Quantifying and rationalizing polarization curves of Zn-air fuel-cells: A simple enabling contribution to device-scale analysis and monitoring

Abstract

Zinc-air fuel cells (ZAFCs) with flowing particulate Zn anode are gaining momentum with respect to cognate and competing flow-battery technologies, such as zinc-air batteries with immobilized zinc anode zinc-bromine, zinc-cerium and zinc-vanadium flow-batteries, owing to better energy storage flexibility and control of recharge, as well as environmental compatibility. Notwithstanding its appeal, the ZAFC concept still exhibits several operational aspects that warrant dedicated studies, mainly regarding the impact of operating conditions on anode passivation and air cathode degradation. To this aim, systematic experimentation with real devices and simple, but robust electroanalytical approaches, that would enable quantitative handling of large corpora of experimental data, are highly desirable. The literature available to date, is mainly concerned with specific issues regarding device engineering or cathode materials, while systematic and quantitative work on the electrochemical response of the device is missing. This work contributes to bridging this gap, proposing a simple and physically transparent method to follow the variations of cell response to changes in operating conditions, through the measurement of polarization curves and their fitting with a very simple, but original and robust macro-electrokinetic equation, enabling a clear correlation between overvoltage components and cell state. In addition to original measurements with a self-built laboratory-scale device, we fitted and analysed all ZAFC polarization curves reported in the literature. Parametric analysis of the model is complemented by materials-science data, targeting selected observables, relevant to polarization curve analysis.