-free catalysts-based PEMWEs17,18. Hence, suppressing cobalt ions leaching and addressing the PEM poisoning problem are critical to enable the practical application of cobalt-based OER electrocatalysts.

Herein, based on a systematic experimental and theoretical investigation of the stability and activity of Co3O4 OER electrocatalysts doped with transition metals and rare earth elements, we developed a chromium (Cr) and neodymium (Nd) co-doped Co3O4 electrocatalyst (CrNd-Co3O4) for PEM water oxidation. Structural and electrochemical characterizations demonstrated that incorporation of Nd enhanced the OER activity in an acidic environment (an overpotential of 395 mV at 10 mA cm-2), while incorporation of Cr significantly improved the intrinsic stability of Co3O4 (200 h at 10 mA cm-2). Despite the reduced cobalt leaching in CrNd-Co3O4 anode, complete elimination of cation-driven contamination in PEM operation remains challenging. To address this universal challenge, we proposed a short-duration (20 minutes), one-time acid circulation strategy applied at the cathode after startup of the device. With this, the PEMWE equipped with CrNd-Co3O4 anode could be operated continuously at 2 A cm-2 for 1,000 hours with an initial cell voltage of 2.27 V and a voltage decay rate of only 32 V h-1. At a harsher operating condition of 4 A cm-2, it could still maintain performance for 1,000 hours with an initial cell voltage of 2.54 V and a voltage decay rate of 3 V h-1, highlighting a pathway for device-level management of leaching-induced degradation in noble-metal-free anode concepts.

Results
Design of Cr, Nd co-doped Co3O4 water oxidation electrocatalyst

A transition metal could be doped to modulate the electronic structure of Co3O4, whereby suppressing the loss of lattice oxygen and cobalt. We screened for candidates via evaluating voltage decay rate and cobalt leaching during chronopotentiometry at a current density of 10 mA cm-2 (Fig. S1). Cr excelled because Cr-doped Co3O4 (Cr-Co3O4) exhibited the lowest voltage decay rate of 1.27 mV h-1 and leached Co2+ concentration of 1.33 ppm (Fig. 1A).

On the other hand, to improve the OER activity of Co3O4, a large-radius lanthanide series element may be incorporated to increase Co electron density near the Fermi level, which in turn shall enhance adsorption of oxygenated species. According to the OER activity volcano plot, Co3O4 lies in the weak adsorption branch, and its weak adsorption to oxygenated intermediates limits the overall OER kinetics19,20. To reveal how thermodynamic profiles correlate with OER kinetics, a thermodynamic-kinetic (TK) model developed by us was applied to determine the adsorption energies of intermediates from electrochemical measurements (Supplementary text 1)19-21. Fig. 1B demonstrated quantitative consistency between the experimental and simulated electrochemical data of representative catalysts, suggesting the valuable guidance provided by the TK model. Nine lanthanide series elements were screened and all of them boosted OER activity of Cr-Co3O4 (Fig. 1C). Among them, Nd-doped Cr-Co3O4 (CrNd-Co3O4) displayed the most promising OER activity with an enhanced adsorption energy of oxygenated intermediates (Fig. S2, S3 and S4, Table S1).

Density functional theory (DFT) calculations were carried out to investigate the doping effects based on the models (Fig. S5, supplementary data 1) derived from the structural characterizations (see below). Surface oxygen vacancy formation energy and Co atom demetallation energy were employed as the indicators for OER stability22-24. Firstly, we conjecture that surface lattice oxygen (OL) loss due to H++e- pair attack involves protonation of surface lattice oxygen (OL-H) and subsequent formation of oxygen vacancy (VO) (Fig. S6). The first step is spontaneous (Fig. 1D), as indicated by the calculated energy change in Co3O4 (-2.41 eV) and Cr-Co3O4 (-1.16 eV). With Cr-doping, the calculated energy barrier for VO formation significantly increased from -0.16 to 0.75 eV (Fig. 1D), suggesting that OL loss was thermodynamically unfavorable for Cr-Co3O4 in contrast to Co3O4. Concurrently, the energy requirement for surface Co demetallation increased from 5.77 in Co3O4 to 6.85 eV in Cr-Co3O4, indicating a more stable surface Co in Cr-Co3O4 (Fig. 1E, Fig. S7). Notably, introduction of Nd hardly compromised the stability of Cr-Co3O4, as implied by the high energy cost for VO formation and Co demetallation (Fig. 1D and 1E). Altogether, the DFT calculations support that doping Cr into Co3O4 lattice stabilizes lattice oxygen and suppresses dissolution of Co under acidic OER conditions.

To understand how doping Nd into Cr-Co3O4 affects its electronic structure and thus OER activity, the free energy profiles of Co3O4, Cr-Co3O4 and CrNd-Co3O4 for OER based on conventional adsorbate evolution mechanism (AEM) were investigated by DFT. The rate determining step (RDS) of OER on Co3O4, Cr-Co3O4 and CrNd-Co3O4 was the formation of *O (Fig. 1F). The free energy barrier of RDS on CrNd-Co3O4 was calculated to be 0.43 eV at 1.23 V (V vs. reversible hydrogen electrode), which is 0.12 eV lower than that in Cr-Co3O4 and 0.44 eV lower than that in Co3O4, indicating strengthened adsorption of oxygenated species, *O in particular, and thus enhanced OER activity on CrNd-Co3O4, agreeing well with the charge density difference analysis (Fig. S8). Moreover, projected density of states (PDOS) analysis revealed a noticeable overlap between Nd 4f and Co 3d orbitals near the Fermi level (EF) (Fig. S9), suggesting strong hybridization between Nd and Co. Consequently, PDOS of Co 3d orbital near EF increased and spread, which favored the interaction between Co 3d and O 2p orbitals of adsorbed oxygenated species, thereby contributing to the enhanced adsorption and more efficient OER. Additionally, d-band center (εd) of Co 3d orbital shifted closer to EF in CrNd-Co3O4 (-0.705 eV), as compared to Cr-Co3O4 (-0.742 eV). Such upshift of εd resulted in less occupied antibonding states of adsorbate O 2p orbital, thus enhancing the adsorption. Notably, DFT calculations are based on idealized surface models under static conditions and do not fully account for dynamic phenomena under actual electrochemical operating conditions. Therefore, the computed results require verification through detailed experimental characterizations. Altogether, co-doping Cr and Nd into Co3O4 produces a PEM water oxidation electrocatalyst with enhanced stability and activity.

Structural characterizations of CrNd-Co3O4

Cr-doping into Co3O4 induced negligible change in X-ray diffraction (XRD) pattern (Fig. S10). Broadening of (311) peak for CrNd-Co3O4 indicated increased amorphization, ascribable to decreased long-range order of the spinel structure induced by large-sized Nd atoms. CrNd-Co3O4 was composed of aggregated nanoparticles as revealed by transmission electron microscopy (TEM, Fig. 2A), consistent with the poly-crystalline concentric circles in selected area electron diffraction pattern (SAED, Fig. 2B). Interplanar spacings of 0.289 and 0.248 nm observed in high-resolution TEM (HRTEM) images could be assigned to (220) and (311) facets of spinel Co3O4, respectively (Fig. S11), agreeing with XRD results. Similarly, HRTEM and SAED analysis (Fig. S12) demonstrated that both Co3O4 and Cr-Co3O4 are predominantly composed of spinel Co3O4 polycrystalline nanoparticles. These structural analyses collectively suggested that introduction of Cr and Nd did not alter the spinel structure. Energy dispersive X-ray spectroscopy (EDX) mapping demonstrated uniform distribution of Cr, Nd, Co and O elements in CrNd-Co3O4 (Fig. 2C) and inductively coupled plasma mass spectroscopy (ICP-MS) determined an atomic ratio of Co:Cr:Nd of 85:8:7 (Fig. S13). From the discussion above, it could be concluded that CrNd-Co3O4 consisted of nanocrystalline spinel Co3O4.

To study the coordination structure, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements were carried out. As demonstrated by high-resolution O 1s XPS spectra, the proportion of lattice oxygen in Cr-Co3O4 (53.0%) was markedly higher than that in Co3O4 (39.2%), suggesting enhanced lattice stability (Fig. S14, Table S2). The widening of lattice oxygen peak for CrNd-Co3O4 signified a decrease in crystallinity, agreeing with the XRD and TEM results. Furthermore, Co 2p XPS spectra of Cr-Co3O4 and CrNd-Co3O4 revealed a ~0.3 eV blue shift as compared with that of Co3O4 (Fig. S15, Table S3), suggesting a lower oxidation state of Co caused by doping. This is further corroborated by a higher ratio of Co2+: Co3+ in doped Co3O4 catalysts (Fig. S15, Table S3). Co K-edge X-ray absorption near-edge structure (XANES) spectra revealed the overlapping pre-edge peaks (1s→3d transition) of Co3O4 and Cr-Co3O4, suggesting that Cr-doping did not disrupt the centrosymmetric local environment of cobalt (Fig. 2D). Notably, CrNd-Co3O4 exhibited an intensified pre-edge feature, indicating lattice distortion of Co3O4 due to incorporation of large Nd. Additionally, CrNd-Co3O4 displayed a subtle red shift in absorption edge energy relative to Co3O4 (Fig. 2D), corresponding to a slightly lower Co oxidation state of 2.60 compared to 2.61 of Co3O4 (Fig. 2E). The k3-weighted Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) spectra at Co K-edge revealed three primary scattering paths: Co-O, Co-Co (octahedral), and Co-Co (tetrahedral) (Fig. 2F). Compared to Co3O4, the decreased Co-Co intensity and unaltered Co-Co intensity in Cr-Co3O4 suggest that Cr substitutes for Co at octahedral sites, consistent with the preservation of centrosymmetric coordination deduced from pre-edge analysis. In comparison, CrNd-Co3O4 showed a marked reduction in Co-Co intensity, indicating that Nd preferentially occupied tetrahedral sites25,26. Moreover, reduced intensity of Co-O first shell in CrNd-Co3O4 implied a decrease in Co-O coordination upon Nd doping (Fig. 2F). Co L-edge XANES spectra presented an increase in the white-line intensity at ~780 eV following Cr and Nd doping, suggesting enhanced 2p→3d transition (Fig. S16). Given the minimal change of Co oxidation state, this enhancement was likely due to an increased number of unoccupied Co 3d states, attributable to greater Co 3d−O 2p orbital interaction.

To probe dynamic structural evolution of CrNd-Co3O4 during OER, in-situ XAS measurements were performed. Co K-edge XANES spectra under various OER operating potentials indicated that the pre-edge intensity remained stable (Fig. 2G), indicating preservation of the centrosymmetric Co coordination and good structural stability of CrNd-Co3O4. With increasing potential, a gradual positive shift of absorption edge was observed, suggesting an increase of Co oxidation state: +2.70 at open circuit voltage (OCV), +2.74 at 1.3 VRHE, and +2.77 at 1.8 VRHE (Fig. 2H). Moreover, an intensified white-line acquired at 1.8 VRHE indicated stronger Co 4p−O 2p interaction during OER. In-situ FT-EXAFS spectra further revealed increased intensities of Co-O and Co-Co shells (Fig. 2I), along with reduction in their bond distances at 1.8 VRHE, implying shortened Co-O bonds and enhanced covalency − factors known to benefit OER activity27,28. On the other hand, Cr K-edge XANES spectra exhibited only a minor edge shift upon potential increase (Fig. S17), suggesting a minimal change of Cr oxidation state. The potential-dependent evolution of the Nd L3-edge white-line (Fig. S18) indicated changes in the unoccupied Nd 5d states and dopant–oxygen covalency under OER-relevant potentials, suggesting that Nd actively participated in the electronic structure of the oxide lattice. This observation is consistent with our DFT results showing enhanced metal–oxygen hybridization in Nd/Cr-incorporated spinel-like motifs. XAS analysis suggests that Co acts as the active site while Nd and Cr dopants modulate its electronic structure.

Electrocatalytic performance

We first evaluated the OER performance of the as-prepared catalysts on Pt-Ti mesh in a 0.1 M HClO4 solution. Linear sweep voltammetry (LSV) showed that CrNd-Co3O4 offered improved activity (an overpotential of 395 mV at 10 mA cm-2) as compared to Cr-Co3O4 and Co3O4 (Fig. 3A). Tafel slope of CrNd-Co3O4 (83.4 mV dec-1) was much lower than that of Co3O4 (99.2 mV dec-1), indicating boosted OER kinetics (Fig. S19). Electrochemically active surface area (ECSA) was estimated by measuring double layer capacitance (Cdl) in the non-Faradaic region (Fig. S20 and Table S4). ECSA-normalized LSV curves demonstrated enhanced intrinsic activity of CrNd-Co3O4 (Fig. S21). CrNd-Co3O4 also displayed an improved OER stability examined in an H-cell at 10 mA cm-2 (Fig. 3B). In addition, the improved S-number of CrNd-Co3O4 compared to that of undoped Co3O4 clearly demonstrated the effect of co-doping on durability enhancement (Fig. S22). Despite that the S-number of CrNd-Co3O4 was about two orders of magnitude lower than that of IrO2, the observed improvement relative to other reported Co3O4-based catalysts indicated that the introduction of Cr and Nd effectively suppressed Co dissolution and extended lifetime.

To explore the potential for industrial applications, a 2 × 2 cm2 single-cell PEMWE was constructed using CrNd-Co3O4 as the anode catalyst and commercial 60 wt.% Pt/C as the cathode catalyst, separated by a commercial proton exchange membrane (FS-990-PK, 90 μm). The electrolyzer was fed with ultra-pure water at the anode and operated at 80 °C. Ion exchange resins were equipped for water circulation system to remove foreign impurities. Current-voltage polarization curve showed that PEMWE could reach 2 A cm-2 and 4 A cm-2 at a cell voltage of 2.544 V and 2.748 V, respectively (Fig. 3C). Note that there was an abnormal inflection in the polarization curve at low current region, after which, the cell voltage dramatically increased. Such abrupt deterioration could be ascribed to cationic contamination of the PEM29,30. To tackle this issue, we have developed a simple strategy, i.e., in-situ acid circulation at the cathode applied when the catalyst has been fully activated (12 hours after the startup of the PEMWE; the in-situ acid circulation lasts for 20-minutes). Catalyst activation is unavoidably accompanied by cation leaching, thus leading to serious PEM poisoning and drastic increase of cell voltages at high current densities. With acid circulation, the performance deterioration was completely absent in the polarization curve of the electrolyzer (Fig. 3C), giving a cell voltage of 2.545 V at a current density of 4 A cm-2 − 203 mV lower than the poisoned counterpart. Such performance also exceeded the PEMWEs with Co3O4 anode (Fig. S23). Additionally, a cell voltage of 2.134 V at 4 A cm-2 was achieved with 100% iR correction, authenticating its high intrinsic performance (Fig. S24).

Long-term stability at industry-relevant current densities was evaluated by chronopotentiometry at 2 A cm-2 and 4 A cm-2 (Fig. 3E and F). The PEMWE displayed a stable cell voltage around 2.27 V at 2 A cm-2 for 1000 hours with an average decay rate of 32 μV h-1 (the discontinuity was caused by power shutdown and re-startup). An even better operational stability was achieved with a cell voltage of 2.54 V at 4 A cm-2 for 1,000 hours with an average decay rate of only 3 μV h-1. Taking total charge Q = j × t (j, current density, A cm-2; t, time, h) and the decay rate as the stability indicators, we benchmarked our CrNd-Co3O4 catalyst with the state-of-the-art nonprecious-metal-based OER catalysts (Fig. S25, Table S5). The Q of our PEMWE operated at 4 A cm-2 reached 4000 A h cm-2, demonstrating its competitive stability compared to others. Under 4 A cm-2, metal concentrations in the anode effluent were detected during the first 300 hours (Fig. 3D). A total metal concentration of <20 ppb validated the high stability of CrNd-Co3O4 anode in the industrial PEMWE.

PEM poisoning and recovery strategy

PEM is highly susceptible to cation contamination, as evidenced by the observation that the performance of a PEMWE with IrO2 anode was significantly compromised by a feed with a trace amount of extraneous cations (3 ppm Co2+) (Fig. S26). For a nonprecious-metal-based anode (CrNd-Co3O4 here), the amount of leached cations (Co2+ here) from catalyst layer was markedly higher than ppm level, most of which migrated towards PEM driven by the electric field (Fig. S27), leading to its poisoning, while the rest diffused to anode feed (Fig. 4A). PEM poisoning is ascribable to the coordination between multivalent cations and sulfonate groups and the subsequent cross-linking of polymer chains, resulting in discontinuous water domain and thus high proton transport resistance within the PEM (Fig. 4A). At a high current, the hindered proton transport causes insufficient proton supply to cathodic hydrogen evolution reaction (HER), making its kinetics sluggish and consequently causing sharp increase of the cell voltage (Fig. 3D)30-32. Furthermore, the cathodic HER pathway may switch from 2H+ + 2e- → H2 to 2H2O + 2e- → H2 + 2OH-. The produced OH- could react with transition metal cations and form insoluble metal hydroxide on cathode, leading to coverage of the active sites thus further performance decline.

For several strategies being proposed to mitigate PEM poisoning30,31,33, either the PEMWE performance cannot be completely recovered or the procedure is not feasible for practical application (Supplementary text 2). Herein, we developed a facile practical strategy, specifically, one-time in-situ circulation of nitric acid at cathode for 20 minutes after operation at 4 A cm-2 for 12 hours since startup of PEMWE (Supplementary text 2). It is highly effective as evidenced by an immediate large voltage drop upon acid circulation (Fig. S28), presumably because Co2+ ions coordinated with sulfonate groups are replaced by protons (Fig. 4A). The efficient removal of cations within the poisoned PEM, corroborated by the cross-sectional elemental distribution of catalyst-coated membrane (Fig. S29), allows continuous water domain being re-constructed for proton transport (Fig. 4A).

To further understand the underlying mechanism, electrochemical impedance spectroscopy (EIS) was used to analyze the interfacial behaviors. EIS data with and without in-situ acid circulation under different current densities ranging from 0.025 to 4 A cm-2 were collected (Fig. S30). Reduced high frequency resistances of the PEMWE with acid circulation indicated the recovered conductivity of the poisoned PEM (Fig. 4B). Furthermore, fitting of the Nyquist plot at 2 A cm-2 based on an equivalent circuit34,35 (Fig. 4C) demonstrated significant reduction in the resistances of proton transfer (100%), charge transfer (94.8%) and mass transfer (88.1%) of the PEMWE with acid circulation (Fig. 4D, Fig. S31 and Table S6). Negligible proton transfer resistance benefited from the thorough reconstruction of continuous water domains and proton transport pathways within the PEM (Fig. 4A). Consequently, the slow kinetics of HER (charge transfer) caused by insufficient protons supply were largely rectified and the mass (water/gas) transport at the interfaces were remarkedly improved.

Methods
Chemicals

All chemicals including cobalt(II) nitrate (Co(NO3)2·6H2O), titanium(IV) tert-butoxide (Ti(C4H9O)4), ammonium metavanadate(V) (NH4VO3), chromium(III) nitrate (Cr(NO3)3·9H2O), manganese(II) nitrate (Mn(NO3)2·4H2O), iron(III) nitrate (Fe(NO3)3·9H2O), nickel(II) nitrate (Ni(NO3)2·6H2O), ammonium molybdate(VI) ((NH4)6Mo7O24·4H2O), ammonium metatungstate(VI) ((NH4)6H2W12O40·xH2O), lanthanum(III) nitrate (La(NO3)3·9H2O), cerium(III) nitrate (Ce(NO3)3·6H2O), praseodymium(III) acetylacetonate (Pr(C5H7O2)3·xH2O), neodymium(III) acetate (Nd(CH3CO2)3·xH2O), samarium(III) acetate (Sm(CH3CO2)3·xH2O), gadolinium(III) acetate (Gd(CH3CO2)3·xH2O), dysprosium(III) acetate (Dy(CH3CO2)3·xH2O), erbium(III) acetate (Er(CH3CO2)3·xH2O), ytterbium(III) acetate (Yb(CH3CO2)3·4H2O), citric acid (C6H8O7), methanol (CH3OH), ethanol (C2H5OH), isopropyl alcohol (C3H8O), iridium oxide (IrO2), and 5 wt.% Nafion perfluorinated resin solution (D-520) were purchased from Sigma-Aldrich and used without further purification. Deionized (DI) water was supplied by Mill-Q system (18.2 MΩ cm).

Synthesis of CrNd-Co3O4

The preparation of Cr, Nd-Co3O4 was based on a conventional sol-gel method. Specifically, 1.163 g Co(NO3)2·6H2O, 160.5 mg Nd(CH3CO2)3·xH2O, 200.0 mg Cr(NO3)3·9H2O and 480 mg citric acid were dissolved in 10 mL ethanol. The mixture was stirred for 30 min to form a homogeneous solution, followed by continuously stirring under 90 °C water bath to evaporate solvent until formation of a uniform gel. Subsequently, the gel was heated in an oven at 120 °C for 1 h to obtain dry gel. It was then calcined in air at 350 °C for 4 h at a heating rate of 5 °C min-1 and the powder was collected after natural cooling. Lastly, the powder was treated with 0.1 M H2SO4 solution for 5 min, followed by DI water washing three times and drying at 60 °C overnight to obtain the final CrNd-Co3O4 product. The transition metals (Mtrans) doped Co3O4 samples were prepared by the same method using corresponding metal salts as precursors, with the atomic ratio of Co:Mtrans = 9:1. The lanthanide series metals (MLa) doped Cr-Co3O4 samples were also similarly prepared by applying corresponding metal salts as precursors, with the atomic ratio of Co:Cr:MLa = 8:1:1.

Materials characterization

The phase and crystal structure of samples were examined by X-ray diffraction (XRD, Bruker D2 phaser) using a Cu Kα radiation source ( = 1.5418 Å). The data processing and analysis of XRD patterns were performed using X’Pert Highscore Plus software. The sample morphology and structure were investigated by field-emission transmission electron microscope (TEM; JEOL 2100F at 200 kV). The surface chemical states of samples were studied by X-ray photoelectron spectroscopy (XPS, Kratos AXIS Supra). All the spectra were calibrated using C 1s peak at 284.8 eV. The metal cations leached out in the electrochemical cell and PEMWE were quantified by inductively coupled plasma mass spectroscopy (ICP-MS). In-situ Co K-edge, Cr K-edge, and Nd L3-edge X-ray absorption spectroscopy (XAS) measurements were performed at the BL32A beamline of the Taiwan Photon Source (TPS), National Synchrotron Radiation Research Center (NSRRC). The experiments were conducted under open-circuit voltage (OCV) and applied potentials of 1.3 and 1.8 V vs. RHE. A custom-designed Teflon cell equipped with a Kapton window was employed to enable X-ray transmission. Spectra were collected from the back side of the electrode to prevent interference from oxygen bubble formation during operation. XAS signals were acquired in partial fluorescence yield mode using a silicon drift detector. Prior to data acquisition, energy calibration was carried out using Co foil for the Co K-edge and Cr foil for both the Cr K-edge and Nd L3-edge. Data processing involved subtraction of the pre-edge baseline followed by normalization of the post-edge region using the Athena software package. Oxidation states were determined by comparing the second derivatives of absorption edge features with those of reference compounds. Extended X-ray absorption fine structure (EXAFS) analysis was performed by applying Fourier transformation to the k3-weighted EXAFS oscillations (2.9-10.8 Å) to resolve contributions from different local bonding environments. All EXAFS spectra are presented without phase correction.

DFT simulations

All DFT calculations were carried out using the Vienna ab initio Simulation Package (VASP, version 6.3.0)36. The exchange-correlation energy was treated with the PBE functional, and ion-electron interactions were described within the PAW formalism37,38. Long-range van der Waals interactions were considered by employing the empirical DFT-D3 correction39. To properly describe the correlated electrons in the metal oxide system, the DFT+U method was applied, with effective U (Ueff = U − J) values of 6, 3, and 2 eV for Nd, Co, and Cr, respectively, following previous reports40-42. Based on XRD results (Fig. S10), our homemade Co3O4 was modelled as a spinel Co3O4 structure; and (400) surface of Co3O4 was chosen because it is the thermodynamically stable surface and was observed in the SAED pattern (Fig. 2B)43,44. In the outermost layer of Co3O4 (400) surface, the coordinatively unsaturated Co atoms at octahedral sites were recognized as the active sites for H2O adsorption and the following OER process (Fig. S5)44,45. XAS results indicated that Cr dopants likely substituted Co atoms at the octahedral sites, and Nd atoms preferred to occupy the tetrahedral sites (Fig. 2F), hence one Cr atom (octahedral site) and one Nd atom (tetrahedral site) were doped in the Co3O4 (400) to form Cr-Co3O4 and CrNd-Co3O4 to simulate the electrochemical OER. Brillouin zone integration was performed using a Monkhorst–Pack k-point mesh of 2 × 2 × 1 for all periodic structures, with a plane-wave cutoff energy of 450 eV. During structural optimization, atoms in the top two layers were fully relaxed, whereas those in the bottom two layers were fixed. Convergence criteria were set to 10⁻⁵ eV for total energy and 0.02 eV/Å for residual forces. The free energy changes for each proton-coupled electron transfer step were determined using the computational hydrogen electrode model developed by Nørskov et al46. The change in Gibbs free energy (ΔG) for all the elementary steps was defined as:

𝛥𝐺= Δ𝐸 +Δ𝐸ZPE +Δ𝐻0→𝑇 −𝑇𝛥𝑆 + 𝐺𝑈 (1)

where ΔE, ΔEZPE, and ΔS denote the variations in electronic energy, zero-point energy, and entropy arising from intermediate adsorption, respectively. The term ΔH0→T accounts for the enthalpy change upon heating from 0 K to temperature T. The entropy associated with the H⁺/e⁻ pair was approximated as half the entropy of gaseous H2 under standard conditions47. The GU represents the applied electrode potential, taken here as 0 V and −1.23 V relative to the standard hydrogen electrode.

The adsorption energy of species (ΔE) was calculated using the following equation:

𝛥𝐸= 𝐸total −𝐸ads −𝐸sub (2)

where Etotal, Eads, and Esub correspond to the total energy of the adsorbate–substrate system, the isolated adsorbate, and the clean substrate, respectively.

The orbital band center (e.g., for p, d, or f states) was obtained from the density of states (DOS) using the following expression:

𝜀 = ∫−∞∞ 𝑛𝑜𝑟𝑏𝑖𝑡𝑎𝑙𝑠(𝜀)𝜀𝑑𝜀 / ∫−∞∞ 𝑛𝑜𝑟𝑏𝑖𝑡𝑎𝑙𝑠(𝜀)𝑑𝜀 (3)

where ε denotes the energy level and n(ε) represents the projected DOS for the orbital of interest.

Three-electrode electrolysis cell

The electrochemical measurements were performed on a CHI 760E electrochemical workstation with a conventional three-electrode electrolysis cell using Pt as the counter electrode, Hg/Hg2Cl2 (saturated KCl) as the reference electrode, and rotating disk electrode (RDE) with a glassy carbon disk (GC, 0.196 cm2, Pine instruments) or platinum-coated titanium mesh (Pt-Ti, Ti thickness: 250 μm, Pt thickness: 0.5 μm; Fuershun Electronic Materials) as the working electrode. The potential of Hg/Hg2Cl2 reference electrode was calibrated using a reversible hydrogen electrode. To prepare the catalyst ink, 5 mg samples and 20 μL of 5 wt.% Nafion solution were added into 980 μL water-isopropanol (v:v=1:1) solution, followed by sonication for 3 h. Two types of electrolysis cells were used: an H-type cell and a five-port cell.

Screening of transition metal dopants

An H-type cell with the anode and cathode chambers separated by a N115 PEM (Dupont, USA) was used as an electrolysis cell. Pt-Ti mesh was applied as the substrate and 0.01 M HClO4 + 0.09 NaClO4 was applied as the electrolyte. 100 μL of the catalyst ink (Mtrans doped Co3O4) was dropped onto a 1×2 cm2 Pt-Ti mesh with the catalyst coating area of 1×1 cm2 on a 60 °C hot plate. The catalyst loading was 500 μg cm-2. The chronopotentiometry (CP) tests were performed for 10 h at the current density of 10 mA cm-2. In the end, 0.5 mL electrolyte in the anode chamber was collected and diluted to 10 mL with DI water for ICP-MS measurements.

Screening of lanthanide series metal dopants

A five-port cell was used as an electrolysis cell. 8 μL of the catalyst ink (MLa doped Cr-Co3O4) was dropped onto a polished GC with a catalyst loading of 200 μg cm-2. 0.1 M HClO4 was used as an electrolyte. Cyclic voltammetry (CV) was scanned between 1.2 and 1.8 VRHE (without iR compensation) at a rate of 50 mV s-1 for 10 cycles to activate the catalyst surface. Subsequently, linear sweep voltammetry (LSV) was conducted at a scan rate of 1 mV s-1 with 100% iR correction and a rotating speed of 1600 rpm. The LSV curves and derived Tafel plots were used for TK modelling and analysis. To obtain the equilibrium potential of the first electron transfer step (OH* formation) by methanol probing method, CV and LSV measurements were carried out under the same conditions except that the electrolyte was changed to 0.1 M HClO4 + 1 M CH3OH.

Electrochemical performance and stability evaluation

The same H-cell was applied with 0.1 M HClO4 as the electrolyte. Co3O4, Cr-Co3O4 or CrNd-Co3O4 coated on 1×2 cm2 Pt-Ti mesh with a catalyst loading of 1 mg cm-2 was applied as the working electrode. The same CV and LSV measurements to ‘screening of lanthanide series metal dopants’ were conducted. Tafel analysis was based on the quasi-steady-state polarization curves collected using a potential-step protocol. The potential was incremented from 1.4 to 1.7 VRHE in steps of 20 mV, and each step was held for 30 s. The steady-state current at each potential was obtained by averaging the current over the last 5 s of the hold once the current drift became negligible. The catalyst stability is evaluated using the S-number (S), defined as the ratio of the produced oxygen (calculated from total charge) to the dissolved Co (calculated from ICP data) under specific OER operating conditions:

S = n(OER)O2 / n(dissolved)Co

Calculation of the electrochemically active surface area

The electrochemically active surface area (ECSA) of catalysts were estimated from the electrochemical double layer capacitance (Cdl) according to:

ECSA=Cdl/CsA (4)

where a typical value of 0.035 mF cm-2 was used for specific capacitance (Cs) in H2SO4 48,49. Cdl was measured from the scan rate-varied CVs in non-Faradic region through the equation ic = vCdl, where v is the scan rate, and ic is the charging current at the middle of the scan region.

Membrane electrode assembly (MEA) and PEMWE fabrication

The MEAs were fabricated by applying catalyst coated membrane (CCM) method. IrO2, Co3O4 or CrNd-Co3O4 was used for anode, commercial Pt/C (60 wt.%; Johnson Matthey) was used for cathode, and FS-990-PK (poly(ketone)-reinforced, 90 μm; FUMASEP) was used as PEM. Although better PEMWE activity could be reached using thinner PEM (e.g., N212, 50 μm), the configuration remains unsuitable for long-term operation at industrial level currents due to safety risks caused by hydrogen crossover. The catalyst ink for anode was prepared by mixing DI water (1.06 g), 1-propanol (0.43 g) and 1,2-propanediol (0.22 g) with an anode catalyst (0.3 g) and ionomer (D31579, 15 wt.%, 0.3 g; Tuoji Topmaterials). For cathode, the catalyst ink was prepared by mixing DI water, isopropanol, Pt/C and ionomer (D2020) and the mass of the ionomers was 62.5 wt.% of the catalyst. Both the anode and cathode inks were homogeneously dispersed by ball milling at a speed of 300 rpm for 4 h. Then the catalyst inks were coated on PTFE films (polytetrafluoroethylene, 50 μm) by a doctor blade coater. After infrared light drying at 120 °C for 3 min, the PTFE films coated with catalysts were pressed on a membrane by decal transfer technique at 165 °C and 2 MPa for 2 min. The catalyst mass loading at anode was 0.8 ± 0.08 mg cm-2 for Co3O4, 1.0 ± 0.1 mg cm-2 for CrNd-Co3O4 or 0.58 ± 0.05 mg cm-2 for IrO2 and the Pt loading at cathode was 0.3 ± 0.03 mg cm-2. The PEMWE cell consisted of a platinum-plated titanium mesh (56% porosity, thickness 0.25 mm; Bekaert) as porous transport layer (PTL) for the anode and a carbon paper (TGP-H-060, Toray Company, Japan) as gas diffusion layer (GDL) for the cathode with the MEA sandwiched between them.

PEMWE measurement

The PEMWE tests were carried out on a four-channel electrolyzer test station (LQ-4C-100, Amoy Island Hydrogen (Xiamen) Technology). The PEMWE with an active area of 4.0 cm2 was operated at 80 °C with ultra-pure water (18.2 MΩ cm) supplied exclusively to the anode at a flow rate of 100 mL·min-1. Ion exchange resins were added to the anode water loop to remove foreign ions. After the startup, the PEMWE was operating at 4 A cm-2 for 12 hours to fully activate the catalyst. The performance was evaluated by galvanostatic tests under different current densities ranging from 0.025 to 4 A cm-2. At each current density, the potential was maintained for 5 min to reach a steady state and minimize the influence of non-Faradic current from charging electrochemical double layer. The steady values were used for plotting polarization curves. To address the problem of cation-leaching induced PEM poisoning, the in-situ acid circulation strategy (lasting for 20 minutes) was employed after 12 hours activation at 4 A cm-2. The same performance measurements were conducted 12 hours after the removal of acid (Supplementary text 2). Durability measurements of the PEMWE with CrNd-Co3O4 anode with acid circulation were conducted at 2 A cm-2 and 4 A cm-2 for 1000 hours.

Electrochemical impedance spectroscopy (EIS) measurements

The EIS measurements were performed on a Solartron EnergyLab impedance analyzer (full scale: 20 A) through galvanostatic impedance at different current densities ranging from 0.025 A cm-2 to 4 A cm-2. Under each current density, EIS test was conducted with an alternating current (AC) amplitude of 10% of the operating current density and with the frequency ranging from 100 kHz to 0.1 Hz. The high frequency resistance (HFR) was obtained from the intercept of the Nyquist plot at high frequency region with the real axis. An equivalent circuit model was used to deconvolute the EIS spectra and analyze the contribution of each process.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, the effects of doping CoOx with transition metals and lanthanide series metal for improving the stability and activity towards OER for PEM water electrolysis technologies are investigated. It is found that Cr and Nd are the best dopants, as supported by DFT calculations. In-situ EXAFS measurements suggest that the coordination of Co is preserved in the CrNd-CoOx electrocatalyst, and that as potential is increased there is a positive shift of absorption edge indicating an increase of Co oxidation state. preservation of thecentrosymmetric Co coordination and excellent structural stability of CrNd-CoOx. The results also point out shortened Co-O bonds and enhanced covalency which could increase the OER activity. This material was tested in a full PEMWE device, showing a stable operation for 1000 h at more 2 A cm-2. This is achieved thanks to a in-situ nitric acid circulation of nitric acid at cathode for 20 minutes after operation at 4 A cm-2 for 12 h after the start-up of the PEMWE, which removes the metal cations coming from catalyst layer leaching and that poison the device performance.\nThe manuscript shows a careful and systematic study supported by different characterization techniques and calculations, showing finally an excellent performance. I could recommend the publication of this work in Nature Communications after addressing the following concerns:\n1) A platinum titatinum-coated mesh is used as support for the working electrode in some experiments. Could not the Pt affect the OER activity measurements for the OER? Some reference measurements only for the Pt-Ti mesh should be presented to demonstrate that the activity from the support does not affect the measurements.\n2) Why the experimental setup for the screening of the transition metal and the lanthanide series metal is so different? (polished glassy carbon support vs. Pt-Ti mesh).\n3) Just comparing Tafel slope values and saying that the one with lower value has better kinetics is not a rigorous kinetic analysis. The change of Tafel slope value should be explained in terms of a change of reaction mechanisms, or possible experimental factors influencing the Tafel slope (formation of bubbles, mass transport, resistance) should be indicated.", "reviewer_2": "This study addresses the high OER activity and durability of Cr- and Nd-doped Co-based oxide catalysts under PEMWE conditions. Specifically, the authors attempted to demonstrate the notable durability of the CrNd-CoOx catalyst and the advantage of acid circulation strategy to overcome the membrane poisoning issue. Despite these efforts, we are concerned that several inconsistencies and a lack of logical coherence prevent us from recommending this study for publication. Our specific comments are outlined below.\n1. One of the most serious logical issues in this paper is the lack of clear structural identification of the synthesized CoOx. The only related data provided are the XRD patterns shown in Supplementary Fig. 9, making it highly unclear what specific crystal structure the material possesses. Consequently, the authors vaguely refer to it as “cobalt oxide (CoOx)” throughout the manuscript without specifying the exact chemical formula. From the XRD pattern in Supplementary Fig. 9, CrNd-CoOx is very likely to be spinel Co3O4 with significantly low crystallinity (i.e. a mixture of a spinel Co3O4 phase and amorphous phases). We strongly believe that providing a precise determination of the catalyst’s crystal structure is the first and most essential step.\n2. Another critical issue is that there are few precise experimental data regarding the crystal structure, and yet the supercell used for the DFT calculations assumes a perfect spinel structure for the CrNd-CoOx sample that is very poorly crystalline. Moreover, the authors assume that the doped Cr occupies the octahedral sites, while Nd substitutes into the tetrahedral sites. However, from a crystal chemistry standpoint, it is considerably unlikely for Nd, one of the lanthanide-series cations having a relatively large ionic radius, to occupy the tetrahedral sites.\n3. In addition to the ambiguity in the crystal structure, the claim that Nd doping enhances Co 3d–O 2p hybridization, derived solely from DFT calculations based on unverified assumptions about the occupation sites of Cr and Nd, lacks logical validity. Without presenting the actual atomic positions of these dopants, it is unlikely that readers will accept such a speculative claim.\n4. One of the puzzling aspects of this study is the choice of dopant elements. The Pourbaix diagrams for Cr and Nd clarify the concern behind this comment. Under OER conditions, Cr tends to easily dissolve as ions across the entire pH range (see the Pourbaix diagram). Nd is also highly corrosive and thus dissolves as ions over a wide pH range (approximately pH 1–6). Therefore, even if DFT calculations suggest that these elements could enhance lattice oxygen stability and suppress metal dissolution in Co3O4, their rapid dissolution and inherent electrochemical instability indicate that their actual contribution would be minimal, contrary to the authors’ claims.\n5. As recently reported (ref. 14 in the manuscript, J. Am. Chem. Soc. 2025, 147, 3517–3528), the S-number of Co3O4 is very low (on the order of 10^3). Therefore, it is essential to evaluate whether the observed improvement in durability truly originates from doping. Assessing catalyst durability solely based on MEA performance, without an objective and quantitative durability comparison as commonly conducted in many recent studies, is considerably inappropriate. A quantitative comparison with reference catalysts, as demonstrated in Science 380, 609–616 (2023), is strongly recommended.\n6. The HNO₃ circulation strategy may be effective for short-term recovery; however, it is likely to cause corrosion, durability degradation, and safety issues in industrial PEMWE systems, making its practical feasibility quite limited.", "reviewer_3": "General Perspective\nThe authors present an innovative approach to enhancing electrolyzer performance in a PEM configuration. While the concept of “in-situ acid recirculation” is interesting, the work is far from an industrial application, as suggested in the introduction. The authors should adopt a more conservative tone when emphasizing the novelty of their approach. This is due to two main concerns: (1) the proposed system replaces the classic IrOx/PEM/Pt/C configuration with a new alternative based on cobalt, chromium, and neodymium—two of which are critical raw materials; and (2) the use of nitric acid is questionable from an industrial perspective. Based on these points, I recommend that the authors temper the way they highlight the innovation and potential impact of their results.\nSpecific Comments and Questions\nThe authors report high current densities, which are indeed desirable at an industrial level. However, the corresponding voltages appear quite high for a PEM electrolyzer. Is the energy penalty justified for the current gains observed?\nWhy was HClO4 used in the three-electrode configuration? I strongly suggest including a comparison with H2SO4. Previous studies have shown that HClO4 solvation can enhance activity and stability for IrOx, but this is not representative of industrial operating conditions. It would be valuable to benchmark against standard electrolytes used in PEM environments.\nIn Figure S2, please consolidate the screening data into a single figure to facilitate comparison. In Figure S3, ensure that all axes and data are clearly labeled—relying solely on the caption is not sufficient.\nThe structural characterization section was particularly strong. I commend the authors for the quality of this analysis.\nCould the authors provide more details about the in-situ measurements? This would help readers better understand the experimental setup and data interpretation.\nElectrochemical Analysis\nHow was the Tafel plot generated? If it was obtained from LSV data, please note that LSV is scan rate–dependent (even at 1 mV/s). I recommend instead collecting Tafel plots from chronoamperometry (CA) or chronopotentiometry (CP) data to ensure accuracy.\nIn Figure 3C, please specify whether the voltage is reported vs. RHE or in a full electrolyzer configuration. Also, consider adding the degradation rate to this figure for clarity.\nWhy was a FUMA membrane used instead of the more common Nafion? Did the authors test an activated Nafion membrane under the same conditions? Typically, Nafion is pretreated with hydrogen peroxide and sulfuric acid at 80 °C to remove impurities and exchange counterions for protons.\nIn Figure S20, iR compensation should not be applied in an electrolyzer configuration, as the membrane inherently minimizes resistance in zero-gap setups. iR correction is more relevant in three-electrode or H-cell systems. Moreover, industrial-scale electrolyzers do not allow for such compensation.\nRegarding the acidification of the cathodic chamber: how was this managed over the 1000 h stability test? Was it a one-time pretreatment, or was it repeated periodically? Please clarify the procedure.\nFinal Remarks\nIn my view, the main novelty of this report lies in the recycling of metal leaching through acidic treatment. The use of neodymium may be an interesting idea, but it is not realistic for industrial implementation. Additionally, there are several existing reports on cobalt-based PEM systems (e.g., Ram R. et al., Science, 384, 1373–1380 (2024)) that the authors should consider discussing for better contextualization."}, "author_response": "We sincerely appreciate the valuable comments and suggestions from the reviewers that shall help us to significantly improve the quality of our manuscript. Below we provide a point-by-point response letter. The original comments are marked in Italics, the responses are marked with BLUE color, and the revisions are marked with RED color.\nReviewer #1:\nIn this work, the effects of doping CoOx with transition metals and lanthanide series metal for improving the stability and activity towards OER for PEM water electrolysis technologies are investigated. It is found that Cr and Nd are the best dopants, as supported by DFT calculations. In-situ EXAFS measurements suggest that the coordination of Co is preserved in the CrNd-CoO electrocatalyst, and that as potential x is increased there is a positive shift of absorption edge indicating an increase of Co oxidation state. preservation of the centrosymmetric Co coordination and excellent structural stability of CrNd-CoO . The results also point out shortened Co-O bonds and enhanced covalency which could increase the OER activity. This material was tested in a full PEMWE device, showing a stable operation for 1000 h at more 2 A cm-2. This is achieved thanks to a in-situ nitric acid circulation of nitric acid at cathode for 20 minutes after operation at 4 A cm-2 for 12 h after the start-up of the PEMWE, which removes the metal cations coming from catalyst layer leaching and that poison the device performance.\nThe manuscript shows a careful and systematic study supported by different characterization techniques and calculations, showing finally an excellent performance. I could recommend the publication of this work in Nature Communications after addressing the following concerns:\nResponse: We thank the reviewer for the positive comments on our work.\n1. A platinum titanium-coated mesh is used as support for the working electrode in some experiments. Could not the Pt affect the OER activity measurements for the OER? Some reference measurements only for the Pt-Ti mesh should be presented to demonstrate that the activity from the support does not affect the measurements.\nResponse: We thank the reviewer for raising the valuable question. The Pt-Ti mesh was used here due to its excellent conductivity and anti-corrosion property under high potential and strong acidic conditions. The OER activity of a Pt-Ti mesh in 0.1 M HClO was measured as shown in Figure R1, and the activity of our reported CrNd- 4 CoO catalyst was included for comparison. The Pt-Ti mesh reached an overpotential x of 570 mV at 10 mA cm-2, which is 175 mV higher than that of CrNd-CoO (395 mV), x indicating that Pt-Ti did not significantly affect the measurements.\n[... Figure R1 omitted ...]\n2. Why the experimental setup for the screening of the transition metal and the lanthanide series metal is so different? (polished glassy carbon support vs. Pt-Ti mesh).\nResponse: We thank the reviewer for raising the helpful question. We originally intended to use the Pt-Ti mesh as the support for all the experiments to simplify the electrochemical setup, but we noticed that the Pt-Ti electrode might experience mass transport issues at high current densities.\nFor screening transition metals, we focused more on the intrinsic stabilities of different candidates at 10 mA/cm2. On one hand, at such a current density, electrochemical activity was mainly controlled by kinetics and less susceptible to mass transport. On the other hand, the mass transport (e.g., bubble diffusion) reached a steady state during stability test, which had negligible effect on the decay rate. Hence, using Pt-Ti as support would neither influence our experimental results nor compromise our conclusions.\nHowever, the screening of lanthanide series elements was based on the adsorption energy profiles of oxygenated species via thermodynamic-kinetic (TK) modelling, which required high-quality electrochemical data under a wide potential range (c.a. 1.4- 1.8 V vs. RHE), where activity could likely involve mass transport issues. Consequently, rotating disk electrode (RDE) with polished glassy carbon was adopted as the working electrode because of the stable and consistent mass transport-controlled current, facilitating the kinetic-controlled current comparison among lanthanide series elements-doped candidates.\nIn conclusion, for the sake of extracting high-quality kinetic fingerprints from electrochemical data for TK modeling, we used RDE for screening lanthanide series elements.\n3. Just comparing Tafel slope values and saying that the one with lower value has better kinetics is not a rigorous kinetic analysis. The change of Tafel slope value should be explained in terms of a change of reaction mechanisms, or possible experimental factors influencing the Tafel slope (formation of bubbles, mass transport, resistance) should be indicated.\nResponse: We sincerely thank the reviewer for the insightful comment. We agree with the reviewer that simply comparing Tafel slopes and concluding “a lower Tafel slope implies better kinetics” is not sufficiently rigorous. Hence, we clarified the limitations of Tafel analysis and strengthened the experimental basis to ensure that the extracted slopes could reflect intrinsic kinetics rather than non-kinetic artefacts.\nFirst, the Tafel slopes were extracted in a low-current regime (< 10 mA cm-2), where bubble accumulation on the electrode surface is minimal and does not measurably affect the polarization behavior. In addition, the fitting was restricted to the kinetically controlled region, avoiding the high-current range where mass-transport limitation and bubble coverage may become significant. Moreover, all polarization data used for Tafel analysis were iR-corrected to minimize the influence of uncompensated resistance.\nSecond, to further reduce potential distortion from capacitive currents and improve the reliability of the kinetic parameters, we collected additional steady-state polarization data using chronoamperometry (CA) and updated Figure 3B accordingly (also shown in Figure R2). The updated Tafel slopes preserve the same qualitative trend: CrNd-CoO exhibits a smaller Tafel slope (83.4 mV/dec) than CoO (99.2 mV/dec) and x x Cr-CoO (95.8 mV/dec). Notably, all the slopes fall within the commonly observed 60 x to 120 mV/dec window, suggesting that the second proton-coupled electron transfer (PCET) step is the rate-determining step. Hence, the smaller Tafel slope of CrNd-CoO x indicates improved charge transfer and faster kinetics of the second elementary step, agreeing well with the DFT calculation results.\n[... Figure R2 omitted ...]\nReviewer #2:\nThis study addresses the high OER activity and durability of Cr- and Nd-doped Co- based oxide catalysts under PEMWE conditions. Specifically, the authors attempted to demonstrate the notable durability of the CrNd-CoO catalyst and the advantage of x acid circulation strategy to overcome the membrane poisoning issue. Despite these efforts, we are concerned that several inconsistencies and a lack of logical coherence prevent us from recommending this study for publication. Our specific comments are outlined below.\nResponse: We thank the reviewer for the valuable comments on our work. Accordingly, we have conducted additional experiments and provided a detailed point-by-point response as shown below.\n1. One of the most serious logical issues in this paper is the lack of clear structural identification of the synthesized CoO . The only related data provided are the XRD x patterns shown in Supplementary Fig. 9, making it highly unclear what specific crystal structure the material possesses. Consequently, the authors vaguely refer to it as “cobalt oxide (CoO )” throughout the manuscript without specifying the exact x chemical formula. From the XRD pattern in Supplementary Fig. 9, CrNd-CoO is very x likely to be spinel Co O with significantly low crystallinity (i.e. a mixture of a spinel 3 4 Co O phase and amorphous phases). We strongly believe that providing a precise 3 4 determination of the catalyst’s crystal structure is the first and most essential step.\nResponse: We sincerely thank the reviewer for highlighting this important point. We fully agree with the reviewer that a clear and accurate structural identification of the synthesized CoO is essential for interpreting catalytic behavior. In the revised x manuscript, we therefore strengthened the structural characterization and clarified the phase assignment and nomenclature.\n(1) Additional structural identification of CoO and Cr-CoO x x\nThe reviewer pointed out that “The only related data provided are the XRD patterns shown in Supplementary Fig. 9, making it highly unclear what specific crystal structure the material possesses.” To provide a more definitive structural identification of CoO x and Cr-CoO , we conducted additional characterization (shown in Figure S12 and x Figure R3), including:\n(a) TEM analysis of CoO and Cr-CoO (Figure R3A and D), revealing that both x x CoO and Cr-CoO consist of clusters of interconnected nanoparticles. x x\n(b) HRTEM lattice fringe analysis (Figure R3B and E), confirming the presence of lattice spacings consistent with spinel Co O (e.g., 0.242 nm for the (311) plane) 3 4 in both CoO and Cr-CoO . x x\n(c) Selected area electron diffraction (SAED) analysis (Figure R3C and F), exhibiting multiple well-defined concentric diffraction rings in agreement with spinel Co O , 3 4 indicating the presence of polycrystalline Co O domains. 3 4\nThese results support that both CoO and Cr-CoO are predominantly composed of x x spinel Co O polycrystalline nanoparticles. Besides, the introduction of Cr did not 3 4 alter the spinel structure, consistent with the XRD patterns as shown in Figure S9. In the revised manuscript, we have added: “In addition, HRTEM and SAED analysis (Fig. S12) demonstrated that both of CoO and Cr-CoO are predominantly composed of x x spinel Co O polycrystalline nanoparticles. These structural analyses collectively 3 4 suggested that introduction of Cr did not alter the spinel structure.”\n[... Figure R3 omitted ...]\n(2) Clarification of the “CoO ” terminology and chemical formula x\nThe reviewer pointed out that “Consequently, the authors vaguely refer to it as “cobalt oxide (CoO )” throughout the manuscript without specifying the exact chemical x formula.” We agree with the reviewer that repeatedly using “CoO ” without explanation x may cause ambiguity. Our initial use of the general term “CoO ” was intended to reflect x this non-stoichiometry and low crystallinity of the doped sample, rather than implying an unknown phase. We have provided Co average oxidation-state information from both XPS and XANES (surface- vs. bulk-sensitive), showing values of ~2.5–2.6 for CrNd–CoO . These data indicated a cobalt-oxide composition close to Co O , but x 3 4 we avoid assigning an exact integer formula solely from average valence because (i) the material is heterogeneous and partially amorphous, and (ii) dopants and oxygen vacancies can lead to non-stoichiometry. We therefore use the more accurate description in the revised manuscript: “Notably, considering the nature of non-stoichiometry and low crystallinity of doped samples, ‘CoO ’ rather than ‘Co O ’ was used to avoid x 3 4 misinterpretation.”\n(3) Phase nature of CrNd–CoO x\nThe reviewer pointed out that “From the XRD pattern in Supplementary Fig. 9, CrNd- CoO is very likely to be spinel Co O with significantly low crystallinity (i.e. a mixture x 3 4 of a spinel Co O phase and amorphous phases).” We agree with the reviewer that the 3 4 broad and weak diffraction features in Fig. S9 suggest that the CrNd-CoO sample x likely contains low-crystallinity spinel Co O domains coexisting with 3 4 disordered/amorphous components. We would like to clarify that lattice fringes in HRTEM image (Fig. S10) and concentric diffraction rings with weak diffuse halos in SAED pattern (Fig. 2B) confirmed the mixture of nanocrystalline spinel Co O and 3 4 amorphous phase in CrNd-CoO . We have revised the discussion to reflect this mixed x structural nature explicitly: “From the discussion above, it could be concluded that CrNd-CoO consisted of nanocrystalline spinel Co O and amorphous phase.” x 3 4\n2. Another critical issue is that there are few precise experimental data regarding the crystal structure, and yet the supercell used for the DFT calculations assumes a perfect spinel structure for the CrNd-CoO sample that is very poorly crystalline. Moreover, x the authors assume that the doped Cr occupies the octahedral sites, while Nd substitutes into the tetrahedral sites. However, from a crystal chemistry standpoint, it is considerably unlikely for Nd, one of the lanthanide-series cations having a relatively large ionic radius, to occupy the tetrahedral sites.\nResponse: We sincerely thank the reviewer for raising this important and insightful comment. We agree with the reviewer that the structural complexity of CrNd-CoO x must be carefully reconciled with the structural assumptions used in our DFT analysis. In the revised manuscript, we clarified the scope and limitations of the DFT model and strengthened the justification of dopant site preference.\n(1) DFT supercell selection\nThe reviewer pointed out that “Another critical issue is that there are few precise experimental data regarding the crystal structure, and yet the supercell used for the DFT calculations assumes a perfect spinel structure for the CrNd-CoO sample that is x very poorly crystalline.” We agree with the reviewer that the CrNd-CoO sample x consists of polycrystalline spinel Co O and amorphous phase, as revealed by 3 4 experimental characterizations (e.g., XRD, SAED, HRTEM). Therefore, a “perfect” spinel supercell cannot represent the full microstructural heterogeneity of the real catalyst.\nOur intention in using a spinel Co O supercell was not to model the long-range 3 4 crystallinity or amorphous fraction, but rather to capture the surface local coordination environment and elementary-step energetics on the spinel-like domains that were experimentally confirmed. In other words, the DFT model provides a local motif-based description of how Cr/Nd substitution perturbs the active-site electronic structure and thermodynamic energetics on spinel-like Co O building blocks, while the 3 4 experimentally observed amorphous component is expected to introduce additional heterogeneity that is beyond the current DFT scope.\n(2) Dopant site preference of Cr and Nd\nThe reviewer pointed out that “Moreover, the authors assume that the doped Cr occupies the octahedral sites, while Nd substitutes into the tetrahedral sites. However, from a crystal chemistry standpoint, it is considerably unlikely for Nd, one of the lanthanide-series cations having a relatively large ionic radius, to occupy the tetrahedral sites.” We appreciate the reviewer’s point regarding ionic size and site preference: lanthanide cations such as Nd3+ typically favor higher coordination environments, making tetrahedral occupancy in an ideal spinel lattice less likely from a crystal-chemistry perspective.\nIn our original manuscript, the proposed Nd site preference was inferred from the FT-EXAFS trend (Fig. 2f), where the Co-Co -related contribution was markedly Tet perturbed in CrNd-CoO relative to the undoped sample. In defect-rich spinel, such x perturbation can arise from (i) partial occupation of tetrahedral-related environments under certain conditions, or (ii) Nd-induced cation redistribution that reduces the order of Co at tetrahedral sites. Notably, there are reports discussing that lanthanides can occupy tetrahedral sites in disordered or low-crystallinity spinels. For example, in Nd- doped ferrites, Nd ion has been shown to preferentially occupy tetrahedral sites in Nd Co Fe O under certain synthesis conditions.1 Such phenomenon was also x 1-x 2 4 observed in Co Zn La Fe O system, where La3+ entered the spinel lattice and 0.5 0.5 x 2-x 4 partially occupied tetrahedral sites, with associated expansion of the tetrahedral sublattice.2 Therefore, the possibility that tetrahedral-site involvement of Nd3+ cannot be excluded in nanocrystalline/disordered spinels. The two references have been included in the revised manuscript to support our claim.\nWe therefore conclude that Nd incorporation in/near tetrahedral environments are plausible in our defect-rich, low-crystallinity spinel-like domains. We hope these responses address the reviewer’s concerns and clarify the structural features of Nd in our material.\n3. In addition to the ambiguity in the crystal structure, the claim that Nd doping enhances Co 3d–O 2p hybridization, derived solely from DFT calculations based on unverified assumptions about the occupation sites of Cr and Nd, lacks logical validity. Without presenting the actual atomic positions of these dopants, it is unlikely that readers will accept such a speculative claim.\nResponse: We sincerely thank the reviewer for raising the insightful comment. We understand the concern raised by the reviewer regarding the speculative nature of our claim without fully verified atomic positions for the dopants. To address this, we would like to clarify that Nd incorporation in/near tetrahedral environments are plausible in our defect-rich, low-crystallinity spinel-like domains. In addition, we agree that Nd L - 3 edge white-line changes primarily reflect variations in the unoccupied Nd 5d states and local coordination/covalency, and therefore should not be used as stand-alone proof of Co 3d–O 2p hybridization. In the revision, we temper the wording and present the in- situ Nd L -edge XANES results as supporting evidence of dopant electronic 3 participation under anodic potentials, which is consistent with modified metal-oxygen covalency predicted by DFT. We have also revised the sentence in the revised manuscript to avoid attributing the white-line increase uniquely to Co 3d–O 2p hybridization: “The potential-dependent evolution of the Nd L -edge white-line (Fig. 3 S18) indicated changes in the unoccupied Nd 5d states and dopant–oxygen covalency under OER-relevant potentials, suggesting that Nd actively participated in the electronic structure of the oxide lattice. This observation is consistent with our DFT results showing enhanced metal–oxygen hybridization in Nd/Cr-incorporated spinel- like motifs.”\n4. One of the puzzling aspects of this study is the choice of dopant elements. The Pourbaix diagrams for Cr and Nd clarify the concern behind this comment. Under OER conditions, Cr tends to easily dissolve as ions across the entire pH range (see the Pourbaix diagram). Nd is also highly corrosive and thus dissolves as ions over a wide pH range (approximately pH 1–6). Therefore, even if DFT calculations suggest that these elements could enhance lattice oxygen stability and suppress metal dissolution in Co O , their rapid dissolution and inherent electrochemical instability indicate that 3 4 their actual contribution would be minimal, contrary to the authors’ claims.\nResponse: We sincerely thank the reviewer for the valuable comments. We fully appreciate the reviewer’s point about the potential dissolution of Cr and Nd under oxygen evolution reaction (OER) conditions and agree that their electrochemical stability is critical to evaluating their role in enhancing catalyst performance.\n(1) Limitations of Pourbaix diagram for doped surfaces\nWe would like to clarify that classical Pourbaix diagrams are constructed for bulk elemental or simple oxides in equilibrium with aqueous solution, and they are most valid for pure surfaces, not for complex doped lattice systems. The thermodynamic maps could not account for local electronic environment and lattice properties modified by dopant incorporation.\nWhen Cr and Nd are substituted into a Co O lattice, the strong covalency and metal– 3 4 oxygen interactions in the spinel structure can shift their effective potentials required for dissolution relative to the isolated ion behavior predicted in a Pourbaix plot. Therefore, although Pourbaix diagrams are useful for initial insight into dissolution tendencies, they cannot directly predict dopant stability in a doped oxide lattice without considering lattice binding, covalency, and kinetic constraints on dissolution.\n(2) Evidence that dopants can improve stability of Co O 3 4\nMultiple recent studies in the literature demonstrate that doping with transition metals or lanthanide elements can enhance the performance and durability of Co O ‑based 3 4 OER catalysts, supporting the rationale behind our dopant selection and showing that doping effects can outweigh simple ionic dissolution tendencies:\n(a) Transition metal doping (taking Cr as example): A recent computational– experimental study screened fourth-row transition metal-doped Co O and 3 4 demonstrated that moderate Cr incorporation could enhance both catalytic activity and durability for acidic OER, improving stability by nearly 2.7 folds with only a small overpotential increase after long‑term testing.3\n(b) Lanthanide metal doping (taking La as example): Chong et al. investigated La and Mn-doped Co O for OER in acidic conditions and highlighted that the long-term 3 4 stability in PEMWE was benefited from a stable La3+-rich surface, i.e., LaO3O_CoTO*.4 This finding suggested that lanthanide doping could modify the surface property and thus improve the stability of Co O . 3 4\n5. As recently reported (ref. 14 in the manuscript, J. Am. Chem. Soc. 2025, 147, 3517– 3528), the S-number of Co3O4 is very low (on the order of 10^3). Therefore, it is essential to evaluate whether the observed improvement in durability truly originates from doping. Assessing catalyst durability solely based on MEA performance, without an objective and quantitative durability comparison as commonly conducted in many recent studies, is considerably inappropriate. A quantitative comparison with reference catalysts, as demonstrated in Science 380, 609–616 (2023), is strongly recommended.\nResponse: We thank the reviewer for the constructive suggestions. We agree with the reviewer that MEA stability alone does not provide an objective, catalyst-intrinsic durability metric. We have therefore revised the manuscript to include a quantitative catalyst durability benchmark, following the stability number (S-number) approach widely used for OER stability comparisons.5\nIn the revision, we have added: “The catalyst stability is evaluated using the S- number (S), defined as the ratio of the produced oxygen (calculated from total charge) to the dissolved Co (calculated from ICP data) under specific OER operating conditions: 𝑆 = 𝑛𝑂2 (𝑂𝐸𝑅) / 𝑛𝐶𝑜(𝑑𝑖𝑠𝑠𝑜𝑙𝑣𝑒𝑑)”\nTo directly address whether durability improvement originates from doping, we included the stability benchmarking for our CoO , CrNd-CoO , and reference catalysts x x (Co O and IrO )6,7, as shown in Figure R4 and Fig. S21. The improved S-number of 3 4 2 CrNd-CoO compared with our CoO and Co O in references suggested the effect of x x 3 4 doping on durability enhancement. We have added: “In addition, the improved S- number of CrNd-CoO compared to that of CoO and Co O indicated the effect of x x 3 4 doping on durability enhancement (Fig. S21).”\n[... Figure R4 omitted ...]\n6. The HNO₃ circulation strategy may be effective for short-term recovery; however, it is likely to cause corrosion, durability degradation, and safety issues in industrial PEMWE systems, making its practical feasibility quite limited.\nResponse: We sincerely thank the reviewer for the constructive comment. We understand the concerns raised by the reviewer. However, in our work, the HNO 3 circulation is not intended as a continuous process, but rather as a short, one-time maintenance procedure to remove leached cations that temporarily poison the membrane during the PEMWE start-up/activation stage.\n(1) The purpose and operating duration of HNO circulation 3\nThe acid circulation strategy is designed to address contamination issues that may arise from catalyst degradation during operation. In our study, CrNd-CoO exhibited x significant cations leaching at the PEMWE startup and activation period (Fig. S25). After the activation, HNO was circulated (one-time, 20 mins) at cathode to remove 3 metal cations and recover its performance. We have clarified this procedure and its rationale in the original manuscript to avoid the impression of long-term acid circulation.\n(2) Corrosion and durability considerations\nWe agree that acids can accelerate corrosion of stack components if used improperly. Accordingly, we (i) limited acid exposure to a short duration (20 mins), (ii) applied it only on the cathode side under controlled conditions. These practical controls are standard engineering approaches for minimizing corrosion risk. Besides, the strong oxidizing nature and low coordination ability of HNO could lead to the formation of 3 dense TiO layer and thus temper the corrosion of Ti PTL and bipolar plates. 2\n(3) Practical feasibility: intermittent maintenance, not continuous operation\nWe agree that it is not suitable for continuous and long-term acid circulation for industrial PEMWE. However, our results demonstrated that only 20-min acid circulation could significantly address contamination problems, making it potentially relevant as an intermittent maintenance action (e.g., after start-up or an abnormal event). For large stacks, if significant cation poisoning occurs, a periodic, short- duration acid circulation strategy can be used to address this issue.\n(4) HNO circulation does not guarantee long-term operation 3\nThe PEMWE with CrNd-CoO anode treated with acid circulation was shown to x operate stably at 2 A/cm2 and 4 A/cm2 (Fig. 3G and H), and only trace dissolved metal cations were detected in the anode effluent during the long-term stability test (Fig. 3F). Such long-term stability benefited from the intrinsic stability of CrNd-CoO . x\nImportantly, Fig. S22 further showed that the acid-circulation strategy could not compensate for intrinsically unstable anode materials: for CoO -based MEA with and x without acid circulation, PEM poisoning could still be observed due to the low intrinsic stability of CoO , causing continuous cations leaching. This comparison clarifies that x the long-term stability observed in our system originates primarily from the intrinsic stability of CrNd-CoO , while the acid circulation serves only as an auxiliary x “decontamination” step during the early-stage leaching window.\nReviewer #3:\nGeneral Perspective\nThe authors present an innovative approach to enhancing electrolyzer performance in a PEM configuration. While the concept of “in-situ acid recirculation” is interesting, the work is far from an industrial application, as suggested in the introduction. The authors should adopt a more conservative tone when emphasizing the novelty of their approach. This is due to two main concerns: (1) the proposed system replaces the classic IrOx/PEM/Pt/C configuration with a new alternative based on cobalt, chromium, and neodymium—two of which are critical raw materials; and (2) the use of nitric acid is questionable from an industrial perspective. Based on these points, I recommend that the authors temper the way they highlight the innovation and potential impact of their results.\nResponse: We thank the reviewer for the valuable comments and suggestions.\nFollowing which, we have replaced strong claim of “industrial applicability” with a more tempered description emphasizing:\n(i) the mechanistic insight into cations-induced contamination in PEMWE;\n(ii) the proof-of-concept demonstration of reversible performance recovery via short- duration decontamination; and\n(iii) the boundaries of applicability and remaining challenges toward scale-up (materials criticality, acid handling, long-term component compatibility).\nCorresponding revisions in “Introduction” part have been made:\n(i) “which was universally observed in many noble-metal-free catalysts-based PEMWEs17,18.”\n(ii) “complete elimination of cation-driven contamination in PEM operation remains challenging.”\n(iii) “To address this universal challenge, we proposed a short-duration (20 minutes), one-time acid circulation strategy applied at the cathode after startup of the device.”\n(iv) “highlighting a pathway for device-level management of leaching-induced degradation in noble-metal-free anode.”\nCorresponding revisions in “Summary” part have been made:\n“towards the practical implementation” was changed to “in understanding durability challenges”\nSpecific Comments and Questions\n1. The authors report high current densities, which are indeed desirable at an industrial level. However, the corresponding voltages appear quite high for a PEM electrolyzer. Is the energy penalty justified for the current gains observed?\nResponse: We sincerely thank the reviewer for raising this important question. We agree with the reviewer that the cell voltages at very high current densities are higher than those of the state-of-the-art IrO -based PEMWEs, and that the key issue is whether 2 the system-level benefit of higher current density compensates for the energy penalty associated with the higher voltage.\nTo quantify energy consumption, specific energy consumption (SEC) is introduced to provide a more comprehensive metric that reflects energy use at different current densities (assuming 100% Faradaic efficiency).\n[... SEC formula omitted ...]\nLet’s assume that our CrNd-CoO -based PEMWE operates at a low current density x (0.5 A/cm2) or a high current density (4 A/cm2):\nUnder 0.5 A/cm2: I = 2 A, V = 2.10 V, t = 1 h, thus SEC = 56.3 kWh/kg H . 2\nUnder 4 A/cm2: I = 16 A, V = 2.54 V, t = 1 h, thus SEC = 68.1 kWh/kg H . 2\nThus, increasing from 0.5 to 4 A cm-2 raises SEC by ~21% (68.1/56.3 = 1.21), which is the energy penalty associated with the higher operating voltage. However, the hydrogen production rate at 4 A cm-2 is 8 (4/0.5 = 8) times higher than that at 0.5 A cm-2. Hence, operating at a higher current density results in significantly higher H production with 2 only a slight increase in energy consumption.\nAdditionally, we would like to highlight that operating at higher current densities could also help reduce the size of the electrolyzer stacks for a fixed H 2 production target, which in turn would lower the balance of plant (BOP) costs (including pumps, tanks, and power electronics), making the system more cost-effective.\n2. Why was HClO used in the three-electrode configuration? I strongly suggest 4 including a comparison with H SO . Previous studies have shown that HClO solvation 2 4 4 can enhance activity and stability for IrO , but this is not representative of industrial x operating conditions. It would be valuable to benchmark against standard electrolytes used in PEM environments.\nResponse: We thank the reviewer for the insightful comment. We initially used HClO 4 in the three-electrode configuration measurement to minimize specific anion adsorption and reduce the electrolyte-induced factors during compositional screening. Despite this, we fully agree with the reviewer that benchmarking in H SO is important for relevance 2 4 to PEM environments, where sulfonic acidity and anion effects are non-negligible.\nAs suggested by the reviewer, we have repeated the key electrochemical measurements of CoO , Cr-CoO and CrNd-CoO in 0.05 M H SO under identical x x x 2 4 protocols and compared their activities in 0.05 M H SO and 0.1 M HClO (with 100% 2 4 4 iR correction), as shown in Figure R5. The qualitative trends across compositions are preserved, while absolute activities are lower in H SO . This is due to the specific 2 4 adsorption of SO 2- on Co sites, partially blocking active sites and suppressing intrinsic 4 OER kinetics.10,11\n[... Figure R5 omitted ...]\n3. In Figure S2, please consolidate the screening data into a single figure to facilitate comparison. In Figure S3, ensure that all axes and data are clearly labeled—relying solely on the caption is not sufficient.\nResponse: We thank the reviewer for the nice suggestions. Accordingly, we have combined the original screening data into a single, unified figure to enhance clarity and facilitate direct comparison. The updated figure (Figure S3) has been added into the revised Supporting Information.\nWe thank the reviewer for pointing out that the axes and data in original Figure S3 are not sufficiently clear. In response, we have updated the figure to ensure that all axes are properly labeled.\n4. The structural characterization section was particularly strong. I commend the authors for the quality of this analysis.\nResponse: We sincerely thank the reviewer for the positive evaluation of our work.\n5. Could the authors provide more details about the in-situ measurements? This would help readers better understand the experimental setup and data interpretation.\nResponse: We thank the reviewer for the nice suggestion. Accordingly, we have added more details about the in-situ XAS measurements to clarify the experimental setup and data analysis. The revisions are shown below:\n“In-situ Co K-edge, Cr K-edge, and Nd L -edge X-ray absorption spectroscopy (XAS) 3 measurements were performed at the BL32A beamline of the Taiwan Photon Source (TPS), National Synchrotron Radiation Research Center (NSRRC). The experiments were conducted under open-circuit voltage (OCV) and applied potentials of 1.3 and 1.8 V vs. RHE. A custom-designed Teflon cell equipped with a Kapton window was employed to enable X-ray transmission. Spectra were collected from the back side of the electrode to prevent interference from oxygen bubble formation during operation. XAS signals were acquired in partial fluorescence yield mode using a silicon drift detector. Prior to data acquisition, energy calibration was carried out using Co foil for the Co K-edge and Cr foil for both the Cr K-edge and Nd L -edge. Data processing 3 involved subtraction of the pre-edge baseline followed by normalization of the post- edge region using the Athena software package. Oxidation states were determined by comparing the second derivatives of absorption edge features with those of reference compounds. Extended X-ray absorption fine structure (EXAFS) analysis was performed by applying Fourier transformation to the k3-weighted EXAFS oscillations to resolve contributions from different local bonding environments. All EXAFS spectra are presented without phase correction.”\nElectrochemical Analysis\n6. How was the Tafel plot generated? If it was obtained from LSV data, please note that LSV is scan rate–dependent (even at 1 mV/s). I recommend instead collecting Tafel plots from chronoamperometry (CA) or chronopotentiometry (CP) data to ensure accuracy.\nResponse: We sincerely thank the reviewer for the nice suggestion. The Tafel plots shown in Figure 3B were derived from LSV data in Figure 3A. We fully agree with the reviewer that LSV curve is scan rate-dependent, and the capacitance contribution can result in the overestimation of the intrinsic activities of electrocatalysts of interest.\nAs per the reviewer’s suggestion, we have collected additional Tafel data using chronoamperometry (CA) to ensure more accurate and stable measurements (as shown in Figure R2) and updated them in Figure 3B.\nIn addition, the measurement protocol has been updated in the revised supporting information: “Tafel analysis was based on the quasi-steady-state polarization curves collected using a potential-step protocol. The potential was increased from 1.4 V to 1.7 V vs. RHE in steps of 20 mV, and each step was held for 30 s. The steady-state current at each potential was obtained by averaging the current over the last 5 s of the hold once the current drift became negligible.”\n7. In Figure 3C, please specify whether the voltage is reported vs. RHE or in a full electrolyzer configuration. Also, consider adding the degradation rate to this figure for clarity.\nResponse: We sincerely thank the reviewer for the valuable suggestion. The potential here was recorded from a three-electrode configuration and reported vs. RHE. The revised figure is shown as Figure R6, which is also updated in Figure 3C. This is now explicitly stated in the title of y-axis to avoid any confusion. Besides, we have also annotated the degradation rates (mV h-1) alongside the voltage data to improve the presentation and facilitate the quantitative comparison of stability of samples.\n[... Figure R6 omitted ...]\n8. Why was a FUMA membrane used instead of the more common Nafion? Did the authors test an activated Nafion membrane under the same conditions? Typically, Nafion is pretreated with hydrogen peroxide and sulfuric acid at 80 °C to remove impurities and exchange counterions for protons.\nResponse: We thank the reviewer for raising the question. In our study, we chose to use the FUMA membrane (FS-990-PK, 90 μm) due to its advantages in terms of excellent mechanical and chemical stability for long-term operation and lower membrane resistance in comparison to Nafion (e.g., N115, 125 μm). On one hand, the FUMA membrane was reinforced by poly(ketone), which was known for their high strength- to-weight ratio, dimensional stability and resistance to chemical degradation.12,13 The inclusion of poly(ketone) enhances the structural integrity of FS-990-PK, particularly under high-stress and high-temperature conditions, ensuring long-term stability and performance. On the other hand, FS-990-PK is 35 μm thinner than N115, enabling the lower Ohmic resistance and decreased overall electrolyzer cell voltage, thus improving energy efficiency and cutting down electricity cost. Although Nafion produces thinner membranes, such as N212 (50 μm), they suffer from high risk of membrane perforation and hydrogen crossover, greatly impeding their application in long-term operating PEMWE under high-current densities.\nAs per the reviewer’s suggestion, we collected the polarization curves of PEMWE equipped with CrNd-CoO anode and N115 membrane with and without acid x circulation (Figure R7). N115 membrane was pretreated and activated under the conditions as suggested by the reviewer. While the absolute activity in the N115-based cell is inferior to the FS-990-PK-based one, qualitative trends of cationic contamination and performance recovery with acid circulation remain unchanged, demonstrating the universality of cationic poisoning and our recovery strategy.\n[... Figure R7 omitted ...]\n9. In Figure S20, iR compensation should not be applied in an electrolyzer configuration, as the membrane inherently minimizes resistance in zero-gap setups. iR correction is more relevant in three-electrode or H-cell systems. Moreover, industrial- scale electrolyzers do not allow for such compensation.\nResponse: We sincerely thank the reviewer for the valuable comments. We acknowledge that iR compensation is more relevant in three-electrode or H-cell systems, where the iR drop across the solution can have a more significant effect on the measured potential. In electrolyzer configurations, particularly with zero-gap setups, the membrane itself minimizes resistance, and thus the need for iR correction is less critical.14\nIt is worth noting that our intention to present the iR compensated performance data of CrNd-CoO is to meticulously exhibit that its intrinsic activity in PEMWE is in x accordance with its activity in three-electrode aqueous system, rather than to improperly consider the iR-free activity as the final performance of PEMWE. Such analysis method to compare iR-compensated activity in PEMWE with intrinsic activity in three-electrode system has been widely acknowledged15,16. Besides, in abstract and main text, all the activities of our PEMWE were properly reported without iR correction.\nTo improve the accuracy of such analysis and address the reviewer’s concern, we have added a clarification note in original Figure S20: “It is worth noting that the performance after iR correction is only used to analyze the intrinsic activity of CrNd- CoO in PEMWE.” x\n10. Regarding the acidification of the cathodic chamber: how was this managed over the 1000 h stability test? Was it a one-time pretreatment, or was it repeated periodically? Please clarify the procedure.\nResponse: We thank the reviewer for raising this question. The acidification of the cathodic chamber was not maintained during the 1000 h stability test. Instead, the HNO 3 circulation was implemented as a one-time, short-duration post-activation treatment during the start-up stage.\nSpecifically, after the initial activation period—during which transient cation leaching from the anode catalyst was most pronounced—we circulated HNO through 3 the cathode chamber once for 20 min to remove dissolved metal cations and mitigate membrane contamination. After this single treatment, the subsequent 1000 h operation was conducted under the standard (acid-free) water-feed conditions, without any further acid addition or periodic acidification, as described in Supplementary Text 2.\nFinal Remarks\n11. In my view, the main novelty of this report lies in the recycling of metal leaching through acidic treatment. The use of neodymium may be an interesting idea, but it is not realistic for industrial implementation. Additionally, there are several existing reports on cobalt-based PEM systems (e.g., Ram R. et al., Science, 384, 1373–1380 (2024)) that the authors should consider discussing for better contextualization.\nResponse: We thank the reviewer for the helpful comments. In the revised manuscript, we reframed the novelty and impact more conservatively, emphasizing recovering performance by removing cations via a short acidic treatment, rather than claiming near-term industrial replacement of Ir-based PEM anodes.\n(1) Positioning of Nd and industrial realism\nWe agree that Nd-containing formulations are unlikely to be the most practical industrial choice, especially given material criticality considerations. We therefore revised the manuscript to present Nd as a model dopant used to interrogate how lanthanide-induced electronic/structural perturbation could influence leaching/poisoning behavior and recovery, rather than as an “industry-ready” dopant.\nIn “Summary” part, we have added: “Although Nd is not intended as a practical industrial dopant due to material criticality considerations, it serves as a model dopant to explore how lanthanide-induced electronic/structural perturbations can affect electrochemical activity and stability.”\n(2) Contextualization with prior cobalt-based PEMWE studies\nWe appreciate the reviewer’s suggestion and expand the Introduction to better position our work relative to recent cobalt-based PEMWE anode advances. We clarify that these studies primarily target intrinsic acidic stability/activity of cobalt-based lattices, whereas our manuscript focuses on a complementary but practically important aspect: cationic contamination and its mitigation via a short acid flush process. This revised framing reflects the maturity level and the realistic scope of the present work, while appropriately acknowledging prior breakthroughs in cobalt-based PEM systems.\nIn “Introduction” part, we have added: “Several strategies like surface engineering10, interfacial water structure modulation11 have been proposed to improve the stability of CoO , while Co2+ leaching could not be eliminated.” x"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed properly my concerns, and therefore I can recommend the publication of the present work in Nature Communications.", "reviewer_2": "Although we appreciate the authors’ responses to our comments, two important issues remain that require clarification.\n1. The manuscript repeatedly uses “CoOx” despite conclusive evidence identifying the material as spinel Co3O4. Figure 2b exhibits characteristic polycrystalline Co3O4 diffraction rings, while HRTEM (Figs. S10 and S12) confirms the (311) lattice spacing (0.242 nm). XPS/XANES-derived oxidation states (2.5-2.6) are consistent with mixed-valent Co3+/Co2+ in stoichiometric spinel Co3O4. The rebuttal’s justification (“non-stoichiometry and low crystallinity”) thus appears to retain vague notation while simultaneously claiming a “predominantly.... polycrystalline Co3O4” phase. This selective precision creates unnecessary reader confusion. Readers reasonably expect precise chemical formulas when structural identification is definitive. The term CoOx implies an undefined composition (e.g., CoO, Co3O4, CoOOH), which contradicts the authors’ own structural and spectroscopic characterizations. This represents the weakest point of the rebuttal. We therefore recommend replacing “CoOx” with “Co3O4,” with any minor non-stoichiometry in doped variants noted parenthetically or discussed in the Supplementary Information.\n2. In Figure R4, the S-number of the CrNd–CoOx catalyst is approximately two orders of magnitude lower than that of the reference IrO2 catalyst. This large disparity indicates that the absolute stability of CrNd–CoOx remains far inferior to that of state-of-the-art Ir-based OER catalysts. Under these circumstances, the claims of high durability in the revised manuscript are not adequately supported by the quantitative data and therefore appear overstated. We recommend that the authors substantially tone down their durability claims and clearly distinguish between relative improvements compared with undoped Co3O4 and absolute stability relative to IrO2 benchmarks.", "reviewer_3": "I really appreciate the authors' efforts to resolve every concern raised by the reviewers. I want to emphasize that every comment and question was intended to help clarify the work and improve the quality of science contributed to the community. From my side, I feel every concern was explained well. Thank you again to the authors for addressing every reply, and especially for conducting the new experimental protocols suggested during the first revision round."}, "author_response": "We sincerely appreciate the valuable comments and suggestions raised by the reviewers that shall help us to significantly improve the quality of our manuscript. Below we provide a point-by-point response letter. The original comments are marked in Italics, the responses are marked with BLUE color, and the revisions are marked with RED color.\nReviewer #1:\nThe authors have addressed properly my concerns, and therefore I can recommend the publication of the present work in Nature Communications.\nResponse: We thank the reviewer for the positive comment on our work.\nReviewer #2:\nAlthough we appreciate the authors’ responses to our comments, two important issues remain that require clarification.\nResponse: We thank the reviewer for raising the valuable comments and suggestions. Accordingly, we have provided a detailed point-by-point response as shown below.\n1. The manuscript repeatedly uses “CoO ” despite conclusive evidence identifying the x material as spinel Co O . Figure 2b exhibits characteristic polycrystalline Co O 3 4 3 4 diffraction rings, while HRTEM (Figs. S10 and S12) confirms the (311) lattice spacing (0.242 nm). XPS/XANES-derived oxidation states (2.5-2.6) are consistent with mixed- valent Co3+/Co2+ in stoichiometric spinel Co O . The rebuttal’s justification (“non- 3 4 stoichiometry and low crystallinity”) thus appears to retain vague notation while simultaneously claiming a “predominantly.... polycrystalline Co O ” phase. This selective 3 4 precision creates unnecessary reader confusion. Readers reasonably expect precise chemical formulas when structural identification is definitive. The term CoO implies an x undefined composition (e.g., CoO, Co O , CoOOH), which contradicts the authors’ own 3 4 structural and spectroscopic characterizations. This represents the weakest point of the rebuttal. We therefore recommend replacing “CoO ” with “Co O ,” with any minor non- x 3 4 stoichiometry in doped variants noted parenthetically or discussed in the Supplementary Information.\nResponse: We sincerely thank the reviewer for the valuable suggestions. We acknowledge that the notation of “CoO ” can be interpreted as vague by readers, particularly when the x dominant phase is identified as spinel Co O with confidence. Hence, we use the term 3 4 “Co O ” to ensure the unambiguous chemical formulation and avoid possible 3 4 misunderstanding. Additionally, the corresponding descriptions about the polycrystalline characteristics of Co O have been revised. 3 4\nRevisions:\n1. All “CoO ” expressions in all texts and figures are replaced by “Co O ”. x 3 4\n2. The sentence “Notably, considering the nature of non-stoichiometry and low crystallinity of doped samples, ‘CoO ’ rather than ‘Co O ’ was used to avoid misinterpretation.” has x 3 4 been deleted.\n2. In Figure R4, the S-number of the CrNd–CoO catalyst is approximately two orders of x magnitude lower than that of the reference IrO catalyst. This large disparity indicates that 2 the absolute stability of CrNd–CoO remains far inferior to that of state-of-the-art Ir-based x OER catalysts. Under these circumstances, the claims of high durability in the revised manuscript are not adequately supported by the quantitative data and therefore appear overstated. We recommend that the authors substantially tone down their durability claims and clearly distinguish between relative improvements compared with undoped Co O and 3 4 absolute stability relative to IrO benchmarks. 2\nResponse: We thank the reviewer for the insightful comment. We agree that it is important to clarify the stability improvements of CrNd-Co O versus undoped Co O . Besides, the 3 4 3 4 intrinsic stability of CrNd-Co O remains substantially inferior to state-of-the-art Ir-based 3 4 OER catalysts under strongly acidic conditions. Some of the language in the previous manuscript may have inadvertently overstated the absolute durability relative to established benchmarks.\nRevisions: We have revised the corresponding descriptions: “In addition, the improved S- number of CrNd-Co O compared to that of undoped Co O clearly demonstrated the effect 3 4 3 4 of co-doping on durability enhancement (Fig.S22). Despite that the S-number of CrNd- Co O was about two orders of magnitude lower than that of IrO , the observed 3 4 2 improvement relative to other reported Co O -based catalysts indicated that the 3 4 introduction of Cr and Nd effectively suppressed Co dissolution and extended the catalytic lifetime.”\nReviewer #3:\nI really appreciate the authors’ efforts to resolve every concern raised by the reviewers. I want to emphasize that every comment and question was intended to help clarify the work and improve the quality of science contributed to the community. From my side, I feel every concern was explained well. Thank you again to the authors for addressing every reply, and especially for conducting the new experimental protocols suggested during the first revision round.\nResponse: We thank the reviewer for the appreciation of our work."}]

Decision letter