Exceptional Electrochemical HER Performance with Enhanced Electron Transfer between Ru Nanoparticles and Single Atoms Dispersed on a Carbon Substrate

: Precisely regulating the electronic structures of metal active species is highly desirable for electrocatalysis. However, carbon with inert surface provide weak metal–support interaction, which is insufficient to modulate the electronic structures of metal nanoparticles. Herein, we propose a new method to control the electrocatalytic behavior of supported metal nanoparticles by dispersing single metal atoms on an O-doped graphene. Ideal atomic metal species are firstly computation-ally screened. We then verify this concept by deposition of Ru nanoparticles onto an O-doped graphene decorated with single metal atoms (e.g., Fe, Co, and Ni) for hydrogen evolution reaction (HER). Consistent with theoretical predictions, such hybrid catalysts show outstanding HER performance, much superior to other reported electrocatalysts such as the state-of-the-art Pt/C. This work offers a new strategy for modulating the activity and stability of metal nanoparticles for electrocatalysis processes.


Introduction
Precise tailoring and controlling of the electronic structure of ac atalyst have long been pursued for achieving outstanding catalytic performance. [1] Conventionally,m anipulation of catalysts surface electronic states can be realized by particle size/shape tuning, hetero-element doping,organic ligands microenvironment engineering,a nd substrate interaction. [2] To date,o xide materials have been considered as favorable substrates to support metal nanoparticles,d ue to their strong electronic metal-support interaction (EMSI), [3] which enables the modulation of the electronic structure and catalytic property of metal nanoparticles.Onthe other hand, carbon materials as substrates possess superior electrical conductivity and high chemical stability. [4] However,c arbon materials with arelatively inert surface can only provide weak EMSI with metal nanoparticles,which is often insufficient to effectively steer the electronic structures of the loaded metal nanoparticles.
To address this challenge,s everal strategies have been proposed for optimizing the EMSI of carbon-based electrocatalysts.F or instance,d oping carbon substrates with nonmetal elements can induce the redistribution of electrons or the spin state of the sp 2 conjugated carbon matrix, thus tailoring the valence orbital energy of the active sites on carbon surface. [5] Besides,c arbon surface functionalization using oxygen-containing groups can also introduce extra electronic states near the Fermi level, which alters its surface reactivity. [6] Thee nhanced EMSI results in strong orbital hybridization between metal nanoparticles and carbon surface,which is beneficial for electron transfer at their interface.
Distinct from non-metal dopants or functional groups for surface modification, metallic dopants carry more versatile electronic states,w hich can be more effective for activating the inert carbon surface,t hus providing additional opportunities for tailoring the metal particle-support interaction and their associated catalytic characteristics.Indeed, metal single atoms have been incorporated into carbon frameworks (i.e., single-atom catalysts) to catalyze as erial of electrochemical processes.F or example,avariety of metal atoms or their clusters (e.g., Fe,C o, Ni, Cu, Pt, Ru, and Pd) have been coordinated on carbon surface for electrocatalysis of water splitting,n itrogen fixation, and CO 2 reduction etc. [7] Besides directly serving as the active sites,these metal centers on the carbon surface,asmentioned above,might have the capability for interacting with secondary metal nanoparticles when they are loaded together on ac arbon substrate,w hich is rarely investigated to the best of our knowledge.
Taking Ru nanoparticles on ac arbon substrate as ap rototype system, our density functional theory (DFT) calculations suggest that enhanced charge transfer occurs at the interface between the substrate and Ru nanoparticles,w hen proper single metal atoms (e.g.,F e, Co,and Ni)are doped in the carbon support. This can effectively reshape the electronic structure of the Ru nanoparticles and enables the optimization of electrocatalytic activity. [8] To experimentally validate this prediction, Ru nanoparticles,ani deal alternative for the high-cost Pt, were loaded on an O-doped graphene substrate decorated with non-noble-metal single atoms (Fe, Co,o rN i) to serve as model electrocatalysts.H ydrogen evolution reaction (HER), which is hindered by the excessively strong binding of Ha toms on pristine Ru surface, [9] was chosen as ap rototype reaction. This hybrid catalyst comprised of both Ru nanoparticles and dispersed metal atoms exhibits an ultralow Tafel slope of 22.8 mV dec À1 and an extremely low overpotential of 13 mV at ac urrent density of 10 mA cm À2 , which is the highest performance reported by far to the best of our knowledge.T his clearly confirms that Ru nanoparticles are the active centers and the atomic Co species enhanced HER activity,w hich is well consistent with the DFT calculation results.T hus,this work offers auniversal strategy for precisely tailoring the electronic structure,a ctivity,a nd stability of metal nanoparticles via an on-destructive and flexible route for various electrocatalytic reactions.

Results and Discussion
First, this strategy was conceptually investigated by DFT calculations.A ss hown in Figure 1a,ahighly stable Ru 55 cluster with cuboctahedral geometry and adiameter of about 1.2 nm was adopted and supported on an O-doped graphitic nanosheet with highly dispersed transition metal single atoms (denoted as M 1 @OG,with M = Fe,Co, and Ni). Theshape and size effects on the activity of metal nanoparticles have been extensively explored [10] and thus will not be considered here. Thee lectronic density of states (DOS) in Figure 1d reveals that the decoration of single metal atoms introduces prominent states near the Fermi level, which activate the inert carbon surface and remarkably enhance its interaction with Ru 55 .S pecifically,t he binding strength of Ru 55 on Fe 1 /Co 1 / Ni 1 @OG substrates becomes over 3.5 eV stronger than that of Ru 55 on OG without metal-atom decoration, as demonstrated in Figure 1c.
This enhanced nanocluster-substrate interaction is attributed to the charge transfer from the dispersed metal atoms (Fe, Co,a nd Ni)t oO G ( Figure 1a). Bader charge analysis suggests that these transferred electrons (0.87-0.93 e)a re carried by the Ca toms nearby the Oa toms (electron-rich C sites in Figure 1a), which would occupy the C pz orbitals and break the p conjugation of the graphitic sheet. As aresult, the carbon substrate is more reactive to interact with Ru 55 , gaining more electrons from Ru 55 in comparison with Ru 55 / OG (Table 1), and inducing significant charge redistribution on the nanocluster.A si llustrated in Figure 1b   HER. Therefore,o ur DFT calculations suggest that single metal atoms and Ru nanoparticles can be synergized via their electronic coupling with O-doped graphene substrate,w hich allows delicate modulation of the activity of Ru nanoparticles.
To verify this hypothesis,R u/M@OG analog materials were fabricated by asimple salt-template method (Supporting Information, Scheme S1). Specifically,N aCl was used as the template,and glucose and metal chlorides were employed as carbon and metal source,r espectively.T hese chemicals were firstly dissolved in deionized water, in which process the metal ions spontaneously coordinate with the -OH group of glucose. Afterward, the mixture was freeze-dried. Ultrathin precursor layers,c ontaining metal ions that were immobilized by glucose molecules,w rapped around the NaCl crystals at this stage.Subsequently,the materials were annealed at 700 8 8Cin an inert atmosphere.D uring this process,g lucose was carbonized into graphene nanosheets.P art of Co ions and Ru ions were still coordinated with Oo rCt of orm single atoms,while the rest was reduced and aggregated into metal nanoparticles.F inally,R u/M@OG samples were obtained after removing of NaCl template by washing in water. For comparison, Ru/OG was synthesized via the same procedure as Ru/M@OG except for the absence of non-noble metal salt in the feeding mixture.
Thes tructure of Ru/Co@OG was characterized by transmission electron microscopy (TEM). As shown in Figure 2 and S1, metal nanoparticles are uniformly dispersed on the surface of carbon nanosheets,and the mean diameter of metal clusters is around 1.5 nm (Figure 2a). Thed istance between crystal planes of metal particles is 0.23 nm, which can be assigned to the (100) facet of Ru (Figure 2b). Under the darkfield and high-resolution TEM (HRTEM) observation, alarge number of bright dots,w hich represent single atoms,c an be clearly seen around the metal nanoparticles ( Figure 2c). Linear scanning result (Figure 2d)s hows that most of the particles contain only Ru element and small amount particles contain Ru and Co element. EDS-based elemental mapping images (Figure 2e)present Ru element mainly distributed on the particles,w hile cobalt element appears more randomly and distributed on the whole sample.According to the above results,R u/Co@OG contains metallic Ru particles,asmall amount of Co-doped Ru particles,and single atoms.Furthermore,t he contents of cobalt in the prepared samples were tuned by increasing the cobalt precursor amount in the feeding system to 1.7 times of Ru/Co@OG,a nd the corresponding sample was named Ru/Co 1.7 @OG.T he size of Ru particles on Ru/Co 1.7 @OG is similar to that of Ru/Co@OG ( Figure S2). ForR u/Ni@OG and Ru/Fe@OG,a lmost the same structures with Ru/Co@OG were observed, as shown in Figure S3.
HAADF-STEM images ( Figure S4) show that Ru particles and atoms coexist on Ru/OG.T he metal contents of synthesized catalysts were characterized by an inductively coupled plasma-optical emission spectrometer (ICP-OES) and shown in Table S1. All the samples are with similar Ru content of 6.9 wt.% to 9.0 wt.%. TheC oc ontent in Ru/ Co@OG and Ru/Co 1.7 @OG is 0.9 wt.% and 1.6 wt.%, respectively.T he ratio of Co content in Ru/Co@OG and Ru/ Co 1.7 @OG is the same as in the precursors.
X-ray photoelectron spectroscopy (XPS) was performed to analyze the chemical state of the metal species on Ru/ Co@OG.T he XPS sweep scan confirmed the presence of C, O, Co,and Ru elements on Ru/Co@OG ( Figure S5). Forthe Ru/Co@OG sample,the high-resolution spectrum of Ru 3p3/ 2 ( Figure 3a)c an be deconvoluted into two species at 462.4 and 465.0 eV,which can be attributed to the metallic Ru and oxidized Ru x+ species,r espectively. [11] Meanwhile,t he peaks at 484.5 and 487.0 eV can be assigned to the same species as well. In contrast, for the Ru/OG material, the peak positions of the corresponding Ru 3p3/2 species are located at 462.1 and 464.2 eV.C ompared with the Ru/OG sample,t he binding energy of the metallic Ru species on Ru/Co@OG was positively shifted by approximately 0.3 eV,i ndicating an electron-deficient feature on the Ru nanoparticles for Ru/ Co@OG than Ru/OG.O nt he other hand, in the highresolution O1s spectra of the Ru/Co@OG sample,o nly one  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,w hich indicates ahigher electron density of Ofor Ru/Co@OG than Ru/OG.O nt he basis of these results,i ti sr easonable 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,a ligning well with our theoretical prediction.
Theelectronic structure of Ru species on Ru/Co@OG was further characterized by synchrotron-based X-ray absorption spectroscopy.I nt he 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 alittle higher than that of the Ru foil, indicating the average valence state is higher than 0. Fourier transformed k 3 -weighted c(k)-function of extended X-ray absorption fine structure (EXAFS) spectrum for Ru/ Co@OG shows two characteristic peaks at around 1.47 and 2.45 (Figure 3d). Theformer can be assigned to the RuÀO bond of the atomic Ru coordinated with Oi nt he graphene substrate,and the latter can be assigned to the metallic Ru-Ru bonds in Ru particles. [13] Thew avelet transforms (WT) of EXAFS spectrum for Ru K-edge ( Figure 3e)ofRu/Co@OG shows the characteristic peak of RuÀOb ond at 3.95 that can be assigned to atomic Ru-O species and Ru-Ru bond at 6.1 that can be contributed to Ru particles,w hich further confirms the co-existence of Ru atoms and Ru particles. Compared with the Ru-Ru bond length of Ru foil, the Ru-Ru bond length of the Ru nanoparticles on Ru/Co@OG is slightly larger,which may be due to the small size of Ru nanoparticles than Ru foil. [14] Thee lectronic structure of Co species was also characterized. According to the XANES of Co K-edge ( Figure S6a), the adsorption edge of Ru/Co@OG is between those of Co foil and CoO,verifying the valence state of Co is between 0to + 2. Tw od istinct peaks appear at 1.5 and 2.4 in the EXAFS spectrum for Co K-edge of Ru/Co@OG sample ( Figure S6b). Thef ormer is due to the Co-O sites of atomic Co coordinated with Oo ng raphene,a nd the latter is attributed to the interaction of Co with Ru in Co-doped Ru particles,r espectively.T he structures of Co-O sites and Codoped Ru particles were fitted (Table S2 and Figure S6c), which was well matched with the measured EXAFS of Ru/ Co@OG.F or Co-O sites,C ow as coordinated with 4o xygen atoms.T he above verifies the existence of atomic Co on Ru/ Co@OG. Based on the above characterization results on microstructure and electronic structure of Ru/Co@OG,i ti sc lear that both atomic Co coordinated by four Oa toms and Ru nanoparticles were dispersed on catalyst substrate.A tomic Co on substrate enhances the charge transfer from Ru nanoparticle to substrates,w hich is fully consistent with our theoretical prediction. Thee nhanced EMSI provides aw ide possibility for tailoring the catalytic activity of Ru particles.
Theelectrocatalytic activity of Ru/Co@OG for HER was assessed in a1 .0 MK OH electrolyte in comparison with Ru/ O-G and commercial Pt/C catalysts.A ss hown in the linear scanning voltammetry (LSV) curves (Figure 4a), Ru/Co@OG possessed the highest current density among the materials in the whole potential range.R emarkably,R u/Co@OG also presented an ultralow overpotential of merely 13 mV at the current density of 10 mA cm À2 (h 10 ), which is much lower than that of Ru/OG (48 mV) and Pt/C (31 mV). In the meantime, Co@OG without the Ru nanoparticle loading only exhibited an egligible HER activity ( Figure S8). This clearly demonstrates that the Ru species on Ru/Co@OG is actually the active sites for HER, while Co species just enhance the HER activity of Ru species.
To further confirm this,the HER activity of Ru/Co 1.7 @OG was measured. Thec orresponding LSV curve for Ru/ Co 1.7 @OG shows a h 10 value of 24 mV,w hich is higher than that of Ru/Co@OG (Figure 4b). Thus,H ER activity of Ru/ Co 1.7 @OG is lower than Ru/Co@OG,which may be attributed to the low atomic Co content due to the aggregation induced by over high addition of Co in the synthesis system. Thus,itis clearly shown that atomic Co plays adecisive role in the HER enhancement of Ru/Co@OG. Subsequently,t he HER kinetics of the hybrid catalysts were analyzed ( Figure 4c). As shown, Ru/Co@OG gives the lowest Tafel slope of 22.8 mV dec À1 .I nc ontrast, the Tafel slope for Ru/Co 1.7 @OG,R u/OG,a nd commercial Pt/C are 27.0, 32.4, and 31.4 mV dec À1 ,r espectively,i ndicating the HER over the catalyst follows the Volmer-Tafel mechanism and the Tafel step is rate-limiting. [15g, 16] Furthermore,t he exchange current density (j 0 )w as also calculated based on Tafel plots.The j 0 value of Ru/Co@OG is 2.93 mA cm À2 ,which is about two times higher than that of Pt/C (1.56 mA cm À2 ). To the best of our knowledge,t he Ru/Co@OG catalyst demonstrates the lowest h 10 and Tafel slope values among the representative HER electrocatalysts in alkaline electrolytes (Figure 4d). In short, the low h 10 and Tafel slope,a nd high j 0 clearly illustrate the superior catalytic activity of Ru/Co@OG for HER. Electrochemical impedance spectroscopy (EIS) measurement was conducted to measure the conductivity of those electrocatalysts.The fitted EIS data ( Figure S9) shows asemicircular shape,a nd the diameters of semicircular reflect the charge transfer resistance of electrocatalysts. [17] It clearly confirms that Ru/Co@OG presented the lowest charge transfer resistance and fastest charge transfer than the other electrocatalysts,l eading to the most efficient HER performance.
Assuming all the Ru sites are active,t he turnover frequencies (TOF) values of Ru/Co@OG is 6.2 s À1 at the overpotential of 100 mV,w hich outperforms most of the reported electrocatalysts by far ( Figure S10). Considering the cost of catalysts,t he 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.T he durability of Ru/Co@OG was evaluated in 1MKOH. After 10 000 cycles during the potential window of À0.07 Vt o À0.38 V, the HER activity exhibits an increase in h 10 of only 3mV, indicating the excellent stability of Ru/Co@OG for HER ( Figure S12).
Thes uperior HER activity of Ru/Co@OG than Ru/OG suggests that the atomically dispersed Co species on the Odoped graphene substrate played akey role for the enhancement of HER activity.F or comparison, the HER activity of Ru/Ni@OG and Ru/Fe@OG were also measured under the same condition as that for Ru/Co@OG.R u/Ni@OG shows as imilar HER activity to that of Ru/Co@OG,a nd Ru/ Fe@OG exhibits al ittle inferior activity with h 10 of 28 mV ( Figure S13). However,a ll of Ru/Co@OG,R u/Ni@OG,a nd Ru/Fe@OG show much higher HER activity than that of Ru/ OG.This further confirms that the introduction of atomic Co, Ni, and Fe species on O-doped graphene substrate can significantly enhance the HER activity.
To differentiate the actual contribution of Ru species in the nanoparticle form and those in single-atoms form on the Ru/Co@OG for HER electrocatalysis,poisoning experiments were carried out using ethylenediaminetetraacetic acid disodium (EDTA) and potassium thiocyanide (KSCN) as the complexing reagents.E DTAi sd ominantly coordinated with Ru single atoms,but KSCN can be coordinated with both Ru nanoparticles and single atoms. [18] Thus,E DTAwill suppress the activity of Ru single atoms,while KSCN can deactivate all the Ru species.Asshown in Figure S14, upon the addition of EDTAi nt he electrolyte,t he HER current density on Ru/ Co@OG only slightly decreased compared with that in the pristine electrolyte without EDTA. In sharp contrast, the introduction of 10 mM KSCN in electrolyte would significantly decrease the HER current density on Ru/Co@OG by poisoning Ru particles and increase the h 10 up to 166 mV.The significant activity gap for the electrocatalysis performance of Ru/Co@OG poisoned by EDTAa nd KSCN thus clearly suggests it is the Ru nanoparticles that are the main active sites for HER, rather than the Ru single atoms on the Co@OG.F urthermore,t he effect of the Ru nanoparticles number on Ru/Co@OG for HER activity was explored. The result shows that the HER activity was significantly decreased, as the decrease of the number of Ru nanoparticles ( Figure S14b). Thus,the decisive role of Ru nanoparticles for HER was further proved.
To better understand the enhanced HER performance of Ru/Co@OG,wecalculated the free adsorption energy for H* species (DG H* )and kinetic barrier for water dissociation (E a ), which are the key parameters to characterize HER activity in alkaline media, [13,19] for the Ru 55 nanocluster supported on various substrates.A sd isplayed in Figure 5a,R u 55 /Co 1 @OG shows the optimal adsorption strength towards Hspecies with DG H* of À0.05-0.05 eV,inwhich the active sites are the center of the triangle forming by interfacial Ru atoms ( Figure S15). Besides,w ater dissociation on Ru 55 /Co 1 @OG is found to be exothermic with an enthalpy change of À0.57 eV and involves asmall kinetic barrier E a = 0.66 eV ( Figure S16), close to that of Ru 55 (0.47 eV), and both can occur readily at ambient condition. However,t he freestanding Ru 55 cluster has much stronger binding strength for H* species (DG H* = À0.27 eV), leading to asluggish Volmer-Tafel mechanism for HER in the alkaline media. In comparison, Ru 55 /OG has over-strong binding with Hs pecies with DG H* = À0.15-À0.11 eV (Figure S15), due to the less electron transfer from the Ru 55 nanocluster to the substrate as mentioned at the beginning (Table 1). Furthermore,t oe xclude the possibility of Co doping into the Ru nanoparticles,wealso considered amodel of alloyed nanocluster Co 11 Ru 44 supported on O-doped graphene.C o 11 Ru 44 /OG provides too strong binding with H* species,h aving DG H* = À0.41-À0.12 eV that is unfavorable for the formation of H 2 from the adsorbed H* species ( Figure S20). Besides,single Co or Ru atoms anchored on Odoped graphene has the too weak or too strong binding capability with H* species,y ielding DG H* = 0.60 and À0.38 eV,respectively,which further confirms the synergistic interaction between Co single atoms and Ru nanoparticles on the O-doped graphene substrate for efficient HER electrocatalysis (see Figure S21-S23 for details). Thea ctivities of Ru 55 nanocluster and single metal atoms on the graphenebased substrates show ac lear linear relation with the d band center of transition metal atoms,asevident in Figure 5b and Figure S24. In particular, the support of Co 1 @OG lowers the d band center of Ru 55 nanocluster to À2.55 eV compared to À2.28 and À2.52 eV for freestanding Ru 55 and Ru 55 /OG, respectively,a gain manifesting the effective tuning of the electronic structure of Ru nanoparticles by the single-metalatom-dispersed graphene substrate.A ccording to the d band theory, [20] al ower d band center corresponds to more occupancyo fa ntibonding state between Ru 55 /Co 1 @OG and H* adsorbate,w hich results in weaker but more optimal H* binding strength for hydrogen evolution. Therefore,both the computed electronic structures and catalytic properties of Ru 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 nanoparticles and the substrate,a ffording the precise design of high-efficiency metal nanoparticles for ac ertain reaction.

Conclusion
We demonstrated that atomically dispersed metal species on the carbon substrate can remotely communicate with the supported metal nanoparticles,inducing synergistic electronic coupling with the nanoparticles and enabling the control of their electrocatalytic activity.P roposed by DFT calculations, it is suggested that modification of O-doped graphene substrate with single metal atoms can effectively enhance the EMSI with the Ru nanoparticles supported on it and redistribute the electron of Ru nanoparticles,r esulting in optimized adsorption free energy of H* species on Ru particles and enhanced HER activity.C onfirmed by experimental results,t he fabricated hybrid of Ru nanoparticles dispersed on metal-doped graphene (Ru/M@OG) indeed exhibited enhanced HER activity than that of Ru/OG.I n alkaline electrolyte,R u/Co@OG showcased exceptional performance with an overpotential of 13 mV at the current density of 10 mA cm À2 and ultralow Tafel slopes of 22.8 mV dec À1 ,o utperforming the commercial Pt/C electrocatalyst. Furthermore,t he metal atoms on the O-doped graphene strengthen the binding of Ru nanoparticle on the substrate to prevent their ripening and enhance stability.This work successfully demonstrated the most active Pt-free catalyst for HER, and provided new insights into the modulation mechanism of EMSI between atomic metal doped carbon and the supported metal nanoparticles to rationally design electrocatalysts.