High-spin transition metal atoms drive acidic oxygen evolution reactions

Domain: chemistry
Published: 2026-02-18
Editorial outcome: major_revision

Abstract
Synthesizing transition metal catalysts to replace precious metal ones such as IrO2 and RuO2 achieving efficient acidic oxygen evolution reaction while balancing intrinsic activity, stability, and cost-effectiveness always been a dream pursued by scientists and industrialists, but still remains a challenge. Here, we present an efficient catalytic system formed by graphdiyne-induced high-spin state cobalt-based oxide (HSS-CoOx/GDY) for enhancing the activity and stability of the acidic oxygen evolution reaction. Experimental and theoretical results demonstrate that the bonding of electron-rich sp-hybridized carbon and Co atoms initiates the Jahn-Teller effect of CoO6 octahedra, which regulates the occupied d-orbital of Co atoms and generates the high-spin Co3+. Such spin occupancy breaks the spin-forbidden effect and optimizes the adsorption/desorption ability of HSS-CoOx/GDY toward key reaction intermediates, thereby promoting the coupling of O-O bonds and the evolution of oxygen gas. The proton exchange membrane water electrolyzers constructed based on this catalyst achieve a current density of 1.0 A cm−2 at a low cell voltage of 1.80 V. This research indicates that graphdiyne has the ability to manipulate the electronic spin states of electrocatalysts.

Introduction
The kinetic bottleneck of the anodic oxygen evolution reaction (OER) is reflected in proton exchange membrane water electrolyzers (PEMWEs), which is confronted with the sluggish four-electron transfer kinetics and the strong reliance on rare and precious metal catalysts such as iridium oxide (IrO2) and ruthenium dioxide (RuO2)1–4, which have always been difficult technical issues to overcome in this field and suppressed the rapid progress of OER technology. For many years, scientists have made tremendous efforts to achieve the replacement of precious metal catalysts with transition metal (TM)-based materials, but the results have always been insignificant. The catalytic performance is fundamentally limited by the spin-forbidden effect of oxygen-containing intermediate5–7. Specifically, to satisfy angular momentum conservation (spin-allowed) during the OER process, the additional energy (~1.0 eV) is required to form the oxygen molecule (O2) with the singlet excited state (1Δg), subsequently transitions to the triplet ground state O2 (3Σg−) (Supplementary Fig. 1)8. Some advanced catalysts attempted to modulate the d-orbital occupancy of the catalytically active centres through spin-state manipulation. An appropriate spin configuration enables a reduction in the energy barrier of O-O coupling via Hund’s rule-driven parallel spin alignment, facilitating the direct coupling of O-O bond9–12. However, our anxiety is that among numerous non-precious metal systems, there is still no clear roadmap for simultaneously optimizing water dissociation (H-OH cleavage) and oxygen dimerization via spin-state control.

After years of exploration, trivalent cobalt (Co3+), which is low-cost and abundant on Earth, has come into the view of scientists. This is because Co3+ has a relatively high intrinsic OER activity, and becomes a promising alternative to Ru/Ir-based oxides8,13–15. Notably, elevating the spin state of Co3+ can effectively activate the eg orbitals, thereby enhancing OER kinetics16. However, conventional strategies predominantly focus on electronic structure tuning rather than spin-state engineering. Although a few Co-based oxides with high-spin Co3+ have been reported, such metastable states are unreliable and rapidly degrade in acidic PEMWE conditions10,17. To address this core issue, discovering a corrosion-resistant catalytic system that stabilizes high-spin Co3+, generates strong interactions between metal and support on the interface, and preserves high-catalytic performances represents a critical pathway toward developing durable acidic OER electrocatalysts. As a novel carbon material, graphdiyne (GDY) possesses distinctive electronic and structural features, including an sp/sp²-hybridized carbon framework, intrinsic nanoporous architecture, high electron mobility, and a π-conjugated system with robust metal-anchoring capability18–21. The electron-rich nature of GDY facilitates charge transfer through metal-carbon bonding, which modulates the d-orbital occupancy of metal atoms22–27. This strong metal-GDY charge transfer might elevate eg orbital occupancy and trigger spin-state transitions in metal centers, consequently optimizing OER performance.

Building upon these issues, we strategically employed OER-inert Co3O4 as a model system to demonstrate GDY’s capability in stimulating and stabilizing high-spin states of Co3+ for enhancing acidic OER performances. Superconducting quantum interference device (SQUID) magnetometry quantified the high-spin Co3+ population is 58.9% in HSS-CoOx/GDY (Total effective magnetic moment μeff = 5.62 μB, eg occupancy = 1.18), contrasting sharply with bulk Co3O4 (μeff = 4.35 μB, eg occupancy = 0.12). Density functional theory (DFT) calculations revealed that high-spin centres reduce the energy barrier of the rate-determining step through the optimized eg orbital configuration, directly accelerating O-O bond formation. This spin-state modulation originates from GDY’s electron-donating π-system and uneven surface charge distribution, which induces the Jahn-Teller effect and weakens crystal field splitting (ΔOh reduced by 0.247 eV), thereby forming and stabilizing the high-spin state. The spin-polarized HSS-CoOx/GDY exhibited a high acidic OER performance with a low overpotential of 221 mV at 10 mA cm−2 (140 mV lower than Co3O4) and good stability (>400 h at 10 mA cm−2, <7.1% activity decay). When integrated into a PEMWE device, the catalyst achieved industrially relevant current densities (1.0 A cm−2 at 1.80 V without iR-correction) with minimal voltage drift at 200 mA cm−2 (<2.87 mV h−1 over 100 h). Mechanistic studies combining pulse voltammetry and in-situ EIS analysis revealed that the enhanced OER performance, compared to its low-spin counterpart, is attributable to a spin-aligned, facilitated electron transfer process.

Results
Orbital configuration and theoretical predictions of HSS-CoOx/GDY
The Co3O4 spinel consists of corner-sharing CoO4 tetrahedra and edge-sharing CoO6 octahedra. According to the ligand field theory, Co2+ (3d7) in the tetrahedral field adopts a high-spin configuration (eg4t2g3) because the ligands have relatively less shielding effect on the d-orbitals of the central Co2+ ion (Fig. 1a)28. Therefore, it is difficult to regulate the spin configuration of the CoO4 tetrahedra, which results in activity that cannot be modified. In contrast, Co3+ in octahedral structures has been widely recognized as the primary active site of the OER29,30. This inference is consistent with the density functional theory (DFT) calculation results (Supplementary Fig. 2). To further increase the catalytic activity of the Co3+ active center, the spin configuration of Co3+ in the CoO6 octahedron is manipulated by GDY. To verify this, DFT calculations were first performed to investigate the effect of GDY on the electronic and crystal configurations of Co3O4. As shown in Fig. 1b and Supplementary Fig. 3, the lattice structure of the CoO6 octahedron is distorted after loading on GDY. The structural distortion shortens the gap between the eg orbital and t2g orbital because of the Jahn-Teller effect, which favors the electron transition from the t2g orbital to the eg orbital (Fig. 1c and Supplementary Fig. 4).

To further reveal the influence of spin states on OER performance, we explored the relationships between the spin states of Co3+ and structural distortions, the spin channels of Co-O bonds, and the spin alignment of *OOH intermediates, respectively. Figure 1d shows that the distortion degree (Lmax/Lmin) of the crystal structure increases with increasing spin state, further demonstrating that distorting the structure of Co3O4 increases the probability of producing high-spin Co3+. Another key change is that the magnetic moment of lattice oxygen (OL) is positively correlated with the spin state of Co3+ (Fig. 1e), meaning that the OL is magnetized by the adjacent Co3+ ions. This change highlights the emergence and intensification of the spin channel between Co3+ and OL, which facilitates increased delocalization of electrons and charge transport during electrocatalysis31,32. In addition, the average magnetization of the oxygen atoms on the adsorbed *OOH species increases as the spin state increases (Fig. 1f). As a result, the high-spin state is conducive to the spin alignment of the oxygen-containing intermediates, and enhances the reaction kinetics by breaking the spin-forbidden effect33. On the basis of these analyses, the Gibbs free energy profiles were calculated to explore the OER performance differences between HSS-CoOx/GDY and Co3O4 (Supplementary Fig. 5). As displayed in Fig. 1g, HSS-CoOx/GDY has a lower energy demand (0.360 eV) in the rate-determining step (RDS) than Co3O4 does (0.453 eV) at 1.23 V. Therefore, DFT calculation has shown that engineering high-spin Co3+ in Co3O4 by GDY has the ability to enhance the OER performance of Co3O4 in acidic media.

Preparation and structural investigation of HSS-CoOx/GDY and Co3O4
Encouraged by the theoretical predictions, we prepared HSS-CoOx/GDY and Co3O4 via simple pulsed electrochemical deposition and a low-temperature thermal oxidation method. In a typical synthesis, Co(OH)x precursor is first electrodeposited on GDY or a carbon cloth substrate, followed by calcination to form HSS-CoOx/GDY or Co3O4 (Fig. 2a). Scanning electron microscopy (SEM) revealed that pure Co3O4 and HSS-CoOx/GDY present an interconnected 3D network (Fig. 2b, c, Fig. 2f–h, and Supplementary Fig. 6a, b), meaning that HSS-CoOx/GDY maintains the morphology of GDY after Co3O4 loading. Compared to HSS-CoOx/GDY, Co3O4 looks much more random and thicker (Fig. 2b, c). Atomic force microscopy (AFM) measurements revealed thicknesses of 4.8 nm for the Co3O4 and 4.3 nm for HSS-CoOx/GDY, respectively (Fig. 2c, 2g). Furthermore, transmission electron microscopy (TEM) image of HSS-CoOx/GDY revealed that Co3O4 nanoparticles were dispersed on the GDY surface (Fig. 2i and Supplementary Fig. 7), while the Co3O4 samples without GDY present the structure of nanosheets (Fig. 2d). High-resolution TEM (HRTEM) images show that the lattice distance of the (311) plane (0.245 nm) in Co3O4 is irregularly expanded to 0.247 nm, 0.250 nm and 0.255 nm in HSS-CoOx/GDY (Fig. 2e and 2j–2l), indicating that the degree of lattice distortion ranges from 0.8% to 4.08%. This conclusion was confirmed by the X-ray diffraction (XRD) patterns (Fig. 2m). In addition, some stepped interfaces exist in HSS-CoOx/GDY (Fig. 2n–q), further confirming the presence of defects and distortions in the crystal structure of HSS-CoOx/GDY. Energy-dispersive spectroscopy (EDS) elemental mapping results confirmed the distribution of the C, O, and Co elements on the entire HSS-CoOx/GDY (Supplementary Fig. 8). From the above analysis, it is concluded that GDY plays a constructive role in constructing the expansion and stepped interfaces of the crystal structure34,35. This situation destroyed the structural symmetry of Co3O4 and redistributed the d-orbital level of Co3+, which could regulate the electronic configuration of the eg orbital.

Characterization of the electronic structure and spin states
The electronic structure and coordination environment were analyzed via Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). Raman spectroscopy of the pure Co3O4 revealed that a significant A1g symmetry peak at 676 cm−1 originating from the vibrational mode of CoIII-O in CoO6 units (Fig. 3a). This peak blue-shifted to 698 cm−1 in HSS-CoOx/GDY, confirming that the structural symmetry of the CoO6 octahedron is broken36,37. Furthermore, the peak for acetylenic bonds in pure GDY (2163 cm−1) positively shifts to 2182 cm−1 in HSS-CoOx/GDY, indicating the occurrence of interactions between Co3O4 and GDY38,39. This phenomenon is also visualized by the differential charge density (Supplementary Fig. 9). To further investigate the interactions between Co3O4 and GDY, the C1s and Co2p XPS spectra of HSS-CoOx/GDY were obtained. In the C1s spectra (Supplementary Fig. 10a), an additional peak at 283.5 eV, which is consistent with the sp-C-Co bond appeared40. In addition, upon composite formation with Co3O4, the binding energy of the sp-C bond in GDY shows a positive shift from 285.0 eV to 285.15 eV. In return, the Co2p XPS spectra clearly exhibited a higher Co2+/Co3+ ratio (0.75) on the surface of HSS-CoOx/GDY than that of Co3O4 (0.51) (Supplementary Fig. S10b). Similarly, the Co K-edge X-ray absorption near-edge structure (XANES) revealed that a lower oxidation state (+2.53) of Co in HSS-CoOx/GDY than in Co3O4 (+2.67) (Fig. 3b and Supplementary Fig. 11). The lower Co valence state provided additional evidence for the presence of high-spin Co3+9. In addition, the charge transfer between GDY and Co atoms generates a strong interaction, increases the intrinsic activity of the Co3+ while stabilizes catalytic active centers22,26,41. Further, the pre-edges of HSS-CoOx/GDY, attributed to the transition from the Co 1s core level to Co 3d states, are stronger than those of Co3O4 (Fig. 3b). This phenomenon indicates that the Co site is not centrosymmetric in HSS-CoOx/GDY. To further investigate the local coordination environment of Co, we analyzed the Fourier transform extended X-ray absorption fine structure (FT-EXAFS) of HSS-CoOx/GDY and Co3O4 (Fig. 3c). By fitting the FT-EXAFS data (Fig. 3d, Supplementary Fig. 12, and Supplementary Table 1), we find that the average coordination number (CN) of Co-O and Cooh-Cooh decreased from 5.06 and 9.73 to 4.11 and 4.97 after the introduction of the GDY substrate, respectively. In stark contrast, the average CN of Cooh-Cotd increased from 3.94 to 6.13, indicating that GDY disrupted the symmetry of the Co3O4 crystal structure. Additionally, the fit results of bond length confirmed that GDY led to the elongation of the Co-O (1.91 Å to 1.94 Å), Cooh-Cooh (2.86 Å to 2.89 Å), and Cooh-Cotd (3.36 Å to 3.40 Å) bonds, the change ratio for bond length is 1.57%, 1.05%, and 1.19%, respectively, which is in good agreement with XRD and HRTEM results. The wavelet transforms of EXAFS (WT-EXAFS) analysis of the Co K-edges show that the k position of the Co-O/C bond in HSS-CoOx/GDY decreases from 7.21 Å−1 to 6.77 Å−1 (Supplementary Fig. 12), which is attributed to the presence of the sp-C-Co bond. These results suggest that GDY has the ability to disrupt the symmetry of the CoO6 octahedron and modify the local environment of Co3+, finally leading to the formation of high-spin Co3+.

In light of the above findings, the superconducting quantum interference design (SQUID) technique was used to delve into the magnetic behavior and spin configuration of HSS-CoOx/GDY, Co3O4 and commercial Co3O4 (Com-Co3O4) (Fig. 3e). First, the magnetic hysteresis (M-H) curves of these samples were measured in a magnetic field from −50,000 Oe to 50000 Oe under ambient conditions. The M-H curves of all the samples show a linear shape without a hysteresis loop and saturation magnetization (Fig. 3f), suggesting paramagnetic behavior42. However, the alteration in their slope signifies a change in magnetic susceptibility (χ, χ = M/H), further implying that their spin configurations have obvious differences. To quantify the electron occupancy of eg orbitals, the temperature-dependent magnetization (M-T) curves were recorded under zero field cooling (ZFC) and field cooling (FC) between temperatures ranging from 4 K to 300 K at H = 150 Oe. As shown in Fig. 3g, the Néel temperatures of HSS-CoOx/GDY are distinctly lower than those of Co3O4 and Com-Co3O4, indicating that GDY alters the magnetic properties of Co3O4. Further, the Curie constant (C) value was obtained by fitting the slopes of the M-T curves above 150 K, and these values are 0.253, 0.423, and 0.458 for HSS-CoOx/GDY, Co3O4 and Com-Co3O4, respectively (Supplementary Fig. 13)43. The total effective magnetic moment (μeff) was also calculated, which was 4.35 μB and 4.18 μB for Co3O4 and Com-Co3O4, respectively (Fig. 3h). These were close to the previously reported practical measured value of Co2+ (4.14 μB) in reported previously, suggesting that almost all of Co3+ ions in both Co3O4 and Com-Co3O4 are in a low-spin state44. Interestingly, the μeff value of HSS-CoOx/GDY is 5.62 μB, indicating the presence of high-spin Co3+. The volume fraction of high-spin Co3+ is determined to be approximately 58.9%, and the corresponding average eg orbital filling is 1.18 (Fig. 3i; more details are provided in Supplementary Table 2)45. These results provide solid evidence that GDY is capable of generating high-spin Co3+ in Co3O4. Notably, the degree of spin polarization has become an interesting descriptor of OER activity because it optimizes the adsorption capacity of reaction intermediates and accelerates charge transfer.

Catalytic performance in acidic media
To explore the influence of high-spin Co3+ on OER activity, the acidic OER performances of HSS-CoOx/GDY and Co3O4 were first evaluated in 0.5 M H2SO4 solution by a traditional three-electrode system (Supplementary Fig. 14). As shown in Fig. 4a, pure Co3O4 has a relatively poor activity, presenting overpotentials of 361 mV and 476 mV to reach current densities of 10 mA cm−2 and 100 mA cm−2, respectively. After engineering the spin state of Co3+ in Co3O4 by GDY, the OER activity was increased. Specifically, HSS-CoOx/GDY only required low overpotentials of 221 mV and 301 mV at 10 mA cm−2 and 100 mA cm−2, respectively. The corresponding Tafel slope for HSS-CoOx/GDY (72.9 mV dec−1) is smaller than that of Co3O4 (128.9 mV dec−1), indicating the fast reaction kinetics (Fig. 4b and Supplementary Fig. 15). In addition, the electrochemical surface area (ECSA) values were extracted by measuring the cyclic voltammetry (CV) curves with different scan rates (Supplementary Fig. 16a-b). The calculated ECSA value for HSS-CoOx/GDY is 8.00 cm−2, which is 3.94 times greater than that of Co3O4 (2.03 cm−2), indicating that the more active sites are exposed at the reaction interface (Supplementary Fig. 16c). To compare their intrinsic OER activity, the polarization curves were normalized by the ECSA values. As shown in Fig. 4b and Supplementary Fig. 16d, the intrinsic activity of HSS-CoOx/GDY (3.00 mA cm−2) was still higher than that of pure Co3O4 (0.33 mA cm−2) at 1.65 V. Furthermore, the catalytic stability of HSS-CoOx/GDY was examined via CV and chronopotentiometry (CP) protocol. As illustrated in Fig. 4c and Supplementary Fig. 17, HSS-CoOx/GDY maintained almost constant activity over 400 h at 10 mA cm−2, and maintained an unattenuated LSV curve after 10,000 cycles of CV testing. Such high stability has also been verified through 18O-labeled in-situ differential electrochemical mass spectrometry (DEMS) measurement (Supplementary Fig. 18) and in-situ XAS (Supplementary Fig. 20). As presented in Supplementary Fig. 19, the ratio of 34O2 to 32O2 and 36O2 to 32O2 in HSS-CoOx/GDY is consistently lower than that of Co3O4, reflecting that the adsorption evolution mechanism dominated the OER process for HSS-CoOx/GDY, thereby enhancing the stability46. In-situ XAS also confirmed that the local environment and valence of Co in HSS-CoOx/GDY undergo slight changes with potential variation, while a severe oscillation occurs in Co3O4 (Supplementary Fig. 21). Furthermore, the characterization of spent HSS-CoOx/GDY after the OER test by TEM-EDS, XPS, and SQUID revealed that no obvious changes in morphology and electronic structure (Supplementary Figs. 22, 23 and Supplementary Table 2), further highlighting that HSS-CoOx/GDY has a high catalytic activity and structural stability. In terms of activity and stability, HSS-CoOx/GDY is competitive with most previously reported Co-based oxide catalysts in acidic media (Supplementary Fig. 24 and Supplementary Table 3).

Motivated by the above results of the three-electrode test, HSS-CoOx/GDY was assembled as an OER catalyst in a real PEMWE system to evaluate its performance (Fig. 4d). The steady-state polarization curves (without iR-correction) revealed much higher PEMWE activity when HSS-CoOx/GDY was used as the anode catalyst than when pure Co3O4 was used (Fig. 4e). Specifically, a cell voltage of 1.80 V delivers a current density of 1.0 A cm−2, which is higher than that of pure Co3O4 (157 mA cm−2). The PEMWE device efficiency reaches 68.3% at 1.0 A cm−2, and the corresponding energy consumption is 4.31 kWh Nm−3 H2, which is comparable with that of the previously reported most noble metal-free OER catalysts (Supplementary Table 4). The durability of the PEMWE cell was determined to be 100 h at 200 mA cm−2 and 130 h at 1.0 A cm−2 (Fig. 4f and Supplementary Fig. 25). However, pure Co3O4 only ran 1.8 h at 200 mA cm−2, and lost all its activity. The degradation rate of HSS-CoOx/GDYǀǀPt/C PEMWE device is 2.87 mV h−1, which is 2.65% of pure Co3O4 (108 mV h−1), confirming that the stability of HSS-CoOx/GDY was better than that of pure Co3O4 (Fig. 4g). These results indicate that GDY ultimately underpinned the performance of Co3O4 by producing the high-spin Co3+, suggesting the promise of using noble metal-free anode catalysts for practical applications in PEMWE setups.

Charge transfer and storage mechanism
Charge transfer and storage on the catalyst surface directly affect the cleavage and formation of chemical bonds, further leading to differences in their performance. On the basis of this premise, it is crucial to understand the transfer and storage mechanism of charge on the catalyst surface to explain the root of catalytic performance. To this end, in-situ electrochemical impedance spectroscopy (EIS) measurements were conducted at various voltages to probe the charge transfer ability at the electrolyte-catalyst interface (Fig. 5a). Compared to Co3O4, HSS-CoOx/GDY shows a more rapid decrease in phase angle in the Bode phase plots (Fig. 5b, Supplementary Fig. 26, and Fig. 5c), indicating that more electrons are used to trigger the OER process rather than being stored.

A pulse voltammetry (PV) protocol was also employed to investigate the dynamic evolution of intermediates on electrocatalysts during the OER process (Fig. 5d)47,48. The applied pulse voltage can directly affect the accumulation and release of charge on the catalyst surface, resulting in a change in current. The obtained current response curve is primarily composed of the transient process corresponding to the evolution of intermediates and the steady-state process of kinetic equilibrium, which includes information on the double layer capacitance (Cdl) and diffusion process49. First, the total charge was obtained by integrating the cathodic charge from the current profile. As displayed in Fig. 5e, the total charge in HSS-CoOx/GDY is linear in potential. Moreover, that of Co3O4 can be divided into three parts: Co2+→Co3+, OER, and Co3+→Co4+. In addition, the slope of the total charge relative to the potential in HSS-CoOx/GDY is 8.30 mC V−1, which is lower than that of Co3O4 (24.99 mC V−1, 206.8 mC V−1, 92.70 mC V−1), indicating that the charge transfer in HSS-CoOx/GDY is faster than that in Co3O4, which is in agreement with the EIS results. To acquire the Cdl-type capacitance and charge, the Dupont and Donne model was subsequently used to fit the transient current response curves. Both the anodic Cdl-type capacitance and charge for HSS-CoOx/GDY first rise rapidly and then begin to decrease after reaching 1.45 V (Fig. 5a, 5f and Supplementary Fig. 27). This phenomenon means that the charge on the HSS-CoOx/GDY surface is sufficient to initiate the OER at 1.45 V, which is consistent with the LSV curve results. However, the anodic Cdl-type capacitance and charge for Co3O4 undergo three processes, including an increase (1.30 to 1.55 V), a decrease (1.55 to 1.68 V), and another increase (1.68 to 1.80 V), which are attributed to the processes of Co2+→Co3+, OER and Co3+→Co4+ (Supplementary Fig. 28). The lifetime of the charge was calculated by dividing the total charge by the OER current (Fig. 5g). Compared with Co3O4, HSS-CoOx/GDY has a shorter charge lifetime and a faster decay, demonstrating that the charges on the HSS-CoOx/GDY surface are difficult to accumulate due to the parallel spin alignment of the reaction intermediate50. These results indicate that HSS-CoOx/GDY with high-spin Co3+ accelerates charge transfer during the reaction process, not only increasing the catalytic reaction activity but also improving the stability by rapid charge transfer (Fig. 5h).

In summary, we demonstrate that Co3O4 anchored on GDY has the comprehensive ability to serve as an alternative to noble metal catalysts, representing the beginning of the challenge from non-noble metals to noble metal OER catalysts in acidic media because of the abundance of high-spin Co3+ in the CoO6 octahedron. The existence of GDY successfully leads to the imbalance of the symmetry of the CoO6 octahedron, causing a decrease in the crystal field splitting energy and promoting the transition of t2g electrons to eg orbitals of Co3+. This transition reconfigures octahedral Co3+ ions from a low-spin state (t2g6eg0) in Co3O4 to a favorable high-spin state (t2g5.188eg0.812) in HSS-CoOx/GDY. High-spin Co3+ possesses high electron delocalization ability and fast charge transfer kinetics, enabling high acidic OER performance. Our research results indicate that the activity and performance of spin-engineered HSS-CoOx/GDY have undergone favorable changes compared to the activity of Co3O4 without a spintronic structure. This work has encouraged us to construct high-performance OER catalysts by manipulating the spin states of earth-abundant metals, which is expected to replace expensive precious metal catalysts for PEMWE setups in the future.

Methods
Materials
Cobalt(II) acetate tetrahydrate (Co(CH3COO)2•4H2O, ≥99.5%), carbon cloth (CC), copper foil (Cu, 0.1 mm thick), hydrochloric acid (HCl, 1.19 g mL−1), nitric acid (HNO3, 1.40 g mL−1), sulfuric acid (H2SO4, 1.84 g mL−1), acetone (CH3COCH3, ≥99.7%), dichloromethane (CH2Cl2, ≥99.5 %) and pyridine (C5H5N, ≥99.9 %), hydrogen peroxide (H2O2, 30 wt%), heavy-oxygen water (H218O, ≥99.9%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Before the experiment, the CC was activated in 8 M HNO3 at 100 °C for 48 h, and then rinsed by deionized water (DI) and CH3COCH3, finally dried at 60 °C for 12 h.

Preparation of porous GDY nanosheet
30 mL of CH2Cl2 and 2 mL of C5H5N containing 20 mg of hexaethynylbenzene (HEB) were injected into a petri dish (Φ 7.0 cm × H 3.5 cm). Then, the cleaned Cu foil (4.5 cm × 4.5 cm, two pieces) clamps the activated CC (3.0 cm × 2.0 cm, two pieces), which were placed together in the above petri dish and kept for 2 days in the dark and at 25 °C. After the reaction was completed, the CC with GDY was cleaned several times with CH3COCH3 and DI water. Subsequently, the CC with GDY was placed in a tube furnace and annealed under Ar atmosphere at 200 °C for 2 h. Cu species in GDY/CC were removed in 1.0 M HCl solution at 80 °C for 2 h, which was followed by drying in a vacuum oven at 60 °C for 12 h, finally obtained the GDY nanosheet.

Preparation of HSS-CoOx/GDY electrode
The HSS-CoOx/GDY electrode was prepared by the pulse electrodeposition method on a CHI660E electrochemical workstation. Firstly, a piece of GDY with 3 cm2 was used as work electrode (WE), graphite plate as counter electrode (CE, Φ 0.6 cm × H 10.0 cm) and Ag/AgCl as a reference electrode (RE, 3.5 M KCl-saturated), immersed 20 mL of Co(CH3COO)2•4H2O solution (0.02 mmol mL−1, 498 mg of Co(CH3COO)2•4H2O was dissolved in 50 mL DI water and titrated into a 100 mL volumetric flask. The solution was prepared fresh for immediate use.). Next, a constant voltage pulse is applied to the GDY. The voltage was kept at −1.0 V for 20 s, then switched to open-circuit voltage (OCP) for 40 s. This cycle was repeated 60 times. The resulting Co(OH)x/GDY electrode was annealed at 250 °C in air for 2 h, and finally formed the HSS-CoOx/GDY electrode.

Preparation of Co3O4 electrode
The same preparation process with the HSS-CoOx/GDY electrode was used to produce the Co3O4 electrode, which only employed bare CC as the substrate.

Material characterizations
The powder X-ray diffraction patterns of HSS-CoOx/GDY and Co3O4 were collected on a PANalytical X’pert with Cu Kα radiation (λ = 1.542 Å) at the ambient temperature. The morphological and structural information of HSS-CoOx/GDY and Co3O4 were investigated by a field emission scanning electron microscope (SEM, Hitachi SU-4800), a transmission electron microscope (TEM), and high-resolution TEM (HRTEM, JEM-2100) at an acceleration voltage of 200 kV. The corresponding energy dispersive X-ray spectrum (EDS) was recorded to detect the species and distribution of elements. The Raman spectroscopy of HSS-CoOx/GDY, Co3O4 and GDY was recorded at a Renishaw-2000 Raman spectrometer with 473 nm excitation laser. XPS measurements were conducted on a Thermo Scientific ESCALab 250Xi instrument with Al Kα light source to uncover the chemical states. The thickness of the samples was determined by an AFM device on a Bruker FASTSCAN BIO in non-contact mode. The K-edge X-ray absorption spectra (XAS) of Co were recorded at the 1W1B beamline of the Beijing Synchrotron Radiation Facility in Beijing, China.

Magnetic measurements
The magnetic properties of the samples were studied by a SQUID magnetometer (MPMS3, Quantum Design, America). The M-H curves were recorded at 300 K in fields between −50,000 Oe and +50,000 Oe. The susceptibilities (χ) were obtained by calculating the slope of M-H plots (χ = M/H). Further, the M-T plots were collected by measuring the variation of magnetization with temperature (4 K to 300 K) with a constant external magnetic field (H = 150 Oe). The Curie constant (C) value was extracted by fitting the M-T curves above 150 K based on the Curie-Weiss law: χ = C/(T–Tc), where Tc is the Curie-Weiss temperature. The effective magnetic moments (μeff) for these samples were subsequently obtained from the expression: μeff = (8C)1/2 μB. Further, the total μeff of samples includes contributions from both magnetic ions, i.e., Co2+ (3d7) in a tetrahedral field and Co3+ (3d6) in an octahedral field, and obeys Eq. (1):
μ2eff = μ2eff,Co3+ + μ2eff,Co2+ (1)
The volume fractions of Co3+ in high-spin (HS) and low-spin (LS) derived from the following Eq. (2)10:
μeff,Co3+ = g [SLS(SLS + 1)VLS + SHS(SHS + 1)VHS]1/2 μB (2)
Where SLS and SHS are the unpaired eg electrons number of Co3+ for the LS and HS states, their values are 0 and 2 for SLS and SHS, respectively; VLS and VHS (VLS + VHS = 1) represent the volume fractions of Co3+ ions in LS and HS states, respectively; g is the Lande g-factor, its value is 2 for electrons.
The eg filling can be further calculated by Eq. (3):
x = 2VHS (3)

Electrochemical measurements
All electrochemical studies were performed on a CHI660E electrochemical workstation by a three-electrode system with HSS-CoOx/GDY or Co3O4 as a WE (0.3 cm × 0.3 cm), Ag/AgCl as a RE (3.5 M KCl-saturated) and graphite plate as a CE (Φ 0.6 cm × H 10.0 cm), cylindrical glass vessels (Φ 3.5 cm × H 7.0 cm) as an electrolytic cell, respectively. All the potentials were converted to a reversible hydrogen electrode (RHE) through the following Eq. (4):
ERHE = EAg/AgCl + 0.197 + 0.059 × pH (4)
Where EAg/AgCl is the potential relative to the Ag/AgCl electrode, 0.197 V is the potential of the Ag/AgCl electrode against the standard hydrogen electrode (SHE). All tests were conducted in 20 mL of 0.5 M H2SO4 solution (pH = 0 ± 0.02, 27.76 mL of concentrated H2SO4 was gradually diluted with DI water. After cooling, the solution was made up to 1.0 L in a volumetric flask with DI water, and stored in a sealed and dark environment, unless otherwise specified. The linear sweep voltammetry (LSV) curves were collected once at a scan rate of 5 mV s−1 from 1.0 V to 1.8 V (without iR-correction) at room temperature (25 °C). The LSV curves were compensated by iR-correction for eliminating the effect of solution resistance, according to Eq. (5):
EiR-correction = ERHE − Imea × Rsol (5)
Where Imea is the measured polarization current, Rsol is the solution resistance, obtained by electrochemical impedance spectroscopy (EIS). The electrochemically active surface area (ECSA) values were estimated by the cyclic voltammetry (CV) method in a non-Faradaic area (1.10 V − 1.30 V vs. RHE without iR-correction) with different scan rates (10 mV s−1, 20 mV s−1, 30 mV s−1, 40 mV s−1, and 50 mV s−1). The electrochemical double-layer capacitance (Cdl) was calculated by plotting the corresponding current density difference (ΔJ = (J+ − J−)/2) at 1.20 V (vs. RHE without iR-correction) against the scan rate, then linear fitting this curve, the slope of this curve is the CDL value. The ECSA value was estimated using the following Eq. (6):
ECSA = Cdl = Cs (6)
Where Cs represents the ideally flat electrode, Cs = 0.035 mF cm−2. In-situ EIS was carried out in the frequency range from 106 Hz to 10−2 Hz at a voltage range of 1.3 V − 1.8 V (vs. RHE without iR-correction) with a 10 mV amplitude. Stability tests were conducted by chronopotentiometry (CP) and the CV method.

Pulse voltammetry (PV) tests
PV tests were performed on a CS310X electrochemical workstation at a low potential (E1 = 1.20 V vs. RHE) for 4 s and a high potential (E2) for 5 s, and then returned to E1 for 4 s. This process was cycled while increasing the potential from 1.20 to 1.80 (V vs. RHE) in 0.02 V/step, and recording the current response. The total charge was obtained by integrating the cathodic charge. The Dupont and Donne model was used to fit the temporal decay profile based on Eq. (7)47–49:
i = (E/Rs) exp(−t / RsCdl) (7)
Where the i is the decaying current; Rs is the electrolyte resistance (Ω); Cdl is the Cdl-type capacitance (F); t is the time (s); E is the potential (V vs. RHE).
The lifetime of the charge was calculated by dividing the total charge by the OER current.

PEMWE tests
The membrane electrode assembly (MEA) was firstly prepared, using Nafion 212 membrane (thickness: 50.8 μm, area: 3.0 cm × 3.0 cm) as solid polymer electrolytes, HSS-CoOx/GDY or Co3O4 as anode catalysts (∼4.0 mg cm−2), and Pt/C (40 wt.%) as cathode catalyst (∼0.4 mg cm−2), Ti fiber with Pt layer and carbon paper as an anode and cathode porous transport layer (PTL), respectively. The Nafion 212 membrane was sequentially treated with 3 wt% H2O2 and 0.5 M H2SO4 at 80 °C for 1 h, and stored in DI water. HSS-CoOx/GDY catalyst was collected by the ultrasonic method. The anode (4.0 mg) and cathode (1.0 mg) catalyst ink were separately prepared by dispersing the catalyst in a mixture of 0.055 mL of Nafion (5 wt%), 1.6 mL of isopropanol, and 0.4 mL of DI water. The catalyst ink was sprayed onto both sides of a Nafion 212 membrane with an active area of 1.0 cm × 1.0 cm. Then, the MEA electrode was assembled into a PEMWE device, and DI water was injected as the electrolyte solution at a flow rate of 30 mL min−1. Finally, the PEMWE setup was run at the temperature of 65 °C, and polarization curves were collected between 0.8 V and 1.8 V in the CS310X electrochemical workstation. The stability was also measured by recording the cell voltage change under a constant current of 200 mA cm−2 and 1.0 A cm−2. The energy efficiency, hydrogen production cost, and energy consumption of the PEMWE setup were evaluated.
Energy consumption calculation: The energy consumption of a PEMWE setup is calculated based on Eq. (8):
Energy consumption = (Ucell × z × F) / (3600 × Vm) (8)
Where Ucell is cell voltage (V) when a current density reaches 1.0 A cm−2; z is the number of electron transfers for generating one hydrogen molecule, its value is 2; F represents Faraday’s constant (96484 C mol−1); Vm is the molar volume of gas under standard conditions (22.4 L mol−1).
Energy efficiency calculation: The energy efficiency of a PEMWE setup is evaluated by the following Eq. (9):
Energy efficiency = 1.23 / Ucell (9)
where 1.23 V is the theoretical thermodynamic voltage for water splitting.

In-situ XAS measurements
In-situ XAS spectroscopy for Co under different potentials (OCP, 1.4 V, 1.5 V, 1.6 V and 1.7 V (vs. RHE) without iR-correction) in flow cell with three-electrode were carried out using the Rapid XAFS 2M (Anhui Absorption Spectroscopy Analysis Instrument Co., Ltd.) by transmission mode at 20 kV and 20 mA, and the Si (533) spherically bent crystal analyzer with a radius of curvature of 500 mm was used for Co. Electrochemical signals are driven by CHI660E electrochemical workstation.

DEMS measurements
To elucidate the OER mechanism, differential electrochemical mass spectrometry (DEMS) measurements were conducted using heavy-oxygen water (H218O) in a CIS‑DEMS‑DM-C device (Hefei insitutechnology Co., Ltd., China) with AOEPF-TB002HS-T membrane (pore size: 0.2 μm). The catalyst ink was prepared by dispersing 1.0 mg of HHS‑CoOx/GDY or Co3O4 in a mixture solution with 0.8 mL of isopropanol, 0.2 mL of DI water, and 0.027 mL of Nafion solution (5 wt%). Subsequently, 0.3 mL of the ink was drop‑cast onto a porous gold (Au) electrode, yielding a catalyst loading of 0.3 mg cm−2. A three‑electrode configuration was employed, with the catalyst‑modified porous Au, an Ag/AgCl electrode, and a pure Pt mesh as WE, RE, and CE, respectively.
Prior to DEMS detection, the catalysts were isotopically labeled with 18O at 1.5 V (vs. Ag/AgCl) for 10 min in 0.5 M H2SO4 containing H218O by CHI660E electrochemical workstation. After labeling, the electrodes were thoroughly rinsed with DI water (H216O) to remove residual H218O. The electrodes were then transferred to a fresh 0.5 M H2SO4 electrolyte with DI water. To ensure comparable current density between HHS‑CoOx/GDY and Co3O4, the LSV potential windows were adjusted to 1.0 V-1.4 V (vs. Ag/AgCl) for HHS‑CoOx/GDY and 1.0 V-1.9 V (vs. Ag/AgCl) for Co3O4. Throughout the LSV process, the evolved oxygen was monitored in real time using mass spectrometry.

Theoretical calculations
All calculations were performed using spin-polarized density functional theory (DFT) with the projector augmented plane-wave (PAW) method, as implemented in the Vienna Ab Initio Simulation Package (VASP). The exchange-correlation potential was modeled using the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA). Long-range van der Waals interactions were accounted for by employing the DFT-D3 correction. A plane-wave cutoff energy of 480 eV was used for all calculations. The energy convergence criterion was set to 10⁻5 eV for the iterative solution of the Kohn-Sham equations.
A unit cell of pristine Co3O4 was constructed by 48 Co atoms and 64 O atoms, and the most stable (311) plane was exposed at the terminal. The slab model of HHS-CoOx/GDY, it contained 72 C atoms, 28 Co atoms, and 44 O atoms. C atoms were used to form the skeleton of GDY. Co and O atoms were applied to build the model of Co3O4, and the Co atoms were connected to the sp-C atoms in the GDY model. During the structural optimization process, Brillouin zone integration was carried out using 3 × 3 × 4 gamma k-point sampling, and all atoms were fully relaxed until the residual forces reached below 0.02 eV/Å to form the most stable structure (Supplementary Data 1). Data analysis and visualization were performed using the VASPKIT and VESTA software packages51. A 20 Å vacuum layer was introduced normal to the slab surface to eliminate interlayer interactions.
Free energy changes (ΔG) were computed via the computational hydrogen electrode (CHE) approach, with potentials referenced to the standard hydrogen electrode by Eq. (10):
ΔG = ΔE + ΔEZPE − T×ΔS + ΔGU (10)
Where ΔE is the binding energy of the intermediates; ΔEZPE is the zero-point energy at 298.15 K; T is the experimental temperature (298.15 K); ΔS is the entropy change; ΔGU is the free energy correction term for the electrode potential, which can be obtained by: ΔGU = −eU, in which U refers to the electrode potential with respect to the standard hydrogen electrode, and e is the transferred charge number.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors report on the development of a high-performance acidic OER system based on high-spin transition metal atoms stabilized by graphdiyne (GDY). Both experimental and theoretical analyses consistently demonstrate that the reversible non-integer charge transfer between GDY and Co species promotes the formation of high-spin Co3+, which optimizes the adsorption-desorption energetics of OER intermediates. The work is further strengthened by the integration of the catalyst into a PEMWE device, delivering 1 A cm-2 at 1.8 V, highlighting its potential for practical applications. Although the results are promising and the concept of spin-state engineering through GDY is of potential interest for developing non-precious metal OER catalysts, several issues require clarification and improvement before the publication in Nature Communications:\n(1) The HRTEM images should explicitly indicate the lattice spacings of GDY for unambiguous structural identification.\n(2) For a clearer comparison on the changes in the thickness before and after the formation of the catalysts, it is suggested that the AFM images for the thicknesses of Co3O4, HSS-CoOx/GDY and GDY be displayed in Figure 2 in the main text.\n(3) In Figure 3c, the peak intensity for Cooh-Cooh in HSS-CoOx/GDY is lower than in Co3O4, whereas the average coordination numbers for Cooh-Cooh in HSS-CoOx/GDY exceeds that of Co3O4. Please carefully check.\n(4) While XPS and differential charge density analyses reveal charge redistribution between GDY and Co, the link between this charge transfer and the observed catalytic enhancement should be more explicitly discussed.\n(5) To avoid ambiguity caused by cell resistance, the iR-corrected polarization data of PEMWE should be provided.\n(6) The methodology for calculating energy efficiency, hydrogen production cost, and energy consumption should be described in sufficient detail (in Methods or Supplementary Information) to ensure reproducibility and fair comparison with literature reports.", "reviewer_2": "Recommendation: Minor Revision\nThe authors present a compelling study on using graphdiyne (GDY) to stabilize high-spin Co^3+ in Co_3O_4, significantly improving acidic oxygen evolution reaction (OER) performance. The integration of advanced experimental characterizations (Raman, XPS, XAS, EXAFS, SQUID, in-situ EIS, pulse voltammetry) with density functional theory (DFT) calculations convincingly demonstrates GDY-induced spin state modulation. This work is impactful for electrocatalysis and spin-related catalysis, offering a pathway toward high-efficiency, noble metal-free anodes for proton exchange membrane water electrolysis (PEMWE).\nTo strengthen the claims, especially regarding high-spin identification, stability, and spin-selective tunnelling, additional clarifications are recommended.\nComments and suggestions\n1. Quantification and validation of the high-spin Co^3+ fraction\nThe SQUID-derived _eff values and Curie-Weiss fitting suggest ~40.6% high-spin Co^3+. Given the coexistence of Co^2+ (high-spin by default), it would be helpful to describe more explicitly:\n1.1. How _eff was deconvoluted into separate Co^2+ and Co^3+ contributions.\n1.2. Whether complementary techniques (e.g., XMCD, variable-temperature XAS pre-edge analysis, etc.) could further substantiate the high-spin assignment.\nThis would ensure that the estimated high-spin fraction is not inadvertently inflated by the Co^2+ background.\n2. Long-term stability of the high-spin state\nThe reliability test (400 h OER) confirms morphological and electronic stability, yet no direct post-test verification of the high-spin fraction is provided. If feasible, performing SQUID or XAS measurements after prolonged acidic OER would offer valuable confirmation that the high-spin configuration is retained.\n3. Interpretation of spin-selective tunnelling\nThe enhancement in charge transfer is clearly supported by EIS and pulse voltammetry. However, the direct attribution to “spin-selected electron tunnelling” is not experimentally isolated. Please clarify whether this term is intended qualitatively or if any quantitative modelling/measurement (e.g., magnetoresistance-type experiments, spin-polarized transport simulations) supports the claim. If primarily qualitative, adopting terminology such as “spin-aligned facilitated electron transfer” may avoid overinterpretation.\n4. Agreement between DFT and experimental data\nThe experimentally derived e_g occupancy (0.812) is in reasonable agreement with DFT, yet the degree of consistency and any systematic deviations should be discussed. Possible sources include temperature effects, structural inhomogeneity, or model approximations.\nMinor suggestions.\n▪ Context for d-orbital engineering: Please emphasize more clearly how GDY differs from other supports in terms of _oh modulation, stability of the high-spin state under acidic OER, and its -electron system’s role in continuous, reversible charge transfer.\n▪ Jahn-Teller distortion parameters: Since the _oh reduction is linked to GDY-induced lattice distortion, it would be useful to include explicit distortion metrics (e.g., bond angle variance, L_max/L_min) from EXAFS or HRTEM directly in the main text.\n▪ Terminology consistency: Ensure uniform notation for “high-spin Co^3+” and orbital fillings (t_2g^n e_g^n) throughout the manuscript.\nIn summary, this is a well-conceived and impactful study with strong experimental-theoretical synergy. Addressing the above points would further reinforce the magnetic-catalytic correlations and ensure that the claims regarding high-spin stabilization and spin-dependent charge transfer are well supported. I therefore recommend minor revision, with the expectation that these clarifications will make the work even stronger for both the catalysis and magnetism communities.", "reviewer_3": "This manuscript reports the preparation of graphdiyne (GDY) supported CoOx as active electrocatalyst for OER in acid. The GDY substrate is suggested to stimulate and stabilize the high-spin states of Co3+ for CoOx nanoparticles. Therefore the spin-polarized HSS-CoOx/GDY exhibits a low OER overpotential of 221 mV at 10 mA cm⁻2. A PEMWE is also constructed and a cell voltage of 1.80 V is recorded at 1.0 A cm−2. However, I have several questions after going through this manuscript.\n1. Carbon-based materials are generally suggested to be unstable during long-term anodic operation in PEMWE. Even though some important structure-performance relationships in acidic OER electrocatalysis have been reported for carbon-based materials, it is still quite challenging to achieve stable response under ampere-level current density for this group of materials in PEMWE. The authors use GDY as the substrate in this manuscript. Can it really be stable at 1.0 A cm−2 in PEMWE during long-term test? Would it be oxidized to CO2 or other species? If not, why would not it be oxidized under such harsh condition with long-term operation? The authors should provide systematic characterization and theoretical discussion to reveal this issue.\n2. The authors suggest that GDY would significantly stimulate the formation of high-spin Co3+ for CoOx (from 0% of Co3O4 to 40.6% of HSS-CoOx/GDY). However, CoOx in this manuscript seems to be nanoparticles with sizes of several nanometers rather than single-atom sites or single-atom layers. The GDY may affect the spin state of the interfacial Co. But how could it affect the whole nanoparticles? Moreover, the size and loading of CoOx on GDY are not studied. Would the ratio of high-spin Co3+ be further increased?\n3. The interaction between GDY and CoOx is not clearly discussed. Why choosing GDY as the substrate? Would some other carbon-based materials such as graphene, oxidized graphene, N-doped carbon nanomaterials and C3N4 also demonstrate similar performance? More theoretical and experimental discussion should be provided.\n4. The valence of Co changes during OER according to the EIS analysis. However, direct experimental evidence is missing. In-situ XAS should be carried out to further analysis the structure and valence change of Co during OER. The spin state of Co should also be characterized to further certify the stable presence of high-spin Co3+ during OER.\n5. In the Introduction section, the authors suggest “The catalytic performance is fundamentally limited by scaling relationships between *OH and *OOH intermediates”. Does the strategy in this manuscript break the scaling relationships? If yes, how is the scaling relationships broken?\n6. 18O-labled DEMS test is suggested to carried out to further illustrate the OER mechanism in this work.\n7. The stability test in PEMWE should also be evaluated under larger current density, such as 1.0 A cm−2.\n8. The calculation method for the price of H2 should be carefully checked. Can it really reduce the cost of H2 to US$ 0.96 per kg H2? Does the authors use the same electricity price as the international standards (US$ 2 kg−1 H2) in the calculation model?\nConsidering these problems, I do not recommend the publication of this manuscript in Nature Communications."}, "author_response": "Dear Reviewers,\nThank you very much for your valuable suggestions and comments regarding our manuscript entitled\n\" High-spin transition metal atoms drive acidic oxygen evolution reactions\" (Manuscript ID:\nNCOMMS-25-52216-T) to Nature communications, which are very helpful for us to improve our\nmanuscript. We have addressed all the concerns and the revisions are highlighted in the text. Based\non your revision suggestions, we have made new version of our manuscript. The point-by-point\nresponse to the comments are listed below.\nReviewer #1 (Remarks to the Author):\nThe authors report on the development of a high-performance acidic OER system based on high-spin\ntransition metal atoms stabilized by graphdiyne (GDY). Both experimental and theoretical analyses\nconsistently demonstrate that the reversible non-integer charge transfer between GDY and Co species\npromotes the formation of high-spin Co3+, which optimizes the adsorption-desorption energetics of\nOER intermediates. The work is further strengthened by the integration of the catalyst into a PEMWE\ndevice, delivering 1 A cm-2 at 1.8 V, highlighting its potential for practical applications. Although the\nresults are promising and the concept of spin-state engineering through GDY is of potential interest\nfor developing non-precious metal OER catalysts, several issues require clarification and\nimprovement before the publication in Nature Communications:\n(1) The HRTEM images should explicitly indicate the lattice spacings of GDY for unambiguous\nstructural identification.\nResponse: Thanks for your kind suggestion. We have added the lattice spacings of GDY in Figure 2i\nand more detailed information is also provided in the revised Supporting information (Fig. R1).\nFig. R1 (a) TEM image of HSS-CoO /GDY, (b-f) HRTEM images of HSS-CoO /GDY.\nx x\n(2) For a clearer comparison on the changes in the thickness before and after the formation of the\ncatalysts, it is suggested that the AFM images for the thicknesses of Co O , HSS-CoO /GDY and\n3 4 x\nGDY be displayed in Figure 2 in the main text.\nResponse: Thanks for your concern. AFM images for the thicknesses of GDY has been added in\nrevised Fig. 2h in the revised manuscript.\n(3) In Figure 3c, the peak intensity for Co -Co in HSS-CoO /GDY is lower than in Co O , whereas\noh oh x 3 4\nthe average coordination numbers for Co -Co in HSS-CoO /GDY exceeds that of Co O . Please\noh oh x 3 4\ncarefully check.\nResponse: According to XAFS theory, the peak intensity of the scattering path is collectively by the\ncoordination number (CN) and the Debye-Waller factor (σ2). Specifically, the peak intensity of the\nscattering path is directly proportional to the coordination number and inversely proportional to the\nDebye-Waller factor. The fitting results of FT-XAFS show that the Debye-Waller factor of the Co -\noh\nCo scattering path in HSS-CoO /GDY is 0.0067, which is significantly higher than that of Co O\noh x 3 4\n(0.0033), indicating that the peak intensity of the Co -Co scattering path in HSS-CoO /GDY will\noh oh x\nbe lower. Therefore, our results are reliable.\n(4) While XPS and differential charge density analyses reveal charge redistribution between GDY\nand Co, the link between this charge transfer and the observed catalytic enhancement should be more\nexplicitly discussed.\nResponse: Thanks for your kind suggestion. The charge transfer between GDY and Co atoms\ngenerates a strong interaction, improves the electrical conductivity, regulate the electronic structures\nof Co, increases the intrinsic activity of the Co3+ while greatly stabilize catalytic active centers (J. Am.\nChem. Soc. 2024, 146, 10573-10580, Angew. Chem. Int. Ed. 2025, e202507269, Angew. Chem. Int.\nEd. 2024, e202416664). More explicitly discussion has been added in revised manuscript as follows:\nThe charge transfer between GDY and Co atoms generates a strong interaction, improves the\nelectrical conductivity, regulate the electronic structures of Co, increases the intrinsic activity of the\nCo3+ while greatly stabilize catalytic active centers.\n(5) To avoid ambiguity caused by cell resistance, the iR-corrected polarization data of PEMWE\nshould be provided.\nResponse: The iR-corrected polarization data of PEMWE should be provided in Fig. R2. After iR\ncorrection, the PEMWE performance of HHS-CoO /GDY as anode catalyst reduced the overall cell\nx\nvoltage to 1.75 V at 1 A cm−2.\nFig. R2 Polarization curves with and without iR-correction for HSS-CoO /GDY and Co O anode-\nx 3 4\nbased electrolyzers at 80 ℃.\n(6) The methodology for calculating energy efficiency, hydrogen production cost, and energy\nconsumption should be described in sufficient detail (in Methods or Supplementary Information) to\nensure reproducibility and fair comparison with literature reports.\nResponse: The methodology for calculating energy efficiency, hydrogen production cost, and energy\nconsumption have been described and added in Methods as follows:\nEnergy consumption calculation: The energy consumption of a PEMWE setup is calculated\nbased on the equation:\nU × z × F\ncell\nEnergy consumption=\n3600 × V\nm\nWhere U is cell voltage (V) when a current density reaches 1 A cm−2; z is the number of\ncell\nelectrons transfers for generating one hydrogen molecule, its value is 2; F represents Faraday's\nconstant (96484 C mol−1); V is the molar volume of gas under standard conditions (22.4 L mol−1).\nm\nEnergy efficiency calculation: The energy efficiency of a PEMWE setup is evaluated by the\nfollowing equation:\n1.23 V\nEnergy efficiency=\nU\ncell\nwhere 1.23 V is the theoretical thermodynamic voltage for water splitting.\nHydrogen production cost: The electricity cost is calculated based on the energy consumption\nand electricity bill:\nU × z × F\ncell\nHydrogen production cost= 0.2778 × × y\nM\nH2\nwhere M represents the relative molecular mass of hydrogen molecule (2 g mol−1); y is electricity\nH2\nbill, which is derived from the previous reports ($0.02 kWh−1).\nReviewer #2 (Remarks to the Author):\nRecommendation: Minor Revision\nThe authors present a compelling study on using graphdiyne (GDY) to stabilize high-spin Co3+ in\nCo O , significantly improving acidic oxygen evolution reaction (OER) performance. The integration\n3 4\nof advanced experimental characterizations (Raman, XPS, XAS, EXAFS, SQUID, in-situ EIS, pulse\nvoltammetry) with density functional theory (DFT) calculations convincingly demonstrates GDY-\ninduced spin state modulation. This work is impactful for electrocatalysis and spin-related catalysis,\noffering a pathway toward high-efficiency, noble metal-free anodes for proton exchange membrane\nwater electrolysis (PEMWE).\nTo strengthen the claims, especially regarding high-spin identification, stability, and spin-selective\ntunnelling, additional clarifications are recommended.\n1. Quantification and validation of the high-spin Co3+ fraction\nThe SQUID-derived μ values and Curie-Weiss fitting suggest ~40.6% high-spin Co3+. Given the\neff\ncoexistence of Co2+ (high-spin by default), it would be helpful to describe more explicitly:\n1.1. How μ was deconvoluted into separate Co2+ and Co3+ contributions.\neff\nResponse: According to the calculation formula of the intrinsic magnetic moment:\nμ= √n(n+2) μ\nB\nthe theoretical magnetic moment of Co2+ in Co O is 3.87 μ (the number of unpaired electrons n\n3 4 B\nwould be 3 for the tetrahedral Co2+ in Co O ) (Nat. Commun. 2024, 15, 1383; Adv. Funct. Mater.\n3 4\n2024, 34, 2315039). However, due to the contribution of the orbital magnetic moment, the measured\nmagnetic moment of Co2+ is actually greater than 3.87 μ (J. Phys.: Condens. Matter 2008, 20,\nB\n015218; J. Appl. Phys. 115, 114309 (2014); ). According to the previous reports, this measured value\nis approximately 4.14 μ (Materials Chemistry and Physics 2011, 125, 347–350; Nano Lett. 2018,\nB\n18, 3934−3942; J. Phys. Chem. Solids 1964, 25, 1), which is very close to the magnetic moment of\nCo2+ in Co O that our measured (4.18 μ ). Therefore, based on the equation (Angew. Chem. Int. Ed.\n3 4 B\n2020, 59, 2313–2317):\nμ2 = μ2 + μ2\neff eff, Co3+ eff, Co2+\nthe contributions of Co3+ in CoO /GDY to the magnetic moment is 3.76 μ , this is 𝜇 =\nx B 𝑒𝑓𝑓,𝐶𝑜3+\n3.76 𝜇 . Using this value, the volume fractions of Co3+ in high-spin state (HS) and low-spin state\n𝐵\n(LS) can be obtained by the following equation (Angew. Chem. Int. Ed. 2021, 60, 14536–14544):\nμ =g√S (S + 1)V +S (S + 1)V μ\neff, Co3+ LS LS LS HS HS HS B\nWhile S (= 0) and S (= 2) are the number of unpaired spins in e orbitals for LS and HS state Co3+,\nLS HS g\nrespectively; V and V (V + V =1) are the volume fractions for Co3+ in LS and HS states,\nLS HS LS HS\nrespectively; g is the Lande g-factor and equals to 2 for electrons.\nBased on these calculations, the value of V for Co3+ is 58.9%. The e filling can be further\nHS g\ncalculated by Equation (Angew. Chem. Int. Ed. 2020, 59, 2313–2317):\nx=2V\nHS\nThe calculated e fillings are 1.18 for Co3+ in CoO /GDY.\ng x\nIn addition, the corresponding discussion has been updated in the revised Supplementary\nInformation.\n1.2. Whether complementary techniques (e.g., XMCD, variable-temperature XAS pre-edge analysis,\netc.) could further substantiate the high-spin assignment.\nThis would ensure that the estimated high-spin fraction is not inadvertently inflated by the Co2+\nbackground.\nResponse: Thanks for your valuable suggestions. X-ray magnetic circular dichroism (XCMD)\ntechnique is a powerful method for analyzing the spin states of specific elements. However, it requires\nlarge-scale facilities such as synchrotron radiation sources. Unfortunately, we do not have the\nnecessary experimental conditions at present. As for variable-temperature XAS pre-edge analysis,\nto our knowledge, this technique cannot precisely determine the spin arrangement of Co3+. SQUID\ntechnology is currently the most common and practical technique for analyzing spin assignment (Nat.\nCommun. 2024, 15, 1383; Angew. Chem. Int. Ed. 2020, 59, 2313-2317; Adv. Mater. 2023, 2302966;\nNat. Commun. 2025, 16, 215). We fully understand the reviewers' concern that the Co2+ background\nmay affect the assessment of the Co3+ spin fraction. However, in our experiments, the measured\nmagnetic moment of Co²⁺ is 4.18 μB, which is nearly identical to previously reported values (4.14\nμB). This indicates that the SQUID technique possesses excellent reproducibility and reliability.\nFurther, the background signal of Co2+ was also subtracted when estimating high-spin fraction of\nCo3+. Therefore, the obtained result is highly reliable.\n2. Long-term stability of the high-spin state\nThe reliability test (400 h OER) confirms morphological and electronic stability, yet no direct post-\ntest verification of the high-spin fraction is provided. If feasible, performing SQUID or XAS\nmeasurements after prolonged acidic OER would offer valuable confirmation that the high-spin\nconfiguration is retained.\nResponse: Thanks for your concern. The magnetic behavior and spin configuration of the spent HSS-\nCoO /GDY (400 h OER test) was also measured by SQUID technique. The result show that, the\nx\nproportion of high-spin Co3+ still reached as high as 51.3% (87.1% of the initial value) after 400 h\nstability test, indicating that the HSS-CoO /GDY has high stability toward acidic OER (Fig. R3).\nx\nThe corresponding discussion has been updated in the revised manuscript and Supplementary\nInformation.\nFig. R3 (a) Magnetic hysteresis loops of HSS-CoO /GDY-400; (b) FC (field cooled) and ZFC (zero\nx\nfield cooled) magnetization curves of HSS-CoO /GDY-400 as a function of temperature at H = 150\nx\nOe. The total μ (d), the calculated volume fraction of Co3+ (e) and the corresponding e orbital filling\neff g\n(f) of HSS-CoO /GDY and HSS-CoO /GDY-400.\nx x\n3. Interpretation of spin-selective tunnelling\nThe enhancement in charge transfer is clearly supported by EIS and pulse voltammetry. However, the\ndirect attribution to “spin-selected electron tunnelling” is not experimentally isolated. Please clarify\nwhether this term is intended qualitatively or if any quantitative modelling/measurement (e.g.,\nmagnetoresistance-type experiments, spin-polarized transport simulations) supports the claim. If\nprimarily qualitative, adopting terminology such as “spin-aligned facilitated electron transfer” may\navoid overinterpretation.\nResponse: Thanks for your kind suggestion. We have revised “spin-selected electron tunnelling” to\n“spin-aligned facilitated electron transfer” in revised manuscript.\n4. Agreement between DFT and experimental data\nThe experimentally derived e_g occupancy (0.812) is in reasonable agreement with DFT, yet the\ndegree of consistency and any systematic deviations should be discussed. Possible sources include\ntemperature effects, structural inhomogeneity, or model approximations.\nResponse: We fully understand your concern that considering more comprehensive factors can better\nreflect the actual situation, yet it is always complex. In our work, DFT calculation model is\nconstructed based on the structural characterization, aiming to be as close as possible to the actual\nstructure. As shown in Fig. R4 (Figure 2 in original manuscript), Co O species are dispersed on the\n3 4\nsurface of GDY, and its thickness is about 0.5 nm, indicating that Co O has a layered structure with\n3 4\n2-3 atomic layers on GDY substrate. Based on these characterization results, we constructed the\nmodel as shown in Fig. R5 and carried out the calculation of the d-band and Gibbs free energy.\nFurthermore, we investigated the influence of the spin state on the OER performance by manually\nincreasing the spin state of Co3+, and confirmed that the spin polarization effect can align the reaction\nintermediates and enhance the OER reaction. More comprehensive factors, including temperature\neffects, structural inhomogeneity, or model approximations, will be one of the key points in our\nsubsequent work.\nFig R4. (a) HRTEM image of HSS-CoO /GDY; (b-d) SEM and AFM images of HSS-CoO /GDY,\nx x\nGDY and Co O .\n3 4\nFig R5. Structural model of HSS-CoO /GDY for DFT calculation.\nx\nMinor suggestions.\n▪ Context for d-orbital engineering: Please emphasize more clearly how GDY differs from other\nsupports in terms of Δ modulation, stability of the high-spin state under acidic OER, and its π-\noh\nelectron system’s role in continuous, reversible charge transfer.\nResponse: Unlike other traditional carbon materials (such as graphene, carbon nanotubes, etc.), GDY\nis a carbon carrier with sp and sp2-C hybridization. The uneven surface charge distribution can easily\ncustomize and regulate the electronic structure of the metal atoms. The electron rich sp-C bond can\nnot only anchor metal atoms, but also act as an electron reservoir to prevent the over-oxidation and\ndissolution of metal atoms, improving the stability of the catalysts. GDY's π-electron system\naccelerates charge transfer, avoids charge accumulation and ensures a continuous electron flow,\nthereby maintains the progress of the catalytic reaction.\n▪ Jahn-Teller distortion parameters: Since the Δ reduction is linked to GDY-induced lattice distortion,\noh\nit would be useful to include explicit distortion metrics (e.g., bond angle variance, L_max/L_min)\nfrom EXAFS or HRTEM directly in the main text.\nResponse: Thanks for your kind suggestion. Based on the HRTEM results, the degree of lattice\ndistortion ranges from 0.8% to 4.08% by (d − d )/d . And, the fitting results of FT-XAFS shown\nmax stad stad\nthat the change ratio for Co-O, Co -Co and Co -Co bond length is 1.57%, 1.05% and 1.19%,\noh oh oh td\nrespectively.\nThe corresponding discussion for explicit distortion metrics has added in revised manuscript.\n▪ Terminology consistency: Ensure uniform notation for “high-spin Co^3+” and orbital fillings\n(t_2g^n e_g^n) throughout the manuscript.\nResponse: We have unified the notation for high-spin Co3+ and orbital fillings (t ne n).\n2g g\nIn summary, this is a well-conceived and impactful study with strong experimental-theoretical\nsynergy. Addressing the above points would further reinforce the magnetic-catalytic correlations and\nensure that the claims regarding high-spin stabilization and spin-dependent charge transfer are well\nsupported. I therefore recommend minor revision, with the expectation that these clarifications will\nmake the work even stronger for both the catalysis and magnetism communities.\nReviewer #3 (Remarks to the Author):\nThis manuscript reports the preparation of graphdiyne (GDY) supported CoO as active\nx\nelectrocatalyst for OER in acid. The GDY substrate is suggested to stimulate and stabilize the high-\nspin states of Co3+ for CoO nanoparticles. Therefore, the spin-polarized HSS-CoO /GDY exhibits a\nx x\nlow OER overpotential of 221 mV at 10 mA cm−2. A PEMWE is also constructed and a cell voltage\nof 1.80 V is recorded at 1.0 A cm−2. However, I have several questions after going through this\nmanuscript.\n1. Carbon-based materials are generally suggested to be unstable during long-term anodic operation\nin PEMWE. Even though some important structure-performance relationships in acidic OER\nelectrocatalysis have been reported for carbon-based materials, it is still quite challenging to achieve\nstable response under ampere-level current density for this group of materials in PEMWE. The\nauthors use GDY as the substrate in this manuscript. Can it really be stable at 1.0 A cm−2 in PEMWE\nduring long-term test? Would it be oxidized to CO or other species? If not, why would not it be\n2\noxidized under such harsh condition with long-term operation? The authors should provide systematic\ncharacterization and theoretical discussion to reveal this issue.\nResponse: Unlike traditional carbon materials, GDY is a novel carbon allotrope with a 2D highly\nconjugated structure composed of a benzene ring (sp2-C) and an acetylene linker (sp-C). The linear\nalkyne bond provides an exceptional channel for electron transfer, and the planar benzene ring offers\nextensive delocalized space. The combination of these two structures creates an anisotropic, highly\nconjugated large π-electron system. In this conjugated system, electrons can move rapidly, exhibiting\nexceptionally high intrinsic carrier mobility, meaning that electrons can be rapidly replenished or\nextracted in the catalytic system during electrocatalytic reactions. This characteristic avoids charge\naccumulation, ensures a continuous electron flow, prevents the oxidation of GDY itself and the\ncatalytic active sites, thereby maintaining stable catalytic reaction.\nHSS-CoO /GDY catalyst was also assembled in the anode of PEMWE setup as an OER catalyst\nx\nand evaluated its stability at a current density of 1.0 A cm−2. As shown in Fig R6, the PEMWE setup\ncan run continuously at a current density of 1.0 A cm−2 for 130 h, and the voltage only increased by\n6.92%, strongly suggesting its robustness under PEMWE operational conditions.\nFig R6. Stability curves of HSS-CoO /GDY||Pt/C at a current density of 1.0 A cm−2.\nx\n2. The authors suggest that GDY would significantly stimulate the formation of high-spin Co3+ for\nCoO (from 0% of Co O to 40.6% of HSS-CoO /GDY). However, CoO in this manuscript seems\nx 3 4 x x\nto be nanoparticles with sizes of several nanometers rather than single-atom sites or single-atom\nlayers. The GDY may affect the spin state of the interfacial Co. But how could it affect the whole\nnanoparticles? Moreover, the size and loading of CoO on GDY are not studied. Would the ratio of\nx\nhigh-spin Co3+ be further increased?\nResponse: In structural characterization, atomic force microscopy (AFM) was employed to measure\nthe thickness of GDY (3.8 nm) and HSS-CoO /GDY (4.3 nm), shown that the thickness of CoO on\nx x\nthe GDY surface is approximately 0.5 nm, indicating that CoO consists of 2-3 atomic layers (Fig.\nx\nR6, Fig. 2 in original manuscript). This phenomenon of GDY as substrate to load the metal atomic\nlayers has also been demonstrated in our previous work (J. Am. Chem. Soc. 2024, 146, 10573-10580;\nAngew. Chem. Int. Ed. 2024, 63, e202406043; Nat. Commun. 2024, 15, 7331). Therefore, GDY can\ndirectly affect the spin state of Co throughout the entire CoO nanolayer.\nx\nIn addition, the size of CoO can affect spin state of Co3+, but this influence is confined to the\nx\ncatalyst surface, which is extremely difficult to define the thickness of catalyst surface (Nat Commun\n2016, 7, 11510). The loading of catalyst usually does not affect the spin state, because the loading of\ncatalyst is normalized when calculating the spin state.\nFig R7. SEM images of HSS-CoO /GDY (a) and GDY (c). AFM images of HSS-CoO /GDY (b)\nx x\nand GDY (d).\n3. The interaction between GDY and CoO is not clearly discussed. Why choosing GDY as the\nx\nsubstrate? Would some other carbon-based materials such as graphene, oxidized graphene, N-doped\ncarbon nanomaterials and C N also demonstrate similar performance? More theoretical and\n3 4\nexperimental discussion should be provided.\nResponse: Compared with other sp2-carbon materials (such as graphene, oxidized graphene, N-doped\ncarbon nanomaterials and C N ), GDY is consisting of sp (acetylene bond) and sp2 (benzene rings)\n3 4\nto form a porous structure with uniformly distributed large pores (2.5 Å) and tunable electronic\nproperties. The natural structural advantage of GDY has triggered a series of superior properties such\nas special catalytic active sites, accelerating charge transfer behavior, facilitating mass transport, and\nalso offering an ideal support for in situ confinement growth of metal atoms and quantum dots. GDY\nhas special charge distribution inhomogeneity and conjugated alkyne-rich characteristics with a\nstrong ability as an electric acceptor or donor; such strong interactions and charge transfer between\ndonor and receptors lead to sustained maintenance of catalytic activity and excellent selectivity and\nstability. Therefore, there is a huge space for the rational design and precise synthesis of GDY-based\nefficient catalysts with defined atomic interface structures by utilizing the unique and attractive\nproperties of GDY. These characteristics of GDY allow it to exhibit many unique and significant\nadvantages in the field of electrocatalysis. In previous work, the catalytic activities of the catalysts\nsupported on the glassy carbon electrode (GCE) and other carbon substrates, such as graphene oxide\n(GO) and multiwall carbon nanotubes (MWCNT), were all tested. As expected, the catalyst with GDY\nas substrate exhibited more catalytic activity than those supported by other carbon substrates (Proc.\nNatl. Acad. Sci. 2022, 119, e2206946119; Angew. Chem. Int. Ed. 2024, e202401501; J. Am. Chem.\nSoc. 2023, 145, 1803−1810).\n4. The valence of Co changes during OER according to the EIS analysis. However, direct\nexperimental evidence is missing. In-situ XAS should be carried out to further analysis the structure\nand valence change of Co during OER. The spin state of Co should also be characterized to further\ncertify the stable presence of high-spin Co3+ during OER.\nResponse: Thanks for your suggestions. we carried out in-situ XAS to further analysis the structure\nand valence change of Co during OER. As shown in Fig R8, the local environment and valence of Co\nin HHS-CoO /GDY undergo slight changes with potential variation, while a severe oscillation occurs\nx\nin Co O . The magnetic behavior and spin configuration of the spent HSS-CoO /GDY (400 h OER\n3 4 x\ntest) was also measured by SQUID technique (Fig. R9). The result show that, the proportion of high-\nspin Co3+ still reached as high as 51.3% after 400 h stability test, indicating that the HSS-CoO /GDY\nx\nhas high stability toward acidic OER.\nFig. R8 Co K-edge XANES spectra of (a) HHS-CoO /GDY and (b) Co O ; Fourier transforms of\nx 3 4\nk3-weighted EXAFS spectra of (c) HHS-CoO /GDY and (d) Co O .\nx 3 4\nFig. R9 (a) Magnetic hysteresis loops of HSS-CoO /GDY-400; (b) FC (field cooled) and ZFC (zero\nx\nfield cooled) magnetization curves of HSS-CoO /GDY-400 as a function of temperature at H = 150\nx\nOe. The total μ (d), the calculated volume fraction of Co3+ (e) and the corresponding e orbital filling\neff g\n(f) of HSS-CoO /GDY and HSS-CoO /GDY-400.\nx x\n5. In the Introduction section, the authors suggest “The catalytic performance is fundamentally limited\nby scaling relationships between *OH and *OOH intermediates”. Does the strategy in this manuscript\nbreak the scaling relationships? If yes, how is the scaling relationships broken?\nResponse: Breaking the scale effect between *OH and *OOH is a way to enhance the OER activity\nby LOM pathway. However, in this work, the scale relationship has not been broken. Our strategy is\nto utilize high-spin Co3+ to break the spin-forbidden effect and reduce the additional energy required\nfor spin flipping caused by O-O coupling (Fig. S10), thereby achieving the aim of lowing the energy\nbarrier for the oxygen evolution reaction. In order to eliminate misunderstandings, we have revised\nthe corresponding descriptions and made changes in the updated manuscript.\nFig R10. The thermodynamic energy barrier of oxygen evolution reaction from the viewpoint\nof spin states.\n6. 18O-labled DEMS test is suggested to carried out to further illustrate the OER mechanism in this\nwork.\nResponse: Thanks for your suggestions. 18O-labled DEMS test shows that the proportion of the\nadsorption evolution mechanism (AEM pathway) for HHS-CoO /GDY during OER reaction is higher\nx\nthan that of Co O , indicating that the AEM pathway dominated the OER process for HHS-\n3 4\nCoO /GDY, rather than the lattice oxygen oxidation mechanism (LOM pathway) (Fig. R11).\nx\nThe corresponding discussion for OER mechanism has added in revised manuscript:\nFig R11. DEMS signals of 32O , 34O , and 36O from the produced oxygen gas by 18O-labeled HHS-\n2 2 2\nCoO /GDY (a) and Co O (b) catalysts. The ratio of 34O /32O (c) and 36O /32O (d) over multiple\nx 3 4 2 2 2 2\ncycles.\n7. The stability test in PEMWE should also be evaluated under larger current density, such as 1.0 A\ncm−2.\nResponse: HSS-CoO /GDY catalyst was also assembled in the anode of PEMWE setup as an OER\nx\ncatalyst and evaluated its stability at a current density of 1.0 A cm−2. As shown in Fig R12, the\nPEMWE setup can run continuously at a current density of 1.0 A cm−2 for 130 h, and the voltage only\nincreased by 6.92%, strongly suggesting its robustness under PEMWE operational conditions.\nFig R12. Stability curves of HSS-CoO /GDY||Pt/C at a current density of 1.0 A cm−2.\nx\n8. The calculation method for the price of H should be carefully checked. Can it really reduce the\n2\ncost of H to US$ 0.96 per kg H ? Does the authors use the same electricity price as the international\n2 2\nstandards (US$ 2 kg−1 H ) in the calculation model?\n2\ntakes into account the electricity cost, excluding expenses such as equipment, catalysts, labor and\nother factors due to their complexity. In addition, the electricity price is $0.02 kWh−1 in the calculation\nmodel, which is the same as the international expected standard (Nat. Nanotechnol. 2021, 16, 1371–\n1377; Energy Environ. Sci. 2024, 17, 5911; Energy Environ. Sci. 2024, 17, 4196; Nat. Commun. 2024,\n15, 4974)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The revised manuscript has addressed the issues raised in the previous round. The authors have provided additional experimental evidence and improved the discussion, which strengthens the overall conclusions.\nI have no further substantive comments and support publication of the manuscript in its current form.", "reviewer_2": "The authors have carefully and comprehensively addressed all of my previous comments concerning the quantification of the high-spin Co3+ fraction, post-OER stability, and interpretation of spin-selective tunnelling. The additional analyses and refined discussion have substantially improved the clarity and scientific soundness of the manuscript. I find the revised version suitable for publication in Nature Communications.", "reviewer_3": "The quality of this manuscript improves after revision. However, questions still remain and my concerns are not fully addressed. Major revisions are still needed before considering its publication in Nature Communications.\n1. I understand GDY is a novel carbon material. But it is still consisted of carbon frameworks. Thus its long-term stability under large current density should be more carefully checked. If the authors can not provide more clearly theoretical calculations to certify the stability of GDY under harsh oxidation condition, the faradaic efficiency of the catalyst under large current density during long-term test should be at least tested and the gas products should be carefully checked to ensure the absence of CO2 and CO.\n2. The authors suggest that the thickness of CoOx on the GDY surface is approximately 0.5 nm. However, this result is not consistent with the TEM results. According to the TEM images, CoOx seems to be particles with sizes of several nanometers. If the authors insist that the CoOx are hundred-nanometer-scale nanosheets with a thickness of 0.5 nm. Evidence on TEM should be provided.\n3. The key factor that induce high-spin of Co species for GDY is still not clearly illustrated. It is well known that GDY has special structure and charge distribution. But what is the key parameter that affect the spin state of Co? How to regulate it in theory? More clear answer should be demonstrated.\n4. For question 8, the response seems quite unprofessional. The authors still suggest that “the price per kg H2 is as low as US$ 0.96 at 1 A cm−2, which is far superior to international standards (US$ 2.0 kg−1 H2)”. How can the price of H2 be reduce to 50% with similar operation voltage if “only takes into account the electricity cost”? Do the authors mean that the commercial water electrolyzes operate at > 3 V in a single cell?"}, "author_response": "Dear Reviewers,\nThank you very much for your valuable suggestions and comments regarding our manuscript\nentitled \"High-spin transition metal atoms drive acidic oxygen evolution reactions\" (Manuscript\nID: NCOMMS-25-52216-A) to Nature communications, which are very helpful for us to improve\nour manuscript. We have addressed all the concerns and the revisions are highlighted in the text.\nBased on your revision suggestions, we have made new version of our manuscript. The point-by-\npoint response to the comments are listed below.\nReviewer #1:\nThe revised manuscript has addressed the issues raised in the previous round. The authors have\nprovided additional experimental evidence and improved the discussion, which strengthens the\noverall conclusions.\nI have no further substantive comments and support publication of the manuscript in its current form.\nResponse: Thanks very much for all your kind assistance.\nReviewer #2:\nThe authors have carefully and comprehensively addressed all of my previous comments concerning\nthe quantification of the high-spin Co3+ fraction, post-OER stability, and interpretation of spin-\nselective tunnelling. The additional analyses and refined discussion have substantially improved the\nclarity and scientific soundness of the manuscript. I find the revised version suitable for publication\nin Nature Communications.\nResponse: Thanks very much for all your kind assistance.\nReviewer #3:\nThe quality of this manuscript improves after revision. However, questions still remain and my\nconcerns are not fully addressed. Major revisions are still needed before considering its publication\nin Nature Communications.\n1. I understand GDY is a novel carbon material. But it is still consisted of carbon frameworks. Thus\nits long-term stability under large current density should be more carefully checked. If the authors\ncan not provide more clearly theoretical calculations to certify the stability of GDY under harsh\noxidation condition, the faradaic efficiency of the catalyst under large current density during long-\nterm test should be at least tested and the gas products should be carefully checked to ensure the\nabsence of CO and CO.\ncurrent density of 1.0 A cm⁻2 during long-term test using the drainage method. Considering the\nsaturated vapor pressure of water (P ) is approximately 4.0 kPa at 29 °C (302.15 K, this is the\nH2O\nambient temperature during testing), the partial pressure of oxygen (P ) is calculated to be about\nO2\n97.3 kPa (101.3 kPa – 4.0 kPa = 97.3 kPa). The theoretical volume of oxygen produced (V ) per\nO2\n10 minutes at a current density of 1.0 A cm⁻2 can be calculated based on the equation:\nV = nRT/P\nO2 O2\nWhere n is the number of oxygen molecules produced by a 1.0 A current over 10 min; R is the gas\nconstant (8.31 J mol⁻1 K⁻1), and T is the temperature (302.15 K).\nThis yields a theoretical V is 40.09 mL per 10 min at the total current of 1.0 A.\nO2\nAs shown in Figure R1, the volumes of the oxygen produced at the reaction of 10, 20, 30, 40,\n50, and 60 min were 39, 79, 118, 156, 195 and 223 mL, respectively. These closely match the\ntheoretical calculation results, with the Faraday efficiency up to 97.5% (Fig R2). The subtle\ndifferences may stem from some bubbles adhering to the walls of the glass cylinder (Fig R1).\nIn addition, to avoid the interference of 44CO and 28N (same molar mass as 28CO) in the air,\n2 2\nPEMWE test were conducted at 1.0 A cm−2 using 10% 18O-labeled water (H 18O) as reactant. The\n2\nanode gas products were analyzed using gas chromatography-mass spectrometry (GC-MS). No\nsignificant signals for 30CO, 46CO and 48CO were detected (Fig. R3a-c). The mass spectrometry\n2 2\nresults showed that the relative intensities of 30CO, 46CO and 48CO were only 0.021%, 0.002%,\n2 2\nand 0.0394% (Fig. R3d), respectively, indicating negligible production of CO and CO. These\n2\nresults shown HHS-CoO /GDY has high stability. This stability is primarily attributed to the rapid\nx\ncharge transport capability of GDY, which prevents excessive oxidation due to charge accumulation.\nThis conclusion has also been supported in this work.\nFig R1. Optical photographs of the volume of O2 produced at different times using the drainage method.\nFig. R2 Comparison of theoretical and actual values of oxygen produced at different times, as well as its Faraday\nefficiency.\nFig. R3 Gas chromatography-mass spectroscopy of gas produced from the anode of PEMWE setup at 1.0 A cm−2\nusing 10% H2 18O as reactant. (a) The m/z signals within the range of 0 to 50. (b) The m/z signals within the range\nof 25 to 32. (c) The m/z signals within the range of 40 to 50. (d) The relative intensity of 30CO, 46CO2, and 48CO2\n(The signal intensity of 32O2 is defined as 100).\n2. The authors suggest that the thickness of CoO on the GDY surface is approximately 0.5 nm.\nx\nHowever, this result is not consistent with the TEM results. According to the TEM images, CoOx\nseems to be particles with sizes of several nanometers. If the authors insist that the CoOx are\nhundred-nanometer-scale nanosheets with a thickness of 0.5 nm. Evidence on TEM should be\nprovided.\nResponse: After careful analysis and review, we confirm that CoO is particles with sizes of several\nx\nnanometers. The previous inaccuracy in thickness interpretation for CoO likely arose from several\nx\nfactors. When assessing thickness using an atomic force microscope (AFM), the measured apparent\nheight is simultaneously influenced by both the tip geometry and substrate interactions. Additionally,\nthe height differences in AFM images do not always correspond to the actual physical thickness, as\nthey are also affected by surface stiffness or adhesion force fluctuations introduced by the formation\nof nanoparticles (CoO nanoparticles). From the sample's perspective, the thickness of GDY may\nx\nalso undergo compression after loading CoO and annealing (The Journal of Physical Chemistry\nx\nC 115, 2611-2615 (2011)). Based on these considerations, obtaining the CoO thickness by directly\nx\ncomparing the height difference between HHS-CoO /GDY and GDY is inaccurate. The\nx\ncorresponding description has been changed in the revised version. Nevertheless, the uneven charge\ndistribution on the GDY surface induces an irregular atomic arrangement at the early stages of CoO\nx\ngrowth, leading to the distortion in the entire crystal structure. This has been confirmed in this work\nand in previous studies (Proc. Natl. Acad. Sci. U.S.A. 119 (36) e2206946119; Angew. Chem. Int.\nEd. 2024, 63, e202316723; Nano Energy 114 (2023) 108622). The Jahn-Teller effect generated by\nthe distorted crystal structure reduces the splitting energy of the octahedral crystal field, thereby\npromoting the electrons transition from the t orbital to the e orbital, ultimately leading to the\n2g g\nformation of the high-spin Co3+.\n3. The key factor that induce high-spin of Co species for GDY is still not clearly illustrated. It is\nwell known that GDY has special structure and charge distribution. But what is the key parameter\nthat affect the spin state of Co? How to regulate it in theory? More clear answer should be\ndemonstrated.\nResponse: Thanks for your concern. The spin state of Co3+ might be affected by the following issues\narisen from GDY, including (i) the electron-rich sp-C bonds in GDY donate electrons to CoO ,\nx\ndifferentially regulating the spin states of Co3+ and Co2+. For the Co3+ in the octahedral field (CoO\n6\nconfiguration), the electron injection increases the electron density around the Co3+, which repels\nthe electron clouds of the surrounding O2⁻ ligands. This alters the covalency of the Co-O bond,\nleading to bond length changes and ultimately inducing a Jahn-Teller distortion. (ii) the uneven\ndistribution of charges on the surface of GDY leads to an uneven atomic arrangement during the\ninitial stage of crystal growth. This ultimately results in a distorted lattice structure, and thereby also\ngenerating the Jahn-Teller effect. Consequently, the crystal field splitting energy (Δ ) is significantly\no\nreduced due to the Jahn-Teller effect, facilitating electron transition from the low-energy t to the\n2g\nhigh-energy e orbitals by reducing electron repulsion (Angew. Chem. Int. Ed. 2021, 60, 14536–\ng\n14544; Chem 4, 1263–1283, June 14, 2018; J. Am.Chem. Soc. 138, 5087–5092.). These\ntransitions prompt a spin-state change from low spin (t 6e 0) to high spin (t 4e 2).\n2g g 2g g\nHowever, Co2+ (3d7) in the tetrahedral field (CoO configuration) is inherently high-spin state\n4\n(e 3t 4) due to the weak ligand field. The further reduction of the Δ is insufficient to alter its\ng 2g o\nestablished spin state.\n4. For question 8, the response seems quite unprofessional. The authors still suggest that “the price\nper kg H is as low as US$ 0.96 at 1 A cm−2, which is far superior to international standards (US$ 2.0\n2\nkg−1 H )”. How can the price of H be reduce to 50% with similar operation voltage if “only takes\n2 2\ninto account the electricity cost”? Do the authors mean that the commercial water electrolyzes\noperate at > 3 V in a single cell?\nResponse: Thanks for your professional suggestion. In our initial assessment for hydrogen costs,\nwe adopted a simplified approach from the published works that only considered the electricity cost,\nand set it at $0.02 kWh−1. We agree that this approach was unsuitable because the actual evaluation\nstrategy is complex and the electricity cost is multifaceted. Therefore, the related description has\nbeen removed in the revised version."}, {"round": 3, "reviewer_comments": {"reviewer_3": "This manuscript can be accepted without further revision."}, "author_response": ""}]

Decision letter