Manganese Dissolution in alkaline medium with and without concurrent oxygen evolution in LiMn2_2O4_4

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

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

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 mechanis

Manganese Dissolution in alkaline medium with and without concurrent oxygen evolution in LiMn2O4

Manganese dissolution during the oxygen evolution reaction (OER) has been a persistent challenge that impedes the practical implementation of Mn-based electrocatalysts including the LixMn2O4 system in aqueous alkaline electrolyte. The investigated LiMn2O4 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 Mn4+ from Mn3+ during OER. We show that control over the amount of Mn4+ in LixMn2O4 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

Characterization of a Modular Flow Cell System for Electrocatalytic Experiments and Comparison to a Commercial RRDE System

Generator-collector experiments offer insights into the mechanisms of electrochemical reactions by correlating the product and generator currents. Most commonly, these experiments are performed using a rotating ring-disk electrode (RRDE). We developed a double electrode flow cell (DEFC) with exchangeable generator and detector electrodes where the electrode width equals the channel width. Commonalities and differences between the RRDE and DEFC are discussed based on analytical solutions, numerical simulations and measurements of the ferri-/ferrocyanide redox couple on Pt electrodes in a potassium chloride electrolyte. The analytical solutions agree with the measurements using electrode widths of 5 and 2 mm. Yet, we find an unexpected dependence on the exponent of the width so that wider electrodes cannot be analysed using the conventional analytical solution. In contrast, all the investigated electrodes show a collection efficiency of close to 35.4% above a minimum rotation speed or flow rate, where the narrowest electrode is most accurate at the cost of precision and the widest electrode the least accurate but most precise. Our DEFC with exchangeable electrodes is an attractive alternative to commercial RRDEs due to the flexibility to optimize the electrode materials and geometry for the desired reaction