Syngas mediated microbial electrosynthesis for CO₂ to acetate conversion using Clostridium ljungdahlii

Carbon dioxide valorization through microbial electrosynthesis (MES) is promising due to it's potential for mild operating conditions and stable long-term performance. Previous MES studies have typically shown poor organic production rates using direct electron transfer mechanisms, prompting the use of electrochemically generated H₂ to provide reducing equivalents for improved carbon utilization via metabolisms like the Wood-Ljungdahal pathway. Still, CO is a more favorable electron donor than H₂ as it provides more thermodynamic reducing power for conversion of CO₂ into valuable products. Here, we incorporated highly selective cobalt phthalocyanine catalysts into new planar 2D and porous 3D MES cathodes to produce syngas rather than H₂ and boost the overall bioconversion rates of CO₂ into value-added products (i.e. acetate and ethanol). Both the 2D and 3D systems were able to consistently generate syngas for over 250 h using microbial media electrolyte, demonstrating excellent stability in challenging electrolyte conditions. However, the 2D planar cathodes required a larger potential to maintain similar current densities as the 3D porous cathodes (-1.8 V vs. -1.2 V vs. Ag/AgCl) and showed a slow decline in CO production (0.23 ml/min–0.09 ml/min) and increase in H₂ (0.01 ml/min–0.13 ml/min) production after 250 h. In comparison, the 3D porous electrodes allowed for more stable CO (0.08–0.06 ml/min) and H₂ (0.16–0.06 ml/min) generation that also led to higher maximum acetate (5.1 vs. 3.8 g/L) and ethanol (1.2 vs. 0.9 g/L) titers. Demonstration of these new cathode materials shows significant progress towards more stable and effective MES operations and delivers useful insight on syngas mediated electron transfer and utilization in bioelectrochemical systems.