Rechargeable zinc−air batteries require air cathodes that support bifunctional electrocatalytic activity for two key oxygen-based reactions: O2 reduction (ORR) for battery discharge and O2 evolution (OER; water oxidation) during subsequent charging. This catalytic pairing can be achieved with two distinct electrocatalysts or with a single bifunctional catalyst that supports both reactions at technologically relevant rates and cell voltages. For single bifunctional catalysts, sustainable electrocatalysts that rely on abundant, low-cost elements struggle to outperform less-sustainable cobalt-versions. We therefore use a two-catalyst design strategy to achieve state-of-the-art bifunctional activity from a cathode that comprises elements with low supply-chain risk. We optimize the ORR and OER performance of select oxides and hydroxides of manganese, iron, and nickel by synthesizing aerogels with high active surface area and soft aggregate morphologies that facilitate improved catalyst distribution within a conventional powder-composite cathode. For example, we synthesize aerogels of α-MnO2 for ORR and Ni2FeOx for OER and fabricate air cathodes using simple milling and pressing techniques. We pair cathodes with our 3D Zn sponge anodes in zinc–air cells and demonstrate remarkably low discharge−charge voltage hysteresis (700 mV at 10 mA cm–2) with cycles lasting longer than 2 h. We present our strategies for: (i) synthesis of oxygen electrocatalysts and their integration within technically relevant air cathodes (ii) fabricating practical Zn−air cells (i.e., no impractical Zn plate anodes), (iii) testing rechargeable Zn−air cells under conditions of high areal capacity (&gt;15 mA h cm−2) and long-term cycling (1 A h cm−2; over 200 h), and (iv) post-analysis of Zn−air cells tested to failure.

Earth abundant elements in oxygen electrocatalysts give cobalt a run for its money

ACS Spring Meeting 2021

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