Crystalline MnO2 Nanodomains in Mesoporous Carbon for Electrochemical Capacitors

MnO2/mesoporous carbon hybrid nanocomposites were synthesized to achieve high values of MnO2 capacitance even at high scan rates of 100 mV/s. High resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray spectroscopy (EDX) illustrated the MnO2 was uniformly distributed throughout the mesoporous carbon structure. The highly loaded nanocomposites contained ~1 nm thick MnO2 nanodomains inside of the mesoporous carbon pores. HRTEM and X-ray diffraction (XRD) showed formation of MnO2 nanocrystals with lattice planes corresponding to birnessite. The electrochemical capacitance of these hybrid materials in aqueous 1 M Na2SO4 electrolyte containing as little as 2 wt % MnO2 materials exhibit a high specific capacitance with respect to MnO2 of ~560 F/gMnO2. Even for 30 wt% MnO2, a high MnO2 capacitance of 137 F/gMnO2 on carbon was observed at 100 mV/s. Sodium ion diffusion coefficients on the order of 10-9–10-10 cm2/s were measured with chronoamperometry. Controlled growth of the thickness and crystallinity of conformal, redox-active MnO2 on mesoporous carbon with high surface areas will facilitate high rate, high energy storage.