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

Abstract

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.