Microbial Electrosynthesis for Sustainable Polyhydroxybutyrate Production from CO2 Using Bismuth Nanoparticles

Microbial electrochemical technologies (MET) are a promising approach that integrates electrochemical and microbial processes to convert CO₂ into value-added chemicals. Herein, Cupriavidus necator is utilized to produce polyhydroxybutyrate (PHB) using electrochemically synthesized formate as the sole carbon source. Formate is generated via CO₂ reduction using a Bi-based electrode in a physiological electrolyte, achieving concentrations of ≈40 mM with a production rate of 0.05 mmol h⁻¹ cm⁻² and the highest faradaic efficiency achieved up to 50.81%. Two MET configurations are evaluated: an integrated system, where CO₂ reduction and fermentation occur in a single reactor, and a drop-in system, where electrochemically produced formate is collected and later is used for fermentation. The drop-in system achieves the highest PHB production, reaching ≈340 mg L⁻¹ within 24 h. By directly utilizing the formate-containing electrolyte as a fermentation medium, this approach simplifies process integration, reduces purification steps, and improves compatibility between electrochemical and microbial systems. These findings highlight the potential of MET as a scalable platform for sustainable biopolymer production from CO₂.