Improved Electrolyte for Negative Side of Zinc-Cerium Redox Flow Batteries
Authors: Amini Kiana , Pritzker Mark 
Published: February 2020

Redox flow batteries (RFBs) are one of the most recent and promising technologies for large-scale energy storage applications. In an RFB, the active materials for the two redox reactions are dissolved in separate electrolytes stored in external tanks. During charge/discharge, these electrolytes are pumped through the cell where the redox reactions occur. Compared to conventional energy storage systems, RFBs are advantageous due to their portability, low maintenance cost, modularity and flexibility [1].
RFBs based on zinc-cerium chemistry are very attractive since they provide the highest cell voltage among flow batteries and thus the highest theoretical energy density [2]. Nevertheless, this system has a number of challenges, the most important of which is the occurrence of side reactions. During charge, hydrogen and oxygen evolution side reactions occur at the negative and positive electrodes, respectively, leading to a lower charge efficiency, wasted energy and eventual premature battery failure [3,4]. During discharge, self-discharge of the zinc deposit due to acid attack also contributes to reduction in the efficiency. Thus, the suppression of these side reactions is necessary for the improvement and ultimate success of zinc-cerium RFBs. Toward this goal, our work focuses on the enhancement of the Zn/Zn(II) redox reaction through the use of a mixed-acid electrolyte. From half-cell experiments on a glassy carbon electrode, we have shown that the introduction of 0.2 – 0.3 mol dm−3 of chloride anions to the conventional methanesulfonic acid electrolyte significantly increases the transport properties of Zn(II), reduces the zinc nucleation overpotential and enhances the exchange current density of the Zn/Zn(II) redox reaction [5]. The same effect is observed when glassy carbon is replaced by polyvinyl ester (PVE), which is a commonly-used negative electrode in Zn-Ce RFBs. The use of this improved electrolyte on the charge, voltage and energy efficiency of a bench-scale zinc-cerium RFB has also been investigated. Similar to results obtained in the half-cell studies, the full-cell experiments show that the mixed methanesulfonate-chloride electrolyte facilitates the zinc redox reaction and thus increases the voltage efficiency of the battery as high as 10%. Additionally, the charge efficiency increases significantly under the conditions where negative half-cell is the limiting reaction. The improvement is more than 20% at an operating current density of 25 mA cm-2. The performance and life-cycle analysis of the battery with this new electrolyte under different operating conditions will also be discussed in this presentation.
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