Characteristic Dual-Domain Structure of Reduced Graphene Oxide and Its Guidance to Higher Specific Capacitance

Reduced graphene oxide (rGO) has been regarded as an attractive material for diverse energy storage applications such as fuel cells, batteries, and supercapacitors. Graphene synthesized by the chemical vapor deposition method or graphene oxide (GO) produced by oxidative exfoliation process from graphite has critical<br/>drawbacks of poor scalability or poor electrical conductivity to be used as electrochemical electrode materials. In contrast, rGO addresses those limitations of graphene and GO by providing considerable conductivities of both electron and ion due to simultaneously existing components of graphene and GO as well as the advantage of high-yield mass production. For successful applications of rGO to high-performance electrochemical energy storage devices as electrode materials, both high electrical conductivity of rGO and high ion accessibility from the electrolyte to rGO are crucial requirements. The electrical conductivity of rGO,<br/>however, has a trade-off property of ion accessibility. If the electrical conductivity were enhanced by increasing the degree of reduction of rGO [28–32], the ion accessibility from the electrolyte to rGO would decrease due to diminished hydrophilicity with the reduction of the GO component. Thus, optimization between electrical conductivity and ion accessibility is necessary for successful applications of rGO to high performance<br/>electrode material. Investigations on the changes of the above two intrinsic properties with the reduction level of rGO had been rarely reported thus far. Furthermore, fundamental and scientific investigations on the nature of rGO surface, e.g., characteristic domain structures of rGO with reduction levels have been never reported.<br/>In this study, the electrical domain structures of rGO were investigated by the conductive atomic force microscopy (C-AFM) technique. This analysis was expected to reveal the amounts and configurations of graphene and GO domains. If the characteristic 2D domain structures were revealed and controlled with reduction level, the optimized domain structure would guide to higher performance of rGO-based electrode<br/>materials. The specific capacitance of rGO freestanding sheets was discussed with their characteristic domain structures. In addition, hybrid composite sheets of Fe2O3 and MnO2 ceramic particles mixed with rGO were examined for further enhanced specific capacitance.