Synthesis of atomically dispersed catalysts via hydrogen embrittlement-like assisted thermal activation for acidic oxygen reduction

Domain: biology
Published: 2026-04-01
Editorial outcome: major_revision

Abstract
Atomically dispersed catalysts based on 3d metals have been extensively explored in the catalytic field, but stabilizing 4d and 5d metals like Ru, Pd, and Pt as single atoms remains a challenge due to their high cohesive energies. Herein, we develop a hydrogen-embrittlement-inspired strategy that leverages H permeation to weaken metal-metal cohesion in 4d/5d metal clusters during high-temperature synthesis. Hydrogen diffuses into the clusters, driving their dissociation into individual atoms, which are subsequently stabilized by nitrogen dopants in carbon supports, resulting in the formation of stable M-N single-atom sites. Taking Ru as a model system, ex-situ microscopy and spectroscopy offer definitive evidence that hydrogen permeation disrupts Ru-Ru bonding interactions, facilitating the conversion of Ru clusters into isolated RuN sites during the H -assisted thermal activation process. Consequently, the prepared NC-Ru-950 catalyst achieves satisfactory activity and stability for acidic oxygen reduction and proton exchange membrane fuel cells. This work introduces a robust and universal strategy for stabilizing 4d and 5d transition metals as single-atom catalysts, offering a promising route to develop high-performance electrocatalysts.

Introduction
Atomically dispersed metal catalysts (ADMCs) represent a revolutionary advancement in heterogeneous catalysis, offering unparalleled atomic utilization and well-defined active sites that frequently culminate in exceptional catalytic efficiency and selectivity1-4. Among them, 3d transition metal ADMCs, primarily Fe-5-9, Co-10,11, and Mn-based systems12,13, have dominated the research landscape owing to their relatively accessible synthesis and promising performance in electrochemical energy conversion reactions. While 4d and 5d transition metal-based nanoparticle and cluster catalysts have been extensively studied and commercialized14-16, their single-atom analogues, especially for noble metals like Ru, Pd, and Pt, have received far less attention. This disparity is attributable to the formidable challenge of stabilizing these metals in atomically dispersed configurations, since 4d and 5d metals exhibit much higher cohesive energies compared to typical 3d metals17-19, making them more prone to form clusters rather than single atoms.

Conventional synthetic routes for ADMCs are generally classified into two main strategies: ‘bottom-up’ and ‘top-down’20. In ‘bottom-up’ strategies, metal precursors are typically introduced onto the support surface, where they undergo adsorption, reduction, and subsequent stabilization at defect or vacancy sites4. Nevertheless, this method is compromised by the pronounced mobility and aggregation tendency for high-cohesive 4d/5d metal species. In contrast to ‘bottom-up’ methods, ‘top-down’ strategies construct atomically dispersed sites by decomposing pre-organized nanostructures or polymeric frameworks21,22, thereby suffering from the high-cohesive energy of metal nanoparticles, which hinders their atomization into single atoms and favors the growth of larger metallic aggregates. Critically, both routes fail to sufficiently disrupt the inherent metal-metal bonding responsible for cluster formation. Given the high intrinsic catalytic potential and substantial cost of 4d and 5d metals, the development of reliable and generalizable strategies to break the strong metal-metal interactions for achieving their atomization is both scientifically pressing and economically desirable.

Herein, we introduce a H -permeation-triggered hydrogen-embrittlement-like strategy to address the inherent aggregation tendency of 4d and 5d metals (Ru, Pd, Pt, and Au et al.) during high-temperature synthesis. In this approach, hydrogen diffuses into the crystal lattice of metal clusters and weakens the metal-metal cohesion, facilitating the thermally driven dissociation of clusters into individual atoms. These liberated atoms are subsequently immobilized by nitrogen dopants in the carbon support, yielding thermodynamically stable M-N single-atom sites. Taking Ru as a representative example, the combination of atomic resolution electron microscopy and X-ray absorption spectroscopy unambiguously demonstrates that hydrogen infiltration into the Ru clusters disrupts Ru-Ru interactions and triggers the transformation of clusters into atomically dispersed Ru-N coordinated sites during the H -assisted thermal activation process. Attributed to the high density and intrinsic robust RuN active centers, the as-prepared NC-Ru-950 delivers encouraging catalytic activity and stability. In an acidic electrolyte, NC-Ru-950 exhibits a half-wave potential (E ) of 0.816 V, a mere 9 mV decline in E after 30,000 cycles. The membrane electrode assembly (MEA) equipped with NC-Ru-950 performs a current density of 44.00 mA cm-2 at 0.881 V (U.S. DOE 2025 target (United States Department of Energy): 44.00 mA cm-2 at 0.9 V ) and a peak power density of 670.9 mW cm-2. More importantly, NC-Ru-950 remains 82.04% retention of its peak power density after 30,000 cycles, higher than that of Fe-NC catalysts.

Results
The structural evolution of Ru species

Firstly, density functional theory (DFT) calculations were conducted to systematically evaluate the period-dependent metal atom detachment energies. As displayed in Fig. 1a, compared to 3d transition metals, 4d/5d transition metals exhibit significantly higher metal detachment energies, establishing a strong thermodynamic preference for clustered configurations over isolated single atoms. Among them, Ru displays the greatest energy barrier for atomic liberation (1.607 eV), thereby motivating its selection as a representative system for exploring noble metal dispersion methodologies. Inspired by the mechanism of hydrogen embrittlement, introducing H into clusters is expected to weaken the interactions between metal atoms (Supplementary Fig. 1). To further investigate the mechanism by which H weakens metal-metal interactions, a small cluster consisting of ten Ru atoms was adopted to represent Ru nanoclusters without H (NC-Ru-without H ), while the same Ru cluster with randomly distributed hydrogen atoms was used to represent the hydrogen-included state (NC-Ru-with H )22,23. The optimized model of NC-Ru-without H and NC-Ru-with H (side view) shows that the Ru-Ru distance is increased in the presence of H , suggesting that the Ru clusters are less tightly packed and thus more prone to collapse after hydrogen permeation (Supplementary Fig. 2). Furthermore, the transition states for the atomization process in the absence and presence of H (Ru +N to Ru +RuN , and Ru H +N to Ru H +RuN ) were calculated via the climbing image nudged elastic band (CI-NEB) method (Fig. 1b, 1c, and Supplementary Fig. 3)24-26. The transformation from Ru and Ru H clusters to atomically dispersed RuN sites requires overcoming energy barriers of 0.477 eV and 0.120 eV, respectively, along with substantial exothermicity of 1.374 eV and 1.350 eV, implying that H plays a key role in lowering the energy barrier for atomization. Regardless of H presence, the system’s energy decreases following the formation of RuN , indicating that thermodynamically stable RuN coordination provides the driving force for the atomization of the clusters23. Notably, under the H environment, hydrogen may penetrate into the Ru clusters, disrupting their interactions in a manner similar to hydrogen embrittlement, inhibiting the growth of Ru clusters and promoting the generation of high-density RuN active sites.

Based on the above theoretical analysis of the role of H , we prepared ZIF-8-derived N-doped carbon (NC) and employed it as a carbon substrate to adsorb Ru(acac) under liquid-phase conditions (Supplementary Fig. 4), followed by controlled thermal treatment in H /Ar (NC-Ru-H /Ar) and pure Ar (NC-Ru-Ar) to decouple hydrogen-mediated effects. The results of inductively coupled plasma optical emission spectrometry (ICP-OES) suggest that the weight percentage of Ru in NC-Ru-H /Ar is higher than in NC-Ru-Ar, implying that the Ru(acac) is more readily captured and reduced under H /Ar flow (Supplementary Fig. 5). Combined with X-ray Photoelectron Spectroscopy (XPS) and X-ray Diffraction (XRD) analyses (Supplementary Fig. 6 and Supplementary Fig. 7), the signal from Ru nanoclusters in NC-Ru-H /Ar is scarcely detected, indicating that Ru primarily exists in the form of single atoms for NC-Ru-H /Ar. Moreover, according to the aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) images of NC-Ru-Ar (Fig. 1d) and NC-Ru-H /Ar (Fig. 1e), NC-Ru-Ar exhibits the coexistence of Ru clusters and Ru single atoms, while NC-Ru-H /Ar contains only Ru single atoms, corroborating that the H /Ar atmosphere facilitates the complete disintegration of Ru clusters into single atoms. Notably, it was observed that the decomposition rate of Ru clusters accelerates with increasing H concentration (Supplementary Fig. 8).

Additionally, the energy dispersive X-ray spectroscopy (EDS) elemental mapping analysis manifests that C, N, and Ru are evenly distributed over the carbon substrates for the NC-Ru-H /Ar catalyst (Fig. 1f). Finally, the oxygen reduction catalytic performance of NC-Ru-Ar and NC-Ru-H /Ar was evaluated. Compared with NC-Ru-Ar, NC-Ru-H /Ar delivers a higher half-wave potential (E ) and faster reaction kinetics (Supplementary Fig. 9 and Supplementary Table 1), validating the beneficial role of H in the formation of atomically dispersed Ru sites.

To gain deeper insight into the evolution of Ru species as a function of temperature under the influence of H , NC adsorbed with Ru(acac) was treated at various temperatures (NC-Ru-T, T = RT, 350℃, 550℃, 750℃, and 950℃). XRD patterns (Supplementary Fig. 10) and Raman spectra (Supplementary Fig. 11) results suggest that the N-doped carbon is stable, making it an ideal carbon support model for studying the formation of Ru single-atom active sites during high-temperature treatment. Thermal gravimetric analysis (TGA) of Ru(acac) reveals a sharp weight loss in the range of 200℃ ~ 350℃, which is primarily attributed to the decomposition of the Ru(acac) (Supplementary Fig. 12). The TEM image of NC-Ru-RT demonstrates the atomic dispersion of Ru species on the NC support (Supplementary Fig. 13). As shown in Supplementary Fig. 14, Fig. 2a-d, and Supplementary Fig. 15, ex-situ AC-HAADF-STEM images and electron energy loss point spectra of four catalysts (NC-Ru-350, NC-Ru-550, NC-Ru-750, and NC-Ru-950) are presented to further comprehend the conversion mechanism from Ru clusters to Ru single atoms. As the temperature increases from 350℃ to 950℃, the average size of Ru clusters gradually decreases from ~ 2 nm to complete disappearance, with the initial emergence of single atoms at 550℃ and a full conversion to atomically dispersed Ru sites at 950℃ (Fig. 2e). According to the Brunauer-Emmett-Teller (BET) analyses of NC and NC-Ru-T (T = RT, 350℃, 550℃, 750℃, and 950℃), (Supplementary Fig. 16 and Supplementary Table 2), Ru(acac) is predominantly confined within NC micropores, as suggested by the reduced specific surface area (NC: 1126.61 m2 g-1, NC-Ru-RT: 764.77 m2 g-1) and micropore volume (NC: 0.39 cm3 g-1, NC-Ru-RT: 0.24 cm3 g-1). With increasing annealing temperature, both gradually recover and eventually approach their initial values at 950 °C, which confirms the structural robustness of the carbon support and complete transformation of Ru clusters to single atoms.

Additionally, ex-situ X-ray photoelectron spectroscopy (XPS) analyses and ex-situ X-ray absorption fine structure (XAFS) analyses were also employed to track the variation in oxidation states and the local coordination environment of Ru species throughout the pyrolysis procedure under H /Ar27,28. The full spectra of various catalysts annealed at multiple temperatures reveal the presence of C, N, O, and Ru elements (Supplementary Fig. 17a). As illustrated in Supplementary Fig. 17b, Fig. 2f, and Supplementary Table 3, the Ru 3p peak can be deconvolved into three valence states: Ru3+ (463.2 eV), Ruδ+ (462.5 eV, 0 < δ < +3), and Ru0 (461.6 eV)29. For NC-Ru-RT, only Ru3+ derived from Ru(acac) can be detected. Afterward, Ru3+ is gradually reduced to Ru0 under H /Ar. Upon reaching 550°C, Ru3+ completely disappears, accompanied by the appearance of Ruδ+. As the temperature continues to rise to 950°C, the content of Ruδ+ increases from 26.27% to 100%, while the content of Ru0 decreases to 0.00%. At low to medium temperatures (room temperature to 350℃), the Ru(acac) precursor (Ru3+) is reduced by H , leading to the cleavage of Ru-ligand bonds and the formation of metallic Ru clusters (Ru0). As the annealing temperature increases from 350℃ to 950℃, H penetrates into the Ru clusters, weakening the Ru-Ru interaction and ultimately driving the Ru clusters to collapse into individual Ru atoms. Subsequently, the detached Ru atoms are captured by the inherent N-defects on the NC support, forming stable Ru-N moieties (Ruδ+). Furthermore, the Bader charge of Ru in NC-Ru(acac) , NC-Ru_cluster, and NC-RuN models is calculated. During the transformation of Ru species from Ru(acac) to Ru_cluster and subsequently to RuN model, the absolute value of the Ru Bader charge first decreases and then increases, while remaining lower than that in Ru(acac) . The trend of the calculated Bader charges is in agreement with the sequence of binding energy shifts observed in XPS spectra (Supplementary Fig. 18). Concurrently, the high-resolution spectra of N 1s display two distinct peaks in Supplementary Fig. 17c, Fig. 2f, and Supplementary Table 4, attributed to graphitic-N at 401.3 eV and pyridinic-N at 398.6 eV. Similar to the results for Ru 3p , a new peak for Ru-N (399.6 eV) begins to appear at 550°C, and its percentage content gradually increases from 5.91% to 19.34% as the temperature rises to 950°C. The above results suggest that the atomization process of Ru clusters begins from 550°C and reaches complete atomic dispersion at 950°C.

Based on the X-ray absorption near-edge structure (XANES) of Ru K-edge in Supplementary Fig. 19, with increasing temperature, the Ru K-edge absorption edge (E ) initially shifts toward lower energy and subsequently moves to higher energy. Ultimately, the E position of Ru K-edge in the NC-Ru-950 catalyst is located between those of NC-Ru-750 and NC-Ru-RT30. According to the linear relationship between the valence state and E , the oxidation state of Ru in NC-Ru-950 is determined to be +2.2. Furthermore, the transformation of the coordination environment of Ru during the thermal activation process was unveiled by Fourier-transformed k2-weighted extended X-ray absorption fine structure (FT-EXAFS) spectra in the R space (Fig. 2i). For NC-Ru-RT, only Ru-O bonds around 1.50 Å are observed. As for NC-Ru-550 and NC-Ru-750, there are two primary scattering paths at ~1.53 Å and ~2.38 Å in the R-space, related to Ru-N bonds and Ru-Ru bonds, respectively. It is noteworthy that, as the temperature increases from 550℃ to 750℃, the peak intensity of Ru-N bonds gradually rises while the peak intensity of Ru-Ru bonds decreases, corroborating the gradual transformation of Ru clusters into Ru single atoms. Finally, upon elevating the heating temperature to 950℃, the complete disappearance of Ru-Ru bonds implies that Ru clusters have entirely converted into Ru single atoms. Combining the quantitative least-squares EXAFS fitting analysis, detailed variations in the coordination structure of Ru species were obtained. As displayed in Supplementary Figs. 20-22, the fitted curves of k-space and R-space for the Ru K-edge of various catalysts closely match the experimental results. Notably, the coordination numbers (CNs) of Ru-N bonds show a progressive increase with elevated temperatures (NC-Ru-550: 3.4±0.3, NC-Ru-750: 3.9±0.2, NC-Ru-950: 4.0±0.3), whereas the CNs of Ru-Ru demonstrate a gradual reduction (NC-Ru-550: 2.4±0.3, NC-Ru-750: 1.4±0.1), denoting the conversion of Ru clusters to Ru-N species (Fig. 2g and Supplementary Table 5). The optimal fitting analysis for NC-Ru-950 distinctly reveals that the CNs of Ru-N bonds are 4.0, with a distance of 2.06 Å. Based on the fitting results, the Ru-Ru bond lengths in NC-Ru-750 and NC-Ru-550 are marginally longer than those of Ru foil, likely due to internal strain generated by a hydrogen embrittlement-like phenomenon within the Ru clusters under H /Ar conditions (Fig. 2h and Supplementary Table 5). The WT-EXAFS can simultaneously provide three-dimensional information, including k-space, R-space, and the intensity of peaks31 (Fig. 2j). It can not only distinguish the distances of coordinating atoms but also differentiate the types of coordinating atoms. The peaks located at k = ~ 5.6 Å-1, k = ~ 5.7 Å-1, and k = ~ 9.4 Å-1 correspond to the scattering paths of Ru-N, Ru-O, and Ru-Ru, respectively. As the temperature increases from room temperature to 950°C, the Ru-O bonds start to disappear initially, followed by the formation of Ru-N and Ru-Ru bonds. Subsequently, the Ru-Ru bonds vanish entirely, leaving only the Ru-N configuration, demonstrating the successful transformation from Ru clusters to RuN sites during the annealing treatment procedure.

In conclusion, the gradual downsizing of Ru clusters and their eventual transformation into atomically dispersed RuN species can be attributed to a synergistic interplay of thermodynamic and kinetic factors under the H /Ar atmosphere (Fig. 2k). Specifically, H molecules readily penetrate into the Ru clusters, facilitating the weakening of Ru-Ru metallic bonds by introducing internal strain, thereby destabilizing the cluster structure and promoting its collapse into single atoms. In parallel, as the temperature rises, the mobility of Ru species is observed to increase, resulting in more frequent collisions between Ru atoms/clusters and the nitrogen-doped carbon support23. These interactions enable individual Ru atoms to anchor at nitrogen sites, which stabilize the atoms and inhibit their re-aggregation. Therefore, the combination of hydrogen-embrittlement-inspired destabilization and support-mediated stabilization collectively drives the structural evolution from Ru clusters to Ru single atoms.

Universality of the hydrogen-embrittlement-like strategy

Building on the above findings, we demonstrated that the hydrogen-embrittlement-like assisted thermal activation strategy can be extended to 4d and 5d transition metals that were challenging to stabilize as single atoms. Using the same approach, we synthesized Pd-NC, Pt-NC, and Au-NC catalysts, with the ICP content of 0.55 wt%, 0.52 wt%, and 0.69 wt%, respectively (Supplementary Fig. 23). Aberration-corrected imaging unambiguously confirmed the isolated atomic dispersion of these metals, while elemental mapping revealed their uniform distribution across the carbon support (Fig. 3a-c). Notably, EXAFS spectra further resolved their local coordination environments (Fig. 3d-f): in stark contrast to the corresponding metal foils, only metal-N coordination signals were observed, with no detectable metal-metal bonds, and this conclusion was further supported by wavelet transform analysis (Fig. 3g-i). These results not only verify the successful synthesis of Pd, Pt, and Au single-atom catalysts but also underscore the broad applicability of the hydrogen-embrittlement-like strategy.

The ORR catalytic activity and stability in RRDE and PEMFCs

The ORR catalytic performances of NC-Ru-T catalysts (T = RT, 350℃, 550℃, 750℃, and 950℃) were evaluated utilizing a rotating ring-disk electrode (RRDE) in O -saturated 0.1 M HClO to illuminate the influence of heating treatment on forming RuN active centers32. As depicted in the staircase cyclic voltammetry curves (Fig. 4a), the NC without adsorbed Ru(acac) exhibits poor ORR activity. After adsorbing Ru(acac) and undergoing the low-temperature annealing treatment, NC-Ru-RT and NC-Ru-350 still exhibit inferior performance, with an E of ~0.440 V and ~0.457 V, respectively, which implies the absence of active centers at these stages. Then, the ORR electrocatalytic activity of catalysts is obviously improved as the pyrolysis temperature rises from 550°C to 950°C, signifying the emergence and the elevated proportion of atomically dispersed RuN active centers, resulting in the boosted ORR activity. Regarding NC-Ru-950, it delivers a high ORR catalytic activity with the E of 0.816 V and the kinetic current density (j ) of 2.008 mA cm-2 at 0.85 V (Supplementary Fig. 24 and Supplementary Table 6), which is slightly higher than those of Fe-NC catalysts we prepared (E = 0.813 V, j = 1.135 mA cm-2 at 0.85 V), and is 50 mV lower than Pt/C catalyst (~0.866 V). In addition, the H O yield of various catalysts decreases as the temperature rises , with the H O yield of NC-Ru-950 being less than 0.5% (Supplementary Fig. 25a). The electron-transfer number (n) of NC-Ru-950 is calculated to be 3.99, signifying that NC-Ru-950 catalyzes the ORR through a high-efficiency 4e- pathway (Supplementary Fig. 25b). The Tafel slope also exhibits a similar trend with temperature changes, among which NC-Ru-950 demonstrates the smallest Tafel slope (82 mV dec-1), indicating the fastest kinetics towards ORR (Supplementary Fig. 26). Additionally, we systematically optimized the content of Ru precursors, which is a key parameter for achieving high activity. As suggested in Supplementary Fig. 27, Supplementary Tables 7, 8, the optimal parameters for producing high-performance Ru-based single-atom catalysts are Ru(acac) precursor and the corresponding content of 2 mg (NC: Ru(acac) = 10:1). To further prove the capture and anchoring effects of N-doping on forming RuN active centers during the pyrolysis process, XC-72 was employed to adsorb Ru(acac) , followed by annealing at a series of temperatures. The nearly negligible performance of the SCV polarization curves (Supplementary Fig. 28) and the distinct Ru clusters (Supplementary Fig. 29) verify the essential role of N-doping in forming the atomic-level dispersed RuN active centers.

Besides, the catalytic stabilities of NC-Ru-950 and Fe-NC were evaluated by the standard accelerated stress test (AST: 0.60 V to 0.95 V, 30,000 potential cycles) and the chronoamperometry at 0.60 V. As exhibited in Fig. 4b and Fig. 4c, NC-Ru-950 exhibits robust stability, with a mere 9 mV decay in E after 30,000 cycles (from 0.816 V to 0.807 V), compared to a 31 mV decay observed for Fe-NC (from 0.813 V to 0.782 V). Furthermore, NC-Ru-950 certifies a high retention rate of over 93.0% of its original current density after being subjected to a constant potential for 300 hours, in stark contrast to Fe-NC, which retains only 34.3% after testing for 50 hours (Fig. 4d). Therefore, the NC-Ru-950 catalyst exhibits competitive stability (Supplementary Table 9). To analyze the reasons for the enhanced stability, the Fenton reactivity of NC-Ru-950 and Fe-NC catalysts was assessed using the ABTS (2, 2’-azinobis-(3-ethylbenzthiazoline-6-sulfonate)) reagent in conjunction with a UV-Vis spectrophotometer. The ABTS can be oxidized by ROS, giving rise to a detectable alteration in absorbance at a wavelength of 417 nm and a visible change in color from colorless to blue-green. Notably, the NC-Ru-950 catalyst exhibits a pale blue color and a lower absorbance value in comparison to Fe-NC, affirming the faint Fenton process for NC-Ru-950 (Fig. 4e and Supplementary Fig. 30). To conclude, the enhanced stability of NC-Ru-950 can be attributed to two dominant aspects: (1) the lower H O yield and (2) the reduced demetallation and Fenton reactivity. These could effectively suppress the demetallation of active centers and the degradation of the carbon support.

Motivated by the aforementioned ORR performance, the NC-Ru-950 catalyst was integrated into the membrane electrode assembly (MEA) to assess the performance and durability under the fuel cell operating conditions. Notably, the MEA featuring the NC-Ru-950 cathode achieves a high current density of 27.52 mA cm-2 at 0.9 V and 44.00 mA cm-2 at around 0.881 V under 1.0 bar H -O (Fig. 5a, Supplementary Fig. 31a). As displayed in Fig. 5b, c, and Supplementary Fig. 31, the peak power density of NC-Ru-950 under the practical H -air (1.0 bar) condition reaches 670.9 mW cm-2, higher than that of Fe-NC catalysts. The MEA performance obtained from the NC-Ru-950 catalyst exhibits competitive performance (Supplementary Table 10). After undergoing a voltage square wave cycle ASTs from 0.6 V (3 s) to 0.95 V (3 s) under H -N flows, NC-Ru-950 delivers robust stability, maintaining 82.04% of its original peak power density after 30,000 cycles, while Fe-NC shows 50.40% decrease in peak power density after only 30,000 cycles (Fig. 5b, c, and Supplementary Table 11).

Furthermore, the electrochemical impedance spectroscopy (EIS) under 1.0 bar H -air condition at 0.65 V was analyzed. In comparison with Fe-NC, NC-Ru-950 demonstrates a smaller charge transfer impedance, unveiling a faster charge transfer and an improved ORR activity (Supplementary Fig. 32). Deeply, we conducted the distribution of relaxation times (DRT) to study the evolution of the electrode properties and the degradation mechanisms of catalysts during the AST process33. Previous works report that ·OH radicals can damage the ionomer, leading to an increased H+ conductivity resistance in the cathode catalyst layer. As suggested in the peak of P2 in Fig. 5d, e, Fe-NC manifests a higher proton transport impedance compared to NC-Ru-950 after aging 30,000 cycles, primarily attributed to the easier demetallation of Fe and its strong Fenton effect34,35. The P3 peak corresponds to the ORR charge-transfer resistance. With increasing aging cycles, both NC-Ru-950 and Fe-NC catalysts exhibited a low-frequency shift and enhanced intensity of the P3 peak (Fig. 5d, e, and Supplementary Table 12); however, the change was smaller for NC-Ru-950 than for Fe-NC, indicating the relatively better stability of NC-Ru-95034,36.

Mechanistic insights into the enhanced activity and stability

Furthermore, DFT calculations were conducted to clarify the factors contributing to the boosted ORR catalytic activity and stability of NC-Ru-95037. Based on the quantitative least-squares EXAFS fitting results, the RuN fragment embedded in the graphene layer was preliminarily constructed (Supplementary Fig. 33a). Considering the possible forms of active centers for the NC-Ru-950 catalyst under actual operating conditions, the models of O-RuN , HO-RuN , HOO-RuN , and H-RuN were constructed and optimized (Supplementary Fig. 33b-e). Interestingly, based on the surface thermodynamic phase diagram in Fig. 6a, we discovered that the energy of RuN with axially adsorbed O is the lowest (O-RuN ) at 0.65 V, indicating that the NC-Ru-950 catalyst stably exists in this structure under the operating voltage38-41. Therefore, in-situ generated O-RuN during the electrochemical process would be considered to be the real catalytic center (Supplementary Fig. 34a). The ORR process on the O-RuN site follows an associative pathway on the O-RuN site with four proton-coupled electron transfer steps, including the protonation of O to form the *OOH, the cleavage of the O-O bond to produce the *O and H O, the generation of *OH, and the subsequent desorption of *OH to form * and H O (* represents the catalytic active center) (Supplementary Fig. 35). Furthermore, in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRASS) is employed to clarify the active site structure and reaction intermediates (Supplementary Fig. 34b and Fig. 6b). At the initial stage of ORR, only the characteristic vibrational peak (*O, ~ 900 cm-1) is detectable42. This observation suggests that the O-RuN structure serves as the real active site. As the potential is lowered, characteristic vibrational peaks of *OOH (~ 1225 cm-1), *O (~ 900 cm-1), and *OH (~ 1650 cm-1) gradually intensify, suggesting that ORR proceeds via an associative four-electron pathway on the O-RuN active site43-45. As shown in Supplementary Fig. 36, the Gibbs free energy diagrams on the O-RuN , RuN , and FeN site are calculated, and the theoretical limiting potential (U ) of O-RuN is 0.96 V, which is higher than that of RuN (0.54 V) and FeN (0.72 V). The electrochemical process related to the desorption of *OH is recognized as the rate-determining step (RDS) for three models (RuN , O-RuN , and FeN ), with O-RuN exhibiting the lowest *OH desorption energy barrier compared to RuN and FeN , suggesting that the axially adsorbed O could modulate the binding energy between the Ru site and oxygen-containing intermediates and further improve the ORR intrinsic activity.

The electronic structure of the Ru center was calculated and analyzed after introducing the axial O. As depicted in Supplementary Fig. 37, the electronic localization functions (ELF) of RuN and O-RuN present distinct differences: RuN exhibits a symmetric charge distribution, while the O-RuN displays significant electronic delocalization and asymmetric charge distribution46. Differential charge density distribution also suggests that the partial electron transfer from the Ru center to the axial O (Supplementary Fig. 38), resulting in the increased Bader charge for O-RuN (RuN : + 0.850 |e|, O-RuN : + 1.524 |e|). In addition, the density of state of the Ru site for RuN and O-RuN was calculated. Due to the modulation of the axial O, the spin polarization of the Ru center is observed to be weakened47(Supplementary Fig. 39a). The peaks of Ru 4d z2 orbitals split into two parts: one above the Fermi level and the other below the Fermi level (Supplementary Fig. 39b, c), optimizing the adsorption strength between the Ru site and *OH intermediate. Meanwhile, in comparison to RuN , O-RuN presents a noticeable downshifted d-band center (centroid of the d-orbital states, ε = - 2.916 eV) and a widened d-orbital width (W = 5.301 eV), which is expected to decrease the adsorption strength of the Ru center and oxygen-containing intermediates and enhance the ORR activity. Moreover, Crystal Orbital Hamilton Population (COHP) analysis was employed to decipher the effect of axial adsorbed O on the desorption of the *OH intermediate48,49. According to the COHP curves (Supplementary Fig. 40) and Integrated Crystal Orbital Hamilton Population (ICOHP) results (Supplementary Table 13), the interaction between Ru and O (*OH) for RuN -OH and O-RuN -OH is primarily attributed to Ru 4d orbitals and O 2p orbitals. After regulating by the axial O, the Ru-O (*OH) interaction of O-RuN -OH performs a more positive ICOHP (-1.732) than that of RuN (-2.804), suggesting the ameliorative *OH desorption on the O-RuN site. In more detail, we investigate the interaction between five Ru 4d orbitals (d , d , d , d x2-y2, and d z2) and O 2p orbitals (Fig. 6c and Supplementary Table 14) of RuN -OH and O-RuN -OH. It is noteworthy that Ru 4d xy and Ru 4d x2-y2 are almost not involved in bonding with O 2p, and Ru 4d z2 plays a decisive role in the Ru-O (*OH) interaction. We discovered the disappearance of the pink peak near -5 eV for O-RuN , indicating that the bonding orbitals between Ru and O (*OH) are filled with fewer electrons, implying an accelerated desorption of *OH from O-RuN compared to RuN . The ICOHP value for the interaction between Ru 4d z2 and O (*OH) noticeably increases from -2.051 (RuN -OH) to -1.246 (O-RuN -OH), which further illustrates a weakened adsorption interaction between Ru and *OH for O-RuN . The underlying cause of this phenomenon is that the axial O ligand modulates the Ru 4d z2 orbitals, thereby reducing the interaction between the Ru 4d z2 and the O 2p (*OH) orbitals (Supplementary Fig. 41).

Furthermore, to clarify the mechanism for the boosted stability of the NC-Ru-950 catalyst, the strength of the Ru-N and Fe-N bonds was evaluated by COHP39. The COHP and ICOHP values of Ru-N , Ru-(N1, N2, N3 and N4), Fe-N , and Fe-(N1, N2, N3 and N4) were calculated average average and presented in Fig. 6d, Supplementary Fig. 42, Supplementary Tables 15, 16. The interaction of Ru-N is primarily attributed to the Ru 4d orbitals and the N 2p orbitals, which is also similar to Fe-N (Fe 3d and N 2p) (Supplementary Fig. 43). In contrast to Fe-N (-2.309), there is a notable enhancement in the interaction between Ru and N, as evidenced by a more negative ICOHP (-3.009), indicating that the stability of Ru-N is relatively higher than that of Fe-N. Besides, the simulated demetallation process of the Ru site and Fe site was depicted in Fig. 6e, indicating that Ru, along with the adsorbed O , transitions from a N -coordinated state to a N -coordinated state. Concurrently, the remaining two nitrogen atoms form N-H bonds with hydrogen ions derived from the electrolyte9. The demetallation barrier energy of the Ru site was predicted to be 1.27 eV, much higher than that of the Fe site (0.52 eV), which further suggests that NC-Ru-950 processes improved stability as opposed to Fe-NC. To further demonstrate the stability of the catalysts, ICP-OES and AC-HAADF-STEM characterizations of the aged NC-Ru-950 and Fe-NC catalysts were performed and analyzed (Fig. 6f-h). Under the harsh testing conditions, NC-Ru-950 exhibited minimal metal leaching (1.5%) and retained its atomically dispersed structure, whereas Fe-NC showed pronounced metal loss (17.9%) and the formation of visible nanoclusters. Taken together, these results underscore the robust stability of NC-Ru-950 under realistic operating conditions, highlighting its promise for durable fuel cell applications.

In summary, a hydrogen-embrittlement-inspired strategy has been proposed and validated as an effective approach to overcome the inherent aggregation tendency of 4d/5d transition metals, thereby enabling the synthesis of homogeneous single-atom site catalysts, including Ru-NC, Pd-NC, Pt-NC, and Au-NC. Using Ru as an example, H could penetrate into Ru clusters and facilitate their collapse into single-atom Ru sites. The experimental results and computational analysis show that as the temperature increases from room temperature to 950°C, the Ru species undergo a transformation from Ru(acac) to Ru species, and finally to Ru single atoms. The prepared NC-Ru-950 catalyst delivers high activity and stability in the tests of rotating ring-disk electrodes (E = 0.816 V, and 9 mV decaying after 30 000 cycles) and fuel cells (P = 670.9 mW cm-2, and 82.04% retention of peak power density after 30 000 cycles), making it competitive among the reported non-Fe and Pt-free catalysts currently. Our strategy makes an important contribution in the preparation of 4d/5d transition metal single-atom catalysts, providing a valuable perspective for the development of high-performance single-atom catalysts for energy conversion technologies.

Methods
Materials

Zinc nitrate hexahydrate (Zn(NO ) · 6H O, 99%), 2-methylimidazole (2-MeIm, ≥98%), Ruthenium(III) acetylacetonate (RuL(acac) , ≥97%), chloroplatinic acid (H PtCl , ≥99.9%), chloroauric acid (HAuCl , ≥98%), palladium chloride (PdCl , ≥99.9%), methanol (CH OH, ≥99.9%), isopropanol (C H OT, ≥99.9%), melamine (C H N , ≥99%), and perchloric acid (HClO , 70.0-72.0%) were purchased from Aladdin. ABTS (2, 2’-azinobis-(3-ethylbenzthiazoline-6-sulfonate)) reagent was bought from Innochem. Nafion (5 wt %) was purchased from Dupon. The commercial Pt/C (20 wt%) was purchased from Suzhou Sinero Technology Co., Ltd. The distilled deionized water (18.2 MΩ·cm) used in all experiments was sourced from an ultra-pure water system. All chemicals were employed directly without further purification.

Fabrication of catalysts

Preparation of ZIF-8 and the derived NC supports:

Firstly, solution A: 3.00 g Zn(NO ) ·6H O was dissolved in 40 mL methanol with stirring uniformly. Solution B: 6.30 g 2-methylimidazole (2-MeIm) was solubilized in 80 mL methanol and mixed homogeneously. Subsequently, solution A was quickly poured into solution B, and the mixture was stirred at room temperature for 24 h (the term “room temperature” throughout this paper refers to 25℃). And then, the samples were separated and washed (CH OH, three times) via centrifugation at 5000 rpm. After drying at 60°C for 6 h, the ZIF-8 precursors were obtained and heated at 1100℃ for 1 h under H /Ar. Ultimately, the carbon supports derived from ZIF-8 were designated as NC.

Preparation of NC-Ru-Ar and NC-Ru-H /Ar:

Firstly, a certain amount of Ru(acac) was uniformly dissolved into 20 mL isopropanol through sonication and stirring. Subsequently, 20 mg ZIF-8 derived NC supports were tardily added into the mixture, followed by ultrasonicating and stirring for 10 h. The product was collected by centrifugation (10 000 rpm, 3 min) and dried at 60°C for 6 h. Finally, the black powder was mixed with melamine at a 1:1 mass ratio through solid-state grinding and then annealed at 950℃ for 1 h under different atmospheres, yielding the samples named as NC-Ru-Ar and NC-Ru-H /Ar.

Preparation of NC-Ru-T:

Firstly, a certain amount of Ru(acac) was uniformly dissolved into 20 mL isopropanol through sonication and stirring. Subsequently, 20 mg ZIF-8 derived NC supports were tardily added into the mixture, followed by ultrasonicating and stirring for 10 h. The product was collected by centrifugation (10 000 rpm, 3 min) and dried at 60°C for 6 h. Finally, the black powder was mixed with melamine at a 1:1 mass ratio through solid-state grinding and then annealed under H /Ar atmosphere for 1 h at a series of temperatures, named as NC-Ru-T (T= 350℃, 550℃, 750℃, and 950℃). To synthesize the XC-72-Ru-T, XC-72 was used instead of the ZIF-8-derived NC. Unless otherwise specified, all samples were annealed under 10% H /Ar atmosphere.

Preparation of Pd-NC, Pt-NC, and Au-NC:

Pd-NC, Pt-NC, and Au-NC catalysts were synthesized using the same procedure as NC-Ru-950, except that the metal precursor was replaced with the corresponding metal salt.

Characterizations

The morphology and nanoparticle size distribution for N-doped carbon supports were analyzed using scanning electron microscopy (SEM) Merlin Compact (Carl Zeiss AG). More precise morphologies were obtained using transmission electron microscopy (TEM) and aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM) via the Tecnai G2 F30 (FEI Company) the Themis G2 300 (FEI Company), respectively. The crystal phase of catalysts was determined by the powder X-ray diffraction (XRD) via a Bruker D8 diffractometer with Cu K X-rays. The content of Ru was detected by an inductively coupled plasma optical emission spectrometer (ICP-OES, iCAP 7400). Raman spectra characterized the structure of carbon via the inVia-Reflex system at a wavelength of 532 nm. We used the Thermo Scientific K-Alpha to conduct X-ray photoelectron spectroscopy (XPS), carefully calibrating the binding energy data using C 1s (284.6 eV). Thermogravimetry Analysis (TGA) was obtained via Rigaku TG/DTA 8121. The N isothermal adsorption/desorption was collected by Quantachrome Autosorb IQ. Ultraviolet-visible absorption spectra were characterized by SPECORD 200PLUS UV-vis spectrophotometer.

Ru K-edge X-ray absorption spectroscopy (XAS) spectra were collected at beamline 12-BM-B at the Advanced Photon Source at Argonne National Laboratory using a double-crystal monochromator. The spectroscopic data were collected using a focused beam in fluorescence mode owing to the low concentration of the element of interest within the sample pellet. Fluorescence detector: Canberra 13-element Ge detector. Ru foil was adopted as a standard sample. The collected data were performed with the Demeter software package, including Athena, Artemis, and Hephaestus.

In situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) was performed on a Nicolet 6700 Ispectrometer equipped with the MCT detector. The catalysts were coated onto a silicon wafer working electrode coated with a thin layer of gold nanoparticles. The electrochemical tests were conducted in O -saturated 0.1 M HClO , adopting the Hg/Hg SO electrode as RE. The potential mentioned in this work was referenced to a reversible hydrogen electrode (RHE) according to the Nernst equation:

E = E + 0.656 + 0.059*pH (1)
(vs. RHE) (vs. Hg/Hg SO )

Electrochemical measurements

The electrocatalytic performance of catalysts was assessed using the rotating ring-disk electrode (RRDE) measurements (CHI760e, Shanghai Chenhua Instrument, China) in a three-electrode system within 0.1 M HClO at room temperature. The 0.1 M HClO electrolyte was prepared fresh prior to each experiment by diluting concentrated high-purity HClO with deionized water, and it was stored in a sealed, dark glass container at room temperature (pH = 1.00 ± 0.01). In this configuration, the catalyst-coated glassy-carbon electrode functioned as the working electrode (WE), a home-made RHE served as the reference electrode (RE), and the carbon rod acted as the counter electrode (CE). Firstly, 2.5 mg catalysts were dispersed in 500 μL mixed solution with a volume ratio of 5:5:0.3 (DI-water: ethanol: 5 wt% Nafion solution) and ultrasonicated for 1 hour. Subsequently, a specific volume of the ink was dropwise added to a glassy-carbon electrode (GCE, with an inner diameter of 5.61 mm and an area of 0.2472 cm2). The mass loading of as-prepared catalysts was 0.6 mg cm-2, and the loading of Pt for the Pt/C was 40 μg cm-2. Finally, the working electrode was subjected to drying at room temperature.

The specific testing methods are as follows: the catalysts were first activated through cyclic voltammetry (CV). The electrocatalytic performance for the oxygen reduction reaction (ORR) was then evaluated using steady-state staircase voltammetry (SCV). The test techniques were as follows: a 30-second quiet time, potential range of 1.05 to 0.05 V, voltage increment of 50 mV, step duration of 30 seconds, and a rotation speed of 900 rpm. Chronoamperometric (I-t) measurements were performed at 0.6 V in O -saturated 0.1 M HClO with a rotation rate of 900 rpm for 300 hours. To assess the stability of the catalysts, the potential was cycled between 0.6 and 0.95 V (vs. RHE) at a scan rate of 100 mV s-1 in O -saturated 0.1 M HClO .

The H O yield, electron-transfer number (n) , the kinetic current density (j ) at 0.85 V and Tafel slope were determined using the following equations:

%HEO =200×
Ir ⁄N
(2)
Id+Ir ⁄N

n=
4Id
(3)
Id+Ir ⁄N

1 1 1
= + (4)
j jk jd

η=a + b log |j | (5)
k

Notes: I and I represent the ring and the disk current, respectively. N denotes the ring current collection efficiency (37%). j, j , and j refer to the measured, kinetic, and diffusion-limiting current densities, respectively. η indicates the overpotential.

Fuel cell tests

The Scribner 850e fuel cell test system was used to evaluate the performance of various catalysts in PEMFCs. A membrane electrode assembly (MEA) with a geometric area of 5 cm2 was fabricated using the Gas Diffusion Electrode method (GDE). The catalyst ink was prepared by mixing catalyst powder with a Nafion ionomer solution and a blend of isopropanol and water, followed by ultrasonicating the mixture for 1 hour. For the cathode NC-Ru-950 and Fe-NC catalysts, the solid content was 6%, with a Nafion ionomer-to-catalyst ratio of 0.7. The catalyst ink (NC-Ru-950 and Fe-NC) was then uniformly applied onto a gas diffusion layer (Toray, XGL-R-055) using a brush. For the anode Pt/C, the solid content was 1%, with a Nafion ionomer-to-catalyst ratio of 0.7. The Pt/C catalyst was sprayed onto the Gore membrane (12 μm, 3.5 cm*3.5 cm, used as received from commercial source without treatment.) using the Catalyst Coated Membrane approach (CCM). The catalyst loading on the cathode for NC-Ru-950 and Fe-NC was 3.5 mg cm-2, while the anode Pt loading was 0.1 mg cm-2. The tests were conducted at 75°C, 65% relative humidity (RH), and 1 bar back pressure for both H /O and H /Air. Polarization and power density curves for all catalysts were obtained under H (500 sccm)/O (2000 sccm) and H (500 sccm)/Air (2000 sccm) conditions. Durability tests were carried out in H (200 sccm)/N (75 sccm) using a square wave cycling protocol (0.6 V for 3 s, 0.95 V for 3 s, 75°C, 65% RH, and 0 bar).

Electrochemical impedance spectroscopy (EIS) tests were performed at H /Air operating conditions within a frequency range of 105 to 10-1 Hz at 0.65 V. The EIS was analyzed using the distribution of relaxation times (DRT). The DRT function was calculated via the Python software DRTtools developed by the Ciucci group33, 50.

DFT calculations

The density functional theory (DFT) calculations were performed using the Vienna Ab Initio Simulation Package (VASP). The Projector Augmented Wave (PAW) pseudopotential was applied to describe the core electrons. For the exchange-correlation energy, the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) was employed. In the case of DFT+U, the U values for TM were taken from the literature51. Meanwhile, this work incorporates implicit solvation models and constant-potential calculations to simulate the electrocatalytic interface52-55. To prevent interactions between periodic structures, a vacuum layer of 15 Å was added in the z-direction. The energy cut-off was set as 400 eV to expand the wavefunction. The Brillouin zone was sampled with a 4 × 4 × 1 k-point mesh based on the Monkhorst-Pack scheme. In the self-consistent-field (SCF) calculations, the convergence criterion for energy was set to 10-6 eV, and the maximum force on any atom was limited to 0.01 eV Å-1. Transition states were identified using the climbing image nudged elastic band (CI-NEB) method, followed by vibrational analysis to confirm the results.

The Gibbs free energy (G) is determined by the following equations:

∆G = ∆E + ∆ZPE ‒ T∆S + ∆G + ∆G (6)
U pH

∆G = ‒ eU (7)
U

Notes: ∆E and ∆ZPE represent the reaction energy change and the change of the zero-point energy (ZPE), respectively. ∆S indicates the difference in entropy between reactants and products at 298.15 K. ∆G refers to the contribution from the applied potential, where e denotes the transferred charge, and U represents the applied potential. The contribution of pH is ignored, and ∆G is neglected in this study.

Implementation of the implicit solvation model:

The solvent environment was simulated using the VASPsol code52,53, which implements a self-consistent implicit electrolyte model within the plane-wave DFT framework. The relative permittivity was set to 78.4 to model the aqueous electrolyte. The effective surface tension parameter was assigned to 0 in VASPsol to neglect the cavitation energy contribution. The linearized Poisson-Boltzmann model with a Debye length of 3.04 Å mimics the compensating charge.

Implementation of constant-potential calculations:

The constant-potential calculations in this work were performed using a grand canonical density functional theory (GC-DFT) framework, as implemented in the CP-VASP code54,55. This approach allows the electron number (n) in the system to vary to maintain a constant electrode potential relative to the standard hydrogen electrode (SHE). As for the grand-canonical ensemble of electronic charges, the electrons should be coupled with a fictitious potentiostat so that the number of electrons fluctuates following the grand-canonical distribution at the preset electrode potential (Φ ). For simplicity, we adjust the electron number according to:

∆n = C (Φ − Φ) (8)

where ∆n is the electron number correction for a given structure and electron number, Φ is the electron potential; C is the “capacitance” parameter that is chosen to be the capacitance of the initial state. Note that the net electronic charges are balanced by the ionic charges in the implicit electrolyte, and thus the total system remains charge neutral. The convergence threshold for the potential is 0.01.

For a given structure and electron number, the electron potential Φ with respect to the stand hydrogen electrode (SHE) can be obtained as:

Φ = [E − E − (E SHE − E SHE)]/e (9)
F es F es

where E is the Fermi level of the system, E is the electrostatic energy in the middle of implicit electrolyte region, and E SHE and E SHE are the corresponding quantities in SHE. E SHE – E SHE is benchmarked to be 4.6 eV for VASPsol52.

It should be noted that these calculations, while providing valuable insights, are based on simplified models and inherent approximations (e.g., in the exchange-correlation functional). The results should therefore be interpreted as providing qualitative trends and mechanistic hypotheses rather than quantitative analyses of the exact behavior under realistic catalytic conditions.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I have read this manuscript by Guo et al who reported the synthesis of atomically dispersed Ru atoms in a Nitrogen-carbon matrix and applied the catalyst for the oxygen reduction reaction. The electrochemical stability is very impressive as well as the characterization of the catalyst at different temperatures by using XAS and EXAFS. Unfortunately, the theory part contains a series of inconsistencies preventing publication in Nature Communications. The authors should reconsider the following aspects.\n- Single-site catalyst are heterogeneous catalysts but with features of homogeneous catalysis. It is not meaningful to discuss d-band center, as there is no band, while orbitals.\n- In single-site catalysis self-interaction error should be in general accounted. I recommend the authors to (at least) perform some tests with hybrid functionals or DFT+U approach to check for the reliability of the results [10.1021/acs.jpcc.8b09430; 10.1039/D4CP00175C].\n- I suggest removing this sentence. “The ΔGpH is an important parameter that indicates how changes in H+ concentration influence free energy. However, numerous studies have demonstrated that pH has no impact on the rate-determining step, as the free energy of the elementary reactions remains independent of pH.” There are a series of studies with opposite results. The authors should state that they neglect the contribution only.\n- The origin of the calculated Pourbaix diagrams is not clear. Have the authors approximated the stability to the bulk Ru or have they explicitly considered the Ru-SAC as reported recently? [10.1021/acscatal.3c04801]\n- Based on Figure S30, it appears that the “poisoning” of the catalyst with O-species implies a decrease of thermodynamic barriers with respect to RuN4. Have the authors considered the free energy to form O* in FeN4 in working electrochemical conditions?\n- Related to the previous point, how do the FeN4 and O-FeN4 free energy profile compare with the observed polarization curves and tafel slopes?", "reviewer_2": "In this work, a novel hydrogen-embrittlement-inspired thermal activation strategy for synthesizing atomically dispersed metal catalysts of 4d/5d metals is demonstrated. This method addresses the significant challenge of stabilizing high-cohesive-energy metals like Ru, Pd, Pt, and Au as single atoms by utilizing H₂ permeation to weaken metal-metal bonds in clusters during high-temperature treatment, facilitating their dissociation into individual atoms anchored by nitrogen sites on a carbon support. Using Ru as a primary example, the structural evolution from clusters to isolated RuN₄ sites was comprehensively studied through ex-situ microscopy and spectroscopy. The resulting Ru-N-C catalyst exhibits exceptional oxygen reduction reaction (ORR) activity and stability in both half-cell measurements and proton exchange membrane fuel cells (PEMFCs), achieving performance that meets DOE targets and surpasses state-of-the-art Fe-N-C catalysts. The universality of the strategy is successfully validated by extending it to other noble metals. Combined with detailed DFT calculations, the work provides fundamental insights into the formation mechanism and the enhanced catalytic properties of the active sites. However, this paper could not be published in the current state and some issues need to be properly addressed.\n1. It is suggested to supplement the TEM image of NC-Ru-RT to further confirm the existence form of Ru.\n2. The signal-to-noise ratio of the Ru 3p XPS spectrum for NC-Ru-Ar is relatively low, which may affect the accuracy of the analysis. It is recommended to retest this sample.\n3. The difference in pore width shown in Figure S13b is not clearly distinguishable. Please redraw the figure to improve clarity.\n4. Regarding the transformation of part of Ru³⁺ to Ru⁰ and finally to Ruδ⁺ in Figure S14b, further explanation of the underlying mechanism is needed. Additional DFT calculations simulating these valence states are suggested to support the discussion.\n5. There appears to be a citation error in the manuscript. The reference in line 202 should point to \"Figure 2i\" instead of \"Figure 3i.\" Please also verify the citations in lines 218, 223, and 225.\n6. The labels \"a/b/c:10k\" in Figure 4f seem to be incorrect. Please check and correct as necessary.\n7. The mass ratio of Ru in the composites is less than 1 wt%, which is relatively low compared to some MOF-derived single-atom catalysts (which can reach up to 40 wt%). Are there any strategies to increase the metal loading? Please provide the corresponding ICP results for all samples in the supporting information.\n8. The synthesis of atomically dispersed catalysts via the thermal treatment of MOF precursors under a H₂ atmosphere is a well-established and widely reported methodology in the literature. Therefore, the novelty of the methos of the synthesized Atomically dispersed metal catalysts is limited.", "reviewer_3": "This paper proposes a thermal activation strategy inspired by hydrogen embrittlement for synthesizing atomically dispersed metal catalysts of 4d/5d metals. The study demonstrates that hydrogen molecule permeation weakens metal-metal cohesion, facilitating the transformation of metal clusters into M-N4 single-atom sites, with Ru serving as a model system for detailed characterization and performance testing. The aspect lies in the creative application of materials science principles (hydrogen embrittlement) to atomic-scale catalyst preparation, offering an interdisciplinary research perspective. The experimental design is systematic, integrating in situ characterization, theoretical calculations, and performance tests. However, there is room for improvement in the depth of mechanistic explanation and further data analysis. The detailed comments are shown below:\n1.The authors propose that hydrogen molecules permeate into metal clusters and weaken metal-metal bonds, but direct evidence of the dynamic hydrogen permeation process at high temperatures is lacking. Could quantitative data on hydrogen concentration and cluster decomposition rates be provided? Additionally, while the successful extension of the strategy to Pd, Pt, and Au is demonstrated, the rationale for selecting these metals is not deeply discussed. Are there specific criteria (e.g., hydrogen solubility, metal-N bond strength) determining whether a metal can be atomically dispersed via this method? For instance, why were Rh, Ir, or Ag not tested? A more systematic discussion on the limitations would strengthen the claim of universality.\n2.The DFT calculations reveal electronic structure changes in Ru-N4 sites but do not explicitly derive the specific energy barriers for hydrogen permeation. It is recommended to supplement with energy barrier calculations for hydrogen diffusion paths and correlate them with experimental activation energies to establish a quantitative theory-experiment relationship.\n3.Although AC-HAADF-STEM images confirm the presence of Ru single atoms, statistical results from large areas (e.g., atomic count distribution or cluster size statistics) are not provided. It is advised to include image analyses from at least three different regions and calculate a uniformity index for atomic dispersion.\n4.The performance of Ru–N–C is primarily compared to Fe–N–C and a few Pt-based catalysts. However, recent studies have reported Ru-based SACs with comparable or superior activity. A more comprehensive benchmarking table including current density, stability, and other performance of recent Ru, Pd, and Pt SACs would better contextualize the advancement claimed in this work.\n5.Although the catalyst exhibits excellent stability over 30,000 cycles, post-mortem analyses (e.g., AC-HAADF-STEM, ICP-OES) after testing are not provided. Evidence of retained atomic dispersion and minimal metal leaching after aging would solidify the stability claims.\n6.Key experimental details are missing, such as error margins in performance metrics (e.g., current density, power density). Including statistical data (e.g., standard deviations from multiple tests) would enhance reliability.\n7.There is an absence of in situ or operando characterization during the ORR. The study relies heavily on ex situ characterizations. In situ XAFS or Raman spectroscopy during ORR could provide direct evidence of the active site structure and reaction intermediates, strengthening the mechanistic conclusions.\n8.The DFT models (e.g., O-RuN4) are constructed based on EXAFS fitting but do not account for actual reaction conditions (e.g., electrolyte effects, applied potential). It is suggested to incorporate implicit solvation models or constant-potential DFT calculations to simulate the electrocatalytic interface more realistically and calibrate with experimental free energy diagrams.\n9.In the “Methods” section, for the preparation of NC-Ru-Ar and NC-Ru-H2/Ar catalysts, it is mentioned that “the black powder was mixed with melamine through solid-state grinding.” Please specify the exact mass ratio of melamine to the material. This ratio likely influences the nitrogen doping level during thermal treatment and consequently the final catalyst performance, making this detail crucial for the reproducibility of the experiments."}, "author_response": "Response to the reviewers' comments on the Manuscript ID: NCOMMS-25-70659-T\nentitled: Synthesis of atomically dispersed metal catalysts via hydrogen\nembrittlement-like assisted thermal activation strategy\nWe thank the reviewers for their constructive and insightful comments. In response, we\nhave studied all the comments and revised the manuscript carefully. Here we present a\ndetailed answer to the review. Our replies are highlighted in blue, while the red text\nsections indicate changes to the manuscript.\nREVIEWER COMMENTS\nReviewer #1\nI have read this manuscript by Guo et al who reported the synthesis of atomically\ndispersed Ru atoms in a Nitrogen-carbon matrix and applied the catalyst for the oxygen\nreduction reaction. The electrochemical stability is very impressive as well as the\ncharacterization of the catalyst at different temperatures by using XAS and EXAFS.\nUnfortunately, the theory part contains a series of inconsistencies preventing\npublication in Nature Communications. The authors should reconsider the following\naspects.\nReply: We are sincerely grateful to the reviewer for the highly positive evaluation of\nour work. We particularly appreciate the recognition of our synthesis strategy for\natomically dispersed Ru active sites, the impressive electrochemical stability, and the\ndetailed XAS/EXAFS characterization of catalysts at different temperatures.\nWe have taken the reviewer’s serious concerns regarding the theoretical part very\nseriously. We fully acknowledge the importance of ensuring consistency and clarity in\nthis part of the manuscript. In this revision, we have thoroughly re-examined and\nrevised the theoretical calculations. Specifically, we employed the more accurate\nDFT+U method, combined with an implicit solvation model and constant-potential\ncalculations. The catalytic reaction pathways (including surface thermodynamic phase\ndiagrams and reaction free-energy diagrams) and the electronic structure of the active\n1\nsites (such as DOS, PDOS, COHP, and ICOHP) were systematically recalculated and\nanalyzed. We have provided point-by-point responses and clarifications below to\nresolve these inconsistencies. We believe the revised theoretical section now aligns\nperfectly with our experimental findings and provides a solid basis for the reported\nmechanisms.\nOnce again, we appreciate the insightful and professional comments provided by\nReviewer #1, which have significantly improved the quality of our manuscript. We hope\nthat the revisions satisfactorily address all concerns.\nQuestion 1. Single-site catalyst are heterogeneous catalysts but with features of\nhomogeneous catalysis. It is not meaningful to discuss d-band center, as there is no\nband, while orbitals.\nAnswer 1: We sincerely appreciate the reviewer’s insightful comment. We fully agree\nthat single-site catalysts, featuring isolated metal centers, exhibit discrete electronic\nstates, which are fundamentally different from the continuous band structure observed\nin bulk metals. However, in this context of our discussion, the term “d-band center”\nrepresents the calculated centroid of the projected density of states (PDOS) of the single\nmetal atom d-orbitals. Mathematically, it represents the first moment of the d-states\ndistribution relative to the Fermi level, which has been widely established in the\ncommunity as an effective descriptor for single-atom catalysts, especially for M-N-C\ncatalysts (Nature Catalysis, Nature Communications, Journal of the American\nChemical Society, Angewandte Chemie International Edition, Advanced Materials,\nChem, and et.al).\nIn the field of single-atom catalysis, the d-band center has been widely adopted as\na conceptual descriptor to correlate the local electronic structure of the metal active\ncenter with the adsorption strength of reaction intermediates. For example, Professor\nEdward H. Sargent and his team demonstrated that the epoxy groups on carbon\nnanotubes induce electron delocalization at the Co center. This effect downshifts the\nd‑band center of Co, thereby weakening the adsorption strength of oxygen‑containing\nintermediates on the Co single‑sites (Nat Catal 6, 234-243 (2023)., Figure R1a));\nspin regulation can promote the desorption of *CO from the active centers (J. Am.\nChem. Soc. 2025, 147, 46, 42539-42548, Figure R1b); Chunyu Du et.al reported that\nMpOH moieties could modulate the electronic structure of single-atom Co site, thereby\ntuning its d-band center, which consequently regulated the adsorption strength between\nthe Co site and reaction intermediates (Nat Commun 14, 4766 (2023), Figure R1c);\nHanqing Yu et.al discussed the correlation between the d-band center of transition metal\n(Cu, Ni, Co, and Fe) single-atom catalysts and the peroxymonosulfate (PMS)\nadsorption energy, and identified the d-band center of active site as the key driver for\npolymerization transfer of pollutants (Nat Commun 15, 2327 (2024), Figure R1d).\n[FIGURE REDACTED]\nFigure R1. (a) PDOS of the CoPc molecule (pink), Co d bands (blue); (b) PDOS of HS Fe(II)Pc–\nand LS O-Fe(II)Pc–; the energy of the Fermi level is set to 0 eV, and the dashed line represents the\nposition of the d-band center; (c) PDOS and corresponding calculated d-band centers of Co 3d for\nd-CoMpOH and s-CoVac models; (d) PDOS of TM (Cu, Ni, Co, and Fe) 3d in TM-SA/PN-g-C N\n3 4\nsystems.\nconvention to provide a qualitative understanding of the adsorption-desorption\nbehavior of intermediates on the Ru site. To avoid any conceptual confusion, we have\nclarified in the revised manuscript that we are referring to the “centroid of the d-orbital\nstates” when discussing the d-band center.\nIn the revised manuscript: Meanwhile, in comparison to RuN , O-RuN presents a\n4 4\nnoticeable downshifted d-band center (centroid of the d-orbital states, ε = - 2.916 eV)\nd\nand a widened d-orbital width (W = 5.301 eV), which is expected to decrease the\nd\nadsorption strength of the Ru center and oxygen-containing intermediates and enhance\nthe ORR activity.\nQuestion 2. In single-site catalysis self-interaction error should be in general accounted.\nI recommend the authors to (at least) perform some tests with hybrid functionals or\nDFT+U approach to check for the reliability of the results [10.1021/acs.jpcc.8b09430;\n10.1039/D4CP00175C].\nAnswer 2: Thanks for your insightful and professional comment. We have adopted the\nreviewer's recommendation by performing comprehensive tests utilizing the DFT+U\napproach. Furthermore, recognizing the complexity of electrochemical reactions during\nthe ORR process, we have extended our validation by incorporating an implicit\nsolvation model and constant potential calculation. The catalytic reaction pathways\n(including surface thermodynamic phase diagrams and reaction free-energy diagrams)\nand the electronic structure of the active sites (such as DOS, PDOS, COHP, and ICOHP)\nwere systematically recalculated and analyzed.\nAfter conducting these additional tests, we found that the results were consistent\nwith the conclusions drawn in the original manuscript, which further supports the\nreliability of the data presented. We have updated the manuscript and the supporting\ninformation to reflect these new calculations, and we believe that these modifications\nwould enhance the reliability of our findings (Figure R2-R10, Table R1-R4).\nIn the revised manuscript and supplementary information: In the case of DFT+U,\nthe U values for TM were taken from the literature[51]. Meanwhile, this work\n4\nincorporates implicit solvation models and constant-potential calculations to simulate\nthe electrocatalytic interface.\nFigure R2 (Figure 6). Fundamental understanding of the excellent electrocatalytic activity and\nstability of NC-Ru-950 based on DFT calculations. (a) The surface thermodynamic phase\ndiagram of NC-Ru-950. (b) In situ ATR-SEIRAS spectra of NC-Ru-950 during the ORR process.\n(c) The project COHP analyses between Ru 4d and O 2p (*OH) for RuN -OH and O-RuN -OH. (d)\n4 4\nCOHP analyses between M (M: Fe/Ru) and N for FeN and O-RuN . (e) The simulated\n4 4\ndemetallation process of the Ru site and the Fe site. (f) The demetallation energy for FeN and O-\n5\na\nR 3\n2\n)Ve\n1 (\nG∆\n0\n-1\n0 .0\nd\n5\n)V\n0 e(\nygrenE\n-5\n-1 0\n-3\nf\n3\n2 )Ve(\nygrenE\n1\n0\nuN 4\n0 .2\ntotal\nd p 3 -2\n-2 -1 0\n-C O H\nN C\n0 .4 0 .6 0 .8\nU (V )\nRHE\nFe-N ave\nIC O H P\n=-2.309\ntotal\nd p 4 -2\n-3 -2 1 -1 2 0 3\nP -C O H\ndem etallation energy\nm etal leaching\n-R u -9 5 0 F e -N\n*O O H *O\n*O H *H slab\n0.65 V\n1 .0\nO -R u-N ave\nIC O H P\n=-3.009\n1 2 3\nP\nC\n2\n2\n1\n1\n5\n0\nb\n5\n0\n5\n0\n).u.a(\nnoissimsnarT\ne\ng\n)%( \ngnihcael\nlateM\n*O\n8 0 0 1 0 0\nF e N 4\nO -R u N\n1 n m\n4\n*O O H *O H\n1 2 0 0 1 4 0 0 1 6 0 0\n-1 a ve n u m b e r (n m )\nN C\n1 8\n-R u-950\n0.6 V 0.7 V\n0.8 V\n0.9 V\n1.0 V\n1.1 V\nO C V\n0 0 2 0 0\n+\n+\nc\n5\nd p Ru4 -O2 yzd p Ru4 -O2 z2d p Ru4 -O2 xz )V\n0 e( ygrenE\n-5\nHO-4NuR-O\n-1 0\nd e m e ta lla tio n\nR u\nd e m e ta lla tio n\nh\n1 n m\npu nips\nHO-4NuR-O\nnwod\nnips\nHO-4NuR\npu nips\nHO-4NuR\nnwod\nnips\nFigure R3 (Figure S36). The surface thermodynamic phase diagram of FeN (a) and Gibbs free\n4\nenergy diagram of O-RuN (b), RuN (c), and FeN (d).\n4 4 4\nFigure R4 (Figure S37). The electronic localization functions (ELF) of RuN 4 and O-\nRuN .\n4\n6\n)Ve(\nygrene\neerF\n)Ve(\nG∆\nc\na\n4\n3\n2\n1\n0\n-1\n0\n6\n5\n4\n3\n2\n1\n0\n-1\n-2\nF i g\nF\n.0\nR\nu\ne N\nu N\nr\n4\n4\ne\n.2\n3\n*O O\nR\nG 0\ne\n.4\nU\nR\na c tio\ni b b\n0 .6\n(V )\nH E\n*O\nn p a th\ns f r e\n*O O *O\n*O H\n*H\ns la b\n0 .6 5\n0 .8\n*O H\nw a y\ne e n e\nH\nV\nU\nU\nU\nr g\n===\n1 .0\n0 .5 4\n1 .2 3\ny d\nV\nV\ni a\n)Ve(\nygrene\neerF\n)Ve(\nygrene\neerF\ng\nb\nd\nr a\n6\n5\n4\n3\n2\n1\n0\n-1\n-2\n6\n5\n4\n3\n2\n1\n0\n-1\n-2\nm\nO -R\nF e N\no f\nu\nR\n4\nu N\n*O O\nR\n*O O\nR\n,\ne a\nH\ne a\nO\nc\nc\ntio\ntio\n- R\n*O\nn p\n*O\nn p\nu\na\na\nN\n*O H\nth w a y\n*O H\nth w a y\n,\na\n4\nn d\nU U\nU\nU\nU\nU\nF\n= 0 = 0\n= 1\n= 0\n= 0\n= 1\ne\nV.9.2\nV.7.2\nN\n63\nV\nV\nV\n4\n.\nFigure R5 (Figure S38). The differential charge density distribution between Ru and O for O-RuN .\n4 4\nthe d-band center and d-band width of the Ru site) and the projected density of state of the Ru site\nfor (b) RuN and (c) O-RuN .\n4 4\n( a\nb\n).u.a(\netats\nfo\nytisneD\ni g\n)\nR u N 1 0\n5\n0\n-5\n-1 0\n-1 0\nu r e S 3\nR u N a\n4\n4\nd\nd\nd\nd\nd\n9\nn\nxy\nyz\n2z\nxz\n2 2 x -y\nT\nd\nh(\nb\n-5\nE\ne p\n) O\na\n).u.a(\netats\nfo\nytisneD\nn e\nr o-\nR\n1 2\n8\n4\n0\n-4\n-8\n-1 2\nrg y\nj eu\n(e\nc t\nN\n-1\nd e\n.\n4\n5\nn s\nS\ni\np\nt\n-1 0\nE n\nS p in u p\nin d o w n\n5\ny o f\ne\ns\n-5\nrg y (e V )\n1 0\n).u\n5 .a(\netats\n0\nfo\nytis\n-5 neD\n-1 0\n-1\nt a t e o f\nR u N\nO -R\nd xy\nd yz\nd 2z\nd xz\nd 2 2 x -y\ns i t e\n4u\nN\n4\n-5\nf o\nE\nr\nn e rg y\nT\nf o\ns h\nb a\n0\n(e V )\nh e d\nr R u\no w s\nn d w\nS\nS p in\ne n s i t y\nN a n\n4\nt h e d\ni d t h\np in u p\nd o w n\n5\no f s t\nd O - R\n- b a n d\no f t h e\na tucR e o f t h\nNe\nn t e r\nu s i t e\ne\ne n\n)\nR\nd\nu s i t e\ne\nF e\nd\ne\nS\nS\np in u p\np in d o w n\nd\ne =d -0 .7 5 1 w =d 4 .5 5 6\ne =d -2 .9 1 6 w =d 5 .3 0 1\nc\nFigure R7 (Figure S40). The COHP between Ru and O (*OH) for (a) RuN -OH and (b) O-RuN -OH.\n4 4\nFigure R8 (Figure S41). The pDOS of Ru 4d and O 2p for (a) RuN 4 -OH and (b) O-RuN 4 -OH.\n8\na\n)Ve(\nygrenE\n5\n0\n-5\n-1 0\nFa\ns p in u p\nto ta l\ns s 5 -2\ns p 5 -2\nd s 4 -2\nd p 4 -2\n-3 -2 -1 0\n-C O\ni g u r e S 3\nn d ( b ) O\nH\n4-\nR u N -O 4 5\n0\n-5\n-1 0\n1 2\nP\nT h e\nR u N\nH\ns p\n-3\nO\nH\nin\nH\nd o w n\nto ta l\ns s 5 -2\ns p 5 -2\nd s 4 -2\nd p 4 -2\n-2 -1\n-C\nP b\ne\nH\nt\nR\nw\nu N\n2\ne\n-O H 4\n3\ne n R u\nb\n5\n)V\n0 e(\nygrenE\n-5\n-1 0\na n\ns p in u p\nto ta l\ns s 5 -2\ns p 5 -2\nd s 4 -2\nd p 4 -2\n-3 -2 -1\n-C\nd O ( * O\nO\nH\n-R u N5\n0\n-5\n-1 0\n) f o\n-O 4\n2\nr\ns H\n-3 3\n( a )\np in d o w n\nto ta l\ns s 5 -2\ns p 5 -2\nd s 4 -2\nd p 4 -2\n-2 -1 0\n-C O\nR u N -\nH\nO\n-R\nH\nu N -O 4\n4\n(b) O-RuN -OH.\n4\n).u.a(\netats\nfo\nytisneD\n5\n4\n3\n2\n1\n0\n-1\n-2\n-3\n-4\n-5-1\ns p\ns p\n0\nin\nin\nu\nd\np\no w n\n-5\nE n e rg y (e V\n0\n)\nR u N -O\n4\nd\nd\nd\n2\nyz\n2z\nxzp\n).u.a(\netats\nfo\nytisneD\n5\n4\n3\n2\n1\n0\n-1\n-2\n-3\n-4\n-5-1\ns p\ns p\n0\nin\nin\nu\nd\np\no w n\n-5\nE n e rg y (e V\n0\n)\nO -R\nd\nd\nd\n2\nu N\nyz\n2z\nxzp\n-O\n4\na b\nFigure R9 (Figure S42). The COHP between Ru and N for NC-Ru-950 (a, c) and the\nCOHP between Fe and N for Fe-NC (b, d).\nc\n6\n4\n2\n)V\n0 e(\nyg\n-2\nrenE\n-4\n-6\n-8\n-1 0\nd\n6\n4\n2\n)V\n0 e(\nyg\n-2\nren\n-4 E\n-6\n-8\n-1 0\nS 4\n-3\n-3\n2\na\nto ta l\ns s 5 -2\ns p 5 -2\nd s 4 -2\nd p 4 -2\n-2 -1 0\n-C O H\nto ta l\ns s 4 -2\ns p 4 -2\nd s 3 -2\nd p 3 -2\n-2 -1 0\n-C O H\nT h e C O\nO\nH\n-R u -N 1\n2 3\nF e -N 1\n2 3\nP b e\n1\n6\n4\n2\n)V\n0 e(\nyg\n-2\nren\n-4 E\n-6\n-8\n-1 0\n-3\n6\n4\n2\n)V\n0 e(\nyg\n-2\nren\n-4 E\n-6\n-8\n-1 0\n-3\nt w e e\n4\n2\nto ta l\ns s 5 -2\ns p 5 -2\nd s 4 -2\nd p 4 -2\n-2 -1 0\n-C O H\nto ta l\ns s 4 -2\ns p 4 -2\nd s 3 -2\nd p 3 -2\n-2 -1 0\n-C O H\nn R u a\nO -R u -N\n1 2\nP\nF e -N\n1 2\nP\nn d N\n2\n3\n2\n3\nf o\nb\n6\n4\n2\n)V\n0 e(\nyg\n-2\nren\n-4 E\n-6\n-8\n-1 0\n6\n4\n2\n)V\n0 e(\nyg\n-2\nren\n-4 E\n-6\n-8\n-1 0\nr N\n-3\n-3\nC\n-2\n-2\n- R\n3\n1\nto ta l\ns s 5 -2\ns p 5 -2\nd s 4 -2\nd p 4 -2\n-1 0\n-C O H\nto ta l\ns s 4 -2\ns p 4 -2\nd s 3 -2\nd p 3 -2\n-1 0\n-C O H\nu - 9 5\nF e\nO -R u\n1 2\nP\nF e\n1 2\nP\n0 ( a\n4\n2\n-N 3\n3\n-N 3\n3\n, c )\n)Ve(\nygrenE\n)Ve(\nygrenE\na\n6\n4\n2\n0\n-2\n-4\n-6\n-8\n-1 0\n6\n4\n2\n0\n-2\n-4\n-6\n-8\n-1 0\nn\n-3\n-3\nd\n-2\n-2\nt h\nto ta l\ns s 5 -2\ns p 5 -2\nd s 4 -2\nd p 4 -2\n-1 0\n-C O H\nto ta l\ns s 4 -2\ns p 4 -2\nd s 3 -2\nd p 3 -2\n-1 0\n-C O H\ne C O\nO\nH\n-R\nF\nP\nu -N\n2\ne -N\n2\nb\n4\n3\n4\n3\ne t w e e n F e a n d N f o r\nFigure R10 (Figure S43). The different orbital interaction proportions between Ru/Fe\nand N for O-Ru-N and Fe-N.\nTable R1 (Table S13). The ICOHP value between Ru and O (*OH) for RuN -OH and\n5s-2s -0.364 -0.366 -0.730\n5s-2p -0.097 -0.094 -0.192\n4d-2s -0.355 -0.385 -0.741\n4d-2p -1.096 -1.708 -2.804\nO-RuN -OH Spin up Spin down Total\n5s-2s -0.314 -0.312 -0.626\n5s-2p -0.067 -0.065 -0.132\n4d-2s -0.237 -0.245 -0.482\n4d-2p -0.727 -1.005 -1.732\nN\ne S\nf o\n4\nr\nT\n-\nhR eu d-\nN\n)%( \negatnecreP\ni f\n1 0\n9\n8\n7\n6\n5\n4\n3\n2\n1\nf e\na n\n0\n0\n0\n0\n0\n0\n0\n0\n0\nr ed n\nF\nte o- rN b i. t a l i n t e r a c t i o\nO -R u -N (a v e ra g\nF e -N (a v e ra g e )\nn p r o p o\ne\nr\n)\nt i o n s b e t w e e n R u / F e\n5s-2s 5s-2p 4d-2s 4d-2p 4s-2s 4s-2p 3d-2s 3d-2p\nTable R2 (Table S14). The ICOHP value between Ru 4d and O 2p (*OH) for RuN -OH and\n4\n4d -2p -0.000 -0.000 -0.000\nxy\n4d -2p -0.022 -0.577 -0.599\nyz\n4d z2-2p -1.039 -1.012 -2.051\n4d -2p -0.033 -0.119 -0.153\nxz\n4d x2-y2-2p -0.000 -0.000 -0.000\nO-RuN -OH Spin up Spin down Total\n4\n4d -2p -0.000 -0.000 -0.000\nxy\n4d -2p -0.024 -0.268 -0.293\nyz\n4d z2-2p -0.627 -0.618 -1.246\n4d -2p -0.079 -0.123 -0.202\nxz\n4d x2-y2-2p -0.000 -0.000 -0.000\nTable R3 (Table S15). The total ICOHP between Ru/Fe and N for NC-Ru-950 and Fe-NC.\nRu-N Spin up Spin down Total\nO-Ru-N1 -1.440 -1.569 -3.009\nO-Ru-N2 -1.441 -1.569 -3.010\nO-Ru-N3 -1.445 -1.573 -3.018\nO-Ru-N4 -1.434 -1.564 -2.998\nO-Ru-N(average) -1.440 -1.569 -3.009\nFe-N Spin up Spin down Total\nFe-N1 -1.122 -1.187 -2.309\nFe-N2 -1.121 -1.188 -2.308\nFe-N3 -1.121 -1.187 -2.308\nFe-N4 -1.122 -1.188 -2.310\nFe-N(average) -1.122 -1.188 -2.309\n(detailed orbital information).\nO-Ru-N(average) Spin up Spin down Total\n5s-2s -0.216 -0.215 -0.431\n5s-2p -0.125 -0.121 -0.246\n4d-2s -0.320 -0.377 -0.697\n4d-2p -0.779 -0.856 -1.635\nFe-N(average) Spin up Spin down Total\n5s-2s -0.262 -0.244 -0.506\n5s-2p -0.234 -0.193 -0.428\n4d-2s -0.178 -0.199 -0.377\n4d-2p -0.447 -0.550 -0.998\nQuestion 3. I suggest removing this sentence. “The ΔGpH is an important parameter\nthat indicates how changes in H+ concentration influence free energy. However,\nnumerous studies have demonstrated that pH has no impact on the rate-determining\nstep, as the free energy of the elementary reactions remains independent of pH” There\nare a series of studies with opposite results. The authors should state that they neglect\nthe contribution only.\nAnswer 3: Thanks for your valuable comment. As suggested by Reviewer #1, the\nsection “The ΔGpH is an important parameter that indicates how changes in H+ concentration influence free energy. However, numerous studies have demonstrated that pH has no impact on the rate-determining step, as the free energy of the elementary reactions remains independent of pH.” has been deleted and replaced with the statement\n“The contribution of pH is ignored, and ∆GpH is neglected in this study.”\nQuestion 4. The origin of the calculated Pourbaix diagrams is not clear. Have the\nauthors approximated the stability to the bulk Ru or have they explicitly considered the\nRu-SAC as reported recently? [10.1021/acscatal.3c04801]\nAnswer 4: Thanks for your insightful comment. We sincerely apologize for the\nmisstatement regarding the Pourbaix diagram after learning the relevant literature\n12\n(10.1021/acscatal.3c04801). Firstly, we would like to clarify the nature of the diagrams\npresented in Figure R11(Figure 6a and Figure S36a). Actually, they are technically\nsurface thermodynamic phase diagrams (plotting Gibbs free energy ∆G vs. potential\nU ), rather than traditional E-pH Pourbaix diagrams used to assess bulk corrosion or\nRHE\npassivation (Proc. Natl. Acad. Sci. U.S.A. 120 (49) e2308458120). In previous reports,\nsome researchers have also referred to the surface thermodynamic phase diagram as a\n“surface Pourbaix diagram” (J. Am. Chem. Soc. 2024, 146, 38, 26442-26453, J. Am.\nChem. Soc. 2025, 147, 28, 24900–24912, J. Mater. Chem. A, 2024, 12, 27974-27978).\nIn the surface thermodynamic phase diagram, we focus specifically on the relationship\nbetween potential and free energy. This analysis is used to identify the\nthermodynamically most stable surface adsorbates (e.g., *OH, *O, *OOH) under\noperating conditions, thereby helping to deduce the true catalytically active site\nstructure during the electrochemical reaction. It primarily relates to activity analysis\nrather than material stability or dissolution. To avoid any misunderstanding, we have\nrevised the terminology throughout the manuscript, replacing “Pourbaix diagram” with\nthe more precise “surface thermodynamic phase diagram”.\nFigure R11. The surface thermodynamic phase diagram for RuN (a) and FeN (b).\n4 4\nRegarding the reviewer’s query on the calculation origin, the Gibbs free energy\nchanges (∆G) were calculated based on the standard equation:\n∆G = G - G - G\ntotal slab adsorbate\nIn this context, the reference state is the clean RuN slab itself, not bulk Ru metal.\n4\n13\n3\n2\n1\n0\n-1\n.0\nu N\n4\n0 .2 0 .4 0 .6 0 .8\n*O O\n*O\n*O H\n*H\ns la b\n0 .6 5\n1 .0\nH\nV\n)V\ne\nG\n(\n∆\nU\nR H E\n(V )\n4\n3\n2\n1\n0\n-1\n.0\ne N\n4\n0 .2 0 .4 0 .6\n*O O\n*O\n*O H\n*H\ns la b\n0 .6 5\n0 .8\nH\nV\n1 .0\n)V\ne\nG\n(\n∆\na b\nU\nR H E\n(V )\nFeN , the potential-dependent stability of intermediates was analyzed. Under fuel cell\n4\nworking potentials (0.65 V), for the RuN structure, the surface is covered by oxygen,\n4\nidentifying O-RuN as the real catalytic active center for NC-Ru-950. And for the FeN\n4 4\nstructure, the surface remains free of adsorbates, identifying the bare FeN moiety as\n4\nthe active center.\nIn the revised manuscript: Interestingly, based on the surface thermodynamic phase\ndiagram in Figure 6a, we discovered that the energy of RuN with axially adsorbed O\n4\nis the lowest (O-RuN ) at 0.65 V, indicating that the NC-Ru-950 catalyst stably exists\n4\nin this structure under the operating voltage [38-41].\nQuestion 5. Based on Figure S30, it appears that the “poisoning” of the catalyst with\nO-species implies a decrease of thermodynamic barriers with respect to RuN4. Have\nthe authors considered the free energy to form O* in FeN4 in working electrochemical\nconditions?\nAnswer 5: Thanks for your valuable comment. As the reviewer suggested, we have\ncalculated the surface thermodynamic phase diagram of FeN . As analyzed in question\n4\n4 (Figure R11), the formation free energy of *O remained significantly positive and the\nFeN site without adsorbing any intermediates is the thermodynamically most stable\n4\nstate under a fuel cell operating voltage of 0.65 V. Therefore, we consider the bare FeN\n4\nto be the true active center for single-atom Fe-NC catalysts (Nature 644, 668-675\n(2025).; Nat Commun 15, 4219 (2024).; Nat Catal 6, 1215-1227 (2023).; Nat Energy 7,\n652-663 (2022)). This conclusion aligns well with several recent studies on Fe\nsingle‑atom catalysts, which consistently adopt FeN as the active center in\n4\ncomputational investigations.\nQuestion 6. Related to the previous point, how do the FeN4 and O-FeN4 free energy\nprofile compare with the observed polarization curves and tafel slopes?\nAnswer 6: Thanks for your professional comment. According to the surface\nthermodynamic phase diagram of FeN , the O-FeN active site theoretically does not\n4 4\nexist during the oxygen reduction reaction process. Therefore, the subsequent\ndiscussion regarding O-FeN is based on a hypothetical scenario.\n4\nFeN as the active center. As shown in Figure R12a, the theoretical limiting potential\n4\n(U ) of O-FeN is 0.34 V, which is much lower than that of FeN (0.72 V). The\nL 4 4\nexperimental SCV polarization curve of Fe-NC catalysts exhibits excellent ORR\nperformance with a half-wave potential (E ) of 0.813 V (Figure R12b). This high\n1/2\nactivity aligns well with the theoretical limiting potential calculated for the bare FeN .\n1(Figure R12c), and the relatively low Tafel slope suggests faster ORR kinetics and\nsuperior performance. In contrast, if the O-FeN site were the true active center, it would\n4\nlead to slower reaction kinetics and a higher Tafel slope.\nTherefore, by combining the thermodynamic phase analysis with the observed\nORR polarization curve and the Tafel slope analysis, we confirm that the FeN moiety\n4\nis the genuine catalytic site in the Fe-NC catalysts.\nFigure R12. (a) The Gibbs free energy diagram on O-FeN active center; (b) The SCV\n4\npolarization curve of Fe-NC catalysts; (c) The Tafel slope curve of Fe-NC catalysts.\n15\n0\n-2\n-4\n0\nin\n.2\nitia l\n0 .4\n)Ve(\nygrene\neerF\n0 .6\n6\n5\n4\n3\n2\n1\n4\n.8\n*O O H\nR e a\n1 .0\nc tio\n*O\nn p a th w\n*O\na\nH\ny\nU\nU\nU\n=== 001 V.3.2\nV\n2-m )\nc\nA\nm\ny\n(\ntis n\ne\nd\ntn\ne\nrru\nC\nP o te n tia\nE\nl (V\n= 0 .8 1/2\nv s . R\n1 3\nH\nV\nE )\nF e -N C\n0\n0\n0\n0\n.9\n.9\n.8\n.8\n5\n0\n5\n0\n-1 .5 -1 .0 -0 .5 0 .0 0\nF\n.5\ne -N C\n1 .0\n)E\nH\nR\n.s\nv V (\nla\nitn\ne\nto\nP\nlo g J[ (m\nk\nA c m -2)]\n85 m\nV dec-1\nb\na\nc\nREVIEWER COMMENTS\nReviewer #2\nIn this work, a novel hydrogen-embrittlement-inspired thermal activation strategy for\nsynthesizing atomically dispersed metal catalysts of 4d/5d metals is demonstrated. This\nmethod addresses the significant challenge of stabilizing high-cohesive-energy metals\nlike Ru, Pd, Pt, and Au as single atoms by utilizing H₂ permeation to weaken metal-\nmetal bonds in clusters during high-temperature treatment, facilitating their dissociation\ninto individual atoms anchored by nitrogen sites on a carbon support. Using Ru as a\nprimary example, the structural evolution from clusters to isolated RuN₄ sites was\ncomprehensively studied through ex-situ microscopy and spectroscopy. The resulting\nRu-N-C catalyst exhibits exceptional oxygen reduction reaction (ORR) activity and\nstability in both half-cell measurements and proton exchange membrane fuel cells\n(PEMFCs), achieving performance that meets DOE targets and surpasses state-of-the-\nart Fe-N-C catalysts. The universality of the strategy is successfully validated by\nextending it to other noble metals. Combined with detailed DFT calculations, the work\nprovides fundamental insights into the formation mechanism and the enhanced catalytic\nproperties of the active sites. However, this paper could not be published in the current\nstate and some issues need to be properly addressed.\nReply: We sincerely thank the reviewer #2 for the positive and insightful assessment\nof our work. We are grateful for the recognition of the novelty and significance of our\nhydrogen-embrittlement-inspired strategy, the comprehensive characterization using\nRu as a model system, the exceptional ORR performance meeting DOE targets, and the\nsuccessful demonstration of the method's universality across other noble metals.\nWe have carefully considered all comments provided by reviewer #2, and have\nundertaken a thorough revision to address each point raised and have undertaken a\nthorough revision to address each point raised. Our detailed point-by-point response\nbelow outlines the specific modifications and additional data/analysis we have\nincorporated. We are once again grateful to the reviewer #2 for their time and\nconstructive feedback, which has been invaluable in enhancing the quality of our work.\nconfirm the existence form of Ru.\nAnswer 1: Thanks for your valuable comment. We have added the TEM image of NC-\nRu-RT Figure R13 (Figure S13) in the Supplementary Information. The absence of Ru\nclusters or nanoparticles in Figure R13 (Figure S13) confirms the atomic-level\ndispersion of Ru throughout the NC-Ru-RT.\nFigure R13 (Figure S13). TEM image of NC-Ru-RT.\nQuestion 2. The signal-to-noise ratio of the Ru 3p XPS spectrum for NC-Ru-Ar is\nrelatively low, which may affect the accuracy of the analysis. It is recommended to\nretest this sample.\nAnswer 2: Thanks for your good suggestions. We have retested the Ru 3p XPS\nspectrum for NC-Ru-Ar. Figure R14 (Figure S6) shows the revised image and the\nmodified version has been included in the Supplementary Information.\nFigure R14 (Figure S6). The deconvoluted high-resolution Ru 3p spectra of NC-Ru-Ar and\n3/2\nNC-Ru-H /Ar.\n2\n).u.a(\nytisnetnI\nF\nT\ni\nh\nP D\ng u\ne d e\nF # 0 6 -0\n2 0\nr e S\nc\n6 6\n6\no\n3\nn\nX\nv\n).u.a(\nytisnetnI\no l u\n0\nD p\np R u 3\n3/2\n4\nt e d h i g h -\nN C\n6 0\ne ta (° )\na t t e r n s o\n6 5\nB in d in g e\nr e s o l u t\n-R u -H /A r\n8 0\nf N C - R\n4 6 0\nn e rg\ni o n\n).u.a(\nytisnetnI\nu - A\nN C -R u -H /A\n4 5 5\ny (e V )\nR u 3 p s p\n3 / 2\nP D F # 0 6 -0 6 6 3\n3 5 4\nr a n d N C - R\nr\nr\n0\nu\ne c t r\n- H\n2\na\ne\n/\nt a\nA\no f N\n4\n(° )\nr .\nN\nR\nC\nu\n-R\nN C\n- A r\nu -H /A\n2\n-R u -A\na\nr\nr\n5 0\nn d N C\nQuestion 3. The difference in pore width shown in Figure S13b is not clearly\ndistinguishable. Please redraw the figure to improve clarity.\nAnswer 3: Thank you very much for your comment. As shown in Figure R15 (Figure\nS16b), we have revised Figure S16b accordingly to ensure clear differentiation of the\npore sizes. The built-in image is an enlarged view of the gray area.\nFigure R15 (Figure S16). (a) The N adsorption/desorption isothermal physisorption curves and (b)\n2\npore size distribution plots of NC and NC-Ru-T (T = RT, 350℃, 550℃, 750℃, and 950℃). (c) The\npore volume and BET surface area of NC and NC-Ru-T (T = RT, 350°C, 550°C, 750°C, and 950°C).\nQuestion 4. Regarding the transformation of part of Ru³⁺ to Ru⁰ and finally to Ruδ⁺ in\nFigure S14b, further explanation of the underlying mechanism is needed. Additional\nDFT calculations simulating these valence states are suggested to support the discussion.\nAnswer 4: Thanks for your constructive comment regarding the valence state\ntransformation of Ru species in our manuscript. We agree that a further explanation of\nthe underlying mechanism is needed. Following the reviewer’s suggestion, we have\nperformed DFT calculations combined with Bader charge analysis to simulate these\ns\nut\nr\n=T\na\n)1\n4 0 0 -g\nLm(\nytitnau\n2 0 0 q\nnoitprosdA\n0\n0 .0 0 .2 0 .4\nR e la tiv e p\n0 .8\n0 .7\n0 .6\n0 .5\n0 .4\n0 .3\n0 .2\n0 .1\n0 .0\nr e S 1 6 ( a ) T h e N a d s\n2\ni b u t i o n p l o t s o f N C a n\nR T , 3 5 0 , 5 5 0 , 7 5 ℃ ℃\n° ° ( T = R T , 3 5 0 C , 5 5 0 C\nT a b l e S 2 B E T s u r f a\n( T = R T , 3 5 0 , 5 5 ℃\nV m ic ro\n3 -1 c m g\nN C 0 .3 9 3 8 4\nN C -R u -R T 0 .2 4 2 5 8\nN C -R u -3 5 0 0 .2 8 6 4 8\nN C -R u -5 5 0 0 .2 9 4 7 8\n0 .6\nP re s s u re\no r p t i o\nd N C -\n0 , 9 ℃\n° , 7 5 0\nc e a r e\n0 , 7 ℃\n%\n0 .7 1 1 9 5\n0 .6 1 3 6 7\n0 .6 0 5 8 1\n0 .6 3 6 8 9\nN C\nN C -R u -R T\nN C -R u -3 5 0\nN C -R u -5 5 0\nN C -R u -7 5 0\nN C -R u -9 5 0\n0 .8 1 .0\nP/\n0\nn / d e s o r p t i o\nR u - T\n5 0 , 1 0 5 0 ℃ ℃°\nC , a n d 9 5 0\na s a n d p o r e\n5 0 , 9 5 0 ℃ ℃\nV m e so\nV + m a c ro\n3 -1 c m g\n6 3 2 5 0 .1 5 9 3 4\n6 0 3 5 0 .1 5 2 7 1\n9 6 5 8 0 .1 8 6 4\n3 9 5 9 0 .1 6 8 0 6\nn\nC\nb\ni s\n) .\n) .\nd i s\n, 1\n0 .2\n0 .3\n0 .3\n0 .3\no t h e\n( c ) T\nt r i b u\n0 5 0 ℃\n%\n8 8 0 4 3 6\n8 6 3 2 3 9\n9 4 1 8 0 3\n6 3 1 0 6 0\nr m\nh e\nt i o)\n7 5\n6 5\n4 2\n4 1\na\np\nn\n.\nl p h y s i s o\no r e v o l u\no f N C a\nV to ta l\n3 -1 c m g\n0 .5 5 3 1 8\n0 .3 9 5 2 9\n0 .4 7 2 8 8\n0 .4 6 2 8 4\nr\nm\nn\np\nd\nt\ne\ni o n c u r v\na n d B E\nN C - R u - T\nS B E T m 2 /g\n1 ,1 2 6 .6 0 6 7\n7 6 4 .7 7 3 6\n9 0 8 .6 8 5 7\n9 3 8 .3 6 9 5\ne\nT\ns\ns\na\nu\nn\nr f\nd\na c\n( b\ne\n)\na\np\nr\no\ne a\nr e\no\ns\nf\ni z\nN\ne\nC a n d N C\nNC NC-Ru-350 NC-Ru-550 NC-Ru-750 NC-Ru-950\n)1-g\n3mc(\nemuloV\neroP\nV\nm icro\nV\nm\n+\neso\nV\nm acro\nV\ntotal\nNC-Ru-RT\nc\n)1-mn\n1-g\nLm(\n)d(Vd\n0\n0\n0\n0\n.1\n.1\n.0\n.0\n5\n0\n5\n0\n0\n0\n0\n1\n.0\n.0\n.0\n4\n2\n0\n1 0\n1 0\nth (n m )\nS\nB E T\nN\nN\nN\nN\nN\nN\nCCCCCC\n-R-R-R-R-R u -R\nu -3\nu -5\nu -7\nu -9\n1 2\n8 0\n4 0\n0\n5 0\n5 0\n5 0\n1\n0 0\n0\n0\n0\n)1-g\n2mc(\naera\necafruS\nTEB\n0\nvalence states.\nFirst, we introduce the existing forms of Ru corresponding to Ru3+, Ru0, and Ruδ⁺,\nrespectively. Ru3+ represents the Ru species within the pristine Ru(acac) molecular\n3\nprecursor. Ru0 corresponds to metallic Ru clusters/nanoparticles formed after the\nreduction of the Ru(acac) precursor. Ruδ⁺ (which we can assign more specifically as\n4\nconfiguration anchored on the nitrogen-doped carbon (NC) support.\nThe transformation mechanism of Ru species involves two sequential steps:\nStep1: Reduction from Ru3+ (in Ru(acac) ) to Ru0 (metallic clusters)\n3\nprecursor is reduced by H , leading to the cleavage of Ru-ligand bonds and the\n2\nformation of metallic Ru clusters. The rapid weight loss in the range of 200 to 350℃\nin the thermogravimetric curve can also confirm this conclusion.\nStep 2: Transformation from Ru0 (clusters) to Ruδ⁺ (single atoms)\nAs the annealing temperature increases from 350℃ to 950℃, H penetrates into\n2\nthe Ru clusters, weakening the Ru-Ru interaction and ultimately driving the Ru clusters\nto collapse into individual Ru atoms. Subsequently, the detached Ru atoms are captured\nby the inherent N-defects on the NC support, forming stable Ru-N moieties.\nThe\nx\nelectron transfer from the Ru atom to the more electronegative N-coordinating\nenvironment results in an electron-deficient state, observed as Ruδ⁺ (approximately\nRu+2.2) in XPS.\nFurthermore, according to the reviewer’s suggestion, DFT calculations were\nperformed to simulate the electronic structure of these different Ru species. We\ncalculated the Bader charge of Ru in various models, and the specific structures are\nshown in Figure R16 (Figure S18). The average Bader charges of NC-Ru(acac) , NC-\n3\nto RuN model, the absolute value of the Ru Bader charge first decreases and then\n4\nincreases, while remaining lower than that in Ru(acac) . The trend of the calculated\n3\nobserved in XPS spectra. While the absolute values of the Bader charge (an electronic\ncharge metric) do not directly equate to the formal oxidation state or XPS binding\nenergy (which measures core-level energy shifts), the consistent trend provides robust\ntheoretical validation that our assignment of the chemical species is correct.\nFigure R16 (Figure S18). The average Bader charges of NC-Ru(acac) , NC-Ru_cluster, and\nBader charges in the revised manuscript and supplementary information:\nAt low to medium temperatures (room temperature to 350℃), the Ru(acac) \n3\nprecursor (Ru3+) is reduced by H , leading to the cleavage of Ru-ligand bonds and the\n2\nformation of metallic Ru clusters (Ru0). As the annealing temperature increases from\n350℃ to 950℃, H penetrates into the Ru clusters, weakening the Ru-Ru interaction\n2\nand ultimately driving the Ru clusters to collapse into individual Ru atoms.\nSubsequently, the detached Ru atoms are captured by the inherent N-defects on the NC\nsupport, forming stable Ru-N moieties (Ruδ+). Furthermore, the Bader charge of Ru in\nx\nNC-Ru(acac) , NC-Ru_cluster, and NC-RuN models is calculated. During the\n3 4\ntransformation of Ru species from Ru(acac) to Ru_cluster and subsequently to RuN\n3 4\nmodel, the absolute value of the Ru Bader charge first decreases and then increases,\nwhile remaining lower than that in Ru(acac) . The trend of the calculated Bader charges\n3\nis in excellent agreement with the sequence of binding energy shifts observed in XPS\nspectra (Figure S18).\n202 should point to \"Figure 2i\" instead of \"Figure 3i.\" Please also verify the citations\nin lines 218, 223, and 225.\nAnswer 5: Thanks for your valuable comment. We have corrected the citation errors\nand marked the corrections in the manuscript. In the revised manuscript, Figure 3i, 3g,\n3h, and 3j have been changed to Figure 2i, 2g, 2h, and 2j, respectively.\nQuestion 6. The labels \"a/b/c:10k\" in Figure 4f seem to be incorrect. Please check and\ncorrect as necessary.\nAnswer 6: Thanks for your good suggestion. In Figure 4f, we have used superscripts a,\nb, and c to represent aging cycles of 10k, 20k, and 30k, respectively. We admit that the\nlack of clarity in our original description caused misunderstanding for the reviewers.\nTo facilitate the reading process for both reviewers and readers, we have revised Figure\nR17 (Figure 4f) as shown below. Yellow, green, blue, and red represent 5k, 10k, 20k,\nand 30k aging cycles, respectively. The NC-Ru-950 catalyst we synthesized\ndemonstrates outstanding stability, surpassing most of the catalysts reported to date.\n4\n(a) SCV polarization curves of NC, Pt/C, and NC-Ru-T (T = RT, 350℃, 550℃, 750℃, and 950℃).\nSCV polarization curves before and after 10k, 20k, and 30k CV cycles of (b) NC-Ru-950 and (c)\nFe-NC (the inset is the values of E after different aging cycles). (d) i-t curves of NC-Ru-950 and\n1/2\nFe-NC catalysts tested in 0.1 M HClO at a constant potential of 0.6 V. (e) The UV-vis absorption\n4\nspectra of 0.1 M HClO solutions consist only ABTS; Fe-NC and ABTS; NC-Ru-950 and ABTS;\n2 2 2 2 2 2\nComparison of the E loss after CV cycles between NC-Ru-950 and the recently reported catalysts.\n1/2\n(Yellow, green, blue, and red represent 5k, 10k, 20k, and 30k aging cycles, respectively.)\nQuestion 7. The mass ratio of Ru in the composites is less than 1 wt%, which is\nrelatively low compared to some MOF-derived single-atom catalysts (which can reach\nup to 40 wt%). Are there any strategies to increase the metal loading? Please provide\nthe corresponding ICP results for all samples in the supporting information.\nAnswer 7: Thanks for your constructive comment. We acknowledge that the\ndetermined Ru loading in NC-Ru-950 catalyst is relatively low compared to some\n22\na\n) 0 2-mc\nAm(\nyt -2 isned\ntnerru -4 C\nd\n1 0 0 )%( \nyt 8 0 isned\ntn\n6 0 erruc\nev 4 0 italeR\n2 0\n35\n) 30 Vm(\ns 25 elcy\n20 c\nVC\n15 retfa\n10 ssol\n5 2/1\nE\n0\nNC-Ru-350 NC-Ru-550 NC-Ru-750\nNC-Ru-950\nPt/C\n0 .2\nP\n0 .4 0\no te n tia l (V\n34.3%\n5 0\n.6\nvs. R H\n0 .8\nE )\n1 0 0\n1 .0\nT\nb\n) 0 2-mc\nAm(\nyt -2 isned\ntnerru -4 C\n1 5 0\nim e (h )\n0\ninitial after 10k C V cycles\nafter 20k C V cycles after 30k C V cycles\nE 1/2\n0.816 0.808 0.808 0.807\ns s s e e e l l l c c c y y y c c c la k k k it 0 0 0 in 2 3 1 i\n.2 0 .4 0\nP o te n tia l (V\n2 0 0\n9 m\n.6\nvs. R H\n2 5 0\nV\nN C -R u-950\n.8 1 .0\n)\n93%\nN C -R u-950 Fe-N C\n3 0 0\n)2-mc\nAm(\nytisned\ntnerruC\necnabrosbA\nc\n0\n-2\n-4\ne\n0 .8\n0 .6\n0 .4\n0 .2\n0 .0\n3 6 0\n0\ninitial Fe-N C after 10k C V cycles\nafter 20k C V cycles after 30k C V cycles\nE 1/2\n0.813 31 m V\n0.796\n0.788 0.782\ns s s e e e l l l c c c y y y c c c la k k k it 0 0 0 in 2 1 3 i\n.2 0 .4 0 .6 0 .8 1 .0\nP o te n tia l (V vs. R H E )\nABTS\nFeNC+ABTS NC-Ru-950+ABTS\nH O +ABTS 2 2 FeNC+H O +ABTS 2 2 NC-Ru-950+H O +ABTS 2 2\n4 0 0 4 4 0 4 8 0\nW a ve le n g th (n m )\nCS F F e e - / N N C -C /Scaad-C F e e O S C 2 A l-F /F e e - A N C C −2DNPC Ru-N/ F G e -7 S 5 A 0 C-MO Fe F - - N 5 C/ScaCeO2 Fe2 N6 HP-Fe F N e 4 (mIm)-NC Ru-SSC Ru1 /N-CPore F s e2 -N- F C e/NC-N F a e C - l SA-NSF F C e-N F - e C -N /P x d /C N - C NC1100 Fe-N-C-S N P C-Ru-950\nf\n5 k c y c le s 1 0 k c y c le s 2 0 k c y c le s 3 0 k c y c le s\nreported MOF-derived single-atom catalysts. We fully understand your concern\nregarding metal loading, as high loading is crucial for the practical application of\ncatalysts. We would like to clarify two points in our response: firstly, the primary\nscientific objective of this work was to validate and elucidate the “hydrogen-\nembrittlement-inspired” mechanism we proposed and to investigate the intrinsic\nactivity and stability of the Ru-NC catalyst prepared based on this strategy. At the\nmechanistic exploration stage, a moderate loading that ensures the formation of high-\npurity single-atom sites was advantageous for us to clearly and unambiguously track\nthe evolution pathway of the metal species using various characterization techniques.\nWe are pleased to observe that even at the 0.51 wt% Ru loading, the obtained Ru-NC\ncatalyst already exhibits excellent performance in acidic electrolyte and PEMFCs,\nsurpassing most single-atom catalysts.\nSecondly, regarding strategies to increase the metal loading, this is indeed a key\nresearch direction for our future work towards application and is an important\nmanifestation of the universality of this synthesis strategy. Based on the understanding\nof the mechanism: H promotes cluster disintegration, and nitrogen sites capture single\n2\nmetal atom. We propose the following promising strategy to further increase the metal\nloading: Proposing a two-step strategy combining chemical doping and confined\nadsorption: initially, a portion of the Ru precursor is chemically doped into the ZIF-8\nframework to form Ru-ZIF-8. And then, Ru-ZIF-8 precursor is pyrolyzed to yield a Ru\nand N co-doped carbon support. Subsequently, a secondary adsorption of Ru precursors\nis performed onto the Ru, N co-doped carbon support via the liquid-phase confined\nadsorption method described in this work. As we know, the maximum single-atom\nloading capacity is generally dictated by the density of available N anchoring sites.\nTherefore, melamine was introduced as a nitrogen source in both heat treatment steps\nto increase the density of N-anchoring sites on the carbon substrate. The synthesis\nstrategy resulted in an increased Ru loading in the final Ru-NC catalyst, which reached\n2.5 wt% as determined by ICP-OES. As displayed in Figure R18, Ru exists in the form\nof single atoms.\nFurthermore, based on some literatures, the development of carbon carriers\npossessing higher specific surface areas and increased meso/micropore volumes, or the\nutilization of functionalized graphene quantum dots as supports (Nat. Chem. 13, 887–\n894 (2021)., Nat Commun 15, 5675 (2024).), combined with our proposed hydrogen-\nembrittlement-assisted thermal activation strategy, is highly promising for further\nincreasing the metal loading of single-atom catalysts. This will be a key focus of our\nfuture research direction.\nAs suggested by the reviewer, we have added the corresponding ICP results for all\nsamples in the supplementary information. As shown in Figure R19 (Figure S23), the\nICP results for Pd-NC, Pt-NC, and Au-NC are 0.55 wt%, 0.52 wt%, and 0.69 wt%,\nrespectively.\nFigure R19 (Figure S23) The ICP results for Pd-NC, Pt-NC, and Au-NC catalysts.\nAs shown in Table R5 (Table S8), the ICP results for NC-Ru-RT, NC-Ru-350,\nand 0.51 wt%, respectively. The ICP results for NC-Ru-0.5 mg, NC-Ru-1.0 mg, NC-\nRu-2.0 mg, NC-Ru-4.0 mg, and NC-Ru-20.0 mg are 0.13 wt%, 0.25 wt%, 0.51 wt%,\n0.97 wt%, and 1.83 wt%, respectively.\nTable R5 (Table S8). The ICP content for series catalysts.\nICP content ICP content\nSample Sample\n(wt%) (wt%)\nNC-Ru-RT 0.49 NC-Ru-0.5 mg 0.13\nNC-Ru-350 0.49 NC-Ru-1.0 mg 0.25\nNC-Ru-550 0.50 NC-Ru-2.0 mg 0.51\nNC-Ru-750 0.50 NC-Ru-4.0 mg 0.97\nNC-Ru-950 0.51 NC-Ru-20.0 mg 1.83\nQuestion 8. The synthesis of atomically dispersed catalysts via the thermal treatment\nof MOF precursors under a H atmosphere is a well-established and widely reported\n2\nmethodology in the literature. Therefore, the novelty of the methods of the synthesized\nAtomically dispersed metal catalysts is limited.\nAnswer 8: Thanks for your insightful comment. Indeed, we agree that the thermal\ntreatment of MOF precursors under H is a widely established approach for preparing\n2\natomically dispersed catalysts. However, we would like to respectfully clarify that the\nmain focus and innovation of our work do not lie in the synthesis method itself, but\nrather in uncovering a previously unexplored hydrogen-embrittlement-inspired\nmechanism for forming atomically dispersed catalysts and the resultant superior\ncatalytic performance achieved through this strategy. Crucially, we wish to emphasize\nonce again that this strategy is particularly effective for 4d/5d transition metals with\nhigh cohesion energy, which are typically difficult to form atomically disperse sites\nusing conventional approaches.\nOur experimental design utilizes ZIF-8 derived N-doped carbon (NC) to adsorb\nRu(acac) , followed by subsequent high-temperature treatment under the H /Ar\n3 2\natmosphere. The purpose of this design is to decouple the formation of the active sites\nfrom the carbonization and nitrogen-doping processes of the support, thereby enabling\n25\na clear elucidation of the formation mechanism of atomically dispersed RuN active\n4\ncenters under the H /Ar atmosphere.\naspects:\n1. The newly proposed “hydrogen-embrittlement-inspired strategy” to\nprepare atomically dispersed metal catalysts, especially for 4d/5d\ntransition metals with high cohesion energy. We have first clarified that\nintroducing H into metal clusters can weaken the cohesive energy and\n2\ninteractions between metal atoms, thereby overcoming the long-standing\nchallenge in conventional synthesis methods of cluster formation and enabling\nthe preparation of high-purity single-atom catalysts. H molecules can\n2\npermeate into metal clusters, promoting their collapse into single atoms, which\nare subsequently captured by nitrogen dopants in carbon support to form stable\nM-N single-atom sites. Taking Ru as a model system, we further observed the\n4\nevolution mechanism of Ru species with increasing temperature under H /Ar\n2\natmosphere. Through a multi-technique characterization approach integrating\nXAFS, XPS, and AC-HAADF-STEM characterizations, the atomic-scale\nevolution of Ru species from Ru(acac) to Ru clusters and eventually to RuN\n3 4\nsingle-atom with the increase of temperature is unambiguously elucidated. We\nsystematically prepared and characterized atomically dispersed Pd-NC, Pt-NC,\nand Au-NC catalysts, further demonstrating the universality of the hydrogen-\nembrittlement-assisted thermal activation strategy.\n2. The remarkable activity and long-term stability of the Ru-NC catalyst in\nboth acidic electrolyte and PEMFCs. The obtained Ru-NC presents excellent\nactivity and stability, with a half-wave potential (E ) of 0.816 V, a mere 9 mV\n1/2\ndecline in E after 30,000 cycles, and a current retention of 93% at 0.60 V\n1/2\nafter 300 hours in acidic media. Additionally, the membrane electrode\nassembly employing Ru-NC achieves a current density of 44.00 mA cm-2 at\n0.881 V (H -O , 1.0 bar, U.S. DOE 2025 target 44.00 mA cm-2 at 0.9 V\niR-free 2 2 iR-\n2\nfree) and a peak power density of 670.9 mW cm-2 (H -air, 1.0 bar), representing\n2\nthe highest level of currently reported non-Fe and Pt-free catalysts. More\nimportantly, Ru-NC maintains 82.04% of its peak power density after 30,000\ncycles of accelerated stress tests, markedly outperforming Fe-NC catalysts.\nIn conclusion, the innovation of this paper lies in two core aspects: the first is the\nproposal and systematic elucidation of the hydrogen-embrittlement-inspired strategy\nfor single-atom catalyst synthesis; the second is the achievement of remarkable activity\nand durability with the resulting Ru-NC catalyst. We believe this work provides a new\nperspective for understanding the crucial role of H in the formation of single-atom\n2\ncatalysts and opens a new pathway for preparing high-performance 4d/5d transition\nmetal single-atom catalysts.\nThis paper proposes a thermal activation strategy inspired by hydrogen embrittlement\nfor synthesizing atomically dispersed metal catalysts of 4d/5d metals. The study\ndemonstrates that hydrogen molecule permeation weakens metal-metal cohesion,\nfacilitating the transformation of metal clusters into M-N4 single-atom sites, with Ru\nserving as a model system for detailed characterization and performance testing. The\naspect lies in the creative application of materials science principles (hydrogen\nembrittlement) to atomic-scale catalyst preparation, offering an interdisciplinary\nresearch perspective. The experimental design is systematic, integrating in situ\ncharacterization, theoretical calculations, and performance tests. However, there is\nroom for improvement in the depth of mechanistic explanation and further data analysis.\nThe detailed comments are shown below:\nReply: We sincerely thank the reviewer#3 for the positive evaluation of our work and\nthe constructive feedback. We greatly appreciate the recognition of the novelty in\napplying the hydrogen embrittlement concept to single-atom catalyst synthesis, as well\nas the systematic experimental design integrating in-situ characterization, theory, and\nperformance tests.\nWe have carefully considered all the comments. In our detailed point-by-point\nresponse, we have thoroughly addressed the specific suggestions for improvement. We\nbelieve these revisions have substantially enhanced the depth and clarity of the\nmanuscript. Once again, we would like to express our gratitude to reviewer #3 for their\napproval and valuable suggestions.\nQuestion 1. The authors propose that hydrogen molecules permeate into metal clusters\nand weaken metal-metal bonds, but direct evidence of the dynamic hydrogen permeation process at high temperatures is lacking. Could quantitative data on\nhydrogen concentration and cluster decomposition rates be provided? Additionally,\nwhile the successful extension of the strategy to Pd, Pt, and Au is demonstrated, the\nrationale for selecting these metals is not deeply discussed. Are there specific criteria\n(e.g., hydrogen solubility, metal-N bond strength) determining whether a metal can be\n28\natomically dispersed via this method? For instance, why were Rh, Ir, or Ag not tested?\nA more systematic discussion on the limitations would strengthen the claim of\nuniversality.\nAnswer 1: Thanks for your insightful and professional comment. First, regarding the\ndynamic hydrogen permeation process, we respectfully acknowledge that direct in-situ\nobservation of H permeation into metal clusters at the atomic scale under high\n2\ntemperature conditions is extremely challenging in the field. However, to address this\nconcern, we have provided indirect but compelling experimental evidence, supported\nby theory, to substantiate the proposed mechanism. As shown in Figure R20a, the\noptimized model of NC-Ru-without H and NC-Ru-with H (side view) confirms that\n2 2\nthe Ru-Ru distance is increased in the presence of H , suggesting that H permeation\n2 2\npromotes structural expansion of the clusters, making them more prone to collapse.\nFurthermore, based on the EXFAS fitting results (Figure R20b), the Ru-Ru bond\nlengths increase from 2.64 Å in the metallic Ru foil, to ~2.68 Å in the sample treated at\n550°C (NC-Ru-550), and further to ~2.70 Å in the sample treated at 750°C under H /Ar\n2\n(NC-Ru-750). This gradual elongation of the Ru-Ru bond with increasing treatment\ntemperature in a H atmosphere provides direct spectroscopic evidence that hydrogen\n2\npermeation weakens the metal-metal bonds, making the clusters more prone to\ndissociation.\nFigure R20. (a) The side view of NC-Ru-without H and NC-Ru-with H . (b) Changes in\n2 2\nRu-Ru distance for Ru foil, NC-Ru-550, and NC-Ru-950.\nFurthermore, to address the reviewer's concern regarding the quantitative data on\n29\nhydrogen concentration and cluster decomposition rates, we conducted a series of\ncontrolled experiments to obtain quantitative data linking H concentration to the\n2\nclusters decomposition rates. We treated the precursor (NC-Ru-RT) at 950°C for 1 hour\nunder atmospheres with varying H concentrations (0%, 2%, 5%, 10% in Ar).\nthe resulting samples. Representative images and the corresponding quantitative\nanalysis are provided in Figure R21. In order to reduce statistical variation, three\nrandom regions per sample were selected for analysis. The results clearly show that as\nthe H concentration increases, the average size of the remaining Ru clusters decreases\n2\nmonotonically, while the density of atomically dispersed Ru sites rises. The cluster\ndecomposition rate, represented by the formation rate of single atoms, increased from\n0.06 sites nm-2 h-1 to 0.8 sites nm-2 h-1 as the H concentration rose, clearly\n2\ndemonstrating the critical role of H in promoting the disintegration of clusters into\n2\nisolated atoms (Figure R22).\na\nb\n2 nm 2 nm\nc d\n2 nm\nFigure R21. AC-HAADF-STEM images of (a) NC-Ru-Ar (original manuscript), (b) NC-Ru-2%\nH /Ar, and (c) NC-Ru-5% H /Ar, (d) NC-Ru-10% H /Ar (original manuscript).\n2 2 2\n30\n)mn(\nezis\nsretsulC\n3\n2\n2\n1\n1\n0\n0\n.0\n.5\n.0\n.5\n.0\n.5\n.0\n0 % H\n2\n2 % H\n2\n5 % H\n2\n1 0 % H\n2\n2\n1\n1\n0\n0\n.0\n.5\n.0\n.5\n.0\n)1-nim\nsetis\n(\netar\nnoitareneG\na b\n2 nm 2 nm\nc\n3.0 1.0\n)\n1\n2.5 -\n2\nh\n-m\n)\nm n\nn2.0 s\n(\ne\nz\ne\ntis\nis1.5 0.5 (\ns e\nr\ne\nta\nts u lC 1.0 r n o ita\n0.5 m\nr\no\nF\n0.0 0.0\n0% H\n2\n2% H\n2\n5% H\n2\n10% H\nFigure S8. AC-HAADF-STEM images of\nIn addition, the r(eav) iNewCe-Rr rua-i2s%es Han/ Aexrc aenllde n(bt )p NoiCn-t Rreug-a5r%di ng the selection criteria for\napplicable metals. Wh2ile factors such as hydrogen solubility and metal-nitrogen (M-N)\nthe formation rate of single atom\nbond strength undoubtedly play a role in the thermodynamics of single-atom trapping,\nsites.\nwe identified the metal cohesive energy (or detachment energy) as the dominant\ndescriptor and the primary barrier for atomizing 4d/5d metals. As illustrated in Figure\nR23, 4d and 5d transition metals (TMs) generally possess much higher detachment\nenergy compared to 3d TMs, making it significantly harder to break their metal-metal\nbonds to form single atoms. Among the plotted 4d and 5d metals, Ru exhibits the\nhighest detachment energy and relatively modest hydrogen solubility compared to Pd,\nrepresenting the “limiting case” or the most difficult metal to disperse atomically. We\nreasoned that if our hydrogen-embrittlement-inspired strategy can successfully\novercome the highest energy barrier of Ru, it should be thermodynamically feasible for\nother 4d/5d metals with lower detachment energies. Pd, Pt, and Au were chosen as\nrepresentative members of the 4d/5d TMs with high detachment energy and great\npractical importance in catalysis to demonstrate the strategy's universality.\nmetals.\nTo further validate this hypothesis and address the reviewer’s specific query, we\nhave extended our synthesis to Rhodium (Rh), Silver (Ag), and Iridium (Ir). As shown\nin Figure R24, the HAADF-STEM images clearly confirm that Rh, Ag, and Ir can also\nbe successfully converted into atomically dispersed species using our method. This\nsuccessful extension to a broader range of metals reinforces our conclusion that the\nstrategy is broadly universal for high-cohesive-energy metals.\nFigure R24. AC-HAADF-STEM images of (a) Rh-NC, (b) Ag-NC, and (c) Ir-NC.\nWe thank the reviewer again for their valuable comments, which have helped\nimproved the paper quality. In the revised manuscript and supplementary information:\nNotably, it was observed that the decomposition rate of Ru clusters accelerates with\nincreasing H concentration (Figure S8).\n2\n32\n2\n1\n1\n0\n0\n.0\n.5\n.0\n.5\n.0\nM n F e C o A g\n)V\ne\n(\ny\ng\nre\nn\ne\ntn\ne\nm\nh\nc\na\nte\nD\nN i C u Z n R u R h P d Ir P t A u\na\n3d TMs 4d TMs 5d TMs\nSingle atom Atomic cluster\nb1.0 c\nNC-Ru-without H\n2\n)V 0.5 E a =0.477 eV\ne (\ny\n0.0\nRu +N\ng re -0.5 10 4\nn\nE-1.0\n-1.5 Ru 9 +RuN 4 IS TS FS\nNC-Ru-with H\n0.5 2\n)V E a =0.120 eV\ne (\ny g re -\n0\n0\n.\n.\n0\nH\n2\nn\nE-1.0\nRuH+RuN\n9 9 4\n-1.5\nIS FS IS TS FS\nd e f HAADF C\n5 nm 5 nm\nN Ru\n2 nm 2 nm 5 nm 5 nm\nFigure R25 (Figure S8). AC-HAADF-STEM images of (a) NC-Ru-2% H /Ar and (b) NC-Ru-5% 2\nH /Ar. (c) The size of clusters and the formation rate of single atom sites. NC-Ru-2% H /Ar and 2 2\nNC-Ru-5% H /Ar catalysts were prepared by heat-treating the NC-Ru-RT precursor at 950 °C for 2\n1 hour under gas flows containing 2% and 5% H₂ (balanced with Ar), respectively.\nQuestion 2. The DFT calculations reveal electronic structure changes in Ru-N4 sites\nbut do not explicitly derive the specific energy barriers for hydrogen permeation. It is\nrecommended to supplement with energy barrier calculations for hydrogen diffusion\npaths and correlate them with experimental activation energies to establish a\nquantitative theory-experiment relationship.\nAnswer 2: Thanks for your insightful and professional comment. First, regarding the\nspecific energy barrier for hydrogen permeation, we employed the Climbing Image\nNudged Elastic Band (CI-NEB) method to calculate the process of hydrogen diffusion\nfrom the surface of the Ru cluster into its interior. As shown in Figure R26 (Figure S1),\nthe diffusion of hydrogen from the cluster surface to the interior is an energetically\ndownhill process, suggesting that the hydrogen atom is thermodynamically favored to\nreside within the Ru cluster, driving the subsequent cluster disintegration. Notably, the\n33\na\n2 nm\nc\nF\n3 .0\n2 .5\n)m\n2 .0 n(\nezi\n1 .5 s\nsrets\n1 .0 ulC\n0 .5\n0 .0\n0\ni g u r e\n( a ) N C\nH / A r .\n2\nt h e f o\n% H\nS 8\n- R u\n( c )\nr m\n2 %\n2\n. A C\n- 2 %\nT h\na t i o\nb\n2 nm\nH 5 % H\n2 2\n- H A A D F - S T E M\nH / A r a n d ( b ) N\n2\ne s i z e o f c l u s\nn r a t e o f s i n g\ns i t e s .\n1 0 %\ni m a g e\nC - R u - 5\nt e r s a n\nl e a t o\nH\n2\ns o f\n%\nd\nm\n1\n0\n0\n.0\n.5\n.0\n)1-h\n2-mn\nsetis\n(\netar\nnoitamroF\ncalculated hydrogen diffusion barrier is 0.256 eV, which is close to the values reported\nin the literature (J Phys Chem C Nanomater Interfaces. 2022 Mar 23; 126(13):5950–\n5959). This relatively low barrier indicates that hydrogen permeation is kinetically\nfeasible under our synthesis conditions.\nFigure R26 (Figure S1). Hydrogen diffusion from the surface of the Ru cluster into its interior using\nCI-NEB.\nSecond, regarding the experimental activation energies and establishing a\nquantitative theory-experiment relationship: We know that applying the Arrhenius\nequation (lnk = -E /RT + lnA) to derive activation energies is the ideal approach to link\na\nkinetic theories with experimental observations. By calculating the reaction rate\nconstant k at different temperatures and fitting the relationship between lnk and 1/T, the\nactivation energy is subsequently determined. However, after careful assessment, we\nbelieve that a reliable experimental activation energy cannot be derived for this system,\nfor the following reasons:\n1. The structural evolution involves multiple coupled steps, making it difficult\nto define a single rate constant k. In our system, the experimentally observed\ntransformation involves a sequence of coupled processes: (i) the reduction of the\nRu(acac) precursor, (ii) H permeation into Ru clusters, (iii) the weakening of Ru-Ru\n3 2\nmetallic bonds leading to cluster collapse/disintegration, and (iv) the subsequent\ntrapping of isolated Ru atoms by N-sites on the carbon support. These steps are\ndynamically coupled, and their dominant pathways vary with temperature. The\n34\nactivities of the reactants, intermediates, and products cannot be quantified, and\ntherefore a physically meaningful the rate constant k cannot be defined. Any Arrhenius-\ntype “apparent activation energy” would thus lack mechanistic significance.\n2. The temperature during thermal treatment is not instantaneously stabilized.\nExperimentally, the synthesis involves a heating ramp where the temperature increase\nis not instantaneous. The duration of this heating ramp is significant relative to the\nthermal dwell time, meaning that it is not negligible in the overall thermal process.\nAs the reaction proceeds continuously during the heating phase, the system operates under\nnon-isothermal kinetics rather than ideal isothermal conditions. Therefore, the observed\nreaction rate at a specific target temperature is convoluted with the thermal history of\nthe ramp, making it impossible to extract a precise, intrinsic rate constant for that\nspecific temperature to fit the Arrhenius plot.\nGiven these factors, we consider that extracting an experimental activation energy\nin this complex structural reconstruction process would be inappropriate and potentially\nmisleading. We hope the reviewer understands the objective limitations in establishing\na quantitative kinetic model for such a complex dynamic system. Nevertheless, we\nsystematically investigated the effects of hydrogen concentration and thermal treatment\ntemperature for the formation of single atoms. We discovered that increasing both\nhydrogen concentration and thermal treatment temperature could accelerate the\ntransformation from clusters to single atoms, demonstrating that the process is\nkinetically controlled. We appreciate your suggestion again, which points out a critical\ndirection for bridging the gap between theory and experiment in single-atom synthesis.\nWe plan to focus on these quantitative kinetic measurements in our future work to\nfurther deepen the mechanistic understanding.\nWe have added the specific energy barriers for hydrogen permeation (Figure R26\n(Figure S1)) in the revised supplementary information:\nThe diffusion of hydrogen from the surface to the interior of the Ru cluster involves\na decrease in energy and proceeds with a low energy barrier (0.256 eV), indicating that\nthe process is both thermodynamically favorable and kinetically viable.\natoms, statistical results from large areas (e.g., atomic count distribution or cluster size\nstatistics) are not provided. It is advised to include image analyses from at least three\ndifferent regions and calculate a uniformity index for atomic dispersion.\nAnswer 3: Thanks for your valuable suggestion. We completely agree that statistical\nanalysis from multiple regions is crucial to rigorously demonstrate the homogeneous\natomic dispersion of Ru. In the manuscript, both Figure 1e and Figure 2d show AC-\nHAADF-STEM images of the NC-Ru-950 catalyst. As suggested by the reviewer,\nwe have also provided an additional AC-HAADF-STEM image for NC-Ru-950 catalyst.\nIn all three images (Figure R27), Ru is present in an atomically dispersed form, and no\nRu nanoclusters were detected. The uniformity index of atomic-level dispersion for the\ncatalyst was characterized by calculating the average atomic density and its standard\ndeviation across the measured regions. The very low relative standard deviation\nstrongly indicates a highly uniform dispersion. The number of single atoms per unit\narea in the three images was quantified as 0.65 nm-2, 0.66 nm-2, and 0.64 nm-2,\nrespectively, resulting in a calculated standard deviation of 0.01. Therefore, Ru single\natoms are homogeneously distributed on the carbon support.\nFigure R27. AC-HAADF-STEM images of NC-Ru-950 catalyst.\nQuestion 4. The performance of Ru–N–C is primarily compared to Fe–N–C and a few\nPt-based catalysts. However, recent studies have reported Ru-based SACs with\ncomparable or superior activity. A more comprehensive benchmarking table including\ncurrent density, stability, and other performance of recent Ru, Pd, and Pt SACs would\nbetter contextualize the advancement claimed in this work.\nAnswer 4: Thanks for your constructive suggestion. We fully agree that a broader\n36\nbenchmarking comparison is important for contextualizing the performance of NC-Ru-\n950 catalyst.\nIn the revised supplementary information, we have expanded benchmarking\nanalysis of catalytic performance, including Ru, Pd, and Pt SACs. Regarding Ru,\nPd,\nand Pt single-atom catalysts (SACs), we have carefully surveyed the recent literature\nas suggested. We note that:\n1. Research on Ru single-atom catalysts has primarily focused on alkaline media,\nwhile studies in acidic electrolytes remain limited (J. Am. Chem. Soc. 2022,\n144, 5, 2197-2207, Energy Storage Mater. 2022, 44, 469-476.).\n2. Pd and Pt single-atom catalysts are rarely utilized for the 4e⁻ ORR pathway that\nleads to H O. Instead, Pd and Pt SACs are predominantly designed for 2e⁻ ORR\n2\nto selectively produce H O , and thus do not serve as suitable benchmarks for\n2 2\nacidic 4e⁻ ORR performance (J. Am. Chem. Soc. 2024, 146, 16, 11181-11192,\nNat Commun 13, 685 (2022)., Adv. Funct. Mater. (2025): e16600., Nat\nCommun 13, 4737 (2022)., J. Phys. Chem. C 2025, 129 (19), 8877-8888.)\n3. Correspondingly, only a limited number of Ru, Pd, and Pt SACs with credible\n4e⁻ ORR metrics (current density, mass activity, stability, etc.) in acid have\nbeen reported, and their ORR performance is generally inferior to state-of-the-\nart Fe-N-C catalysts.\nNevertheless, in response to the reviewer’s suggestion, we have added the\nbenchmarking table summarizing the available Ru, Pd, and Pt SACs, including their\nactivity and stability in the revised supplementary information (Table R6 (S9) and\nTable R7 (S10)). Meanwhile, Figure R28 (4f) and Figure R29 (5d) are revised. This\nadditional comparison further highlights the advancement of NC-Ru-950 catalyst under\nacidic conditions and in proton exchange membrane fuel cells.\nN-C catalysts in acid media.\nActivity Stability\nCatalyst E Chronoamperometry Reference\n1/2\nCV cycles\n(V vs. RHE) (i - t)\nNC-Ru-950 0.816 Loss 9 mV (30k) 93% 300 h at 0.6 V This work\nCS Fe/N–C 0.820 Loss 8 mV (5k) / 11\nFeSA/FeAC−2DNPC 0.81 Loss 15 mV (10k) 78.9% 20 h at 0.75 V 22\nFe-N /C-NC1100 0.811 Loss 25 mV (30k ) / 33\nX\nFe–NC/Scaad-CeO2 0.822 Loss 14 mV (10k) / 44\nFe–NC/ScaCeO2 0.812 Loss 24 mV (10k) / 54\nFe SAC-MOF-5 0.83 Loss 23 mV (10k) 97.1% 7.3 h at 0.5 V 65\nFe-N-C-SP 0.80 Loss 30 mV (30k) / 76\nFe/NC-NaCl 0.832 Loss 22 mV (20k) 83.6% 16 h at 0.8 V 87\nFe -N-C 0.78 Loss 20 mV (20k) / 98\nNC\nFe N 0.84 Loss 24 mV (10k) / 1110\n2 6\nFe(mIm)–NC 0.84 Loss 28 mV (10k) / 1211\nFe-SA-NSFC 0.82 Loss 24 mV (20k) / 1312\nCl-Fe-NC 0.82 Loss 15 mV (10k) ~90% 50 h at 0.7 V 1413\nHP-FeN 0.8 Loss 26 mV (10k) / 1514\nRu-N/G-750 0.75 Loss 18 mV (10k) 78.9% 2.7 h at 0.70 V 1716\nPt1-N/BP 0.760 / / 1817\nPd-N-C SACs ~0.42 / / 1918\nPt-N-C SACs ~0.41 / / 2018\nPd/N,S-HCS ~0.45 / / 2119\nRu /N-C 0.835 Loss 18 mV (20k) / 2220\nPt /N-C ~0.67 / / 2320\nPd /N-C ~0.70 / / 2420\nNote: E1/2 represents the half-wave potential and ~ denotes approximate values that were not\nexplicitly stated by the authors but extracted from the paper.\nreported Fe-N-C and Fe-free-N-C catalysts.\nPeak power\nCurrent density Stability\ndensity under 1\nCatalyst under 1 bar H -air (Square Reference\n2\nbar H -air\n2\n(mA cm-2) wave cycle)\n(mW cm-2)\n670.9 (BOL) 82.04% (30 k\nNC-Ru-950 77.5 at 0.8 V This work\n550.4 (EOL) cycles) in P\nFe–S–NC/Fe C 27.0 at 0.8 V 535 (BOL) / 121\n3\n401 (BOL) 72.0% (30 k\nFe−NC/Scaad-CeO2 65.5 at 0.8 V\niR-free\n24\n288 (EOL) cycles) in P\n366 (BOL) 47.0% (30 k\nFe−NC/ScaCeO2 ~48.5 at 0.8 V\niR-free\n34\n170 (EOL) cycles) in P\nFeCl −NC-1000 ~35.8 at 0.8 V 280 (BOL) / 422\nFe-N /C-NC1100 11.57 at 0.8 V 312 (BOL) / 63\nX\nFeNC-CVD-750 ~42.9 at 0.8 V 370 (BOL) 723\n450 (BOL) 80.0% (70 k\nFe-N-C ~56.9 at 0.8 V 824\nCVM\n410 (EOL) cycles) in P\n20Mn-NC ~31.5 at 0.8 V 180 (BOL) / 925\nCo(mIm)–NC(1.0) ~24.5 at 0.8 V 320 (BOL) / 1011\n(Au-Co) DP-NPAs ~7.4 at 0.8 V 360 (BOL) / 1126\n390 (BOL) 96.0% (15 k\n1% Co-N-C + Co 60 at 0.8 V 1227\n~375 (EOL) cycles) in P\nRu-SSC ~34.7 at 0.8 V ~260 (BOL) / 1315\n400 (BOL) 68.0% (30 k\nRu /N-C ~10.4 at 0.8 V 1420\n~272 (EOL) cycles) in P\nNote: P represents the peak power density in fuel cells, and ~ denotes approximate values that were\nnot explicitly stated by the authors but extracted from the paper.\n4\n(a) SCV polarization curves of NC, Pt/C, and NC-Ru-T (T = RT, 350℃, 550℃, 750℃, and 950℃).\nSCV polarization curves before and after 10k, 20k, and 30k CV cycles of (b) NC-Ru-950 and (c)\nFe-NC (the inset is the values of E after different aging cycles). (d) i-t curves of NC-Ru-950 and\n1/2\nFe-NC catalysts tested in 0.1 M HClO at a constant potential of 0.6 V. (e) The UV-vis absorption\n4\nspectra of 0.1 M HClO solutions consist only ABTS; Fe-NC and ABTS; NC-Ru-950 and ABTS;\n2 2 2 2 2 2\nComparison of the E loss after CV cycles between NC-Ru-950 and the recently reported catalysts.\n1/2\n(Yellow, green, blue, and red represent 5k, 10k, 20k, and 30k aging cycles, respectively.)\n40\na\n) 0 2-mc\nAm(\nyt -2 isned\ntnerru -4 C\nd\n1 0 0 )%( \nyt 8 0 isned\ntn\n6 0 erruc\nev 4 0 italeR\n2 0\n35\n) 30 Vm(\ns 25 elcy\n20 c\nVC\n15 retfa\n10 ssol\n5 2/1\nE\n0\nNC-Ru-350 NC-Ru-550 NC-Ru-750\nNC-Ru-950\nPt/C\n0 .2\nP\n0 .4 0\no te n tia l (V\n34.3%\n5 0\n.6\nvs. R H\n0 .8\nE )\n1 0 0\n1 .0\nT\nb\n) 0 2-mc\nAm(\nyt -2 isned\ntnerru -4 C\n1 5 0\nim e (h )\n0\ninitial after 10k C V cycles\nafter 20k C V cycles after 30k C V cycles\nE 1/2\n0.816 0.808 0.808 0.807\ns s s e e e l l l c c c y y y c c c la k k k it 0 0 0 in 2 3 1 i\n.2 0 .4 0\nP o te n tia l (V\n2 0 0\n9 m\n.6\nvs. R H\n2 5 0\nV\nN C -R u-950\n.8 1 .0\n)\n93%\nN C -R u-950 Fe-N C\n3 0 0\n)2-mc\nAm(\nytisned\ntnerruC\necnabrosbA\nc\n0\n-2\n-4\ne\n0 .8\n0 .6\n0 .4\n0 .2\n0 .0\n3 6 0\n0\ninitial Fe-N C after 10k C V cycles\nafter 20k C V cycles after 30k C V cycles\nE 1/2\n0.813 31 m V\n0.796\n0.788 0.782\ns s s e e e l l l c c c y y y c c c la k k k it 0 0 0 in 2 1 3 i\n.2 0 .4 0 .6 0 .8 1 .0\nP o te n tia l (V vs. R H E )\nABTS\nFeNC+ABTS NC-Ru-950+ABTS\nH O +ABTS 2 2 FeNC+H O +ABTS 2 2 NC-Ru-950+H O +ABTS 2 2\n4 0 0 4 4 0 4 8 0\nW a ve le n g th (n m )\nCS F F e e - / N N C -C /Scaad-C F e e O S C 2 A l-F /F e e - A N C C −2DNPC Ru-N/ F G e -7 S 5 A 0 C-MO Fe F - - N 5 C/ScaCeO2 Fe2 N6 HP-Fe F N e 4 (mIm)-NC Ru-SSC Ru1 /N-CPore F s e2 -N- F C e/NC-N F a e C - l SA-NSF F C e-N F - e C -N /P x d /C N - C NC1100 Fe-N-C-S N P C-Ru-950\nf\n5 k c y c le s 1 0 k c y c le s 2 0 k c y c le s 3 0 k c y c le s\nFigure R29 (Figure 5). The MEA performance of NC-Ru-950 and Fe-NC catalysts. (a) The\ndetermination at 0.9 V and 44.00 mA cm-2under 1.0 bar H -O for NC-Ru-950, respectively.\niR-free 2 2\nPolarization and power density curves of (b) NC-Ru-950 (BOL and after 30k cycles AST) and (c)\nFe-NC (BOL and after 30k cycles AST). (d) Comparison of the MEA performance between NC-\nRu-950 and recently reported catalysts under H -air conditions. DRT analyses for NC-Ru-950 (e)\n2\nand Fe-NC (f) during the AST process.\nQuestion 5. Although the catalyst exhibits excellent stability over 30,000 cycles,\npost-mortem analyses (e.g., AC-HAADF-STEM, ICP-OES) after testing are not provided.\nEvidence of retained atomic dispersion and minimal metal leaching after aging would\nsolidify the stability claims.\nAnswer 5: Thank you very much for your comment. The reviewer rightly points out\nthat post-mortem analyses are crucial for solidifying the stability claims.\nWe are pleased to clarify that in the original manuscript, we placed the relevant\ndata in Figure 6, which may have led the reviewer to overlook it. Specifically, the post-\nmortem characterization of the aged catalyst, including atomic-level dispersion and\nmetal leaching information, was presented in Figures 6f, 6g, and 6h (Figure R30). As\nshown in Figure 6f, the metal leaching behavior of the NC-Ru-950 and Fe-NC catalysts\nwas evaluated based on ICP-OES results. NC-Ru-950 catalyst exhibits a much lower\n41\na\n)V(\neerf-Ri egatloV\nd\n)2-mc\nWm(\nytisned rewop kaeP\nb\n0 .9 8 1.0 0.8\nH -air fuel cell N C -R u-950 N C -R u-950 H -O fuel cell 2 2 2\n0 .9 6 ) 0.8 2-m 0.6c\n0 .9 4\nW ) D O E target 2025 V 0.6 ( ( -2 44.00 m A cm y 0 .9 2 e t 0.4 g i @ 0.9 V s iR-free a n t e 0.4 l 0 .9 0 o d V\nr -2 44.00 m A cm e B O L w 0.2 @ 0.881 V 0 .8 8 iR-free o 0.2 10k cycles P\n20k cycles\n0 .8 6 30k cycles\n0.0 0.0\n0.00 0.02 0.04 0. 0 0 . 6 0 0 0 . . 5 08 1.0 1.5 2.0 2.5\n-2 -2 C u rre n t d e n sity (A cm ) C u rre n t d e n sity (A cm )\ne\n0.12 600 Initial E O L 10k cycles\n0.10 20k cycles ) ) Fe-S -N C /Fe C 500 Ω Ω 3 30k cycles Fe-N -C ( ( CVM 0.08 ) ) τ τ ad-CeO R u /N -C Fe−N C /S ca 2n n 1 Pores FeN C -C V D -750 400 l l ( ( 0.06 1% C o-N -C + C o γ γ (A u-C o) D P -N P A s CeO Fe−N C /S ca 2 300 Fe S A C -M O F-5 C o(m Im ) 0.04 Fe-N / FeC l−N C -1000 X -N C (1.0) O R R 2 C -N C 1100 R u-S S C + H Transport 0.02 200 P 3 P 2 P1 20M n-N C\n0.00\n0 10 20 30 40 50 60 70 80 -4 -3 -2 -1 0 10 10 10 10 10\n-2 i @ 0 .8 V H -a ir fu e l ce ll (m A cm ) τ (s) 2\nc\n1.0 0.8\nB O L H -air fuel cell Fe-N C 2 10k cycles\n20k cycles ) 0.8 2 30k cycles -m 0.6\nc\nW )V 0.6 ( ( y e t 0.4 g is a n t e 0.4 lo d V\nrew 0.2 o 0.2 P\n0.0 0.0\n0.0 0.5 1.0 1.5 2.0 2.5\n-2 C u rre n t d e n sity (A cm )\nf\nFe-N C H -air fuel cell 0.14 2\n0.12 Initial 10k cycles\n0.10 20k cycles 30k cycles 0.08 0.06 O R R 0.04 + H Transport P 3 P 2 P1 0.02\n0.00\n-4 -3 -2 -1 0 10 10 10 10 10\nτ (s)\nmetal leaching ratio (1.5%) compared to Fe-NC (17.9%) catalyst. And AC-HAADF-\nSTEM images of post-aging states for NC-Ru-950 and Fe-NC catalysts were displayed\nin 6g and 6h. NC-Ru-950 catalyst maintains its atomic dispersion, in contrast to the Fe-\nNC catalyst, which shows significant nanocluster formation. In summary, the NC-Ru-\n950 catalyst demonstrates excellent stability.\nFigure R30 (Figure 6). (f) The demetallation energy for FeN and O-RuN and metal leaching\n4 4\nresults for NC-Ru-950 and Fe-NC after 30k aging in MEA. The AC-HAADF-STEM images for\nNC-Ru-950 (g) and Fe-NC (h) after 30k aging in MEA.\nWe have revised the manuscript: To further demonstrate the stability of the catalysts,\nICP-OES and AC-HAADF-STEM characterizations of the aged NC-Ru-950 and Fe-\nNC catalysts were performed and analyzed (Figure 6f, Figure 6g, and Figure 6h). Under\nthe harsh testing conditions, NC-Ru-950 exhibited minimal metal leaching (1.5%) and\nretained its atomically dispersed structure, whereas Fe-NC showed pronounced metal\nloss (17.9%) and the formation of visible nanoclusters.\nQuestion 6. Key experimental details are missing, such as error margins in performance\nmetrics (e.g., current density, power density). Including statistical data (e.g., standard\ndeviations from multiple tests) would enhance reliability.\nAnswer 6: Thank you for your valuable comment. We agree that providing statistical\ninformation is essential to improve the reliability and rigor of the experimental results.\nIn the revised supplementary information, we have supplemented the statistical data\naccordingly. As shown in Figure R31 (Figure S31), error bars represent the standard\ndeviation for three separate measurements, and a smaller standard deviation indicates\nthe repeatability and reliability of the experimental data.\n42\na\nR 3\n2\n)Ve\n1 (\nG∆\n0\n-1\n0 .0\nd\n5\n)V\n0 e(\nygrenE\n-5\n-1 0\n-3\nf\n3\n2 )Ve(\nygrenE\n1\n0\nuN 4\n0 .2\ntotal\nd p 3 -2\n-2 -1 0\n-C O H\nN C\n0 .4 0 .6 0 .8\nU (V )\nRHE\nFe-N ave\nIC O H P\n=-2.309\ntotal\nd p 4 -2\n-3 1 -2 2 -1 3 0\nP -C O H\ndem etallation energy\nm etal leaching\n-R u -9 5 0 F e -N\n*O O H *O\n*O H *H slab\n0.65 V\n1 .0\nO -R u-N ave\nIC O H P\n=-3.009\n1 2 3\nP\nC\n2\n2\n1\n1\n5\n0\nb\n5\n0\n5\n0\n).u.a(\nnoissimsnarT\n)%( \ngnihcael\nlateM\n8 0 0\n*O\n1 0 0 0\nW\n*O O H\n1 2 0 0\na v e n u m\n1 4\nb\n0\ne\n0\nr (n\n1\nm\n*O H\n6 0 0\n-1)\nN\n8\n-R u -9 5 0\n0 .6 V\n0 .7 V\n0 .8 V\n0 .9 V\n1 .0 V\n1 .1 V\nO C V\n0 0 2 0 0 0\nHO-NuR\n4\npu nips\nHO-NuR\n4\nnwod\nnips\nRu4dz2 -O2p\nRu4dxz-O2p\n0\n-5\n-10\n)Ve(\nygrenE\n5\n0\n-5\n-1 0\n-3 -2\nto ta l\nd p 3 -2\n-1 0\n-C O H\nF e -N ave\nIC O H P\n= -2 .3 0 9\n1 2 3\nP\n-3 -2\nto ta l\nd p 4 -2\n-1 0\n-C O H\nO -R u -N ave\nIC O H P\n= -3 .0 0 9\n1 2 3\nP\n)Ve(\nygrenE\nHO-NuR-O\n4\npu nips\nHO-NuR-O\n4\nnwod\nnips\nFigure R31 (Figure S31). Error analysis of MEA performance metrics (current density, power\ndensity). Error bars represent the standard deviation for three separate measurements. (a) The\ndetermination at 0.9 V and 44.00 mA cm-2under 1.0 bar H -O for NC-Ru-950, respectively.\niR-free 2 2\nPolarization and power density curves of (b) NC-Ru-950 (BOL and after 30k cycles AST) and (c)\nFe-NC (BOL and after 30k cycles AST).\nQuestion 7. There is an absence of in situ or operando characterization during the ORR.\nThe study relies heavily on ex situ characterizations. In situ XAFS or Raman\nspectroscopy during ORR could provide direct evidence of the active site structure and\nreaction intermediates, strengthening the mechanistic conclusions.\nAnswer 7: We sincerely appreciate the reviewer's insightful suggestion regarding the\nabsence of in situ or operando characterization during the oxygen reduction reaction\nprocess. We fully agree that such measurements are crucial for a definitive\nunderstanding of the active site structure and reaction mechanism.\nWe initially attempted the suggested in situ Raman spectroscopy; however, we\n43\nb\n)V(\negatloV\n1\n0\n0\n0\n0\n0\n.0\n.8\n.6\n.4\n.2\n.0\n.0\n-a\n2\nir fu e l c\n0 .5\ne\nC\na 0 .9 8\nH -O fu e l c e ll\n2 2\n0 .9 6\n) 0 .9 4 V(\neer\n0 .9 2 f-Ri\nega\n0 .9 0\ntloV\n0 .8 8\n0 .8 6\n0 .0 0 0 .0 2 0 .0 4\nC u rre n t d e n s ity\nc 1 0 .8\nll N C -R u -9 5 0\n0 )2 0 .6 -mc\n) W V 0\n( ( e y 0 .4 g ti a s\nt n 0 lo e\nV d\nre\n0 .2 B O L w 0 o 1 0 k c y c le s\nP\n2 0 k c y c le s\n3 0 k c y c le s\n0 0 .0\n1 .0 1 .5 2 .0 2 .5\n-2 u rre n t d e n s ity (A c m )\nF i g u r e S 3 1 . E r r o r a n a l y\np e r f o r m a n c e m e t r i c s\nd e n s i t y , p o w e r d e n s i t y )\nr e p r e s e n t t h e s t a n d a r d d\nt h r e e s e p a r a t e m e a s u r e m\nN C -R u -9 5 0\n0 .0 6 0 .0 8\n-2 (A c m )\n.0\nB O L H -a ir fu e l c e ll F\n2 1 0 k c y c le s\n2 0 k c y c le s .8 3 0 k c y c le s\n.6\n.4\n.2\n.0\n0 1 2\n-2 C u rre n t d e n s ity (A c m )\ns i s o f M E A\n( c u r r e n t\n. E r r o r b a r s\ne v i a t i o n f o r\ne n t s .\ne e -N C\n0\n0\n0\n0\n0\n.8\n)2-m .6\nc\nW(\nyt .4 isned\nrew\n.2 oP\n.0\nencountered an experimental challenge: in 0.1 M HClO electrolyte, the strong\n4\nfluorescence interference caused by ClO - under a 532 nm excitation laser made it\n4\ndifficult to obtain reliable spectral features, thereby hindering the detection of reaction\nintermediates on the catalyst surface. To overcome this limitation and provide the\ndesired direct mechanistic evidence, we successfully performed in situ attenuated total\nreflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). As\nshown in Figure R32, at the initial stage of ORR, only the characteristic vibrational\npeak (*O, ~ 900 cm-1) is detectable (Nat Commun 15, 1675 (2024)). This observation\nstrongly suggests that the O-RuN structure serves as the real active site. As the\n4\npotential is lowered, characteristic vibrational peaks of *OOH (~ 1225 cm-1), *O (~ 900\ncm-1), and *OH (~ 1650 cm-1) gradually intensify, suggesting that ORR proceeds via an\nassociative four-electron pathway on the O-RuN active site (Nat Commun 16, 8963\n4\n(2025)., Angew. Chem. Int. Ed., 2025, 64(37), e202508674., Angew. Chem. Int. Ed.,\n2025, 64(34): e202509790.)\na b c\nd Figure R32 (Figure 6). (b) eIn situ ATR-SEIRAS spectra of NC-Ru-950 during ORR process\nFeN 4\nIn the revised manuscript, we have added the da+ta of in F si e tu attenuated total\ndemetallation\nreflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and the\nO-RuN\n4\ncorresponding descriptions:\n+ Ru\nFurthermore, in situ attenuated total reflectance surface-enhanced infrared\ndemetallation\nabsorption spectroscopy (ATR-SEIRAS) is employed to clarify the active site structure\nand reaction intermediates (Figure 6b). At the initial stage of ORR, only the\nf g h\ncharacteristic vibrational peak (*O, ~ 900 cm-1) is detectable[42]. This observation\n44\n1 nm 1 nm\n)Ve(\nG∆\n3\n2\n1\n0\n-1\n)Ve(\nygrenE\nR\n0 .0\n3\n2\n1\n0\nu N 4\n0 .2\nN C -R\n0 .4 0 .6\nU (V )\nR H E\nd e m e ta lla tio n\nm e ta l le a c h in\nu -9 5 0\ne\ng\nn\n0 .8\ne rg y\nF e -N C\n*O O *O\n*O H\n*H\ns la b\n0 .6 5\n1 .0\nH\nV\n2\n2\n1\n1\n5\n0\n5\n0\n5\n0\n).u.a(\nnoissimsnarT\n)%( \ngnihcael\nlateM\n8 0 0\n*O\n1 0 0 0\nW\n*O O H\n1 2 0 0\na v e n u m\n1 4\nb\n0\ne\n0\nr (n\n1\nm\n*O H\n6 0 0\n-1)\nN\n8\n-R u -9 5 0\n0 .6 V\n0 .7 V\n0 .8 V\n0 .9 V\n1 .0 V\n1 .1 V\nO C V\n0 0 2 0 0 0\nHO-NuR\n4\npu nips\nHO-NuR\n4\nnwod\nnips\nRu4dz2 -O2p\nRu4dxz-O2p\n0\n-5\n-10\n)Ve(\nygrenE\n5\n0\n-5\n-1 0\n-3 -2\nto ta l\nd p 3 -2\n-1 0\n-C O H\nF e -N ave\nIC O H P\n= -2 .3 0 9\n1 2 3\nP\n-3 -2\nto ta l\nd p 4 -2\n-1 0\n-C O H\nO -R u -N ave\nIC O H P\n= -3 .0 0 9\n1 2 3\nP\n)Ve(\nygrenE\nHO-NuR-O\n4\npu nips\nHO-NuR-O\n4\nnwod\nnips\nstrongly suggests that the O-RuN structure serves as the real active site. As the\n4\npotential is lowered, characteristic vibrational peaks of *OOH (~ 1225 cm-1), *O (~ 900\ncm-1), and *OH (~ 1650 cm-1) gradually intensify, suggesting that ORR proceeds via an\nassociative four-electron pathway on the O-RuN active site[43-45].\nand stability of NC-Ru-950 based on DFT calculations. (a) The surface thermodynamic phase\ndiagram of NC-Ru-950. (b) In situ ATR-SEIRAS spectra of NC-Ru-950 during ORR process. (c)\nThe project COHP analyses between Ru 4d and O 2p (*OH) for RuN -OH and O-RuN -OH. (d)\n4 4\nCOHP analyses between M (M: Fe/Ru) and N for FeN and O-RuN . (e) The simulated\n4 4\ndemetallation process of the Ru site and the Fe site. (f) The demetallation energy for FeN and O-\n45\na\nR 3\n2\n)Ve\n1 (\nG∆\n0\n-1\n0 .0\nd\n5\n)V\n0 e(\nygrenE\n-5\n-1 0\n-3\nf\n3\n2 )Ve(\nygrenE\n1\n0\nuN 4\n0 .2\ntotal\nd p 3 -2\n-2 -1 0\n-C O H\nN C\n0 .4 0 .6 0 .8\nU (V )\nRHE\nFe-N ave\nIC O H P\n=-2.309\ntotal\nd p 4 -2\n-3 -2 1 -1 2 0 3\nP -C O H\ndem etallation energy\nm etal leaching\n-R u -9 5 0 F e -N\n*O O H *O\n*O H *H slab\n0.65 V\n1 .0\nO -R u-N ave\nIC O H P\n=-3.009\n1 2 3\nP\nC\n2\n2\n1\n1\n5\n0\nb\n5\n0\n5\n0\n).u.a(\nnoissimsnarT\n)%( \ngnihcael\nlateM\n8 0 0\n*O\n1 0 0 0\nW\n*O O H\n1 2 0 0\na v e n u m\n1 4\nb\n0\ne\n0\nr (n\n1\nm\n*O H\n6 0 0\n-1)\nN\n8\n-R u -9 5 0\n0 .6 V\n0 .7 V\n0 .8 V\n0 .9 V\n1 .0 V\n1 .1 V\nO C V\n0 0 2 0 0\n+\n+\nc\n5\nd p Ru4 -O2 yzd p Ru4 -O2 z2d p Ru4 -O2 xz )V\n0 e( ygrenE\n-5\nHO-4NuR-O\n-1 0\nd e m e ta lla tio n\nR u\nd e m e ta lla tio n\nh\n1 n m\npu nips\nHO-4NuR-O\nnwod\nnips\nHO-4NuR\npu nips\nHO-4NuR\nnwod\nnips\nFigure R37 (Figure S36). The surface thermodynamic phase diagram of FeN (a) and Gibbs free\n4\nenergy diagram of O-RuN (b), RuN (c), and FeN (d).\n4 4 4\nConsidering the possible forms of active centers for the NC-Ru-950 catalyst under\nactual operating conditions, the models of O-RuN , HO-RuN , HOO-RuN , and H-\n4 4 4\nRuN were constructed and optimized (Figure S33b-e). Interestingly, based on the\n4\nsurface thermodynamic phase diagram in Figure 6a, we discovered that the energy of\nRuN with axially adsorbed O is the lowest (O-RuN ) at 0.65 V, indicating that the NC-\n4 4\nRu-950 catalyst stably exists in this structure under the operating voltage [38-41].\nAs shown in Figure S36, the Gibbs free energy diagrams on the O-RuN , RuN ,\n4 4\nand FeN site are calculated, and the theoretical limiting potential (U ) of O-RuN is\n0.96 V, which is higher than that of RuN (0.54 V) and FeN (0.72 V).\n4 4\n……In the case of DFT+U, the U values for TM were taken from the literature[51].\nMeanwhile, this work incorporates implicit solvation models and constant-potential\ncalculations to simulate the electrocatalytic interface……\nQuestion 9. In the “Methods” section, for the preparation of NC-Ru-Ar and NC-Ru-\n49\n)Ve(\nygrene\neerF\n)Ve(\nG∆\nc\na\n4\n3\n2\n1\n0\n-1\n0\n6\n5\n4\n3\n2\n1\n0\n-1\n-2\nF i g\nF\n.0\nR\nu\ne N\nu N\nr\n4\n4\ne\n.2\n3\n*O O\nR\nG 0\ne\n.4\nU\nR\na c tio\ni b b\n0 .6\n(V )\nH E\n*O\nn p a th\ns f r e\n*O O *O\n*O H\n*H\ns la b\n0 .6 5\n0 .8\n*O H\nw a y\ne e n e\nH\nV\nU\nU\nU\nr g\n===\n1 .0\n0 .5 4\n1 .2 3\ny d\nV\nV\ni a\n)Ve(\nygrene\neerF\n)Ve(\nygrene\neerF\ng\nb\nd\nr a\n6\n5\n4\n3\n2\n1\n0\n-1\n-2\n6\n5\n4\n3\n2\n1\n0\n-1\n-2\nm\nO -R\nF e N\no f\nu\nR\n4\nu N\n*O O\nR\n*O O\nR\n,\ne a\nH\ne a\nO\nc\nc\ntio\ntio\n- R\n*O\nn p\n*O\nn p\nu\na\na\nN\n*O H\nth w a y\n*O H\nth w a y\n,\na\n4\nn d\nU U\nU\nU\nU\nU\nF\n= 0 = 0\n= 1\n= 0\n= 0\n= 1\ne\nV.9.2\nV.7.2\nN\n63\nV\nV\nV\n4\n.\nH2/Ar catalysts, it is mentioned that “the black powder was mixed with melamine\nthrough solid-state grinding.” Please specify the exact mass ratio of melamine to the\nmaterial. This ratio likely influences the nitrogen doping level during thermal treatment\nand consequently the final catalyst performance, making this detail crucial for the\nreproducibility of the experiments.\nAnswer 9: Thanks for your good suggestion. As recommended, we have now specified\nthe exact mass ratio of melamine to the precursor material in the revised manuscript\n(Methods section). In our synthesis, melamine and the black powder were mixed at a\nmass ratio of 1:1 (melamine: precursor). This ratio was consistently used for all NC-\nRu-Ar and NC-Ru-H₂/Ar samples.\nIn the revised manuscript:\nFinally, the black powder was mixed with melamine at a 1:1 mass ratio through\nsolid-state grinding and then annealed at 950℃ for 1 h under different atmospheres,\nyielding the samples named as NC-Ru-Ar and NC-Ru-H\n2\n/Ar.\nFinally, the black powder was mixed with melamine at a 1:1 mass ratio through\nsolid-state grinding and then annealed under H /Ar atmosphere for 1 h at a series of\n2\ntemperatures, named as NC-Ru-T (T= 350℃, 550℃, 750℃, and 950℃).\n50"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have made a substantial number of improvements to their manuscript.\nI am very afraid to say that I cannot recommend this manuscript for publication in Nature Communications, as the theory part looks not fully under control.\nThe authors reported that they have reinforced their DFT calculations: \"In the case of DFT+U, the\nU values for TM were taken from the literature51 589 . Meanwhile, this work incorporates implicit solvation models and constant-potential calculations to simulate the electrocatalytic interface. \"\nUnfortunately, I was unable to find any reference or explanation for the implicit solvation scheme adopted. Most importantly, there is not explanation for the way constant potential calculations have been made. Constant potential (DFT?) is a very advanced methodology, most often invoking somehow the introduction of Grand Canonical formalism. It should be described the way the method has been implemented, and the added value with respect to the \"classical\" computational hydrogen electrode (CHE) approach within the constant charge formalism, the standard methodology.", "reviewer_2": "The manuscript has been revised carefully. All the comments are addressed. It can be accepted at this state.", "reviewer_3": "The authors have addressed most of the points raised. However, one issue requires further analysis and clarification regarding the XPS data for Ru.\nIn their response to Reviewer 2's second question, the authors provided new XPS data. In Figure S6, they state that the signal from Ru nanoclusters in NC-Ru-H2/Ar​ is scarcely detected, concluding that Ru exists primarily as single atoms in this sample. Yet, their own fitting for the NC-Ru-Ar​ sample includes a Ru (0) peak. More importantly, the newly provided spectrum for NC-Ru-H2/Ar​ shows a distinct shoulder in the 462-459 eV binding energy region, which indicates the presence of a signal very close to that of metallic Ru (0). The current fitting does not adequately account for this spectral feature. The authors need to carefully re-examine and clarify this discrepancy in their XPS data analysis."}, "author_response": "Response to the reviewers' comments on the Manuscript ID: NCOMMS-25-70659B\nentitled: Synthesis of atomically dispersed metal catalysts via hydrogen\nembrittlement-like assisted thermal activation strategy (Second round)\nWe thank the reviewers for their constructive and insightful comments. In response, we\nhave thoroughly studied the comments from Reviewers #1 and #3 and have carefully\nrevised the manuscript accordingly. Here we present a detailed answer to the review.\nOur replies were highlighted in blue, and the revised content has been highlighted in\nthe main text and supplementary information.\nREVIEWER COMMENTS\nReviewer #1\nThe authors have made a substantial number of improvements to their manuscript.\nI am very afraid to say that I cannot recommend this manuscript for publication in\nNature Communications, as the theory part looks not fully under control.\nThe authors reported that they have reinforced their DFT calculations: \"In the case of\nDFT+U, the U values for TM were taken from the literature51 589 . Meanwhile, this\nwork incorporates implicit solvation models and constant-potential calculations to\nsimulate the electrocatalytic interface. \"\nUnfortunately, I was unable to find any reference or explanation for the implicit\nsolvation scheme adopted. Most importantly, there is not explanation for the way\nconstant potential calculations have been made. Constant potential (DFT?) is a very\nadvanced methodology, most often invoking somehow the introduction of Grand\nCanonical formalism. It should be described the way the method has been implemented,\nand the added value with respect to the \"classical\" computational hydrogen electrode\n(CHE) approach within the constant charge formalism, the standard methodology.\nResponse to reviewer #1’s comment:\nWe sincerely thank the reviewer for raising these critical methodological points,\nwhich have prompted us to provide a more thorough and transparent description of our\n1\ncomputational approach. Below, we provide a detailed response addressing the\nconcerns regarding the implicit solvation scheme and the constant-potential\nmethodology.\n1. Implementation of the implicit solvation model\nAs the reviewer#1’s suggestion, we have added references and related explanations\nregarding the implicit solvation model. The solvent environment was simulated using\nthe VASPsol code (J. Chem. Phys. 151, 234101 (2019), J. Chem. Phys. 140, 084106\n(2014)), which implements a self-consistent implicit electrolyte model within the plane-\nwave DFT framework. The model and corresponding code are well-established and\nwidely adopted in the research community. The relative permittivity was set to 78.4 to\nmodel the aqueous electrolyte. The effective surface tension parameter was assigned to\n0 in VASPsol to neglect the cavitation energy contribution. The linearized Poisson-\nBoltzmann model with a Debye length of 3.04 Å mimics the compensating charge.\n2. Implementation of constant-potential calculations\nBased on the reviewer#1’s suggestion, we have provided the details of the constant-\npotential calculations.\nThe constant-potential calculations in this work were performed using a grand\ncanonical density functional theory (GC-DFT) framework, as implemented in the CP-\nVASP code (J. Am. Chem. Soc. 145, 25352-25356 (2023)., J. Am. Chem. Soc. 143,\n9423-9428 (2021).). This approach allows the electron number (n) in the system to vary\nto maintain a constant electrode potential relative to the standard hydrogen electrode\n(SHE). As for the grand-canonical ensemble of electronic charges, the electrons should\nbe coupled with a fictitious potentiostat so that the number of electrons fluctuates\nfollowing the grand-canonical distribution at the preset electrode potential (Φ ). For\nd\nsimplicity, we adjust the electron number according to:\nn = C ( − )\nd\nwhere ∆n is the electron number correction for a given structure and electron number,\nΦ is the electron potential; C is the “capacitance” parameter that is chosen to be the\ncapacitance of the initial state. Note that the net electronic charges are balanced by the\n2\nionic charges in the implicit electrolyte, and thus the total system remains charge neutral.\nThe convergence threshold for the potential is 0.01.\nFor a given structure and electron number, the electron potential Φ with respect to\nthe stand hydrogen electrode (SHE) can be obtained as:\nΦ = [E − E − (E SHE − E SHE)]/e\nF es F es\nwhere E is the Fermi level of the system, E is the electrostatic energy in the middle\nF es\nof implicit electrolyte region, and E SHE and E SHE are the corresponding quantities in\nF es\nSHE. E SHE – E SHE is benchmarked to be 4.6 eV for VASPsol (J. Chem. Phys. 151,\nF es\n234101 (2019)).\nTo demonstrate the practical implementation and output of this methodology, we\npresent the potential-dependent data for the representative RuN and O-RuN sites in\n4 4\nFigure R1. As shown in Figure R1a and R1c, a decrease in the number of electrons in\nthe system leads to an increase in the system potential. Furthermore, the data in Figure\nR1b and R1d indicate that the energy of the system changes nonlinearly with the applied\npotential.\n3\n1\n1\n0\n0\n0\n0\n0\n.2\n.0\n.8\n.6\n.4\n.2\n.0\n-4 .0 -3 .5 -3 .0 -2 .5 -2 .0 -1 .5 -1 .0 -0 .5\n)V\n(\nla\nitn\ne\nto P\n n\nR\nR\nR\nR\nR\nu\nu\nu\nu\nu\nN\nN\nN\nN\nN\n-s 4-O\n4-O\n4-O\n4-H\n4\nla\nO\nH\nb\nH\nO-RuN-slab 1.2 4\nO-RuN-OOH\n4\n1.0 O-RuN 4 -O\nO-RuN-OH\n4\n)V0.8\n( la\nitn 0.6\ne\nto0.4 P\n0.2\n0.0\n-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5\nn\n-6\n-6\n-6\n-6\n-6\n-6\n-6\n-6\n-6\n-6\n4\n5\n5\n5\n5\n5\n6\n6\n6\n6\n8\n0\n2\n4\n6\n8\n0\n2\n4\n6\n0 .0 0 .5\nR\nR u\n1 .0\nR\nR\nu\nN\nu N -H\n4\nu N -O\n-O O H\n4\n)V\ne\n(\ny\ng\nre\nn E\nP o te n tia l (V )\nR u N -s 4 la b\na b\nc d\n-654\nO-RuN-slab\n-656 4\n-658\n)V -660\ne (\ny\n-662 O-RuN 4 -O\ng re -664\nn E-666 O-RuN 4 -OH\n-668\n-670 O-RuN-OOH\n4\n-672\n0.0 0.5 1.0\nPotential (V)\nFigure R1. (a) The relationship between electrode potential of system and the electron number\ncorrection (∆n) for RuN and corresponding reaction intermediates. (b) Calculated energies of RuN\n4 4\nand corresponding reaction intermediates as a function of the applied electrode potential. (c) The\nrelationship between electrode potential of system and the electron number correction (∆n) for O-\nRuN and corresponding reaction intermediates. (d) Calculated energies of O-RuN and\n4 4\ncorresponding reaction intermediates as a function of the applied electrode potential.\n3. Added value over the standard computational hydrogen electrode (CHE)\napproach\nIn the first version of the manuscript, we employed the standard computational\nhydrogen electrode (CHE) model. Subsequently, following Reviewer 3's suggestion to\nsimulate the electrocatalytic interface more realistically, we performed the constant-\npotential DFT calculations combining with implicit solvation models. Compared with\nthe standard CHE approach, the added value of the constant-potential method with an\nimplicit solvation model is summarized as follows:\nThe CHE method assumes a fixed charge (constant electron number, n) and applies\na posteriori potential correction. This neglects the explicit influence of the applied\npotential on the electronic structure of the catalyst (e.g., Fermi level shifts, orbital\noccupations), leading to an oversimplified linear response. The advantage of the GC-\nDFT method lies in its constant-potential approach, which maintains a target electrode\npotential relative to the reversible hydrogen electrode (RHE) by adjusting the electron\nnumber (n). This enables self-consistent calculations of electron density and adsorption\nenergies at each potential, captures nonlinear effects such as changes in adsorption\nconfiguration or reaction pathways with varying potential, and accurately describes\npotential-driven shifts in frontier orbitals. These orbital shifts directly influence the\nadsorption strength of intermediates and the overall reaction kinetics (J. Am. Chem. Soc.\n145, 25352-25356 (2023)., J. Am. Chem. Soc. 143, 9423-9428 (2021).).\nTherefore, the constant-potential method self-consistently calculates the electronic\nstructure and reaction energetics at each target potential, mirroring the actual conditions\nof an electrochemical experiment. This enables a more direct and physically realistic\nconnection to experimental data.\nIn summary, the reviewer’s insightful comments have been instrumental in\n4\nstrengthening the methodological rigor of our manuscript. By incorporating a detailed\ndescription of the implicit solvation model and the grand canonical constant-potential\nframework, we have significantly improved the clarity and theoretical foundation of\nour computational work. This version effectively addresses the reviewer’s concerns and\nclearly demonstrate the validity of our DFT simulations. We thank the reviewer again\nfor their constructive guidance.\nReviewer #2\nThe manuscript has been revised carefully. All the comments are addressed. It can be\naccepted at this state.\nResponse to reviewer #2’s comment:\nThank you very much for your careful review and constructive comments, which have\nsignificantly improved our manuscript.\nThe authors have addressed most of the points raised. However, one issue requires\nfurther analysis and clarification regarding the XPS data for Ru.\nIn their response to Reviewer 2's second question, the authors provided new XPS data.\nIn Figure S6, they state that the signal from Ru nanoclusters in NC-Ru-H2/Ar is\nscarcely detected, concluding that Ru exists primarily as single atoms in this sample.\nYet, their own fitting for the NC-Ru-Ar sample includes a Ru (0) peak. More\nimportantly, the newly provided spectrum for NC-Ru-H2/Ar shows a distinct shoulder\nin the 462-459 eV binding energy region, which indicates the presence of a signal very\nclose to that of metallic Ru (0). The current fitting does not adequately account for this\nspectral feature. The authors need to carefully re-examine and clarify this discrepancy\nin their XPS data analysis.\nResponse to reviewer #3’s comment:\nWe sincerely thank the reviewer #3 for the careful observation and valuable\nsuggestion regarding the Ru XPS analysis. Regarding the issue mentioned above, we\nprovide the following response:\nIn the original Figure S6, the binding energy of Ru(0) is located at 461.60 eV,\nwhich does not fully correspond to the shoulder peak for NC-Ru-H /Ar in the original\n2\nspectrum. The distinct shoulder in the 462-459 eV binding energy region is attributed\nto measurement noise. This primarily stems from the intrinsically weak XPS signal for\natomically dispersed Ru, which can be explained by several factors: firstly, the Ru 3p\norbital has a significantly smaller photoionization cross-section under the Al Kα source\ncompared to the 2p orbitals of common 3d transition metals, leading to a lower inherent\nphotoelectron yield. Secondly, the Ru 3p spectral region suffers from interference due\nto a pronounced background step created by intense MNN Auger peaks, severely\ndegrading the signal-to-noise ratio (SNR) and complicating peak fitting. Finally,\ncombined with the low Ru loading (<1 wt%) and the limited probing depth of XPS\n(typically 5-10 nm), the detectable signal is substantially diminished, resulting in a low\nSNR where minor spectral fluctuations can resemble faint spectral features.\nRu‑H /Ar sample by significantly extending the acquisition time and increasing the\n2\nnumber of scans. The newly obtained spectrum exhibits a markedly improved SNR and\nan absence of any Ru(0) peak. The corresponding data have been updated in the\nSupplementary Information (Figure R2(Figure S6)).\nFigure R2(Figure S6). The deconvoluted high-resolution Ru 3p spectra of NC-Ru-Ar and NC-\n3/2\nRu-H /Ar.\ntechniques—including HAADF-STEM images (Figure R3a) showing exclusively\nisolated Ru atoms and EXAFS spectra (Figure R3b) revealing no Ru–Ru\ncoordination—it is confirmed that Ru species in NC-Ru-H /Ar exist exclusively in an 2\natomically dispersed form, with no detectable Ru clusters or nanoparticles.\nFigure R3. (a) AC-HAADF-STEM image and (b) the FT-EXAFS spectra in the R space for NC-\nRu-H /Ar (NC-Ru-950 and NC-Ru-H /Ar refer to the same sample in this work).\n2 2\n).u.a(\nytisnetnI\nR\nu 3\nT\nP\nF i\np\n3/2\n4 6 5\nB in d\nh e d e c\nD F # 0 6 -0 6 6 3\n2 0\ng u r e S 7\nin\no\nN\n4 6 0\ng e n e rg y (e V )\nn v o l u t e d h\n4 0 6 0\nX R D p a t t e\nC -R u -H /A r 2\nN C -R u -A r\n4 5 5\ni g h - r e s o l u\nN C -R u -H /A r 2\nN C -R u -A r\n8 0\nr n s o f N C -\nt\nR\ni\nu\no\n-\nn\nA\np R u 3\n3/2\nR u 3\nP D F # 0 6\n3 5\nr a n d\n4 6 5\nB in\np\n3 / 2\n-0 6 6 3\nN C\nd in\ns p\n4 0\n- R\n4 6 0\ng e n e rg\ne c t r a\nu - H /\n2\ny (e V )\no f N\n4 5\n(° )\nr .\nN\nR\nC\nu\n-R\nN C\n- A r\nu -H /A\n2\n-R u -A\na\nr\nr\n5 0\nn d N C\nIn summary, the combination of direct imaging (HAADF-STEM), spectroscopic\nevidence (EXAFS), and high-SNR XPS data conclusively supports that Ru in NC-Ru-\nH /Ar is atomically dispersed. The initial shoulder feature stems from noise under low-\naccordingly. Thank you again for prompting this rigorous re-examination, which has\nstrengthened our manuscript."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors addressed the raised points.", "reviewer_3": "The revised manuscript can be accepted."}, "author_response": ""}]

Decision letter