Mechanistic Study of Hydroxide Ion Consumption in Ultralean PVA Hydrogel Electrolytes for High-Energy-Density Micro Zinc-Air Batteries

Micro zinc-air batteries (micro-ZABs) are a promising power source for miniature devices, attracting great attention due to their high energy density, biodegradability, and safety. Operating ZABs in an ultralean, minimum volume electrolyte regime is a productive approach to maximize energy density. However, an ultralean electrolyte ZAB is more susceptible to unfavorable reactions than one containing a larger amount of electrolyte. In particular, side reactions that result in hydroxide ion (OH-) consumption significantly hinder the electrochemical performance of the micro-ZAB. The mechanisms of such side reactions are studied through titrations and electrochemical impedance spectroscopy (EIS). The experimental results demonstrate that both carbonation and zincate accumulation contribute to the consumption of OH- in ultralean alkaline hydrogel electrolytes and can significantly impact electrochemical performance. Under the conditions studied, for an alkaline polyvinyl alcohol (PVA) hydrogel electrolyte, the average consumption rate of OH- due to carbonation is measured to be 5.22 x 10(-7) mol min(-1) cm(-2). A diffusion-reaction model is developed to understand the carbonation process. Adopting parameters from the literature on aqueous systems, the model shows good agreement with experimental results, suggesting that the carbonation process of PVA alkaline hydrogel electrolytes is similar to that of alkaline aqueous electrolytes. The electrochemical performance of the micro-ZAB is modeled based on the consumption rates of the OH- and is shown to be in good agreement with experimental data.