Increasing the Electrolyte Capacity of Alkaline Zn–Air Fuel Cells by Scavenging Zincate with Ca(OH)₂

Abstract

The use of calcium hydroxide for scavenging zincate species is demonstrated to be a highly effective approach for increasing the electrolyte capacity and improving the performance of the zinc–air fuel cell system. A fundamental approach is established in this study to quantify the formation of calcium zincate as the product of scavenging and the amount of water compensation necessary for optimal performance. The good agreement between predicted and experimental results proves the validity of the proposed theoretical approach. By applying the results of theoretical predictions, both the electrolyte capacity and the cell longevity have been increased by more than 40 %. It is also found that, using Ca(OH)2 to scavenge zincate species in concentrated KOH solutions, affects mostly the removal of zincate, rather than ZnO, from the electrolyte, whereas the presence of excess, free, mobile H2O plays a key role in dissolving ZnO and forming zincate. The results obtained in this study demonstrate that the proposed approach can widely and effectively be applied to all zinc–air cell systems during their discharge cycle.

Discharge performance of Zn‐air fuel cells under the influence of Carbopol 940 thickener

“…The solubility of Zn in the anode was dependent on electrolyte concentration. Zhu et al noticed that the electrode potential decreased with increasing electrolyte concentrations but the current and conductivity increased till 30%, but then reduced with further concentration upsurge. Thus, for efficient ZAFC performance, the electrolyte concentrations were determined between 20% and 40%.…”

Observation of Water Consumption in Zn–air Secondary Batteries

“…Zn 2+ ions convert to Zn(OH) 4 2− (zincate) in the basic solution and subsequently convert to ZnO [12][13][14]. The formation of ZnO and related reactions are not discussed in this work.…”

Zinc regeneration in rechargeable zinc-air fuel cells—A review

“…It should be noted that the solution chemistry of zincate species is rather complex [53], which brings more challenges in getting correct values of activity, and therefore manipulating concentrations during deposition in such solutions is rather hard. Still, all information necessary for finding functional relationships between the alkaline electrolyte concentration, ZnO solubility, electrode potential, solution conductivity and the limiting current of zinc electrode is presented in Fig.10.…”

“…It should also be noted that oxodihydroxo-zincate species are known to evolve easily into ZnO, the latter being an insulator causes problems in electrochemical cells. Therefore, the best way to eliminate its negative effect would be dissolving oxodihydroxo-zincate species by slowly adding water into the spent solution, which is recently realized in practice and called the "concentration fine-tuning" of the spent solution [53].…”

“…Light scattering was applied to fresh and supersaturated zincate solutions [52,53], revealing that colloids are not involved in the formation of such species. The different zinc-hydroxide zincate species being present in the spent solution can demonstrate different kinetics in the course of zinc deposition [25,52].…”