Synthesis of Co-BTC nanoparticles:
All chemicals are of analytical grade and used without purification. Co-BTC used as MOF precursors was prepared according to the method described in the related literatures with some modications. For synthesis of Co-BTC, 0.2 g C4H6CoO4 ·4H2O and 1.2 g PVP was dissolved into the mixed solution of ethanol (20mL) and deionized water (20 mL), formed solution A, which was put on the magnetic stirrer with a low speed. 0.36 g H3BTC was dissolved into the mixed solution of ethanol (20 mL) and deionized water (20 mL), formed solution B. Then solution B was poured into solution A with a constant speed by using an injector (10 mL). The mixed solution was kept stirring until the precipitation formed. Finally, the product was centrifuged after 24 hours' standing and washed four times by ethanol beforedrying at 60 C in vacuum.
Synthesis of CoSO4 nanoparticles:
Solution A: The as-prepared 40 mg of Co-BTC was dissolved in 20 ml ethanol solution and dissolved by magnetic stirring. Solution B: 240 mg Na2S·9H2O was dissolved in 10 ml deionized water solution to form a colorless transparent solution after continuous stirring. The solution B is slowly poured into solution A, and then it is sonicated for 30 min to make the mixture disperse uniformly. After that, the solution was transferred into a 35 ml reactor, heated to 130℃ in the oven, and kept for 6 hours. The reaction solution was centrifuged and washed alternately with deionized water and ethanol solution for 5 times finally dried in a vacuum. The obtained solid was designated as S-0.
Synthesis of Ag-CoSO4 nanohybrids:
25 mg of the as-prepared CoSO4 nanoparticles were dissolved in 25 ml deionized water to form uniformly dispersed solution after stirring. And then 0.5 ml, 1.0 ml, 2.0 ml and 4.0 ml AgNO3 (100 ml containing 22.4 mg of AgNO3) solution was dropwise added to above solution with vigorous stirring for 40 min at room temperature. The mixed solution was transferred to a 30 ml autoclave, heated to 160 ℃ and kept for 6 h. The obtained product was centrifuged and washed with deionized water and ethanol solution for 3 times and dried in a vacuum oven. The obtained samples were designated as S-1, S-2, S-3, and S-4, respectively.
Transmission electron microscopy (TEM) measurements were performed on a Hitachi H-7650 and JEOL JEM-2100F field emission transmission electron microscope and a HRTEM (JEOL-2011) was operated at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were acquired on a JEOL JSM-6700 M scanning electron microscope, The powder XRD data were obtained on a Japan RigakuD/MAX-γA X-ray diffractometer using Cu-Kα radiation (λ=1.54178Å) with 2θ range of 20-80°. Fourier transform infrared (FT-IR) spectra were determined by a Magna-IR 750 spectrometer. Raman spectra were recorded with a LabRAM HR Raman spectrometer ranging from 500 to 3000 cm-1. XPS was conducted on an ESCALAB 250 X-ray photoelectron spectrometer instrument. XPS was performed on an ESCALAB 250 X-ray photoelectron spectrometer using Al Ka radiation. Co XANES were obtained at soft x-ray magnetic circular dichroism station in national synchrotron radiation laboratory in USTC, Hefei.
All of the electrochemical measurements were performed in a three-electrode system on an electrochemical workstation (CHI660D) in 1M KOH electrolyte. To prepare the working electrode, typically, 4 mg of catalyst and 30μL Nafion solution (Sigma Aldrich, 5 wt %) were dispersed in 1 mL ethanol solution, and then sonicated it for 40min to form a homogeneous dispersion. Then 5 uL of the dispersion was dropped onto a glassy carbon electrode with 3 mm diameter (loading 0.285 mg/cm2). While a Ag/AgCl (in 3 M KCl solution) electrode and a platinum foil were served as the reference electrode and counter electrodes, respectively. All of the potentials were calibrated to a reversible hydrogen electrode (RHE). The linear sweep voltammetry (LSV) were applied at a scan rate of 5 mV s-1 from 0 V to 0.7 V (vs. RHE) into 1 M KOH electrolyte with a stable flow of N2 gas maintained over the electrolyte during the OER experiment. In order to investigate the stability of the samples, the cyclic voltammetry (CV) were applied at a sweep rate of 50 mV/s in 1 M KOH solution in the potential from 0 V to 0.7 V (vs. RHE) for 1000 cycles. In addition, we used the commercial RuO2 catalyst was used as a referencet to evaluate the catalytic activity of the our catalyst.
Supplementary Figures and Tables
Table S1. Chemical compositions of samples after etching process prepared at different annealing temperatures by XPS measurement
Table S2. Comparison of the OER activity between S-2 with some cobalt-based OER catalysts in basic condition from literatures.
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