# Techno-Economic Potential of Plasma-Based CO\({}_{2}\) Splitting in Power-to-Liquid Plants
## 2 State of the Art
### Oxygen Separation--Review and Experimental  
Reducing carbon dioxide yields oxygen-rich gas, from which the oxygen must be extracted before further use is possible to inhibit recombination. The amount of oxygen in the process gas was investigated in the PlasmaFuel project, and a 1% to 2% concentration was set as a maximum value that can be processed by the downstream Fischer-Tropsch synthesis. The influence of the oxygen content was investigated by [PERSON] et al. [42]. To reduce the oxygen content to 1%, a variety of technologies can be applied. Commonly, adsorption technologies or nanoporous membranes are used; however, both are not able to deliver high-purity gases or extract traces of oxygen from gases. A lot of research is currently being undertaken on high-temperature solid electrolyte ion pumps, which offer high selectivity for oxygen. However, these systems require a catalyst as well, which can lead to coking issues when carbon monoxide is present. Finding high-temperature solutions that offer high current density while being robust is an ongoing field of research. Low-temperature electrolysis can resolve this issue in theory; however, a fitting catalyst is also required that is stable in the presence of carbon-containing gases. Different oxygen separation approaches are characterized in the following. The challenge of oxygen separation lies in the gas composition of the mixed gas from which the oxygen is to be separated. Originally, a zirconium dioxide electrode was considered for this task. Commercially available zirconium dioxide membranes use a catalyst to split the molecular oxygen into atomic oxygen, which can be conducted. This is typically a platinum-group metal. Such catalysts are poisoned by carbon monoxide (CO) and carbon dioxide (CO\({}_{2}\)), i.e., their active surface area decreases rapidly. Research reveals several potential alternative technologies. These include nanoporous membranes, which enable low-temperature applications. They are operated by diffusion, i.e., a pressure difference. The main disadvantage is their low separation efficiency due to the similar permeability of O\({}_{2}\), CO, and CO\({}_{2}\) of the membrane. Better separation efficiencies are achieved by electrochemical methods. For example, a zirconium dioxide membrane has high selectivity and is commercially available but requires a working temperature \(T>600\)\({}^{\circ}\)C. Working temperature, selectivity, and flow rate are, therefore, relevant parameters. Perovskite membranes belong to the group of mixed ionic electric conductors (MIEC) and can be manufactured both monolithically and asymmetrically [43]. At low temperatures of \(T=150\)\({}^{\circ}\)C, they can be used as a catalyst for oxygen uptake. For significantly higher temperatures, the function of a separation membrane is also possible, although the asymmetric membrane has, so far, only been CO\({}_{2}\)-resistant to a limited extent. In addition, a perovskite membrane is conceivable as an electrode for electrochemical substance separation, but developments of this kind would have to be pursued in cooperation with research groups experienced in this field. Gas diffusion electrodes are another option and are used in the chemical industry. They achieve sufficient fluxes at low operating temperatures and sufficient selectivity. The question of a suitable catalyst also arises here. Pressure swing adsorption or temperature swing adsorption can both be used to separate oxygen from the gas mixture but require a large amount of energy in the form of heat [44]. The last technology considered is the molten carbonate fuel cell (MCFC) membrane, which can be used in reverse mode for oxygen separation. Currently, this patented membrane is only sold by one company at a very high price and is consequently not commercially available in a cost-efficient way.