Defect-engineered Fe-Ti anodes with oxygen-vacancy-associated spin activity promote electroactive biofilm formation and interfacial electron transfer

Extracellular electron transfer (EET) across the biofilm-anode interface governs current generation and electron recovery in microbial fuel cells (MFCs). Although engineering the electronic structure of bioanodes offers a promising strategy for accelerating EET, the contribution of electrode spin states remains poorly understood. Here, oxygen-vacancy-rich, spin-polarized Fe-Ti anodes (SP-Fe-Ti) and vacancy-suppressed, spin-depressed controls (SD-Fe-Ti) were prepared and then compared to determine how their distinct electronic structures influenced electroactive biofilm development and interfacial electron transfer during 73 d of MFC operation. Our results showed that SP-Fe-Ti increased the full-period mean current density and the stable-period mean current density. The cumulative charge density of SP-Fe-Ti increased from 2780.7 ± 600.0 to 5163.2 ± 459.2 kC m^-2, 1.86 to 4.5-fold higher than that of SD-Fe-Ti. Voltammetric and impedance analyses revealed SP-Fe-Ti had a stronger turnover, non-turnover, and EPS-associated redox responses for SD-Fe-Ti, together with a lower fitted interfacial resistance. The spin-active interface of SP-Fe-Ti also promoted biofilm development, increasing the attached protein content by 3.30-fold, selectively enriching bound extracellular polymeric substances (EPS), and decreasing the membrane-compromised cell fraction from 63.29 ± 5.26 % to 26.49 ± 1.67 %. More importantly, SP-Fe-Ti-derived EPS exhibited a normal apparent kinetic isotope effect, KIE(H/D), of 2.94, whereas SD-Fe-Ti-derived EPS showed a pronounced inverse response of 0.050, indicating potentially distinct protonation- and hydrogen-bond-dependent redox environments. Collectively, these findings highlight spin‑electronic engineering as a promising avenue for tuning microbe‑electrode interactions and improving microbial bioelectrochemical energy conversion.