Degradation of α-MnO2 in Zn-air battery gas-diffusion electrodes: An investigation based on chemical-state mapping

Abstract

Rechargeable zinc-air batteries (RZABs) could enable cheap and safe high energy density electrochemical energy storage, but durability issues impair their real-life implementation, both Zn anodes and oxygen gas-diffusion electrodes (GDEs) exhibiting cyclability issues. This study focusses on the electrochemical operation and durability of GDEs based on α-MnO₂ nanowires and Ni-core / NiO shell nanoparticles (Ni@NiO) nanoparticles (NPs) as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts. Specifically, we concentrated on chemical-state mapping with sub-micrometre resolution of electrocatalyst clusters extracted from GDEs operated under well-characterized electrochemical conditions closely representative of real-life conditions. This approach is enabled by synchrotron-based Scanning Transmission X-ray Spectro-Microscopy (STXSM), complemented by high-resolution Transmission Electron Microscopy (TEM) to assess the correlated morphological and structural changes of electrocatalyst units. Specifically, this work analyses the performance of monofunctional and bifunctional GDEs aged electrochemically under ORR or ORR/OER cycling conditions in 6 M KOH electrolyte without and with Zn²⁺ addition. In brief, GDE activity was found to correlate with the spatial correlation Mn(III) and Mn(IV), the species that favour the 4-electron ORR mechanism. Instead, degradation is accompanied by the formation of Mn(II)- and Mn(IV)-rich islands of dead electrocatalyst, Mn(III)/Mn(IV) uncorrelation, resulting from cathodic an anodic stress due to high anodic polarization in monofunctional GDEs or to the formation of inactive Zn²⁺-containing phases. Ni@NiO NPs reduce anodic stress stabilizing Mn(III)/Mn(IV) correlation.