Oxygen bifunctional catalysts for rechargeable zinc-air batteries

Abstract

Zinc-air redox flow batteries have a 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 VRHE to 1.7 VRHE, under which no catalyst material has been shown to survive. Ni and Co are two of the few elements expected to be thermodynamically stable at anodic potentials. At the same time, NiCo2O4 is known to enable catalysis of both O2 reduction, and evolution reactions, making it a good candidate as an O2 bifunctional catalyst. Despite their appreciable activity, reversible NiCo2O4 electrodes quickly lose their O2 reduction activity. Where the subject of catalyst stability is mostly evaluated for a single reaction, we employ the strategy of understanding a nickel cobalt oxide (NiCo2O4) catalyst at potentials of both the O2 reduction and evolution reactions. Before studying the parameters that control the activity of NiCo2O4, the benchmarked O2 reduction and evolution activities of Pt/C, and IrOx catalysts are reproduced. These experiments lay the background to compare to the activity of a commercial Co3O4, literature-based co-precipitated spinel NiCo2O4, and commercial NiCo2O4 with rock salt structure. Ni is observed to play a crucial role in catalysing the O2 reduction, in good agreement with previous literature. Furthermore, it is concluded that rock salt NiCo2O4 is the more active material towards the O2 evolution, whereas spinel NiCo2O4 offers the best compromise between both O2 reduction and evolution reactions. Over time, the activity of oxide catalysts can be affected by dissolution, crystal structure and oxidation state changes, as well as the uptake or adsorption of particular impurities, such as Fe. A combination of electrochemical degradation tests, post-mortem characterization techniques, and in-situ spectroelectrochemistry are employed...