*2.4. Electrochemical Tests*

The CV cycle test and steady-state polarization curve test of the electrolyte were performed using a CHI 760B electrochemical workstation (Shanghai Chenhua Instrument, Shanghai, China). We recorded the current versus potential curves using a three-electrode

electrochemical cell in the CV cycling test with a scan rate range of 10–200 mV s−<sup>1</sup> in a potential range of −0.6–1.8 V at 25 ◦C, in which the graphite electrode (surface area of 3.14 mm2), saturated calomel electrode, and platinum electrode (surface area of 1 cm2) are the working electrode, reference electrode, and counter electrode, respectively. The steady-state polarization curve was tested with a potential range of 0.49–0.56 V and a scan rate of 1 mV s−1, because the current and voltage are closer to a straight line under the condition of lower potential. Electrochemical impedance spectroscopy (EIS) was also obtained at room temperature with the sinusoidal excitation voltage of the electrolyte of 5 mV and the frequency range between 0.01 Hz and 100 kHz. Prior to each electrochemical measurement, the working electrode was polished with SiC paper and then washed with distilled water. The reference electrode was washed with distilled water and the solution in salt bridge was replaced before use. Platinum plate electrode is cleaned with distilled water and ultrasonically treated.
