Modelling of Zinc-Air Fuel Cell and Alkaline Zinc Electrolyzer

Abstract

A zinc-air fuel cell (ZAFC) is attracting widespread interest for electricity generation. Furthermore, ZAFC combining with alkaline zinc electrolyzer is a potential candidate for an energy storage system. This work aims to develop dynamic mathematical models of ZAFC and alkaline zinc electrolyzer and to investigate the effects of operating parameters on the cell performance. The ZAFC was devised as a tubular cell with electrolyte flow inside. Potassium hydroxide (KOH) solution mixed with zinc powder was used as the electrolyte. Stainless steel mesh and nickel foam were used as current collectors of anode and cathode, respectively. Manganese dioxide mixed with graphite used as the catalyst was coated on nickel foam. The electrolyzer cell was devised as electroplating cell with stirred electrolyte. KOH solution mixed with zinc oxide (ZnO) was used as the electrolyte of the electrolyzer. Copper sheet and nickel foam were used as the current collectors of cathode and anode, respectively. The developed models were implemented in MATLAB and validated with experimental data. Good agreement between the predicted data and experimental data was obtained for both ZAFC and electrolyzer. The proposed models were then used to investigate the effects of four parameters: KOH concentration, flow, zincate ion concentration and conductive carbon. By increasing KOH concentration, the cell voltage increased. Nevertheless, at the KOH concentration above 7 M, the cell voltage dropped as the KOH concentration increased because of the adverse effects on ionic conductivity and anodic exchange current density. ZAFC performance improved by using high flow rate. For long-time operation, using higher flow rate could maintain zinc content and electrolyte concentration better than that of lower flow rate. Higher concentration of zincate ion resulted in lower Nernst potential and cell voltage. A Higher amount of ZnO provided lower electrode conductivity and active area of zinc electrode. Adding carbon did not improve cell performance significantly. For the electrolyzer, the highest ionic conductivity was also observed at 7 M. Therefore, this concentration provided lowest cell voltage at high current. KOH concentration lower or greater than 7 M provided higher cell voltage. Below the saturation limit of zincate ion, increasing zincate ion concentration decreased cell voltage. Increasing zincate ion concentration above the saturation limit provided no significant improvement. For current efficiency (C.E.) of zinc electrolyzer, higher current density provided higher C.E. in all conditions. Moreover, increasing concentration of both KOH and zincate ion increased C.E.