Electrochemical modification of a pyrolytic graphite sheet for improved negative electrode performance in the vanadium redox flow battery

The vanadium redox flow battery is a promising technology for buffering renewable energies. It is recognized that negative electrode is the limitation in this device where there are problems of slow heterogeneous electron transfer (HET) of V3+/2+ and parasitic H2 evolution. Any methods aimed at addressing one of these barriers must assess the effects on the other. We examine electrochemical enhancement of a common commercially available material. Treatment of Panasonic pyrolytic graphite sheets is through oxidation at 2.1 V vs. Ag/AgCl for 1 min in 1 M H2SO4. This increases the standard HET rate for V3+/2+ from 3.2 × 10−7 to 1 × 10−3 cm/s, one of the highest in literature and shifts voltammetric reductive peak potential from −1.0 V to −0.65 V in 50 mM V3+ in 1 M H2SO4. Infrared analysis of the surfaces indicates formation of COH, CO, and CO functionalities. These groups catalyze HET with V3+/2+ as hypothesized by Skyllas-Kasacos. Also of significance is that electrode modification decreases the fraction of the current directed towards H2 evolution. This proportion decreases by two orders of a magnitude from 12% to 0.1% as measured at the respective voltammetric peak potentials of −1.0 V (pristine) and −0.65 V (modified).