Mechanisms of Direct Electron Transfer Governed by Redox‐Active Conductive Carrier with Superior Wettability in Anaerobic Biofilms

Biofilms cultivated on conductive carriers emerge as promising systems for enhancing anaerobic wastewater treatment, while the underlying electron transfer mechanisms remain insufficiently elucidated. Herein, a redox-active conductive carrier composed of carbon felt functionalized with tannic acid-modified iron-biochar (TA-FeBC) with superior wettability is engineered to regulate direct electron transfer (DET) at the anaerobic biofilm-carrier interface. It possesses a redox potential significantly lower than CO₂/CH₄ and an exceptional electron-donating capacity of 16.6 µmol e^-1 g^-1, collectively creating a strong thermodynamic driving force for DET-driven methanogenesis. In contrast to conventional direct interspecies electron transfer (DIET), the redox-active conductive TA-FeBC carrier may act as an exogenous electron donor, channeling electrons directly into anaerobic biofilm through cytochrome c (CytC)-mediated pathway. Notably, the novel mechanism reduces the electron transfer resistance of anaerobic biofilm by over 50-fold compared to anaerobic suspended sludge. The higher flat-band potentials of anaerobic biofilm (-0.131 V) compared with the redox-active conductive TA-FeBC carrier (-0.301 V) favors a steep redox gradient, enabling Methanobacterium to directly harvest electrons from the carrier, as evidenced by a stable 110 µA cm^-2 cathodic current. This study provides the first integrated experimental evidence for CytC-mediated DET governed by an engineered conductive carrier, offering new avenues for rational design of redox-active carriers in bioelectrochemical and anaerobic systems.