A Combined Theoretical-Experimental Study of Energy Storage Behavior of Reduced Graphene Oxide/Manganese Oxide Supercapacitor

Abstract

Miniaturized electrochemical capacitors, such as micro-supercapacitors, have emerged as a new class of micro-power sources. However, they have limited uses in micro-energy storage because of their low volumetric energy densities. Here, compact structured Birnessite (MnO2)/reduced graphene oxide (RGO) hybrid films (Fig.1) consisting of multi-valent Mn ions were prepared via a two-step fabrication process involving a simple chemical redox reaction and post-thermal annealing to form a multi-layer structured film. With this versatile method, GO sheets can act as either a growth template for the highly capacitive MnO2 nanosheets or as a mechanical support for the production of compact hybrid films. Enhanced electrochemical characterization of the MnO2/RGO hybrid film revealed a remarkable ultrahigh volumetric capacitance (493 F/cm3 at 10 mV/s), ultrahigh energy and power density (13.3 mWh/cm3 at 2.5 A/cm3 and 22.6 W/cm3 at 58 A/cm3, respectively) and a semi-permanent cycle life (97% capacitance retention) in an aqueous electrolyte system. We also elucidated operating mechanisms of MnO2/RGO capacitor using state-of-the-art density-functional theory calculations with MnO2 nanoparticle/RGO sheet models. Various materials properties of MnO2/RGO systems including stoichiometric deviation, defect energetics, electron transfer, and kinetic behavior of cations and their contributions to their capacitance have been explored in atomic level. Figure 1