Insights into all-vanadium redox flow battery: A case study on components and operational conditions

Highlights: • VRFB performance depends on the operational conditions and components such as. • Flow rate, current density, electrode compression and membrane. • AEM FAP450 performs similar to Nafion^® 212 and to Nafion^® 115, at lower cost. • Thermal treating carbon electrodes may improve energy efficiency up to 67%. • Lower efficiencies were obtained at high flow rates due to mass transfer limitations. This works reports the effect of operation conditions on the performance of a 25 cm² vanadium redox flow cell (VRFC) featuring commercial carbon felt (CF) as electrode and different commercial membranes, namely electrolyte flow rate and charge-discharge current density. The effect of electrode compression and thermal treatment was also assessed. The battery performance was evaluated based on the charge-discharge curves and average efficiencies during 20 charge-discharge cycles; the capacity fading was evaluated after 20 cycles. The limiting current was obtained through polarization curves and the ohmic resistance of the battery was obtained by electrochemical impedance spectroscopy (EIS). The electrodes yielding the highest energy efficiency (EE), ca. 85%, at 40 mA cm⁻² current density were thermally treated at 390 °C or 415 °C during 12 h under air. This result represents an improvement of more than 50% compared with the as-received electrodes. Additionally, energy efficiency increased ca. 10% when electrode compression rose from 8% to 25%. Increasing the flow rate from 37.5 cm³ min⁻¹ to 150 cm³ min⁻¹ EE was also enhanced from 76% to 84% at 40 mA cm⁻². The type of membrane affects the performance: a comparison between a commercial anion exchange membrane (AEM) and Nafion^® series cation exchange membranes (CEM) showed that AEM, generally, yields higher EE than all Nafion^® membranes tested; however, the capacity fading of AEM is much higher than CEM. The information provided in this work shows that, by tuning operational conditions and using suitable components, it is possible to increase the performance of a VRFC without additional costs.