catalyst for efficient overall water splitting in alkaline medium

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1 Supplementary Information Amorphous Co 2 B grown on CoSe 2 nanosheets as a hybrid catalyst for efficient overall water splitting in alkaline medium Yaxiao Guo, a,b Zhaoyang Yao, c Changshuai Shang, a,b and Erkang Wang a* a State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin , China. ekwang@ciac.ac.cn b University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing, , China. c Department of Chemistry, University of Washington, Seattle, 98105, USA. Content Experimental Section.S-2 Figure S1 S-5 Figure S2 S-5 Figure S3 S-5 Figure S4 S-6 Table S1..S-6 Figure S5 S-6 Table S2..S-6 Figure S6 S-7 Figure S7 S-7 Table S3..S-8 Figure S8 S-8 Figure S9 S-9 Figure S10..S-9 References..S-9 1

2 Experimental Section Materials and Reagents Co(OAc) 2 H 2 O( 98%), Na 2 SeO 3 (99%), NaBH 4 ( 96.0%), Co(NO 3 ) 2 6H 2 O, Diethylenetriamine (DETA, 99.0%), Nafion solution (5 wt% ) and graphite rod ( %) were purchased from Sigma-Aldrich. Coiled Platinum Counter Electrode Kit (AFCTR5) was purchased from PINE. Absolute ethanol ( 99.7%) and KOH (82%) were obtained from Beijing Chemical Reagent (China). The ultra-pure water was prepared by the Millipore Milli-Q water purification system (18.2 MΩ ). All reagents were used directly without further purification. Characterizations The scanning electron microscopy (SEM) images were obtained using a PHILIPS XL-30 ESEM with an accelerating voltage of 20 kv. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), Fast Fourier transform (FFT), high-angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM), and STEM-energy dispersive X-ray (EDX) mapping were taken by using a JEM-2010 (HR) microscope operated at 200 kv. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250 with Al Kα radiation (pass energy, 20.0 ev; energy step size. 1.0 ev; total acq. time: 1 min 0.1 s). X-ray diffraction (XRD) spectra were obtained using a Bruker D8 ADVANCE instrument with Cu Kα radiation (40 kv, 40 ma). Raman spectroscopy was performed on a customized LabRAM HR Evolution Raman system (HORIBA Scientific) with an excitation wavelength of nm. Inductively coupled plasma optical emission spectrometry (ICP-OES) was performed on X Series 2 (Thermo Scientific, USA). GC analysis was carried out on GC-2014C (Shimadzu Co.) with thermal conductivity detector and nitrogen carrier gas. Pressure data during electrolysis were recorded using a CEM DT-8890 Differential Air Pressure Gauge Manometer Data Logger Meter Tester with a sampling interval of 1 point per second. The Nyquist plots (EIS) were performed on Zahner Zennium. All electrochemical measurements were performed on CHI 620a. 2

3 Electrochemical Measurements The electrode modification process is as following. Typically, 4 mg of sample were dispersed in 1 ml water-ethanol solution with volume ratio of 3:1 by sonicating to form a homogeneous ink. Then 7 µl of the dispersion and 3 µl Nafion solution (0.5 wt% ) were loaded onto a glassy carbon electrode of 3 mm diameter (loading ca mg cm 2 ) and then dried in air at room temperature. The oxygen evolution reaction (OER) electrochemical measurements were performed in a three-electrode system at an electrochemical workstation (CHI 620a). Linear sweep voltammetry (LSV) at a scan rate of 5 mv s 1 was conducted in 1.0 M KOH (sparged with pure O 2 ) using a Ag/AgCl (saturated KCl) electrode as the reference electrode, a coiled platinum as the counter electrode and the glassy carbon electrode with various samples as the working electrode, respectively. The hydrogen evolution reaction (HER) electrochemical measurements were performed in a three-electrode system at an electrochemical workstation (CHI 620a). Linear sweep voltammetry (LSV) at a scan rate of 5 mv s 1 was conducted in 1.0 M KOH (sparged with pure N 2 ) using a Ag/AgCl (saturated KCl) electrode as the reference electrode, a graphite rod as the counter electrode and the glassy carbon electrode with various samples as the working electrode, respectively. For the complete watersplitting test, two piece of carbon fiber cloth (CFC, 1.0 cm 0.5 cm, geometric area = 1.0 cm 2 ) modified with Co 2 B/CoSe 2 with a loading mass of mg cm -2 as both the anode and cathode. Before prepare the electrodes, the CFC was carefully pre-treated with concentrated HNO 3 to remove impurity of surface, and then acetone, ethanol and deionized water were used for several times to ensure the surface of the CFC was well cleaned. Then the CFC electrodes were prepared according to the literature. 1 The CFC, still moist, was placed on a clean glass slide and drop coated with Co 2 B/CoSe 2 ink (100 µl 4 mg ml -1 ) and Nafion solution (80 µl 0.5 wt% ). Dropping 10 µl each time, and every side of the CFC was dropped nine times in turn. The modified CFC electrodes were then dried in an oven at 60 o C for 30 3

4 min. After drying, the electrodes were first moistened by dipping in a mixture of ethanol and water (50:50 v/v), then multiple times in ultrapure water. All the data were recorded after applying a number of potential sweeps until the electrodes were stable. All of the potentials were calibrated according to the following equation E =E V *pH. All the potentials reported in our RHE Ag/AgC l manuscript were against RHE. The exchange current densities (j 0 ) were derived by extrapolation of Tafel plots to zero. According to the Tafel equation η =a+b log j, where η (V) represents the applied overpotential, j (ma cm -2 ) represents the resulting current density, b (V dec -1 ) represents the Tafel slope and a (V) is the intercept. When η =0, this j is the exchange current density j 0. The double-layer capacitances ( C ) were estimated by CV at various scan rates dl ( mv s 1 ) to evaluate the effective surface area of various catalysts. The ohmic resistance used for ir-correction was obtained from electrochemical impedance spectroscopy (EIS) measurements under the open circuit potential with frequencies ranging from 100 mhz to 100 KHz with an AC voltage of 5 mv. The impedance data were fitted to a simplified Randles circuit to extract the series resistances ( R ) and charge-transfer resistances ( R ). s The electrochemical stability of different catalysts was evaluated by amperometric i-t curve test at static overpotential for 30h. ct 4

5 Figure S1 Raman spectra of Co2B/CoSe2. There are no characteristic peaks of Co2B in the Raman spectrum of Co2B, verifying its amorphous character. In addition, four sharp peaks at 188, 463, 504, and 672 cm 1 correspond to the characteristic peaks of CoSe2 phase. Figure S2 SEM images of (a) CoSe2/DETA, (b) Co2B and (c) Co2B/CoSe2. As shown in Figure S2a, CoSe2/DETA nanosheets are thin and flexible, with a flake-like morphology. Instead of the aggregated flake hierarchical structure of Co2B product (Figure S2b), much smaller Co2B nanosheets are uniformly grown on the surface of CoSe2/DETA nanosheets (Figure S2c). Figure S3 (a) TEM and (b) HRTEM images of Co2B/CoSe2. 5

6 Figure S4 Energy dispersive X-ray (EDX) spectra of Co 2 B/CoSe 2. Table S1 The atomic percentage (at.%) of Co, Se and B elements in different catalysts obtained from ICP-OES Catalyst ICP-OES Co Se B Co/Se/B CoSe 2 /DETA /1/0 Co 2 B /0/0.52 Co 2 B/CoSe /0.42/0.32 Figure S5 O1s XPS spectra in Co 2 B/CoSe 2. Table S2 Electrochemical parameters of different catalysts for OER. Tafel slopes Catalyst [mv dec 1 ] Co 2 B M KOH CoSe 2 /N-doped graphene M KOH Co 3 O 4 /NiCo 2 O M KOH nanocages 3 Mn 3 O 4 /CoSe M KOH 6 η j=10 [mv] Electrolyte

7 composite 4 Co 3 S M KOH nanosheet 5 Hollow Co 3 S M KOH nanosheets 6 CoP/NF M KOH Co 2 B/CoSe 2 (current work) M KOH Figure S6 The OER performance of different loading amount of Co 2 B on CoSe 2. (a) Linear sweep voltammograms and (b) Tafel plots. Figure S7 Electrochemical cyclic voltammogram of (a) CoSe 2 /DETA, (b) Co 2 B and (c) Co 2 B/CoSe 2 at different scanning rates. (d) The C dl obtained at 1.4 V vs. RHE. (e) Nyquist 7

8 plots and (f) Current versus time during the long term (30 h) with a constant potential (320 mv) of Co 2 B/CoSe 2 for OER. Table S3 Electrochemical parameters of different catalysts for HER. Catalyst Tafel slopes [mv dec 1 ] η j=10 [mv] Electrolyte B Co 2 B M KOH MoB 8 59 ~210 1 M KOH Se CoSe M KOH 9 NiSe M KOH 10 Amorphous MoS x N/A M KOH S MoS 2+x /FTO 11 N/A M KOH Co-S/FTO 12 N/A M KOH P CoP/CC M KOH MoP ~135 1 M KOH C Mo 2 C 8 54 ~200 1 M KOH Co-NRCNTs 15 N/A M KOH current work Co 2 B/CoSe M KOH Figure S8 The HER performance of different loading amount of Co 2 B on CoSe 2. (a) Linear sweep voltammograms and (b) Tafel plots. Incorporating with different amount of Co 2 B on CoSe 2, Co 2 B/CoSe 2 catalysts also exhibit different HER activities in 1M KOH. Originally, with increasing Co 2 B concentration, the performance of the hybrid catalyst increases. It is interesting to note that as the continuing increase of the Co 2 B loading amount, the HER performance of the catalysts decline. The results show that Co 2 B/CoSe 2 exhibits the best HER activity when the precursor of Co is 0.5 mm. 8

9 Figure S9 Electrochemical cyclic voltammogram of (a) CoSe 2 /DETA, (b) Co 2 B and (c) Co 2 B/CoSe 2 at different scanning rates. (d) The C dl obtained at V vs. RHE. (e) Nyquist plots and (f) Current versus time during the long term (30 h) with a constant potential (300 mv) of Co 2 B/CoSe 2 for HER. Figure S10 The amount of H 2 and O 2 theoretically calculated and experimentally measured versus time for Co 2 B/CoSe 2 at 2.0 V for 60 min. The results show that the Faradaic efficiency of hydrogen and oxygen evolution are close to 100%, and the volume ratio of H 2 and O 2 is close to 2. References [1] Masa, J.; Weide, P.; Peeters, D.; Sinev, I.; Xia, W.; Sun, Z.; Somsen, C.; Muhler, M.; 9

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