Reducing Oxygen Stress and Improving Hydrogen Availability Boosts Microbial Electrosynthesis by Clostridium ljungdahlii.

Abstract

Microbial electrosynthesis (MES) holds great promise for converting carbon dioxide (CO₂) into building blocks of the (bio)chemical industry. Its advancement is hindered by limited process control and an incomplete understanding of the oxygen (O₂) stress response of biocatalysts or key engineering parameters like the availability of hydrogen (H₂). With Clostridium ljungdahlii as a model acetogen for strict anaerobic MES from CO₂, the effect of O₂ stress and H₂ availability using 1-L electrobioreactors is showcased, providing high process control and relevance for follow-up engineering and scaling. Using a combinatorial approach of two cathode materials, three anode types, and various current regimes ranging from -5 to -80 mA, MES performance is boosted by overcoming O₂ stress and insufficient H₂ distribution at high current. It is demonstrated that a large-surface-area carbon fiber fabric cathode combined with O₂ evolution anodes flushed with nitrogen (N₂) allows the highest reproducible acetate concentration of 12.44 ± 1.56 g L^-1 and maximum acetate production rate of 0.6 ± 0.1 g L^-1 d^-1 reported for MES from CO₂ using a pure culture. There is certainly room for improved process control at this and even larger scales, showing that the ceiling of strict anaerobic MES is far from being reached.