Enhanced electrocatalytic performance of phosphorus and metal oxide-modified graphite electrodes for all-vanadium redox flow batteries

Abstract

This study evaluates the electrocatalytic performance of graphite powder (GP) electrodes modified with CoO, NiO, and oxygen-rich phosphorus functional groups (P-GP) to improve the sluggish VO₂⁺/VO²⁺ redox kinetics in vanadium redox flow batteries (VRFBs). Although these electrocatalysts have been extensively applied to graphite felt, their comparative activity on graphite powder, a lower-cost, structurally distinct material, has not been systematically examined. Their intrinsic catalytic behavior was assessed through cyclic voltammetry, electrochemical impedance spectroscopy, charge–discharge cycling, and polarization tests. Pristine GP delivered a coulombic efficiency (CE) of 89.11%, voltage efficiency (VE) of 88.01%, energy efficiency (EE) of 78.43%, and power density of 49 mW cm⁻². CoO- and NiO-modified GP improved performance with CE values of 92.5% and 90.79%, and power densities of 154 and 149 mW cm⁻², respectively. The P-GP electrode showed the best results, with CE of 94.41%, VE of 93.97%, EE of 88.72%, and power density of 167 mW cm⁻², while retaining ∼96% CE over 90 cycles. This study highlights phosphorus-functionalized and metal oxide modification (CoO, NiO) in enhancing the electrocatalytic performance of graphite powder electrodes, while providing a clear comparative assessment of their intrinsic electrocatalytic activity on graphite powder electrodes.