High pseudocapacitance of MnO ₂ nanoparticles in graphitic disordered mesoporous carbon at high scan rates

Nanocomposites composed of MnO ₂ and graphitic disordered mesoporous carbon (C _MnO2/C) were synthesized for high total specific capacitance and redox pseudocapacitance (C _{MnO 2}) at high scan rates up to 200 mV s ^-1. High resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray spectroscopy (EDX) demonstrated that MnO ₂ nanodomains were highly dispersed throughout the mesoporous carbon structure. According to HRTEM and X-ray diffraction (XRD), the MnO ₂ domains are shown to be primarily amorphous and less than 5 nm in size. For these composites in aqueous 1 M Na ₂SO ₄ electrolyte, C _MnO2 reached 500 F/g _MnO2 at 2 mV s ^-1 for 8.8 wt% MnO ₂. A capacitance fade of only 20% over a 100-fold change in scan rate was observed for a high loading of 35 wt% MnO ₂ with a C _MnO2 of 310 F/g _MnO2 at the highest scan rate of 200 mV s ^-1. The high electronic conductivity of the graphitic 3D disordered mesoporous carbon support in conjunction with the thin MnO ₂ nanodomains facilitate rapid electron and ion transport offering the potential of improved high power density energy storage pseudocapacitors.