  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.
