Pseudocapacitive Behavior of (Fe, Cu) Based Co ₃ O ₄ as High‐Performance Electrode Materials for Solid‐State Stiff and Flexible Supercapacitors

Detailed investigation into the electrochemical behavior of Fe‐ and Cu‐substituted Co 3 O 4 , prepared via a hydrothermal technique in the form of FeCo 2 O 4 and CuCo 2 O 4 at 150 °C, is reported in this study. X‐ray diffraction analyses of all three compositions confirmed the formation of highly crystalline, single‐phase spinels with cubic symmetry. Scanning electron microscopy images revealed distinct morphologies with spherical, flower‐like, and clustered grains. Substitution of Cu and Fe increased the specific capacitance (Csp) from 607 to 1350 F g −1 and 410 to 1237 F g −1 in stiff and flexible electrodes, respectively. Calculations of Csp from cyclic voltammetry (CV) profiles revealed a strong dependence on electrolyte molarity for all spinels studied, irrespective of whether stiff or flexible current collectors were used. A shift in anodic and cathodic peak positions in the CV profiles with increasing scan‐rates indicates rapid Faradaic kinetics at the electrode/electrolyte interface and the dominance of a capacitance‐controlled process. Electrochemical impedance spectroscopy analysis demonstrated low internal resistance, below 0.5 Ω, in all spinels, affirming the easy movement of electrolyte ions toward electrochemically active sites. Thus, this study illustrates the potential of these spinels as efficient electrode materials for supercapacitor applications that could be used in wearable electronics employing a KCl electrolyte and flexible copper current collectors.