Pure α-MnO₂ and activated carbon-MnO₂ (AC-MnO₂) films coated on Ni foam by electrophoretic deposition were applied as a supercapacitor electrode. The specific capacitance of AC-MnO₂ films (155.03 F g⁻¹) surpasses those of the pure AC (110.62 F g⁻¹) and pure MnO₂ film in the 1 M NaOH electrolyte. EDX and XPS detect an increase in the Na content and the reduction of Mn⁴⁺ to Mn³⁺ on the discharged MnO₂ electrode (at 0.0 V), whereas a decrease in the Na content and the oxidation of Mn³⁺ to Mn⁴⁺ were obtained on the charged MnO₂ electrode (at 0.45 V). Computational simulation of the Na inserted α-MnO₂ structure displays the connection of Na to O atoms and the increasing electron density on Mn atoms. EDX of the charged AC-MnO₂ (at −1.0 V) film detects a rise in the Na and a fall in the O contents, but the discharged AC-MnO₂ film (at 0.0 V) shows a decrease in Na and increase in O contents. The AC-MnO₂ film could retain 82.29% of the initial specific capacity after 10,000 cycles. Four series-supercapacitor coin cell assembled from the AC-MnO₂ anode and MnO₂ cathode delivers a power density of 2.79 kW kg⁻¹ and an energy density of 168.8 Wh kg⁻¹.