The electrochemical reduction of CO₂ (CO₂RR) is a sustainable technology that can be used to convert CO₂ into valuable products when electricity is generated from renewable sources. The focus of application-oriented research is currently mainly on the electrolysis of CO₂ to produce hydrogen-rich fuels or chemical feedstock material. In contrast, our project aims to produce stable, carbon-rich products (e.g. oxalate and carbon flakes) that can be safely and permanently disposed of in geological repositories. This negative emission technology has been developed to sustainably remove CO₂ from the global cycle.

The continuous electrochemical formation of solid carbon flakes from CO₂ has been reported on liquid GaInSn-M alloys (M: Ce, V) in water containing DMF^1,2. To further develop this approach, we studied GaInSn with and without additional Cerium alloying in DMF/H₂O/ TBAPF₆ electrolyte. Pure GaInSn shows a significant activity for CO₂RR to carbon monoxide (CO) and formate, depending on the content of water. We found evidence that the chemical composition of the liquid GaInSn interface changes with the applied electrode potential and water content, what can explain the observed product selectivity³. Our results suggest that the carbon monoxide (CO) generated on GaInSn serves as an intermediate product in the formation of carbon flakes on the alloyed cerium particles⁴. However, our study shows that numerous challenges still need to be overcome for this technically complex approach, which currently shows rather comparatively low carbon production rates.

As an alternative, the reduction of CO₂ to oxalate on solid, activated lead electrodes in anhydrous propylene carbonate (PC+TEA-Cl) was investigated. Faraday efficiencies for CO formation of over 80% at current densities of over 20 mA/cm² make it suitable for use as a cathode in a standalone solar-powered electrolyser. For this purpose, the electrolyzer was coupled with 5-junction or 3-junction solar cells and operated under 1sun-AM-1.5G illumination. To keep the required cell voltage sufficiently low, sacrificial electrodes were used and investigated as anodes. Using Zn anodes, we achieved solar-to-carbon conversion efficiencies of over 10%, around five to ten times higher than it is observed in natural photosynthesis. This demonstrates that our technical approach could require a smaller valuable land area than biomass-based methods.