Interfacial water regulation on Ru single atoms doped Co_3O_4 toward efficient electrochemical hydrogenation of quinoxaline

Domain: chemistry
Published: 2026-01-21
Editorial outcome: major_revision

Abstract
Electrochemical hydrogenation of quinoxaline presents a promising alternative to traditional methods, yet is suffering from low current density and Faradaic efficiency due to the hampered hydrogenation process. Herein, we develop a cocatalytic system of Ru single atoms doped Co3O4 nanosheet (RuSA/ns-Co3O4) to optimize the interfacial H2O behavior by tuning the Ru single atoms concentration for accelerating the electrochemical hydrogenation of quinoxaline, which enables remarkable Faradaic efficiency of 82% toward 1,2,3,4-tetrahydroquinoxaline at high current density of 200 mA cm-2. Detailed experimental and theoretical studies reveal that Ru single atoms trigger interfacial charge redistribution, inducing an asymmetric local electric field that reconstructs interfacial H2O molecules into an H-down configuration. This reorientation remodels the hydrogen-bonded water network, shortens the distance between hydrogen atoms and the Co3O4 surface, regulates K+•H2O availability, and enhances H2O dissociation to supply H*. Consequently, the membrane electrode assembly electrolyser exhibits a long-term stability of >100 h at 200 mA cm-2. Our findings highlight the prospect of interfacial water microenvironment for electrochemical hydrogenation of unsaturated N-heterocyclic compounds, with promising applications for the electrosynthesis of other valuable chemicals.

Introduction
The hydrogenation of unsaturated N-heterocyclic compounds holds significant importance in liquid organic hydrogen carrier (LOHC) technology and pharmaceutical synthesis, attracting extensive research interest due to its application potential1–3. For example, quinoxaline hydrogenation to 1,2,3,4-tetrahydroquinoxaline as one of the most common hydrogen storage reactions, which is dependent on thermochemcial hydrogenation using H2 or organic hydrogen donor as the source of protons and electrons under high temperature and pressure conditions4,5, resulting in significant energy waste and byproduct (Fig. 1a). In contrast, renewable energy-powered electrochemical hydrogenation (ECH) using water as the green hydrogen source provides an efficient and sustainable approach to achieve hydrogenation of unsaturated compounds under ambient conditions6–8. Unfortunately, the sluggish water dissociation and poor proton supply rate cause the insufficient kinetics of the hydrogenation process, the current quinoxaline to 1,2,3,4-tetrahydroquinoxaline conversion via the ECH process suffers from the unsatisfactory current densities (<50 mA cm−2) and Faradaic efficiency, as well as poor stability9–12, thus resulting in industrial-incompatible production rates. To achieve high efficiency and selectivity in ECH, adequate proton supply is the prerequisite for guaranteeing the electrochemical hydrogenation steps of substrates13–15. However, the production of protons is required to overcome the high thermodynamic energy barrier of H2O dissociation16–18, inhibiting the hydrogenation efficiency and selectivity in alkaline environments (Fig. 1b). Actually, interfacial H2O not only dissociate into adsorbed active hydrogen (H*) on the electrode surface to provide the hydrogen source but also form an hydrogen-bond (HB) networks to affect proton transfer19,20. In view of this, the unique characteristics of interfacial H2O structure can greatly determine the thermodynamics and kinetics of active hydrogen generation, which can be readily influenced by the local microenvironment21–23. Currently, most of the previous reports focus on the surface modification/electrolyte additives to modulate active hydrogen (H*) behavior for enhancing the proton supply24–26. However, excessive addition of additives could reduce the conductivity of the electrolyte and hinder proton transport27–29. In comparison, regulating the interactions between the catalyst surface and interfacial H2O via local electric fields to reorient H2O molecules and reconstruct H-bond networks represents an excellent strategy for controlling water dissociation activity (Fig. 1c)30,31. However, those methods cannot precisely regulate the structure of H-bond networks, and its correlation with ECH performance remains unclear. Therefore, it is highly important to modulate the local microenvironment of H2O molecules on the electrocatalyst surface to meet the H* demands of ECH reactions by regulating the dissociation activity of interfacial H2O, which has been rarely explored.

Here, we construct a series of Ru single-atom doped Co3O4 nanosheet electrocatalysts (denoted as RuSA/ns-Co3O4) to precisely regulate the availability of K+•H2O for accelerating the hydrogenation process of quinoxaline at an industrial-level current density. By integrating theoretical simulations and calculations, and in situ spectra characterizations, revealing the RuSA/ns-Co3O4 cocatalytic system generates an asymmetric localized electric field on the catalytic surface, which could induce interfacial H2O molecules into an H-down reconfiguration, facilitating stepwise H2O activation and efficient H* generation for boosting the hydrogenation of quinoxaline. As the proof-of-concept electrocatalyst, the RuSA/ns-Co3O4-0.7% with the optimal K+•H2O ratio enables efficient ECH of quinoxaline to a 1,2,3,4-tetrahydroquinoxaline with selectivity of ~100% and Faradaic efficiency of 82% at 200 mA cm−2, surpassing recently reported state-of-the-art electrocatalysts. This enables membrane electrode assembly (MEA) stable operation for 100 h at a current density of 200 mA cm−2. Importantly, the RuSA/ns-Co3O4 catalyst also shows excellent universality for electrochemical hydrogenation of various unsaturated N-heterocyclic compounds. These results indicate the great potential of the electrochemical hydrogenation of unsaturated N-heterocyclic compounds as a next-generation sustainable hydrogen storage technology32.

Results
Theoretical investigations
Density functional theory (DFT) calculations were first performed to analyse the redistribution of charge density, indicating that the introduction of Ru single atoms leads to a strong and asymmetric atomic localized electric field (Fig. 2a). Ab initio molecular dynamics (AIMD) simulations were further conducted to explore atomic structures of interfacial H2O on RuSA/Co3O4 and Co3O4 surfaces (Fig. 2b, c). The local electric field around the RuSAs affects the configuration of H2O molecules and the distributions of H and O at the interface. The average distance between H and the surface of Co atoms is significantly reduced from 1.60 Å to 1.38 Å (Fig. 2d), while the distance between O and catalyst surface is not changed significantly in these two cases (Fig. 2e). This evidence further indicates that asymmetric atomic localized electric fields facilitate the regulation of interfacial water orientation and enable precise manipulation of the reaction microenvironment. We further employed DFT calculations to investigate the energy barrier for H2O dissociation on the Co3O4 and RuSA/Co3O4 (Fig. 2f, Supplementary Figs. 1–3). The results show that the energy barrier of H2O dissociation on the Co atoms of RuSA/Co3O4 is 0.44 eV, which is much lower than that Co atoms of Co3O4 (0.63 eV) and Ru atoms of RuSA/Co3O4 (0.91 eV). These findings indicate that the interfacial H2O reorientation induced by local electric field at the active site can optimize the H2O dissociation process, significantly reducing the kinetic barrier for forming H*.

Synthesis and characterizations of RuSA/ns-Co3O4
Inspired by the concept, such Ru single atoms doped Co3O4 nanosheets were prepared through a facile chemcial dealloying and annealing process (Supplementary Fig. 4). Typtically, the precursor Al Co Ru alloy ribbons were prepared by single-roller melt spinning. Then the metallic Al of these precursor alloys was completely dealloyed in a KOH solution to obtain Ru doped CoOx nanosheets (Supplementary Figs. 5 and 6). Finally, uniform Ru-doped Co3O4 nanosheets were obtained after annealing at 300 °C under Ar atmosphere, demonstrating that the introduction of Ru did not alter the morphology of the Co3O4 nanosheets (Fig. 3a, Supplementary Fig. 7). The X-ray diffraction (XRD) patterns show typical spinel structure with Fd-3m space groups for ns-Co3O4 and RuSA/ns-Co3O4 (Supplementary Fig. 8a)33. Owing to the introduction of the Ru atoms, the diffraction peaks of Ru/ns-Co3O4 slightly shifted to a higher angle than that of ns-Co3O4, suggesting the atomic-level Ru was well incorporated into the Co3O4 crystal lattice and causes the local structure contraction (Supplementary Fig. 8b)34. Aberration–corrected high–angle annular dark–field scanning transmission microscopy (HAADF–STEM) image of RuSA/ns-Co3O4 reveals clear lattice fringes with a spacing of 1.82 Å, which can assigned to the interplanar spacing of the (331) plane of Co3O4 (Fig. 3b). Isolated dots with particularly high intensity were clearly observed, which are assigned to Ru atoms because of the higher atomic number of Ru (Supplementary Fig. 9). These results reveal that Ru atoms are doped into the lattice structure of Co3O4. Furthermore, the energy-dispersive spectroscopy (EDS) mapping images indicate a homogenous distribution of atomic Ru species in the Co3O4 nanosheets (Supplementary Fig. 10). Subsequent composition evaluation of RuSA/ns-Co3O4 by inductively coupled plasma optical emission spectrometry (ICP-OES) confirms that Ru has an atomic ratio of 0.7% (Supplementary Table 1). X-ray absorption spectroscopy (XAS) was conducted with the aim of distinguishing the electronic structure and the local atomic environment of RuSA/ns-Co3O4. The normalized Ru K-edge X-ray absorption near-edge structure (XANES) spectrum is positioned between those of Ru foil and RuO2 (Fig. 3c), indicating the distinctive electronic structure of Ruδ+ (0<δ<4), as confirmed by the X-ray photoelectron spectroscopy (XPS) results of Ru 3p in the RuSA/ns-Co3O4 (Supplementary Fig. 11). The Fourier transform (FT) of the Ru K-edge extended X-ray absorption fine structure (EXAFS) spectra of RuSA/ns-Co3O4 was further analyzed, exhibiting two distinct peaks at 1.55 Å and 2.45 Å, attributed to Ru-O and Ru-Ru/Co coordinations, respectively (Fig. 3d). This confirms the synthesis of the Ru-incorporated structure without the formation of Ru nanoclusters or nanoparticles. Intriguingly, the Ru-O peak position in RuSA/ns-Co3O4 catalyst, slightly higher than in the pure RuO2, indicates a manipulated local structure with varied Ru-O bond lengths and lattice distortion in breaking the structural symmetry of the ns-Co3O4 matrix35. Further analysis via EXAFS wavelet transform (WT) discerns radial distance resolution and differentiates backscattering atoms in k-space (Fig. 3e). Notably, two main maxima at 5.0 Å−1 and 7.2 Å−1 have been shown on RuSA/ns-Co3O4, which represent Ru-O and Ru-Co units35,36, respectively, further indicating the atomic dispersion of Ru atoms in RuSA/ns-Co3O4 with the absence of locally aggregated RuO2 cluster structure. Furthermore, a model-based EXAFS fitting of RuSA/ns-Co3O4 indicates that no Ru-Ru scattering can be discovered and the second scattering can be attributed to Ru-Co with a distinct bond distance, which represents Ru-O-Co in the crystal structure (Fig. 3f and Supplementary Fig. 12)34. Therefore, we conclusively demonstrate that Ru species exist in the form of single-atom sites with homogeneous dispersion throughout the Co3O4 matrix. The photon energy of RuSA/ns-Co3O4 shows a negative shift of pre-peak (7709 eV) than Co3O4 (Fig. 3g), indicating that the atomic-level Ru doping leads to the local atomic arrangement of Co atoms and the transfer of electrons between ns-Co3O4 and Ru, as verified by the Co 2p3/2 XPS results (Supplementary Fig. 13). The corresponding Co K-edge EXAFS of Co3O4 and RuSA/ns-Co3O4 reveal three distinct peaks, identified as Co-O, octahedral Co-Co (CoOh-CoOh) and tetrahedral Co-Co bonds (Co-CoTd), respectively (Fig. 3h)34,35. Compared with Co3O4, RuSA/ns-Co3O4 shows a slightly increased ratio of CoOh-CoOh coordination to Co-CoTd coordination. This indicates a slightly increased ratio of octahedron to tetrahedron sites35. This evidence further substantiates that the incorporated Ru species preferentially occupies the octahedron site in the spinel Co3O4 structure, which enhances its structural stability.

Electrochemical hydrogenation performance measurement
We first assessed the ECH performance in a standard three-electrode H-cell configuration in 1 M KOH with 50 mM quinoxaline. The different Ru loadings in the Co3O4 nanosheets could be created through tuning the compositions of precursor alloys (Supplementary Figs. 8, 14, 15 and Supplementary Table S1). Linear sweep voltammetry (LSV) curves reveal that the Ru SAs catalyst exhibits significantly higher current density than ns-Co3O4, indicating that Ru single atoms can promote the hydrogenation activity and kinetics of quinoxaline (Supplementary Fig. 16). In particular, it is found that the catalytic activities and FE of Ru SAs catalysts are superior to ns-Co3O4, illustrating that the introduction of Ru single atoms is more favourable for ECH of quinoxaline, especially at large current densities (Fig. 4a, b and Supplementary Figs. 17–19). Specifically, RuSA/ns-Co3O4-0.7% presents a high FE of 82% at 200 mA cm−2, which is higher than those of RuSA/ns-Co3O4-0.5% (~42%) and RuSA/ns-Co3O4-1.0% (~32%) (Fig. 4c). Notably, an early 100% carbon balance was achieved, verifying the exclusive conversion of quinoxaline to the target product 1,2,3,4-tetrahydroquinoxaline (Supplementary Figs. 20). Such a high FE and large current density over RuSA/ns-Co3O4-0.7% still have high levels relative to other reported state-of-the-art electrocatalysts (Fig. 4d and Supplementary Table 1)9–12,37. These results indicate that the RuSA/ns-Co3O4-0.7% could enhance quinoxaline hydrogenation activity at industrial-level current densities, primarily attributed to the dynamic equilibrium between active hydrogen generation and consumption established on the catalyst surface. Significantly, the RuSA/ns-Co3O4 exhibits high electrochemical hydrogenation performance toward other N-heterocyclic compounds, achieving a conversion rate of >90% and a selectivity of ~100% (Supplementary Figs. 21–27), which is significantly superior to that of ns-Co3O4, thus validating the generality of the proposed strategy.
To further validate the potential of RuSA/ns-Co3O4 in practical applications, the electrochemical activity and stability of RuSA/ns-Co3O4-0.7% were investigated with MEA electrolyser at different industrial-level current densities (Fig. 4e, f). In comparison with the high cell voltage of water splitting without the addition of quinoxaline, the cell voltages are significantly decreased after adding quinoxaline, indicating that the ECH of quinoxaline process is kinetically more favorable (Supplementary Fig. 28). Notably, RuSA/ns-Co3O4-0.7% maintained the high FE of 1,2,3,4-tetrahydroquinoxaline at 200 mA cm−2 for 100 h under continuous electrolysis (Fig. 4g). Importantly, 15.7 g of 1,2,3,4-tetrahydroquinoxaline as the hydrogenated product is obtained (Fig. 4h), further highlighting the high potential for industrial-level ECH of quinoxaline to 1,2,3,4-tetrahydroquinoxaline. Meanwhile, the morphology and compositions of RuSA/ns-Co3O4-0.7% can well retained without obvious damage after the long-term stability test (Supplementary Figs. 29–32), which confirms their ability to maintain high current density and FE due to the structural stability.
In addition, we conducted a preliminary technoeconomic analysis (TEA) using a model adapted in the literature38,39, which includes the costs of capital, maintenance, installation, input chemicals, and operation, to investigate the feasibility of this coupling system (Supplementary Fig. 33, Supplementary Note 1 and Supplementary Table 3). The cost and revenue of the ECH||OER system were calculated to be ~ $400.3136/ton quinoxaline and ~ $1033/ton 1,2,3,4-tetrahydroquinoxaline, respectively, thus the profit was calculated to be $632.6864/ton 1,2,3,4-tetrahydroquinoxaline (Supplementary Fig. 34).

In situ investigations for interface water
Given that H* only originates from interfacial water in alkaline environments, regulating interfacial water behavior is crucial for promoting H2O-H bond dissociation and providing sufficient H*40. In situ Raman spectroscopy was firstly performed to reveal the precise mechanism of interfacial water manipulation by Ru SAs. The broad O-H stretching peak of interfacial water can be Gaussian fitted into three distinct components, Raman peaks located at ~3200, ~3400, and ~3600 cm−1 are attributed to 4-coordinated hydrogen-bonded water (4HB-H2O), 2-coordinated hydrogen-bonded water (2HB-H2O), and free water (K+•H2O), among which the K+•H2O with the weakest hydrogen-bond interaction require the lowest activation energy to break the covalent O–H bond of H2O (Fig. 5a)41–43. Compared with that of ns-Co3O4, the larger proportion of K+•H2O on RuSA/ns-Co3O4 at different potentials indicates weaken hydrogen-bond networks at the electrode–electrolyte interface (Fig. 5a and Supplementary Fig. 35). Besides, with the potential decreases, the proportion of K+•H2O on RuSA/ns-Co3O4 slightly increased, whereas it is depleted on ns-Co3O4 (Fig. 5b). This different behavior is attributed to Ru SAs induced asymmetric local electric field, which could beneficial for the reconfiguration of interfacial water and disrupt the connectivity of hydrogen-bond network in the electric double layer. Because of the lack of constraint of hydrogen bonds, K+•H2O is more inclined to transform into the H-down structure at negative potentials than 2HB-H2O and 4HB-H2O (Supplementary Fig. 36)30, thereby leading to facilitated dynamic evolution from hydrogen bond water to K+•H2O, and constructing continuous K+•H2O enriched environment. For these catalysts, 4HB-H2O remains the predominant species across all detected potentials, ensuring efficient proton transfer during the water dissociation process (Supplementary Fig. 37). In contrast, the 2-HB•H2O species decreases gradually with increasing Ru single atom concentration, indicating that 2-HB•H2O gradually transforms into K+•H2O (Supplementary Fig. 38)44. Additionally, the vibrational frequencies of the adsorbates vary with electrode potentials, which are attributed to the vibrational Stark effect45. A steeper Stark slope indicates that K+•H2O is more sensitive to the electric field at the cathodic potentials than 4HB-H2O and 2HB-H2O (Supplementary Fig. 39)46. Kinetic isotopic effect (KIE) tests were further conducted to validate the contribution of asymmetric local electric field to the enhanced H2O dissociation. The KIE values were calculated by comparing the current density measured in H2O and D2O, revealing the proton transfer kinetics during the H2O splitting process47. As a result, the RuSA/ns-Co3O4 shows the much lower KIE value than the ns-Co3O4, indicating that asymmetric local electric field can accelerate the H2O dissociation process and ensure the H* supply (Fig. 5c, Supplementary Fig. 40). In addition, the electron paramagnetic resonance (EPR) spectrum from RuSA/ns-Co3O4 exhibited a stronger signal than that in ns-Co3O4 at the same applied potential, indicating a higher capability for H* generation on the RuSA/ns-Co3O4, which in line with the KIE results (Fig. 5d)48. Taking the sequence of Ru single atom concentrations, we have determined herein combination with the above in situ Raman results and electrochemical performance to accurately establish the relationship between the performance and the structure of H-bond networks, achieving precise control over the activity of ECH. Under high current densities, increasing single-atom concentrations induces the weakening of hydrogen-bond networks with large K+•H2O ratios, accompanied by an increase in the competing HER. Conversely, decreasing single-atom concentrations induces insufficient H* supply and suppresses the kinetics of quinoxaline hydrogenation. This can be further verified in Fig. 5e, where the partial current density of 1,2,3,4-tetrahydroquinoxaline shows a volcano-shaped dependence on the Ru single atom concentrations of these catalysts. The RuSA/ns-Co3O4-0.7% with optimal single atom concentrations reaches the summit of the volcano, which stands out for having excellent industrial-level current densities and durability due to the balance of hydrogen transfer from the reaction microenvironment by tuning the K+•H2O ratios at the electrode–electrolyte interface.

Mechanism investigations for quinoxaline hydrogenation
To investigate the origins of the marked quinoline hydrogenation performance of RuSA/ns-Co3O4 catalyst, we performed in-situ attenuated total reflection surface enhanced infrared absorption (ATR-SEIRAS) spectroscopy to monitor quinoline hydrogenation intermediates under the different potentials (Fig. 6a, b). As the ECH of quinoxaline reaction progresses, the C–N stretching vibration at 1208 cm−1 and the C=C backbone vibration at 1560 cm−1 show upward band bends, attributing to the N adsorption of quinoxaline and the hydrogenation of the pyrazine ring, respectively. The vibration peaks of –CH2– at 1496, 2853, and 2960 cm−1 bend downward, which indicates the hydrogenation of the pyrazine ring of quinoxaline. In addition, the generation of *C8H7N2 radical intermediate was also observed by the downward bend of the stretching vibration of N–H at 2929 cm−1 and the bending vibration of N–H at 1631 cm−1. The changes observed in the N–H, –CH2–, C=C, C–N, and O–H signals demonstrate the effective adsorption and hydrogenation process of quinoxaline on catalyst sites49. However, compared with ns-Co3O4, RuSA/ns-Co3O4 exhibits a notably strong adsorption reaction generation peak, which is attributed to the more vigorous H2O dissociation on RuSA/ns-Co3O4 to generate more H* for the ECH of quinoxaline to increase product formation, which is further confirmed by the H* scavenging experiment with tert-butyl alcohol (TBA) (Supplementary Fig. 41). In situ X-ray absorption spectroscopy (XAS) measurements were further conducted to analyze the catalytic behavior of RuSA/ns-Co3O4 in 1 M KOH with quinoxaline. As the applied potential was decreased from the open circuit potential (OCP) to −0.2 V vs. RHE, the absorption edge of the Ru K-edge XANES spectra progressively shifted toward lower energies, indicating a decrease in the oxidation state of Ru (Fig. 6c). Moreover, compared to the OCP, the first coordination shell structure of the Ru-O bond showed no significant changes at potentials of −0.1 V and −0.2 V vs. RHE, indicating that quinoxaline molecules did not adsorb on the Ru sites. Notably, the FT-EXAFS spectra show a slight shift in the peak position of the second coordination shell associated with the Ru-Co bond under applied potentials. These shifts imply that the H2O dissociation and quinoxaline hydrogenation at Co sites alter the local electronic structure and atomic configuration, and indirectly modulate the FT-EXAFS signal of Ru through electronic coupling and structural communication via the Ru-Co bond (Fig. 6d). Therefore, these results indicate that Co sites serve as the main active sites for the ECH of quinoxaline. The in situ experimental results demonstrate that the Ru SAs primarily enhanced H2O dissociation dynamics while generating sufficient H*, thereby effectively driving the ECH of quinoxaline to 1,2,3,4-tetrahydroquinoxaline on the Co3O4 matrix.
The hydrogenation reaction mechanism of quinoxaline was further investigated through DFT calculations. The 1,2,3,5,6- and 1,2,4,5,6-H* addition pathways are two possible pathways for electrochemical hydrogenation of quinoxaline in water (Supplementary Fig. 42). According to theoretical calculations, the 1,2,3,5,6-H* addition pathway is more favorable (Supplementary Figs. 43–47). Free energies for the distinct hydrogenation sequences on the two catalysts were calculated (Fig. 6e). Upon analyzing the reaction free energies of the pathway, it is observed that the first hydrogenation of quinoxaline exhibits reaction barriers of 0.29 and 0.20 eV for the Co3O4 and RuSA/Co3O4, respectively. Notably, the highest activation barrier of this first hydrogenation step serves as the rate-determining step for both catalysts. However, in the second hydrogenation step, there is a significant disparity in the activation energies between these two catalysts. Co3O4 exhibits a substantially lower reaction barrier of 0.01 eV compared to RuSA/Co3O4 (0.07 eV). Nevertheless, for the subsequent steps of hydrogenation, the energy barrier of RuSA/Co3O4 is significantly lower than that of Co3O4, offering a much faster kinetics for the hydrogenation. Besides, H* is preferentially hydrogenated for quinoxaline rather than undergoing self-coupling to release H2, which accounts for the high FE of the ECH process (Supplementary Figs. 48–51). Therefore, Ru SAs can optimize the hydrogenation step and improve the hydrogenation reaction activity of quinoxaline, as experimentally confirmed by the quasi-insitu EPR results (Supplementary Fig 52).

Methods
Materials
Aluminum foil (99.999%), Ruthenium foil (99.99%) and Cobalt foil (99.99%) were purchased from Beijing Jiaming Platinum Nonferrous Metals Co., Ltd. Quinoxaline (98%), Isoquinoline (97%), Pyridine (99.5%), Quinoline (99%), and Tert-butyl alcohol (99.5%) were purchased from Adamas. Ethyl Acetate (99.7%) and KOH (95%) were purchased from Greagent. All chemicals were used without further purification.

Preparation of RuSA/Co3O4 nanosheet alloy. Al80Co20-xRux (x=0, 0.1, 0.2, 0.3) ribbons were prepared via a melt-spinning process. The precursor ribbons were then prepared via a two-step method. Firstly, 200 mg of Al80Co20-xRux ribbons was put into 200 mL of 2 M KOH aqueous solution to remove Al in the ribbons. The etching reaction was at room temperature for around 10 h until no apparent bubbles were observed and obtained Ru/ns-CoOx. The as-prepared product was washed with ultra-pure water and alcohol several times. After the washing process, the product was dried in a vacuum oven at 60 °C for 24 h. The second-step calcination was carried out in air at 300 °C for 3 h.

Characterizations. XRD patterns of the samples were conducted by using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ=1.5418 Å). Morphology and chemical composition were collected via MIR3 TESCAN SEM equipped with an Oxford energy dispersive X-rayspectroscope. HAADF-STEM and EDS mapping were conducted on a JEM-ARM200F with double spherical aberration (Cs) correctors for both the probe forming and image-forming objective lenses at an accelerating voltage of 200 kV. The chemical state and composition of the samples were characterized using XPS (Thermo Scientific Escalab 250Xi) with an Al Kα monochromatic (150 W, 20 eV pass energy, 500 μm spot size). NMR spectra were obtained on a Bruker Avance III HD 400-MHz spectrometer with CDCl3 as the solvent. EPR spectra were recorded with a Bruker EMX plus-6/1 spectrometer.

In situ XAS electrochemical measurements. The XAS experiments were carried out at BL11B at Shanghai Synchrotron Radiation Facility. A home-made Teflon electrochemical cell with an electrochemical workstation (Ivium, Compact Stat.) was employed for in situ XAS measurement under the sensitive fluorescence model (Supplementary Fig. 53). With adding quinoxaline to a cell filled with 1.0 M KOH electrolyte. A graphite rod was used as the counter electrode, and a Hg/HgO electrode was used as the reference electrode. The carbon paper loaded with catalyst as the working electrode was in contact with Kapton tape to the observation window of the cell. During the experimental measurement, different potentials of OCP, −0.1 and −0.2 V vs. RHE were applied to the system, and each potential was maintained to collect spectra for 10 min. The acquired XAS data were processed using Athena software. All the voltages indicated in the “Methods” section has not been iR corrected.

In situ ATR-SEIRAS measurements. Electrochemical in situ ATR-SEIRAS measurements were performed in internal reflection mode on a Thermo Fisher Nicolet iS50 (equipped with an ElectroChemIR unit from Pike Technologies). The experiments were carried out in a three-electrode system: The catalyst ink was dripped onto a monocrystalline silicon covered with an Au membrane, which served as the working electrode, Hg/HgO is introduced as a reference electrode near the working electrode, and a Pt wire is used as the counter electrode. We first reduced the catalyst in 1 M KOH for 10 min at −0.1 V vs. RHE, then 50 mM quinoxaline was added in the electrolyte for in situ ATR-SEIRAS test. The potential range was from OCP to −0.7 V (vs. RHE), with spectra collected at a spectral resolution of 4 cm‒1 with at least 32 coadded scans. The CHI760E electrochemical workstation was used to control the potential during the ATR-SEIRAS test.

In situ Raman measurements. Electrochemical in situ Raman measurements were carried out by Renishaw inVia Qontor Raman microscope. The excitation wavelength of the semiconductor laser was 532 nm. All the Raman measurements were performed with a 50× microscope objective. Raman frequencies were calibrated using silicon wafers before the experiment. A three-electrode Raman cell with an embedded quartz window was used for Raman experiments, with a platinum wire as counter electrode and Hg/HgO electrode as reference electrode. A glassy carbon electrode loaded with a catalyst was used as a working electrode. 1 M KOH solution was used as electrolyte. The CHI760E electrochemical workstation was used to control the potential during the Raman test.

Electrochemical measurements. The electrochemical measurements were carried out on CHI760E workstation. All the experiments were carried out at room temperature. All electrolytes used in the tests were freshly prepared and used immediately. Electrochemical measurements were carried out in a divided two-compartment electrochemical cell (Tianjin Aida Hengsheng Technology Development Co., Ltd.) consisting of a working electrode, a carbon rod counter-electrode, and a Hg/HgO reference electrode (Tianjin Aida Hengsheng Technology Development Co., Ltd.). The cathode cell and anode cell containing 10 mL 1.0 M KOH, were separated by an anion exchange membrane (A40-HCO3, 40 μm). 0.5 mmol of quinoxaline was added into the cathode and stirred to form a homogeneous solution. Then, chronoamperometry was carried out at a given constant current and until reaching 193 °C. The liquid products were extracted with ethyl acetate and then quantified by gas chromatography (Shimadzu, GC-2010 Plus) equipped with a flame ionization detector (FID). In a typical procedure of the fabrication of the working electrode, the catalyst ink was prepared by dispersing 5 mg of catalysts into a mixture solution of 0.48 mL ethanol and 20 μL of Nafion solution (5%, w/w, Alfa Aesar) with sonication for 30 min. Fifty microliters of the electrocatalyst ink was loaded onto a carbon paper with an area of 1×1 cm2 by drop-coating with the loading mass of catalyst is 0.50 mg cm−2. The as-prepared catalyst film was dried at room temperature. And ECH of quinoxaline durability measurements at large current densities were measured in a commercial 1×1 cm2 MEA electrolyzer with a two-electrode system, in which IrOx/Ti foam served as the anode electrode. The catalysts (1.5 mg cm−2) and Nafion (5%) were ultrasonically mixed and then were drop-coating on anion-exchange membrane (A40-HCO3, 40 μm), which served as the cathode electrode. The anion exchange membrane was soaked in 0.5 M KOH aqueous solution for 2 h and rinsed repeatedly with ultrapure water for pretreatment. And an anion-exchange membrane was located between the cathode and anode to separate chambers. The full-cell potentials in the two-electrode MEA system were shown without iR correction. During ECH of quinoxaline in MEA electrolyzer, the anode electrolyte (1.0 M KOH) and cathode electrolyte (1.0 M KOH + 50 mM quinoxaline) flowed through the anode and cathode at a flow rate of 20 mL min−1. All potentials measured were calibrated to RHE using the following equation: ERHE = EHg/HgO + 0.098 + 0.059 × pH (pH 13.7±0.3).

Quantitative reductive product. The conversion (%), selectivity (%), and Faradaic efficiency (FE, %) of 1,2,3,4-tetrahydroquinoxaline were calculated using Eqs. (1)—(3):
Conversion (%) = (mol of formed product / mol of initial reactant) × 100% (1)
Selectivity (%) = (mol of formed product / mol of consumed reactant) × 100% (2)
FE (%) = (n × m × F / I × t) × 100% (3)
where n = Number of transferred electrons
m = Amount of substance
F = Faradaic’s constant
I = Total current
t = Electrolysis time

Scavenge of high active H* with t-BuOH. A total of 0.5 mmol of quinoxaline was added into the electrolytic cell with 10 mL KOH for the following hydrogenation with/without t-BuOH. Chronoamperometry was carried out at −100 mA cm−2 for 193 °C in 1 M KOH.

Calculate H2O dissociation. The Co3O4 (222) was built by cleaving the Co3O4 2×2×2 supercell, which includes seven atomic layers and carries Co atoms on the top layer. The RuSA/Co3O4 (222) was modeled by replacing one top octahedral Co atom with Ru atom. In the cases of Co3O4 (222) and RuSA/Co3O4 (222), one and two reactants were considered, respectively, i.e., H2O on the top of Co atom for the former and H2O on the top of Ru atom or Co atom (that is close to Ru atom) for the latter. A ~12 Å vacuum was added along the direction normal to the surface. The search of transition states (TS) for H2O dissociation was performed by spin-unrestricted density functional theory (DFT) calculations in the CASTEP code50, before which the reactants and products were optimized with the convergence tolerances of energy, maximum force, and maximum displacement of 2.0×10−5 eV/atom, 0.05 eV/Å, and 0.002 Å, respectively. The complete LST/QST was adopted as the search protocol, where the LST (linear synchronous transit) was first performed and then followed by repeated conjugate gradient minimizations and QST (quadratic synchronous transit) maximizations until a TS was located. In these calculations, the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional was used for the description of the exchange and correlation effects; the Grimme method for the DFT-D correction51,52; 500 eV for the energy cutoff; 1×1×1 k-points meshes for Brillouin zone sampling; and OTFG ultrasoft for the pseudopotential.

Analysis of electron density difference. Based on the optimized Co3O4 (222) and RuSA/Co3O4 (222) models, the electron density difference with respect to the sum of the atomic densities was analyzed by using the CASTEP code50.

Examination of free-energy evolution during the hydrogenation processes. Based on the computational hydrogen electrode (CHE) model, in which the chemical potential of a proton-electron pair is taken by the one-half of the chemical potential of H2 (g) at standard hydrogen electrode conditions53,54, the difference of Gibbs free energies between two reaction intermediates were evaluated by:
ΔG(n+1)H = G*(n+1)H − G*nH − 1/2GH2 (4)
where G*(n+1)H, G*nH, and GH2 denote the Gibbs free energy of the adsorption system after adding one H, the adsorption system before adding one H, and the H2 molecule. The spin-unrestricted DFT calculations were carried out in the DMol3 code, where the related adsorption systems and H2 molecule were first optimized and then their frequencies were calculated and analyzed to obtain the free energies at 298.15 K.

Ab initio molecular simulations of H2O molecules. A layer with 24 H2O molecules was placed on the Co3O4 (222) and RuSA/Co3O4 (222) surfaces modeled above, respectively. After adding a vacuum larger than ~12 Å out of the surface, geometry optimizations were first carried out with the convergence tolerances of energy, maximum force, and displacement of 2.0×10−5 Ha, 0.004 Ha/Å, and 0.005 Å, respectively. The relevant spin-unrestricted DFT calculations were executed in the DMol3 code and at the GGA PBE level51,55,56. The DFT Semi-core Pseudopots (DSPP) was employed for core treatment; a smearing of 0.05 Ha for the orbital occupation; the Grimme method for the DFT-D correction52; and 1×1×1 k-points meshes for Brillouin zone sampling. Then, ab initio molecular dynamics simulations were performed on the two optimized systems, respectively, with the NVT (constant number of molecules, volume, and temperature) ensemble at a temperature of 298.15 K for 2.5 ps. Here, the timestep was set as 1 fs and the Massive GGM thermostat was adopted for temperature control.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript by Y. Tan and colleagues describes the RuSA/ns-Co3O4 can efficient electrochemical hydrogenation of\nquinoxaline. The asymmetric local electric field induced by Ru single atoms facilitates the formation of an H-down\nconfiguration in interfacial water and promotes the H2O dissociation rate; therefore, high reaction performance was achieved\nat industrial-grade current densities. The work presented in the manuscript is intriguing as the authors elucidated the\nmechanism through in-situ experiments combined with DFT calculate to support the catalytic performance. This work\nappears suitable for publication in Nature Communications. Nevertheless, I would like to offer some comments and several\nquestions to enhance the manuscript's quality.\n(1) In principle, Ru single atom doping causes lattice expansion of Co3O4, and the diffraction peak should shift towards\nlower values. Why does the diffraction peak shift towards higher values in this work?\n(2) To further validate the accuracy of the in-situ Raman spectroscopy data on interface water, the authors should\ncomplement the study with in-situ infrared spectroscopy measurements. Nat Commun 16, 6459 (2025), Angew. Chem. Int.\nEd. 2025, e202514557\n(3) Similarly, the authors presented the ratio of K•H2O and 4HB-H2O in the in-situ Raman data, yet omitted the\ncorresponding ratio data for 2HB-H2O.\n(4) The authors demonstrated that the K•H2O content is directly proportional to the Ru single-atom concentration. So, if the\nRu single-atom concentration continues to increase, will the K•H2O content also rise accordingly?\n(5) The signal intensity of Ru 3p in Supplementary Fig. 29 is too weak. To ensure data accuracy, it is recommended to retest.\n(6) For the stability assessment, the author demonstrated the material’s outstanding stability via XPS, XRD, and SEM\ncharacterizations. Notably, EDS data after the reaction remains required.\n(7) In the theoretical calculation modeling, the author did not specify the Ru doping sites within the calculation section,\nwhether tetrahedral or octahedral.\n(8) The author should compare the kinetics of quinoxaline hydrogenation and the HER through LSV, which is critical for the\nhigh Faradaic efficiency.\n(9) The EDS data from Supplementary Fig. 9 and Supplementary Fig. 14 feature an unlabeled peak at ~1.5 keV. What\nelement does this peak represent?\n(10) This question is only discussed with the authors, during the electrochemical hydrogenation of quinoxaline, why did\n1,2,3,4-tetrahydroquinoxaline emerge as the sole product, while the fully hydrogenated product did not appear?", "reviewer_2": "The authors investigated the Ru-doped Co3O4 nanosheet for the electrochemical hydrogenation of quinoxaline. It was\nproposed that the reconfiguration of the interfacial hydrogen-bonded water network enabled efficient hydrogenation of\nquinoxaline. Nevertheless, the selective hydrogenation of quinoxaline to 1,2,3,4-tetrahydroquinoxaline remains ambiguous.\nConsequently, the manuscript cannot be published at this time. The following comments warrant consideration:\n1.The authors posited that the induced interfacial H2O molecules facilitated efficient H* generation. Why does the reactive\nH* not accelerate the formation of H2, but instead promote the hydrogenation of quinoxaline?\n2.How can the selective hydrogenation of quinoxaline to 1,2,3,4-tetrahydroquinoxaline be reinforced without generating\nunwanted byproducts?\n3.The notion of obtaining Ru single atoms through a facile chemical dealloying and annealing process is indeed\nquestionable.\n4.How can it be confirmed that Ru effectively substitutes the Co sites?", "reviewer_3": "This work presents the concept of regulating the interfacial water network via Ru single-atom concentration to enhance\nelectrochemical hydrogenation of quinoxaline. However, to fully validate its claims and ensure its impact, substantial\nrevisions are required to address critical gaps in the experimental evidence, data interpretation, and material\ncharacterization.\n1. The authors convincingly demonstrate that Ru SAs promote water dissociation to generate H*. However, a critical\nparadox remains entirely unaddressed: why do these H* species preferentially participate in the kinetically more complex\nmulti-step hydrogenation of quinoxaline rather than simply combining to form H2, the thermodynamically favored and\nkinetically facile HER pathway? The manuscript provides no insight into the factors that might suppress HER.\n2. The manuscript highlights the high FE for the target product but fails to quantitatively address the competing HER. Direct\nquantification of H2 production (H2 FE) under the exact ECH reaction conditions are needed. This is essential to understand\nthe catalyst’s selectivity and to identify the destination of electrons when the FE for 1,2,3,4-tetrahydroquinoxaline was below\n100%.\n3. The consistent omission of quinoxaline conversion rate prevents a full assessment of the catalytic efficiency and practical\nutility. The conversion (%) and yield must be provided for all key performance tests alongside FE data.\n4. They claimed that the rationale for the volcano-shaped activity dependence on Ru concentration (Fig. 5e). However, the\nquantification method is inadequate. EDS is a semi-quantitative technique with a typical error of >5-10%, which is\nunacceptably high for accurately determining the low loadings of single atoms (0.6 at%, 1.0 at%, 1.4 at%) that are central to\nthe argument.\n5. The demonstration of stability in an MEA electrolyzer is a strength. However, the energy efficiency and practical\nadvantage of the process remain unclear. The LSV curves of the ECH || OER system (Supplementary Fig. 27) should be\ndirectly compared with the LSV of conventional water electrolysis (HER || OER) using the same MEA setup and electrodes.\nThis comparison is critical to demonstrate whether the ECH process occurs at a lower cell voltage than water splitting, which\nis a key indicator of its potential energy efficiency.\n6. While not always mandatory, a preliminary Techno-Economic Analysis (TEA) would significantly strengthen the impact of\nthe work.\n7. The DFT calculations present a single hydrogenation pathway for quinoxaline without justifying its likelihood over other\nplausible sequences.\n8. Please enhance details on electrode fabrication ATR-SEIRAS and provide the details for in-situ Raman.", "reviewer_4": "In this manuscript, the authors report the RusA/ns-Co₃O₄ catalyst to enhance the electrochemical hydrogenation (ECH) of\nquinoxaline to 1,2,3,4-tetrahydroquinoxaline via the interfacial water regulation. The decoration of Ru creates an asymmetric\nlocal electric field that modulates the interfacial water structure, thus facilitating the dissociation of H2O to generate H*,\nfurther leading to a higher Faradaic efficiency at high current densities. Multiple in situ and ex situ characterizations and\ntheoretical calculations are performed to unveil the promotion mechanism. While the enhanced performance of the RusA/ns-\nCo₃O₄ composite is promising, I find that many of the mechanistic claims lack convincing experimental support. Therefore,\nthis manuscript should be carefully revised and reorganized before submitting to Nature Communications.\n1. The author states that proton transfer is a critical factor influencing hydrogenation reactions. As the authors noted, \"For\nthese catalysts, 4HB-H2O remains the predominant species across all detected potentials\", what is the specific role of\nRusA/ns-Co₃O₄ in facilitating proton transfer?\n2. What exactly does the promotional effect of Co?\n3. The article exclusively focuses on the generation of H*, without considering the catalyst's influence on quinoxaline\nactivation.\n4. Further analysis of Figure 6d is required.\n5. The Ru-O-Co pathway shown in Figure 3d lacks a reference sample or supporting literature.\n6. There are several spelling errors in the article, for example: \"Faraday efficiency\" in line 15 should be \"Faradaic efficiency\"\nand \"implications\" in line 31 should be \"applications\"."}, "author_response": "Responses to the Referees' Comments\nWe thank the referees for their valuable comments and positive endorsement to our\nmanuscript. We have carefully considered the referees’ comments and revised the\nmanuscript accordingly. Our responses and corresponding revisions are as follows:\nReviewer #1\nThis manuscript by Y. Tan and colleagues describes the Ru /ns-Co O can efficient\nSA 3 4\nelectrochemical hydrogenation of quinoxaline. The asymmetric local electric field\ninduced by Ru single atoms facilitates the formation of an H-down configuration in\ninterfacial water and promotes the H O dissociation rate; therefore, high reaction\n2\nperformance was achieved at industrial-grade current densities. The work presented in\nthe manuscript is intriguing as the authors elucidated the mechanism through in-situ\nexperiments combined with DFT calculate to support the catalytic performance. This\nwork appears suitable for publication in Nature Communications. Nevertheless, I would\nlike to offer some comments and several questions to enhance the manuscript's quality.\nResponse: Thank you for your positive comments on our manuscript. We have revised\nour manuscript accordingly.\nComment 1. In principle, Ru single atom doping causes lattice expansion of Co O , and\n3 4\nthe diffraction peak should shift towards lower values. Why does the diffraction peak\nshift towards higher values in this work?\nResponse: Thank you for your comments.\nWhen Ru atoms are doped into ns-Co O , their atomic radius is larger than that of Co\n3 4\nions in the lattice. This not only induces lattice expansion but also generates\ncompressive stress. If the compressive stress outweighs the lattice expansion effect,\nlattice contraction will occur. Although the atomic radius of Ru is larger, compressive\nstress dominates in Ru /ns-Co O system. Therefore, compared to ns-Co O , the (311)\nSA 3 4 3 4\nmain diffraction peak of Ru /ns-Co O shifts toward higher angles. This anomalous\nSA 3 4\nphenomenon has been observed in similar literature,1-3 suggesting the local structure\ncontraction.\nRef. [1] Lu, Y. et al. Tuning the Selective Adsorption Site of Biomass on Co O by Ir\n3 4\nSingle Atoms for Electrosynthesis. Adv. Mater. 33, 2007056 (2021).\nRef. [2] Cao, J. et al. Tungsten single atoms incorporated in cobalt spinel oxide for\nhighly efficient electrocatalytic oxygen evolution in acid. Energy Environ. Sci., 17,\n5911-5921 (2024).\nRef. [3] Cai, L. et al. Atomically asymmetrical Ir-O-Co sites enable efficient chloride-\nmediated ethylene electrooxidation in neutral seawater. Angew. Chem. Int. Ed. 64,\ne202417092 (2025).\nComment 2. To further validate the accuracy of the in-situ Raman spectroscopy data on\ninterface water, the authors should complement the study with in-situ infrared\nspectroscopy measurements. Nat Commun 16, 6459 (2025), Angew. Chem. Int. Ed.\n2025, e202514557\nResponse: We appreciate the questions from this reviewer. We thank that the reviewer\nprovides the latest literatures (Ref. 20, 40. in the revised manuscript). Following these\ncomments, we further confirmed the interfacial water behavior by in-situ infrared\nspectroscopy. The results showed that as the doping content of Ru single atoms\nincreased, the content of K•H O also increased, which is consistent with the in-situ\nsupporting information)\nFig. R1. (a) In-situ ATR-SEIRAS spectra of interfacial water on the various catalysts\nunder the -0.1 V. Gaussian fitting of three O-H stretching modes of 4-HB•H O, 2-\n2 2\nproportions of K•H O from the peak areas on ns-Co O with different Ru SA loading\n2 3 4\nat -0.1 V vs. RHE.\nComment 3. Similarly, the authors presented the ratio of K•H O and 4HB-H O in the\n2 2\nin-situ Raman data, yet omitted the corresponding ratio data for 2HB-H O.\nmanuscript accordingly. As the increase of Ru single atoms concentration, the content\nof 2HB-H O gradually decreases, indicating that 2HB-H O is gradually transformed\n2 2\ninto K•H O (Fig. R2, see supplementary Fig. 35 in revised supporting information).1\n2 3 4\nRu /ns-Co O -0.5%, (c) Ru /ns-Co O -0.7%, (d) Ru /ns-Co O -1.0%.\nSA 3 4 SA 3 4 SA 3 4\nRef. [1] Li, M. et al. Manipulating interfacial water configuration via constructing\nasymmetric structure unit for hydrogen production in alkaline seawater. Adv. Funct.\nMater. e14517 (2025). DOI: 10.1002/adfm.202514517.\nComment 4. The authors demonstrated that the K•H O content is directly proportional\n2\nto the Ru single-atom concentration. So, if the Ru single-atom concentration continues\nto increase, will the K•H O content also rise accordingly?\nFollowing your comments, we further increased the Ru proportion to 1.4% (Table R1),\nand XPS characterization results indicated that Ru remained in a single-atom state (Fig.\nR3). When the amount of Ru increased to 1.4% (Ru /ns-Co O -1.4%), the content of\nSA 3 4\nK•H O began to decrease, mainly due to the shorter of single atom distance and mutual\n2\ninfluence between local electric fields (Fig. R4). Since this ratio falls outside the scope\nof our discussion, we will not conduct further research on it.\nTable R1. Element content by ICP-OES.\nAtomic Percentage (%)\nCatalyst\nCo Ru\nRu /ns-Co O -1.4% 98.64 1.36\nSA 3 4\nFig. R3. High resolution XPS spectra of Ru /ns-Co O -1.4% of Ru 3p.\nSA 3 4\nFig. R4. (a) Potential-dependent in situ Raman spectra of the O-H stretching mode of\ninterfacial water on Ru /ns-Co O -1.4%. (b) Estimated proportions of K•H O from\nSA 3 4 2\nthe peak areas on ns-Co O with different Ru SA laoding.\n3 4\nComment 5. The signal intensity of Ru 3p in Supplementary Fig. 29 is too weak. To\nensure data accuracy, it is recommended to retest.\nResponse: We appreciate the reviewer for this nice suggestion. Following your\nsuggestion, we conducted XPS testing for Ru /ns-Co O -0.7% after the reaction. The\nSA 3 4\nresults are as follows (Fig. R5, see supplementary Fig. 27 in revised supporting\ninformation).\nFig R5. High resolution XPS spectra of Ru /ns-Co O -0.7% after reaction.\nSA 3 4\nComment 6. For the stability assessment, the author demonstrated the material’s\noutstanding stability via XPS, XRD, and SEM characterizations. Notably, EDS data\nafter the reaction remains required.\nResponse: Thank you for the kind suggestion. To more accurately verify the changes\nin element content before and after the reaction, we conducted ICP-OES testing (Fig.\nR6, see supplementary Fig. 29 in revised supporting information). The results showed\nthat the element content was essentially unchanged, proving its excellent stability.\nFig. R6. Element content of Ru /ns-Co O -0.7% after reaction by ICP-OES.\nSA 3 4\nComment 7. In the theoretical calculation modeling, the author did not specify the Ru\ndoping sites within the calculation section, whether tetrahedral or octahedral.\nResponse: We thank the reviewer for the insightful questions. For the modeling, the\nRu doping was carried out by replacing the octahedral Co atom with Ru atom. We have\nspecified this point in the Computational Methodology section.\nComment 8. The author should compare the kinetics of quinoxaline hydrogenation and\nthe HER through LSV, which is critical for the high Faradaic efficiency.\nResponse: We appreciate the reviewer's professional comments.\nIn MEA, foam iridium as an anode for the oxygen evolution reaction (OER) and the\nRu /ns-Co O -0.7% catalyst act as the cathode for quinoxaline hydrogenation reaction.\nSA 3 4\nThe polarization curves of Ru /ns-Co O -0.7% in 1 M KOH electrolyte with and\nSA 3 4\nwithout quinoxaline was illustrated in Fig. R7 (see supplementary Fig. 25 in revised\nsupporting information). In comparison with the high cell voltage of water splitting\nwithout the addition of quinoxaline, the cell voltages are significantly decreased after\nadding quinoxaline, indicating that the ECH of quinoxaline process is kinetically more\nfavorable than HER.\nFig. R7. LSV curves of the HER||OER and Quinoxaline-ECH||OER systems at a scan\nrate of 50 mV s−1.\nComment 9. The EDS data from Supplementary Fig. 9 and Supplementary Fig. 14\nfeature an unlabeled peak at ~1.5 keV. What element does this peak represent?\nResponse: We sincerely appreciate your valuable comment. A series of electrocatalysts\nwith varying Ru single-atom ratios were prepared via chemical dealloying followed by\nannealing. During the chemical dealloying process, residual Al is inevitably retained,\nand a corresponding characteristic peak of Al is observed at approximately 1.5 keV.\nEDS is limited by its low accuracy and fails to provide the precise content of Ru single\natoms. Therefore, we conducted ICP-OES analysis to determine this precise content,\nwhich is shown in supplementary Table 1 in revised supporting information.\nComment 10. This question is only discussed with the authors, during the\nelectrochemical hydrogenation of quinoxaline, why did\n1,2,3,4-tetrahydroquinoxaline\nemerge as the sole product, while the fully hydrogenated product did not appear?\nResponse: We thank the reviewer for presenting this nice question. PEZ et al. predicted\nthe standard hydrogenation enthalpies of π-conjugated compounds with varying\nnumbers of rings via computational methods. Their calculations revealed that\nincorporating heteroatoms, particularly nitrogen atoms, into the carbon rings\nsignificantly lowers this thermodynamic quantity.1 The reversible hydrogenation of\nquinoxaline should be thermodynamically more advantageous due to the presence of\ntwo nitrogen atoms in an aromatic ring.2, 3 Therefore, the main product of quinoxaline\nhydrogenation is 1,2,3,4-tetrahydroquinoxaline. However, due to the highly stable\naromatic system formed by the delocalized large π-bond of the benzene ring in\nquinoxaline, breaking the conjugated structure requires extremely high energy, making\nit difficult to generate decahydroquinoxaline. Therefore, in this study, 1,2,3,4-\ntetrahydroquinoxaline was identified as the sole final product of the electrocatalytic\nhydrogenation of quinoxaline.\nRef. [1] PEZ, G. P. et al. Hydrogen storage by reversible hydrogenation of piconjugated\nsubstrates. US7351395. US patent, 2008-04-01.\nRef. [2] Oka, K. et al. Reversible hydrogen fixation and release under mild conditions\nby poly(vinylquinoxaline). ACS Appl. Polym. Mater. 2, 2756−2760 (2020).\nRef. [3] Aakko-Saksa, P. T. et al. Liquid organic hydrogen carriers for transportation\nand storing of renewable energy – Review and discussion. J. Power Sources 396,\n803–823 (2018).\nReviewer #2\nThe authors investigated the Ru-doped Co O nanosheet for the electrochemical\n3 4\nhydrogenation of quinoxaline. It was proposed that the reconfiguration of the interfacial\nhydrogen-bonded water network enabled efficient hydrogenation of quinoxaline.\nNevertheless, the selective hydrogenation of quinoxaline to 1,2,3,4-\ntetrahydroquinoxaline remains ambiguous. Consequently, the manuscript cannot be\npublished at this time. The following comments warrant consideration:\nResponse: We sincerely thank the reviewer for those valuable comments and\nsuggestions, which have greatly helped improve the quality of this manuscript. Based\non the reviewer’ comments, we have provided a thorough response and made detailed\nrevisions to the manuscript. We hope that the revised manuscript meets the publication\nrequirements.\nComment 1. The authors posited that the induced interfacial H O molecules facilitated\n2\nefficient H* generation. Why does the reactive H* not accelerate the formation of H ,\n2\nbut instead promote the hydrogenation of quinoxaline?\nResponse: We sincerely appreciate your valuable comment.\nTo confirm this point, we have carried out additional DFT calculations and\nexperimental measurements. We first calculated the adsorption energy of H atoms on\ndifferent sites of Ru /Co O (Fig. R1, see supplementary Fig. 45 in revised supporting\nSA 3 4\ninformation). The calculation results indicate that H atoms have the most stable\nadsorption on the Ru-Co bridge and Ru-Co-Co fcc sites (Fig R2, see supplementary\nFig. 46 in revised supporting information). Therefore, we further calculated the energy\nbarriers of the HER at these two sites (Fig. R3, see supplementary Fig. 47 in revised\nsupporting information). Our DFT calculations on the HER pathway have shown that\nthe reactive free energy for the rate determining step of HER is higher than that for the\nhydrogenation of quinoxaline (Fig. R4, see supplementary Fig. 48 in revised supporting\ninformation). Therefore, H* is preferentially hydrogenated for quinoxaline rather than\nundergoing self-coupling to release H , which accounts for the high FE of the ECH\n2\nprocess.\nIn addition, we have also proven this through our experiments. In MEA, the\npolarization curves of Ru /ns-Co O -0.7% in 1 M KOH electrolyte with and without\nSA 3 4\nquinoxaline was illustrated in Fig. R5 (see supplementary Fig. 25 in revised supporting\ninformation). In comparison with the high cell voltage of water splitting without the\naddition of quinoxaline, the cell voltages are significantly decreased after adding\nquinoxaline, indicating that the ECH of quinoxaline process is kinetically more\nfavorable than HER. In addition, the experimental results also indicate that the Faraday\nefficiency of H is relatively low, suggesting that the HER is suppressed (Fig. R6, see\nFig. R1. H atom adsorption on different sites of Ru /Co O . (a) Ru-Co bridge, (b) Ru-\nSA 3 4\nCo-Co fcc, (c) Ru-Co-Co hcp, (d) Ru top.\nFig. R2. The adsorption energy of H atoms on different sites of Ru /Co O .\nSA 3 4\nFig. R3. The HER pathway on different sites of Ru /Co O . (a) Ru-Co bridge, (b) Co-\nSA 3 4\nCo bridge, (c) Ru-Co-Co fcc, (d) Co-Co-Co fcc.\nFig. R4. LSV curves of the HER||OER and Quinoxaline-ER||OER systems at a scan\nrate of 50 mV s−1.\nFig. R5. LSV curves of the HER||OER and Quinoxaline-ECH||OER systems at a scan\nrate of 50 mV s−1.\nFig. R6. The FE of 1,2,3,4-tetrahydroquinoxaline and H over Ru /ns-Co O -0.7% in\nComment 2. How can the selective hydrogenation of quinoxaline to 1,2,3,4-\ntetrahydroquinoxaline be reinforced without generating unwanted byproducts?\nResponse: We are grateful to the reviewer for this insightful question. There are three\npossible products from the electrocatalytic hydrogenation of quinoxaline: 1,2-\ndihydroquinoxaline, 1,2,3,4-tetrahydroquinoxaline and decahydroquinoxaline (Fig.\nR7). PEZ et al. predicted the standard hydrogenation enthalpies of π-conjugated\ncompounds with varying numbers of rings via computational methods. Their\ncalculations revealed that incorporating heteroatoms, particularly nitrogen atoms, into\nthe carbon rings significantly lowers this thermodynamic quantity.1 The reversible\nhydrogenation of quinoxaline should be thermodynamically more advantageous due to\nthe presence of two nitrogen atoms in an aromatic ring.2, 3 Therefore, the main product\nof quinoxaline hydrogenation is 1,2,3,4-tetrahydroquinoxaline. Owing to the sufficient\nsupply of H* in the reaction system, 1,2-dihydroquinoxaline is rapidly hydrogenated to\nform 1,2,3,4-tetrahydroquinoxaline. However, due to the highly stable aromatic system\nformed by the delocalized large π-bond of the benzene ring in quinoxaline, breaking\nthe conjugated structure requires extremely high energy, making it difficult to generate\ndecahydroquinoxaline. Therefore, in this study, 1,2,3,4-tetrahydroquinoxaline was\nidentified as the sole final product of the electrocatalytic hydrogenation of quinoxaline.\nFig. R7. Electrocatalytic hydrogenation pathway of quinoxaline.\nRef. [1] PEZ, G. P. et al. Hydrogen storage by reversible hydrogenation of piconjugated\nsubstrates. US7351395. US patent, 2008-04-01.\nRef. [2] Oka, K. et al. Reversible hydrogen fixation and release under mild conditions\nby poly(vinylquinoxaline). ACS Appl. Polym. Mater. 2, 2756−2760 (2020).\nRef. [3] Aakko-Saksa, P. T. et al. Liquid organic hydrogen carriers for transportation\nand storing of renewable energy – Review and discussion. J. Power Sources 396,\n803–823 (2018).\nComment 3. The notion of obtaining Ru single atoms through a facile chemical\ndealloying and annealing process is indeed questionable.\nResponse: Thank you very much for this comment. We believe there are four main\nreasons why Ru single atoms can be obtained through simple chemical dealloying and\nannealing processes.\n1. Precursor alloy design: In the Al-Co-Ru ternary alloy (such as Al Co Ru ), Al\n80 19.8 0.2\nserves as the sacrificial phase, Co as the structural phase, and Ru as the inert doped\nphase. The alkaline solution preferentially dissolves Al, while Co is oxidized to CoO ;\nx\nRu, due to its chemical inertness, retains its atomic state (Fig. R8, see supplementary\nFig. 4 in revised supporting information).1 The subsequent annealing further promoted\nthe diffusion of Ru, enabling Ru atoms to be embedded in the Co O lattice in a more\n3 4\nhighly dispersed form, which was also been reported in other literature.2, 3\nFig. R8. Schematic illustration of the preparation of Ru /ns-Co O .\nSA 3 4\n2. Local concentration limit: The extremely low Ru content (0.5–1.0 at%) not only\nprevents Ru atoms from aggregating during the dealloying process but also forces them\nto disperse as single atoms in the oxide matrix (Table R1, see supplementary Table 1\nin revised supporting information).4\nTable R1. Element content by ICP-OES.\nAtomic Percentage (%)\nCatalyst\nCo Ru\nRu /ns-Co O -0.5% 99.54 0.46\nSA 3 4\nRu /ns-Co O -0.7% 99.26 0.74\nSA 3 4\nRu /ns-Co O -1.0% 99.02 0.98\nSA 3 4\n3. Nanosheet Constraint: The thickness of the nanosheets generated by dealloying is\nabout 10 nm, and their confined interlayer space restricts the migration of Ru atoms and\nphysically blocks aggregation (Fig. R9, see Fig. 3a, supplementary Fig. 5 in revised\nmanuscript and supporting information).\nFig. R9. SEM image of ns-Co O and Ru /ns-Co O .\n3 4 SA 3 4\n4. Strong electronic interaction: The charge transfer between Ru atoms and Co O\n3 4\nforms a strong electronic interaction, which thus firmly anchors Ru atoms to the Co O\n3 4\nlattice, thereby stabilizing the single-atom structure (Fig. R10, see supplementary Fig.\n11 in revised manuscript and supporting information).5\nFig. R10. Co 2p XPS spectra Ru /ns-Co O and ns-Co O .\nSA 3 4 3 4\nRef. [1] Gao, J.J. et al. Platinum single atoms/clusters stabilized in transition metal\noxides for enhanced electrocatalysis. Electrochim. Acta 297, 155e162 (2019).\nRef. [2] Li, D. et al. Isolated octahedral Pt-induced electron transfer to ultralow-content\nruthenium-doped spinel Co O for enhanced acidic overall water splitting. J. Am. Chem.\n3 4\nSoc. 146, 28728−28738 (2024).\nRef. [3] Yang, Z. et al. Phase-engineered Bi-RuO single-atom alloy oxide boosting\n2\noxygen evolution electrocatalysis in proton exchange membrane water electrolyzer.\nAdv. Mater. 37, 2417777 (2025).\nRef. [4] Tang, X. et al. Free-standing nanoporous high-entropy phosphides as high-\nperformance water splitting catalysts. Int. J. Hydrogen Energy 117, 292–299 (2025).\nRef. [5] Lin, X. et al. Electronic tuning of RuO polarizes metal–oxygen redox for\n2\nproton exchange membrane water electrolysis. Nat. Commun. 16, 8709 (2025).\nComment 4. How can it be confirmed that Ru effectively substitutes the Co sites?\nResponse: We appreciate the reviewer for this nice suggestion.\nThree possible configurations of Ru single atoms in Co O are considered:\n3 4\nsubstitution of Co sites, substitution of O sites, and interstitial doping.\nFirstly, from the perspective of charge balance: in Co O , Co exists as both Co2+\n3 4\n(tetrahedral sites) and Co3+ (octahedral sites), while Ru in our system shows a valence\nstate of Ruδ+ (0 < δ < 4) (Fig. R11a, see Fig. 3c in revised manuscript).1 This charge\nrange (close to Co2+/Co3+) enables Ru to stably substitute Co sites, whereas substituting\nO2‒ sites would cause severe charge imbalance, which is thermodynamically\nunfavorable.\nSecondly, Co O has a spinel structure, which is a close-packed structure; therefore,\n3 4\ndoping at interstitial sites is energetically disadvantageous. Additionally, the Ru-O bond\nlength (1.94 Å) is close to the Co-O bond length (1.90 Å) as measured by FT-EXAFS\n(Fig. R11b, see Fig. 3d in revised manuscript), which further confirms that interstitial\ndoping is not possible. Therefore, Ru can only substitute for Co sites, which is further\nvalidated by our experimental results as follows:\nThe aberration-corrected high-angle annular dark field scanning transmission\nmicroscope (HAADF-STEM) image of Ru /ns-Co O clearly isolated bright spots\nSA 3 4\nwith particularly high intensity (Fig. R12a, see Fig. 3b in revised manuscript),\nindicating that Ru atoms occupy specific lattice positions. The line scan profile (Fig.\nR12b, see supplementary Fig. 7 in revised supporting information) further confirms\nthese bright spots are located at the original Co lattice sites of ns-Co O , directly\n3 4\nsupporting Ru substitution for Co.\nThe Fourier transform (FT) of the Ru K-edge extended X-ray absorption fine\nstructure (EXAFS) spectra of Ru /ns-Co O was further analyzed, clear Ru-Co\nSA 3 4\ncoordination bonds were observed, but no Ru-Ru bonds were detected. This provides\ndirect evidence that Ru atoms are surrounded by Co atoms; that is, Ru atoms replace\nCo sites and form Ru-O-Co linkages through oxygen bridges (Fig. R11b, see Fig. 3d\nin revised manuscript).2 Further analysis via EXAFS wavelet transform (WT) discerns\nradial distance resolution and differentiates backscattering atoms in k-space (Fig. R11c,\nsee Fig. 3e in revised manuscript). Notably, two main maxima at 5.0 Å−1 and 7.2 Å−1\nhave been shown on Ru /ns-Co O , which represent Ru-O and Ru-O-Co units,\nSA 3 4\nrespectively, further indicating the atomic dispersion of Ru atoms in Ru /ns-Co O\nSA 3 4\nwith the absence of locally aggregated RuO cluster structure. Additionally, a model-\n2\nbased EXAFS fitting of Ru /ns-Co O indicates that no Ru-Ru scattering can be\nSA 3 4\ndiscovered and the second scattering can be attributed to Ru-O-Co with a distinct bond\ndistance, which further proves that Co sites are substituted by Ru (Fig. R13, see Fig 3f.\nin revised manuscript).\nTherefore, based on advanced electron microscopy and spectroscopic techniques,\nwe conclude that Ru species are confirmed to exist as single atoms substituting for Co\nin the Co O matrix.\n3 4\nFig. R11. (a) Ru K-edge XANES of Ru foil, RuO , and Ru /ns-Co O . (b) Fourier\ntransformed magnitudes of Ru K-edge EXAFS spectra for Ru foil, RuO , and Ru /ns-\nCo O . (c) WT-EXAFS Ru K-edge spectra of Ru foil, Ru /ns-Co O and RuO .\n3 4 SA 3 4 2\nFig. R12. (a) HAADF-STEM images of Ru /ns-Co O . (b) The line intensity profile\nSA 3 4\nalong A and B direction in (a).\nFig. R13. k2-weighted Fourier transformed EXAFS at the Ru K-edge of the Ru /ns-\nSA\nCo O with EXAFS fitting.\n3 4\nRef. [1] Luo, W. et al. Controlled electrocatalytic glycerol upgrading to glyceraldehyde\nin near neutral media by cobalt oxide lattice activation. Angew. Chem. Int. Ed. 64,\ne202505059 (2025).\nRef. [2] Zuo, S. et al. Local compressive strain-induced anticorrosion over isolated Ru-\ndecorated Co O for efficient acidic oxygen evolution. Nat. Commun. 15, 9514 (2024).\n3 4\nReviewer #3\nThis work presents the concept of regulating the interfacial water network via Ru\nsingle-atom concentration to enhance electrochemical hydrogenation of quinoxaline.\nHowever, to fully validate its claims and ensure its impact, substantial revisions are\nrequired to address critical gaps in the experimental evidence, data interpretation, and\nmaterial characterization.\nWe sincerely thank the reviewer for this incisive comment, which highlights critical\naspects that need strengthening to enhance the validity and impact of our work. We\nfully acknowledge the gaps in experimental evidence, data interpretation, and material\ncharacterization, and we deeply appreciate the guidance on improving these aspects.\nWe have carefully responded to each of your comments and reorganized the manuscript\naccordingly. We believe these revisions have significantly addressed the critical gaps\nand enhanced the rigor of our work. Should the reviewer deem it necessary, we are\nwilling to perform further experiments or analyses, and we look forward to your\nvaluable feedback.\nComment 1. The authors convincingly demonstrate that Ru SAs promote water\ndissociation to generate H*. However, a critical paradox remains entirely unaddressed:\nwhy do these H* species preferentially participate in the kinetically more complex\nmulti-step hydrogenation of quinoxaline rather than simply combining to form H ,\nthe 2\nthermodynamically favored and kinetically facile HER pathway? The manuscript\nprovides no insight into the factors that might suppress HER.\nResponse: We appreciate you for this constructive recommendation. We sincerely\nappreciate your valuable comment. To confirm this point, we have carried out additional\nDFT calculations and experimental measurements. We first calculated the adsorption\nenergy of H atoms on different sites of Ru /Co O (Fig. R1, see supplementary Fig.\nSA 3 4\n45 in revised supporting information). The calculation results indicate that H atoms\nhave the most stable adsorption on the Ru-Co bridge and Ru-Co-Co fcc sites (Fig. R2,\nsee supplementary Fig. 46 in revised supporting information). Therefore, we further\ncalculated the energy barriers of the HER at these two sites (Fig. R3, see supplementary\nFig. 47 in revised supporting information). Our DFT calculations on the HER pathway\nhave shown that the reactive free energy for the rate determining step for HER is higher\nthan that for the hydrogenation of quinoxaline (Fig. R4, see supplementary Fig. 48 in\nrevised supporting information). Therefore, H* is preferentially hydrogenated for\nquinoxaline rather than undergoing self-coupling to release H , which accounts for the\n2\nhigh FE of the ECH process.\nIn addition, we have also proven this through our experiments. In MEA, the\npolarization curves of Ru /ns-Co O -0.7% in 1 M KOH electrolyte with and without\nSA 3 4\nquinoxaline was illustrated in Fig. R5 (see supplementary Fig. 25 in revised supporting\ninformation). In comparison with the high cell voltage of water splitting without the\naddition of quinoxaline, the cell voltages are significantly decreased after adding\nquinoxaline, indicating that the ECH of quinoxaline process is kinetically more\nfavorable than HER. In addition, the experimental results also indicate that the Faraday\nefficiency of H is relatively low, suggesting that the HER is suppressed (Fig. R6, see\nFig. R1. H atom adsorption on different sites of Ru /Co O . (a) Ru-Co bridge, (b) Ru-\nSA 3 4\nCo-Co fcc, (c) Ru-Co-Co hcp, (d) Ru top.\nFig. R2. The adsorption energy of H atoms on different sites of Ru /Co O .\nSA 3 4\nFig. R3. The HER pathway on different sites of Ru /Co O . (a) Ru-Co bridge, (b) Co-\nSA 3 4\nCo bridge, (c) Ru-Co-Co fcc, (d) Co-Co-Co fcc.\nFig. R4. (a) Gibbs free energy diagram for HER on the surface of Ru /Co O at\nSA 3 4\ndifferent sites. (b) The free-energy diagram for electrochemical hydrogenation of\nquinoxaline to 1,2,3,4-tetrahydroquinoxaline on Ru /Co O and Co O .\nSA 3 4 3 4\nFig. R5. LSV curves of the HER||OER and Quinoxaline-ECH||OER systems at a scan\nrate of 50 mV s−1.\nFig. R6. The FE of 1,2,3,4-tetrahydroquinoxaline and H over Ru /ns-Co O -0.7% in\nComment 2. The manuscript highlights the high FE for the target product but fails to\nquantitatively address the competing HER. Direct quantification of H production (H\n2 2\nFE) under the exact ECH reaction conditions are needed. This is essential to understand\nthe catalyst’s selectivity and to identify the destination of electrons when the FE for\n1,2,3,4-tetrahydroquinoxaline was below 100%.\nResponse: We are truly grateful for this constructive suggestion. Following your\nsuggestion, we have supplemented the corresponding experimental data. The results are\nshown in Fig. 4 in revised manuscript and supplementary Fig. 16, 17 in revised\nsupporting information.\nComment 3. The consistent omission of quinoxaline conversion rate prevents a full\nassessment of the catalytic efficiency and practical utility. The conversion (%) and yield\nmust be provided for all key performance tests alongside FE data.\nResponse: We sincerely thank you for this nice suggestion. We are sorry for the loose\nof this description. In this study, we evaluated the electrocatalytic hydrogenation\nperformance of quinoxaline through the consumption of theoretical charges under a\nconstant current density. Given the 100% selectivity of 1,2,3,4-tetrahydroquinoxaline\nin this reaction, the Faraday efficiency of 1,2,3,4-tetrahydroquinoxaline aligns with the\nconversion and yield of quinoxaline.\nYield (%) = Conversion (%) × Selectivity (%)\nBased on your previous question (Comment 2), we believe that using Faraday\nefficiency can better reflect the data comparison between reaction products and H .\ndependence on Ru concentration (Fig. 5e). However, the quantification method is\ninadequate. EDS is a semi-quantitative technique with a typical error of >5-10%, which\nis unacceptably high for accurately determining the low loadings of single atoms (0.6\nat%, 1.0 at%, 1.4 at%) that are central to the argument.\nResponse: We are grateful for the valuable suggestions provided by the reviewer. As\nEDS is unreliable for accurate Ru quantification, following your suggestion, we instead\ndetermined the Ru content of the catalyst precisely using ICP-OES. The results are as\nfollows (see supplementary Table 1 in revised supporting information).\nTable R1. Element content by ICP-OES.\nAtomic Percentage (%)\nCatalyst\nCo Ru\nRu /ns-Co O -0.5% 99.54 0.46\nSA 3 4\nRu /ns-Co O -0.7% 99.26 0.74\nSA 3 4\nRu /ns-Co O -1.0% 99.02 0.98\nSA 3 4\nComment 5. The demonstration of stability in an MEA electrolyzer is a strength.\nHowever, the energy efficiency and practical advantage of the process remain unclear.\nThe LSV curves of the ECH || OER system (Supplementary Fig. 27) should be directly\ncompared with the LSV of conventional water electrolysis (HER || OER) using the same\nMEA setup and electrodes. This comparison is critical to demonstrate whether the ECH\nprocess occurs at a lower cell voltage than water splitting, which is a key indicator of\nits potential energy efficiency.\nResponse: We sincerely appreciate the reviewer’s constructive suggestions. In MEA,\nfoam iridium as an anode for the oxygen evolution reaction (OER) and the Ru /ns-\nSA\nCo O -0.7% catalyst act as the cathode for quinoxaline hydrogenation reaction.\nThe 3 4\npolarization curves of Ru /ns-Co O -0.7% in 1 M KOH electrolyte with and without\nSA 3 4\nquinoxaline was illustrated in Fig. R6 (see supplementary Fig. 25 in revised supporting\ninformation). In comparison with the high cell voltage of water splitting without the\naddition of quinoxaline, the cell voltages are significantly decreased after adding\nquinoxaline, indicating that the ECH of quinoxaline process is much more energy\nefficiency.\nFig. R6. LSV curves of the HER||OER and Quinoxaline-ECH||OER systems at a scan\nrate of 50 mV s−1.\nComment 6. While not always mandatory, a preliminary Techno-Economic Analysis\n(TEA) would significantly strengthen the impact of the work.\nResponse: We thank the reviewer for presenting this nice question. The preliminary\ntechnical and economic analysis results are as follows.\nNote 1\nTo evaluate the economic potential of the coupling system in practice, a Techo-\neconomic Analysis (TEA) of the coupling reaction process based on the Sargent Group\nmodel.1-4 The Surgent group techno-economic evolution model provides industrial-\ngrade production cost per ton of products ($/ton 1,2,3,4-tetrahydroquinoxaline). Of note,\nthe electricity price selected in the present manuscript was based on the recently\npublished literature.5 The electrolyzer cost per area is a function of current density and\nthe operating cell voltage.\nBelow is the list of assumptions made for the calculations.\n1. The capital costs of electrolyzer is sensitive to the operating current density, we\nassume a cost of $10000 per m2 of electrolyzer. The total catalyst and membrane\ncost are 5 % of the electrolyzer cost.2\n2. The electricity price was considered to be 0.05 $ KWh−1.\n3. The life of the factory is assumed to be 30 years. The capacity factor is expected to\nbe operational on any given day, is assumed to be 0.8, which means the plant will\nbe operational 19.2 hours a day.2\n4. Input chemicals include quinoxaline, potassium hydroxide and water. The output\nproducts include 1,2,3,4-tetrahydroquinoxaline and H .\n2\n5. The cost of electrolyte was set as 30% of the cost of the solid raw material.\n6. Both operation and maintenance cost are assumed to be 10% of the capital costs.\n7. Separation cost is assumed to be 30% of the electricity cost.\nThe calculation process\nTaking the 1.00 ton daily capacity of 1,2,3,4-tetrahydroquinoxaline (TQ) as an example,\nthe need 1.347 ton quinoxaline. Obtain 330 kg of O at the anode with 200 A cm−2,\nthe 2\nvoltage was 2.45 V (all based on FE = 72%).\n1. Capital costs:\n1) The electrolyser cost.\nElectrolyzer cost per ton TQ = Electrolyzer area m2× 10000 $\nRequired current (I)\nArea of electrolyser =\nCurrent density (i)\nn(TQ)× N × F 7452 mol × 4e- × 96485 C/mol\nQ = = = 3.99 × 109\nFE 72%\nQ\nI =\ndaily operation time of power plant × capacity factor\n3.99×109\n= = 5.78×104 A\n19.2 h × 3600 s\n5.78×104 1\nElectrolyzer cost per ton TQ = × × 10000 $ = 2.89×105 $\n0.2 A cm-2 10000 m2\n2) Catalyst and membrane costs:\nCatalyst and membrane = Electrolyzer cost × 5% = 2.89 × 105$ × 5% = 1.44 × 104 $\nElectrolyzer cost + Catalyst and membrane\nCapital cost = = \nLifetime of plant\n2.89×105 $ + 1.44 × 104 $\n= 27.7 $\n30 years × 365 day\n2. Maintenance cost:\nMaintenance cost = Capital cost × 10% = 27.7 $ × 10% = 2.77 $\n3. Balance of plant:\nBalance of plant = Capital cost × Balance of plant factor = 27.7 $ × 0.35 = 9.698 $\n4. Installation cost:\nInstallation cost = Capital cost × Lang factor = 27.7 $ × 0.2 = 5.54 $\n5. Electricity cost:\nI × U 5.78 × 104 A × 2.45 V\nPower = = = 141.61 KW\nEnergy use per day (kWh) = Power × Time in a day (h) × capacity\n= 141.61 KW × 24 h × 0.8 = 2718.912 kWh\nElectricity cost of per day = Energy use per day × Cost per kWh = 2718.912 × 0.05\n$ = 135.9456 $\n6. Separation cost:\nSeparation cost = Electricity cost × 30% = 135.9456 $ × 30% = 40.78 $\n7. Operating cost:\nOperating cost = Capital cost × 10% = 27.7 $ × 10% = 2.77 $\n8. Input chemicals cost:\nInput chemicals cost = 1.347 ton quinoxaline (1.347 × 100) + Alkaline electrolyte\n(134.7 × 30%) = 175.11 $\n9. Total cost:\nTotal cost = Capital cost + Maintenance cost + Balance of plant + Installation cost +\nElectricity cost + Separation cost + Operating cost + Input chemicals cost\n= 27.7 $ + 2.77 $ + 9.698 $ + 5.54 $ + 135.9456 $ + 40.78 + 2.77 $ + 175.11 $ =\n400.3136 $\n10. Product revenue:\nProduct revenue = 1 ton TQ (1 × 1000) + 330 kg O × (100 $ × 0.33 ton) = 1033 $\nmodel adapted in the literature, which includes the costs of capital, maintenance,\ninstallation, input chemicals, and operation, to investigate the feasibility of this\ncoupling system (Note 1, Table R2 and Fig. R7, see supplementary Fig. 30,\nsupplementary Table 3 and Note 1 in revised supporting information). The cost and\nrevenue of the ECH||OER system were calculated to be ∼$400.3136/ton quinoxaline\nand ∼$1033/ton 1,2,3,4-tetrahydroquinoxaline, respectively, thus the profit was\ncalculated to be $632.6864/ton 1,2,3,4-tetrahydroquinoxaline (Fig. R8, see\nsupplementary Fig. 31 in revised supporting information).\nFig. R7. Model used for calculating the plant-gate levelized cost of the process.\nTable R2. Price of feedstocks and products.\nProduct Price ($/ton) Ref.\nFeedstocks\nQuinoxaline 100 a\nProducts\n1,2,3,4-Tetrahydroquinoxaline 1000 b\nO\n2\n100 2\na: Taken from online trade market (https://www.echemi.com/shop-\nus20200308083104833/products.html?keywords=91-19-0).\nb: Taken from online trade market (https://www.echemi.com/shop-\nus20250711150018648/products.html?keywords=3476-89-9).\nFig. R8. Technoeconomic analysis of ECH||OER systems.\nRef. [1] De Luna, P. et al. What would it take for renewably powered electrosynthesis\nto displace petrochemical processes? Science 364, eaav3506 (2019).\nRef. [2] Leow, W. et al. Chloride-mediated selective electrosynthesis of ethylene and\npropylene oxides at high current density. Science 368, 1228-1233 (2020).\nRef. [3] Zhou, H. et al. Electrocatalytic upcycling of polyethylene terephthalate to\ncommodity chemicals and H fuel. Nat. Commun. 12, 4679 (2021).\nalkenols over a bimetallic CuAu alloy catalyst. ACS Catal. 14, 5675−5684 (2024).\nRef. [5] Li, J. et al. Electrocatalytic nitrate reduction to ammonia coupled with organic\noxidation. Chem. Catal. 3, 100638 (2023).\nComment 7. The DFT calculations present a single hydrogenation pathway for\nquinoxaline without justifying its likelihood over other plausible sequences.\nResponse: We appreciate you for this valuable recommendation. The 1,2,3,5,6- and\n1,2,4,5,6-H* addition pathways are two possible pathways for electrochemical\nhydrogenation of quinoxaline in water, as schematically illustrated in Fig. R9 (see\nsupplementary Fig. 39 in revised supporting information). Therefore, the energy profile\ndiagrams of each of the two pathways toward ECH of quinoxaline on Ru /ns-Co O\nSA 3 4\nwere analyzed in detail (Fig. R10, 11 see supplementary Fig. 40-43 in revised\nsupporting information). According to theoretical calculations, the 1,2,3,5,6-H*\naddition pathways is more favorable (Fig. R12, see Fig. 6e in revised manuscript and\nsupplementary Fig. 44 in revised supporting information).\nFig. R9. The proposed possible reaction paths for electrocatalytic hydrogenation\npathway of quinoxaline.\nFig. R10 | The configurations along ECH of quinoxaline pathway on Co O slab from\n3 4\ntop view through 1,2,4,5,6-H* hydrogenation pathway. The cyan, white, blue and\nred balls represent Co, H, Ru and O atoms, respectively.\nFig. R11 | The configurations along ECH of quinoxaline pathway on Ru /Co O slab\nSA 3 4\nfrom top view through 1,2,4,5,6-H* hydrogenation pathway. The cyan, white, blue and\nred balls represent Co, H, Ru and O atoms, respectively.\nFig. R12. The free-energy diagram for electrochemical hydrogenation of quinoxaline\nto 1,2,3,4-tetrahydroquinoxaline on Ru /Co O and Co O through (a) 1,2,3,5,6-H*\nSA 3 4 3 4\naddition pathways, (b) 1,2,4,5,6-H* addition pathways.\nComment 8. Please enhance details on electrode fabrication ATR-SEIRAS and provide\nthe details for in-situ Raman.\nResponse: We highly value your valuable comment. For your convenience, we provide\na point-by-point response to these comments:\nIn Situ ATR-SEIRAS Measurements. Electrochemical in situ ATR-SEIRAS\nmeasurements were performed in internal reflection mode on a Thermo Fisher Nicolet\niS50 (equipped with an ElectroChemIR unit from Pike Technologies). The experiments\nwere carried out in a three-electrode system: The catalyst ink was dripped onto a\nmonocrystalline silicon covered with an Au membrane, which served as the working\nelectrode, Hg/HgO is introduced as a reference electrode near the working electrode,\nand a Pt wire is used as the counter electrode. We first reduced the catalyst in 1 M KOH\nfor 10 min at -0.1 V vs. RHE, then 50 mM quinoxaline was added in the electrolyte for\nin situ ATR-SEIRAS test. The potential range was from OCP to −0.7 V (vs. RHE), with\nspectra collected at a spectral resolution of 4 cm‒1 with at least 32 coadded scans. The\nCHI 760E electrochemical workstation was used to control the potential during the\nATR-SEIRAS test.\nIn Situ Raman Measurements. Electrochemical in situ Raman measurements were\ncarried out by Renishaw inVia Qontor Raman microscope. The excitation wavelength\nof the semiconductor laser was 532 nm. All the Raman measurements were performed\nwith a 50× microscope objective. Raman frequencies were calibrated using silicon\nwafers before experiment. A three-electrode Raman cell with an embedded quartz\nwindow was used for Raman experiments, with platinum wire as counter electrode and\nHg/HgO electrode as reference electrode. A glassy carbon electrode loaded with\ncatalyst was used as a working electrode. 1 M KOH solution was used as electrolyte.\nThe CHI 760E electrochemical workstation was used to control the potential during the\nRaman test.\nReviewer #4\nIn this manuscript, the authors report the Ru /ns-Co O catalyst to enhance the\nSA 3 4\nelectrochemical hydrogenation (ECH) of quinoxaline to 1,2,3,4-tetrahydroquinoxaline\nvia the interfacial water regulation. The decoration of Ru creates an asymmetric local\nelectric field that modulates the interfacial water structure, thus facilitating the\ndissociation of H O to generate H*, further leading to a higher Faradaic efficiency at\n2\nhigh current densities. Multiple in situ and ex situ characterizations and theoretical\ncalculations are performed to unveil the promotion mechanism. While the enhanced\nperformance of the Ru /ns-Co O composite is promising, I find that many of the\nSA 3 4\nmechanistic claims lack convincing experimental support. Therefore, this manuscript\nshould be carefully revised and reorganized before submitting to Nature\nCommunications.\nWe sincerely appreciate the reviewer’s valuable comment, which points out a key\nimprovement direction for our manuscript. We fully agree that robust experimental\nsupport is essential for validating mechanistic claims, and we apologize for the\ninsufficient evidence provided in the initial version. We have carefully revised the\nmanuscript in response to your comment, and we hope these revisions address your\nconcerns and enhance the rigor of our work. We look forward to your further feedback.\nComment 1. The author states that proton transfer is a critical factor influencing\nhydrogenation reactions. As the authors noted, \"For these catalysts, 4HB-H O remains\n2\nthe predominant species across all detected potentials\", what is the specific role of\nRu /ns-Co O in facilitating proton transfer?\nSA 3 4\nResponse: We thank the reviewer for this comment.\nProton transfer in aqueous systems predominantly adheres to the Grotthuss mechanism,\nwhere protons transport through a “hopping” mode along the network of hydrogen\nbonds formed by water molecules, facilitating efficient proton migration from the\nelectrode surface to the electrolyte.1 According to literature reports, the energy required\nfor water dissociation decreases with a reduction in the degree of hydrogen bonding,\nwhile proton transfer increases with an increase in the degree of hydrogen bonding.2\nTherefore, the four-coordinated water with the strongest hydrogen bond network\ndetermines the proton transfer rate. According to in-situ Raman results, the content of\nfour-coordinated water always dominates under different catalyst ratios and potentials,\nindicating that the proton transfer rate maintains high stability during the reaction,\nproceeds efficiently, and is less affected by changes in reaction conditions.\nRef. [1] Deng, L. et al. Lewis acid-mediated interfacial water supply for sustainable\nproton exchange membrane water electrolysis. J. Am. Chem. Soc. 146, 35438−35448\n(2024).\nRef. [2] Yang, C. et al. Bioinspired Sulfo oxygen bridges optimize interfacial water\nstructure for enhanced hydrogen oxidation and evolution reactions. Nat. Commun. 16,\n6459 (2025).\nComment 2. What exactly does the promotional effect of Co?\nResponse: Thank you very much for this comment.\nHerein, Co sites of Ru /Co O plays the important roles. First, it promotes the\nSA 3 4\ndissociation of water molecules to generate active hydrogen (H*). Second, it facilitates\nthe adsorption and activation of quinoxaline, which is then hydrogenated by the H*\ngenerated at adjacent Co sites. We have validated this conclusion through a combination\nof experiments and DFT calculations.\nWe studied the energy barriers for H O dissociation on Co O and Ru /Co O\n2 3 4 SA 3 4\nthrough DFT calculations. The results indicate that the energy barrier for H O\n2\ndissociation on Co atoms in Ru /Co O is 0.44 eV, which is much lower than that of\nSA 3 4\nCo atoms in Co O (0.63 eV) and Ru atoms in Ru /Co O (0.91 eV) (Fig. R1, see Fig.\n3 4 SA 3 4\n2f in revised manuscript). This result indicates that hydrolysis mainly occurs at the Co\nsites of the catalyst. In addition, our experimental results demonstrate that both Ru\nSA\n/ns-\nCo O and Co O exhibit activity toward quinoxaline hydrogenation, providing\n3 4 3 4\nevidence that Co serves as the active site for this reaction (Fig. R2, see Fig. 4a, b in\nrevised manuscript).\nFig. R1. DFT calculation for the energy barriers of H O dissociation process on the\nSA 3 4 3 4\nFig. R2. (a) The FE of 1,2,3,4-tetrahydroquinoxaline and H over Ru /ns-Co O -0.7%\nin 1 M KOH with 50 mM quinoxaline passed 193 C at different current density. (b) The\nFE of 1,2,3,4-tetrahydroquinoxaline and H over ns-Co O in 1 M KOH with 50 mM\n2 3 4\nquinoxaline passed 193 C at different current density.\nComment 3. The article exclusively focuses on the generation of H*, without\nconsidering the catalyst's influence on quinoxaline activation.\nResponse: We are grateful to the reviewer for this nice comment.\nThe experimental and DFT results indicate that doping with Ru single atoms can\neffectively activate quinoxaline, thereby significantly enhancing its hydrogenation\nactivity. We verified that Ru single atom doping could promote the ECH of quinoxaline\nby quasi-in situ electron paramagnetic resonance (EPR) measurements using 5,5-\ndimethyl-1-pyrrolidine-N-oxide (DMPO) as a carbon radical trap. As shown in Fig. R3\n(see supplementary Fig. 49 in revised supporting information), after adding quinoxaline\nto the electrolyte, the Ru /ns-Co O exhibits a much stronger carbon radical signal\nSA 3 4\nthan ns-Co O , which is because Ru single atom doping can effectively activate\n3 4\nquinoxaline.1\nThe hydrogenation reaction mechanism of quinoxaline was further investigated\nthrough DFT calculations (Fig. R4, see Fig. 6e in revised manuscript). The free energies\nof different hydrogenation sequences over the two catalysts were calculated. After\nanalyzing the reaction free energies of these pathways, it was observed that the rate-\ndetermining step reaction barrier of Ru /Co O for quinoxaline hydrogenation was\nSA 3 4\nsignificantly lower than that of Co O . This indicates that Ru /Co O can afford faster\n3 4 SA 3 4\nhydrogenation kinetics. Therefore, Ru single atom doping can effectively activate the\nquinoxaline substrate and enhance its hydrogenation activity.\nFig. R3. Quasi-in situ EPR spectra detected from the electrochemical reactions over ns-\nCo O and Ru /ns-Co O -0.7% in 1 M KOH with quinoxaline at −0.1 V vs RHE using\n3 4 SA 3 4\nDMPO as spin-trapping agent.\nFig. R4. The free-energy diagram for electrochemical hydrogenation of quinoxaline to\n1,2,3,4-tetrahydroquinoxaline on Ru /Co O and Co O through 1,2,3,5,6-H* addition\nSA 3 4 3 4\npathways.\nRef. [1] Ren, Q. et al. Promoting electrocatalytic semihydrogenation of alkynols to\nalkenols over a bimetallic CuAu alloy catalyst. ACS Catal. 14, 5675-5684 (2024).\nComment 4. Further analysis of Figure 6d is required.\nResponse: We sincerely appreciate your suggestion. We have performed further\nanalysis on Fig. 6d, and the results of this analysis are as follows.\nMoreover, compared to the OCP, the first coordination shell structure of the Ru-O\nbond showed no significant changes at potentials of -0.1 V and -0.2 V vs. RHE,\nindicating that quinoxaline molecules did not adsorb on the Ru sites. Notably, the FT-\nEXAFS spectra show a slight shift in the peak position of the second coordination shell\nassociated with the Ru-O-Co bond under applied potentials. These shifts imply that the\nH O dissociation and quinoxaline hydrogenation at Co sites alter the local electronic\n2\nstructure and atomic configuration, and indirectly modulate the FT-EXAFS signal of\nRu through electronic coupling and structural communication via the Ru-O-Co bond\n(Fig. 6d). Therefore, these results indicate that Co sites serve as the main active sites\nfor the ECH of quinoxaline.\nComment 5. The Ru-O-Co pathway shown in Figure 3d lacks a reference sample or\nsupporting literature.\nResponse: We sincerely appreciate the reviewer’s question. We would like to\napologize for any inconvenience caused to the reviewers in the process of reading our\nmanuscript. The supporting literature we provide is as follows.\nRef. [35] Zuo, S. et al. Local compressive strain-induced anticorrosion over isolated\nRu-decorated Co O for efficient acidic oxygen evolution. Nat. Commun. 15, 9514\n3 4\n(2024).\nRef. [36] Li, D. et al. Isolated octahedral Pt-induced electron transfer to ultralow-\ncontent ruthenium-doped spinel Co O for enhanced acidic overall water splitting. J.\n3 4\nAm. Chem. Soc. 146, 28728−28738 (2024).\nComment 6. There are several spelling errors in the article, for example: \"Faraday\nefficiency\" in line 15 should be \"Faradaic efficiency\" and \"implications\" in line 31\nshould be \"applications\".\nResponse: We appreciate you for this valuable recommendation. We have made\nrevisions in the manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "All our concerns have been addressed. This manuscript is recommended to be accepted in the present form.", "reviewer_2": "Although the authors have made every effort to address the reviewers’comments and revise the manuscript, the manuscript\nstill presents the following issues that necessitate further refinement and enhancement by the author.\n1.The electrochemical hydrogenation of quinoxaline exclusively yielded 1,2,3,4-tetrahydroquinoxaline. This result remains\nperplexing. The authors should provide relevant data concerning carbon balance.\n2.Typically, during the preparation process of Raney Ni or Raney Co, aluminum is removed in an alkaline environment,\nallowing the metal to predominantly retain its metallic state, contrary to the authors’ assertion that Co is oxidized.\nFurthermore, during the calcination process, Co is likely to undergo further oxidation, while Ru should also be oxidized,\nrather than existing in its elemental form.\n3.The assertion that Ru substitutes Co sites is contentious. In the synthesis of the materials discussed in this manuscript, Ru\nmerely indicates the modification of Co3O4, with characterization results revealing the predominant presence of Ru-o-Co\nbonds. It is recommended that the authors provide further clarification or amend the manuscript accordingly.", "reviewer_3": "The comments have been well resolved.", "reviewer_4": "I appreciate the authors's efforts in addressing my concerns. Upon reviewing the responses and the revised manuscript, I\nfind that the manuscript has been significantly improved. I am satisfied with the improvements and recommend its\npublication."}, "author_response": "Responses to the Referees' Comments\nWe thank the referees for their valuable comments and positive endorsement of our\nmanuscript. We have carefully considered the referees' comments and revised the\nmanuscript accordingly. Our responses and corresponding revisions are as follows:\nReviewer #2\nAlthough the authors have made every effort to address the reviewers’ comments and\nrevise the manuscript, the manuscript still presents the following issues that necessitate\nfurther refinement and enhancement by the author.\nResponse: We sincerely appreciate the reviewer's continued scrutiny. We fully\nacknowledge the points raised below and have conducted a thorough revision to address\neach concern in detail. The point-by-point responses and corresponding modifications\nare provided here and in the revised manuscript.\nComment 1. The electrochemical hydrogenation of quinoxaline exclusively yielded\n1,2,3,4-tetrahydroquinoxaline. This result remains perplexing. The authors should\nprovide relevant data concerning carbon balance.\nResponse: Thank you for raising this point regarding carbon balance. We agree that\nconfirming carbon balance is crucial. During the electrocatalytic hydrogenation of\nquinoxaline, only 1,2,3,4-tetrahydroquinoxaline is produced. Given that both the\nreactant (quinoxaline) and the product (1,2,3,4-tetrahydroquinoxaline) have high\nboiling points, neither compound volatilizes under ambient conditions. Additionally,\nboth are chemically stable and do not undergo decomposition. Combined with our\nexperimental data, as shown in Fig R1 (see supplementary Fig. 20 in revised supporting\ninformation), these observations collectively confirm that the carbon balance of the\nreaction system approaches 100%.\nFig R1. Carbon balance of quinoxaline ECH under different current densities.\nComment 2. Typically, during the preparation process of Raney Ni or Raney Co,\naluminum is removed in an alkaline environment, allowing the metal to predominantly\nretain its metallic state, contrary to the authors’ assertion that Co is oxidized.\nFurthermore, during the calcination process, Co is likely to undergo further oxidation,\nwhile Ru should also be oxidized, rather than existing in its elemental form.\nResponse: We sincerely appreciate the reviewer’s insightful comments. For the\nconvenience of your review, we will address your questions one by one in the following\nsections.\nThe precursor alloy of conventional Raney Ni or Raney Co is typically Al Ni ,\n50 50\nwhich evolves into a porous architecture upon chemical corrosion. However, the\nrelatively large pore sizes and ligaments in this structure restricts the exposure of Ni/Co\nactive sites, with the majority of metallic species residing in the bulk phase. As a\nconsequence, only the surface Ni/Co atoms are susceptible to oxidation.1, 2\nIn this work, the designed precursor alloy (Al Co Ru ) features an Al-rich\n80 19.8 0.2\nphase, which exhibits a vigorous reaction with concentrated KOH solution. Corrosion\nof this alloy yields a nanosheet structure with an ultrahigh specific surface area, thereby\nenabling the exposure of abundant fresh Co atoms (Fig R2, see supplementary Fig. 5 in\nrevised supporting information). These coordinatively unsaturated Co atoms are readily\noxidized via reaction with dissolved oxygen in the alkaline medium.\nAdditionally, the corrosion process was conducted under ambient air conditions.\nThe intense reaction dynamics facilitate efficient oxygen exchange between the\nsolution and the surrounding air, which further accelerates the oxidation of Co atoms\n(Fig R3, see supplementary Fig. 6 in revised supporting information). Notably, the\nsynthetic strategy of fabricating metal oxides via alkaline corrosion of Al-rich alloys\nhas been documented in previous literature, which provides robust support for the\nfeasibility of our method.3,4\nAt present, the intrinsic mechanism governing the formation of the nanosheet\nstructure during the corrosion of Al-rich alloys has not been fully elucidated in the\npresent study. Systematic investigations into this structural evolution mechanism will\nbe performed in our follow-up work.\nFurthermore, we fully concur with the reviewer’s comments regarding the\nevolution of the oxidation state during the calcination process. We wish to clarify that\nin our final catalyst Ru /ns-Co O , Ru exists predominantly in an oxidized state rather\nSA 3 4\nthan as zerovalent metal. Under the oxidizing atmosphere of calcination, Ru atoms are\noxidized to high-valence cations (Ruδ+). Owing to their highly matched ionic radius\nwith that of Co3+, these Ruδ+ ions can occupy the octahedral sites of Co3+ in the spinel\nlattice, thereby constructing a robust Ru–O–Co bonding network. The oxidized state of\nRu was unambiguously verified via X-ray absorption near-edge structure (XANES) and\nX-ray photoelectron spectroscopy (XPS) measurements (Fig R4, see Fig. 3c in revised\nmanuscript and supplementary Fig. 11 in revised supporting information). Thus, the\nreviewer’s perspective is highly consistent with our conclusion that Ru is oxidized and\ndoped as single atoms within the Co O lattice.\n3 4\nFig R2. SEM image of Ru/ns-CoO .\nx\nFig R3. XRD patterns of Ru/ns-CoO .\nx\nFig R4. (a) Ru K-edge XANES of Ru foil, RuO , and Ru /ns-Co O . (b) High resolution XPS\nspectra of Ru /ns-Co O of Ru 3p.\nSA 3 4\n[1] Zhang, J. et al. Preparation and catalytic performance of an efficient Raney nickel\ncatalyst for syngas methanation. J Mater Sci 54, 14197–14208 (2019).\n[2] Wang, H. et al. Undercoordinated site-abundant and tensile-strained nickel for low-\ntemperature CO methanation. ACS Catal. 8, 1207−1211 (2018).\nx\n[3] Ishikawa, Y. et al. Ultrafine sodium titanate nanowires with extraordinary Sr ion-\nexchange properties. Nano Lett. 15, 2980−2984 (2015).\n[4] Jin, Z. et al. Eight-component nanoporous high-entropy oxides with low Ru\ncontents as high-performance bifunctional catalysts in Zn-air batteries. Small 18,\n2107207 (2022).\nComment 3. The assertion that Ru substitutes Co sites is contentious. In the synthesis\nof the materials discussed in this manuscript, Ru merely indicates the modification of\nCo3O4, with characterization results revealing the predominant presence of Ru-O-Co\nbonds. It is recommended that the authors provide further clarification or amend the\nmanuscript accordingly.\nResponse: We sincerely thank the reviewer for this critical comment, which prompts\nus to clarify a key point of terminology that is central to understanding the structure of\nour catalyst. We apologize for the inconsistency between the manuscript title \"Ru Single\nAtoms Decorated Co O \" and the expression \"Ru substitutes Co sites\" in the main text,\n3 4\nwhich may have caused confusion or controversy among readers and reviewers.\nOur comprehensive characterization data unambiguously reveal a more specific\nand accurate picture: Ru species are atomically dispersed and incorporated into the\nCo O crystal lattice, forming Ru–O–Co coordination bonds. This is a state of doping\n3 4\nat the atomic scale, distinct from surface adsorption.\nThe evidence supporting lattice incorporation is twofold:\n(1) The aberration-corrected high-angle annular dark-field scanning transmission\nelectron microscopy (HAADF-STEM) image of Ru /ns-Co O clearly resolves\nSA 3 4\ndiscrete bright spots with exceptionally high intensity, indicating that Ru atoms occupy\nspecific lattice sites (Fig R5a, see supplementary Fig. 7 in revised supporting\ninformation).1, 2 Furthermore, the line-scan cross-sectional profile distinctly reveals\nisolated high-intensity peaks, demonstrate that Ru single atoms occupy the lattice\nposition of the Co atoms.3 Notably, these Ru-related signals coincide precisely with the\npositions of the original Co atomic columns (Fig R5b, see supplementary Fig. 9 in\nrevised supporting information). Collectively, these results confirm that Ru atoms\noccupy the Co lattice sites and are atomically dispersed within the Co O lattice.\n3 4\n(2) In addition, the Co K-edge EXAFS spectra (Fig. R3) exhibited characteristic\npeaks assignable to scattering events within the spinel structure, specifically from\noctahedrally coordinated Co (Co –Co ) and tetrahedrally coordinated Co (Co–Co )\nOh Oh Td\n(Fig. R6, see Fig. 3h in revised manuscript). Concurrently, the Ru K-edge EXAFS\nspectrum (Fig. R7, see Fig. 3d in revised manuscript) revealed a distinct peak at\napproximately 2.45 Å. This peak corresponds to a Ru–Co scattering path,\ncorresponding to the Ru–O–Co configuration in the spinel crystal structure, providing\ndirect evidence that Ru single atoms are incorporated into the host lattice and occupy\nthe octahedral Co sites.4 This coordination environment is fundamentally distinct from\nthat of surface-adsorbed Ru species, a finding fully consistent with the aforementioned\nanalytical results.5‒7 These results reveal that Ru occupies the lattice sites of Co atoms\nand is atomically dispersed in Co O .\n3 4\nTo resolve the terminology inconsistency and reflect our findings with maximum\nprecision, we will make the following revisions to the manuscript: In the title, we will\nmodify it to “Interfacial Water Regulation on Ru Single Atoms Doped Co O Toward\n3 4\nEfficient Electrochemical Hydrogenation of Quinoxaline”. Throughout the text, we will\nconsistently use terms such as \"Ru single atoms doped into the Co O lattice\" and\n3 4\nmaintain terminological uniformity in all relevant descriptions.\nWe believe these revisions will eliminate ambiguity and accurately convey that Ru\nis an integral part of the Co O host lattice, which is the foundational reason for its\n3 4\nexceptional catalytic properties.\nFig R5. (a) HAADF-STEM images of Ru /ns-Co O . (b) The line intensity profile along A and B\nSA 3 4\ndirection in (a).\nFig R6. Fourier transformed magnitudes of Co K-edge EXAFS spectra for ns-Co O and Ru /ns-\n3 4 SA\nCo O .\n3 4\nFig R7. Fourier transformed magnitudes of Ru K-edge EXAFS spectra for Ru foil, RuO ,\n2\nand\nRu /ns-Co O .\nSA 3 4\n[1] Wang, J. et al. Breaking the yield–selectivity trade-off in polystyrene waste\nvalorization via tandem depolymerization and hydrogenolysis. Nat. Nanotechnol.\n(2025). DOI: 10.1038/s41565-025-02069-x.\n[2] Zhu, W. et al. Direct dioxygen radical coupling driven by octahedral ruthenium–\noxygen–cobalt collaborative coordination for acidic oxygen evolution reaction. J. Am.\nChem. Soc. 145, 17995–18006 (2023).\n[3] Luo, W. et al. Controlled electrocatalytic glycerol upgrading to glyceraldehyde in\nnear neutral media by cobalt oxide lattice activation. Angew. Chem. Int. Ed. 264,\ne202505059 (2025).\n[4] Zhu, Y. et al. Iridium single atoms incorporated in Co O efficiently catalyze the\n3 4\noxygen evolution in acidic conditions. Nat. Commun. 13, 7754 (2022).\n[5] Jiang, J. et al. Efficient electrochemical-enzymatic conversion of PET to formate\ncoupled with nitrate reduction over Ru-doped Co O catalysts. Angew. Chem. Int. Ed.\n3 4\n64, e202421240 (2025).\n[6] Rong, c. et al. Uncoordinated single-site Ru confined in spinel Co O lattice for\n3 4\nhigh-performance and low-cost PEM water electrolysis. Adv. Funct. Mater. e16742\n(2025). DOI: 10.1002/adfm.202516742.\n[7] Gao, L. et al. Embedding Ru single atom catalysts on Co O for efficient hydrazine\n3 4\noxidation and direct hydrazine fuel cells. Appl. Catal. B: Environ. 358, 124287 (2024)."}, {"round": 3, "reviewer_comments": {"reviewer_2": "The revised manuscript can be published at present."}, "author_response": ""}]

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