Exceptional Electrochemical HER Performance with Enhanced

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How to cite: Electrocatalysis International Edition: doi.org/10.1002/anie.202103557 German Edition: doi.org/10.1002/ange.202103557 Exceptional Electrochemical HER Performance with Enhanced Electron Transfer between Ru Nanoparticles and Single Atoms Dispersed on a Carbon Substrate Panpan Su+, Wei Pei+, Xiaowei Wang, Yanfu Ma, Qike Jiang, Ji Liang,* Si Zhou,* Jijun Zhao, Jian Liu,* and Gao Qing (Max) Lu

Abstract: Precisely regulating the electronic structures of metal their strong electronic metal–support interaction (EMSI),[3] active species is highly desirable for electrocatalysis. However, which enables the modulation of the electronic structure and carbon with inert surface provide weak metal–support inter- catalytic property of metal nanoparticles. On the other hand, action, which is insufficient to modulate the electronic struc- carbon materials as substrates possess superior electrical tures of metal nanoparticles. Herein, we propose a new method conductivity and high chemical stability.[4] However, carbon to control the electrocatalytic behavior of supported metal materials with a relatively inert surface can only provide weak nanoparticles by dispersing single metal atoms on an O-doped EMSI with metal nanoparticles, which is often insufficient to graphene. Ideal atomic metal species are firstly computation- effectively steer the electronic structures of the loaded metal ally screened. We then verify this concept by deposition of Ru nanoparticles. nanoparticles onto an O-doped graphene decorated with single To address this challenge, several strategies have been metal atoms (e.g., Fe, Co, and Ni) for hydrogen evolution proposed for optimizing the EMSI of carbon-based electro- reaction (HER). Consistent with theoretical predictions, such catalysts. For instance, doping carbon substrates with non- hybrid catalysts show outstanding HER performance, much metal elements can induce the redistribution of electrons or superior to other reported electrocatalysts such as the state-of- the spin state of the sp2 conjugated carbon matrix, thus the-art Pt/C. This work offers a new strategy for modulating the tailoring the valence orbital energy of the active sites on activity and stability of metal nanoparticles for electrocatalysis carbon surface.[5] Besides, carbon surface functionalization processes. using oxygen-containing groups can also introduce extra electronic states near the Fermi level, which alters its surface Introduction reactivity.[6] The enhanced EMSI results in strong orbital hybridization between metal nanoparticles and carbon sur- Precise tailoring and controlling of the electronic struc- face, which is beneficial for electron transfer at their interface. ture of a catalyst have long been pursued for achieving Distinct from non-metal dopants or functional groups for outstanding catalytic performance.[1] Conventionally, manip- surface modification, metallic dopants carry more versatile ulation of catalysts surface electronic states can be realized electronic states, which can be more effective for activating by particle size/shape tuning, hetero-element doping, organic the inert carbon surface, thus providing additional opportu- ligands microenvironment engineering, and substrate inter- nities for tailoring the metal particle-support interaction and action.[2] To date, oxide materials have been considered as their associated catalytic characteristics. Indeed, metal single favorable substrates to support metal nanoparticles, due to atoms have been incorporated into carbon frameworks (i.e.,

[*] Dr. P. Su,[+] Dr. Y. Ma, Prof. J. Liu Prof. J. Liu State Key Laboratory of Catalysis, Dalian Institute of Chemical DICP-Surrey Joint Centre for Future Materials, University of Surrey Physics, Chinese Academy of Sciences Guildford, Surrey, GU2 7XH (UK) 457 Zhongshan Road, Dalian 116023 (China) E-mail: [email protected] E-mail: [email protected] Prof. G. Q. Lu W. Pei,[+] Prof. S. Zhou, Prof. J. Zhao University of Surrey Key Laboratory of Materials Modification by Laser, Ion and Electron Guildford, Surrey, GU2 7XH (UK) Beams (Dalian University of Technology), Ministry of Education [+] These authors contributed equally to this work. Dalian 116024 (China) Supporting information and the ORCID identification number(s) for E-mail: [email protected] the author(s) of this article can be found under: Dr. X. Wang, Prof. J. Liang https://doi.org/10.1002/anie.202103557. Key Laboratory for Advanced Ceramics and Machining Technology of 2021 The Authors. Angewandte Chemie International Edition Ministry of Education, Tianjin University published by Wiley-VCH GmbH. This is an open access article under Tianjin 300350 (China) the terms of the Creative Commons Attribution License, which E-mail: [email protected] permits use, distribution and reproduction in any medium, provided Dr. Q. Jiang the original work is properly cited. Division of Energy Research Resources, Dalian Institute of Chemical Physics, Chinese Academy of Sciences 457 Zhongshan Road, Dalian 116023 (China)

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Ü These are not the final page numbers! Angewandte Research Articles Chemie single-atom catalysts) to catalyze a serial of electrochemical processes. For example, a variety of metal atoms or their clusters (e.g., Fe, Co, Ni, Cu, Pt, Ru, and Pd) have been coordinated on carbon surface for electrocatalysis of water [7] splitting, nitrogen fixation, and CO2 reduction etc. Besides directly serving as the active sites, these metal centers on the carbon surface, as mentioned above, might have the capability for interacting with secondary metal nanoparticles when they are loaded together on a carbon substrate, which is rarely investigated to the best of our knowledge. Taking Ru nanoparticles on a carbon substrate as a prototype system, our density functional theory (DFT) calculations suggest that enhanced charge transfer occurs at the interface between the substrate and Ru nanoparticles, when proper single metal atoms (e.g., Fe, Co, and Ni) are doped in the carbon support. This can effectively reshape the electronic structure of the Ru nanoparticles and enables the optimiza- tion of electrocatalytic activity.[8] To experimentally validate this prediction, Ru nanoparticles, an ideal alternative for the high-cost Pt, were loaded on an O-doped graphene substrate decorated with non-noble-metal single atoms (Fe, Co, or Ni) to serve as model electrocatalysts. Hydrogen evolution Figure 1. a) Atomic structures of Ru55 supported on O-doped graphene reaction (HER), which is hindered by the excessively strong with dispersed metal atoms obtained by DFT calculations. The C, O, binding of H atoms on pristine Ru surface,[9] was chosen as metal, and Ru atoms are shown in gray, red, purple, and turquoise a prototype reaction. This hybrid catalyst comprised of both colors, respectively. b) Bader charge analysis projected to Ru55 cluster. The positive (negative) charge indicates the electron loss (gain) from Ru nanoparticles and dispersed metal atoms exhibits an the Ru atoms. c) Binding energies (Ebind, left axis) of Ru55/M1@OG and ultralow Tafel slope of 22.8 mVdecÀ1 and an extremely low freestanding Ru55, and its corresponding adsorption free energy of H* À2 overpotential of 13 mV at a current density of 10 mAcm , species (DGH*, right axis). d) Density of states (DOS) for Ru55/ which is the highest performance reported by far to the best of M1@OG. The occupied states were shadowed. The dashed line our knowledge. This clearly confirms that Ru nanoparticles indicates the Fermi level for each system. are the active centers and the atomic Co species enhanced HER activity, which is well consistent with the DFT calculation results. Thus, this work offers a universal strategy suggests that these transferred electrons (0.87–0.93e) are for precisely tailoring the electronic structure, activity, and carried by the C atoms nearby the O atoms (electron-rich C stability of metal nanoparticles via a non-destructive and sites in Figure 1a), which would occupy the C pz orbitals and flexible route for various electrocatalytic reactions. break the p conjugation of the graphitic sheet. As a result, the

carbon substrate is more reactive to interact with Ru55,

gaining more electrons from Ru55 in comparison with Ru55/ Results and Discussion OG (Table 1), and inducing significant charge redistribution

on the nanocluster. As illustrated in Figure 1b, for Ru55/

First, this strategy was conceptually investigated by DFT M1@OG, the Ru atoms close to the interfacial region show calculations. As shown in Figure 1a, a highly stable Ru55 a more positive charge density of 0.20–0.22e (compared to cluster with cuboctahedral geometry and a diameter of about 0.13e for Ru55/OG), indicative of a weakened binding 1.2 nm was adopted and supported on an O-doped graphitic strength with H* species and possibly higher activity for nanosheet with highly dispersed transition metal single atoms (denoted as M @OG, with M = Fe, Co, and Ni). The shape and 1 Table 1: Key parameters of Ru nanoclusters supported on various size effects on the activity of metal nanoparticles have been graphene-based substrates.[a] extensively explored[10] and thus will not be considered here. Model e CT CT D [eV] The electronic density of states (DOS) in Figure 1d reveals Ebind d 1 GH* [eV] [eV] (e) (e) that the decoration of single metal atoms introduces prom- inent states near the Fermi level, which activate the inert Ru55 – À2.28 – 0.03 À0.27 carbon surface and remarkably enhance its interaction with Ru55/G 6.98 À2.26 1.41 0.19 À0.23 Ru55/OG 9.88 À2.52 1.58 0.13 À0.11 Ru55. Specifically, the binding strength of Ru55 on Fe1/Co1/ Ru55/Fe1@OG 14.44 À2.59 1.65 0.20 0.02 Ni1@OG substrates becomes over 3.5 eV stronger than that of Ru55/Co1@OG 13.78 À2.55 1.63 0.21 0.05

Ru55 on OG without metal-atom decoration, as demonstrated Ru55/Ni1@OG 13.41 À2.60 1.61 0.22 0.04 in Figure 1c. Co11Ru44/OG 6.62 À2.14 1.56 À0.15 À0.41 This enhanced nanocluster-substrate interaction is attrib- [a] Binding energy (Ebind), d band center (ed), total charge transfer from uted to the charge transfer from the dispersed metal atoms nanocluster to substrate (CT), and the local charge density (CT1) and free

(Fe, Co, and Ni) to OG (Figure 1a). Bader charge analysis adsorption energy for H* species (DGH*) on the active sites for HER.

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HER. Therefore, our DFT calculations suggest that single images (Figure 2e) present Ru element mainly distributed on metal atoms and Ru nanoparticles can be synergized via their the particles, while cobalt element appears more randomly electronic coupling with O-doped graphene substrate, which and distributed on the whole sample. According to the above allows delicate modulation of the activity of Ru nanoparticles. results, Ru/Co@OG contains metallic Ru particles, a small To verify this hypothesis, Ru/M@OG analog materials amount of Co-doped Ru particles, and single atoms. Further- were fabricated by a simple salt-template method (Supporting more, the contents of cobalt in the prepared samples were Information, Scheme S1). Specifically, NaCl was used as the tuned by increasing the cobalt precursor amount in the template, and glucose and metal chlorides were employed as feeding system to 1.7 times of Ru/Co@OG, and the corre-

carbon and metal source, respectively. These chemicals were sponding sample was named Ru/Co1.7@OG. The size of Ru

firstly dissolved in deionized water, in which process the metal particles on Ru/Co1.7@OG is similar to that of Ru/Co@OG ions spontaneously coordinate with the -OH group of glucose. (Figure S2). For Ru/Ni@OG and Ru/Fe@OG, almost the Afterward, the mixture was freeze-dried. Ultrathin precursor same structures with Ru/Co@OG were observed, as shown in layers, containing metal ions that were immobilized by Figure S3. glucose molecules, wrapped around the NaCl crystals at this HAADF-STEM images (Figure S4) show that Ru par- stage. Subsequently, the materials were annealed at 7008Cin ticles and atoms coexist on Ru/OG. The metal contents of an inert atmosphere. During this process, glucose was synthesized catalysts were characterized by an inductively carbonized into graphene nanosheets. Part of Co ions and coupled plasma-optical emission spectrometer (ICP-OES) Ru ions were still coordinated with O or C to form single and shown in Table S1. All the samples are with similar Ru atoms, while the rest was reduced and aggregated into metal content of 6.9 wt.% to 9.0 wt.%. The Co content in Ru/

nanoparticles. Finally, Ru/M@OG samples were obtained Co@OG and Ru/Co1.7@OG is 0.9 wt.% and 1.6 wt.%, re- after removing of NaCl template by washing in water. For spectively. The ratio of Co content in Ru/Co@OG and Ru/

comparison, Ru/OG was synthesized via the same procedure Co1.7@OG is the same as in the precursors. as Ru/M@OG except for the absence of non-noble metal salt X-ray photoelectron spectroscopy (XPS) was performed in the feeding mixture. to analyze the chemical state of the metal species on Ru/ The structure of Ru/Co@OG was characterized by trans- Co@OG. The XPS sweep scan confirmed the presence of C, mission electron microscopy (TEM). As shown in Figure 2 O, Co, and Ru elements on Ru/Co@OG (Figure S5). For the and S1, metal nanoparticles are uniformly dispersed on the Ru/Co@OG sample, the high-resolution spectrum of Ru 3p3/ surface of carbon nanosheets, and the mean diameter of metal 2 (Figure 3a) can be deconvoluted into two species at 462.4 clusters is around 1.5 nm (Figure 2a). The distance between and 465.0 eV, which can be attributed to the metallic Ru and crystal planes of metal particles is 0.23 nm, which can be oxidized Rux+ species, respectively.[11] Meanwhile, the peaks assigned to the (100) facet of Ru (Figure 2b). Under the dark- at 484.5 and 487.0 eV can be assigned to the same species as field and high-resolution TEM (HRTEM) observation, a large well. In contrast, for the Ru/OG material, the peak positions number of bright dots, which represent single atoms, can be of the corresponding Ru 3p3/2 species are located at 462.1 and clearly seen around the metal nanoparticles (Figure 2c). 464.2 eV. Compared with the Ru/OG sample, the binding Linear scanning result (Figure 2d) shows that most of the energy of the metallic Ru species on Ru/Co@OG was particles contain only Ru element and small amount particles positively shifted by approximately 0.3 eV, indicating an contain Ru and Co element. EDS-based elemental mapping electron-deficient feature on the Ru nanoparticles for Ru/ Co@OG than Ru/OG. On the other hand, in the high- resolution O1s spectra of the Ru/Co@OG sample, only one

Figure 3. a),b) High-resolution XPS spectra of the Ru 3p (a) and O 1s (b) for Ru/Co@OG and Ru/OG. c) Fourier transformed k3-weighted Figure 2. a),b) Low magnification TEM and HRTEM images. c(k)-function of the EXAFS spectra. d) XANES spectra for Ru K-edge c) HAADF-STEM image. d) Linear scan of Co and Ru element. e) Co, and e) the wavelet transforms for the k3-weighted EXAFS signals of Ru

Ru, and overlay element mapping for Ru/Co@OG electrocatalysts. foil, RuO2 and Ru/Co@OG samples.

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Ü These are not the final page numbers! Angewandte Research Articles Chemie oxygen species can be found at 532.4 eV, which belongs to the C-O-C configuration in the graphene substrate (Figure 3b).[12] Compared with the value of Ru/OG (533.1 eV), this peak position of Ru/Co@OG negatively shifted by 0.7 eV, which indicates a higher electron density of O for Ru/Co@OG than Ru/OG. On the basis of these results, it is reasonable to deduce that the atomic Co species on OG could promote the electron transfer from Ru particles to Co-O co-doped graphene, leading to an enhanced EMSI between Ru particles and substrate, aligning well with our theoretical prediction. The electronic structure of Ru species on Ru/Co@OG was further characterized by synchrotron-based X-ray absorption spectroscopy. In the X-ray absorption near edge structure (XANES) of Ru (Figure 3c), it can be found that the K-edge position of Ru in Ru/Co@OG is a little higher than that of the Ru foil, indicating the average valence state is higher than 0. Fourier transformed k3-weighted c(k)-function of extended

X-ray absorption fine structure (EXAFS) spectrum for Ru/ À2 Figure 4. a) LSV curves. b) The overpotential at 10 mAcm (h10). Co@OG shows two characteristic peaks at around 1.47 and c) Tafel plots of Ru-based catalysts and commercial Pt/C. d) Compar-

2.45 (Figure 3d). The former can be assigned to the RuÀO ison of h10 and tafel slope of Ru/Co@OG with representatively bond of the atomic Ru coordinated with O in the graphene reported HER electrocatalysts.[11a, 13,15] substrate, and the latter can be assigned to the metallic Ru-Ru bonds in Ru particles.[13] The wavelet transforms (WT) of EXAFS spectrum for Ru K-edge (Figure 3e) of Ru/Co@OG possessed the highest current density among the materials in shows the characteristic peak of RuÀO bond at 3.95 that the whole potential range. Remarkably, Ru/Co@OG also can be assigned to atomic Ru-O species and Ru-Ru bond at presented an ultralow overpotential of merely 13 mV at the À2 6.1 that can be contributed to Ru particles, which further current density of 10 mAcm (h10), which is much lower than confirms the co-existence of Ru atoms and Ru particles. that of Ru/OG (48 mV) and Pt/C (31 mV). In the meantime, Compared with the Ru-Ru bond length of Ru foil, the Ru-Ru Co@OG without the Ru nanoparticle loading only exhibited bond length of the Ru nanoparticles on Ru/Co@OG is slightly a negligible HER activity (Figure S8). This clearly demon- larger, which may be due to the small size of Ru nanoparticles strates that the Ru species on Ru/Co@OG is actually the than Ru foil.[14] active sites for HER, while Co species just enhance the HER The electronic structure of Co species was also charac- activity of Ru species. terized. According to the XANES of Co K-edge (Figure S6a), To further confirm this, the HER activity of Ru/Co1.7@OG the adsorption edge of Ru/Co@OG is between those of Co was measured. The corresponding LSV curve for Ru/ foil and CoO, verifying the valence state of Co is between 0 to Co1.7@OG shows a h10 value of 24 mV, which is higher than + 2. Two distinct peaks appear at 1.5 and 2.4 in the that of Ru/Co@OG (Figure 4b). Thus, HER activity of Ru/

EXAFS spectrum for Co K-edge of Ru/Co@OG sample Co1.7@OG is lower than Ru/Co@OG, which may be attributed (Figure S6b). The former is due to the Co-O sites of atomic to the low atomic Co content due to the aggregation induced Co coordinated with O on graphene, and the latter is by over high addition of Co in the synthesis system. Thus, it is attributed to the interaction of Co with Ru in Co-doped Ru clearly shown that atomic Co plays a decisive role in the HER particles, respectively. The structures of Co-O sites and Co- enhancement of Ru/Co@OG. doped Ru particles were fitted (Table S2 and Figure S6c), Subsequently, the HER kinetics of the hybrid catalysts which was well matched with the measured EXAFS of Ru/ were analyzed (Figure 4c). As shown, Ru/Co@OG gives the Co@OG. For Co-O sites, Co was coordinated with 4 oxygen lowest Tafel slope of 22.8 mVdecÀ1. In contrast, the Tafel atoms. The above verifies the existence of atomic Co on Ru/ slope for Ru/Co1.7@OG, Ru/OG, and commercial Pt/C are Co@OG. 27.0, 32.4, and 31.4 mVdecÀ1, respectively, indicating the Based on the above characterization results on micro- HER over the catalyst follows the Volmer-Tafel mechanism structure and electronic structure of Ru/Co@OG, it is clear and the Tafel step is rate-limiting.[15g,16] Furthermore, the that both atomic Co coordinated by four O atoms and Ru exchange current density (j0) was also calculated based on À2 nanoparticles were dispersed on catalyst substrate. Atomic Tafel plots. The j0 value of Ru/Co@OG is 2.93 mAcm , which Co on substrate enhances the charge transfer from Ru is about two times higher than that of Pt/C (1.56 mAcmÀ2). To nanoparticle to substrates, which is fully consistent with our the best of our knowledge, the Ru/Co@OG catalyst demon- theoretical prediction. The enhanced EMSI provides a wide strates the lowest h10 and Tafel slope values among the possibility for tailoring the catalytic activity of Ru particles. representative HER electrocatalysts in alkaline electrolytes

The electrocatalytic activity of Ru/Co@OG for HER was (Figure 4d). In short, the low h10 and Tafel slope, and high j0 assessed in a 1.0 M KOH electrolyte in comparison with Ru/ clearly illustrate the superior catalytic activity of Ru/Co@OG O-G and commercial Pt/C catalysts. As shown in the linear for HER. scanning voltammetry (LSV) curves (Figure 4a), Ru/Co@OG

Electrochemical impedance spectroscopy (EIS) measure- (Figure S14b). Thus, the decisive role of Ru nanoparticles for ment was conducted to measure the conductivity of those HER was further proved. electrocatalysts. The fitted EIS data (Figure S9) shows a semi- To better understand the enhanced HER performance of circular shape, and the diameters of semicircular reflect the Ru/Co@OG, we calculated the free adsorption energy for H* [17] charge transfer resistance of electrocatalysts. It clearly species (DGH*) and kinetic barrier for water dissociation (Ea), confirms that Ru/Co@OG presented the lowest charge trans- which are the key parameters to characterize HER activity in [13,19] fer resistance and fastest charge transfer than the other alkaline media, for the Ru55 nanocluster supported on

electrocatalysts, leading to the most efficient HER perfor- various substrates. As displayed in Figure 5a, Ru55/Co1@OG mance. Assuming all the Ru sites are active, the turnover frequencies (TOF) values of Ru/Co@OG is 6.2 sÀ1 at the overpotential of 100 mV, which outperforms most of the reported electrocatalysts by far (Figure S10). Considering the cost of catalysts, the price activity of HER for Ru/Co@OG and Pt/C was calculated and shown in Figure S11. At the overpotential of 100 mV, the price activity for Ru/Co@OG is 15 times greater than that of the commercial Pt/C. The durability of Ru/Co@OG was evaluated in 1 M KOH. After 10000 cycles during the potential window of À0.07 V to Figure 5. a) Free energy diagrams for hydrogen evolution at zero À0.38 V, the HER activity exhibits an increase in h10 of only potential and pH 0 on various catalysts by DFT calculation. b) DGH* as 3 mV, indicating the excellent stability of Ru/Co@OG for a function of the d band center of transition metal atoms (The inset is HER (Figure S12). a schematic illustration of bond formation between the supported The superior HER activity of Ru/Co@OG than Ru/OG metal nanoparticle and adsorbed H* species). suggests that the atomically dispersed Co species on the O- doped graphene substrate played a key role for the enhancement of HER activity. For comparison, the HER activity of shows the optimal adsorption strength towards H species with

Ru/Ni@OG and Ru/Fe@OG were also measured under the DGH* of À0.05–0.05 eV, in which the active sites are the center same condition as that for Ru/Co@OG. Ru/Ni@OG shows of the triangle forming by interfacial Ru atoms (Figure S15).

a similar HER activity to that of Ru/Co@OG, and Ru/ Besides, water dissociation on Ru55/Co1@OG is found to be

Fe@OG exhibits a little inferior activity with h10 of 28 mV exothermic with an enthalpy change of À0.57 eV and involves

(Figure S13). However, all of Ru/Co@OG, Ru/Ni@OG, and a small kinetic barrier Ea = 0.66 eV (Figure S16), close to that

Ru/Fe@OG show much higher HER activity than that of Ru/ of Ru55 (0.47 eV), and both can occur readily at ambient

OG. This further confirms that the introduction of atomic Co, condition. However, the freestanding Ru55 cluster has much

Ni, and Fe species on O-doped graphene substrate can stronger binding strength for H* species (DGH* = À0.27 eV), significantly enhance the HER activity. leading to a sluggish Volmer-Tafel mechanism for HER in the

To differentiate the actual contribution of Ru species in alkaline media. In comparison, Ru55/OG has over-strong

the nanoparticle form and those in single-atoms form on the binding with H species with DGH* = À0.15–À0.11 eV (Fig-

Ru/Co@OG for HER electrocatalysis, poisoning experiments ure S15), due to the less electron transfer from the Ru55 were carried out using ethylenediaminetetraacetic acid diso- nanocluster to the substrate as mentioned at the beginning dium (EDTA) and potassium thiocyanide (KSCN) as the (Table 1). Furthermore, to exclude the possibility of Co complexing reagents. EDTA is dominantly coordinated with doping into the Ru nanoparticles, we also considered a model

Ru single atoms, but KSCN can be coordinated with both Ru of alloyed nanocluster Co11Ru44 supported on O-doped [18] nanoparticles and single atoms. Thus, EDTA will suppress graphene. Co11Ru44/OG provides too strong binding with

the activity of Ru single atoms, while KSCN can deactivate all H* species, having DGH* = À0.41–À0.12 eV that is unfavor-

the Ru species. As shown in Figure S14, upon the addition of able for the formation of H2 from the adsorbed H* species EDTA in the electrolyte, the HER current density on Ru/ (Figure S20). Besides, single Co or Ru atoms anchored on O- Co@OG only slightly decreased compared with that in the doped graphene has the too weak or too strong binding

pristine electrolyte without EDTA. In sharp contrast, the capability with H* species, yielding DGH* = 0.60 and introduction of 10 mM KSCN in electrolyte would signifi- À0.38 eV, respectively, which further confirms the synergistic cantly decrease the HER current density on Ru/Co@OG by interaction between Co single atoms and Ru nanoparticles on

poisoning Ru particles and increase the h10 up to 166 mV. The the O-doped graphene substrate for efficient HER electro- significant activity gap for the electrocatalysis performance of catalysis (see Figure S21–S23 for details). The activities of

Ru/Co@OG poisoned by EDTA and KSCN thus clearly Ru55 nanocluster and single metal atoms on the graphene- suggests it is the Ru nanoparticles that are the main active based substrates show a clear linear relation with the d band sites for HER, rather than the Ru single atoms on the center of transition metal atoms, as evident in Figure 5b and

Co@OG. Furthermore, the effect of the Ru nanoparticles Figure S24. In particular, the support of Co1@OG lowers the d

number on Ru/Co@OG for HER activity was explored. The band center of Ru55 nanocluster to À2.55 eV compared to

result shows that the HER activity was significantly de- À2.28 and À2.52 eV for freestanding Ru55 and Ru55/OG, creased, as the decrease of the number of Ru nanoparticles respectively, again manifesting the effective tuning of the

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Ü These are not the final page numbers! Angewandte Research Articles Chemie electronic structure of Ru nanoparticles by the single-metal- Conflict of interest atom-dispersed graphene substrate. According to the d band theory,[20] a lower d band center corresponds to more The authors declare no conflict of interest. occupancy of antibonding state between Ru55/Co1@OG and H* adsorbate, which results in weaker but more optimal H* Keywords: electrocatalysis · electronic structures · binding strength for hydrogen evolution. Therefore, both the hydrogen evolution · metal nanoparticles · metal– computed electronic structures and catalytic properties of Ru carbon interactions nanoparticles on O-doped graphene substrates demonstrate that the decoration of single metal atoms on the substrate can significantly enhance the EMSI between the supported metal [1] a) L. Liu, A. Corma, Chem. Rev. 2018, 118, 4981 – 5079; b) A. nanoparticles and the substrate, affording the precise design Fukuoka, P. L. Dhepe, Chem. Rec. 2009, 9, 224 – 235. of high-efficiency metal nanoparticles for a certain reaction. [2] a) S. J. Tauster, Acc. Chem. Res. 1987, 20, 389 – 394; b) W. Zhu, R. Michalsky, . Metin, H. Lv, S. Guo, C. J. Wright, X. Sun, A. A. Peterson, S. Sun, J. Am. Chem. Soc. 2013, 135, 16833 – Conclusion 16836; c) Y. Li, W. Shen, Chem. Soc. Rev. 2014, 43, 1543 – 1574; d) X. Xie, Y. Li, Z.-Q. Liu, M. Haruta, W. Shen, Nature 2009, 458, 746 – 749; e) X. Ren, M. Guo, H. Li, C. Li, L. Yu, J. Liu, Q. Yang, We demonstrated that atomically dispersed metal species Angew. Chem. Int. Ed. 2019, 58, 14483 – 14488; Angew. Chem. on the carbon substrate can remotely communicate with the 2019, 131, 14625 – 14630; f) D. Gao, Y. Zhang, Z. Zhou, F. Cai, X. supported metal nanoparticles, inducing synergistic electronic Zhao, W. Huang, Y. Li, J. Zhu, P. Liu, F. Yang, G. Wang, X. Bao, coupling with the nanoparticles and enabling the control of J. Am. Chem. Soc. 2017, 139, 5652 – 5655; g) Y. Zhang, X. 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&&&& www.angewandte.org 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021, 60,2–9 Ü

Ü These are not the final page numbers! Angewandte Research Articles Chemie Research Articles

Electrocatalysis Carbon-based substrates with an inert surface provide weak metal–support P. Su, W. Pei, X. Wang, Y. Ma, Q. Jiang, interactions that are insufficient to effec- J. Liang,* S. Zhou,* J. Zhao, J. Liu,* tively modulate the electronic structures G. Q. Lu &&&& — &&&& of the loaded metal nanoparticles. Here we show that atomic metal species on the Exceptional Electrochemical HER carbon substrate can remotely commu- Performance with Enhanced Electron nicate with the supported metal nano- Transfer between Ru Nanoparticles and particles, inducing synergistic electronic Single Atoms Dispersed on a Carbon coupling with the nanoparticles and ena- Substrate bling the control of their electrocatalytic activity.

Angew. Chem. Int. Ed. 2021, 60, 2 – 9 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org &&&& These are not the final page numbers! Ü Ü