Challenges in the Application of Manganese Oxide Powders as OER Electrocatalysts: Synthesis, Characterization, Activity and Stability of Nine Different MnxOy Compounds

Abstract

Manganese oxides are seen as potential electrocatalysts for the alkaline oxygen evolution reaction (OER). To find the most suitable OER catalyst among the large number of known manganese oxide compounds, several comparative studies of selected MnxOy materials in water oxidation catalysis were reported in recent years with, in some cases, conflicting results. In this study, nine different manganese oxide powders differing in structure and/or composition were synthesized, characterized and compared regarding their OER activity and stability using a consistent set of experimental parameters. It turned out that the activity generally depends strongly on the manganese oxide compound. α-MnO2 manganese oxides of the hollandite-type were found to be more active than those with a lower oxidation state or other crystal structures. The most active catalyst cryptomelane, α-(K)MnO2, reached a current density of 10 mA/cm2 at 1.77±0.02 V in LSV measurements. At a potential of 1.8 V, the current density was approximately 15 mA cm−2. In contrast, the samples with the lowest activity exhibited values less than 1 mA cm−2 at the same potential. The stability experiments revealed a fast decrease in activity of all samples within the first minutes of measurement and an almost complete activity loss after 60 min. Conductivity differences are discussed as a likely reason for the observed differences in performance.