Electronic Supporting Information

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1 Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A. This journal is The Royal Society of Chemistry 2018 Electronic Supporting Information Synthesis of Amorphous Boride Nanosheets by Chemical Reduction of Prussian Blue Analogs for Efficient Water Electrolysis Ting He, a Jean Marie Vianney Nsanzimana, b Ruijuan Qi, c Jun-Ye Zhang, a Mao Miao, a Ya Yan, d Kai Qi, a Hongfang Liu, a and Bao Yu Xia a,* a Key laboratory of Material chemistry for Energy Conversion and Storage (Ministry of Education), School of Chemistry and Chemical Engineering, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology (HUST), 1037 Luoyu Road, Wuhan , PR China b School of Chemical and Biomedical Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore , Singapore. c School of Information Science and Technology, East China Normal University, 500 Dongchuan Road, Shanghai , PR China d School of Materials Science and Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai , PR China Corresponding author: byxia@hust.edu.cn (B. Y. Xia)

2 Experimental Section 1 Material Cobalt chloride hexahydrate, sodium citrate, sodium borohydride, and commercial Platinum (20%) on graphitized carbon (20% Pt/C) were purchased from Sigma-Aldrich. Potassium hydroxide from J. T. Baker and Nafion 5% in lower aliphatic alcohols and water was purchased from Sigma-Aldrich. All chemical reagents were used in the experiments directly without further treatment. 2 Method Synthesis of Co-Ni PBA stacked nanoplates: In a typical procedure, 1.6 mmol of cobalt chloride hexahydrate and 0.6 mmol of sodium citrate were dissolved in 40 ml of DI water to form solution A. 0.8 mmol of potassium tetracyanonickelate (II) was dissolved in 40 ml of DI water to form solution B. Next, blend solutions A and B under magnetic stirring for 5 min. The obtained homogeneous solution was aged for 24 h at room temperature. After collected by centrifugation and washed several times with water and ethanol, the precipitates were dried at 60 o C overnight. Synthesis of metal boride(oxide): The as-obtained CNBO-NS were prepared by chemical reduction of the as-prepared PBA stacked nanoplates at room temperature. In a typical synthesis, 60 mg Co-Ni PBA was dissolved in 20 ml DI water under a continuous and vigorous stirring to form a homogeneous mixture. Afterward, 20 ml of sodium borohydride (NaBH 4 ) solution (0.1 mol L -1 ) was added and the mixture was maintained at room-temperature for 1.5 hours under stirring. The final products were collected by centrifuging and washing with DI water for several times, and then dried overnight for further characterizations. Different metal borides were synthesized by at a range of reduction times. Characterizations. The crystal phase of metal borides was characterized by X-ray diffraction (XRD) patterns on Empyrean (PANalytical B.V. with Cu-Kα radiation). The scanning electron microscopy (SEM) was carried out on JEOL JSM-7100F at an accelerating voltage of 15 kv. Transmission electron microscopy (TEM) was performed on TecnaiG2 20 (Philips) at an accelerating voltage of 200 kv assembled with a linear scanning

3 EDX spectrum. X-ray photoelectron spectroscopy (XPS) experiment was implemented on a Kratos AXIS Ultra DLD-600W XPS system with a monochromatic Al Kα ( ev) X-ray source. Electrochemical Measurements: All the electrochemical tests were carried out in a three-electrode system on an electrochemical workstation (Autolab 302N). In the three-electrode configurations, a glassy carbon electrode was served as the working electrode, while the calibrated Hg/HgO electrode (1.0 M KOH) and a graphite rod were used as the reference electrode and the counter electrode, respectively. All measured potentials were converted to the reversible hydrogen electrode (RHE), according to the equation; E RHE = E Hg/HgO *pH. A homogeneous catalyst ink dispersion was obtained by sonicating the samples (5 mg) and Nafion/alcohol mixed solution (500 μl). The prepared ink (10 μl) was dropped onto the working electrode and dried at room temperature naturally, the mass loadings of the catalyst were about 0.5 mg cm -2. The electrochemical measurements were investigated in 1.0 M KOH medium. Linear sweep voltammetry (LSV) measurements were performed at the scan rate of 5 mv s -1 at a potential range 0 V to -0.4 V for cathodic hydrogen evolution and from 1.3 V to 1.7 V for anodic oxygen evolution. The overall electrochemical water splitting was detected by LSV measurements at the potential from 1.2 to 2.0 V. The stability test was performed at a current density of 10 ma cm -2 for 24 h. All the data presented were corrected for ir losses and background current. Electrochemical impedance spectroscopy (EIS) detection was conducted in the frequency range from 100 khz to 0.01 Hz with an ac perturbation of 5 mv when the potential was 1.55 V vs RHE. The electrochemical double-layer capacitance was determined from the CV curves measured in a potential range without redox processes according to the following equation: C dl = I c /ν, where C dl, I c, and ν are the doublelayer capacitance (mf cm -2 ) of the electroactive materials, charging current (ma cm -2 ), and scan rate (mv s - 1 ). Hence, cyclic voltammogram (CV) cycles used for C dl determination was performed in the potential range from 0.22 V to 0.12 V at the scan rate of 20, 40, 60, 80, 100, 200 mv s -1, respectively. And the drainage method is applied in faradaic efficiency test. The faradaic efficiency calculation efficiency is: F=(m*n*F)/Q, m denotes the moles of product, n denotes transferred electrons, F is faradaic constant and Q is the consumed

4 charge.

5 Figure and Captions Figure S1. (a) SEM and (b) XRD of Co-Ni PBA precursor. Figure S2. XRD patterns of amorphous metal borides annealed at different temperatures..

6 Figure S3. SEM images of CNBO-NS abtained at 0.5h (a,b) and 3h (c,d).

7 Figure S4. (a) TEM image and corresponding, inset of (a) is HRTEM, (b) EDX spectrum accompanying the atomic ratio of corresponding elements. Figure S5. XPS spectra of Ni 2p (a) and Co 2p (b) of Co-Ni PBA precursor.

8 Figure S6. XPS spectra of Ni 2p (a), Co 2p (b), B 1s (c) and O 1s (d) of CNBO-NS product obtained at 3h reduction time.

9 Figure S7. (a) Polarization curves, (b) corresponding Tafel plots and (c) Nyquist plot EIS of CNBO-NS obtained at different reaction time.

10 Figure S8. Polarization curves of CNBO-NS and their annealed samples at different temperatures. Figure S9. Electrochemical capacitance measurements to determine the surface area of the obtained electrodes in 1.0 M KOH electrolyte. The capacitive current density on CNBO-NS obtained at 1.5h (a) and 3h (b) from double layer charging can be measured from cyclic voltammograms in a potential range where no Faradic reaction occur, and (c) the measured capacitive current plotted as a function of scan rate.

11 Figure S10. Electrochemical capacitance measurements to determine the surface area of the obtained electrodes in 1.0 M KOH electrolyte. The capacitive current density on CNBO-NS obtained at 1.5h (a) and PBA precursor (b) from double layer charging can be measured from cyclic voltammograms in a potential range where no Faradic reaction occur, and (c) the measured capacitive current plotted as a function of scan rate. Figure S11. (a) Polarization curves and (b) corresponding Tafel plots of the amorphous boride of different reaction time in 1.0 M KOH solution.

12 Figure S12. Tafel plots of the amorphous boride and Co-Ni PBA in 1.0 M KOH solution. Figure S13. Faradic efficiency of hydrogen evolution.

13 Figure S14. (a) LSV of CNBO-NS couple (without ir corrected), PBA couple and Pt/C//IrO 2. (b) Chronopotentiometry test during 24h electrolysis by the CNBO-NS pair. Inset is the photograph of CNBO- NS couple assembled electrolysis cell.

14 Movie S1 Movie S1. CNBO-NS pair electrolysis at current density=10 ma cm -2 in 1.0 M KOH solution.

15 Table S1. OER activity of recent reported catalysts. Catalysts Electrolyte Overpotential (η 10 )(mv) Tafel slope (mv dec -1 ) Reference CNBO-NS 1.0 M KOH This work Co 2 B M KOH Adv. Energy Mater. 2016, 6, Co Ni B@NF 1.0 M KOH J. Mater. Chem. A 2017, 5, Ni-Co mixed oxide cages FeNi@single layer graphene Co 3 O 4 /NiCo 2 O 4 nanocages Hollow Co 3 S 4 nanosheets 1.0 M KOH Adv. Mater. 2016, 28, M KOH Energy Environ. Sci. 2016, 9, M KOH J. Am. Chem. Soc. 2015, 137, M KOH ACS Nano 2014, 8, Ni x B-300-GC 1.0 M KOH 380 / Adv. Energy Mater. 2017, 7, Co-B i NS/G 1.0 M KOH Angew. Chem. Int. Ed. 2016, 55, Ni-B 1.0 M KOH Angew. Chem. Int. Ed. 2017, 56, CoP@N-doped Graphene 1.0 M KOH Nanoscale 2016, 8, FeB M KOH Adv. Energy Mater. 2017, 7, Ni 3 Se 2 -GC 1.0 M KOH Energy Environ. Sci. 2016, 9, 1771 NiCoP/NF 1.0 M KOH Nano Lett. 2016, 16, 7718 NiCo LDH 1.0 M KOH 367 / Nano Lett. 2015, 15, 1421

16 Table S2. HER activity of recent reported catalysts. Catalysts Electrolyte Overpotential (η 10 )(mv) Tafel slope (mv dec -1 ) Reference CNBO-NS 1.0 M KOH This work (Ni,Co)Se 2 -GA 1.0 M KOH ACS Catal. 2017, 7, NiCoFe LDHs 1.0 M KOH ACS Energy Lett. 2016, 1, 445 Co Ni B@NF 1.0 M KOH 205 / J. Mater. Chem. A. 2017, 5, NiB x film 1.0 M KOH Nano Energy 2016, 19, 98 Ni-Co-P M KOH Chem. Commun. 2016, 52, CoP nanowire arrays 1.0 M KOH J. Am. Chem. Soc. 2014, 136, Co 2 B M KOH Adv. Energy Mater. 2016, 6, NiB Foil 1.0 M KOH 306 / Nano Energy 2016, 19, 98 FeCoNi 1.0 M KOH ACS Catal. 2017, 7, 469 NiCo 2 S 4 NWs/NF 1.0 M KOH Adv. Funct. Mater. 2016, 26, 4661 CP@Ni-P 1.0 M KOH Adv. Funct. Mater. 2016, 26, 4067 Ni-P 1.0 M KOH Chem. Commun. 2016, 52, Ni 3 Se 2 nanoforest 1.0 M KOH Nano Energy 2016, 24, 103

17 Table S3. Cell voltage of different catalysts for overall water electrolysis. Catalysts Electrolyte Cell voltage (V) Current density (ma cm -2 ) Reference CNBO-NS 1.0 M KOH This work (Ni,Co)Se M KOH ACS Catal. 2017, 7, NiSe 1.0 M KOH Angew. Chem. Int. Ed. 2015, 54, 9351 NiFe LDH/NF 1.0 M KOH Science 2014, 345, 1593 NiCo 2 O 4 hollow microcuboids 1.0 M KOH Angew. Chem. Int. Ed. 2016, 55, 1 Co Ni B@NF 1.0 M KOH J. Mater. Chem. A 2017, 5, Co/NBC-900@NF 1.0 M KOH Adv. Funct. Mater. 2018, 28, Co 2 B M KOH Adv. Energy Mater. 2016, 6, CoMnCH 1.0 M KOH J. Am. Chem. Soc. 2017, 139, 8320 FeCoNi 1.0 M KOH ACS Catal. 2017, 7, 469 Ni 5 P M KOH Angew. Chem. Int. Ed. 2015, 54, Ni 3 S 2 /NF 1.0 M KOH J. Am. Chem. Soc. 2015, 137, 14023