50-100 μm-thick pseudocapacitive electrodes of MnO₂ nanoparticles uniformly electrodeposited in carbon nanotube papers

To overcome the tradeoff between the gravimetric capacitance and loading density of pseudocapacitive MnO₂, we electrodeposited MnO₂ nanoparticles on the carbon nanotube (CNT) surfaces in 18-37 μm-thick self-supporting CNT papers. We examined the electrodeposition conditions including constant potential, constant current, and potential pulses, and obtained MnO₂-CNT hybrid electrodes containing MnO₂ nanoparticles uniformly deposited at 60-90 wt% with an expanded CNT matrix. The MnO₂-CNT hybrid electrode with a thickness of 62 μm, density of 1.09 g cm^-3, areal mass of 6.75 mg cm^-2, and 82 wt% MnO₂ load showed a total gravimetric capacitance of 120 and 51 F_total g_electrode^-1, volumetric capacitance of 131 and 56 F_total cm^-3 and areal capacitance of 0.81 and 0.34 F_total cm^-2 at scan rates of 2 and 200 mV s^-1, respectively. The large thickness, moderately high mass density, and fairly conductive CNT matrix realized such high values of gravimetric, areal and volumetric capacitances that are important for practical devices.