Correlations between manganese valence and catalytic oxygen evolution of LixMn2O4

This doctoral thesis deals with the mechanistic insights in catalysing the oxygen evolution reaction (OER) in alkaline solution by using a tuneable LixMn2O4 spinel as a model catalyst, motivated by the challenging step to control the OER for the long-time storage of electricity from renewables in chemical energy carriers. By using a RRDE-setup it was possible to identify the origin of the total electrode current as the sum of the oxygen evolution and the manganese loss at a defined reference potential and could evaluate its impact on the catalytic behaviour for different particle sizes in NaOH (pH 13). The initial observed disk current decay is assigned to the manganese loss reaction while a constant amount of oxygen is detected at the ring electrode. Changing the electrolyte to LiOH reveals a different behaviour of the catalyst as pronounced redox peaks in electrolytes below pH 14 indicates an oxidation by delithiation. This in-situ oxidation during the OER was confirmed by XAS and has an impairing influence on the catalytic activity as implied by the shifted onset of the oxygen detection current to higher overpotentials. In contrast to the initial material, ex-situ delithiated LixMn2O4 particles have a different origin of the total disk current. Thus, the Faradaic efficiency increases from 75(2)% to 96(5)% because of a negligible corrosion process. These results highlight the meaning of side reaction as they may influence the catalytic activity, as demonstrated in the delithiation reaction, thus it leads to a chemical different catalyst and in contrast to that the manganese loss reaction which has no effect on the OER, however could get negligible which leads to an increase in the Faradaic efficiency. The Faradaic efficiency may help to identify model catalysts, which do not have a significant side reaction current contribution. This could be the pathway to a deeper mechanistic insight, as these catalysts only produced the focused product, and no side reactions may interfere with the OER mechanism.2022-06-2