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.