H₂ mediated mixed culture microbial electrosynthesis for high titer acetate production from CO₂

Microbial electrosynthesis (MES) converts CO₂ into value-added products such as volatile fatty acids (VFAs) with minimal energy use, but low production titer has limited scale-up and commercialization. Mediated electron transfer via H₂ on the MES cathode has shown a higher conversion rate than the direct biofilm-based approach, as it is tunable via cathode potential control and accelerates electrosynthesis from CO₂. Here we report high acetate titers can be achieved via improved in situ H₂ supply by nickel foam decorated carbon felt cathode in mixed community MES systems. Acetate concentration of 12.5 g L^-1 was observed in 14 days with nickel-carbon cathode at a poised potential of -0.89 V (vs. standard hydrogen electrode, SHE), which was much higher than cathodes using stainless steel (5.2 g L^-1) or carbon felt alone (1.7 g L^-1) with the same projected surface area. A higher acetate concentration of 16.0 g L^-1 in the cathode was achieved over long-term operation for 32 days, but crossover was observed in batch operation, as additional acetate (5.8 g L^-1) was also found in the abiotic anode chamber. We observed the low Faradaic efficiencies in acetate production, attributed to partial H₂ utilization for electrosynthesis. The selective acetate production with high titer demonstrated in this study shows the H₂-mediated electron transfer with common cathode materials carries good promise in MES development.