Efficient H₂ production in a ZnFe₂O₄/g-C₃N₄ photo-cathode single-chamber microbial electrolysis cell

Photo-assisted single-chamber microbial electrolysis cells (MECs) incorporating semiconductor cathodes are attractively promising for exclusive hydrogen without CH₄ and CO₂. However, the unsustainable, high cost, and unstable metal catalysts on the cathodes along with the intricacies behind the interplay of circuital current, light illumination, and bacterial communities on both electrodes are poorly understood. Herein, photo-assisted single-chamber MECs incorporating ZnFe₂O₄/g-C₃N₄ cathodes are demonstrated to achieve efficient production of exclusive hydrogen (0.11 ± 0.01 m³/m²/day; 1.70 ± 0.04 m³/m³/day) with a solar-to-hydrogen conversion efficiency of 4.08 ± 0.17% and an energy efficiency relative to electrical input of 233 ± 5%. The ZnFe₂O₄/g-C₃N₄ structured cathodes exhibited appreciable higher photocurrents than the controls (g-C₃N₄: 4.3-fold; ZnFe₂O₄: 3.3-fold), and negligible leaking of Fe and Zn after the 4th-cycle operation. Circuital current and light illumination were proven to varying degree shape both electrodes for building up functional bacterial communities with metabolic regulation at the prolonged operation of 12 batch cycles. Energy metabolism and carbohydrate metabolism along with membrane transport, signal transduction, and cell motility based on PICRUSt functional prediction further confirmed the photo-assisted single-chamber MECs for efficient hydrogen production. This study provided a sustainable, cost-effective, and efficient approach for achieving high rates of exclusive hydrogen production and offered new insights for ingenious interplay of circuital current, light illumination, and bacterial communities for efficient hydrogen production in the photo-assisted single-chamber MECs. KEY POINTS: • ZnFe₂O₄/g-C₃N₄ cathodes of single-chamber MECs achieve efficient H₂ production. • Light irradiation and circuit current shape bacterial communities on both electrodes. • Circuital current contributes to less leaking of Fe and Zn, and thus system stability.