Enhancing the performance of all-vanadium redox flow batteries by decorating carbon felt electrodes with SnO₂ nanoparticles

An all-vanadium redox flow battery (VRFB) is an attractive candidate as an electrochemical energy storage system that uses conversion technology for applications that range from those requiring only a few kilowatts to those that must perform on a megawatt scale. Issues to be resolved, however, include problems with increasing the rates of charge/discharge (due to an increase in overpotentials) and cycling stability (due to the irreversibility of redox reactions at the electrodes as well as crossover of the vanadium species) that have prevented a broader market penetration of VRFB systems. One of the strategies to overcome these problems may be the introduction of electrocatalysts to the electrode surface to improve the reaction kinetics of the positive and negative redox couples, thus enabling the achievement of higher levels of power density. Therefore, carbon felt electrodes decorated with SnO₂ nanoparticles were evaluated in this study. The performance of VRFBs at a high current density of 150 mA cm⁻² with SnO₂-deposited carbon felts returned an energy efficiency of 77.3%, with a corresponding increase in discharge capacity of 23.7% over a pristine electrode. Cycling stability of the system was also improved almost 2.7-fold compared with that of a pristine electrode at 50 mA cm⁻². The electrocatalytic activity of SnO₂ nanoparticles facilitates a reduction in the overpotentials, which enables charge/discharge reactions at faster rates, which was confirmed by cyclic voltammetry and electrochemical impedance spectroscopy. Furthermore, confirmation of the formation of clusters of SnO₂ nanocrystals as well as their chemical and physical stability after cycling (as probed by various characterization techniques including synchrotron-based X-ray absorption) supports their feasibility as a stable, efficient and cost-effective electrocatalyst for use in VRFB systems.