Voltage Loss Analysis of Zinc-Cerium Redox Flow Batteries
Authors: Amini Kiana , Pritzker Mark 
Published: December 2020

Redox flow batteries (RFBs) are a relatively new generation of electrochemical devices suitable for large-scale energy storage applications. The separation between the electrolyte storage tanks and the electrochemical cell in RFBs simplifies the battery scale-up and facilitates the energy/power ratio tuning. In comparison to current energy storage devices, RFBs are beneficial due to their portability, flexibility and low maintenance cost [1].
Among the different types of RFBs investigated, those based on zinc and cerium are very attractive due to the large negative and positive electrode potentials in an aqueous media. Thus, zinc-cerium RFBs are capable of providing one of the highest cell voltages (~ 2.4 V) among flow batteries and a large theoretical energy density [2]. To date, Zn-Ce RFBs have primarily been investigated galvanostatically to determine their charge, voltage and energy efficiencies and attempts have been made to suppress the rate of the hydrogen and oxygen evolution side reactions [3-6]. In order to further optimize the performance of these batteries and to elucidate the future pathways to enhance their efficiency, the sources of voltage loss in the battery during discharge must be identified and the role of the positive and negative half-cells in the voltage loss determined. Toward this goal, we have conducted in
situ polarization and EIS experiments on a full-cell Zn-Ce RFB with reference electrodes inserted in the system [7]. The insertion of the reference electrodes in the RFB enables us to decouple the contribution of negative and positive electrodes to the total performance loss. At low and intermediate current densities, the main contributor to the voltage loss during discharge is the kinetic overpotential of the negative Zn/Zn2+ half-cell. On the other hand, at high current densities, mass transfer limitations at the positive Ce3+/Ce4+ half-cell cause a large potential drop in the system. From in situ kinetic studies, we have measured an exchange current density of ∼ 7.4×10−3 A cm−2 for Zn oxidation and ∼ 24.2×10−3 A cm−2 for Ce4+ reduction, which is consistent with our previous findings from battery operation that the kinetics of the negative electrode reaction is slow compared to that of the positive electrode at low-to-intermediate current densities. The use of an alternative mixed methanesulfonate-chloride negative electrolyte to reduce the kinetic overpotential of the negative half-cell reaction and the influence of the flow rate on the mass-transfer rate of the positive half-cell reaction have also been investigated and will be discussed in this presentation.
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