The oxygen paradox in alkaline zinc-air battery via the competition of an unidentified anodic chemical reduction with cathodic kinetics

Can the fuel in use adversely affect the performance of an energy storage system? Here, oxygen (O2) in zinc-air (O2) battery (ZAB) is shown to impact the anodic process adversely, leading to early cell failure, though its amount limiting can affect the cathodic reaction kinetics. An unexplored chemical reduction process of dissolved O2 on metallic zinc in alkaline medium leading to peroxide radical (O2˙¯) species generation is proven, by electron paramagnetic resonance spectroscopy (EPR) and hydrodynamic voltammetry studies, followed by the formation of ZnO passivation layer on the Zn surface causing cell death. Keeping similar discharge times, early ZnO passivation occurred on the ZAB with its anode kept in an O2 rich environment than in the other one of O2 free environment. A slight modification in the cell design ensuring the limited availability of O2 at anode provided ~5 times extended cyclability (~7 hours to ~35 hours) than the ZAB with no such control leading to enhanced specific capacity. Presence of O2 in the anodic compartment leads to early cell failure from passivating ZnO formation impeding charge transfer. Limiting the O2 transfer from the cathodic side to anodic compartment can extend the cell cyclability and enhance discharge capacity. As alkaline ZABs are proposed for applications ranging from portable hearing aids to mini-grids, with self-discharge from the hydrogen evolution reaction (HER) on metallic Zn as a major roadblock, this study unravels predominant parasitic chemical process by O2 on the Zn surface, highlighting the importance of cathodic O2 crossover inhibition in the alkaline ZAB.