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

TL;DR

This study investigates manganese dissolution mechanisms in LiMn2O4 during oxygen evolution, revealing how surface oxidation influences dissolution and proposing strategies to mitigate catalyst degradation.

Contribution

The paper provides detailed atomistic insights into Mn dissolution during OER in LiMn2O4, highlighting the role of surface Mn4+ species and suggesting control strategies to reduce degradation.

Findings

Mn dissolution occurs independently of and during OER.

Surface Mn4+ formation correlates with increased Mn dissolution.

Controlling Mn4+ levels can mitigate OER-triggered dissolution.

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.