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

Gas fermentation using homoacetogenic consortia to convert CO2 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 CO2 fermentation enhancement using a high-pressure gas fermentation (HPGF) reactor embedded with electrodes, effectively overcoming CO2 solubility barriers and addressing sustainability through an innovative approach. CO2 fermentation with H2 as the electron donor was conducted in pressurized fermenters (PFs) at varying partial pressures (pCO2-2, -3, and -5 bar), while pressured electro-fermentation (PEF) used electrodes to replace H2. The pCO2-PEF-5 condition achieved the highest acetic acid productivity of 2.8 g/L, followed by pCO2-PEF-3 at 2.65 g/L, representing 1.2 and 1.18 times higher yields than the best condition of PFs (pCO2-PF-3, 2.1 g/L), respectively. Additionally, PEF systems enhanced solventogenic activity, with ethanol production reaching 1.4 g/L in pCO2-PEF-5. The substitution of H2 with electrodes in CO2 fermentation improved fixation and conversion rates (pCO2-PEF-5: 67 mg/L/h, 77%), demonstrating a viable strategy for enhanced CO2 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 pCO2-PEF-5, further supporting CO2 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 CO2 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 CO2 emissions, helps in maintaining biosphere integrity, reduces atmospheric CO2, and contributes minimally to nitrogen and phosphorus flows. This study signifies the sustainability of the PEF strategy for scaling CO2 conversion processes. The integration of electro-fermentation not only addresses mass transfer limitations but also enhances carbon fixation efficiency and metabolic productivity.