Tandem Acidic CO2 Electrolysis Coupled with Syngas Fermentation: A Two-Stage Process for Producing Medium-Chain Fatty Acids

The tandem application of CO₂ electrolysis with syngas fermentation holds promise for achieving heightened production rates and improved product quality. However, the significant impact of syngas composition on mixed culture-based microbial chain elongation remains unclear. Additionally, effective methods for generating syngas with an adjustable composition from acidic CO₂ electrolysis are currently lacking. This study successfully demonstrated the production of medium-chain fatty acids from CO₂ through tandem acidic electrolysis with syngas fermentation. CO could serve as the sole energy source or as the electron donor (when cofed with acetate) for caproate generation. Furthermore, the results of gas diffusion electrode structure engineering highlighted that the use of carbon black, either alone or in combination with graphite, enabled consistent syngas generation with an adjustable composition from acidic CO₂ electrolysis (pH 1). The carbon black layer significantly improved the CO selectivity, increasing from 0% to 43.5% (0.05 M K⁺) and further to 92.4% (0.5 M K⁺). This enhancement in performance was attributed to the promotion of K⁺ accumulation, stabilizing catalytically active sites, rather than creating a localized alkaline environment for CO₂-to-CO conversion. This research contributes to the advancement of hybrid technology for sustainable CO₂ reduction and chemical production.