Polarization Behaviors and Morphology during Zinc Deposition and Dissolution in Zinc Flow Battery

Abstract

A zinc flow battery is one of the candidates of secondary batteries for stationary energy storage system, and a zinc-air flow battery is the most promising device because of its high theoretical energy density, low cost of anode material, air-breathing cathode, and high safety of non-flammable aqueous electrolyte. The flow of the electrolyte, which is mostly KOH solutions, is believed to be effective to suppress the generation of zinc dendrite and non-uniform distribution of zinc and zinc oxide, while there has been little information on the effects of the charge-discharge conditions and the dimensions, design, and flow parameters of the cell on the polarization behaviors of the zinc anode and on the morphology during zinc deposition and dissolution in zinc flow secondary batteries.1,2 This paper reports the results on the potential measurements of the zinc anode at the current density up to 500 mA/cm2 and the observation of the morphology of zinc growth and dissolution during charge and discharge.The zinc anode was electrodeposited zinc alloy film supplied by Mitsui Mining & Smelting, Co., Ltd, which produces less than 1/100 of hydrogen compared to conventional rolled zinc film, indicating a high hydrogen overpotential and a quite low self-discharge rate. Two-electrode or three-electrode cells were equipped with the zinc film anode and a zinc wire reference electrode, of which the electrolyte was 6 mol/L KOH solutions saturated with ZnO. The electrolyte was pumped into the cell and the flow rate was changed. The potential of the zinc anode during charge and discharge was monitored while constant current electrolysis was performed at 100 mA/cm2 to 500 mA/cm2. The morphological change for zinc deposition and dissolution was observed by video camera during charge and discharge and by SEM after the operation.The results on the polarization of the zinc anode obtained at different current densities are shown in Fig. 1, in which the increase in polarization at the current density from 100 mA/m2 to 500 mA/cm2 was less than 50 mV for both of charge and discharge at 25 mL/min, suggesting that the electrolyte flow enhances the mass transport of zincate ions and suppresses the concentration overpotential. This is also supported by the fact that charge failed with no electrolyte flow. The dependence of the anode’s potential on the flow rate also indicated that the potential hardly increased when the flow rate changed from 250 mL/min to 25 mL/min. The observation of zinc deposition and dissolution was further carried out and some interesting features on the relationship between the morphology of zinc deposits and the operating conditions was found, implying the conditions to suppress zinc dendrite formation.This work was partly supported by JSPS Grant-in-Aid for Scientific Research No. 22H02187.References1) A. L. Zhu, D. P. Wilkinson, X. Zhang, Y. Xing, A. G. Rozhin, S. A. Kulinich, J. Energy Storage, 8, 35-50 (2016).2) A. Abbasi, S. Hosseini, A. Somwangthanaroj, R. Cheacharoen, S. Olaru, S. Kheawhom, Sci. Data, 7, 196 (2020). Figure 1