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

Abstract

This works reports the effect of operation conditions on the performance of a 25 cm(2) 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 degrees C or 415 degrees 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 cm(3) min(-1) to 150 cm(3) 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 (R) series cation exchange membranes (CEM) showed that AEM, generally, yields higher EE than all Nafion (R) 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.