Boosting electron transfer for high-performance microbial fuel cell with Cu₂O-modified cathodes: Regulated pathway,...

Microbial fuel cell (MFC) with Cu₂O nanoparticle (Cu₂O NP)-modified cathodes (Cu₂O@CA) were constructed to investigate the output voltage and nitrate (NO₃^--N) bioreduction performance during Phases I-Ⅵ. The results demonstrated that the Cu₂O@CA-MFC achieved an output voltage of 339-674 mV and a NO₃^--N transformation rate of 0.022 g/(m³·h)-0.026 g/(m³·h) during Phase Ⅴ, whereas no significant effect on COD degradation was observed over the entire operation period. The synthesized Cu₂O NP exhibited a porous structure, high specific surface area, low electron transfer resistance, and excellent oxygen reduction reaction via a 4-electron (4e^-) reaction path from O₂ to H₂O. Moreover, the Cu₂O NP increased the enrichment of functional microorganisms related to NO₃^--N reduction and electricity generation (e.g., Thauera and Rhodopseudomonas palustris). Phylogenetic Investigation of Communities by Reconstruction software (PICRUSt) prediction analysis revealed that the functional genes combined with electron transfer systems (e.g., MtrABC, hoxU, and ubiA-G) and NO₃^--N bioreduction (e.g., narG, narH, narI, and narB) were upregulated, thereby increasing the activity of electron transport systems and NO₃^--N reductases. Additionally, the downregulation of ATPF1ABCEG and ATPF0ABC together with the increase in O₂ diffusion resistance redirected electron flux towards NO₃^--N bioreduction rather than towards O₂ reduction. On the basis of electrochemical characterization, microbial metabolism, and functional gene analysis, the underlying mechanisms on the output voltage and NO₃^--N reduction efficiency of the MFC were elucidated. These improvements were attributed mainly to the improved electron transfer and utilization efficiency, which reduced the cathodic potential loss.