Optimizing the size and electronic effects of core-shell heterostructures via well-constructed Ru clusters encapsulated in N-doped carbon layers
Min Jie Wang , Jiao Yang , Lishan Peng , Yongjie Bai , Zehui Liu , Xiaoliang Yang , Huijuan Lu , Bingjie Zhou , Ningtao Jiang , Guoxu He , Han-Ming Zhang , Liwei Mi , Yonghui Deng
Efficient access to clean hydrogen with its high energy density is a prerequisite for the rapid development of the hydrogen economy [1,2]. As a promising large-scale technology for the production of high-purity and low-carbon hydrogen in an alkaline environment, water electrolysis (WE), which occurs via the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER), has attracted increasing interest and attention [3–5]. However, the lack of a robust OER at the counter electrode and the high cost of equipment corrosion have plagued the commercialization of WE conducted under acidic reaction conditions [6–8]. These problems are largely avoided by performing water splitting under alkaline conditions, although few OER electrocatalysts are known to be suitable for large-scale WE under these conditions [8–11]. Unfortunately, under alkaline conditions, the HER kinetics are two orders of magnitude slower than in acidic environments, even when using the state-of-the-art Pt/C catalyst [12,13]. Thus, it is urgently necessary, yet highly challenging, to develop electrocatalysts capable of boosting the HER performance in alkaline environments.
Ru-based electrocatalysts have been attracting attention as promising substitutes for Pt-based HER catalysts because of their affordability (5% of the cost for Pt) and potentially high activity derived from the metal-hydrogen bond energy (65 kcal/mol) closely resembles that of the benchmark Pt (62 kcal/mol) [12,14,15]. The alkaline HER involves water dissociation (the Volmer step) and hydrogen adsorption (the Heyrovsky or Tafel step) [16,17]. According to previous reports, Ru clusters can effectively drive the former step, but their strong binding energy to *H intermediates (Ru-H bonds) severely slows down hydrogen desorption [15,18,19]. Despite the suitability of Ru single-atom (RuSA)-based catalysts for hydrogen adsorption, their weaker water-dissociation ability significantly hinders the Volmer step [18,20,21]. In this regard, incorporating RuSA into nearby Ru clusters appears to be a valid strategy for facilitating the overall HER. However, it is difficult to optimize the specific distance between RuSA and nearby Ru clusters. In addition, regulation of the electronic structure could effectively modulate the d-band center of metal sites and the adsorption/desorption of reaction intermediates (*H, interfacial H2O, etc.) [22,23], which results in the optimization of apparent hydrogen binding energy (HBEapp) [11,24,25]. This is an essential strategy for tuning the intrinsic activity of catalytic sites and can be accomplished using techniques such as heteroatom doping [26–28], defect engineering [29–31], and heterogeneous interfaces [32–38]. Among these, the use of heterostructures is generally regarded as the most effective alternative, especially for those with core-shell architectures. The effectiveness of this approach has its origins in the enhanced interfacial charge effect of strong core-shell interactions and the potential existence of multiple active sites due to high geometric overlapNotable examples are Ru2P/WO3@NPC [39], RuCo@NC [40], Ru-Ru2P@NPC [41], RhP2/Rh@NPG [42], and MoP-Ru2P/NPC [43]. Computational results uncovered the heterostructure could enhance the Ru-N interactions and well optimize Ru-H bonds, facilitating the HER process. Electrocatalysts with the aforementioned properties are expected to optimize the adsorption/desorption of the HER intermediate species, thereby accelerating the kinetic process. The size effect induced by Ru cores with different sizes on the core-shell interaction is also crucial and is known to promote high-performance HER [22]. However, these beneficial effects are usually compromised by changes in the particle size, which invariably induce alterations in the electronic effects.
Inspired by these considerations, we developed a set of well-defined core-shell heterostructured Ru-based catalysts composed of Ru nanoparticles coated by nitrogen-doped carbon (denoted as Ru@NC). These Ru@NC active components were evenly dispersed on porous N-doped carbon nanosheets with copious meso–/macropores features for improving active site exposure and mass transfer. Ru@NC-3h, with the optimal geometric and electronic effects, exhibited both superior HER activity and desirable durability, with a high mass-specific activity of 6.68 A/mgRu. Density functional theory (DFT) calculations revealed that the enhanced Ru-N interaction between highly active Ru cores and thin N-doped shells upshifted the d-band center of Ru sites to optimize interfacial H2O and *H adsorption and accelerate the HER kinetics. This finding is in good agreement with the results of the attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) analysis, which indicated that the new heterostructures facilitate the generation of hydrogen-containing intermediates and alleviate heavy-current polarization. This work provides a new perspective for constructing core-shell heterostructures for energy conversion and applications.
The preparation of the Ru@NC catalysts is shown in Fig. S1 (Supporting information) schematically, of which size-controlled Ru nanoclusters are fully encapsulated within an N-doped graphene shell layer, and evenly supported on a sheet-like carbon matrix. The catalysts were prepared in a hybrid methanol system containing the nitrogen sources 1-(2-cyanoethyl)−2-phenylimidazole (CEPI) and dicyandiamide (DCD). After adding Ru3+ in the form of the metal salt, in-situ polymerization in this solution yielded the Ru-N precursor, induced by strong hydrogen bonding/electrostatic interaction among the functional groups (NH2, OH, C═NH, and C≡N, etc.) and Ru3+.
Subsequently, the Ru-N precursor was converted into the target Ru@NC by pyrolysis under 550 ℃ in the air atmosphere to yield sheet-shaped feather-like particles, and subsequently under 900 ℃ in the N2 atmosphere for different times to form the N-doped shell layer, respectively. Four catalysts, i.e., Ru@NC-1h, Ru@NC-6h, and Ru@NC-9h, were synthesized using the above strategy by varying the duration of the annealing process in the N2 atmosphere (described in Supporting information). The bulk crystal structures were investigated using X-ray diffraction (XRD, Fig. S1b). The five characteristic diffraction peaks at 38.4°, 42.2°, 44.0°, 58.3°, and 69.4° were assigned to the (100), (002), (101), (102), and (110) planes of metallic hexagonal close-packed (hcp) Ru (hcp-Ru: PDF #06–0663), respectively, implying the successful fabrication of the Ru@NC electrocatalysts.
Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) were used to explore the morphologies of the newly prepared catalysts (Fig. 1). Both Ru@NC-1h (Fig. 1a) and Ru@NC-3h (Fig. 1b) exhibit typical porous sheet-like carbon frameworks with copious meso–/macropores of micrometer-scale dimensions. Even Ru@NC-6h (Fig. 1c) exhibited excellent structural retention under strict carbonization. Notably, this open structure offers excellent active site exposure and facilitates the mass transfer of the intermediate reaction species (absorbed water (*H2O) and hydrogen (*H)). In addition, it improves the reaction kinetics and alleviates heavy-current polarization [41,42]. According to the TEM observations, Ru@NC-1h (Figs. 1d and e) is characterized by the coexistence of large metal particles and small clusters (ca. 3.1 nm). Paradoxically, a narrow size distribution and uniformly dispersed clusters existed in Ru@NC-3h (Figs. 1h and i) with a mean size of ca. 2.2 nm compared to that of Ru@NC-6h (ca. 2.5 nm, Figs. 1l and m). This may be attributed to the higher stability of a cluster encapsulated in N-doped carbon layers (Figs. 1f, j and n) and its relatively smaller work function than that of large nanoparticles [43]. Furthermore, these thin layers (ca. 3 to 7) are conducive to accelerating electron transfer from the metal cores to the outward shell layers, which enhances the metal-N interactions. This could optimize the adsorption/desorption behavior of the intermediate reaction species, as suggested by the ATR-SEIRAS results, to accelerate the reaction kinetics [43]. Figs. 1p and q reveal the absence of any clusters in Ru@NC-9h, with a sharp recombination due to the loss of carbon protective layers. The fast Fourier transformation (FFT) of the high-resolution TEM (HRTEM) results (Figs. 1f, j, n and q) reveals the corresponding d-spacing (Figs. 1g, k, o and r) to be 0.214 nm, which could be indexed to the hcp (002) plane in Ru@NC-1h, Ru@NC-3h, Ru@NC-6h, and Ru@NC-9h, respectively. Moreover, energy-dispersive X-ray spectroscopy (EDS, Fig. 1s) demonstrated that C, N, O, and Ru were homogeneously distributed, and indicated the formation of the expected Ru@NC-3h
The pore structure parameters (PSPs) for the as-prepared electrocatalysts were comprehensively investigated by the N2 adsorption/desorption test. As shown in Fig. S2 (Supporting information), the close resemblance between the isotherms of the three respective catalysts, Ru@NC-1h, Ru@NC-3h, and Ru@NC-6h, implies that they have a similar specific surface areas (SPAs) and porosities. That is, the insets show equivalent SPAs (153–170 m2/g, especially 27 m2/g for Ru@NC-9h), as determined by the Brunauer-Emmett-Teller (BET) method from the corresponding type-Ⅳ isotherms with apparent H3-type hysteresis loops at moderate relative pressure (P/P0 = 0.45–0.90) suggesting the existence of multiscale mesopores [44–46]. In addition, the high relative pressure (P/P0 ~0.99) suggested sharp capillary condensation and the existence of large mesopores and/or an open macroporous structure. Furthermore, the Barrett-Joyner-Halenda (BJH) pore size distribution curves in Fig. S2b confirm the existence of porous structures that contain micro-/meso–/macropores with sizes of ca. 1.2/3.6, and 33.6/58.5 nm. Together with the SEM observations (Figs. 1a–c), these results confirmed that the hierarchical porous features remained. This could effectively improve long-range mass transfer and active site exposure. Considering their similar PSPs, the HER performance mainly depended on the corresponding intrinsic activity.
Apart from the bulk structure, the chemical states of the key surface elements Ru and N were also probed in detail via X-ray photoelectron spectroscopy (XPS). In Fig. 2a, an obvious characteristic peak located at 461.8 eV on the high-resolution Ru 3p spectrum was assigned to Ru 3p3/2 for Ru@NC-3h. The positive shift in the binding energy position (BEP) by 0.27 eV (Ru@NC-1h) and 0.13 eV (Ru@NC-6h) implied outward charge transfer [47–49]. The high-resolution N 1s spectrum was split into five characteristic peaks for Ru@NC-1h/3h/6h (Fig. 2b), corresponding to pyrrole-N (397.4 eV), Ru-N (398.4 eV), pyridine-N (399.3 eV), graphite (401.1 eV), and oxide-N (402.8 eV) [44,50]. However, the Ru-N peak was absent from the spectrum of Ru@NC-9h, possibly due to the harsher annealing conditions. Furthermore, the BEP of N in Ru@NC-3h was negatively shifted by 0.31 and 0.12 eV for Ru@NC-1h and Ru@NC-6h, respectively, which confirmed the enriched charge state.
The maximum charge accumulation from the Ru to the N sites in Ru@NC-3h indicated strong Ru-support interactions, consistent with the thin core-shell heterointerface and conducive to charge transfer (Figs. 2a and b) [12,13,40,42]. XPS analysis showed large amounts of Ru (N) on the surface: 14.1 (5.2), 14.3 (6.0), 14.9 (4.3), and 31.2 (2.2) wt% for Ru@NC-1h/3h/6h/9h (Fig. 2c), respectively. This resulted in the highest Ru-N concentration (1.13 wt%) based on Ru-Nrelative multiplied by the N content, suggesting that these sites were the most inherently active in the HER. Based on the TEM, XRD, and XPS results, the Ru@NC-3h heterostructure has the optimal combination of size and electronic effects, which act synergistically to accelerate the HER kinetics by optimizing the adsorption/desorption of the intermediate reaction species [15,51,52].
The electrochemical HER performances of the as-prepared catalysts and the benchmark Pt/C were evaluated by linear sweep voltammetry (LSV) with a typical three-electrode system in 1.0 mol/L KOH as the electrolyte. As shown in Fig. 3a, for Ru@NC-3h to drive a typical current density of 10 mA/cm2 required an overpotential of merely 48 mV (η10), lower than that of Pt/C (58 mV) and Ru@NC-1h (71 mV), respectively, and far superior to that of the other prepared catalyst samples (> 100 mV). However, Ru@NC-1h required 389 mV to afford 250 mA/cm2, superior to Pt/C (489 mV) and the same as that of Ru@NC-6h. That is, a porous system with ample pores on multiple levels effectively promotes mass transfer and sharply lowers the high-current polarization. Apart from this, Ru@NC-3h demonstrated lower overpotentials of 296 and 455 mV at 250 and 500 mA/cm2, respectively. This confirmed the excellent HER performance over a wide range of current densities compared with the data from recent reports in Table S1 (Supporting information). Compared with Ru@NC-1h, the smaller Ru metal core in Ru@NC-3h could enhance the electron transfer and strengthen the synergistic Ru-N interactions, thereby effectively lowering the energy barrier of the reaction, which is vital for faster reaction kinetics [15,49,53]. As shown in Fig. 3b, the larger Tafel slope of Ru@NC-6h (103 mV/dec), smaller than that of Ru/C (140 mV/dec), suggested that the Volmer step (H2O + e- → H* + OH-) is the rate-determining step (RDS) [54,55]. The smallest Tafel slope 30 mV/dec, measured for Ru@NC-3h, was similar to those of Ru@NC-1h (35 mV/dec) and Pt/C (30 mV/dec) and implied a Volmer-Tafel mechanism with the elementary reaction (2H* → H2) as the RDS [54,56]. These observations resulted from the synergetic interactions at the Ru-N sites, which profoundly improved the sluggish water dissociation reaction. Electrochemical impedance spectroscopy (EIS, Fig. 3c) revealed the lowest charge-transfer resistance (2.1 Ω), thereby confirming the faster HER kinetics. The average turnover frequency (TOF) values were calculated based on the number of Ru sites in a working electrode via inductively coupled plasma mass spectrometry (ICP-MS). Ru@NC-3h exhibits the largest TOF value of 3.6 s-1, exceeding that of Ru/C (1.1 s-1), Ru@NC-9h (0.12 s-1), Ru@NC-6h (1.6 s-1) and Ru@NC-1h (2.2 s-1) at −0.4 V (vs. RHE), coincident with an outperforming intrinsic HER process. The mass activity (MA, Fig. 3d) was additionally investigated and normalized to the bulk loading of Ru. Ru@NC-3h had the highest MA of 6.68 A/mg, 7.03 times that of Pt/C (0.95 A/mg), with that of Pt being 23.6 times that of Ru in terms of cost. This MA is far higher than that of its counterparts with MAs of 0.65, 2.37, and 4.47 A/mg for Ru/C, Ru@NC-6h, and Ru@NC-1h, respectively.
The durability, another key indicator of excellent HER electrocatalysts, was explored in detail. The LSV curve for Ru@NC-3h (Fig. 3e), remained almost the same after the accelerated durability test (ADT) over 3000 cyclic voltammetry (CV) cycles, attesting to its outstanding durability. In contrast, Ru@NC-1h (∆η10 = 15 mV and ∆η200 = 31 mV, Fig. S3 in Supporting information) and Ru@NC-6h (∆η10 = 28 mV and ∆η200 = 52 mV, Fig. S4 in Supporting information) all exhibited obvious attenuation. The voltage output of the chronoamperometric test for Ru@NC-3h at a typical current density of 10 mA/cm2 for 150 h (Fig. S6 in Supporting information) and an industrial 500 mA/cm2 for 24 h (Fig. 3f) remained stable, further confirming the superior retention. The integrity of the core-shell structure (TEM images in the insets in Figs. 3e and f) ensured catalytic stability. Furthermore, the KSCN poisoning results (Fig. 3g) demonstrate a marked increase in the overpotential gap from 17 mV to 53 mV upon injection of 10 mmol/L KSCN into Ru@NC-3h to drive 10 and 100 mA/cm2. These values surpass those of Pt/C (143 and 182 mV, respectively) and are indicative of robust structural stability [48].
Most importantly, the improved charge transfer between the Ru cores and thin N-doped carbon shells strengthened the Ru-N interactions. In addition, the outer shells effectively protected the Ru sites in the interior to largely shield them from the poisoning effect. The electrochemically active surface area (ESCA, Fig. 3h), based on the double-layer capacitance Cdl (Fig. S5 in Supporting information), had a relatively high value of 975, suggesting that Ru@NC-3h has a larger number of active sites. Apart from this, Ru@NC-3h required a lower overpotential than the other catalysts to drive the same current density from ECSA-normalized LSV curves. This implies excellent inherent activity, which is beneficial for electrochemical durability [44,48]. These results indicate that enhanced Ru-N interaction resulting from engineered core-shell structure to optimize size and electronic structures is responsible for exceptional HER performance.
The in-situ ATR-SEIRAS technique was used to study the reaction mechanism of the Ru-based catalysts to investigate the reason for their improved catalytic performance. As shown in Fig. 4a, the peak at ca. 1650 cm-1 in Ru@NC-3h, accounts for the H-O-H deformation vibrations in interfacial water [57,58], which, as a proton donor, provides hydrogen-containing intermediates during catalytic reactions. This promotion of the reaction kinetics is confirmed by the obvious blueshift of 18 wavenumbers compared to that of Ru@NC-9h (1632 cm-1, Fig. 4b). In addition, two potential-dependent vibrations for Ru-H and O—H were observed by increasing the potential stepwise from 0.2 V to ~0.25 V. In Fig. 4a, the peaks in the range of ca. 2084–2011 cm-1 for Ru@NC-3h are assigned to the stretching vibration of *H adsorbed in the Volmer step, and correspond to a moderate Stark tuning rate of 41 cm-1 V-1 [59,60]. Nevertheless, the 2.8 times higher Stark tuning rate of 115 cm-1 V-1 of Ru@NC-9h (Fig. 4c) confirms its stronger *H binding, and is in agreement with the increase in the wavenumber from 2084 cm-1 to 2133 cm-1. This is not conducive to the timely desorption of key *H intermediate species, which in turn poison the active sites [59,60]. In addition, the peaks at ca. 3250–3450 cm-1 were ascribed to the O-H stretching vibration of the interfacial water in the diffusion layer [61]. The higher Stark tuning rate of 109 cm-1 V-1 for Ru@NC-3h compared to that of 48 cm-1 V-1 for Ru@NC-9h (Fig. 4d) indicates the enhanced adsorption of interfacial water. This could ensure *H production and facilitate mass transfer from *H to Ru, thereby improving the heavy-current polarization [2,61].
The structure-activity relationship between the Ru-N interaction and HER activity was probed by conducting DFT calculations on the NC, Ru1.0, Ru0.4@NC, Ru1.0@NC, and Ruslab@NC models (Fig. S7 in Supporting information). As indicated by the partial density of states (PDOS, Fig. 4e), the d-band center of Ru1.0@NC (Ed = 1.37 eV) was shifted upward compared to that of Ru1.0 (Ed = 1.59 eV), due to the hybridization between the p-band of N and the d-band of Ru, which leads to the higher d-electron energy and increased electron delocalization. Fig. 4f shows the charge density difference and Bader charge of Ru1.0@NC. In the side and top views of Ru1.0@NC, compared to Ru0.4@NC and Ruslab@NC (Fig. S7c), the enforced electron transfer from Ru to N was confirmed by the increased cyan clouds near Ru atoms and the yellow clouds near adjacent N atoms. These results corresponded well with the Bader charge loss of 1.26 e- and PDOS for Ru sites. The enforced RuN interaction, as well as the higher d-band center (1.37 eV), makes it easier to provide electrons to hydrogen-containing intermediates, and effectively optimized the adsorption/desorption of the reaction intermediate species [15,44,54].
Generally, the adsorption behavior of water molecules (H2O) and protons (H+) determines the inherent activity of the alkaline HER. As shown in Fig. 4g, the three Ru@NC models have similar *OH adsorption energies (G*OH), whereas the adsorption energy of H2O on the Ru1.0@NC model is the strongest among all the Ru@NC models. The strongest H2O adsorption behavior indicates optimal water dissociation on Ru1.0@NC, which is in agreement with the higher Stark tuning rate of the interfacial water measured with ATR-SEIRAS (Fig. 4h). The Gibbs free energy for the *H ad/-desorption step (G*H, Fig. 4h) for the pure Ru1.0 and NC models was determined to be negative at 0.37 eV and 0.56 eV, respectively, indicating very high H adsorption strength. The application of the coating of NC on Ru1.0 to form Ru1.0@NC further decreases G*H to 0.17 eV, suggesting that the electronic effect of the incorporation of NC is mainly responsible for modulating the *H ad-/desorption on the surface of the Ru-based catalyst. Moreover, the G*H of Ru1.0@NC was closer to the thermoneutral value compared with that of Ru0.4@NC (0.22 eV) and Ruslab@NC (0.69 eV), which have different particle sizes. This result is coincident with the Stark tuning rate of the Ru-H bond (Fig. 4c), and also indicates the important role of the size effect on the H ad-/desorption barrier. Overall, the enhanced size and electronic Ru-N interactions effectively optimized the water dissociation and hydrogen ad-/desorption steps to ultimately accelerate the reaction kinetics.
In conclusion, we successfully synthesized a series of Ru@NC heterostructures with size-controlled Ru cores encapsulated in shells of N-doped graphene layers. Notably, Ru@NC-3h, which had the optimal size and electronic properties, demonstrated superior catalytic activity with a high mass activity of 6.68 A/mg, and strong durability. Detailed DFT calculations confirmed that the robust Ru-N hetero-interfacial interactions between the Ru cores and NC shells reinforced the interfacial interaction with H2O and lowered *H adsorption. These predictions were in excellent agreement with the ATR-SEIRAS results, thereby confirming that the enhanced size and electronic Ru-N interactions bolster the inherent reaction kinetics. Additionally, the porous sheet-like carbon framework offered abundant mesopores and macropores, enhanced active-site exposure, and smooth mass transfer. This work introduces a promising approach for the development of noble-metal-based functional materials for applications such as water electrolysis, hydrogen oxidation, and fuel cells.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Min Jie Wang: Writing – original draft, Methodology, Data curation. Jiao Yang: Software, Formal analysis, Data curation. Lishan Peng: Writing – review & editing, Supervision, Funding acquisition. Yongjie Bai: Data curation. Zehui Liu: Data curation. Xiaoliang Yang: Data curation. Huijuan Lu: Investigation. Bingjie Zhou: Investigation. Ningtao Jiang: Investigation. Guoxu He: Supervision. Han-Ming Zhang: Investigation. Liwei Mi: Writing – review & editing, Supervision, Funding acquisition. Yonghui Deng: Formal analysis.
This work was supported by the National Natural Science Foundation of China (Nos. 22209087, 22209186, 22479149), the Key Science and Technology Project of Henan Province (No. 242102231035), Young Backbone Teacher Training Program of Henan Province Undergraduate Colleges (No. [2024](186)), "Double Thousand Plan" of Jiangxi Province (No. jxsq2023101056), Key Research and Development Program of Jiangxi Province (Nos. 20223BBG74004, 20232BBG70003), and Youth Innovation Promotion Association, Chinese Academy of Sciences (No. 2023343).
Supplementary material associated with this article can be found, in the online version, at doi:
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Figure 1 (a–c) SEM and (d-f, h-j, l-n) TEM images of Ru@NC-1h, Ru@NC-3h, and Ru@NC-6h, respectively. (e, i, m) Overlays: Cluster-size distribution fitting curves. (g, k, o, r) Specific d-spacing based on the respective FFT results. (p, q) HRTEM images of Ru@NC-6h. (s) EDS mapping images of Ru@NC-3h.
Figure 2 High-resolution XPS (a) Ru 3p and (b) N 1s spectra. (c) Rectangular columns in violet, gray, red, and magenta represent the mass percentage of elemental Ru and N, Ru-Nrelative, and Ru-N based on the XPS results. Ru-Nrelative accounted for the relative mass percentage of Ru-N in terms of N 1s, and Ru-N was determined by multiplying Ru-Nrelative by the N content.
Figure 3 (a–d) LSV curves, Tafel slope, EIS spectra, and mass activity for the corresponding catalysts. (e) LSV curves before and after the accelerated durability test (ADT) (f) Chronoamperometric curve on industrial 500 mA/cm2 current density. Insets: TEM images after corresponding durability tests. (g) KSCN poisoning test. (h) LSV curves normalized to the ECSA in colored rectangular columns based on the double-layer capacitance Cdl (Fig. S5).
Figure 4 In-situ ATR-SEIRAS spectra recorded from 0.2 V to ~0.25 V for (a) Ru@NC-3h and (b) Ru@NC-9h The Stark tuning rate, determined from changes in the (c) RuH band wavenumber and (d) O-H stretching wavenumber of interfacial water. DFT data on the structural models (Fig. S7). (e) Partial density of states based on Ru1.0@NC and Ru1.0. (f) Charge density difference on Ru1.0@NC (the cyan and yellow regions represent the charge depletion and accumulation space, where the gray and blue spheres stand for Ru and N atoms, respectively). Inset: Bader charge. (g) Calculated H2O and *OH adsorption energy. (h) *H adsorption energy.
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