Beyond activity: a perspective on diagnosing instability of reversible O2 catalysts for metal–air batteries

Abstract

Zinc–air redox flow batteries have high potential to penetrate the stationary energy storage market, due to the abundancy, and low cost of active species – oxygen and zinc. However, their technological fruition is limited by the development of reversible O₂ electrodes operating at potentials between 0.6 V_RHE to 1.7 V_RHE, under which no catalyst material has been shown to be stable over long durations. Despite heavy research on the topic of reversible O₂ catalysis, little is known about the parameters controlling the stability of the bifunctional catalyst. Several research accounts assess the activity of reversible O₂ catalysts, but only a small portion cover degradation mechanism over such a large potential window. In this perspective, we summarize our current understanding of material challenges for Zn–air batteries, reversible O₂ catalyst integration strategies, and electrochemical behaviour, with a particular focus on catalyst stability. Nickel cobalt oxide (NiCo₂O₄), a promising yet understudied system, is used as an example material for investigations at potentials of both the O₂ reduction (ORR) and evolution (OER) reactions. We also report original data employing ex situ X-ray diffraction, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy, as well as electrochemical measurements to study the activity of NiCo₂O₄. Furthermore, electrochemical accelerated stress tests are coupled with post-mortem transmission electron microscopy, inductively coupled plasma, and X-ray photoelectron spectroscopy to study the dissolution, compositional changes and amorphization of the top surface 5 nm of the catalyst surface. In situ X-ray absorption spectroscopy revealed irreversible oxidation of Co centres in NiCo₂O₄ during OER, which explains the reduction in activity of the ORR after the catalyst was exposed to anodic OER potentials. This methodology provides a broader method to screen reversible O₂ catalyst stability and enables us to summarize future strategies to improve the activity and stability of reversible O₂ catalysts and electrodes.