Fig. 5. (a) CV curves of undoped and Cu-doped Mn₃O₄ samples at 50 mV s⁻¹ and (b) CV curves of 1.5 at% Cu-doped Mn₃O₄ sample at different current density. (c) The plots of the specific capacitance as a function of Cu doping concentration at different scan rates. (d) Charge–discharge curve of 1.5 at% Cu-doped Mn₃O₄ sample. (e) Cycle performances of 1.5 at% Cu-doped Mn₃O₄ at the current density of 2 A g⁻¹.