Cabau-Peinado, O.; Winkelhorst, M.; Stroek, R.; de Kat Angelino, R.; Straathof, A.; Masania, K.; Daran, J.-M. G.; Jourdin, L.

Microbial electrosynthesis allows the electrochemical upgrading of CO2. However, higher productivities and energy efficiencies are needed to reach a viability that can make the technology transformative. Here 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. 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 3cathode and 69 kgC m-3cathode day-1, at 60-97% and 30-35% faradaic and energy efficiencies, respectively. Most notably, the volumetric productivity resembles those achieved in lab-scale and industrial syngas (CO-H2-CO2) fermentation and chain elongation fermentation. This work highlights key design parameters for efficient electricity-driven microbial CO2 reduction. There is need and room to improve the rates of electrode colonization and microbe-specific kinetics to scale-up the technology.