# Techno-Economic Potential of Plasma-Based CO\({}_{2}\) Splitting in Power-to-Liquid Plants
## 2 State of the Art
### Oxygen Separation--Review and Experimental
The possible processes for oxygen removal are summarized graphically in Figure 3.  
After evaluating the possible technologies for extracting oxygen from a mixed gas stream, a separation process for PlasmaFuel has been selected. Because of its innovation potential and technical advantages, the gas diffusion electrode (GDE) was investigated. Gas diffusion electrodes (GDE) are used in the fuel cell industry. As a multi-layer porous electrode made of active material and a hydrophobic layer, it enables the separation of oxygen (O\({}_{2}\)) from a material flow. A mixed gas of CO\({}_{2}\), CO, and O\({}_{2}\), in which the O\({}_{2}\) is to be removed, is used as an example. The working principle and design of a GDE separation device is depicted in Figures 4 and 5, respectively. Oxygen gets into the adjacent electrolyte through the multi-layered GDE (cathode). The hydrophobic separator on the gas side ensures that no liquid electrolyte can penetrate the gas system. The active material of the GDE consists of a catalyst (e.g., silver oxide, manganese oxide) with graphite. The graphite gives the electrode sufficient porosity to allow gas and electrolyte to penetrate. The electrolyte used was 17% potassium carbonate solution (K\({}_{2}\)CO\({}_{3}\)). The gas diffusion electrode currently under investigation uses graphite as the support and manganese dioxide (MnO\({}_{2}\)) as the catalyst for oxygen reduction catalyst. MnO\({}_{2}\) is a weak catalyst commonly used in air-fed batteries. It is an electrochemical separation process of oxygen (O\({}_{2}\)) from a mixed gas stream of predominantly carbon dioxide (CO\({}_{2}\)) and carbon monoxide (CO), and the catalyst used is selective for the reduction of O\({}_{2}\). Oxygen is transported through the electrolyte in the form of OH\({}^{-}\) ions. Due to the high conductivity of aqueous electrolytes, the system can achieve high current densities at low temperatures (\(T<90\)\({}^{\circ}\)C). By using a carbonate-based alkaline electrolyte, side reactions with the electrolyte are mitigated. Using a hydroxide-based electrolyte might work better initially, but they will quickly react to carbonates in situ. Figure 4: Scheme of the working principle of a gas diffusion electrode as an oxygen removal device. Figure 3: Possible processes for oxygen separation: electrochemically, mixed ionic electric conductor, adsorption, and diffusion. The power needed to separate oxygen through the GDE is defined through:  
\[P_{GDE}=U_{GDE}\cdot I_{GDE}=U_{GDE}\cdot\dot{n}_{G2}\cdot F\cdot z \tag{4}\]  
According to the reaction equation at the GDE surface  
\[\frac{1}{2}\mathrm{O}_{2}+\mathrm{H}_{2}\mathrm{O}+2\mathrm{e}^{-}\to 4 \mathrm{OH}^{-} \tag{5}\]  
\(z=2\) is fixed. A current density of \(i=18\) mA/cm\({}^{2}\) was achieved with N\({}_{2}\)/O\({}_{2}\) mixtures. When CO-rich gas from the plasma reactor was used, the current density dropped to 65% of the initial value before stabilizing. The operation was demonstrated successfully for a period of 10 h. The obtained results suggest that a higher current density could be achieved using a modified catalyst since the oxygen reduction and gas formation produce the highest potential.