Zinc deposition and dissolution in sulfuric acid onto a graphite–resin composite electrode as the negative electrode reactions in acidic zinc-based redox flow batteries

Abstract

Electrodeposition and dissolution of zinc in sulfuric acid were studied as the negative electrode reactions in acidic zinc-based redox flow batteries. The zinc deposition and dissolution is a quasi-reversible reaction with a zinc ion diffusion coefficient of 4.6 × 10⁻⁶ cm² s⁻¹ obtained. The increase of acid concentration facilitates an improvement in the kinetics of zinc electrodeposition–dissolution process. But too high acid concentration would result in a significant decrease in charge efficiency. The performance of the zinc electrode in a three-electrode system with magnetic stirring was also studied as a function of Zn(II) ion concentration, sulfuric acid concentration, current density, and the addition of additives in 1 M H₂SO₄ medium. The optimum electrolyte composition is suggested at high zinc(II) concentration (1.25 M) and moderate sulfuric acid concentration (1.0–1.5 M) at a current density range of 20–30 mA cm⁻². Whether in acid-free solution or in sulfuric acid solution with or without additives, no dendrite formation is observed after zinc electrodeposition for 1 h at 20 mA cm⁻². The energy efficiency is improved from 77 % in the absence of additives in 1 M H₂SO₄ medium to over 80 % upon the addition of indium oxide or SLS–Sb(III) combined additive as hydrogen suppressants.