Bulk Aging of Graphite-Polypropylene Current Collectors Induced by Electrochemical Cycling in the Positive Electrolyte of Vanadium Redox Flow Batteries

Abstract

Carbon-based bipolar plates (BPP) are essential components of the vanadium redox flow battery (VRFB) stack guaranteeing electrical conductivity and physical separation of adjacent cells. They are usually made of composite materials with complex structures containing conductive carbon components and polymer binder materials. In order to investigate their aging by simulation of repetitive charging/discharging conditions in areas with sluggish electrolyte flow, commercially available graphite-polypropylene BPPs were subjected to galvanodynamic treatment under diffusion limitation conditions in a three electrode electrochemical cell with positive electrolyte (VO2+/VO2+) using different states of charge (SOC). The current was swept between 100 and −100 mA/cm2. After aging, cyclic voltammetry (CV) in diluted H2SO4 was used to analyze the change in double layer capacitance and redox activity of the BPPs. Morphological investigations of the surface texture and bulk material were performed by means of scanning electron and confocal microscopies as well as by X-ray computed microtomography. After 3000 sweeping cycles in the positive electrolyte at high SOC the BPP showed an increase in double layer capacitance as well as higher roughness and increase in open pore volume in depths up to 100 μm. The aging effects are ascribed to oxidation and corrosion of the surface.