Facile chemical activation of graphite felt by KMnO4 acidic solution for vanadium redox flow batteries

Abstract

The activation method is proposed to enhance the electroactivity of the graphite felt (GF), to be used as positive half-cell electrode for vanadium redox flow batteries (VRFBs). Two following activation approaches are used 1) functionalization of electrode surface by the chemical treatment with acidic solution of the KMnO4 and 2) deposition of KMnO4 derived particles on the surface of GF. The intrinsic heterogeneous electronic transfer constant (k°) of V(V)/V(IV) system is enhanced by 2.6 and 6.1 times for the oxidation (V(IV) → V(V)) and reduction (V(V) → V(IV)) reactions, respectively. The performed electrolysis in the half-cell shows the improved reversibility of the electrode. The treated electrode is also evaluated on the stack level by using 100 cm2 GF in each electrolytic section of the battery, by charge and discharge cycles. The results clearly indicate the lowering of anodic and cathodic overvoltage, i.e. the average charging voltage decreases from 1.86 V to 1.76 V, while the obtained voltage during discharge increases from the 0.59 V to 0.8 V. The proposed activation method is operationally simple; and activated electrode shows the promising performance as positive half-cell electrode in VRFBs.