First-principle investigations of nitrogen-, boron-, phosphorus-doped graphite electrodes for vanadium redox flow batteries

An issue that limits the large-scale application of vanadium redox flow batteries (VRFBs) is their low power density, which is associated with the slow reaction kinetics of vanadium redox couples. To enhance the activities of the electrode toward vanadium redox couples, modifying carbon electrode surfaces with heteroatom doping is an effective strategy. In this work, we investigate the catalytic activity of nitrogen (N), boron (B) and phosphorus (P) doped graphite electrodes for VRFBs via density functional theory calculations. A layer of graphene is adopted to represent the surface of a graphite electrode. It is found that water adsorption is stronger with hydrogenated pyridinic N-doped and pyrrolic N-doped graphene than that of graphitic N-, B- and P-doped graphene, while the density of state of all the modified graphene remains metallic features. These results indicate good wettability and electronic conductivity of heteroatom doped graphite electrodes for VRFBs. To further evaluate their catalytic activity towards the V ²⁺ /V ³⁺ redox reaction, the metrics of energy difference between inner-sphere and outer-sphere adsorption modes for V(H ₂ O) ₆ ²⁺ and V(H ₂ O) ₆ ³⁺ are considered. An interesting finding is that for the P-doped graphene surface, the catalytic activity for both V ²⁺ and V ³⁺ ions can be significantly improved, suggesting a promising method for developing carbon electrodes for VRFBs.