Electrode-Assisted\nPressurized CO<sub>2</sub> Fermentation\nfor Acetic Acid and Ethanol Production: Enhanced Carbon Fixation,\nMetabolic Efficiency, and Sustainability in Carbon-Negative Bioprocesses

Gas fermentation using homoacetogenic consortia to convert\nCO<sub>2</sub> into sustainable fuels and chemicals has emerged as\na promising\nbiotechnological route toward carbon neutrality. However, a significant\nchallenge is the low gas–liquid mass transfer rates due to\nthe limited solubility of C1 gases. This study investigates CO<sub>2</sub> fermentation enhancement using a high-pressure gas fermentation\n(HPGF) reactor embedded with electrodes, effectively overcoming CO<sub>2</sub> solubility barriers and addressing sustainability through\nan innovative approach. CO<sub>2</sub> fermentation with H<sub>2</sub> as the electron donor was conducted in pressurized fermenters (PFs)\nat varying partial pressures (pCO<sub>2</sub>-2, -3, and -5 bar),\nwhile pressured electro-fermentation (PEF) used electrodes to replace\nH<sub>2</sub>. The pCO<sub>2</sub>-PEF-5 condition achieved the highest\nacetic acid productivity of 2.8 g/L, followed by pCO<sub>2</sub>-PEF-3\nat 2.65 g/L, representing 1.2 and 1.18 times higher yields than the\nbest condition of PFs (pCO<sub>2</sub>-PF-3, 2.1 g/L), respectively.\nAdditionally, PEF systems enhanced solventogenic activity, with ethanol\nproduction reaching 1.4 g/L in pCO<sub>2</sub>-PEF-5. The substitution\nof H<sub>2</sub> with electrodes in CO<sub>2</sub> fermentation improved\nfixation and conversion rates (pCO<sub>2</sub>-PEF-5: 67 mg/L/h, 77%),\ndemonstrating a viable strategy for enhanced CO<sub>2</sub> conversion.\nThe thermodynamic analysis indicated more spontaneous synthesis of\nacetic acid and ethanol in PEF systems compared with PF systems. Bioelectrochemical\nassessments revealed higher charge transfer rates, with a faradaic\nefficiency of 48% in pCO<sub>2</sub>-PEF-5, further supporting CO<sub>2</sub> conversion. Especially, key genes in the Wood–Ljungdahl\npathway (WLP) were upregulated in PEF systems, confirming that electro-fermentation\ninfluences metabolic pathways favoring carbon fixation and solvent\nproduction. A life cycle assessment (LCA) highlighted a net emission\nreduction of −7 kg CO<sub>2</sub> equiv in PEF-5 and lower\nimpact across endpoint categories, highlighting the carbon-negative\npotential of this approach. From a planetary boundary framework perspective,\nthis process operates within the Holocene state by reducing CO<sub>2</sub> emissions, helps in maintaining biosphere integrity, reduces\natmospheric CO<sub>2</sub>, and contributes minimally to nitrogen\nand phosphorus flows. This study signifies the sustainability of the\nPEF strategy for scaling CO<sub>2</sub> conversion processes. The\nintegration of electro-fermentation not only addresses mass transfer\nlimitations but also enhances carbon fixation efficiency and metabolic\nproductivity.