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 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.