The electrochemical testing was performed using a Gamry Potentiostat 3000 in a two-electrode configuration. Cyclic voltammetry (CV) tests were conducted at 1 mV/s scan rate in the potential range of 0 to -2 V. Electrochemical impedance spectroscopy (EIS) was performed at different DC voltages (-1.0 V, -1.2 V, -1.4 V, -1.6 V, -2.0 V) in the frequency range of 10 5 Hz to 10 -3 Hz. Galvanostatic charging and discharging (GCD) was performed at a constant current density of 50 mA.g -1 to test the gas adsorption-desorption behavior.