Reducing Oxygen Stress and Improving Hydrogen Availability Boosts Microbial Electrosynthesis by Clostridiumljungdahlii

Microbial electrosynthesis (MES) holds great promise for converting carbon dioxide (CO 2 ) into building blocks of the (bio)chemical industry. Its advancement is hindered by limited process control and an incomplete understanding of the oxygen (O 2 ) stress response of biocatalysts or key engineering parameters like the availability of hydrogen (H 2 ). With Clostridium ljungdahlii as a model acetogen for strict anaerobic MES from CO 2 , the effect of O 2 stress and H 2 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 O 2 stress and insufficient H 2 distribution at high current. It is demonstrated that a large‐surface‐area carbon fiber fabric cathode combined with O 2 evolution anodes flushed with nitrogen (N 2 ) 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 CO 2 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.