Use of Mixed Methanesulfonic Acid/Sulfuric Acid as Positive Supporting Electrolyte and the Study of Ion Crossover in Zn-Ce Redox Flow Battery

The development of a highly efficient and inexpensive energy storage technology is central to the operation of a sustainable smart grid and widespread utilization of intermittent renewable energy sources. An emerging and promising grid-scale technology for electrochemical energy storage is the redox flow battery (RFB). A unique and particularly valuable aspect of the design of RFBs is that their energy and power capacities can be scaled independently. Some of their other advantages include short response time, long life-cycles and no detrimental effects associated with being fully charged or discharged [1]. The Zn-Ce RFB, first developed by Plurion Inc (UK) in 2005 [2], has an open-circuit voltage as high as 2.4 V which is one of the highest among the various RBFs. To further improve the efficiency of Zn-Ce RBFs, the effect of different positive supporting electrolytes on the performance of a bench-scale Zn-Ce RFB has been studied. The effectiveness of mixed methanesulfonic/sulfuric acid, mixed methanesulfonic/nitric acid and pure methanesulfonic acid has been assessed and compared on the basis of the cyclic voltammetry response for the Ce(III)/Ce(IV) redox couple and galvanic charge-discharge of a bench-scale Zn-Ce RFB. We have observed the Ce(III)/Ce(IV) reaction to exhibit faster kinetics and the battery to attain higher coulombic efficiency and long-term performance over ~40 charge-discharge cycles in the mixed 2 mol/L MSA–0.5 mol/L H2SO4 electrolyte compared to that achieved in the commonly used 4 mol/L pure MSA electrolyte due to lower H+ crossover and the prevention of CeO2 precipitation. The coulombic efficiency fade rate in the mixed MSA-H2SO4 electrolyte is 0.55% per cycle over 40 charge-discharge cycles, while the fade rate is 1.26% in the case of 4 mol/L MSA. Furthermore, the positive electrode reaction is no longer the limiting half-cell reaction even at the end of long-term battery charge-discharge operation [3]. Our results show that a mixed MSA–H2SO4 acid electrolyte may be a better option for the positive side of a Zn-Ce RFB as a large-scale energy storage device. At the same time, undesired ion crossover is a practical issue that cannot be neglected since it decreases the concentrations of electroactive species in both negative and positive electrolytes and can cause battery self-discharge and thereby lower battery efficiency and longevity. The concentrations of electroactive species in both the positive and negative electrolytes after each charge-discharge cycle have been monitored as a function of cycle number. We have found that as much as 48.6% of the Zn(II) ions are transported through the Nafion 117 membrane from the negative to the positive electrolyte and 11.1% of Ce(III) ions are transported from the positive to the negative electrolyte by the end of 30 cycles (~55 hours of battery operation). A simple and efficient method to mitigate this problem is to add a certain amount of Zn(II) to the positive electrolyte and Ce(III) to the negative electrolyte at the outset of operation in order to reduce the concentration gradients between the two sides. From our cyclic voltammetry results, the addition of Zn(II) ions into the positive electrolyte has no deleterious effect on the Ce(III)/Ce(IV) redox couple and in fact appears to slightly reduce the separation between the reduction and oxidation peaks, suggesting better reversibility of the positive electrode reaction. In addition, experiments involving 30 cycles of battery charge-discharge have been conducted using Zn(II)-containing pure MSA positive electrolyte as well as Zn(II)-containing MSA-H2SO4 mixed acid electrolytes to analyze battery efficiency. Our results show that both the coulombic and voltage efficiencies are increased compared to that attained using the original electrolytes without Zn(II) initially added to the positive electrolytes. The energy efficiency increases by 19.7% in the case of the Zn(II)-containing pure MSA electrolyte and 6.1% for the Zn(II)-containing H2SO4-MSA mixed acid electrolyte (see Figure 1). Another benefit of adding Zn(II) ions to the MSA pure positive electrolyte is that it prevents the precipitation of CeO2 routinely observed to occur in MSA-only electrolytes after 10 – 15 charge-discharge cycles, which further enhances the energy density of the Zn-Ce RFB. References Nguyen, T. and R.F. Savinell, Flow Batteries. Electrochemical Society Interface, 2010. 19(3), 54. Clarke, R., B. Dougherty, S. Harrison, P. Millington, and S. Mohanta, US Patent 2004/0202925 A1. Cerium Batteries, 2004. Yu, H., M. Pritzker, and J. Gostick, Use of Mixed Methanesulfonic Acid/Sulfuric Acid as Positive Supporting Electrolyte in Zn-Ce Redox Flow Battery. J. Electrochemical Society, 2023. 170(2), 020536. Figure 1

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