Carbon monoxide separation from industrial waste gases could contribute largely to carbon circularity. Traditional separation technologies are unable to separate CO from N₂ selectively. Instead, electroactive carriers show promise in selective separation of CO from N₂, where CO binds a complex in one oxidation state and releases in another oxidation state. We study Cu(i)/Cu(ii)-chloride complexes as potential carrier materials with high binding affinity to CO, good solubility and low energy consumption of the process. We show that the electrolyte composition of a copper chloride system affects the binding affinity and stability of the copper carrier (Cu⁺). Cyclic voltammetry measurements reveal that the CO binding constant increase from the previously reported 1600 M⁻¹ for 1 M KCl to 5500 M⁻¹ for 0.5 M CaCl₂. However, this increase in binding constant is not reflected to the same extent in the CO capture capacity, showing a smaller increase in CO capture. In general, the binding constant decreases with chloride concentration, while the Cu⁺ stability window increases. This highlights a trade-off that needs to be considered for electrolyte selection in electrochemical CO separation with copper chlorides.

Electrochemical CO2 reduction has the potential to use excess renewable electricity to produce hydrocarbon chemicals and fuels. Gas diffusion electrodes (GDEs) allow overcoming the limitations of CO2 mass transfer but are sensitive to flooding from (hydrostatic) pressure differences, which inhibits upscaling. We investigate the effect of the flooding behavior on the CO2 reduction performance. Our study includes six commercial gas diffusion layer materials with different microstructures (carbon cloth and carbon paper) and thicknesses coated with a Ag catalyst and exposed to differential pressures corresponding to different flow regimes (gas breakthrough, flow-by, and liquid breakthrough). We show that physical electrowetting further limits the flow-by regime at commercially relevant current densities (≥200 mA cm-2), which reduces the Faradaic efficiency for CO (FECO) for most carbon papers. However, the carbon cloth GDE maintains its high CO2 reduction performance despite being flooded with the electrolyte due to its bimodal pore structure. Exposed to pressure differences equivalent to 100 cm height, the carbon cloth is able to sustain an average FECO of 69% at 200 mA cm-2 even when the liquid continuously breaks through. CO2 electrolyzers with carbon cloth GDEs are therefore promising for scale-up because they enable high CO2 reduction efficiency while tolerating a broad range of flow regimes.

Electrochemical CO2 reduction is a promising process to store intermittent renewable energy in the form of chemical bonds and to meet the demand for hydrocarbon chemicals without relying on fossil fuels. Researchers in the field have used gas diffusion electrodes (GDEs) to supply CO2 to the catalyst layer from the gas phase. This approach allows us to bypass mass transfer limitations imposed by the limited solubility and diffusion of CO2 in the liquid phase at a laboratory scale. However, at a larger scale, pressure differences across the porous gas diffusion layer can occur. This can lead to flooding and electrolyte breakthrough, which can decrease performance. The aim of this study is to understand the effects of the GDE structure on flooding behavior and CO2 reduction performance. We approach the problem by preparing GDEs from commercial substrates with a range of structural parameters (carbon fiber structure, thickness, and cracks). We then determined the liquid breakthrough pressure and measured the Faradaic efficiency for CO at an industrially relevant current density. We found that there is a trade-off between flooding resistance and mass transfer capabilities that limits the maximum GDE height of a flow-by electrolyzer. This trade-off depends strongly on the thickness and the structure of the carbon fiber substrate. We propose a design strategy for a hierarchically structured GDE, which might offer a pathway to an industrial scale by avoiding the trade-off between flooding resistance and CO2 reduction performance.