Angewandte Eine Zeitschrift der Gesellschaft Deutscher Chemiker Chemie www.angewandte.de Akzeptierter Artikel Titel: Rational Design of Single Mo Atoms Anchored on N-doped Carbon for Effective Hydrogen Evolution Reaction Autoren: Yadong Li, Wenxing Chen, Jiajing Pei, Chunting He, Jiawei Wan, Hanlin Ren, Youqi Zhu, Yu Wang, Juncai Dong, Shubo Tian, Weng-Chon Cheong, Siqi Lu, Lirong Zheng, Xusheng Zheng, Wensheng Yan, Zhongbin Zhuang, Chen Chen, Qing Peng, and Dingsheng Wang Dieser Beitrag wurde nach Begutachtung und Überarbeitung sofort als "akzeptierter Artikel" (Accepted Article; AA) publiziert und kann unter Angabe der unten stehenden Digitalobjekt-Identifizierungsnummer (DOI) zitiert werden. Die deutsche Übersetzung wird gemeinsam mit der endgültigen englischen Fassung erscheinen. Die endgültige englische Fassung (Version of Record) wird ehestmöglich nach dem Redigieren und einem Korrekturgang als Early-View-Beitrag erscheinen und kann sich naturgemäß von der AA-Fassung unterscheiden. Leser sollten daher die endgültige Fassung, sobald sie veröffentlicht ist, verwenden. Für die AA-Fassung trägt der Autor die alleinige Verantwortung. Zitierweise: Angew. Chem. Int. Ed. 10.1002/anie.201710599 Angew. Chem. 10.1002/ange.201710599 Link zur VoR: http://dx.doi.org/10.1002/anie.201710599 http://dx.doi.org/10.1002/ange.201710599 10.1002/ange.201710599 Angewandte Chemie COMMUNICATION Rational Design of Single Mo Atoms Anchored on N-doped Carbon for Effective Hydrogen Evolution Reaction Abstract: Highly efficient electrochemical hydrogen evolution reaction (HER) provides a promising pathway to resolve energy and environment problems. Herein, we designed an electrocatalyst with single Mo atoms (Mo-SAs) supported on N-doped carbon with outstanding HER performance. The structure of the catalyst was probed by aberration-corrected scanning transmission electron microscopy (AC-STEM) and X-ray absorption fine structure (XAFS) spectroscopy, indicating the formation of Mo-SAs anchored with one nitrogen atom and two carbon atoms (Mo1N1C2). Importantly, the Mo1N1C2 catalyst displayed much more excellent activity compared with Mo2C and MoN, and better stability than commercial Pt/C. Density functional theory (DFT) calculation revealed that the unique structure of Mo1N1C2 moiety played a crucial effect to improve the HER performance. This work opens up new opportunities for the preparation and application of highly active and stable Mo-based HER catalysts. Hydrogen is an attractive substitute for traditional fossil fuels, and electrochemical hydrogen evolution reaction (HER) is thought to be a method to generate hydrogen effectively, in which the catalysts play a dominant role.  For HER, Pt-based nanomaterials are considered as the best and practical catalysts, with low overpotential, small Tafel slope and high exchange current density, but blocked in their rare source, fancy price and poor electrochemical stability. Recently, a variety of earthabundant Pt-free catalysts have been intensively researched [*] Dr. W. Chen[+], Dr. J. Wan, H. Ren, Dr. Y. Zhu, S. Tian, W. Cheong, Prof. C. Chen, Prof. Q. Peng, Prof. D. Wang, Prof. Y. Li Department of Chemistry, Tsinghua University, Beijing 100084 China E-mail: firstname.lastname@example.org email@example.com J. Pei[+], S. Lu, Prof. Z. Zhuang State Key Lab of Organic-Inorganic Composites and Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China Dr. C. He[+] MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China Dr. Y. Wang Shanghai Synchrontron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Science, Shanghai 201800, China Dr. J. Dong, Dr. L. Zheng, Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China Dr. X. Zheng, Prof. W. Yan National Synchrotron Radiation Laboratory (NSRL), University of Science and Technology of China, Hefei, Anhui 230029, China [+] These authors contributed equally to this work. Supporting information for this article is given via a link at the end of the document. and developed as HER catalysts under acidic or alkaline conditions. Among them, Mo-based catalysts (MoxC, MoxN, etc.) have attracted extensive attention due to their excellent catalytic performance. Although a number of efforts have been made, it is still extremely necessary and great challenge to understand the nature of the catalytic activity at atomic level for rational design of highly efficient Mo-based catalysts. Recently, single metal atom catalysts (SACs) are of great interest in heterogeneous catalysis, mainly due to their high atom utilization and unique catalytic performance. Furthermore, SACs open a new door for us to explore catalytic process at atomic scale. However, to the best of our knowledge, the preparation and investigation of single Mo atoms (Mo-SAs) catalyst hasn’t been implemented yet. Herein, for the first time, we successfully achieved synthesis of Mo-SAs catalyst, with the help of chitosan, which is one of the most abundant biopolymers in nature. The local atomic structure of the catalyst is confirmed as Mo1N1C2 moiety by employing synchrotron-radiation-based X-ray absorption spectroscopy (SRXAS). The as-prepared catalyst exhibited highly efficient activity for HER in alkaline condition, with a low overpotential (onset overpotential ηonset = 13 mV, and overpotential at a current density of -10 mA cm-2 η10 = 132 mV) and a small Tafel slope (90 mV dec-1) in 0.1 M KOH solution, which is much better than that of Mo2C and MoN. The Mo-SAC is among the excellent non-Pt HER catalysts in alkaline media (Table S1). Additionally, the catalyst also displayed excellent stability, with negligible activity degradation after 1000 CV cycles. DFT calculation reveals the different structure-activity relationships between Mo1N1C2 and Mo2C or MoN. Moreover, this synthetic strategy for Mo-SAs can also be applied to other metals (Cu, Pd, Pt, etc.), which gives new opportunities for the research of SACs. The single Mo atom catalyst (Mo-SAC) was prepared via combining templated and pyrolysis method, using sodium molybdate and chitosan (hydrophilic and with fruitful uncoordinated amine groups) as precursors (Figure S1). The synthesis detail was described in the supplement materials. The morphology of the catalyst was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM images in Figure S2 showed that the obtained Mo-SAC possessed amounts of spherical voids after removal of SiO2 templates, leading to a porous N-doped carbon framework. Brunauer-Emmett-Teller (BET) absorptiondesorption isotherms also demonstrated that the catalyst contained a highly open porous structure, with specific surface area of 583 m2/g. It was emphasized that 3D porous structure was of advantage to the accessibility of active sites and the mass transport during the electrochemical catalytic process.  The TEM and HRTEM images in Figure 1a and 1c further showed that the obtained foam was fabricated by ultrathin Ndoped carbon walls. Moreover, small particles or clusters of Mo This article is protected by copyright. All rights reserved. Accepted Manuscript Wenxing Chen+, Jiajing Pei+, Chunting He+, Jiawei Wan, Hanlin Ren, Youqi Zhu, Yu Wang, Juncai Dong, Shubo Tian, Weng-Chon Cheong, Siqi Lu, Lirong Zheng, Xusheng Zheng, Wensheng Yan, Zhongbin Zhuang, Chen Chen, Qing Peng, Dingsheng Wang, Yadong Li 10.1002/ange.201710599 Angewandte Chemie COMMUNICATION peak located at 1.3 Å, which was mainly attributed to the scattering of Mo-N or Mo-C coordination. Moreover, the scattering peaks derived from Mo-Mo coordination were not observed, in comparison with Mo foil, Mo2C and MoN. These demonstrated the atomic dispersion of Mo in the N-doped carbon substrate. Figure 2. (a) XANES and (b) FT-EXAFS curves of the Mo1N1C2 at Mo K-edge, (c) WT-EXAFS of the Mo1N1C2, Mo foil, Mo2C and MoN, (d) FT-EXAFS fitting curves of the Mo1N1C2 at Mo K-edge (FT range: 2.0-10.5 Å-1, fitting range: 0.52.5 Å), the insert is the k space fitting curve of the Mo1N1C2, (e) atomic structure model of the Mo1N1C2. Figure 1. (a) TEM image of the Mo1N1C2, the insert is SAED pattern, (b) EDS maps revealing the homogeneous distribution of Mo and N on the carbon support, (c) HRTEM image and (d) AC-STEM image of the Mo1N1C2. Synchrotron-radiation-based hard X-ray absorption fine structure (XAFS) measurements were performed to further investigate the structure of Mo-SAC at atomic level. Generally speaking, the average oxidation state of Mo species can be described through the absorption threshold position of Mo Kedge. In the XANES curves (Figure 2a as well as Figure S5), the position of the sample located between Mo2C and MoN, indicating the oxidation state of Mo was between the two references. As we know, Fourier transform (FT) is a fundamental step for the data extraction and interpretation of EXAFS spectra. The FT-EXAFS of the sample was illustrated in Figure 2b. We found that the sample exhibited only one obvious FT Wavelet transform (WT) is thought to be a wonderful supplement for FT. And owing to the powerful resolution in both k and R spaces, WT-EXAFS was recently employed to probe the atomic dispersion of SACs. The Mo K-edge WTEXAFS of the sample was illustrated in Figure 2c, as well as the comparison of the q–space magnitudes for FEFF-calculated k3weighted EXAFS paths in Figure S6. The WT contour plots of the sample displayed only one intensity maximum at 7.2 Å-1, corresponding to the Mo-N/C coordination. Additionally, the WT signals related to Mo-Mo contribution weren’t detected, compared with the WT plots of Mo foil, Mo2C and MoN. These further demonstrated the formation of the Mo-SAs. Quantitative EXAFS fitting was performed to extract the structural parameters, and the fitting results were exhibited in Figure 2d, Figure S7 and Table S2. The first shell of the central atom Mo displayed a coordination number of four. Based on the EXAFS fitting, it was thought that the Mo-SAs were atomically anchored in the Ndoped porous carbon matrix, three-fold coordinated by one N atom and two C atoms (Mo1N1C2). Additionally, one O2 molecule This article is protected by copyright. All rights reserved. Accepted Manuscript species weren’t observed in the field of view. The selected area electron diffraction (SAED) pattern (the insert in Figure 1a) exhibited the poor crystallinity of the N-doped carbon frame, as well as the Powder X-ray diffraction (PXRD) pattern in Figure S3a. Energy-dispersive X-ray spectroscopy (EDS) in a scanning transmission electron microscope (STEM) suggested the uniform distributions of Mo and N on the carbon frame (Figure 1b). The atomic dispersion of Mo on the N-doped carbon substrate could be monitored directly by STEM, equipped with a probe spherical aberration corrector. The Mo-SAs were confirmed by monodispersed bright dots marked with red circles for better observation (Figure 1d). X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption near-edge structure (XANES) spectroscopy were applied to probe the valence state and electronic structure of N and C in the sample (Figure S4). The Mo content in the catalyst was 1.32 wt %, according to the coupled plasma optical emission spectrometry (ICP-OES) analysis. 10.1002/ange.201710599 Angewandte Chemie COMMUNICATION Figure 3. Electrocatalytic HER performance of the Mo1N1C2 in alkaline condition (0.1 M KOH). (a) HER polarization curves for the Mo1N1C2 in comparison with Mo2C, MoN and 20% Pt/C. (b) Overpotential for Mo1N1C2 compared with Mo2C, MoN and 20% Pt/C. (c) Tafel plots and (d) Nyquist plots of the Mo1N1C2, Mo2C, MoN and 20% Pt/C. (e) TOF value of the Mo1N1C2. (f) The long-term durability measurements of the Mo1N1C2. The polarization curves were recorded initially and after 1000 CV cycles between 0 and -0.25 V (vs RHE) at 50 mV S-1. We investigated the electrochemical HER activity of the Mo1N1C2 at alkaline condition (in 0.1 M KOH solution). A typical three-electrode setup was adopted to conduct the electrocatalytic measurements (Figure S9). For comparison, the HER performance of Mo2C, MoN and Pt/C (20 wt %) was also recorded under the same condition. The polarization curves with iR correction (the specific percentage of the correction is 100 %) were exhibited in Figure 3a and 3b, respectively. The Mo 1N1C2 displayed an onset overpotential (ηonset) of 13 mV, which was much lower than that of the counterparts (107 mV for Mo2C and 121 mV for MoN). Additionally, the Mo1N1C2 needed only an overpotential (η10) of 132 mV to reach the current density of 10 mA cm-2. The polarization curves of the sample and references at different loading weight were also displayed in Figure S10S12. Tafel slope is usually employed to investigate the reaction kinetics during HER process, and a smaller Tafel slope is more advantageous for HER catalysis. As shown in Figure 3c, the Tafel slope of the Mo1N1C2 was as low as 90 mV per decade, much smaller than that of Mo2C (102 mV per decade) and MoN (163 mV per decade). The Nyquist plots (Figure 3d) indicated that the charge transfer resistance of the Mo1N1C2 was lower in comparison with Mo2C or MoN, suggesting the Mo1N1C2 had a faster charge-transfer capacity during HER. The turnover frequency (TOF) was then investigated (Figure 3e). The TOF value of the Mo1N1C2 at thermodynamic potential (0 V vs RHE) was calculated to be 0.047 S-1 by using exchange current density. The values of Mo1N1C2 at the overpotentials of 50, 100 and 150 mV were 0.148, 0.465 and 1.46 S-1, respectively. Besides the HER activity, the stability is another critical factor to evaluate an electrocatalyst. To survey the stability of the Mo1N1C2 in alkaline environment, long-term cyclic voltammetry (CV) and chronopotentiometry measurements were conducted. In Figure 3f, the polarization curves measured before and after 1000 CV cycles ranging from 0 V to -0.25 V (vs RHE) showed negligible degradation for both the HER overpotential and the cathodic current density, indicating that the Mo 1N1C2 was of superior stability against long-period electrocatalytic processes, agreeing well with the results obtained from the chronometry curves at the current density of 10 mA cm-2 (Figure S13-S14). The structural stability of the Mo1N1C2 sample was further confirmed by SEM, TEM, HAADF-STEM and XAFS measurements (Figure S15-S18 and Table S2). The high durability of the Mo-SAC is due to the unique structure of the Mo1N1C2 moiety, leading to strong interaction between active Mo atoms and the N-doped carbon matrix. When tested in acidic media (in 0.5 M H2SO4 solution), the Mo1N1C2 catalyst also exhibited improved activity (Figure S19-S25). The catalyst displayed a low overpotential (η onset = 48 mV, and η10 = 154 mV) and a small Tafel slope (86 mV dec -1). The performance is among the good activity for electrocatalysts based on non-noble metal materials (Table S3). Moreover, the catalyst also exhibited excellent stability toward HER at acidic condition (Figure S26S28). Figure 4. (a) Gibbs free energy for H* adsorption on different catalysts of Mo1N1C2, Mo2C and MoN. (b) The calculated DOS of the Mo1N1C2. The black dashed line denotes the position of the Fermi level. The adsorption free energy of H* (ΔGH*) is a key descriptor of the HER activity in both alkaline and acidic conditions, and generally the closer to zero, the better.[1b, 10] ΔGH* largely depends on the geometric and electronic structures of the catalysts. The ΔGH* values of the Mo catalysts were investigated using the density functional theory (DFT) method, as shown in Figure 4a. Six possible active sites of the Mo 1N1C2 This article is protected by copyright. All rights reserved. Accepted Manuscript was considered to adsorb on the Mo-SA (as illustrated in Figure 2e). The best-fit EXAFS results of Mo foil, Mo2C and MoN were exhibited in Figure S8 and Table S2. Combining the morphological information with the corresponding structure characterizations, the presence of individual Mo atoms was well confirmed. 10.1002/ange.201710599 Angewandte Chemie COMMUNICATION Additionally, the developed strategy was also effective in the synthesis of other M-SA (M=Cu, Pd, Pt, etc.) materials. The typical EDS maps, HAADF-STEM images, XANES and EXAFS curves of the as prepared Cu-SA (Figure S34-S36), Pd-SA (Figure S37-S39) and Pt-SA (Figure S40-S42) were exhibited, and the EXAFS fitting results were also listed in Table S4. We found that the above single metal atoms (Cu, Pd and Pt) in the N-doped carbon frames were normally three-fold coordinated by N and C atoms, ignoring the adsorbed O2 molecules on the single metal atom. These demonstrated the generality of the synthetic strategy. In conclusion, we successfully prepared a catalyst with MoSAs dispersed on N-doped carbon, with high catalytic activity and stability for hydrogen evolution reaction. The structure of the catalyst was characterized by electronic microscopy and XAFS measurements. The unique catalytic properties of Mo1N1C2 for HER was further investigated by DFT calculations. This work opens up new opportunities for the preparation and application of high-efficiency and stable Mo-based HER catalysts. measurement. Chun-Ting He is thankful to the National Postdoctoral Program for Innovative Talents (BX201600195). Keywords: single Mo atom • N-doped carbon • hydrogen evolution reaction • electrocatalyst            Acknowledgements This work was supported by China Ministry of Science and Technology under Contract of 2016YFA (0202801), and the National Natural Science Foundation of China (21521091, 21390393, U1463202, 21471089, 21671117, 11405252). We thank the BL14W1 in SSRF, BL10B and BL12B in NSRL for XAS  a) S. Chu, A. Majumdar, nature 2012, 488, 294-303; b) Y. Zheng, Y. Jiao, M. Jaroniec, S. Z. Qiao, Angew. Chem., Int. Ed. 2015, 54, 52-65; c) S. Lu, Z. Zhuang, Sci. China Mater. 2016, 59, 217-238. a) M. Gong, D.-Y. Wang, C.-C. Chen, B.-J. Hwang, H. Dai, Nano Res. 2016, 9, 28-46; b) I. Roger, M. A. Shipman, M. D. Symes, Nat. Rev. Chem. 2017, 1, 0003; c) J. Wang, F. Xu, H. Jin, Y. Chen, Y. Wang, Adv. Mater. 2017, 29, 1605838; d) Y. Ito, W. Cong, T. Fujita, Z. Tang, M. Chen, Angew. Chem. Int. Ed. 2015, 54, 2131-2136; e) Y. Li, P. Liu, L. Pan, H. Wang, Z. Yang, L. Zheng, P. Hu, H. Zhao, L. Gu, H. Yang, Nat. Commun. 2015, 6, 8064. a) H. B. Wu, B. Y. Xia, L. Yu, X.-Y. Yu, X. W. D. Lou, Nat. commun. 2015, 6, 6512; b) X. Wang, B. Xia, Y. Yan, M. Miao, J. Pan, T. He, Chem. Eur. J. 2017, 23, 10947-10961; c) J. Xie, S. Li, X. Zhang, J. Zhang, R. Wang, H. Zhang, B. Pan, Y. Xie, Chem. Sci. 2014, 5, 4615-4620; d) Z. Xing, Q. Liu, A. M. Asiri, X. Sun, Adv. Mater. 2014, 26, 5702-5707; e) G. Ou, P. Fan, X. Ke, Y. Xu, K. Huang, H. Wei, W. Yu, H. Zhang, M. Zhong, H. Wu, Nano Res., DOI 10.1007/s12274-017-1684-2. a) B. Qiao, A. Wang, X. Yang, L. F. Allard, Z. Jiang, Y. Cui, J. Liu, J. Li, T. Zhang, Nat. chem. 2011, 3, 634-641; b) P. Liu, Y. Zhao, R. Qin, S. Mo, G. Chen, L. Gu, D. M. Chevrier, P. Zhang, Q. Guo, D. Zang, Science 2016, 352, 797-800; c) P. Yin, T. Yao, Y. Wu, L. Zheng, Y. Lin, W. Liu, H. Ju, J. Zhu, X. Hong, Z. Deng, Angew. Chem., Int. Ed. 2016, 55, 1080010805; d) X. Huang, Y. Xia, Y. Cao, X. Zheng, H. Pan, J. Zhu, C. Ma, H. Wang, J. Li, R. You, Nano Res. 2017, 10, 1302-1312; e) L. Fan, P. F. Liu, X. Yan, L. Gu, Z. Z. Yang, H. G. Yang, S. Qiu, X. Yao, Nat. Commun. 2016, 7, 10667; f) J. Deng, H. B. Li, J. P. Xiao, Y. C. Tu, D. H. Deng, H. X. Yang, H. F. Tian, J. Q. Li, P. J. Ren, X. H. Bao, Energy Environ. Sci. 2015, 8, 1594. a) J. Deng, H. Li, S. Wang, D. Ding, M. Chen, C. Liu, Z. Tian, K. Novoselov, C. Ma, D. Deng, Nat. commun. 2017, 8, 14430; b) H. Sun, L. Mei, J. Liang, Z. Zhao, C. Lee, H. Fei, M. Ding, J. Lau, M. Li, C. Wang, Science 2017, 356, 599-604. a) W.-F. Chen, C.-H. Wang, K. Sasaki, N. Marinkovic, W. Xu, J. Muckerman, Y. Zhu, R. Adzic, Energy Environ. Sci. 2013, 6, 943-951; b) W.-F. Chen, S. Iyer, S. Iyer, K. Sasaki, C.-H. Wang, Y. Zhu, J. T. Muckerman, E. Fujita, Energy Environ. Sci. 2013, 6, 1818-1826. D. E. Sayers, E. A. Stern, F. W. Lytle, Phys. Rev. Lett. 1971, 27, 1204. H. Funke, A. Scheinost, M. Chukalina, Phys. Rev. B 2005, 71, 094110. a) H. Fei, J. Dong, M. J. Arellano-Jiménez, G. Ye, N. D. Kim, E. L. Samuel, Z. Peng, Z. Zhu, F. Qin, J. Bao, Nat. commun. 2015, 6, 8668; b) X. Wang, W. Chen, L. Zhang, T. Yao, W. Liu, Y. Lin, H. Ju, J. Dong, L. Zheng, W. Yan, J. Am. Chem. Soc. 2017, 139, 9419-9422; c) M. Zhang, Y. Wang, W. Chen, J.-C. Dong, L. Zheng, J. Luo, J. Wan, S. Tian, W.-C. Cheong, D. Wang, J. Am. Chem. Soc. 2017, 139, 10976-10979. Y. Jiao, Y. Zheng, M. Jaroniec, S. Z. Qiao, Chem. Soc. Rev. 2015, 44, 2060-2086. a) J.-S. Li, Y. Wang, C.-H. Liu, S.-L. Li, Y.-G. Wang, L.-Z. Dong, Z.-H. Dai, Y.-F. Li, Y.-Q. Lan, Nat. commun. 2016, 7, 11204; b) Y.-T. Xu, X. Xiao, Z.-M. Ye, S. Zhao, R. Shen, C.-T. He, J.-P. Zhang, Y. Li, X.-M. Chen, J. Am. Chem. Soc. 2017, 139, 5285-5288. Q. Gong, Y. Wang, Q. Hu, J. Zhou, R. Feng, P. N. Duchesne, P. Zhang, F. Chen, N. Han, Y. Li, C. Jin, Y. Li, S.-T. Lee, Nat. commun 2016, 7, 13216. This article is protected by copyright. All rights reserved. Accepted Manuscript had been considered (Figure S29). For comparison, the ΔGH* values of the Mo2C, MoN and N-doped graphene were also involved (Figure S30-S32). The calculated ΔGH* values of the most favorable active sites for Mo1N1C2, Mo2C, MoN and Ndoped graphene were 0.082, -0.260, -0.401 and 0.672 eV, respectively. Obviously, the Mo1N1C2 possessed the lowest absolute value of ΔGH*, being consistent with the best experimental HER performance among the research objects. These indicated that Mo-SA was more nicely beneficial to the HER process than those Mo in the bulk phases. The density of states (DOS) was calculated to further study the electronic structure of the Mo1N1C2 (Figure 4b and Figure S33). We found that the DOS of Mo1N1C2 near the Fermi level was much larger than that of Mo2C and MoN, leading to a higher carrier density for the advantage to charge transfer during HER process, which is consistent with our charge transfer resistance measurements. Moreover, the DOS near the Fermi level in Mo1N1C2 was majorly contributed by the Mo d-orbital, while the contributions of p-orbital of N and C can be neglected (Figure 4b), indicating that the individual Mo dispersion as well as special coordination environment can effectively improve the delectron domination near the Fermi level and optimize the catalytic activity. In a word, combining the controlled experimental tests and theoretical calculations, we reasonably discovered the perfect HER performance of the Mo1N1C2 catalyst. 10.1002/ange.201710599 Angewandte Chemie COMMUNICATION COMMUNICATION Wenxing Chen, Jiajing Pei, Chunting He, Jiawei Wan, Hanlin Ren, Youqi Zhu, Yu Wang, Juncai Dong, Shubo Tian, WengChon Cheong, Siqi Lu, Lirong Zheng, Xusheng Zheng, Wensheng Yan, Zhongbin Zhuang, Chen Chen, Qing Peng, Dingsheng Wang*, Yadong Li* Page No. – Page No. Rational Design of Single Mo Atoms Anchored on N-doped Carbon for Effective Hydrogen Evolution Reaction This article is protected by copyright. All rights reserved. Accepted Manuscript A catalyst with Mo-SAs dispersed on N-doped carbon was prepared, with high catalytic activity and stability for hydrogen evolution reaction. The structure of the catalyst was characterized by electronic microscopy and XAFS measurements. The unique catalytic properties for HER was further investigated by DFT calculations. This work opens up new opportunities for the preparation and application of high-efficiency and stable Mobased HER catalysts.