State of charge and power rating gains in industrial-scale vanadium redox flow batteries through thermal activation of electrodes

Abstract

Vanadium redox flow batteries (VRFB) are typically envisaged for application in large-scale energy storage systems. High-power operation of VRFBs reduces the cost of such installations but also significantly affects the energy storage capacity owing to markdown in the operable range of the state of charge (SoC). This situation can be remedied to some extent by minimizing the area-specific resistance (ASR) of the VRFB cells. In the present study, the application of thermal activation of the electrodes was studied on an industrial-scale VRFB cell with an active electrode area of 936 cm2. The carbon felt electrode was heat treated at 350, 400, and 450 °C for 30 h. Following physical and electrochemical characterization of the heat-treated carbon felt, electrochemical testing of the full cell was carried out through charge-discharge cycling studies conducted at current densities of 60, 90, 120 and 150 mA/cm2. Comparative studies were also conducted with an unactivated carbon felt. Significant reduction in ASR was observed with the optimally heat-treated carbon felt, leading to nearly 20 % expansion in the operating SoC range at a current density of 120 mA/cm2 and 34 % enhancement in the rated power of the cell. Long-term durability studies over 200 cycles and post-test inspection and physical characterization confirmed the stability of thermal activation.