Reduced graphene oxide as a charge reservoir of manganese oxide: Interfacial interaction promotes charge storage property of MnO_x-based micro-supercapacitors

Reduced graphene oxide (rGO)-based micro-supercapacitors (MSCs) have emerged as a new type of micro-energy storage device. However, the low volumetric energy density of rGO hampers the application of MSCs in miniaturized energy storage devices. Hybridization of pseudocapacitive materials with rGO is a potential approach to increase the energy density of MSCs. Herein, a densely packed hybrid film of birnessite-type manganese oxide (K-MnO_x) supported by rGO is developed, and hybrid-film-based MSCs are found to show a high volumetric capacitance (490 F/cm³) that is ∼ 1.2 and 19 times greater than those of rGO and K-MnO_x-based MSCs, respectively. A semi-permanent cycle life with capacitance retention of 97% after 10,000 cycles is observed. Moreover, a charge reservoir concept is introduced, which explains the origin of the high pseudocapacitance of the K-MnO_x/rGO hybrid in a unique way. It is observed that synergistic interaction among the charge reservoir (rGO) and electron transfer channel (K-MnO_x, which becomes conductive at the interface) facilitates the charging and discharging of K ions, with minimum deviation of the Mn oxidation states. This new charge reservoir concept would serve as a stepping stone toward designing novel hybrid devices with high energy storage capabilities.