Bio-electro CO2 recycling into added value compounds:insights, routes, strategies and performance improvements

In recent years, the decarbonization of industry has emerged as a key strategy for fostering a sustainable and eco-friendly economy. One promising approach is the conversion of CO2, a major contributor to climate change, into valuable products such as organic compounds. Among the various processes explored, microbial electrosynthesis (MES) and anaerobic fermentation have shown great potential. MES uses microorganisms to transform CO2 into useful chemicals, powered by renewable energy, while anaerobic fermentation further converts simple organic compounds into more complex, valuable molecules, enhancing process efficiency.

While most research has focused on converting CO₂ into simpler compounds like acetic acid or methane, this PhD research aims to optimize MES for producing higher-value products such as ethanol and longer-chain carboxylic acids. Extensive experimentation identified optimal conditions for ethanol production, including low pH, high hydrogen partial pressure, and acetic acid concentrations above 6.0 g L^-1. Under these conditions, continuous ethanol production rates of up to 10.95 g m^-2 d^-1 were achieved, with final concentrations reaching 5.28 g L^-1.

Advancements in low-gap reactor designs improved the ethanol-to-acetic acid molar ratio to 2.11, enhancing efficiency and reducing energy consumption, bringing MES technology closer to commercial viability.

Additionally, the production of butyric and caproic acids was explored using low-gap MES cells. Butyric acid was produced with 78% selectivity, at a power requirement of 34.6 kWh kg^-1 —half of what was previously reported. A two-step system integrating MES with anaerobic fermentation was developed, producing caproic acid at 0.74 g L^-1 d^-1 with over 90 % selectivity. An innovative membrane-submerged fermenter system allowed to further increase selectivity to 94 %, with production rates of 3.1 g L^-1 d^-1.

This research demonstrates the viability of MES and anaerobic fermentation for transforming waste CO₂ into valuable chemicals. By optimizing conditions and integrating advanced reactor designs, this work contributes significantly to the development of technologies for large-scale industrial applications, paving the way for a more sustainable future.