Kinetic relevancy of surface defects and heteroatom functionalities of carbon electrodes for the vanadium redox reactions in flow batteries

Heteroatom doping of carbon electrodes is an extensively practiced approach to enhance electrokinetics of vanadium redox reactions in flow batteries, because the doped heteroatom functionalities are conventionally considered as the catalytic active sites. In this study, we conducted p type (boron), n type (nitrogen or oxygen), and p-n type (boron and nitrogen) heteroatom doping on graphite felt electrodes, and thoroughly studied their intrinsic electrokinetic effects by collective and quantitative structure-property correlation analysis. The studies reveal that the apparent kinetic enhancements observed by the p type and the n type heteroatom doping are primarily due to the surface lattice defects on carbon electrodes rather than the furnished heteroatom functionalities. Markedly, however, the B and N co-doping give rise to exceptional enhancements in the intrinsic vanadium redox kinetics, both for the VO2+/VO2+ and V2+/V3+ redox reactions, with 2-4 fold greater electrocatalytic activities than those predicted by the increase of electrochemical surface area by the surface lattice defects. Consequently, the p-n type B and N co-doped GF electrodes offers significant enhancements in the efficiency and energy storage capacity of the vanadium redox flow battery that cannot be achieved by the p type (B) or the n type (N and O) heteroatom doping.