Biohybrid CO₂ electrolysis for the direct synthesis of polyesters from CO₂

Converting anthropogenic CO₂ to value-added products using renewable energy has received much attention to achieve a sustainable carbon cycle. CO₂ electrolysis has been extensively investigated, but the products have been limited to some C_1-3 products. Here, we report the integration of CO₂ electrolysis with microbial fermentation to directly produce poly-3-hydroxybutyrate (PHB), a microbial polyester, from gaseous CO₂ on a gram scale. This biohybrid system comprises electrochemical conversion of CO₂ 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 CO₂ 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 cm² 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 CO₂.