Tandem Electrolyzer–Chemostats for Synthesizing Bioplastics from CO ₂ and H ₂ O

Harnessing renewable energy to convert anthropogenic CO2 to valuable products is central to establishing a sustainable carbon cycle. Here, we present a continuous electrobiocatalytic platform for converting CO2 to Bioplastic by using an external water-splitting electrolyzer integrated with a two-stage cascade of continuous stirred-tank bioreactors (CSTBs) arranged in tandem, a system-level architecture that has not been previously reported. A proton exchange membrane (PEM) electrolyzer produces H2 for the acetogenic bacterium Sporomusa ovata, which fixes CO2 into acetate in CSTB 1, achieving a steady-state productivity of 293 ± 17 mg L–1 h–1. The acetate is continuously and directly supplied to CSTB 2 and subsequently metabolized by the facultative chemolithoautotroph Cupriavidus necator for the biosynthesis of poly(3-hydroxybutyrate) (PHB) biopolymers. Under steady-state conditions, the electrolyzer/CSTB 1/CSTB 2 system achieves a PHB productivity of 2.76 ± 0.24 mg L–1 h–1, which provides a quantitative benchmark for a fully continuous, electrolyzer-driven CO2-to-PHB process. This work presents an electromicrobial approach integrating environmental remediation with chemical syntheses from CO2 and H2O.