2.3. Electrochemical Testing

The electrochemical properties of the obtained materials were evaluated in two-electrode and three-electrode systems, respectively, using 6 M KOH aqueous solution as the electrolyte. To fabricate the working electrode, the active materials, polyvinylidene fluoride and carbon black were mixed according to the mass ratio 8:1:1 in moderate N-methyl-2-pyrrolidone solvent. After stirring for 6 h, the slurry was coated on a nickel foam and vacuum baked at 80 °C for 24 h. The mass loading of the active material for a single electrode was about 3 mg·cm⁻². An Hg/HgO electrode and a platinum foil were used as the reference electrode and counter electrode in the three-electrode system. In the two-electrode system, two PRK3N1 electrodes were employed to assemble the symmetric supercapacitor with a separator. A Metrohm Autolab PGSTAT302N electrochemical workstation (Metrohm, Herisau, Switzerland) was applied to carry out galvanostatic charge/discharge (GCD), Cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) tests.

The specific capacitance (C_g) was calculated by the following Equation (1) according to the discharge curve:

where k = 1 in a three-electrode system and k = 2 in a two-electrode system, I (A) is the discharge current, ∆t (s) the discharge time, m (g) is the mass of active materials in each electrode, and ∆V (V) is the discharge voltage range removal of the IR drop [19,20].

The power density (P) and energy density (E) for the two-electrode system were calculated by the following Equations (2) and (3):

where C (F·g⁻¹) is the specific capacitance in the two-electrode system calculated from the Equation (1), ∆t (s) is the discharge time, and ∆V (V) is the discharge voltage range excluding the IR drop [21,22].