Improving Performance of Hybrid Zn-Ce Redox Flow Battery by Controlling Ion Crossover and Use of Mixed Acid Positive Electrolyte

Abstract

In this study, the crossover of the electroactive species Zn(II), Ce(III), Ce(IV) and H⁺ across a Nafion 117 membrane was measured experimentally during the operation of a bench-scale hybrid Zn-Ce redox flow battery containing 0.8 mol/L cerium methanesulfonate in 4 mol/L methanesulfonic acid (MSA) or 2 mol/L MSA–0.5 mol/L H₂SO₄ mixed acid on the positive side and 1.5 mol/L ZnMSA in 1 mol/L MSA on the negative side. As much as 36% of the initial Zn(II) ions transferred from the negative to the positive electrolyte and 42.5% of the H⁺ in the positive electrolyte crossed over to the negative electrolyte after 30 charge-discharge cycles. Both of these phenomena contributed to the steady fade in battery performance over the course of operation. Based on these findings, additional experiments were conducted in which different amounts of Zn(II) were intentionally added to the positive electrolytes. This action was shown to have several beneficial effects: by reducing the crossover of Zn(II) from the negative electrolyte to the positive electrolyte, the battery coulombic and voltage efficiencies both improved, the decay of battery performance over the 30 charge-discharge cycles was reduced, the kinetics of the Ce(III)/Ce(IV) redox couple were enhanced, and inhibition of O₂ evolution was observed. The average energy efficiency over 30 charge-discharge cycles was increased by 19.7% by adding 0.6 mol/L Zn(II) to 4 mol/L MSA positive supporting electrolyte and 6.4% by adding 0.4 mol/L Zn(II) to 2 mol/L MSA–0.5 mol/L H₂SO₄.