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Highly Efficient and Selective Generation of Ammonia and Hydrogen on a Graphdiyne-based Catalyst Lan Hui, Yurui Xue, Huidi Yu, Yuxin Liu, Yan Fang, Chengyu Xing, Bolong Huang, and Yuliang Li J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03004 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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Journal of the American Chemical Society

Highly Efficient and Selective Generation of Ammonia and Hydrogen on a Graphdiyne-based Catalyst Lan Hui,† Yurui Xue,*,† Huidi Yu,† Yuxin Liu,† Yan Fang,† Chengyu Xing,† Bolong Huang,‡ and Yuliang Li*,†,§ †Institute

of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China.

‡Department

of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China. §University of Chinese Academy of Sciences, Beijing 100049, PR China. ABSTRACT: The emergence of zero-valent atom catalysts has been highly attractive for catalytic science. For many years, scientists have explored the stability of zero-valent atom catalysts and demonstrated their unique properties. Here, we describe an atom catalyst (AC) with atomically dispersed zero-valent molybdenum atoms on graphdiyne (Mo0/GDY) with a high mass content of Mo atoms (up to 7.5 wt.%) that was synthesized via a facile and scalable process. The catalyst shows both excellent selectivity and activity in the electrochemical reduction of nitrogen (ECNRR) and in the hydrogen evolution reaction (HER) in aqueous solutions at room temperature and pressure. It is noted that this catalyst is the first bifunctional AC for highly efficient and selective ammonia and hydrogen generation. The catalytic process of our catalyst is well-understood, the structure is defined and the performance is excellent, providing a solid foundation for the generation and application of the new generation of catalysts.

1. INTRODUCTION Scientists have sought to produce a zero-valent metal atom catalyst (AC), and such a zero-valent catalyst can enable an accurate understanding of the phenomena and processes at the atomic level that occur on the surface and interface in catalytic reactions. This catalyst is the key for promoting the progress of catalytic science that may lead to a series of catalysts with unique performance and trigger the catalyst industrial revolution. Highly selective catalysts are always desired and are the basis for solving major problems in catalytic science.1 The Selective and active electrochemical reduction of nitrogen (or water) into carbon-free fuel, NH3 (or H2), has great promise for addressing the ever-increasing energy and environmental concerns.2 It is well-known that the electrochemical nitrogen reduction reaction (ECNRR) usually proceeds at potentials very close to that of the hydrogen evolution reaction (HER).3 The competition between the adsorption of N2 (N2 + *→*N2) in the ECNRR and the adsorption of H (H+ + e− +*→*H) in the HER on catalytic active sites has been a focus of recent research.3a Recently, some electrocatalysts have been reported to improve N2-adsorption and activation processes,4 but the NH3 yield and Faradaic efficiency of the ECNRR remain at very low levels, and these catalysts are quite inert to the HER.3b,3d-f,5 To date, highly active electrocatalysts have not been reported.6 These limitations strongly hinders the development of high-efficiency electrocatalysis and the utilization of energy and resources. Therefore, we must create new ideas and new structures of the catalysts from the source in order to accelerate the development and utilization of new energy and resources. Individual zero-valent atoms are ideal catalytic model systems due to their unique and attractive properties (such as electronic structure, high catalytic activity and selectivity).7 Well-separated individual metal atoms have been demonstrated to have improved catalytic performances in various reactions,

including oxygen reduction reaction,8 HER,9 oxygen evolution reaction,9a,10 CO oxidization,11 and CO2 activation.12 However, due to the complex and harsh synthetic conditions, the conventional single-atom catalysts have unknown chemical and electronic structures that are not conducive to the identification of active sites and the determination of the catalytic mechanism. Recently, by using graphdiyne13 (GDY, a new 2D carbon material comprising of sp-/sp2-cohybridized carbon) as a supporting material, our group first synthesized zero-valent meal atom catalysts (ACs) that show excellent activity and stability in electrocatalysis.7c Compared with the conventional single-atom catalysts, ACs have well-defined chemical/electronic structures and known valence states. These features provide a unique opportunity to identify the active sites during catalysis and therefore determine the catalytic mechanism at the atomic level and also afford the best model system for the fundamental and conceptual study of electrocatalysts. However, to the best of our knowledge, there have been no reports specifically focusing on the ECNRR-HER bifunctional electrode based on zero-valence metal atom catalysts. Here, we report a new class of zero-valent molybdenum GDY-based ACs (denoted as Mo0/GDY) that can directly function as efficient ECNRR and HER electrocatalysts. According to the experimental and theoretical results, the Mo0/GDY AC has a well-defined chemical/electronic structure and valence state. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) clearly shows that the Mo atoms are anchored close to the corner of the alkyne ring. Our density functional theory (DFT) calculations reveal that the strong electron-rich GDY environment preserves Mo0 by a strong p-d coupling. During the ECNRR process, fast reversible Mo-C1 charge transfer not only suppresses the forward HER process but also ensures an extremely high rate of

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N-hydrogenation associated with the thermodynamic trend. Experimentally, Mo0/GDY exhibited very high selectivity and NH3 yield rates at room temperature and pressure. The stability toward ECNRR is excellent in both neutral (0.1 M Na2SO4) and acidic (0.1 M HCl) electrolytes. Very interestingly, Mo0/GDY can also drive efficient HER in non-N2-staturated solutions. For example, in 0.5 M H2SO4, Mo0/GDY exhibits higher HER activity than that of the commercial 20wt.% Pt/C and conventional electrocatalysts.

Figure 1. Electronic structures and activities for ECNRR catalysis. (a) 3D real spatial orbital contour plots for HOMO and LUMO near the EF for Mo0/GDY. (b) PDOSs evolution of site-dependent energetic preference N2-fixation on the Mo0/GDY system. (c) PDOSs of the s, p, and d orbitals from Mo-(C1, C2) bonding motifs from GDY-Mo. (d) PDOSs of porbitals of C0, and C1-C4. The C0 is the C-site of the benzene ring and C1-C4 are labeled sequentially following the C0 along the C-chain bonding with Mo. (e) PDOSs comparison for the adsorption/desorption of NH3 on the Mo0/GDY system. (f) PDOSs comparison for the H-adsorption on the Mo0/GDY system. (g) ECNRR energetic pathway on the GDY-Mo. (h) Formation energies of H-adsorbate on C-sites of Mo0/GDY. (i) H-chemisorption energies on C-sites of Mo0/GDY. 2. RESULTS AND DISCUSSION 2.1. Theoretical calculations. We started with computational calculations aimed to understand and identify the active sites beneficial for the high activity of Mo0/GDY.14 For greater context of the computational models, we have also carried out the additional comparison between different anchoring sites in order to support the presented results (Figure S1, Supporting Information). The bonding and anti-bonding orbitals near the Fermi level (EF) demonstrate that the Mo-site is the active site. The Mo-site transfers the charge and modifies the charge distribution from the C-sites, playing the role as an electron-rich center for active electron-transfer and bonding for stable

adsorption for HER, N2-fixation and ECNRR (Figure 1a). We further analyze the favorable electronic configuration for optimal N2-adsorption. Compared with different N2-adsorption energies, the corresponding projected partial density of states (PDOSs) reflect different behaviors. Generally, the most stable adsorbed N2 state should lead to the highest N-2pσ* (antibonding) state and the deepest localized N-2pσ state. However, both Mo and C can play the role of the bonding site for adsorption. Therefore, we find that there is a direct correction between the N2-adsorption preference and the N-2pσ→2pσ* splitting gap. The energetically preferred adsorption configuration is Mo-N≡N, which leads to the N-2pσ state being localized at EV of -6.8 eV (blue shaded) and the anti-bonding state remaining high at EV of +3.6 eV. The most energetically unfavorable configuration is Mo-N=N-C, which suppresses the N-2pσ state toward even deeper energy levels with the N-2pπ long-pair state simultaneously participating in the N-C bonding. This phenomenon leads to a nearly five times larger effect for the most stable configuration (Mo-N≡N) and shows the large splitting gap contrast of 10.4 eV (Mo-N≡N) and 2.65 eV (MoN=N-C) (Figure 1b). We further interpret the prominent electronic activities given by Mo0/GDY. From the PDOSs of Mo0/GDY, the Mo-4d and C-p orbitals dominantly control the electron-transfer activities. The two dominant C-bonding and anti-bonding orbitals pin the Mo-4d bands in the middle crossover the EF. This phenomenon will not only robustly preserve the Mo-4d valence electronic states for the various catalytic steps of ECNRR but also indicates that the Mo-site can easily accumulate the active electrons from the C-sites for further efficient electron transfer between Mo and N (Figure 1c). PDOSs of individual C-sites have also been decomposed and analyzed. We find the C1 site clearly shows the bonding and anti-bonding splitting, confirming the stable Mo-C1 interaction. We find the C1 and C2 sites possess intermediate levels of p-electron population, promoting the electron-transfer pathway between the Mo and GDY system (Figure 1d). The electronic variations of the adsorption/desorption of NH3 on Mo0/GDY are compared. For NH3 adsorption, the Mo-4d band is lower and is slightly electron-rich. The N-2pπ long-pair peak is also lower and suppressed by the Coulomb potentials from the Mo-4d electrons. For the desorption, the Mo-4d band recovers while both N-2pσ and N-2pπ state lie higher, indicating itinerant behavior above Mo0/GDY surface (Figure 1e). Further electronic variation of pure H-adsorption is also demonstrated. We find that only Mo-4d bands are clearly modified even with H-adsorption on the C1 site. This result confirms that the electron-transfer between Mo and C1 sites is reversible. The C1 electronic character from PDOS is almost unchanged with the anti-bonding state pinned at EV+4.5 eV. This result implies that both the Mo and C1 sites can simultaneously stabilize H adsorption and N2-fixation (Figure 1f). We next address the energetic preference of the ECNRR. The Mo-site dominated preferable pathway (associative pathway) is considered. The system has the potential of U=-0.71 V and shows purely downhill energetic trend. The combination of the Mo- and C-sites promotes selectivity via enhancing the electron transfers. Between these two parts, the Mo-site shows prominent N2-fixation in energy gain, and the C-sites along the C-chain contribute favorable H-adsorption. For U=-0.71 V, the overall energy gain is -4.36 eV where the NH3-desorption encounters a small barrier of 0.36 eV for each NH3. For the U=0 V, the path shows a downhill trend starting from the initial N2-


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Figure 2. Synthesis and structural configuration evolution of catalysis process. adsorption to the formation of [*NH=NH+4(H++e-)]. The subsequent hydrogenation step [*NH-NH2+3(H++e-)] determines the overall reaction barrier of 1.29 eV. The formation of [*NH2-NH2+2(H++e-)] also shows an uphill step albeit with a lower barrier of 0.47 eV. Therefore, the efficient desorption of H at the C1 sites determines the barrier acting as the potential determining step (PDS) for the N-intermediate [NH=NH] hydrogenation (Figure 1g). To provide supporting information for the reaction pathway, we also conducted additional calculations on another possible ECNRR mechanism (distal pathway) that deliver a slightly larger onset potential (Figure 1g). In this mechanism, the rate-determining step is the adsorption of the second hydrogen that leads to the applied potential of U=-0.96 V for the realization of the ECNRR. Therefore, the associative mechanism is more preferred than the distal mechanism during the ECNRR, significantly enhancing the presented calculation results for a more comprehensive understanding of the ECNRR reaction pathways. The manuscript shows updated free energies diagram of reaction pathways. We also benchmark the site-dependent H-adsorption energies and confirm that both C1 and C2 are the energetically preferred adsorption sites. Additional chemisorption energies of H show that C1 is the optimal active site for H-adsorption for proton-electron charge exchange (Figure 1h and 1i). Further examinations of the local structural configurations of the ECNRR process demonstrate that the homopolar N-bond dissociation occurs at the step of [NH2---NH3+(H++e-)], preventing the overbinding effect of N-intermediate. This result agrees with the trend that the intermediate nitrogen-nitrogen bonding (i.e., N=N and N-N) variation ensures the energetic compensation when the HER is suppressed or H-desorption from the local active adsorption sites occurs (Figures S2 and S3, Supporting Information).

dimensional (3D) GDY nanosheets array electrode (Figure S4, Supporting Information). The as-synthesized 3D GDY electrode was immersed in a solution of molybdenum pentachloride. After a facile solvothermal reduction process, the Mo0/GDY ACs were generated (Figure 2, see Method for details). Inductively coupled plasma mass spectrometry (ICPMS) demonstrates the presence of Mo in the obtained samples with an average loading of 7.5 wt.%. Scanning electron microscopy (SEM, Figure 3a; Figure S5, Supporting Information), transmission electron microscopy (TEM, Figure 3b; Figure S6, Supporting Information), high-resolution TEM (HRTEM, Figure 3c) and X-ray diffraction (XRD, Figure S7, Supporting Information) results show no formation of Mo nanoparticles. Energy-dispersive X-ray spectroscopy reveals that the Mo atoms are uniformly dispersed in Mo0/GDY (Figure 3d). The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Mo0/GDY (Figures 3e-g) clearly show isolated and uniformly dispersed bright dots. These bright dots represent the heavy element atoms on graphdiyne, confirming the successful anchoring of Mo atoms on GDY. These HAADF-STEM images were recorded at different regions of different batches of samples (Figure S8, Supporting Information, for additional images), demonstrating the high repeatability of the Mo0/GDY catalysts. Figures 3h-j show the clear views of the local coordination environment of the Mo0 atoms in the sample, providing evidence for the possible anchoring configurations of the Mo atoms on GDY.

2.2 Synthesis and structural characterization. In light of the findings described above, we prepared the atom catalyst Mo0/GDY through a facile one-step method with the three-



Figure 3. Structural characterization. (a) SEM, (b) TEM and (c) high-resolution TEM images of Mo0/GDY. (d) STEM and corresponding element mapping images of C (red) and Mo (green). (e-g) HAADF-STEM images of the freshly prepared Mo0/GDY samples. (h-j) Different configurations of the individual Mo atoms anchored on GDY structure. (k) EXAFS spectra of Mo0/GDY (red line) and Mo foil (black line) at the Mo K-edge. (l) Normalized Mo K-edge XANES spectra of Mo0/GDY and Mo foil (inset: calculated first derivative curves). X-ray adsorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopy measurements were carried out to verify that Mo atoms are anchored in the sample. As shown in Figure 3k, EXAFS shows only a single prominent peak centered at 1.2 Å due to Mo-C interactions, and no peak corresponding to Mo-Mo contribution is observed. This result verifies the existence of isolated Mo atoms only. An examination of Figure 3l shows that the XANES spectra of Mo0/GDY and Mo reference are almost the same, and the pre-edge derivative of XANES of Mo0/GDY catalysts are similar to that of metallic Mo, confirming the zero-valence state of the Mo atoms. The quality of the samples was determined by X-ray photoelectron spectroscopy (XPS, Figure S9, Supporting Information) and Raman spectra (Figure S10, Supporting Information). Together, the observation of the sp2 to sp carbon atom ratio of 0.5 and the observation of the C≡C unit vibration band at 2105 cm1 in Raman spectrum reveal the structural integrity of GDY after Mo atoms decoration. The intensity ratio of the D and G bands increases from 0.70 to 0.76, suggesting the formation of more defective sites in the sample. In addition, the top of the valence-band (TVB) of Mo0/GDY is closer to the Fermi level compared with pristine GDY, indicating that Mo0/GDY is more conductive than pure GDY (Figure S11, Supporting Information). These properties are beneficial for enhancing the catalytic activities of Mo0/GDY.

Figure 4. ECNRR performance of Mo0/GDY. (a) UV-Vis absorption spectra of the 0.1 M Na2SO4 electrolytes after ECNRR at different potentials for 2 h. (b) FEs and (c) YNH3 at different applied potentials in 0.1 M Na2SO4. (d) YNH3 and FEs of NH3 production of different batches of Mo0/GDY samples. (e) UV-vis adsorption spectra of Mo0/GDY tested under N2 (red line) and Ar (black line) saturated electrolytes. (f) Amounts of NH3 generated with pure GDY and the Mo0/GDY electrode after 2-h electrolysis at −1.2 V under ambient conditions. (g) FEs and (h) YNH3 at different applied potentials in 0.1 M HCl. (i) Amounts of NH3 generated with pure GDY and the Mo0/GDY electrode after 2-h electrolysis at −0.1 V under ambient conditions. 2.3 Electrocatalytic Activities of Mo0/GDY. The electrocatalytic performance characteristics of Mo0/GDY were evaluated for the N2 reduction reaction in different electrolyte solutions (0.1 M Na2SO4 and 0.1 M HCl) under ambient conditions in a two-compartment cell separated by a Nafion 211 membrane (Figure S4, Supporting Information). The NH3 yield was determined by the spectrophotometric method (Figure 4a and Figure S12, Supporting Information). The Faradaic efficiencies (FEs, Figure 4b) and NH3 yield rates (YNH3, Figure 4c) were obtained at different applied potentials in 0.1 M Na2SO4. The ECNRR performance of Mo0/GDY increases with the increase in the negative potential to -1.2 V versus the saturated calomel electrode (SCE), at which point Mo0/GDY exhibits the highest YNH3 and FE. With a further increase in the negative potentials, the YNH3 and FEs of ECNRR decrease rapidly. This result reveals that the ECNRR dominates at low negative potentials, whereas the HER becomes dominant at more negative potentials, as indicated by the calculation results (Figure 1) and the polarization curves (Figure S13, Supporting Information). As shown in Figure 4d, the YNH3 of the Mo0/GDY catalyst varies in the range of 113.4-145.4 μg h1mgcat.1, and the FE changes from 15.2% to 21.0%. These values are significantly larger than those reported for the ECNRR electrocatalysts such as vanadium nitride B4C/CPE,5d Au/TiO2,16 and a-Au/CeOx–RGO17 in previous work (Table S1, Supporting Information), although the ECNRR performances of the Mo0/GDY is close to that of Ru/NC catalyst18 and BiNCs.19 The facts that no NH3 can be detected in Ar-saturated electrolyte (Figure 4e) and GDY exhibits negligible activities for the ECNRR (Figure 4f) indicate that the measured NH3 is produced mainly from the reduction of the N2 gas. 15N-labeling experiments were further conducted (Figure S14, Supporting


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Journal of the American Chemical Society Information). The results showed that NH3 is produced by electrochemical reduction of N2 on the Mo0/GDY catalyst. No hydrazine (N2H4) is detected in the N2-saturated electrolyte (Figure S15, Supporting Information), demonstrating the high catalytic selectivity of Mo0/GDY toward the ECNRR. The XPS spectra (Figure S16, Supporting Information), XAFS spectra (Figure S17, Supporting Information), and HAADF-STEM images (Figure S18, Supporting Information) obtained after the ECNRR in Na2SO4 electrolyte reveal that the Mo atoms are still found in the zero-valence state and are atomically dispersed on the GDY structure after the ECNRR tests. The XPS C 1s spectrum also shows that there is no structural variation in the GDY structure (Figure S16c, Supporting Information). These findings suggest the robust stability of the Mo0/GDY during the electrochemical testing process. Next, we measured the ECNRR performance characteristics of Mo0/GDY in 0.1 M HCl electrolyte (Figures 4g, and 4h and Figures S19 and S20, Supporting Information). Compared to pristine GDY, Mo0/GDY shows a better catalytic activity with the higher YNH3 of 2.0 μgNH3 h1mgcat.1 and FE of 15.6% at 0.1 V versus SCE (Figure 4i). In addition to the high activity, Mo0/GDY also exhibits an excellent catalytic selectivity with no N2H4 production (Figure S21, Supporting Information). Figure S22 suggests that no NH3 is detected in Ar-saturated 0.1 M HCl, implying that the formation of ammonia is due to the electrochemical reduction of nitrogen. This result was further confirmed by the 15N-labeling experiments (Figure S14b, Supporting Information). The HAADF-STEM images (Figure S23, Supporting Information) XPS spectra (Figure S24, Supporting Information) and XAFS spectra (Figure S17, Supporting Information) of the Mo0/GDY samples measured after ECNRR in Na2SO4 electrolyte confirm that the Mo atoms on GDY are stable and well-preserved during the electrochemical testing. Mo0/GDY shows a poorer ECNRR catalytic activity in 0.1 M HCl compared to that in 0.1 M Na2SO4, which might be due to its better HER behavior in acidic solution (discussed in detail in the following section).

Figure 5. HER performance characteristics. (a) Polarization curves and (b) mass activities of Mo0/GDY and 20 wt% Pt/C. (c) Overpotential at 10 mA cm2 of the catalysts. (d) Timedependent current density curve of Mo0/GDY recorded at 20 mA cm2. Encouraged by our previous work, the graphdiyne-based ACs are found to be efficient in the HER process. Naturally, we suspected that the Mo0/GDY catalyst might have a high catalytic performances toward HER. As shown in Figure 5a,

Mo0/GDY shows a higher HER activity with the smaller overpotential of 48 mV at 10 mA cm2 than 20wt.% Pt/C (59 mV). Compared with Mo0/GDY, the pure GDY has lower HER activity (Figure S25, Supporting Information). This result confirms that the Mo0/GDY leads to enhanced HER activity, in accordance with the predicted GH (Figure S26, Supporting Information). Mo0/GDY shows a small Tafel slope of 33 mV dec1 (Figure S27, Supporting Information), which is slightly larger than that for Pt/C. This result might be due to the competitive adsorption of H* and OH* on the Mo-C active sites at 0 V (RHE). Figure 5b shows the mass activities of Mo0/GDY and 20 wt.% Pt/C. At the overpotential of 0.2 V, the mass activity of Mo0/GDY (15.10 A mgmetal–1) is approximately 2.6 times larger than that of commercial Pt/C (5.89 A mgmetal–1). It is noteworthy that compared with the conventional single atom catalysts, Mo0/GDY showed an overwhelming advantage in the HER activity (Figure 5c). The accelerated degradation measurements and the chronoamperometric test were performed to evaluate the long-term stability of the Mo0/GDY (Figure 5d). The Mo0/GDY shows negligible variation in current density over 100 h of continuous electrolysis, revealing its reliable stability during the HER process. Detailed characterizations (XAFS: Figure S17; XPS: Figure S28; HAADF-STEM images: Figure S29, Supporting Information) of the Mo0/GDY samples after HER measurements further confirm the structural stability of Mo0/GDY in the HER process. The electrochemical impendence spectra of Mo0/GDY were measured and analyzed using the R(QR)(QR) equivalent circuit model (Figure S30 and Table S2, Supporting Information). Both the solution resistance (Rs, 3.14 Ω) and charge transfer resistance (Rct, 1.299 Ω) are much lower than those of 20 wt.% Pt/C (Rs=12.57 Ω, Rct=42.68 Ω), suggesting the favorable HER kinetics of Mo0/GDY. Furthermore, the layered double capacitance of Mo0/GDY was obtained from the cyclic voltammetry method (Figure S31, Supporting Information), which is more than 6-times larger than that of pristine GDY (1.9 mF cm-2 vs 0.3 mF cm-2; Figure S32, Supporting Information). This sharp contrast shows that the atomically anchored Mo0 on GDY can create numerous active sites, enlarging the electrochemical active surface area and finally boosting the HER kinetics.20 3. CONCLUSIONS In summary, we have successfully synthesized the ECNRR/HER bifunctional zero-valent atom catalyst on GDY, Mo0/GDY that exhibits highly accurate chemical/electronic structures, and we have determined its valence state. GDY allows the firm anchoring of Mo atoms in the alkyne ring. The obtained catalyst has a high Mo atoms content (7.5 wt.%) and shows excellent catalytic activities and selectivities for electrochemical reduction of N2 into NH3 at ambient conditions in acidic and neutral electrolytes. For example, in 0.1 M Na2SO4, the NH3 yield rate and Faradaic efficiency can reach 145.4 μg h1mgcat.1 and >21%, respectively, which are larger than the corresponding values of the previously reported ECNRR catalysts. Furthermore, Mo0/GDY samples can also function as an efficient HER electrocatalyst with better HER activity and stability than the commercial 20 wt.% Pt/C in acidic conditions.

ASSOCIATED CONTENT



Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Figures provide additional information on the samples. Tables give the catalytic performances and EIS parameters of samples.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Nature Science Foundation of China (21790050, 21790051 and 21771156), the National Key Research and Development Project of China (2016YFA0200104), the Key Program of the Chinese Academy of Sciences (QYZDY-SSW-SLH015), the Early Career Scheme (ECS) fund (Grant No. PolyU 253026/16P) from the Research Grant Council (RGC) in Hong Kong, and the National Postdoctoral Program for Innovative Talents (BX20190332). We thank the XAFS station (beam line 1W1B) of the Beijing Synchrotron Radiation Facility. We would like to thank Professor Jun Luo (Director of Center for Electron Microscopy, Vice Dean of Institute for New Energy Materials & Low-Carbon Technologies, Tianjin University of Technology, China) for his assistance in the HAADF imaging measurements.

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