As part of MECS, the Research Group "Electrochemical Energy Conversion" headed by Alexander Opitz is working on the electrochemical reduction of gases such as CO₂, CO and N₂ in solid oxide cells. The aim is to gain fundamental insights into the reaction mechanisms and material properties in order to develop more efficient and longer-lasting solutions for chemical energy storage. The group's approach to achieving this goal is to understand the relevant surface processes at the atomic level. To this end, in-situ experiments are carried out that combine electrochemical measurements with surface analytical methods. This approach is intended to provide in-depth knowledge of the functioning and optimisation of materials for chemical energy storage.

a) High-temperature CO₂ electrolysis

The reduction of CO₂ to CO in solid oxide electrolysis cells (SOECs) is favoured both thermodynamically and kinetically at the high operating temperatures of SOECs (600-800 °C). One problem, however, is the possible subsequent reduction of CO, which can lead to carbon deposits (coking) on the electrode surface. Such deposits can impair the catalytic activity of the electrode or even cause irreversible mechanical damage.

In order to overcome these challenges, Alexander Opitz's group is investigating mixed-conducting oxides (e.g. cerium oxide or perovskite-based materials). The aim is to use targeted doping strategies to improve the CO selectivity of the materials and at the same time minimise the tendency for carbon formation. A variety of electrochemical and surface analytical methods such as NAP-XPS, AES and IR spectroscopy are used to gain a detailed understanding of the point defects, reaction intermediates (e.g. carbonates) and their interactions.

b) Hydrogenation reactions in proton-conducting solid oxide cells

Another focus is on the hydrogenation of molecules such as CO₂, CO or N₂ on mixed-conducting electrode materials that offer both proton and electron conduction. Through cathodic polarisation at moderate temperatures of 200-500 °C, very high effective hydrogen pressures can be achieved (realised in the material through high chemical potentials of H⁺ and e^-). This represents a high thermodynamic driving force for hydrogenations. In order to actually utilise this driving force for the corresponding desired reactions (such as the synthesis of ammonia from N₂), catalysts are required that can provide the corresponding activity in combination with the mixed-conducting electrode. The CoE MECS offers the ideal environment for investigating potential catalyst candidates.

c) Perovskite-based model (electro)catalysts

A third thematic branch of the group within the MECS cluster deals with mixed-conducting perovskites, which form the basis for model investigations as well-defined thin films. On the one hand, this involves so-called exsolution catalysts. These are catalytically active transition metal nanoparticles that are released from the perovskite through targeted strong reduction and the associated partial decomposition of the perovskite. Alexander Opitz's group is using cathodic polarisation to achieve this reduction, which also opens up the possibility of influencing the oxidation state of the particles by changing the applied electrochemical polarisation. This allows the catalytic activity of the particles to be switched electrochemically. Gaining a fundamental understanding of the elementary physical phenomena involved in this switching process is the main driving force behind our investigations.

A very recent topic that the group is pursuing on model perovskite thin films is the investigation of point defects on free-standing membranes. Here, the collaboration with other groups in the CoE MECS offers unprecedented opportunities to gain direct insights into the defect chemistry of mixed conductors.

A highly attractive form of electrolysis relies on Solid Oxide Electrolysis Cells (SOECs), which can electrolyze both water vapor and CO₂, splitting them into H₂ and CO (respectively) and O₂. Besides high efficiencies, advantages include the avoidance of highly critical materials (such as Ir, Pt, Co, among others), the use of non-critical electrolytes (based on ZrO₂), and the potential for combined power and heat generation. However, long-term stability still requires optimization, with processes at both anodes and cathodes being involved. Additionally, efficiency is targeted to be further enhanced by reducing the overpotentials of electrochemical reactions at the electrodes.

Within the framework of the cluster, extensive research is conducted, particularly on model electrodes. This aims to elucidate degradation mechanisms (e.g., mechanical damage due to high oxygen pressures during anodic reactions) and identify kinetically slow processes, while also generating kinetically optimized electrodes through surface modifications with other oxides. Crucial to this endeavor is the combination of highly controlled sample preparation (e.g., through pulsed laser deposition), electrochemical measurements with mechanistic interpretation, and integration with high-quality analytics, including XPS, AES, SIMS, XRD, and TEM.

In related activities regarding the applied methods mentioned above, efforts are made to bridge to another type of energy storage. Lithium-ion batteries are the most important type of rechargeable batteries, suitable for short-term electrical energy storage in the power grid. Although oxygen exchange should not occur during normal battery operation, interactions with O2 come into play in two instances: during unwanted cathode overcharging and even more so during thermal runaway, as well as during material fabrication at elevated temperatures. The parallels (and differences) in oxygen stoichiometry changes between Li-ion battery electrodes and solid oxide cell electrodes are still largely unexplored, and within this cluster, efforts are made to bridge the gap to the world of grid storage batteries. By combining various approaches from two research communities, contributions are aimed at improving battery properties and safety.