Trifunctional endogenous mediator orchestrates efficient biocathodes via synergistic electron transfer and enzyme catalytic site modulation

Microbial catalysts offer compelling advantages for oxygen reduction reaction (ORR) in microbial fuel cell (MFC) cathodes, including reduced costs and extended operational lifespans. However, their practical application remains limited by insufficient intrinsic activity at catalytic protein sites and restricted charge accessibility, both of which constrain ORR kinetics. Here, we report the development of an efficient trifunctional bioendogenous system based on menaquinone-7 (MK-7), enriched from Bacillus subtilis natto (natto digester strain (ND)) through a straightforward fermentation strategy. The engineered MK-7 simultaneously performs three critical functions: (i) facilitating mediated electron transfer between bacteria and electrodes, (ii) regulating the in-situ formation of size-controlled conductive polydopamine nanostructures that enhance direct electron transfer pathways, and (iii) modulating the electronic structure of cytochrome c (Cyt c) to activate its catalytic center and optimize O₂ adsorption capacity. Through these synergistic effects, our engineered nano-hybrid ND-FM@sPDA (FM is fermentation and sPDA is size-controlled conductive polydopamine) achieves an oxygen reduction current density of 3.83 mA·cm⁻², representing a 1.54-fold enhancement over pristine ND (2.48 mA·cm⁻²). MFCs constructed with the ND-FM@sPDA biocathode deliver a peak power density of 412 μW·cm⁻², surpassing previously reported microbial catalysts for similar applications. This work elucidates novel regulatory mechanisms for optimizing biocatalysts at the molecular level and provides critical insights for advancing sustainable bioelectrocatalytic technologies with enhanced performance.