Antimony-decorated graphite felt electrode of vanadium redox flow battery in mixed-acid electrolyte: Promoting electrocatalytic and gas-evolution inhibitory properties

Abstract

• The shift of electrode gas-evolution potential by antimony. • Utilizing the electrocatalytic properties of antimony in stabilizing the electrode. • Improving vanadium redox flow battery efficiency by facilitating redox reactions. Facilitation of redox reactions and inhibition of gas evolution in the graphite felt electrode of vanadium redox flow battery (VRFB) is investigated by adding antimony ions to the electrolyte. The electrolyte is used as a mixture of sulfuric acid and hydrochloric acid to stabilize the vanadium ion. Antimony has quasi-reversible reactions prior to the electrochemical reactions of vanadium ion pairs. In general, as shown by the results of scanning electron microscopy, metallic antimony is decorated on the surface of graphite felt during charge–discharge cycles. The results of full-window cyclic voltammetry indicate an increase in the cyclic stability of samples containing antimony, especially at a concentration of 7.5 mM. This is due to the electrocatalytic properties of antimony and the facilitation of vanadium electrochemical reactions. Ease of reactions is also closely related to the improvement of vanadium ion diffusion coefficients in samples containing antimony. The reduction peak of V 3+ to V 2+ in the bare sample practically disappears after a few cycles, which is due to the superiority of hydrogen gas evolution. Linear sweep voltammograms confirm the effectiveness of antimony in shifting the potential position of hydrogen and oxygen evolution. Wettability and contact angle tests show more water adsorption by antimony-decorated graphite felts compared to the bare one which has a direct effect on improving the performance of the electrodes. Although antimony is added to anolyte in VRFB and has the most beneficial effect on the negative electrode, antimony cross-contamination does not have a bad effect on positive electrode performance and even it also stabilizes the reduction–oxidation currents ratio on positive electrode based on cyclic voltammetry results. In general, the presence of antimony increases all three types of coulombic, voltage, and energy efficiency in VRFB.