Tuning electronic structures of non-precious ternary alloys. encapsulated in graphene layers for optimizing overall water splitting

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1 Tuning electronic structures of non-precious ternary alloys encapsulated in graphene layers for optimizing overall water splitting activity Yang Yang,a Zhiyu Lin,a Shiqi Gao,a Jianwei Su,a Zhengyan Lun,a Guoliang Xia,a Jitang Chena, Ruirui Zhang, and Qianwang Chen*a,b a Hefei National Laboratory for Physical Science at Microscale, Department of Materials Science & Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei , China. b High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei , China Corresponding Author * for Q.W. Chen: cqw@ustc.edu.cn

2 Figure S1 (a-c) SEM images of Co3[Co(CN)6]2,Ni3[Co(CN)6]2 and Ni3[Fe(CN)6]2, respectively.(d-e) SEM images of Co, CoNi and FeNi, respectively. (g-i) TEM images of Co, CoNi and FeNi, respectively. Figure S2 (a b) enlarged high resolution transmission electron microscopy (HRTEM) images of FeCoNi-2 where interplanar spacing of the graphene layers were measured in the image, (c f) STEM image of the single FeCoNi nanocrystal and the images of elemental mapping of Fe,Co and Ni. Figure S3 (a b) enlarged high resolution transmission electron microscopy (HRTEM) images of

3 FeCoNi-3 where interplanar spacing of the graphene layers were measured in the image, (c f) STEM image of the single FeCoNi nanocrystal and the images of elemental mapping of Fe,Co and Ni. Figure S4 (a) enlarged pattern of ternary alloys in Figure 3 (b) XRD pattern of FeCoNi-2 after acid treatment. Figure S5 Raman spectra of various binary and ternary alloys.

4 Figure S6 The XPS results of N1s spectrum of FeCoNi-1, FeCoNi-2 and FeCoNi-3. Figure S7 EDX spectrum of the (a) FeCoNi-1, (b) FeCoNi-2 and (c) FeCoNi-3 Figure S8 OER linear sweep voltammograms of FeCoNi-2 and mixture of FeCo and CoNi.

5 Figure S9 Enlarged durability test of FeCoNi-2 in alkaline electrolyte for 10000th cycles in Figure 5a. Figure S10 (a) OER linear sweep voltammograms of FeCoNi-2 measured by SCE and Ag/AgCl electrodes (b) Durability test of FeCoNi-2 used SCE electrode for 10000th cycles. Figure S11 (a) HER linear sweep voltammograms of FeCo and Pt/C measured in 1M and 0.1M KOH (b) OER linear sweep voltammograms of FeCoNi-2 and Ir/C measured in 1M and 0.1M KOH.

6 Figure S12 TEM images of (a) FeCo electrocatalyst after 10000th HER cycling test (b) enlarged FeCo sample after cycling (c) FeCoNi-2 electrocatalyst after 10000th OER cycling test (d) enlarged FeCoNi-2 sample after cycling. Figure S13 Optimized structures of (a) HO* (b) O* and (c) HOO* adsorbed on nitrogen doped graphene encapsulating Co cluster.

7 Figure S14 The free energy profiles of FeCo, FeCoNi and CoNiFe. Figure S15 CV curves of (a) FeCo for HER and (b) FeCoNi-2 for OER with scan rate of 50mV/s. Table S1 The element compositions of ternary alloys estimated from XPS measurements. Samples C (At%) N (At%) O (At%) Fe (At%) Co (At%) Ni (At%) FeCoNi FeCoNi FeCoNi Table S2 The element compositions of ternary alloys estimated from ICP measurements. Samples Fe (wt%) Co (wt%) Ni (wt%) FeCoNi FeCoNi FeCoNi Table S3 The element compositions of ternary alloys estimated from EDX measurements. Samples Fe (wt%) Co (wt%) Ni (wt%) FeCoNi FeCoNi FeCoNi

8 Table S4 The BET surface area of the prepared ternary alloys. Samples BET surface area (m 2 /g) FeCoNi FeCoNi FeCoNi Table S5 Total number of transferred electrons from metal to graphene based on Bader charge analysis. Models Co FeCo FeCoNi CoNiFe Transferred electron(e - ) Table S6 Calculated free energies of different intermediates and overpotential of various models. Models G O* (ev) G HO* (ev) G HOO* (ev) η(v) Graphene Co FeCo FeCoNi CoNiFe Table S7 Summary of our catalysts and some of representative non-precious OER catalysts in literatures (references are listed in main article, CP and NF refers to catalysts loading on carbon paper and Ni foam, respectively). Catalysts Electrolyte Loading amount FeCoNi-2 FeCoNi-2 (mg/cm 2 ) 1M KOH (CP) Overpotential at 10 ma/cm 2 (vs. RHE) 325mV 288mV Reference This work This work FeCoNi-2 0.1M KOH mV This work Co 3 O 4 NCs 1M KOH 0.35(CP) ~320mV Ref 45 NiFe LDH/NGF 0.1M KOH mV Ref 43 Fe@N C M KOH mV Ref 44 Co@N C 1M KOH mV Ref 9 Compact MoO 2 1M KOH 3.4(NF) 330mV Ref 49 Ni 3 S 2 1M NaOH 1.6(NF) 260mV Ref 42 NiCo 2 O 4 1M KOH mV Ref 46 N-Co 9 S 8 /G 0.1M KOH mV Ref 47 Co//PCP@NG 0.1M KOH mV Ref 29 Co@CoO/NG 1M KOH 2(CP) 315mV Ref 52

9 Table S8 Summary of our catalysts and some of representative non-precious HER catalysts in literatures under basic solution (references are listed in main article, CP and NF refers to catalysts loading on carbon paper and Ni foam, respectively). Catalysts FeCo FeCo Electrolyte Loading amount (mg/cm 2 ) 1M KOH (CP) Overpotential at 10 ma/cm 2 (vs. RHE) 211mV 149mV Reference This work This work FeCo 0.1M KOH mV This work Ni 3 S 2 1M NaOH 1.6 (NF) 223mV Ref 42 MnNi 0.1M KOH mV Ref 48 Co 3 O 4 NCs 1M KOH 0.35(CP) 380mV Ref 45 Co-NRCNTs 1M KOH mV Ref 6 Compact MoO 2 1M KOH 3.4(NF) 124mV Ref 49 Co-P/NC 1M KOH mV Ref 50 Co 9 S 2 /CNFs 1M KOH Not given 190mV Ref 4 MoB 1M KOH 2.3 ~210mV Ref 51 N-Co@G 0.1M NaOH mV Ref 53 Co@Co-N-C 0.1M KOH mV Ref 54 Material synthesis: The Co was synthesized by direct annealing of Co 3 [Co(CN) 6 ] 2 under N 2 atmosphere at 600, while binary alloys of FeCo, CoNi, FeNi were obtained by annealing of Fe 3 [Co(CN) 6 ] 2,Ni 3 [Co(CN) 6 ] 2 and Ni 3 [Fe(CN) 6 ] 2 under the same condition with a heating rate of 5 C min -1 and kept for 4 h. FeCoNi ternary alloys were obtained by annealing of Ni 3 [Co(CN) 6 ] 2 encapsulated Fe 3 [Co(CN) 6 ] 2. In a typical synthesis process of FeCoNi-1, 59.8 mg of obtained Fe 3 [Co(CN) 6 ] 2 seed and 33.3 mg of K 3 Co(CN) 6 were dissolved in 20 ml water to form a homogeneous dispersion as solution A mg NiCl 2 6H 2 O and 600mg PVP were dissolved in 20 ml water to form solution B. Solution B was slowly and regularly added to solution A through a syringe to form a colloid solution. The molar ratio of total Fe, Co and Ni in the colloid solution was 4:4:2. The whole reaction was performed at room temperature with agitated stirring. After 60 min, the reaction was aged for 24 h at room temperature without any interruption. The resulting precipitate was filtered and washed several times with distilled water and finally dried in air at 60 C. The final precursor was grinded uniformly and annealed under N 2 atmosphere at 600 C for 4 h. FeCoNi-2 and FeCoNi-3 were obtained under the same method and condition except that the molar ratio of total Fe, Co and Ni in the colloid solution was 3:4:3 and 2:4:4. Material characterization: The powder XRD patterns of the samples were recorded with an X-ray diffractometer

10 (Japan Rigaku D/MAX-γA) using Cu-Ka radiation (λ= A) with 2θ range of FESEM images were taken on a JEOL JSM-6700 M scanning electron microscope. TEM images were collected from Hitachi H-800 transmission electron microscope using an accelerating voltage of 200 kv, and a HRTEM (JEOL-2011) was operated at an accelerating voltage of 200 kv. XPS was performed on an ESCALAB 250 X-ray photoelectron spectrometer using Al Ka radiation. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was carried out in Optima 7300 DV. The samples for ICP analysis were first heated at 500 C for 4 h in air, followed by treatment in concentrated chloroazotic acid for several hours until the material was fully dissolved. The specific surface area was evaluated at 77 K (Micromeritics ASAP 2020) using the Brunauer Emmett Teller (BET) method applied to the adsorption branch. Calculation details: We perform DFT calculations using the Vienna Ab Initio Simulation Package (VASP), 1,2 the generalized gradient approximation (GGA) of Perdew Becke Ernzerhof (PBE) is used for the exchange-correlation functional. 3 A graphitic carbon cage C 240 encapsulated 55 metal atoms was used as the model of graphene encapsulated alloys, which performed well in previous study. 4-6 The cut-off energies for plane waves is 400 ev, providing a convergence of 10 4 ev in total energy and 0.05 ev/å in Hellmann Feynman force on each atom. The free energy of the adsorbed state is calculated as G H* = E H* + E ZPE T S, where the hydrogen binding energy E H* was calculated by E H* = E H-slab E slab 1/2 E H2. As for OER, the free energies of the intermediates at K were obtained using G= E+ ZPE-T S+eU according to previous work, 7 where E is the binding energy of adsorption species HO*, O* and HOO*, ZPE, S and U are the zero point energy changes, entropy changes and applied potentials, respectively. Electrochemical measurement details: The electrochemical measurements were performed in a three-electrode system on an electrochemical workstation (CHI 760E) in 1 M KOH electrolyte. The catalysts dispersed onto a glassy carbon rotating disk electrode (PINE, PA, USA) were used as a working electrode, while 3M Ag/AgCl and a platinum foil served as the reference and counter electrodes, respectively. The GC RDE has a diameter of 5 mm and a geometric area of cm 2. Typically, 8 mg of catalyst and 60µL Nafion solution (Sigma Aldrich, 5 wt %) were dispersed in 2 ml ethanol solution by sonicating for 1 h to form a homogeneous ink. Then 16.2 µl of the dispersion was loaded onto the polished rotating disk electrode with 5 mm diameter (loading 0.32 mg cm 2 ). Linear sweep voltammetry with a scan rate of 2 mv s 1 was conducted with a flow of N 2 gas maintained over the electrolyte during the HER experiment to eliminate dissolved oxygen. All of the potentials were calibrated to a reversible hydrogen electrode (RHE) for HER. Linear sweep voltammetry with a scan rate of 5 mv s 1 was conducted with a flow of O 2 gas maintained over the electrolyte during the OER experiment to eliminate dissolved oxygen (rpm:1600). Commercial Pt/C catalysts (Alfa Aesar, Pt:

11 20 % wt) and RuO 2 were used as a reference to evaluate the electrocatalytic performance of various samples. Cyclic voltammetry (CV) was conducted between 0 and 0.7 V versus for Ag/AgCl for times with a scan rate of 0.1V s 1 to investigate the cycling stability of OER. The catalysts with higher loading amount were measured on carbon fiber paper. In general, 1 mg catalyst and 250 µl ethanol dispersion solution (4 mg ml 1 ) were mixed well by ultrasonicating for 30 min. Then, the uniform suspension was dropped on carbon paper with an area of 1 cm 2 and left to dry (this yielded an approximate catalyst loading of 1 mg cm 2 on carbon paper). Rererence: 1. K. Hoshino, F. Shimojo, J Phys-Condens Mat 1996, 8, G. Kresse, D. Joubert, Phys Rev B 1999, 59, Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys Rev Lett 1996, 77, Deng, J.; Ren, P.; Deng, D.; Bao, X. Angewandte Chemie International Edition 2015, 54, Cui, X.; Ren, P.; Deng, D.; Deng, J.; Bao, X. Energ Environ Sci 2016, 9, Deng, J.; Ren, P.; Deng, D.; Yu, L.; Yang, F.; Bao, X. Energ Environ Sci 2014, 7, Man, I. C.; Su, H. Y.; Calle-Vallejo, F.; Hansen, H. A.; Martinez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Norskov, J. K.; Rossmeisl, J. Chemcatchem 2011, 3,