Converting carbon dioxide (CO₂) into valuable resources contributes to industry decarbonization and fosters circular economy. Main biological processes for CO₂ conversion are microbial electrosynthesis (MES) and anaerobic fermentation (AF). MES is based on the electro-reduction of CO₂ catalyzed by microorganisms. MES are particularly promising as i) do not consume chemical reagents and ii) provide reducing power in-situ, which can be obtained from renewable sources. Up to date, MES studies have mostly focused on CO₂ conversion to acetic acid or methane. This doctoral thesis goes one step further and targets more valuable compounds such as ethanol and longer chain carboxylic acids (butyric and caproic acids). Additionally, it studies for the first time the coupling of microbial electrosynthesis with anaerobic fermentation processes and the implementation of a membrane-based system. The goal is to increase production rates, reduce energy consumption and improve product selectivity of the target compounds by optimizing operational conditions.

Ethanol is used as fuel, chemical solvent and in the synthesis of several organic compounds, among other applications. This thesis aimed to trigger ethanol bioelectrochemical production from CO₂ via solventogenesis (by means of Clostridium species). Meritxell Romans investigated at laboratory scale new materials for the cell (i.e. stainless steel), optimized reactor configurations (i.e. low-gap electrically efficient cell stacks) and new operation strategies. Moreover, she implemented an automated control system. This allowed defining the optimal operational parameters for enhancing ethanol production: low pH (< 5.0), low partial CO₂ pressures (pCO₂ < 0.1 atm), high hydrogen partial pressure (pH₂ > 2.0 atm) and acetic acid concentration above 6.0 g L^-1. Following this approach, she achieved for the first time a continuous ethanol production rate of 777,7 mg per liter of cell and day (mg L^-1 d^-1) and a production selectivity of 60%.

Butyric and caproic acids are used to produce numerous compounds for the chemical, energy and pharmaceutical sectors. As with ethanol production, Meritxell Romans investigated the use of low-gap MES cells to produce butyric acid from CO₂ and reached 78% selectivity with a power demand of 34.6 kWh kg^-1, two-fold lower than previously required. On the other hand, she designed and developed a new two-step integrated system in which the ethanol and acetic acid produced in a microbial electrosynthesis cell act as a feedstock of an anaerobic fermentation process for chain elongation to butyric and caproic acid. This configuration produced caproic acid at a rate of 0.74 g L^-1 d^-1. Finally, she developed an innovative membrane-submerged fermenter system, in which ethanol and acetic acid ratios are tweaked to the desired ones by using non-porous membranes, achieving up to 94 % selectivity and production rates of 3.1 g L^-1 d^-1. The invention has been patented.

Such remarkable results will contribute to the upscale and commercialization of microbial electrosynthesis technologies and their integration with fermentation processes to convert CO₂ into valuable chemicals. The impact of this circular approach is high, both in terms of industry decarbonization and reduction of dependency of fossil resources. The thesis was supervised by Dr Sebastia Puig and Dr Maria Dolors Balaguer from LEQUIA research group (University of Girona), and by Dr. Paolo Dessì (University of Naples Federico II).