Biohybrid CO 2 electrolysis for the direct synthesis of polyesters from CO 2 .

Converting anthropogenic CO 2 to value-added products using renewable energy has received much attention to achieve a sustainable carbon cycle. CO 2 electrolysis has been extensively investigated, but the products have been limited to some C 1-3 products. Here, we report the integration of CO 2 electrolysis with microbial fermentation to directly produce poly-3-hydroxybutyrate (PHB), a microbial polyester, from gaseous CO 2 on a gram scale. This biohybrid system comprises electrochemical conversion of CO 2 to formate on Sn catalysts deposited on a gas diffusion electrode (GDE) and subsequent conversion of formate to PHB by Cupriavidus necator cells in a fermenter. The electrolyzer and the electrolyte solution were optimized for this biohybrid system. In particular, the electrolyte solution containing formate was continuously circulated through both the CO 2 electrolyzer and the fermenter, resulting in the efficient accumulation of PHB in C. necator cells, reaching a PHB content of 83% of dry cell weight and producing 1.38 g PHB using 4 cm 2 Sn GDE. This biohybrid system was further modified to enable continuous PHB production operated at a steady state by adding fresh cells and removing PHB. The strategies employed for developing this biohybrid system will be useful for establishing other biohybrid systems producing chemicals and materials directly from gaseous CO 2 .