Anthraquinone disulfonate as a stable redox mediator for efficient air-cathodes at neutral pH in dual-chamber microbial fuel cells

Microbial Fuel Cells (MFCs) are commonly developed as organic-matter oxidizing bioanodes with abiotic air cathodes. However, O₂ reduction requires active aeration and/or the use of expensive catalysts using noble metals. In this study, 2,7-anthraquinone disulfonate (2,7-AQDS), an organic redox mediator commonly used in aqueous redox flow battery systems (AORFBs), served as a redox-stable intermediate for oxygen reduction. Dual-chamber MFC pilots were developed with 2,7-AQDS in the catholyte under both anoxic and aerobic conditions and compared to pilots with ferricyanide catholytes. In both conditions, cyclic voltammetry studies confirmed similar and efficient electroactivity despite the proximity AQDS formal redox potential to that of acetate oxidation. Mediated air-cathodes achieved open-circuit voltage (OCV) of 510 mV and current densities of 140 μA/cm², nearly double those of air-only cathodes (72 μA/cm²), while delivering a 33% higher power density (12 mW/m² vs. 8 mW/m²). Passive catholyte aeration enabled continuous reoxidation of reduced 2,7-AQDS at 8.8 × 10^-8 mol/s, exceeding the AQDS reduction rate by the bioanode (2.5 × 10^-10 mol/s), thus ensuring effective self-regeneration and stable AQDS concentration. These results demonstrate that AQDS coupled with passive oxygen supply sustains biofilm activity with enhances current and power, and allow long-term / low-maintenance MFC operation and organic-matter oxidation.