Existing and emerging technologies in biohydrogen production: mechanisms and influential factors for sustainable energy transition.

Biological hydrogen (Bio-H₂) production is increasingly recognized as a key pathway for sustainable energy transition and circular resource utilization. This review presents a comprehensive and comparative synthesis of the three principal biological hydrogen production routes: dark fermentation (DF), photo-driven processes (biophotolysis and photo-fermentation), and bioelectrochemical systems (BESs), including microbial electrolysis and MFC-integrated configurations. The analysis systematically evaluates metabolic pathways, reactor configurations, operational parameters, and performance indicators to establish a unified framework for assessing technological progress and scalability. Among the reviewed pathways, DF emerges as the most technologically mature and operationally robust approach due to its high production rates, process simplicity, and compatibility with diverse organic wastes, with reported yields reaching up to 3.9 mol H2/mol glucose. Photo-driven systems offer higher theoretical conversion efficiencies and potential for direct solar-to-hydrogen integration, but their practical implementation remains constrained by light utilization, oxygen sensitivity, and process control complexity. Bioelectrochemical technologies provide improved electron recovery and enhanced process controllability, enabling higher resource recovery efficiencies, although their large-scale deployment is still limited by material costs and system optimization challenges. The review further identifies key performance determinants, including substrate characteristics, microbial interactions, environmental conditions, and reactor design, and highlights emerging strategies such as metabolic engineering, hybrid system integration, and process intensification. By providing a comparative performance perspective and identifying critical research gaps, this work offers strategic guidance for advancing biohydrogen technologies toward economically viable and scalable real-world applications.