Electrochemical mechanisms of activated carbon, α-MnO2 and composited activated carbon-α-MnO2 films in supercapacitor applications

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.