Enhancing high-rate capability of MnO2 film electrodeposited on carbon fibers via hydrothermal treatment

•Electrodeposited MnO2 films were firstly subjected to hydrothermal post-treatment.•The hydrothermal post-treatment in a K2SO4 solution decreases the valence of Mn.•The K2SO4-treated film keeps the ramsdellite structure and water content.•The K2SO4-treated film exhibits a good high-rate capability.•Suitable ratio of Mn4+ to Mn3+ is important for high-rate capability of the films.

MnO2 films electrodeposited on carbon fibers were subjected to hydrothermal treatment in deionized water and a K2SO4 solution. The effects of hydrothermal treatment on the structures and capacitive properties of the films were systematically evaluated. Compared to the as-deposited and K2SO4-treated films, the water-treated film exhibits a much lower capacitance at 2 mV s−1 due to the phase transformation from ramsdellite to manganite during the hydrothermal treatment. The K2SO4-treated film maintains the ramsdellite structure although the average oxidation state of Mn in the film is decreased to some extent. At the charge-discharge current density of 1C, the K2SO4-treated and as-deposited films show specific capacitances of 281.9 and 274.7 F g−1, respectively. But the K2SO4-treated film displays a capacitance of 135 F g−1 at 15C and a good high-rate capability which is more than 3 times as high as that of the as-deposited one. These results and the related kinetic analysis suggest that the suitable ratio of Mn4+ to Mn3+ plays an important role in determination of the high-rate capability of the film.