Synergistic induced charge transfer switch by oxygen vacancy and pyrrolic nitrogen in MnFe₂O₄/g-C₃N₄ heterojunctions for efficient transformation of bicarbonate to acetate in photo-assisted MES

Inorganic carbon (HCO₃^-) was efficiently converted into acetate (204.4 ± 0.5 mM with a coulombic efficiency of 96 ± 3% over 24 days operation) in a photo-assisted microbial electrosynthesis system (MES) using a urea-treated MnFe₂O₄/g-C₃N₄ cathode and the nonphotosynthetic bacteria Serratia marcescens Q1. The remarkable photocatalytic performance of MnFe₂O₄/g-C₃N₄ heterojunction was resulted from the charge transfer mechanism switch (from type II to Z-scheme) induced by the synergistic effect of oxygen vacancies and pyrrolic N after urea treatment. The increased pyrrolic N was conductive to photoinduced electron transfer while the oxygen vacancies provided a higher fraction of surface-active sites for H₂ evolution, which was metabolized in-situ with bicarbonate by S. marcescens Q1 to yield acetate via the Wood–Ljungdahl pathway. This study provides a simple and feasible strategy for switching the photocatalytic charge transfer in a spinel-based heterojunction and offers new insights for ingeniously synthesizing photocatalysts with high CO₂ conversion in MES.