Microbial electrosynthesis from CO2 reaches productivity of syngas and chain elongation fermentations

Sustainable production of carbon-based products is urgently needed.A novel directed flow-through microbial electrosynthesis (MES) reactor was designed and characterized for carbon dioxide (CO2) conversion to C2–C6 carboxylates.Three-times denser biofilm, volumetric current density, and productivity were achieved compared with the state of the art.Biomass-specific production rates were maintained over more than 200 days, yet still an order of magnitude lower than that achieved by acetogens in syngas fermentation.Volumetric productivity in MES was comparable with that from syngas fermentation.Clostridium luticellarii and Eubacterium limosum were the dominant species. Carbon-based products are essential to society, yet producing them from fossil fuels is unsustainable. Microorganisms have the ability to take up electrons from solid electrodes and convert carbon dioxide (CO2) to valuable carbon-based chemicals. 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 over 248 days. We demonstrate a threefold higher biofilm concentration, volumetric current density, and productivity compared with the state of the art. Most notably, the volumetric productivity (VP) resembles those achieved in laboratory-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. Graphical abstract [Display omitted] Microbial electrosynthesis (MES) of carboxylic acids from CO2 and electricity has been validated for over a decade, now reaching Technology Readiness Levels 3/4 in laboratory settings. However, process optimization is needed before demonstrating an industrial prototype. Key challenges for full-scale implementation include ensuring production stability. Critical areas to investigate and demonstrate are: (i) the impact of CO2 feed stream composition and properties; (ii) the short- and long-term effects of renewable electricity supply intermittency; and (iii) the flexibility of MES operations and the integrated process, including up- and downstream processes. Moreover, a com