Gas Diffusion Electrode Design and Conditioning with a Manganese(III/IV) Oxide Catalyst for Reversible Oxygen Reduction/Evolution Reactions

Significant efforts have been made to develop cost-efficient, nonprecious metal catalysts for reversible oxygen electrodes [i.e., oxygen reduction/evolution reactions (ORR/OER)]. However, typically, their fast degradation during cycling between the OER and ORR potential domains (i.e., battery charge/discharge) has been a critical challenge for rechargeable metal-air batteries or reversible fuel cells. Herein, we used gas diffusion electrodes (GDEs) with a core-shell Mn/Mn3O4 (referred to as MnOx) nanocatalyst, and we investigated the impact of the gas diffusion layer properties (teflonation and HNO3 pretreatment) and MnOx catalyst layer composition on the bifunctional activity and durability under alkaline conditions. Raman spectroscopy corroborated by X-ray photoelectron spectroscopy (XPS) results and average oxidation state calculations showed that the performance degradation during galvanostatic cycling is due to phase transition and oxidation of Mn3O4 to gamma-MnO2, which can further oxidize at high anodic potentials (>= 1.5 V-RHE) to catalytically inactive MnO4-. We found that the incorporation of carbon additives in the catalyst layer as electronic conductivity boosters has an additional beneficial effect on the cycling durability of the MnOx GDEs. The mixture of graphene and Vulcan XC 72 (1:1 w/w) in the catalyst layer improved 6-fold the galvanostatic cycling durability in accelerated degradation experiments. Another degradation mode occurs when the electrode is cycled to high reduction current densities generating inactive Mn(II) species. Electrode activation protocols based on cyclic voltammetry can further improve the bifunctional activity and stability of the MnOx GDEs.