Enhancing the Performance of Vanadium Redox Flow Batteries Using Carbon Cloth Electrode Decorated with C/WOx Nanocomposites

Abstract

Given the growing world population and the industrial revolution, there is an immense global energy demand, which has heavily been established by fossil fuels for several decades. However, the depletion rate of fossil fuel reserves and their environmental hazards have led the global energy systems to shift to using renewable energy sources. Even though the roles of solar and wind energy are expected to increase over the coming years, their intermittent behavior requires their integration with electrical energy storage systems for frequency balancing and efficient utilization. Whereas conventional energy storage techniques (e.g., pumped hydro, compressed air, flywheel, and thermal energy) have large footprints, require big investments and lack flexibility, electrochemical energy storage methods, have higher energy density and better flexibility depending on their design. Compared to other electrochemical systems (e.g., Li-ion batteries and supercapacitors), vanadium redox flow batteries are thought to be an optimal solution to create smart grids when combined with renewable energy sources, owing to their scalability, site versatility, long lifetime and ease of operation. Nevertheless, there are some issues concerning their efficiency and further market penetration, including (i) low solubility of vanadium ions in acidic electrolyte leading to low energy density; (ii) crossover of vanadium ions through membrane leading to self-discharge and low capacity; and (iii) slow kinetics of vanadium ions reactions at the electrodes. Even though the widely used carbon-based electrodes have high stability in acidic solutions and high conductivity, modifications are needed to improve the kinetics of the electrocatalytic reaction, which is the aspect being concerned in this study. Thermal treatment of carbon cloth showed faster electrode kinetics at 0.1 M and 1 M vanadium ion concentration in pure H2SO4 and H2SO4/HCl. Incorporation of carbon and tungsten oxide nanostructures also demonstrated excellent electrocatalytic activity, resulting in high energy efficiency. A proposed description of the poorly understood mechanism of this electrochemical reaction will be presented, along with these findings, allowing for a big step forward towards a more efficient vanadium redox flow battery.