Marked Electrocatalytic Effects of Two-Step Boron and Oxygen Atomic Doping of Carbon Electrodes for Vanadium Redox Flow Battery

The marked electrocatalytic effects of two-step B and O atomic doping of graphite-felt (GF) electrodes for VO2+/VO2+ and V2+/V3+ redox reactions in a vanadium redox flow battery (VRFB) are studied. The p- and n-type heteroatom doping is conducted via thermochemical reactions varying the doping sequence and temperature, and the effects of the structural configuration and the amount of the heteroatom functional groups doped on the GF surface are quantitatively investigated. The kinetic significance of the B and O surface centers for the vanadium redox reactions is identified. The two-step thermochemical heteroatom doping in the sequence of O followed by B induces the preferential formation of the most kinetically active BC3 surface sites, because the O doping step develops surface defects on the sp2 carbon lattice of the GF electrodes for promoting B doping as the BC3 structural configuration. The B/O-GF electrode obtained by the two-step O and B doping exhibits one order of magnitude greater catalytic activity for the vanadium redox reactions than the pristine-GF and affords an about 8-fold increase in the specific charge-discharge capacity of the VRFB compared to that obtained with the pristine-GF. The voltage efficiency of the VRFB with the B/O-GF electrode (89%, 50 mA cm-2) is much higher than that achievable by the pristine-GF (75%), demonstrating the marked effects of the two-step B and O atomic co-doping of the GF electrode for the vanadium redox reactions in the VRFB.