*2.4. Electrochemical Measurements*

Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) of single electrodes were performed using a potentiostat (VSP, Bio-Logic). The electrolyte is a 1 M H2SO4 aqueous solution that purged N2 gas for 20 min. Measurements are conducted through a three-electrode system. A reference electrode was a saturated calomel electrode (SCE), and a counter electrode was a platinum wire. For voltammetry measurements, the cell potential was scanned at 5 mV/s and 20 mV/s. When the scan rate was 5 mV/s, it was measured for 5 cycles, and when it was 20 mV/s, it was measured for 4990 cycles. Galvanostatic charge-discharge behavior was measured in current density, from 0.5 to 20 A/g. EIS was measured from 100 kHz to 100 mHz.
