Manganese Dissolution in alkaline medium with and without concurrent oxygen evolution in LiMn$_2$O$_4$

Abstract: Manganese dissolution during the oxygen evolution reaction (OER) has been a persistent challenge that impedes the practical implementation of Mn-based electrocatalysts including the LiMn$_x$O$_4$ system in aqueous alkaline electrolyte. The investigated LiMn$_2$O$_4$ particles exhibit two distinct Mn dissolution processes; one independent of OER and the other associated to OER. Combining the bulk sensitive X-ray absorption spectroscopy, surface sensitive X-ray photoelectron spectroscopy and electrochemical detection of Mn dissolution using rotating ring-disk electrode, we explore the less understood Mn dissolution mechanism during OER. We correlate near-surface oxidation with the charge attributed to dissolved Mn, which demonstrates increasing Mn dissolution with the formation of surface Mn4+ species under anodic potential. The observed stronger dissolution during the OER is attributed to the formation of additional Mn$^{4+}$ from Mn$^{3+}$ during OER. We show that control over the amount of Mn4+ in Li$_x$Mn2O$_4$ before the onset of the OER can partially mitigate the OER-triggered dissolution. Overall, our atomistic insights into the Mn dissolution processes are crucial for knowledge-guided mitigation of electrocatalyst degradation, which can be broadly extended to manganese-based oxide systems.