Microstructure Effects on the Electrochemical Properties of Graphene/Manganese Oxide Porous Electrodes

Abstract

Manganese oxide (MnO2) nanowires as guest hosts with ultra-large reduced graphene oxide (RGO) sheets form a sandwich structure and may provide electrochemical properties that cannot be achieved with either material alone. Ultra-large graphene oxide (GO) sheets exhibit an order of magnitude larger length to diameter ratio compared to most studies with smaller GO sheets (~70,000 compared to ~2,000) and demonstrate potential for improved electrode design. Understanding the self-assembly properties of the combination of MnO2 nanowires and ultra-large RGO sheets was explored by two methods: a) mixing both dispersions that have been synthesized separately to form a mixture and b) growing the nanowires along the ultra-large GO sheets to form a hybrid material. The pure and multicomponent dispersions were characterized by spectroscopy and microscopy techniques to characterize the materials and gain insight into their dispersion microstructures. The dispersions were then freeze-dried into aerogels with 3D porous architectures. Tuning the initial dispersion parameters, including concentration, MnO2 content, and assembly results in different morphologies and physical and electrochemical properties of the aerogels. Results from this work convey that the hybrid exhibits a more uniform distribution of the nanowires than that of the mixture and the porous electrodes consist of mesopores (diameters between 2 and 50 nm) with interconnected pathways. Furthermore, the electrochemical performance of the hybrid and mixture electrodes suggests that there may be stronger interfacial contact between the RGO sheets and MnO2 nanowires in the hybrid due to the nanowires’ nucleation and growth along the sheets as opposed to the mixture. Comparison of the dispersion and aerogel morphologies as well as electrochemical performance provided insights into the relationships between material type, initial dispersion microstructure, and electrode properties.