Electrode-Assisted Pressurized CO₂ Fermentation for Acetic Acid and Ethanol Production: Enhanced Carbon Fixation, Metabolic Efficiency, and Sustainability in Carbon-Negative Bioprocesses

Abstract

Gas fermentation using homoacetogenic consortia to convert CO 2 into sustainable fuels and chemicals has emerged as a promising biotechnological route toward carbon neutrality. However, a significant challenge is the low gas−liquid mass transfer rates due to the limited solubility of C1 gases. This study investigates CO 2 fermentation enhancement using a high-pressure gas fermentation (HPGF) reactor embedded with electrodes, effectively overcoming CO 2 solubility barriers and addressing sustainability through an innovative approach. CO 2 fermentation with H 2 as the electron donor was conducted in pressurized fermenters (PFs) at varying partial pressures (pCO 2 -2, -3, and -5 bar), while pressured electro-fermentation (PEF) used electrodes to replace H 2 . The pCO 2 -PEF-5 condition achieved the highest acetic acid productivity of 2.8 g/L, followed by pCO 2 -PEF-3 at 2.65 g/L, representing 1.2 and 1.18 times higher yields than the best condition of PFs (pCO 2 -PF-3, 2.1 g/L), respectively. Additionally, PEF systems enhanced solventogenic activity, with ethanol production reaching 1.4 g/L in pCO 2 -PEF-5. The substitution of H 2 with electrodes in CO 2 fermentation improved fixation and conversion rates (pCO 2 -PEF-5: 67 mg/L/h, 77%), demonstrating a viable strategy for enhanced CO 2 conversion. The thermodynamic analysis indicated more spontaneous synthesis of acetic acid and ethanol in PEF systems compared with PF systems. Bioelectrochemical assessments revealed higher charge transfer rates, with a faradaic efficiency of 48% in pCO 2 -PEF-5, further supporting CO 2 conversion. Especially, key genes in the Wood−Ljungdahl pathway (WLP) were upregulated in PEF systems, confirming that electro-fermentation influences metabolic pathways favoring carbon fixation and solvent production. A life cycle assessment (LCA) highlighted a net emission reduction of −7 kg CO 2 equiv in PEF-5 and lower impact across endpoint categories, highlighting the carbon-negative potential of this approach. From a planetary boundary framework perspective, this process operates within the Holocene state by reducing CO 2 emissions, helps in maintaining biosphere integrity, reduces atmospheric CO 2 , and contributes minimally to nitrogen and phosphorus flows. This study signifies the sustainability of the PEF strategy for scaling CO 2 conversion processes. The integration of electro-fermentation not only addresses mass transfer limitations but also enhances carbon fixation efficiency and metabolic productivity.