Anion-tuned d-p hybridization breaks activity-stability trade-off in single-atom hydrogen evolution catalysts

Anion-tuned d-p hybridization breaks activity-stability trade-off in single-atom hydrogen evolution catalysts | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Anion-tuned d-p hybridization breaks activity-stability trade-off in single-atom hydrogen evolution catalysts Yida Deng, Rouna Jia, Zongyan Liu, Yang Wang, Feiyu Li, Jingyang Zhao, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7039226/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Single-atom catalysts hold great promise for hydrogen evolution reactions due to their maximal atomic utilization and discrete energy levels. Modulating metal-support interactions is a powerful strategy for tailoring the electronic structure and catalytic performance of single-atom catalysts. However, achieving precise control and gaining mechanistic insight into these interactions, especially at the orbital level, remains challenging and often controversial. Here, we construct a model system of rhodium single-atom catalysts, in which isolated Rh atoms are anchored on a series of molybdenum sulfide selenide supports (Rh SA -MoS x Se 2−x , 0 ≤ x ≤ 2), enabling gradient-continuous modulation of metal-support d-p orbital interactions through systematic tuning of the support p-band structure. We demonstrate that the d-band center of Rh single-atoms exhibits a volcano-type relationship with key HER descriptors, such as hydrogen and hydroxide binding energies, where hybridization-induced d-band position optimizes intermediate adsorption/desorption kinetics, and strengthened Rh-S/Se covalent interactions enhance durability. The apex Rh SA -MoSSe catalyst, with optimal d-p orbital hybridization, achieves superior HER activity and exceptional stability simultaneously. This work offers fundamental insights into the band structure-activity relationships of SACs and establishes a rational design framework for high-efficiency electrocatalysis through support-mediated d-p orbital hybridization. Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis Physical sciences/Chemistry/Catalysis/Electrocatalysis Single Atom Catalysts Metal-Support Interaction d-p Orbital Hybridization Volcano-Type Relationship Anionic Regulation Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Renewable-electricity-driven water electrolysis offers a sustainable hydrogen production pathway 1,2 , yet alkaline hydrogen evolution reaction (HER) suffers from sluggish kinetics due to coupled water dissociation and proton-transfer barriers 3-6 . While volcano relationships link HER activity to hydroxide (OH) 7-9 and hydrogen (H) 10,11 adsorption energies, simultaneously optimizing both intermediates remains fundamentally challenging. Single atom catalysts (SACs) hold great promise due to their maximal atomic utilization and tunable coordination environment 12–15 , but their activity and long-term stability still require significant enhancement. Metal-support interactions (MSIs) are crucial for stabilizing single atoms on the support and modulating the d-band structure of single-atom sites, while also influencing the local electronic structure of the support in SACs 16-19 . Precise control of MSIs is essential for enhancing catalyst stability and optimizing the binding energies of reactants and intermediates, which govern the catalytic performance of SACs. To date, MSIs research has predominantly focused on the active single-atom metal, with the critical role of support materials often overlooked. Conventional strategies, such as varying the type or increasing the loading of single atoms, escalate catalyst costs and face limitations in activity enhancement. Given that the coordination number, chemical bonding, and spatial environment of single-atom sites are dictated by the properties of the support, tailoring the support’s composition and phase structure offers an effective approach to modulate the geometry and electronic structure of SACs 20-22 . This has been demonstrated in heteroatom (e.g., N, P, O, and S) carbon-based SACs, where activity and selectivity are significantly altered by adjusting the extent of heteroatom coordination 23-26 . Compared to relatively inert carbon-based supports, transition metal compounds, with their tunable band structures and covalent bonding capabilities, serve as ideal platforms for stabilizing single atoms 27-30 . However, achieving precise MSIs control through atomic-level compositional gradients remains challenging, impeding a clear understanding of structure-performance relationships. Notably, the role of d-p orbital hybridization in anion-engineered supports remains largely unexplored, representing a critical gap in the orbital-level design of SACs. Herein, we establish an anion-gradient Rh SACs platform (Rh SA -MoS x Se 2-x , 0 ≤ x ≤2) enabling continuous d-p orbital hybridization via p-band center engineering. Combined with detailed experimental characterization and theoretical simulations, we reveal that bidirectional Rh-chalcogen coupling continuously tunes the d-band center of Rh single-atoms, exhibiting a volcano-type correlation with H and OH adsorption energies. Optimal d-p orbital hybridization in Rh SA -MoSSe simultaneously balances intermediate adsorption-desorption kinetics and reinforces Rh-S/Se covalent bonding, leading to exceptional long-term stability under industrially relevant fluctuating power conditions (> 300 h). This work resolves the orbital hybridization-electrocatalysis nexus, offering a universal design paradigm beyond activity-stability compromises. Results and Discussion To gain a comprehensive understanding of Metal-support interactions (MSIs), we construct a model system of Rh single-atom catalysts (SACs), in which the d-band structure of the Rh centers can be effectively modulated by tuning the anion composition in the support (Fig. 1 a). Conversely, the anchored Rh atoms also influence the p-band structure of the support anions, establishing a bidirectional electronic coupling that optimizes the adsorption energy barrier for reactants and intermediates (Supplementary Fig. 1). First principal density functional theory (DFT) calculations were conducted to investigate the MSIs effects based on the MoS x Se 2−x and Rh SA -MoS x Se 2−x (0 ≤ x ≤ 2) structure models (Supplementary Figs. 2 and 3). The density of states (DOS) reveals that the incorporation of Rh single-atoms significantly increases the DOS of MoS x Se 2−x near the Fermi level (Supplementary Fig. 4). Moreover, the band gap of Rh SA -MoSSe (0.85 eV) is notably narrower than those of Rh SA -MoS 2 (0.92 eV), Rh SA -MoSe 2 (1.03 eV), MoS 2 (1.70 eV), MoSSe (1.54 eV), and MoSe 2 (1.56 eV), thereby promoting accelerated electron transfer and optimizing the adsorption and desorption of reaction intermediates. Charge density difference analyses reveal distinct charge redistribution between Rh single-atoms and coordinated S/Se atoms, resulting in a marked reduction of electron density around Rh atoms (Fig. 1 b). This redistribution enhances the attraction of electronegative OH species, thereby strengthening local OH adsorption. Bader charge analysis of Rh atoms anchored on MoS x Se 2−x supports indicates varying electron transfer, with charges following the trend: Rh SA -MoS 2 (-0.22 e) ˂ Rh SA -MoSSe (-0.14 e) < Rh SA -MoSe 2 (-0.01 e). These results suggest that electron donation from Rh atoms to the MoS x Se 2−x supports can be systematically tuned by adjusting the S: Se ratio, enabling precise modulation of the Rh electronic structure. As shown in Fig. 1 c, projected density of states (PDOS) analysis of Rh SA -MoS 2 , Rh SA -MoSSe, and Rh SA -MoSe 2 reveals significant orbital hybridization between Rh d orbitals and the p orbitals of S and Se atoms, highlighting the role of d-p coupling in mediating metal-support interactions. Moreover, the d-band center of Rh atoms can be precisely and continuously modulated via d-p orbital hybridization between the Rh single atom and the coordinated S/Se atoms in the MoS x Se 2−x supports. According to d-band theory, the tunability of the Rh d-orbital states directly alters the adsorption energies of key intermediates, thereby influencing the catalytic activity for HER. Moreover, the d-band center of Rh in the Rh SA -MoS x Se 2−x series exhibits a volcano-type relationship with both the free energy of hydrogen adsorption (ΔG H* ) and hydroxide adsorption (ΔG OH* ), with Rh SA -MoSSe located at the volcano apex, displaying optimal adsorption strengths for both species (H and OH) (Fig. 1 d and Supplementary Figs. 5 and 6). To further elucidate the impact of MSIs, we evaluated ΔG H* and ΔG OH* at all possible and thermodynamically stable adsorption sites on both pristine MoS x Se 2−x and regions surrounding the Rh atoms in Rh SA -MoS x Se 2−x . It is worth noting that the introduction of Rh single-atom substantially enhances the adsorption of H and OH at Rh sites, synergistically improving both the thermodynamics and kinetics of HER (Figs. 1 e and 1 f). Inspired by these DFT results, we synthesized Rh single-atoms anchored on a series of MoS x Se 2−x with different S and Se ratios to validate the findings. The synthesis strategy for Rh SA -MoS x Se 2−x is illustrated in Supplementary Fig. 7. First, MoS x Se 2−x with different S and Se contents (denoted as MoS x Se 2−x , 0 ≤ x ≤ 2) were prepared via a hydrothermal method followed by annealing. Then Rh single-atoms were anchored on the MoS x Se 2−x substrates (denoted as Rh SA -MoS x Se 2−x , 0 ≤ x ≤ 2) using an electrochemical deposition method. The Rh loadings were comparable across the samples (Rh SA -MoS 2 : 0.35 wt.%, Rh SA -MoS 1.5 Se 0.5 : 0.42 wt.%, Rh SA -MoSSe: 0.51 wt.%, Rh SA -MoS 0.5 Se 1.5 : 0.38 wt.%, and Rh SA -MoSe 2 : 0.43 wt.%), as determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Scanning electron microscope (SEM) (Supplementary Fig. 8) and transmission electron microscopy (TEM) (Supplementary Figs. 9 and 10) characterizations revealed that all Rh SA -MoS x Se 2−x exhibit a similar nanoflower morphology, which promotes the exposure of active sites and enhances mass transfer. Energy-dispersive X-ray (EDX) spectroscopy elemental analysis confirms the uniform distribution of Rh, Mo, and Se in Rh SA -MoS x Se 2−x . As shown in Supplementary Figs. 11 and 12, the X-ray diffraction (XRD) patterns and corresponding Raman spectra indicate that the synthesized Rh SA -MoS x Se 2−x possess the same crystal structure as the 2H phase of MoS x Se 2−x 31,32 , and no distinct Rh-containing phases detected after Rh electrodeposition. In addition, high-resolution transmission electron microscopy (HRTEM) results (Supplementary Figs. 13 and 14) further confirm that the crystal structures of Rh SA -MoS x Se 2−x remain well-preserved following Rh decoration. The structural models of the Rh SA -MoS 2 , Rh SA -MoSSe, and Rh SA -MoSe 2 catalysts are presented in Figs. 2 a-c, respectively. The aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) detects the atomically dispersed Rh atoms (bright spots) on the MoS x Se 2−x nanosheets (Figs. 2 d-f). Furthermore, the different intensity profiles along the dashed yellow rectangles in the HAADF-STEM images confirm the single-atom dispersion of Rh atoms. As depicted in Fig. 2 g, the Fourier transformed extended X-ray absorption fine spectroscopy (FT-EXAFS) spectra of Rh in Rh SA -MoS 2 , Rh SA -MoSSe, and Rh SA -MoSe 2 display characteristic peaks at 1.8, 2.0, and 2.1 Å, corresponding to Rh-S, Rh-S/Se, and Rh-Se bonds 22 , 33 , respectively. In contrast to the Rh foil reference, the absence of a metallic Rh-Rh scattering signal rules out the presence of Rh nanoparticles or clusters, confirming the atomic dispersion of Rh species in Rh SA -MoS 2 , Rh SA -MoSSe, and Rh SA -MoSe 2 . Furthermore, Wavelet transformed EXAFS (WT-EXAFS) contour plots reveal single intensity maxima for Rh in Rh SA -MoS 2 , Rh SA -MoSSe, and Rh SA -MoSe 2 at approximately 6.7, 8.0, and 9.4 Å −1 (Fig. 2 h), corresponding to the Rh-S, Rh-S/Se, and Rh-Se coordination paths 34 , respectively. Notably, both the coordination path lengths and k-space positions of Rh-S/Se exhibit a linear correlation with the S: Se ratio in Rh SA -MoS x Se 2−x (Fig. 2 i), providing further evidence that Rh single-atoms are covalently bonded to S/Se and that their coordination environment can be precisely tuned by adjusting the Se: S ratio in MoS x Se 2−x substrates. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) analyses were performed to elucidate the electronic structures and chemical compositions of Rh SA -MoS x Se 2−x catalysts. As shown in Fig. 3 a, the synergistic electronic interactions among Rh, S, and Se in Rh SA -MoS x Se 2−x are characterized through S-Rh-S, S-Rh-Se, and Se-Rh-Se coordination units. In all Rh SA -MoS x Se 2−x catalysts, the predominant interaction between the fully occupied π-symmetry (t 2g ) d-orbitals of Rh x+ (0 < x < 3) and bridging S 2− /Se 2− arises from electron-electron repulsion. Given the higher ionic electronegativity of S 2− (1.94) compared to Se 2− (1.91), the d-orbital electronic structure and oxidation states of Rh single-atoms can be precisely and systematically tuned by adjusting the S: Se ratio in Rh SA -MoS x Se 2−x . Rh K-edge XANES spectra (Fig. 3 b) reveal a positive shift in edge absorption energies of Rh single-atoms with increasing Se content, confirming effective modulation of the Rh electronic structure through variation of the Se: S ratio. Quantitative analysis of the XANES data indicates oxidation states of Rh single-atom in Rh SA -MoS 2 , Rh SA -MoSSe, and Rh SA -MoSe 2 are + 1.67, + 1.36, and + 0.74, respectively (Fig. 3 c). Relative to Rh 0 foil (4d 8 5s 1 ) and Rh Ⅲ 2 O 3 (4d 5 5s 1 ) standards, the d-band hole count for Rh SA -MoSSe is estimated to be 3.36, lower than that of Rh SA -MoS 2 (3.67) but higher than Rh SA -MoSe 2 (2.74), reflecting moderate d-orbital vacancy induced by d-p orbital hybridization in Rh SA -MoS x Se 2−x 35 . High-resolution Rh 3d XPS spectra (Fig. 3 d) show a negative shift in binding energies with increasing Se contents, consistent with XANES results. Both the Rh oxidation state and Rh 3d 5/2 binding energy exhibit a linear correlation with the S: Se ratio in Rh SA -MoS x Se 2−x (Fig. 3 e). High-resolution Mo 3d, S 2p, and Se 3d spectra further confirm the successful synthesis of Rh SA -MoS x Se 2−x variants with distinct S:Se ratios 31 , 35 (Fig. 3 f and Supplementary Fig. 15). Collectively, these electronic structure characterizations demonstrate that the d-band structure of Rh single-atoms can be precisely tuned by modulating the S: Se ratio of MoS x Se 2−x supports via MSIs. The electronic properties of Rh SA -MoS x Se 2−x and MoS x Se 2−x were further probed through work function (W f ) measurements using ultraviolet photoelectron spectroscopy (UPS). As shown in Fig. 3 g and Supplementary Fig. 16, W f displays a volcano-type dependence on the S: Se ratio for both Rh SA -MoS x Se 2−x and MoS x Se 2−x , with Rh SA -MoSSe exhibiting a significantly lower W f (3.54 eV) compared to Rh SA -MoS 2 (3.75 eV), Rh SA -MoSe 2 (4.04 eV), and pristine MoS x Se 2−x substrates (5.51 ~ 5.71 eV). The incorporation of Rh single-atoms and precise tuning of the S: Se ratio in MoS x Se 2−x markedly enhance electron transfer capabilities, thereby facilitating the adsorption and activation of reactants. Electrochemical measurements were performed to elucidate the impact of the d-band structure of Rh single-atoms on alkaline HER performance. As shown in the 90% iR-compensated linear sweep voltammetry (LSV) curves (Fig. 4 a and Supplementary Fig. 17), Rh SA -MoSSe exhibits exceptional HER activity, achieving a remarkably low overpotential of 42 mV at a current density of 10 mA cm − 2 , outperforming Rh SA -MoS 2 (122 mV), Rh SA -MoS 1.5 Se 0.5 (92 mV), Rh SA -MoS 0.5 Se 1.5 (69 mV), Rh SA -MoSe 2 (90 mV), pristine MoS x Se 2−x substrates, and commercial Pt/C catalysts. Specifically, Rh SA -MoSSe demonstrates a high mass activity of 3.48 A mg − 1 (normalized to Rh loading) at an overpotential of 100 mV, approximately 50-fold higher than that of commercial Pt/C (0.07 A mg − 1 ). Tafel plots were analyzed to probe HER kinetics 36 (Supplementary Fig. 17), revealing that Rh SA -MoSSe possesses a significantly lower Tafel slope (56 mV dec − 1 ) compared to Rh SA -MoS 2 (111 mV dec − 1 ), Rh SA -MoS 1.5 Se 0.5 (94 mV dec − 1 ), Rh SA -MoS 0.5 Se 1.5 (84 mV dec − 1 ), Rh SA -MoSe 2 (99 mV dec − 1 ), Pt/C (40–121 mV dec − 1 ), and MoS x Se 2−x substrates, indicating accelerated Volmer reaction kinetics. The electrochemically active surface areas (ECSA) were estimated via double-layer capacitances ( C dl ) measurements derived from cyclic voltammogram (CV) (Supplementary Figs. 18 and 19). Rh SA -MoSSe exhibits the highest C dl (92.6 mF cm − 2 ) compared to Rh SA -MoS 2 (52.4 mF cm − 2 ), Rh SA -MoS 1.5 Se 0.5 (65.5 mF cm − 2 ), Rh SA -MoS 0.5 Se 1.5 (74.4 mF cm − 2 ), Rh SA -MoSe 2 (52.5 mF cm − 2 ), and pristine MoS x Se 2−x substrates, suggesting a greater abundance of exposed active sites. Moreover, polarization curves of Rh SA -MoS x Se 2−x and MoS x Se 2−x samples, normalized to ECSA, confirm the superior intrinsic activities of Rh SA -MoSSe (Supplementary Fig. 20). The turnover frequency (TOF) of Rh SA -MoSSe at -100 mV vs. RHE (18.58 s − 1 ) surpasses that of Rh SA -MoS 2 (2.27 s − 1 ), Rh SA -MoS 1.5 Se 0.5 (6.05 s − 1 ), Rh SA -MoS 0.5 Se 1.5 (11.45 s − 1 ), Rh SA -MoSe 2 (6.32 s − 1 ), and most reported noble metal-based and transition metal dichalcogenide (TMD) catalysts 14 , 20 , 35 , 37 – 43 (Fig. 4 c and Supplementary Table 1). Notably, the S: Se ratio in Rh SA -MoS x Se 2−x exhibits a volcano-type relationship with HER activity, Tafel slope, C dl , intrinsic activity, and TOF, with Rh SA -MoSSe at the volcano’s apex displaying optimal overall HER performance (Supplementary Figs. 21 and Fig. 4 b). This underscores the effectiveness of precisely tuning the d-band structure of Rh single-atoms to enhance HER activity and kinetics. To further quantify the impact of MSIs modulation via the S:Se ratio variation in MoS x Se 2−x supports, the enhancement rates of Rh SA -MoS x Se 2−x relative to pristine MoS x Se 2−x substrates were evaluated for activity, Tafel slope, ECSA, and intrinsic activity, based on the following equation: The enhancement rate (δ Enhance rate ) is defined as the ratio of the performance metrics (activity, Tafel slope, ECSA, and intrinsic activity) for Rh SA -MoS x Se 2−x (N RhSA−MoSxSe2−x ) relative to those for pristine MoS x Se 2−x (N MoSxSe2−x ). Notably, the enhancement rates for activity, Tafel slope, ECSA, and intrinsic activity exhibit a volcano-type dependence on the S: Se ratio, with the highest enhancement rate observed at an S: Se ratio of 1:1 (volcano’s apex) (Fig. 4 d). The strong interaction between OH* species and catalysts surfaces is well-established for accelerating water dissociation in the HER 4 , 7 . To probe the OH adsorption capacity at CO adsorption sites 9 , CO-stripping voltammetry was employed to assess the water dissociation capabilities of the Rh SA -MoS x Se 2−x catalysts (Supplementary Fig. 22a). Among them, Rh SA -MoSSe exhibited the lowest onset potential for CO oxidation (0.28 V), compared to Rh SA -MoS 2 (0.31 V), Rh SA -MoS 1.5 Se 0.5 (0.30 V), Rh SA -MoS 0.5 Se 1.5 (0.30 V), and Rh SA -MoSe 2 (0.32 V), indicating a stronger Rh-OH interaction and enhanced water dissociation kinetics. The H adsorption ability of Rh SA -MoS x Se 2−x catalysts was evaluated through the desorption of underpotential-deposited hydrogen (H upd ) 9 , 10 , 44 . As shown in Supplementary Fig. 22b, Rh SA -MoSSe displayed a notably low H upd potential (0.12 V), compared to Rh SA -MoS 2 (0.26 V), Rh SA -MoS 1.5 Se 0.5 (0.15 V), Rh SA -MoS 0.5 Se 1.5 (0.15 V), and Rh SA -MoSe 2 (0.16 V), facilitating H recombination during HER. Furthermore, the S: Se ratio in Rh SA -MoS x Se 2−x catalysts exhibited a volcano-type correlation with both OH and H adsorption strengths, with Rh SA -MoSSe at the volcano’s apex demonstrating optimal adsorption capabilities for both H and OH (Fig. 4 e), consistent with theoretical predictions. Long-term durability of Rh SA -MoSSe was assessed via chronopotentiometry, demonstrating negligible potential decay over 500 h at a high current density of 1 A cm − 2 (Supplementary Fig. 23). Furthermore, post-stability testing revealed minimal Rh leaching into the electrolyte. The catalyst’s stability across varying current densities was evaluated through multi-step current experiments, where the potential rapidly stabilized at each step, indicating efficient mass transport at the electrode surface during the HER (Fig. 4 f). These findings underscore the catalyst’s robust electrochemical stability and potential for industrial applications. Structural integrity of Rh SA -MoSSe post long-term HER testing was investigated using XRD, SEM, HAADF-STEM, and XPS. XRD analysis (Supplementary Fig. 24) confirmed that the crystal structure of Rh SA -MoSSe remained intact after extended HER operation. SEM (Supplementary Fig. 25) and HAADF-STEM (Supplementary Fig. 26) images verified that Rh SA -MoSSe retained its original morphology and atomic dispersion of Rh after 500 h testing. XPS analysis (Supplementary Fig. 27) further revealed that the oxidation states of Rh, Mo, S, and Se in Rh SA -MoSSe remained stable and unaltered post testing. Collectively, these structural characterizations demonstrate that Rh SA -MoSSe exhibits exceptional HER activity and stability, attributed to the precise tuning of the Rh single-atoms d-band structure, which optimizes intermediate adsorption energies and significantly enhances alkaline HER performance. Operando electrochemical impedance spectroscopy (EIS) was conducted to probe the charge transfer kinetics and elucidate the HER 45 . The equivalent circuit for both Rh SA -MoS x Se 2−x and pristine MoS x Se 2−x comprises four components: electron transfer from the cathode to the interface (R 1 , part 1), accumulation of reaction intermediate (Volmer step, R 2 , part 2), charge transfer during the interfacial reaction (Heyrovsky step, R 3 , part 3), and electrolyte resistance (R s , part 4) (Fig. 5 a). The low-frequency region is primarily associated with the Volmer step, while the high-frequency region reflects electron transfer from the catalyst’s inner layer to surface-active sites, driven by distinct relaxation time. Bode plots at overpotential of 100 mV (Fig. 5 b) reveal significantly reduced phase angles at low frequencies for Rh SA -MoS x Se 2−x compared to MoS x Se 2−x supports, indicating that Rh single-atoms enhance the Volmer step kinetics. Analysis of Nyquist plots and optimized fitting parameters (Figs. 5 c-f) shows that Rh SA -MoS x Se 2−x catalysts exhibit substantially lower charge transfer resistances than their pristine MoS x Se 2−x counterparts. Notably, R 1 , R 2 , and R 3 display a volcano-type dependence on the S: Se ratio in both Rh SA -MoS x Se 2−x and MoS x Se 2−x , with Rh SA -MoSSe at the volcano’s top exhibiting the lowest R 1 , R 2 , and R 3 values, signifying accelerated HER charge transfer kinetics and a rapid Faradaic reaction at the catalyst-electrolyte interface. Operando EIS measurements at varying applied biases further clarify the charge transfer dynamics and HER mechanism of Rh SA -MoS x Se 2−x . Bode plot (Fig. 5 g) for Rh SA -MoS 2 , Rh SA -MoSSe, and Rh SA -MoSe 2 reveal a single phase-angle peak spanning low-frequency (Volmer step) and mid-frequency (Heyrovsky step) regions as the applied bias decreases, suggesting that Rh SA -MoS x Se 2−x follows a mixed Heyrovsky-Volmer and Tafel-Volmer mechanism rather than a singular Heyrovsky-Volmer pathway. Remarkably, Rh SA -MoSSe exhibits a smaller phase angle and a faster phase-angle decrease rate (0.05 to -0.05 V) compared to Rh SA -MoS 2 and Rh SA -MoSe 2 (Fig. 5 h), further confirming its superior HER kinetics. To elucidate the enhanced alkaline HER activity of Rh SA -MoSSe, operando infrared absorption (IR) spectroscopy and in situ Raman spectroscopy were employed to investigate adsorption site and binding energy dynamics of reaction intermediates under HER operating conditions. As shown in Fig. 6 a, with decreasing bias potential, distinct absorption peaks emerged at approximately 1030, 1621, and 3200–3600 cm − 1 . The peak at ~ 1030 cm − 1 is attributed to S/Se-OH formation, while the broad peak at 3200–3600 cm − 1 and the peak located at ~ 1621 cm − 1 for Rh SA -MoS 2 , Rh SA -MoSSe, and Rh SA -MoSe 2 correspond to the O-H stretching and 𝛿(H-O-H) bending modes of interfacial water molecules, respectively 44 , 46 . Moreover, water-related peaks for Rh SA -MoSSe appeared at a more positive potential (0.15 V) compared to Rh SA -MoS 2 (0.1 V) and Rh SA -MoSe 2 (0.1 V), indicating that Rh SA -MoSSe facilitates water dissociation more effectively. The vibrational Stark effect, which describes potential-dependent shifts in adsorbate vibrational frequencies 3 , reveals that S/Se-OH in Rh SA -MoSSe is more responsive to the local electric field than in Rh SA -MoS 2 and Rh SA -MoSe 2 , as evidenced by steeper Stark slopes (Fig. 6 b). In-situ Raman spectroscopy further probed the catalytic processes in Rh SA -MoS x Se 2−x during alkaline HER. As presented in Fig. 6 c, Raman peaks at ~ 1522 and ~ 1395 cm −1 for Rh SA -MoSSe emerged at 0.15 V, corresponding to adsorbed OH species generated during the HER 39 . In contrast, OH-related Raman peaks for Rh SA -MoS 2 and Rh SA -MoSe 2 appeared at 0.1 V, underscoring the stronger OH binding and accelerated water dissociation kinetics of Rh SA -MoSSe. Additionally, the Stark slopes for * OH in Rh SA -MoSSe indicate greater sensitivity to the local electric field compared to Rh SA -MoS 2 and Rh SA -MoSe 2 (Fig. 6 d). These findings suggest that precise modulation of the S: Se ratio optimizes OH adsorption at Rh single atom sites, thereby enhancing H 2 O adsorption and dissociation (Fig. 6 e). Gibbs free energy differences for the alkaline HER process in Rh SA -MoS x Se 2−x and MoS x Se 2−x were calculated in Supplementary Figs. 28–37. H 2 O adsorption at Rh sites in Rh SA -MoS x Se 2−x is thermodynamically barrier-free and more favorable than at S/Se sites in Rh SA -MoS x Se 2−x or MoS x Se 2−x . The rate-determining step (RDS) for alkaline HER in both systems is H₂O dissociation. Significantly, the incorporation of Rh single-atom in Rh SA -MoS x Se 2−x lowers the energy barriers for H 2 O dissociation and enhances the adsorption/desorption behavior of OH and H, synergistically improving HER thermodynamics and kinetics (Fig. 6 f). Furthermore, the water dissociation free energy of Rh SA -MoS x Se 2−x exhibits a volcano-type relationship with the S: Se ratio, with the lowest energy barrier observed at an S: Se ratio of 1:1 (volcano’s apex). Conclusion In summary, we establish an anion-gradient engineered Rh SACs platform (Rh SA -MoS x Se 2-x ) for precisely manipulating d-p orbital hybridization at the atomic scale. Through bidirectional Rh-chalcogen coupling, the d-band center of single-atom Rh is continuously tuned, exhibiting a volcano correlation with H and OH adsorption energies. The apex catalyst, Rh SA -MoSSe, achieves optimized orbital hybridization, simultaneously balancing intermediate adsorption/desorption kinetics and strengthening Rh-S/Se covalent bonding-thereby delivering outstanding HER performance, with a low overpotential of 42 mV at 10 mA cm - 2 and excellent long-term stability under industrially relevant fluctuating power conditions (> 300 h). This work resolves the fundamental linkage between orbital hybridization and electrocatalytic energetics, providing a universal design principle to transcend activity-stability trade-offs in single-atom catalysis. Declarations Supporting Information Author Contributions Notes The authors declare no competing financial interest. Acknowledg e ments This work was financially supported by National Natural Science Foundation of China (22369005, 52302236, W2521028, 52231008, and 52301011), the Key Research and Development Program of Hainan Province (ZDYF2024GXJS006), International Science & Technology Cooperation Program of Hainan Province (GHYF2023007). References Yu, Z. Y. et al. Clean and Affordable Hydrogen Fuel from Alkaline Water Splitting: Past, Recent Progress, and Future Prospects. Adv. Mater. 33 , 2007100 (2021). Yu, P. C. et al. Nitrogen-Mediated Promotion of Cobalt-Based Oxygen Evolution Catalyst for Practical Anion-Exchange Membrane Electrolysis. J. Am. Chem. Soc. 146 , 20379-20390 (2024). Wang, Y. H. et al. In situ Raman spectroscopy reveals the structure and dissociation of interfacial water. Nature 600 , 81-85 (2021). Shah, A. H. et al. The role of alkali metal cations and platinum-surface hydroxyl in the alkaline hydrogen evolution reaction. Nat. Catal. 5 , 923-933 (2022). Subbaraman, R. et al. 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Creating High-entropy Single Atoms on Transition Disulfides through Substrate-induced Redox Dynamics for Efficient Electrocatalytic Hydrogen Evolution. Angew. Chem . Int. Ed. 63 , e202405017 (2024). Tsounis, C. et al. Pt Single Atom Electrocatalysts at Graphene Edges for Efficient Alkaline Hydrogen Evolution. Adv. Funct. Mater. 32 , 2203067 (2022). Wang, K. et al. Cu-Doped Heterointerfaced Ru/RuSe 2 Nanosheets with Optimized H and H 2 O Adsorption Boost Hydrogen Evolution Catalysis. Adv. Mater. 35 , 2300980 (2023). Yin, Y. et al. Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets. J. Am. Chem. Soc. 138 , 7965-7972 (2016). Zhu, S., Qin, X., Yao, Y. & Shao, M. pH-Dependent Hydrogen and Water Binding Energies on Platinum Surfaces as Directly Probed through Surface-Enhanced Infrared Absorption Spectroscopy. J. Am. Chem. Soc. 142 , 8748-8754 (2020). Chen, W. et al. Deciphering the alternating synergy between interlayer Pt single-atom and NiFe layered double hydroxide for overall water splitting. Energy Environ. Sci. 14 , 6428-6440 (2021). Yan, Z. T., Tao, S., Wang, J., Lu, X. L. & Lu, T. B. Unlocking Efficient Alkaline Hydrogen Evolution Through Ru-Sn Dual Metal Sites and a Novel Hydroxyl Spillover Effect. Adv. Mater. 36 , 2411942 (2024). Additional Declarations There is NO Competing Interest. Supplementary Files SP20250618.docx Supplementary Information floatimage7.jpeg TOC Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7039226","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":502609484,"identity":"39597897-9294-457f-9a7e-412d895026df","order_by":0,"name":"Yida Deng","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA50lEQVRIie3RMQrCMBSA4VcCcQnOcSh6hEhBHUrv4WYIdHQpFMeCUDd3UfAKLV4gkqGbroKLUyfBjm7aFsQt7SiYfwgvkI8EAmAy/WIIGAAuh05UTWBF7QlRbQl8CBX1rpmwDKUFhO68P33IgIBrJxLlNy1ROKBw8oPhVYBDwHcSicdMTwijVqx4uq2J4okkmDYQ52nFL55uVEVerciovEXyPUUVkc2kp3A4mZ0ET4hgwx0TzkbhkZZ0z+pwKUKP71fHnN0Xnr3OlrmWDGS5zKoXSsCs/kykO1/Wj74DujUcNplMpj/tDUqvRs0htOb3AAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-8890-552X","institution":"Hainan University","correspondingAuthor":true,"prefix":"","firstName":"Yida","middleName":"","lastName":"Deng","suffix":""},{"id":502609485,"identity":"b8812394-7a7d-41a5-95b2-125c3902d5dd","order_by":1,"name":"Rouna Jia","email":"","orcid":"","institution":"Hainan University","correspondingAuthor":false,"prefix":"","firstName":"Rouna","middleName":"","lastName":"Jia","suffix":""},{"id":502609486,"identity":"bfb2d501-32e4-4012-b5be-abcee2939c2f","order_by":2,"name":"Zongyan Liu","email":"","orcid":"","institution":"Hainan University","correspondingAuthor":false,"prefix":"","firstName":"Zongyan","middleName":"","lastName":"Liu","suffix":""},{"id":502609487,"identity":"d9eb74e6-5bc3-46fc-8eca-e511dc9e7e1b","order_by":3,"name":"Yang Wang","email":"","orcid":"","institution":"Hainan University","correspondingAuthor":false,"prefix":"","firstName":"Yang","middleName":"","lastName":"Wang","suffix":""},{"id":502609488,"identity":"b61071f1-3c99-46e8-8035-72a47c7ec31d","order_by":4,"name":"Feiyu Li","email":"","orcid":"","institution":"Hainan University","correspondingAuthor":false,"prefix":"","firstName":"Feiyu","middleName":"","lastName":"Li","suffix":""},{"id":502609489,"identity":"425305ce-9245-4131-b9d6-08c04dcaf0ee","order_by":5,"name":"Jingyang Zhao","email":"","orcid":"","institution":"Hainan University","correspondingAuthor":false,"prefix":"","firstName":"Jingyang","middleName":"","lastName":"Zhao","suffix":""},{"id":502609490,"identity":"e9566f07-ae8a-4352-9a5f-1609af61f0ab","order_by":6,"name":"Zhong Huang","email":"","orcid":"","institution":"Hainan University","correspondingAuthor":false,"prefix":"","firstName":"Zhong","middleName":"","lastName":"Huang","suffix":""},{"id":502609491,"identity":"fcd2e8d3-2a6d-470f-9f26-fdc0ecec96ec","order_by":7,"name":"Wen Yue","email":"","orcid":"","institution":"China University of Geosciences","correspondingAuthor":false,"prefix":"","firstName":"Wen","middleName":"","lastName":"Yue","suffix":""},{"id":502609492,"identity":"88065d61-d8df-46c7-b06d-0476bb0b8ca2","order_by":8,"name":"Haozhi Wang","email":"","orcid":"https://orcid.org/0000-0002-3543-2319","institution":"Hainan University","correspondingAuthor":false,"prefix":"","firstName":"Haozhi","middleName":"","lastName":"Wang","suffix":""},{"id":502609493,"identity":"296a3d1e-a8e3-4214-b932-2969d13ada6e","order_by":9,"name":"M.X. Huang","email":"","orcid":"https://orcid.org/0000-0002-8038-431X","institution":"The University of Hong Kong","correspondingAuthor":false,"prefix":"","firstName":"M.X.","middleName":"","lastName":"Huang","suffix":""}],"badges":[],"createdAt":"2025-07-03 14:31:08","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7039226/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7039226/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":89467912,"identity":"b6a60bdd-4107-49a1-b5a4-5ba0a95ff242","added_by":"auto","created_at":"2025-08-20 08:55:12","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1210414,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTheoretical design of Rh single-atom catalysts.\u003c/strong\u003e (a) Schematic illustration of modulating MSIs in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e by varying the S: Se ratio of MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e supports. (b) Charge density difference image for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e, with yellow and blue regions indicating charge accumulation and decrease, respectively. (c) Calculated PDOS for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e, where red dashed lines denote the d-band center of Rh, orange dashed lines indicate the p-band center of S, and green dashed lines represent the p-band center for Se. (d) Correlation between the S: Se ratio and ΔG\u003csub\u003eOH*\u003c/sub\u003e, and ΔG\u003csub\u003eH*\u003c/sub\u003e in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e. (e) H adsorption strength and (f) OH adsorption strength at S/Se sites in pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e, Rh sites, and adjacent S/Se sites in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7039226/v1/a42d07d6463eed4319601cd2.png"},{"id":89467913,"identity":"bcc66f1c-8639-4619-b9a2-354537fa757e","added_by":"auto","created_at":"2025-08-20 08:55:12","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":2154101,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructural characterization of Rh\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eSA\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e-MoS\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003ex\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eSe\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2-x \u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003ecatalysts.\u003c/strong\u003e Structural models of (a) Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, (b) Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and (c) Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e. HAADF-STEM images of (d) Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, (e) Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and (f) Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e, with corresponding intensity profiles derived from the regions marked by red dashed rectangles (yellow dashed circles highlight selected single-atomic Rh sites). (g) Rh K-edge EXAFS spectra in R-space for Rh foil, Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e. (h) WT-EXAFS contour plots of Rh for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e. (i) Correlation between the S: Se ratio, Rh-S/Se coordination path length, and the k-space positions in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7039226/v1/34f176414ba17a315634e719.png"},{"id":89467919,"identity":"2b32e2c0-71be-436f-879d-c11408656757","added_by":"auto","created_at":"2025-08-20 08:55:12","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":739261,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectronic structure and composition analysis of Rh\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eSA\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e-MoS\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003ex\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eSe\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2-x\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e catalysts.\u003c/strong\u003e (a) Schematic depiction of electronic interactions among Rh, S, and Se in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e. (b) Rh K-edge XANES spectra for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e. (c) Fitted average oxidation states of Rh derived from XANES spectra. (d) Rh 3d and (f) Mo 3d XPS spectra for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e1.5\u003c/sub\u003eSe\u003csub\u003e0.5\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e0.5\u003c/sub\u003eSe\u003csub\u003e1.5\u003c/sub\u003e, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e. (e) Correlation between the S: Se ratio, Rh oxidation state, and Rh 3d\u003csub\u003e5/2\u003c/sub\u003e binding energy in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e. (g) Dependence of the work function on the S: Se ratio for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e and pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7039226/v1/f332a629e88f61cd9b53ecf7.png"},{"id":89468282,"identity":"f7dd9ff3-3b39-4761-9fa4-973a6abfa6fe","added_by":"auto","created_at":"2025-08-20 09:03:12","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":597071,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrocatalytic HER performance of Rh\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eSA\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e-MoS\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003ex\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eSe\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2-x\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e in Ar-saturated 1 M KOH solution at 25 ℃.\u003c/strong\u003e (a) HER polarization curves for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e1.5\u003c/sub\u003eSe\u003csub\u003e0.5\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e0.5\u003c/sub\u003eSe\u003csub\u003e1.5\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e, and Pt/C catalysts, with 90% iR compensation. (b) Radar chart illustrating Overpotential (at 10 mA cm\u003csup\u003e-2\u003c/sup\u003e), Tafel slope, TOF (at -100 mV vs. RHE), \u003cem\u003eC\u003c/em\u003e\u003csub\u003edl\u003c/sub\u003e, and intrinsic activity for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e1.5\u003c/sub\u003eSe\u003csub\u003e0.5\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e0.5\u003c/sub\u003eSe\u003csub\u003e1.5\u003c/sub\u003e, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e. (c) TOF values of Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe (red dot) compared with previously reported HER electrocatalysts at -100 mV vs. RHE. (d) Enhancement rates of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e relative to pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e in terms of HER activity, Tafel slope, number of active sites, and intrinsic activity. (e) Correlation between the S: Se ratio and the H and OH adsorption strengths of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e. (f) Stability evaluation of Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe under step-change current density of 10, 50, 100, and 200 mA cm\u003csup\u003e-2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7039226/v1/0a80ebef3058a3a72aa479e4.png"},{"id":89468280,"identity":"8366fe4a-eb93-4c74-969f-4716044f480d","added_by":"auto","created_at":"2025-08-20 09:03:12","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":762372,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectron transfer characteristics of Rh\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eSA\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e-MoS\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003ex\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eSe\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2-x\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e catalysts.\u003c/strong\u003e (a) Equivalent circuit-fitted EIS date and schematic representation of the mass and charge transfer processes for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e and pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e. (b) Corresponding Bode plots and (c) Nyquist plots of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e and MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e at 0.1 V vs. RHE. Correlation between the S: Se ratio and the resistance components (d) R\u003csub\u003e1\u003c/sub\u003e, (e) R\u003csub\u003e2\u003c/sub\u003e, and (f) R\u003csub\u003e3\u003c/sub\u003e in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e and MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e. (g) In situ bode plots for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e. (h) Frequency dependence of phase changes in Bode plots for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7039226/v1/89fb2e35aeb465b68e258703.png"},{"id":89467921,"identity":"760bff9f-74c2-42b7-9a03-f4ff945e69e3","added_by":"auto","created_at":"2025-08-20 08:55:12","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":878316,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMechanism insights into catalytic activity.\u003c/strong\u003e (a) Operando attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectra of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e. (b) Frequency dependence of S/Se-OH modes variations in ATR-FTIR spectra at different potentials. (c) In situ Raman spectra of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e. (d) Frequency dependence of stretching mode variations in Raman spectra at different potentials. (e) Schematic illustration of the water dissociation trends for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e. (f) Water dissociation energies at S/Se sites in pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e, Rh sites, and adjacent S/Se sites in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7039226/v1/4c93291a8a2bb8c0b87ff572.png"},{"id":89471154,"identity":"ab5fe32a-3ce7-4b6e-ab7e-f3ab44553a3d","added_by":"auto","created_at":"2025-08-20 09:27:16","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":7225697,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7039226/v1/9405fc17-7160-4b3a-8eb2-0ada0636ffa3.pdf"},{"id":89467929,"identity":"6d52987e-fbfa-4fea-9822-df8e5aafd994","added_by":"auto","created_at":"2025-08-20 08:55:13","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":32222720,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SP20250618.docx","url":"https://assets-eu.researchsquare.com/files/rs-7039226/v1/f2c81096447858df98f3a780.docx"},{"id":89467915,"identity":"f36b0214-d7c1-470d-84ed-bf5fefc811b2","added_by":"auto","created_at":"2025-08-20 08:55:12","extension":"jpeg","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":635267,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTOC\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7039226/v1/a288bfc9683005e01182713d.jpeg"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Anion-tuned d-p hybridization breaks activity-stability trade-off in single-atom hydrogen evolution catalysts","fulltext":[{"header":"Introduction","content":"\u003cp\u003eRenewable-electricity-driven water electrolysis offers a sustainable hydrogen production pathway\u003csup\u003e1,2\u003c/sup\u003e, yet alkaline hydrogen evolution reaction (HER) suffers from sluggish kinetics due to coupled water dissociation and proton-transfer barriers\u003csup\u003e3-6\u003c/sup\u003e. While volcano relationships link HER activity to hydroxide (OH)\u003csup\u003e7-9\u003c/sup\u003e and hydrogen (H)\u003csup\u003e10,11\u003c/sup\u003e adsorption energies, simultaneously optimizing both intermediates remains fundamentally challenging. Single atom catalysts (SACs) hold great promise due to their maximal atomic utilization and tunable coordination environment\u003csup\u003e12\u0026ndash;15\u003c/sup\u003e, but their activity and long-term stability still require significant enhancement.\u003c/p\u003e\n\u003cp\u003eMetal-support interactions (MSIs) are crucial for stabilizing single atoms on the support and modulating the d-band structure of single-atom sites, while also influencing the local electronic structure of the support in SACs\u003csup\u003e16-19\u003c/sup\u003e. Precise control of MSIs is essential for enhancing catalyst stability and optimizing the binding energies of reactants and intermediates, which govern the catalytic performance of SACs. To date, MSIs research has predominantly focused on the active single-atom metal, with the critical role of support materials often overlooked. Conventional strategies, such as varying the type or increasing the loading of single atoms, escalate catalyst costs and face limitations in activity enhancement. Given that the coordination number, chemical bonding, and spatial environment of single-atom sites are dictated by the properties of the support, tailoring the support\u0026rsquo;s composition and phase structure offers an effective approach to modulate the geometry and electronic structure of SACs\u003csup\u003e20-22\u003c/sup\u003e. This has been demonstrated in heteroatom (e.g., N, P, O, and S) carbon-based SACs, where activity and selectivity are significantly altered by adjusting the extent of heteroatom coordination\u003csup\u003e23-26\u003c/sup\u003e. Compared to relatively inert carbon-based supports, transition metal compounds, with their tunable band structures and covalent bonding capabilities, serve as ideal platforms for stabilizing single atoms\u003csup\u003e27-30\u003c/sup\u003e. However, achieving precise MSIs control through atomic-level compositional gradients remains challenging, impeding a clear understanding of structure-performance relationships. Notably, the role of d-p orbital hybridization in anion-engineered supports remains largely unexplored, representing a critical gap in the orbital-level design of SACs.\u003c/p\u003e\n\u003cp\u003eHerein, we establish an anion-gradient Rh SACs platform (Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e, 0 \u0026le; x \u0026le;2)\u0026nbsp;enabling continuous d-p orbital hybridization via p-band center engineering. Combined with detailed experimental characterization and theoretical simulations, we reveal that bidirectional Rh-chalcogen coupling continuously tunes the d-band center of Rh single-atoms, exhibiting a volcano-type correlation with H and OH adsorption energies. Optimal d-p orbital hybridization in Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe simultaneously balances intermediate adsorption-desorption kinetics and reinforces Rh-S/Se covalent bonding, leading to exceptional long-term stability under industrially relevant fluctuating power conditions (\u0026gt; 300 h). This work resolves the orbital hybridization-electrocatalysis nexus, offering a universal design paradigm beyond activity-stability compromises. \u0026nbsp;\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eTo gain a comprehensive understanding of Metal-support interactions (MSIs), we construct a model system of Rh single-atom catalysts (SACs), in which the d-band structure of the Rh centers can be effectively modulated by tuning the anion composition in the support (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Conversely, the anchored Rh atoms also influence the p-band structure of the support anions, establishing a bidirectional electronic coupling that optimizes the adsorption energy barrier for reactants and intermediates (Supplementary Fig.\u0026nbsp;1). First principal density functional theory (DFT) calculations were conducted to investigate the MSIs effects based on the MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e and Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e (0\u0026thinsp;\u0026le;\u0026thinsp;x\u0026thinsp;\u0026le;\u0026thinsp;2) structure models (Supplementary Figs.\u0026nbsp;2 and 3). The density of states (DOS) reveals that the incorporation of Rh single-atoms significantly increases the DOS of MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e near the Fermi level (Supplementary Fig.\u0026nbsp;4). Moreover, the band gap of Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe (0.85 eV) is notably narrower than those of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e (0.92 eV), Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (1.03 eV), MoS\u003csub\u003e2\u003c/sub\u003e (1.70 eV), MoSSe (1.54 eV), and MoSe\u003csub\u003e2\u003c/sub\u003e (1.56 eV), thereby promoting accelerated electron transfer and optimizing the adsorption and desorption of reaction intermediates. Charge density difference analyses reveal distinct charge redistribution between Rh single-atoms and coordinated S/Se atoms, resulting in a marked reduction of electron density around Rh atoms (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). This redistribution enhances the attraction of electronegative OH species, thereby strengthening local OH adsorption. Bader charge analysis of Rh atoms anchored on MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e supports indicates varying electron transfer, with charges following the trend: Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e (-0.22 e) ˂ Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe (-0.14 e)\u0026thinsp;\u0026lt;\u0026thinsp;Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (-0.01 e). These results suggest that electron donation from Rh atoms to the MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e supports can be systematically tuned by adjusting the S: Se ratio, enabling precise modulation of the Rh electronic structure. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec, projected density of states (PDOS) analysis of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e reveals significant orbital hybridization between Rh d orbitals and the p orbitals of S and Se atoms, highlighting the role of d-p coupling in mediating metal-support interactions. Moreover, the d-band center of Rh atoms can be precisely and continuously modulated via d-p orbital hybridization between the Rh single atom and the coordinated S/Se atoms in the MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e supports. According to d-band theory, the tunability of the Rh d-orbital states directly alters the adsorption energies of key intermediates, thereby influencing the catalytic activity for HER. Moreover, the d-band center of Rh in the Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e series exhibits a volcano-type relationship with both the free energy of hydrogen adsorption (ΔG\u003csub\u003eH*\u003c/sub\u003e) and hydroxide adsorption (ΔG\u003csub\u003eOH*\u003c/sub\u003e), with Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe located at the volcano apex, displaying optimal adsorption strengths for both species (H and OH) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed and Supplementary Figs.\u0026nbsp;5 and 6). To further elucidate the impact of MSIs, we evaluated ΔG\u003csub\u003eH*\u003c/sub\u003e and ΔG\u003csub\u003eOH*\u003c/sub\u003e at all possible and thermodynamically stable adsorption sites on both pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e and regions surrounding the Rh atoms in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e. It is worth noting that the introduction of Rh single-atom substantially enhances the adsorption of H and OH at Rh sites, synergistically improving both the thermodynamics and kinetics of HER (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee and \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eInspired by these DFT results, we synthesized Rh single-atoms anchored on a series of MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e with different S and Se ratios to validate the findings. The synthesis strategy for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e is illustrated in Supplementary Fig.\u0026nbsp;7. First, MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e with different S and Se contents (denoted as MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e, 0\u0026thinsp;\u0026le;\u0026thinsp;x\u0026thinsp;\u0026le;\u0026thinsp;2) were prepared via a hydrothermal method followed by annealing. Then Rh single-atoms were anchored on the MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e substrates (denoted as Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e, 0\u0026thinsp;\u0026le;\u0026thinsp;x\u0026thinsp;\u0026le;\u0026thinsp;2) using an electrochemical deposition method. The Rh loadings were comparable across the samples (Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e: 0.35 wt.%, Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e1.5\u003c/sub\u003eSe\u003csub\u003e0.5\u003c/sub\u003e: 0.42 wt.%, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe: 0.51 wt.%, Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e0.5\u003c/sub\u003eSe\u003csub\u003e1.5\u003c/sub\u003e: 0.38 wt.%, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e: 0.43 wt.%), as determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Scanning electron microscope (SEM) (Supplementary Fig.\u0026nbsp;8) and transmission electron microscopy (TEM) (Supplementary Figs.\u0026nbsp;9 and 10) characterizations revealed that all Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e exhibit a similar nanoflower morphology, which promotes the exposure of active sites and enhances mass transfer. Energy-dispersive X-ray (EDX) spectroscopy elemental analysis confirms the uniform distribution of Rh, Mo, and Se in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e. As shown in Supplementary Figs.\u0026nbsp;11 and 12, the X-ray diffraction (XRD) patterns and corresponding Raman spectra indicate that the synthesized Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e possess the same crystal structure as the 2H phase of MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e\u003csup\u003e31,32\u003c/sup\u003e, and no distinct Rh-containing phases detected after Rh electrodeposition. In addition, high-resolution transmission electron microscopy (HRTEM) results (Supplementary Figs.\u0026nbsp;13 and 14) further confirm that the crystal structures of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e remain well-preserved following Rh decoration.\u003c/p\u003e\u003cp\u003eThe structural models of the Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e catalysts are presented in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-c, respectively. The aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) detects the atomically dispersed Rh atoms (bright spots) on the MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e nanosheets (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed-f). Furthermore, the different intensity profiles along the dashed yellow rectangles in the HAADF-STEM images confirm the single-atom dispersion of Rh atoms. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg, the Fourier transformed extended X-ray absorption fine spectroscopy (FT-EXAFS) spectra of Rh in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e display characteristic peaks at 1.8, 2.0, and 2.1 \u0026Aring;, corresponding to Rh-S, Rh-S/Se, and Rh-Se bonds\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e, respectively. In contrast to the Rh foil reference, the absence of a metallic Rh-Rh scattering signal rules out the presence of Rh nanoparticles or clusters, confirming the atomic dispersion of Rh species in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e. Furthermore, Wavelet transformed EXAFS (WT-EXAFS) contour plots reveal single intensity maxima for Rh in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e at approximately 6.7, 8.0, and 9.4 \u0026Aring;\u003csup\u003e\u0026minus;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh), corresponding to the Rh-S, Rh-S/Se, and Rh-Se coordination paths\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e, respectively. Notably, both the coordination path lengths and k-space positions of Rh-S/Se exhibit a linear correlation with the S: Se ratio in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei), providing further evidence that Rh single-atoms are covalently bonded to S/Se and that their coordination environment can be precisely tuned by adjusting the Se: S ratio in MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e substrates.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eX-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) analyses were performed to elucidate the electronic structures and chemical compositions of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e catalysts. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, the synergistic electronic interactions among Rh, S, and Se in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e are characterized through S-Rh-S, S-Rh-Se, and Se-Rh-Se coordination units. In all Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e catalysts, the predominant interaction between the fully occupied π-symmetry (t\u003csub\u003e2g\u003c/sub\u003e) d-orbitals of Rh\u003csup\u003ex+\u003c/sup\u003e (0\u0026thinsp;\u0026lt;\u0026thinsp;x\u0026thinsp;\u0026lt;\u0026thinsp;3) and bridging S\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Se\u003csup\u003e2\u0026minus;\u003c/sup\u003e arises from electron-electron repulsion. Given the higher ionic electronegativity of S\u003csup\u003e2\u0026minus;\u003c/sup\u003e (1.94) compared to Se\u003csup\u003e2\u0026minus;\u003c/sup\u003e (1.91), the d-orbital electronic structure and oxidation states of Rh single-atoms can be precisely and systematically tuned by adjusting the S: Se ratio in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e. Rh K-edge XANES spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb) reveal a positive shift in edge absorption energies of Rh single-atoms with increasing Se content, confirming effective modulation of the Rh electronic structure through variation of the Se: S ratio. Quantitative analysis of the XANES data indicates oxidation states of Rh single-atom in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e are +\u0026thinsp;1.67, +\u0026thinsp;1.36, and +\u0026thinsp;0.74, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). Relative to Rh\u003csup\u003e0\u003c/sup\u003e foil (4d\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e5s\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e) and Rh\u003csup\u003eⅢ\u003c/sup\u003e\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (4d\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e5s\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e) standards, the d-band hole count for Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe is estimated to be 3.36, lower than that of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e (3.67) but higher than Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (2.74), reflecting moderate d-orbital vacancy induced by d-p orbital hybridization in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e\u003csup\u003e35\u003c/sup\u003e. High-resolution Rh 3d XPS spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed) show a negative shift in binding energies with increasing Se contents, consistent with XANES results. Both the Rh oxidation state and Rh 3d\u003csub\u003e5/2\u003c/sub\u003e binding energy exhibit a linear correlation with the S: Se ratio in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee). High-resolution Mo 3d, S 2p, and Se 3d spectra further confirm the successful synthesis of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e variants with distinct S:Se ratios\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef and Supplementary Fig.\u0026nbsp;15). Collectively, these electronic structure characterizations demonstrate that the d-band structure of Rh single-atoms can be precisely tuned by modulating the S: Se ratio of MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e supports via MSIs. The electronic properties of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e and MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e were further probed through work function (W\u003csub\u003ef\u003c/sub\u003e) measurements using ultraviolet photoelectron spectroscopy (UPS). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg and Supplementary Fig.\u0026nbsp;16, W\u003csub\u003ef\u003c/sub\u003e displays a volcano-type dependence on the S: Se ratio for both Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e and MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e, with Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe exhibiting a significantly lower W\u003csub\u003ef\u003c/sub\u003e (3.54 eV) compared to Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e (3.75 eV), Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (4.04 eV), and pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e substrates (5.51\u0026thinsp;~\u0026thinsp;5.71 eV). The incorporation of Rh single-atoms and precise tuning of the S: Se ratio in MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e markedly enhance electron transfer capabilities, thereby facilitating the adsorption and activation of reactants.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eElectrochemical measurements were performed to elucidate the impact of the d-band structure of Rh single-atoms on alkaline HER performance. As shown in the 90% iR-compensated linear sweep voltammetry (LSV) curves (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and Supplementary Fig.\u0026nbsp;17), Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe exhibits exceptional HER activity, achieving a remarkably low overpotential of 42 mV at a current density of 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, outperforming Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e (122 mV), Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e1.5\u003c/sub\u003eSe\u003csub\u003e0.5\u003c/sub\u003e (92 mV), Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e0.5\u003c/sub\u003eSe\u003csub\u003e1.5\u003c/sub\u003e (69 mV), Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (90 mV), pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e substrates, and commercial Pt/C catalysts. Specifically, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe demonstrates a high mass activity of 3.48 A mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (normalized to Rh loading) at an overpotential of 100 mV, approximately 50-fold higher than that of commercial Pt/C (0.07 A mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Tafel plots were analyzed to probe HER kinetics\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e (Supplementary Fig.\u0026nbsp;17), revealing that Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe possesses a significantly lower Tafel slope (56 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) compared to Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e (111 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e1.5\u003c/sub\u003eSe\u003csub\u003e0.5\u003c/sub\u003e (94 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e0.5\u003c/sub\u003eSe\u003csub\u003e1.5\u003c/sub\u003e (84 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (99 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), Pt/C (40\u0026ndash;121 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), and MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e substrates, indicating accelerated Volmer reaction kinetics. The electrochemically active surface areas (ECSA) were estimated via double-layer capacitances (\u003cem\u003eC\u003c/em\u003e\u003csub\u003edl\u003c/sub\u003e) measurements derived from cyclic voltammogram (CV) (Supplementary Figs.\u0026nbsp;18 and 19). Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe exhibits the highest \u003cem\u003eC\u003c/em\u003e\u003csub\u003edl\u003c/sub\u003e (92.6 mF cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) compared to Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e (52.4 mF cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e1.5\u003c/sub\u003eSe\u003csub\u003e0.5\u003c/sub\u003e (65.5 mF cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e0.5\u003c/sub\u003eSe\u003csub\u003e1.5\u003c/sub\u003e (74.4 mF cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (52.5 mF cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), and pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e substrates, suggesting a greater abundance of exposed active sites. Moreover, polarization curves of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e and MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e samples, normalized to ECSA, confirm the superior intrinsic activities of Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe (Supplementary Fig.\u0026nbsp;20). The turnover frequency (TOF) of Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe at -100 mV vs. RHE (18.58 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) surpasses that of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e (2.27 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e1.5\u003c/sub\u003eSe\u003csub\u003e0.5\u003c/sub\u003e (6.05 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e0.5\u003c/sub\u003eSe\u003csub\u003e1.5\u003c/sub\u003e (11.45 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (6.32 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), and most reported noble metal-based and transition metal dichalcogenide (TMD) catalysts\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e,\u003cspan additionalcitationids=\"CR38 CR39 CR40 CR41 CR42\" citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec and Supplementary Table\u0026nbsp;1). Notably, the S: Se ratio in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e exhibits a volcano-type relationship with HER activity, Tafel slope, \u003cem\u003eC\u003c/em\u003e\u003csub\u003edl\u003c/sub\u003e, intrinsic activity, and TOF, with Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe at the volcano\u0026rsquo;s apex displaying optimal overall HER performance (Supplementary Figs.\u0026nbsp;21 and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). This underscores the effectiveness of precisely tuning the d-band structure of Rh single-atoms to enhance HER activity and kinetics. To further quantify the impact of MSIs modulation via the S:Se ratio variation in MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e supports, the enhancement rates of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e relative to pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e substrates were evaluated for activity, Tafel slope, ECSA, and intrinsic activity, based on the following equation:\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\n\u003cp\u003eThe enhancement rate (δ\u003csub\u003eEnhance rate\u003c/sub\u003e) is defined as the ratio of the performance metrics (activity, Tafel slope, ECSA, and intrinsic activity) for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e (N\u003csub\u003eRhSA\u0026minus;MoSxSe2\u0026minus;x\u003c/sub\u003e) relative to those for pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e (N\u003csub\u003eMoSxSe2\u0026minus;x\u003c/sub\u003e). Notably, the enhancement rates for activity, Tafel slope, ECSA, and intrinsic activity exhibit a volcano-type dependence on the S: Se ratio, with the highest enhancement rate observed at an S: Se ratio of 1:1 (volcano\u0026rsquo;s apex) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed).\u003c/p\u003e\u003cp\u003eThe strong interaction between OH* species and catalysts surfaces is well-established for accelerating water dissociation in the HER\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. To probe the OH adsorption capacity at CO adsorption sites\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e, CO-stripping voltammetry was employed to assess the water dissociation capabilities of the Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e catalysts (Supplementary Fig.\u0026nbsp;22a). Among them, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe exhibited the lowest onset potential for CO oxidation (0.28 V), compared to Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e (0.31 V), Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e1.5\u003c/sub\u003eSe\u003csub\u003e0.5\u003c/sub\u003e (0.30 V), Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e0.5\u003c/sub\u003eSe\u003csub\u003e1.5\u003c/sub\u003e (0.30 V), and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (0.32 V), indicating a stronger Rh-OH interaction and enhanced water dissociation kinetics. The H adsorption ability of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e catalysts was evaluated through the desorption of underpotential-deposited hydrogen (H\u003csub\u003eupd\u003c/sub\u003e)\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. As shown in Supplementary Fig.\u0026nbsp;22b, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe displayed a notably low H\u003csub\u003eupd\u003c/sub\u003e potential (0.12 V), compared to Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e (0.26 V), Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e1.5\u003c/sub\u003eSe\u003csub\u003e0.5\u003c/sub\u003e (0.15 V), Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e0.5\u003c/sub\u003eSe\u003csub\u003e1.5\u003c/sub\u003e (0.15 V), and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (0.16 V), facilitating H recombination during HER. Furthermore, the S: Se ratio in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e catalysts exhibited a volcano-type correlation with both OH and H adsorption strengths, with Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe at the volcano\u0026rsquo;s apex demonstrating optimal adsorption capabilities for both H and OH (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee), consistent with theoretical predictions.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eLong-term durability of Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe was assessed via chronopotentiometry, demonstrating negligible potential decay over 500 h at a high current density of 1 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (Supplementary Fig.\u0026nbsp;23). Furthermore, post-stability testing revealed minimal Rh leaching into the electrolyte. The catalyst\u0026rsquo;s stability across varying current densities was evaluated through multi-step current experiments, where the potential rapidly stabilized at each step, indicating efficient mass transport at the electrode surface during the HER (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). These findings underscore the catalyst\u0026rsquo;s robust electrochemical stability and potential for industrial applications. Structural integrity of Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe post long-term HER testing was investigated using XRD, SEM, HAADF-STEM, and XPS. XRD analysis (Supplementary Fig.\u0026nbsp;24) confirmed that the crystal structure of Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe remained intact after extended HER operation. SEM (Supplementary Fig.\u0026nbsp;25) and HAADF-STEM (Supplementary Fig.\u0026nbsp;26) images verified that Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe retained its original morphology and atomic dispersion of Rh after 500 h testing. XPS analysis (Supplementary Fig.\u0026nbsp;27) further revealed that the oxidation states of Rh, Mo, S, and Se in Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe remained stable and unaltered post testing. Collectively, these structural characterizations demonstrate that Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe exhibits exceptional HER activity and stability, attributed to the precise tuning of the Rh single-atoms d-band structure, which optimizes intermediate adsorption energies and significantly enhances alkaline HER performance.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eOperando electrochemical impedance spectroscopy (EIS) was conducted to probe the charge transfer kinetics and elucidate the HER\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e. The equivalent circuit for both Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e and pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e comprises four components: electron transfer from the cathode to the interface (R\u003csub\u003e1\u003c/sub\u003e, part 1), accumulation of reaction intermediate (Volmer step, R\u003csub\u003e2\u003c/sub\u003e, part 2), charge transfer during the interfacial reaction (Heyrovsky step, R\u003csub\u003e3\u003c/sub\u003e, part 3), and electrolyte resistance (R\u003csub\u003es\u003c/sub\u003e, part 4) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). The low-frequency region is primarily associated with the Volmer step, while the high-frequency region reflects electron transfer from the catalyst\u0026rsquo;s inner layer to surface-active sites, driven by distinct relaxation time. Bode plots at overpotential of 100 mV (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb) reveal significantly reduced phase angles at low frequencies for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e compared to MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e supports, indicating that Rh single-atoms enhance the Volmer step kinetics. Analysis of Nyquist plots and optimized fitting parameters (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec-f) shows that Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e catalysts exhibit substantially lower charge transfer resistances than their pristine MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e counterparts. Notably, R\u003csub\u003e1\u003c/sub\u003e, R\u003csub\u003e2\u003c/sub\u003e, and R\u003csub\u003e3\u003c/sub\u003e display a volcano-type dependence on the S: Se ratio in both Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e and MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e, with Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe at the volcano\u0026rsquo;s top exhibiting the lowest R\u003csub\u003e1\u003c/sub\u003e, R\u003csub\u003e2\u003c/sub\u003e, and R\u003csub\u003e3\u003c/sub\u003e values, signifying accelerated HER charge transfer kinetics and a rapid Faradaic reaction at the catalyst-electrolyte interface. Operando EIS measurements at varying applied biases further clarify the charge transfer dynamics and HER mechanism of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e. Bode plot (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg) for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e reveal a single phase-angle peak spanning low-frequency (Volmer step) and mid-frequency (Heyrovsky step) regions as the applied bias decreases, suggesting that Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e follows a mixed Heyrovsky-Volmer and Tafel-Volmer mechanism rather than a singular Heyrovsky-Volmer pathway. Remarkably, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe exhibits a smaller phase angle and a faster phase-angle decrease rate (0.05 to -0.05 V) compared to Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eh), further confirming its superior HER kinetics.\u003c/p\u003e\u003cp\u003eTo elucidate the enhanced alkaline HER activity of Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, operando infrared absorption (IR) spectroscopy and in situ Raman spectroscopy were employed to investigate adsorption site and binding energy dynamics of reaction intermediates under HER operating conditions. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, with decreasing bias potential, distinct absorption peaks emerged at approximately 1030, 1621, and 3200\u0026ndash;3600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The peak at ~\u0026thinsp;1030 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is attributed to S/Se-OH formation, while the broad peak at 3200\u0026ndash;3600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and the peak located at ~\u0026thinsp;1621 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e correspond to the O-H stretching and \u0026#120575;(H-O-H) bending modes of interfacial water molecules, respectively\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e,\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. Moreover, water-related peaks for Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe appeared at a more positive potential (0.15 V) compared to Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e (0.1 V) and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (0.1 V), indicating that Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe facilitates water dissociation more effectively. The vibrational Stark effect, which describes potential-dependent shifts in adsorbate vibrational frequencies\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e, reveals that S/Se-OH in Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe is more responsive to the local electric field than in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e, as evidenced by steeper Stark slopes (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). In-situ Raman spectroscopy further probed the catalytic processes in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e during alkaline HER. As presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec, Raman peaks at ~\u0026thinsp;1522 and ~\u0026thinsp;1395 cm\u003csup\u003e\u0026minus;1\u003c/sup\u003e for Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe emerged at 0.15 V, corresponding to adsorbed OH species generated during the HER\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. In contrast, OH-related Raman peaks for Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e appeared at 0.1 V, underscoring the stronger OH binding and accelerated water dissociation kinetics of Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe. Additionally, the Stark slopes for \u003csup\u003e*\u003c/sup\u003eOH in Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe indicate greater sensitivity to the local electric field compared to Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003e2\u003c/sub\u003e and Rh\u003csub\u003eSA\u003c/sub\u003e-MoSe\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed). These findings suggest that precise modulation of the S: Se ratio optimizes OH adsorption at Rh single atom sites, thereby enhancing H\u003csub\u003e2\u003c/sub\u003eO adsorption and dissociation (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee). Gibbs free energy differences for the alkaline HER process in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e and MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e were calculated in Supplementary Figs.\u0026nbsp;28\u0026ndash;37. H\u003csub\u003e2\u003c/sub\u003eO adsorption at Rh sites in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e is thermodynamically barrier-free and more favorable than at S/Se sites in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e or MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e. The rate-determining step (RDS) for alkaline HER in both systems is H₂O dissociation. Significantly, the incorporation of Rh single-atom in Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e lowers the energy barriers for H\u003csub\u003e2\u003c/sub\u003eO dissociation and enhances the adsorption/desorption behavior of OH and H, synergistically improving HER thermodynamics and kinetics (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef). Furthermore, the water dissociation free energy of Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e exhibits a volcano-type relationship with the S: Se ratio, with the lowest energy barrier observed at an S: Se ratio of 1:1 (volcano\u0026rsquo;s apex).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, we establish an anion-gradient engineered Rh SACs platform (Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2-x\u003c/sub\u003e) for precisely manipulating d-p orbital hybridization at the atomic scale. Through bidirectional Rh-chalcogen coupling, the d-band center of single-atom Rh is continuously tuned, exhibiting a volcano correlation with H and OH adsorption energies. The apex catalyst, Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe, achieves optimized orbital hybridization, simultaneously balancing intermediate adsorption/desorption kinetics and strengthening Rh-S/Se covalent bonding-thereby delivering outstanding HER performance, with a low overpotential of 42 mV at 10 mA cm\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e and excellent long-term stability under industrially relevant fluctuating power conditions (\u0026gt;\u0026thinsp;300 h). This work resolves the fundamental linkage between orbital hybridization and electrocatalytic energetics, providing a universal design principle to transcend activity-stability trade-offs in single-atom catalysis.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eSupporting Information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNotes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing financial interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledg\u003c/strong\u003e\u003cstrong\u003ee\u003c/strong\u003e\u003cstrong\u003ements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was financially supported by National Natural Science Foundation of China (22369005, 52302236, W2521028, 52231008, and 52301011), the Key Research and Development Program of Hainan Province (ZDYF2024GXJS006), International Science \u0026amp; Technology Cooperation Program of Hainan Province (GHYF2023007).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eYu, Z. 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Mater.\u003c/em\u003e \u003cstrong\u003e36\u003c/strong\u003e, 2411942 (2024).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Single Atom Catalysts, Metal-Support Interaction, d-p Orbital Hybridization, Volcano-Type Relationship, Anionic Regulation","lastPublishedDoi":"10.21203/rs.3.rs-7039226/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7039226/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSingle-atom catalysts hold great promise for hydrogen evolution reactions due to their maximal atomic utilization and discrete energy levels. Modulating metal-support interactions is a powerful strategy for tailoring the electronic structure and catalytic performance of single-atom catalysts. However, achieving precise control and gaining mechanistic insight into these interactions, especially at the orbital level, remains challenging and often controversial. Here, we construct a model system of rhodium single-atom catalysts, in which isolated Rh atoms are anchored on a series of molybdenum sulfide selenide supports (Rh\u003csub\u003eSA\u003c/sub\u003e-MoS\u003csub\u003ex\u003c/sub\u003eSe\u003csub\u003e2\u0026minus;x\u003c/sub\u003e, 0\u0026thinsp;\u0026le;\u0026thinsp;x\u0026thinsp;\u0026le;\u0026thinsp;2), enabling gradient-continuous modulation of metal-support d-p orbital interactions through systematic tuning of the support p-band structure. We demonstrate that the d-band center of Rh single-atoms exhibits a volcano-type relationship with key HER descriptors, such as hydrogen and hydroxide binding energies, where hybridization-induced d-band position optimizes intermediate adsorption/desorption kinetics, and strengthened Rh-S/Se covalent interactions enhance durability. The apex Rh\u003csub\u003eSA\u003c/sub\u003e-MoSSe catalyst, with optimal d-p orbital hybridization, achieves superior HER activity and exceptional stability simultaneously. This work offers fundamental insights into the band structure-activity relationships of SACs and establishes a rational design framework for high-efficiency electrocatalysis through support-mediated d-p orbital hybridization.\u003c/p\u003e","manuscriptTitle":"Anion-tuned d-p hybridization breaks activity-stability trade-off in single-atom hydrogen evolution catalysts","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-20 08:55:08","doi":"10.21203/rs.3.rs-7039226/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"d78b0c4f-1e2a-4813-b9f0-2e8487a268ae","owner":[],"postedDate":"August 20th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":53390796,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis"},{"id":53390797,"name":"Physical sciences/Chemistry/Catalysis/Electrocatalysis"}],"tags":[],"updatedAt":"2026-05-05T06:50:32+00:00","versionOfRecord":[],"versionCreatedAt":"2025-08-20 08:55:08","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7039226","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7039226","identity":"rs-7039226","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}