LiMn2O4 as a Model System to Understand the Mn Dissolution Mechanism during Water Oxidation

Abstract

LiMn2O4 as a model system to understand the Mn dissolution mechanism during water oxidation Omeshwari Bisen1,2, M. Baumung1,2, F. Schönewald2, C. A. Volkert2, M. Risch1,2 1 Nachwuchsgruppe Gestaltung des Sauerstoffentwicklungsmechanismus, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1,14109 Berlin, Germany 2 Institut für Materialphysik, Georg-August-Universität Göttingen, Germany Email: omeshwari.bisen@helmholtz-berlin.de The water splitting provides an attractive avenue for chemical energy storage by producing the green hydrogen, but it suffers from the inefficiency of the oxygen evolution reaction (OER) and the degradation of the electrocatalyst under the harsh oxidizing conditions of the reaction. Manganese oxides are attractive due to their abundance and as a non-critical raw material, but they are known to degrade quickly during the OER, which hinders their usage severely. [1] Therefore, it is equivalently important to focus on the understanding of degradation processes during the OER. [2]In this study and our previous studies, we used the battery material LiMn2O4 as a model system for the OER in alkaline media where the Mn valence can be tuned with little effect on structure. [3, 4] LiMn2O4 exhibits a core-shell cubic-spinel structure with tetrahedral antisite Mn2+ defects in the shell. [5] We have used the rotating ring-disk electrode (RRDE) setup to obtain the product current of manganese corrosion and oxygen evolution by applying the desired potential at the Pt ring (Fig. 1a). We monitored the dissolution of Mn from the LiMn2O4 (Fig. 1b) and found two onsets of exponential currents, indicating the two distinct Mn loss processes; one independent of the OER and one associated to it, which is further investigated by Tafel slopes. The ex-situ X-ray absorption spectroscopy (Fig. 1c) and X-ray photoelectron spectroscopy (Fig. 1d) represents the oxidation of Mn moieties after voltage cycling including the OER, and loss of Mn2+ and Li+. 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 and observed the distinct atomistic origin before and after OER onset.[6] It is highly desirable to understand the atomistic insights into the Mn dissolution processes for the knowledge-guided mitigation of electrocatalyst degradation during water oxidation, which can be broadly extended to manganese-based oxide systems.