Study and Development of the Zinc Electrode for the Alkaline Zinc-Nickel Redox Flow Battery

Abstract

Cost effective energy storage solutions are increasingly in demand for electrical grid and renewable energy applications. The zinc-nickel redox flow battery (RFB) is a promising technology due to its potential cost competitiveness. However, a number of obstacles must be overcome before the zinc-nickel RFB can be commercialised. Many of these relate to the zinc electrode in alkaline electrolytes. Therefore, this work is focused on development of the alkaline zinc electrode, aiming to improve performance in the zinc-nickel RFB by addressing the issues of zinc morphology and hydrogen evolution. Sixteen electrolyte additives are screened, tetraethylammonium hydroxide (TEAH) at 15 mM L-1 concentration being identified as promising. Zinc-nickel flow cell cycling tests provide coulombic and energy efficiencies of 97.8 % and 86.0 % respectively, with compact zinc morphologies obtained after 80 cycles. Increasing KOH and ZnO concentrations are shown to be beneficial to both the zinc and nickel electrodes. An electrolyte of 6 M L-1 KOH and 0.5 M ZnO is identified as optimal at 293 K. The effect of zinc electrode substrate material on coulombic efficiency is shown to relate to hydrogen evolution reaction (HER) overpotentials. A graphite composite with 5 % polyvinylidenefluoride (PVDF) (BMA5, Eisenhuth) demonstrates the highest coulombic efficiency (96.7 %) and most negative HER onset potential (-1.595 V vs. Hg/HgO). The effect of electrolyte velocity on zinc morphology is related to Reynolds numbers. At 20 mA cm-2 in an electrolyte containing 0.5 M ZnO compact zinc depositions result from Reynolds numbers over 2000. The ratio of applied current density to limiting current density is used as an indicator of zinc morphology, with values of 0.4 and below providing compact zinc depositions. Zinc-nickel flow cell cycling employing the selected electrolyte composition, zinc electrode substrate material and operational parameters yields average coulombic and energy efficiencies of 98.3 % and 86.6 % respectively over 200 stable cycles. This is comparable to the highest efficiencies previously reported but is achieved at an increased current density of 20 mA cm-2, resulting in improved power density. The electrolyte flow rate is also reduced by 64 %, representing a significant reduction in required pumping energy.