PEM Electrolysis in a Stirred-Tank Bioreactor Enables Autotrophic Growth of Clostridium ragsdalei with CO₂ and Electrons

Acetogenic bacteria produce CO₂-based chemicals in aqueous media by hydrogenotrophic conversion of CO₂, but CO is the preferred carbon and electron source. Consequently, coupling CO₂ electrolysis with bacterial fermentation within an integrated bio-electrocatalytical system (BES) is promising, if CO₂ reduction catalysts are available for the generation of CO in the complex biotic electrolyte. A standard stirred-tank bioreactor was coupled to a zero-gap PEM electrolysis cell for CO₂ conversion, allowing voltage control and separation of the anode in one single cell. The cathodic CO₂ reduction and the competing hydrogen evolution enabled in-situ feeding of C. ragsdalei with CO and H₂. Proof-of-concept was demonstrated in first batch processes with continuous CO₂ gassing, as autotrophic growth and acetate formation was observed in the stirred BES in a voltage range of −2.4 to −3.0 V. The setup is suitable also for other bioelectrocatalytic reactions. Increased currents and lower overvoltages are however required. Atomically-dispersed M−N−C catalysts show promise, if degradation throughout autoclaving can be omitted. The development of selective and autoclavable catalysts resistant to contamination and electrode design for the complex electrolyte will enable efficient bioelectrocatalytic power-to-X systems based on the introduced BES.