Rigorous assessment of electrochemical rechargeability of alkaline Zn-air batteries

Zn-air batteries (ZABs) are experiencing a resurgence of attention due to competitive energy density and inherent safety. Multiple recent reports suggest that optimized bifunctional catalysts resolve the century-old challenge of ZABs and enable electrochemical rechargeability, but it is still difficult to ponder and scale-up. Herein, a rigorous assessment of electrochemical rechargeability masked by stable voltage profiles is presented. First, polarization testing as an essential technique for rapidly benchmarking the electrochemical performance of ZABs is evaluated. Then, the cycling stability tests at different depth of discharge (DOD) show that the Zn depletion rate per cycle increases significantly with increasing DOD, reaching 10% at a DOD of 50%. Further, differential electrochemical mass spectrometry is introduced for in-situ monitoring oxygen released during charging. Stable voltage profiles may miss critical information indicating effective charging, especially with transition metal oxides as catalysts and cycling using small DOD, the cycling process may involve only valence conversion of transition metals rather than oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Finally, electrolyte changes masked by the stable voltage profile are investigated. Hopefully, this work provides useful guidelines for realizing the electrochemical rechargeability of air-based batteries including ZABs.