Drastic Improvement in Capacity-Retention and Polarization of Vanadium Redox Flow Battery with Hydrophilic Co3O4 Nanostructure Modified Activated Graphite Felt Electrodes

Abstract

Metal oxides supported on carbon materials are reported as catalysts for the positive and negative electrodes of vanadium redox flow battery (VRFB). In this study, thermally activated graphite felt (TGF) is decorated with Co3O4 nanostructure by a low-temperature hydrothermal method. The functional groups on the TGF are believed to nucleate the Co3O4 particles establishing a covalent bridging between them. The bridge improves the electron tunnelling across the electrolyte/electrolyte interface, reducing the overpotential of vanadium redox reactions. The covalent bridge, coupled with the enhanced surface area of the nanostructured-Co3O4 coated TGF (TGF/Co-100–12), are responsible for improved VO2 +/VO2+ and V3+/V2+ redox kinetics in VRFB. A 25 cm2 VRFB employing TGF/Co-100–12 electrodes, compared to TGF, enhances the specific capacity from ∼ 38 Ah l−1 to ∼ 45 Ah l−1 and energy efficiency (EE) from 81 to 87.6% at 100 mA cm−2 and its capacity retention (after 50 cycles) is ∼ 100% higher than that of TGF based VRFB. Besides, a two-cell stack is demonstrated with an EE of 84% and 89% of initial capacity even after 50 cycles, and 3% loss in EE vis-à-vis single cell is mainly due to the additional contact resistance arising out of coupling the cells.