Promoting Multi-Carbon Fatty Acids Production from Microbial Chain Elongation of CO₂/H₂ with Carbonaceous Materials

Conversion of CO2 and H2 into biomethane is a promising technology within the concept of power-to-X. However, the integration of high-value-added products can enhance the economic viability and diversify future biobased production chains. Herein, an integrated biosystem converting CO2/H2 into acetic acid, which is further converted to C4+ fatty acids, was demonstrated with the aim to assess the role of carbonaceous materials (model nanomaterial graphene and cost-effective biochar) and external electron donor (ethanol). With the supplementation of ethanol as the electron donor, carbonaceous materials significantly increased the production and selectivity of butyrate (C4) and caproate (C6) due to enhanced microbial growth and chain elongation efficiency. The highest concentration of butyrate (2512.35 ± 60.05 mg/L) was observed in the graphene-added group, while the highest concentration of caproate (396.74 ± 14.83 mg/L) was observed in the biochar group, suggesting different mechanisms induced by carbonaceous materials. Microbial community analysis revealed that carbonaceous materials primarily enriched members of the genus Clostridium_sensu_stricto_12, which are identified as typical carbon chain elongating microorganisms. Functional prediction revealed that carbonaceous materials increased the relative abundance of functional genes encoding crucial enzymes involved in both the Wood–Ljungdahl pathway and the fatty acid biosynthesis pathway. This study demonstrated an integrated approach for the efficient conversion of CO2 to value-added green chemicals.