Assessment of three-dimensional nitrogen-doped mesoporous graphene functionalized carbon felt electrodes for high-performance all vanadium redox flow batteries

Abstract

We demonstrate the synthesis of three-dimensional (3D) nitrogen doped mesoporous graphene-functionalized carbon felt (NMG-CF) via a facile hydrothermal process. These NMG-CF act as electrocatalysts in an all-vanadium redox flow battery (VRFB). NMG-CF exhibits a uniform distribution of nitrogen atoms and spectroscopic studies indicate successful N-doping in the form of pyridinic-N, pyrrolic-N, quaternary-N and oxidic-N configurations. NMG-CFs show superior electrocatalytic activity towards V2+/V3+ and VO2+/VO2+ redox couples than activated-CF (A-CF) and mesoporous graphene-CF (MG-CF). Furthermore, NMG-CF exhibits 14–16% greater energy and voltage efficiencies than A-CF at 150 mA cm−2, with an excellent rate capability and cycling stability at current densities of 50–275 mA cm−2. These enhancements are attributed to improved hydrophilicity, 3D N-doped mesoporous structures, enhanced specific surface area and rapid charge/electron transfer. Moreover, N-doping generates defects acting as active sites and alters the electronic and chemisorption properties of NMG due to the large electronegativity difference between C and N atoms, making NMG-CFs more electrochemically accessible than A-CF and MG-CF. Notably, electrocatalytic activity is dependent not only on N-doping content but also on nitrogen configuration. The above results reveal the great potential of NMG-CFs as advanced electrode materials for VRFB.