Influence of Edge- and Basal-Plane Sites on the Vanadium Redox Kinetics for Flow Batteries

The reaction kinetics of V-II/V-III and (VO2+)-O-IV/VVO2+ redox on carbon electrodes in sulfuric acid limit the development of vanadium redox flow batteries (VRFB) with high power and efficiency characteristics. Cyclic voltammetry and symmetric flow cell measurements on selectively masked graphite foil and highly oriented pyrolytic graphite electrodes revealed that edge carbon sites provide faster kinetics for V-II/V-III and (VO2+)-O-IV/VVO2+ redox than basal carbon, especially at low vanadium concentrations. The understanding was used to explain the marked enhanced kinetics of carbon paper electrodes with heat-treatments in air relative to that without, which was supported by X-ray photoelectron spectroscopy measurements that showed much higher amounts of surface functional groups on the heat-treated carbon upon exposure to the VV species in the electrolyte. Of particular significance to note is that markedly enhanced kinetics for the V-II/V-III redox for the heat-treated carbon were found at both low and high vanadium concentrations, while similar enhancement was found for the (VO2+)-O-IV/VVO2+ redox for low vanadium concentrations but much smaller increased kinetics were noted for high vanadium concentrations required for practical flow batteries. This result was further confirmed by symmetric flow cell measurements that show much higher currents for the V-II/V-III electrolyte using heat-treated carbon in comparison to the as-received, while comparable currents were found for (VO2+)-O-IV/VVO2+ electrolyte, indicating that the redox kinetics of V-II/V-III can be limiting for VRFBs using as-received carbon (low edge carbon and oxygen functional groups). These findings provide new insights and strategies for carbon electrode designs for high-power VRFBs.