Improving CO2 Electromethanogenesis: The Role of Carbon Dots in Biofilm Development and Extracellular Electron Transfer

Bioelectrochemical approaches for transforming CO₂ into low-carbon extracellular chemicals can be beneficial for storing energy, reducing greenhouse gas emissions, and promoting sustainable practices. Addressing challenges such as low biofilm adhesion and slow electron transfer dynamics within the biofilm–electrode interface is crucial for improving the bioelectroconversion of CO₂ into CH₄. Therefore, this study investigates the technical feasibility of supplying carbon dots (CDs), a porous and highly conductive nanomaterial, to enhance biofilm adhesion behaviors and electron transfer dynamics within biofilm–electrode interactions for maximizing the bioelectroconversion capability of CO₂ to CH₄. With the addition of carbon dots, the methane production rate increased by 35.3% (64.0 ± 12.9 mL·L_reactor^–1·d^–1) and charge transfer resistance decreased by 8.7%. Supplementing with carbon dots improved the metabolic processes of methanogenic microorganisms, resulting in increases of 18.7%, 23.5%, and 19.8% in aromatic proteins, fulvic acids, and DNA content in biofilm, respectively. The 25.6% increase in biomass led to the formation of a more stable and active biofilm structure, improving the adhesion and activity of methane-producing microbes. Remarkably, the abundance of archaea, particularly hydrogenotrophic methanogens, like Methanobacterium, soared to a significant proportion of 43.6%. Carbon dots increase the proportion of the Mtr gene family linked to nanowire synthesis, regulating environmental conditions and promoting the secretion of beneficial metabolites, thereby enhancing microbial biofilm formation and providing a solid foundation for process stability and longevity. The findings of this study endorse the development of sustainable CO₂ upgrading technologies and provide useful insights into microbial metabolism, electron transfer, and biofilm structure.