Long-term operation of microbial electrosynthesis cell reducing CO₂ to multi-carbon chemicals with a mixed culture avoiding methanogenesis

In microbial electrosynthesis (MES), CO₂ can be reduced preferably to multi-carbon chemicals by a biocathode-based process which uses electrochemically active bacteria as catalysts. A mixed anaerobic consortium from biological origin typically produces methane from CO₂ reduction which circumvents production of multi-carbon compounds. This study aimed to develop a stable and robust CO₂ reducing biocathode from a mixed culture inoculum avoiding the methane generation. An effective approach was demonstrated based on (i) an enrichment procedure involving inoculum pre-treatment and several culture transfers in H₂:CO₂ media, (ii) a transfer from heterotrophic to autotrophic growth and (iii) a sequential batch operation. Biomass growth and gradual acclimation to CO₂ electro-reduction accomplished a maximum acetate production rate of 400 mg L_catholyte ^− 1 d^− 1 at − 1 V (vs. Ag/AgCl). Methane was never detected in more than 300 days of operation. Accumulation of acetate up to 7–10 g L^− 1 was repeatedly attained by supplying (80:20) CO₂:N₂ mixture at − 0.9 to − 1 V (vs. Ag/AgCl). In addition, ethanol and butyrate were also produced from CO₂ reduction. Thus, a robust CO₂ reducing biocathode can be developed from a mixed culture avoiding methane generation by adopting the specific culture enrichment and operation procedures without the direct addition of chemical inhibitor.