Electro-Mechanical and Electrochemo-Mechanical Coupling of Reduced Graphene Oxide-Based Electrodes

Abstract

Modern applications such as flexible electronics and structural energy storage require the application of external strain on electrodes. Although emphasis has been given to the development of strong and flexible electrodes, few studies have been conducted on the effect of applied stress to the electrochemical capabilities of electrodes and the exploration of internal stress development during cycling, which in turn may lead to electrochemical degradation. In this dissertation, we developed a new experimental setup for combined mechanical and electrical or electrochemical testing. This method was used with reduced graphene oxide (rGO), which is often used for supercapacitor electrodes due to its high electrical conductivity and large surface area, and as a conductive additive for battery electrodes. The change of resistance of rGO films was measured under tensile and bending stresses. Also, a model was developed to calculate the compressive piezoresistance of films by mathematically decomposing bending into its tensile and compressive components. For rGO films, the compressive piezoresistance was found to be much greater than the tensile. Micrography revealed that extensive damage occurred on the compressive side of the bending specimen, resulting in a large increase of resistivity. Additionally, electrochemo-mechanical coupling of rGO supercapacitor electrodes was measured in-situ. The coupling of capacitance, charge, stress, and strain for individual rGO electrodes was observed using our newly built instrumentation. The dependence of capacitance on the application of tensile strain was measured. The in-situ increase and decrease of tensile stress with the adsorption and release of ions onto the electrodes was measured. Finally, structural cathodes for zinc-ion batteries were developed, combining the inherent safety of zinc-ion batteries, with the multifunctionality of structural energy storage. For this study, rGO was combined with MnO2, a typical active material for Zn-ion batteries and with branched aramid nanofibers (BANF) as a binder. The addition of BANFs allowed for largely improved electrochemical and ultimate tensile properties. In-situ electrochemo-mechanical tests were also performed, showing that applied strain decreases the capacitance, but no measurable stresses develop during electrochemical cycling. We anticipate that our new approach and method will provide a new characterization platform for observed coupled electro-mechanical and electrochemo-mechanical phenomena.