High rate CO₂ valorization to organics via CO mediated silica nanoparticle enhanced fermentation

Carbon dioxide (CO₂) valorization to high energy density fuels presents an economically sound approach to directly couple waste mitigation with sustainable energy production, yielding tremendous environmental and societal benefits. Here, a new two-stage hybrid system was developed that pairs highly efficient electrochemical CO₂ reduction (CO₂-R) to carbon monoxide (CO) with acetogenic bacteria for the rapid production of green chemicals. This original design seeks to solve issues in mediator production and utilization evident in related studies. A progressive silver-based gas diffusion electrode with anion exchange membrane sustained current densities of 200 mA/cm² for over 240 h at a time and produced CO at excellent Faradaic efficiencies of over 80% at a cell voltage ca. 3.2 V. The effluent gas from the CO₂ electrolyzer was connected to a bioreactor containing Clostridium ljungdahlii where the blend of CO/CO₂ was converted into a mixture of mainly acetate and ethanol. This demonstrates the first time this organism has been tested in a two-stage hybrid CO₂-R system. Competitive acetate and ethanol production rates of 17.87 ± 7.1 and 3.23 ± 1.4 mg/L/h were achieved under autotrophic conditions. In addition, mercapto-modified silica nanoparticles were developed to increase gas mass transfer and resulted in 217% and 224% increases in acetate and ethanol production rates, respectively. This system presents a viable approach to produce sustainable fuels and chemicals in a process that exploits commercially scalable C1 electron carries.