Microbial Electrosynthesis from CO₂ reaches Productivity of Syngas and Chain Elongation Fermentations

Microbial electrosynthesis (MES) allows the electrochemical upgrading of CO₂. However, higher productivities and energy efficiencies are needed to reach a viability that can make the technology transformative. In this study we show how a biofilm-based microbial porous cathode in a directed flow-through electrochemical system can continuously reduce CO₂ to even-chain C2-C6 carboxylic acids during 248 days. Robustness and stability of the biocatalysts are major advantages for industrial application. Our novel reactor mitigated mass transport limitations. We demonstrate a 3 fold higher biofilm concentration, volumetric current density, and productivity than the state of the art, up to a new record of  35 kA m^-3_cathode and 69 kg_C m^-3_cathode day^-1, at 60-97% and 30-35% faradaic and energy efficiencies, respectively. The highest butyric and hexanoic acids concentration ever reported in MES were achieved (17.4 g L^-1 and 3.2 g L^-1, respectively). Moreover, we identified the dominant microbial species responsible for the elongation of CO₂ to C2-C6 carboxylates, namely Clostridium luticellarii and Eubacterium limosum. None of these species have been reported in MES to date and may represent the future ‘work-horses’ that push the boundaries of MES and possibly other gas fermentation technologies. Most notably, the volumetric productivity resembles those achieved in lab-scale and industrial syngas (CO-H₂-CO₂) fermentation and chain elongation fermentation. This study represents a milestone in developing MES as a competitive technology, marking a promising outcome that warrants further scaling-up of MES technology.