4.6. Electrochemical measurements
For HER/OER, the electrochemical workstation CHI 660e (Shanghai Chenhua Co., China) was used to collect the electrochemical properties of various catalysts in traditional three electrode systems and alkaline environments (1 M KOH). Among them, the electrocatalyst, Hg/HgO, and Pt sheet in the three electrode system were used as the working electrode (1 × 0.5 cm2), reference electrode and counter electrode, respectively. Please note that the customized reference electrode in in-situ Raman and CV systems was used to study the surface electrochemical reconstruction process, which was selected as the customized Ag/AgCl. In addition, during the pre-testing phase, the Hg/HgO electrode and Ag/AgCl electrode were calibrated at room temperature. The potentials versus Hg/HgO and Ag/AgCl reference electrode were converted to reversible hydrogen electrode (RHE): ERHE = EHg/HgO + 0.098 + 0.059 × pH and ERHE = EAg/AgCl + 0.1971 + 0.059 × pH. The PH value of 1 M KOH was determined to be 13.5 by PH device. In note, HER and OER data were corrected through 90 % IR compensation, aiming to accurately obtain the intrinsic activity of the catalyst. For HER, CV curve was obtained by the scan rate of 100 mV s−1 for 60 cycles between -0.5 and -2.0 V vs Hg/HgO for activating catalysts before the LSV test. Similarly, CV test was obtained by the scan rate of 100 mV s−1 for 60 cycles between 0 and 1.2 V vs Hg/HgO for activating catalysts before the LSV test for OER. In detail, LSV curves were selected by a scan rate of 1 mV s−1, the other parameters of LSV test (such as potential range, etc.) were consistent with the activation process. For HER, CV curves at different scan rate between -0.75 and -0.85 V vs Hg/HgO were used to calculate ECSA, which were selected as 20 mV s-1, 40 mV s-1, 60 mV s-1, 80 mV s-1 and 100 mV s-1. Similarly, CV curves at different scanning rate (2 mV s-1, 4 mV s-1, 6 mV s-1, 8 mV s-1 and 10 mV s-1) between 0.22 and 0.32 V vs Hg/HgO were used to calculate electrochemical active surface area (ECSA) for OER. Furthermore, the LSV curve was normalized by the double layer capacitance obtained from CV data. The relationship between the logarithmic form of current density and overpotential was used to fit the Tafel slope. For overall water splitting, the catalyst grown on CC was used as the cathode and anode in two electrode system to study the electrocatalytic performance. Considering the complex electrolyte environment of overall water splitting in chemical industry, the electrochemical signal of overall water splitting has not been calibrated by the IR compensation process. The charge transport behavior on the catalyst surface was studied by electrochemical impedance spectroscopy (EIS). Among them, the application potentials in HER and OER processes were set to -1.1 V and 0.63 V vs Hg/HgO, respectively. The frequency range and amplitude for EIS testing were selected to be 0.01 ~ 1×105 Hz and 0.005 V. For HER/OER, the stability of the catalyst was estimated in multiple current densities over a short period of time. Besides, for overall water splitting, the stability measurements (i-t) were recorded at constant current density (10 and 100 mA cm-2) for 100 h to evaluate the stability of electrocatalysts.