Real-Time Monitoring of Oxygen Released During Charging of Alkaline Zn-air Batteries with a Redox Mediator

Abstract

The main challenges for alkaline Zn-air batteries (ZABs) are their high charging potential and low energy efficiency. The introduction of a redox mediator (RM) is a novel approach, but the effects on the oxygen evolution reaction (OER) are lacking at the device level. Herein, the effects of RMs on ZABs are comprehensively investigated through a differential electrochemical mass spectrometer (DEMS) online gas monitoring system. First, the monitoring system is proven to be effective in capturing oxygen during charging. Then, four electrolytes depending on the RM (KI) concentration are prepared. The duration of the oxygen signal decreases with an increase of the RM concentration, and disappears completely when the concentration reaches 0.1 M. It can be deduced that the charging process at a small depth or with a high concentration of the RM is only a conversion of the RM rather than the OER. Further, the duration without oxygen release decreases with cycles, indicating the decreasing amount of the RM. Therefore, KI is not strictly an RM but a sacrificial agent, and considerable efforts are required to find a reliable RM for achieving low-potential charging. Hopefully, this work provides novel insights to evaluate the effects of RM for practical applications.