Influence of Surface Treatments on the Electrochemical Reactivity of VO2+/VO2 + Redox Couple Using Carbon Fiber Ultramicroelectrodes

Abstract

Several surface treatments for flow battery electrodes have been reported in the literature to enhance performance of the all-vanadium redox flow battery (RFB). An increase in carbon-oxygen functional groups has been correlated with an increase in rate of electron transfer for the VO2+/VO2 + redox reaction and thus, enhanced performance of the all-vanadium RFB. It is proposed that surface oxide groups act as electron transfer mediators by providing reaction or adsorption sites. Thus far, treatments reported in the literature are difficult to compare, as the evaluation of each treatment varies. The use of carbon fiber ultramicroelectrodes, prepared from treated felts, can be used as a definitive basis for comparing treatments and investigating the true effect of surface alterations. Reaction kinetics of the VO2+/VO2 + reaction can then be calculated and compared across treatments. First, fabrication and sample mounting methods for the carbon fibers were verified by comparison to platinum ultramicroelectrodes. Then the role of the surface structure of carbon felts for the aqueous all-vanadium redox flow battery (RFB) was investigated. The graphite felt had undergone thermal, chemical, or electrochemical treatments in order to alter surface structure. The structure and composition of the felts was analyzed using x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Electrochemical performance was evaluated using cyclic voltammetry (CV), pulse voltammetry (PV), and electrochemical impedance spectroscopy (EIS). Thermally treated felts (300oC for 30hrs in air) exhibited a loss of surface functional groups, but an increase in double layer capacitance attributed to an increase in the active surface area (9.9 times that of untreated). This suggests that available surface area plays a larger role than the specific functional groups on the surface of the carbon electrodes in increasing the electrokinetics of the VO2+/VO2 + redox reaction.