Improving performance of hybrid Zn–Ce redox flow battery by controlling ion crossover and use of mixed acid positive electrolyte

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. For the conditions considered in this study, 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 reduce the crossover of Zn(II) from the negative side to the positive side, improve both the battery coulombic and voltage efficiencies and reduce the decay of battery performance over the 30 charge–discharge cycles. The average energy efficiency over 30 charge–discharge cycles was increased by 19.7% by adding 0.6 mol L⁻¹ Zn(II) to 4 mol L⁻¹ MSA positive supporting electrolyte and 6.4% by adding 0.4 mol L⁻¹ Zn(II) to 2 mol L⁻¹ MSA–0.5 mol L⁻¹ H₂SO₄.