Hydrogen from wastewater through photosynthetic bacteria-based electrolysis

Hydrogen plays a pivotal role to limit global warming to 1.5 degrees C, offering solutions for sectors such as energy and heavy industry. However, most of the hydrogen production currently depends on fossil fuels, while green hydrogen production faces economic constraints. Bioelectrochemical systems, such as microbial electrolysis cells, offer an alternative route by coupling wastewater treatment with sustainable hydrogen generation. This review critically examines recent advances in bacteria-based cathodes, with a specific focus on purple non-sulfur bacteria as photoelectroactive biocatalysts. Chemical and physical principles governing photo-assisted microbial electrolysis cells are elucidated, including charge transfer mechanisms, energy recovery pathways, and the role of light-driven metabolism in cathodic hydrogen evolution. The performance of purple non-sulfur bacteria-based systems is analyzed under different electrode polarization and illumination conditions, and critically compared with current hydrogen production technologies. Despite demonstrating hydrogen production costs in the range of approximately 1.1-4.5 $ kg-1, photosynthetic bacteria-based electrolysis systems remain at low technology readiness levels (TRL 2-3), reflecting the gap between laboratory validation and industrial deployment. In addition, the most representative wastewater substrates investigated to date are reviewed with respect to process efficiency and scalability. Finally, a future-oriented roadmap is outlined, highlighting key challenges for real-world implementation and the potential of digital tools, artificial intelligence, and digital twin approaches to optimize system control and accelerate the integration of bacteria electrolysis into energy-positive wastewater treatment plants.