N- and P-co-doped Graphite Felt Electrode for Improving Positive Electrode Chemistry of the Vanadium Redox Flow Battery

In this study, N- and P-co-doped carbon derived from microcrystalline cellulose, and diammonium hydrogen phosphate ((NH4)2HPO4) were employed as the catalyst for VO2+/VO2+ redox reaction. Doping with P enhanced the onset potential and the presence of N (in the form of pyridinic- and pyrrolic-N) and P together enhanced the peak current for the VO2+/VO2+ redox reaction. Further, the doping improved the wettability of the catalyst, thereby increasing the interfacial area available for the reaction. The use of N- and P-co-doped catalyst coated graphite felts in the vanadium redox flow battery (VRFB) yielded higher energy efficiency and discharge capacity, especially at higher current densities in contrast to un-doped graphite felt (GF). 100 cycles of charge-discharge with minimal capacity fade were demonstrated while using N- and P-co-doped catalyst at the positive electrode of the VRFB.