Insights into rechargeable Zn-air batteries for future advancements in energy storing technology

Owing to its high theoretical specific energy density, low cost, abundance and environmental friendliness, the rechargeable Zn-Air batteries (ZAB) are becoming the most prevalent candidate as energy storage devices for consumer electronics, and electric vehicles. Nevertheless, the interaction of O2 as a fuel with the components of ZAB is highly challenging for practical implementations of this technology. The underlying electrochemical reactions in ZAB involving multi-electron transfer, adsorption/evolution of O2, and dissolution of Zn metal in electrolyte, need robust-electrocatalyst and stable Zn/electrolyte interface. This prominently evokes the need for an in-depth study of electrocatalytic reactions occurring at the electrode/electrolyte interphases as well as the physiochemical features of membranes in ZAB. Therefore, this review provides significant insights into the fundamentals of Zn air battery system in terms of the underlying electrochemical mechanism, composition/structural performance relationship of different battery components. A detailed section has been devoted in summarizing the evaluating factors for battery performance including power density, polarization curves, columbic efficiency and correlation of catalyst's redox activity (Eonset, Ehalf-way, and Jd) with the device performance parameters (OCV, Ohmic losses, and Pmax). Moreover, representative studies of in-situ/operando characterizations have also been summarized to reveal the structural stability, reaction kinetics, formation of by-products, and morphological evolution. The intriguing advanced features of ZABs including flexibility, photo-recharge ability, economic feasibility, fast charging, high energy density, improved stability and hybrid Zn battery systems are particularly discussed. For the accomplishment of these functionalities, the chemical heterogeneity and structural modifications of materials (electrode, electrolyte and membranes) with improved electrical conductivity, reduced energy barrier, increased reactive surface area, and improved mass transport behavior at the nanoscale have been anticipated. This material survey could be highly beneficial for the development and modification of new catalysts in the field of electrocatalysis. Additionally, for the prospect of green energy technology, the economic viability and environmental sustainability of ZAB are also highlighted. Lastly, based on the discussion of recent achievements, some challenges and outlooks for maturing the rechargeable Zn air battery technology at the academic level and at the industrial scale are also set forth.