Long-term operation of microbial electrosynthesis cell reducing CO 2 to multi-carbon chemicals with a mixed culture avoiding methanogenesis.

In microbial electrosynthesis (MES), CO ₂ 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 CO ₂ reduction which circumvents production of multi-carbon compounds. This study aimed to develop a stable and robust CO ₂ 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 H ₂ :CO ₂ media, (ii) a transfer from heterotrophic to autotrophic growth and (iii) a sequential batch operation. Biomass growth and gradual acclimation to CO ₂ electro-reduction accomplished a maximum acetate production rate of 400mgL _catholyte ^-1 d ^-1 at -1V (vs. Ag/AgCl). Methane was never detected in more than 300days of operation. Accumulation of acetate up to 7-10gL ^-1 was repeatedly attained by supplying (80:20) CO ₂ :N ₂ mixture at -0.9 to -1V (vs. Ag/AgCl). In addition, ethanol and butyrate were also produced from CO ₂ reduction. Thus, a robust CO ₂ 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.
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