Investigation of Hydroxylated Carbon Felt Electrode in Vanadium Redox Flow Battery by Using Optimized Supporting Electrolyte

Traditional vanadium batteries use pure sulfuric acid as electrolyte, but H۲SO۴ does not absorb enough vanadium ions to make the electrolyte an efficient energy source. This study investigates the effect of hydroxylation process on electrochemical and operational properties of carbon felt electrode in VOSO۴ solution with an optimized supporting electrolyte (a mixture of six parts HCl and ۲.۵ parts H۲SO۴). Carbon felt electrode was hydroxylated with mixed acids of H۲SO۴ and HNO۳ in a stainless steel autoclave for ۶ h. Then thermal treatment of electrode was performed at ۴۰۰ oC for ۵h. Obtained results of cyclic voltammograms showed that when the carbon felt was hydroxylated, both oxidation and reduction peak currents were increased remarkably and the peak potential separation is decreased from ۳۵۶ mV to ۲۴۶ mV, suggesting that the electrochemical activity and the kinetic reversibility on HCF electrode were improved compared to the pristine one. According to results of electrochemical impedance spectra, charge transfer resistance (Rct) was calculated to be ۶۴۸ Ω for pristine carbon felt. The obtained Rct at hydroxylated electrode (۱۷۶ Ω) shows a decrease of about ۷۳ % in Rct. Charge-discharge profiles of two cells assembled with the pristine carbon felt (cell A), and hydroxylated carbon felt (cell B) showed that energy, voltage and coulombic efficiencies were significantly improved by using the hydroxylated electrodes inside the cell of vanadium redox flow battery.