(397a) Fundamental Studies on the Origin of Reduced Graphene Oxide Enhancements in Energy Storage Applications

Graphene oxide (GO) offers unique opportunities to generate conductive composites consisting of electrochemically-active nanomaterials useful in energy storage applications.  The electronegative oxygen moieties present on the GO surface provide negatively-charged species to which positive cationic precursors common to energy storage materials such as metal oxides can adsorb.  This mechanism for producing electrode composites yields high surface coverage of the 2-D GO sheets. After reduction of GO to reduced graphene oxide (RGO), which can be accommodated in a multitude of ways, conductivity is restored and the active materials are in direct contact with an electron shuttle for effective delivery of electrons to otherwise poorly-conducting metal oxide intercalation materials.  At the same time the drying of RGO sheets during solid electrode preparation enhances the natural wrinkling of the graphene sheets and produces a 3-D porous structure through which Li⁺ ions are able to more rapidly diffuse.  Here we present a systematic study of the electrochemical enhancements RGO offers to electrode architectures in energy storage applications.  Specifically, we build a composite electrode consisting of MnO₂ nanowires and RGO and test the electrodes in lithium half-cell construct using an in-house cell design allowing for post-characterization of the electrode. Coulometric techniques were utilized to study the kinetics of the electron transfer reactions, Li⁺ ion diffusion and adsorption, and the electron storage capabilities of the RGO sheets. Cyclic voltammetry in conjunction with XRD also shows a self-healing mechanism for oxygen deficient sites on the MnO₂ nanowires when composited with RGO. Finally, Transmission and scanning electron microscopy were used to characterize the surface morphology of the MnO₂ nanowires and the RGO composite.