Review of over two decades of research on dark and photo fermentation for biohydrogen production - A combination of traditional, systematic, and bibliometric approaches

Biohydrogen represents a highly feasible and environmentally sustainable fuel alternative for the world's growing energy needs. However, technological advancements are still required for large-scale hydrogen utilization, particularly in determining the most advantageous technological path for recovering affordable and renewable hydrogen. Researchers have conducted numerous studies to determine the optimal technology for producing large-scale biohydrogen. However, no comprehensive systematic and bibliometric review presents an overview of the various production modes, i.e., the photo and dark fermentation techniques. This paper reviews recent research on biohydrogen production using dark and photofermentation modes, spanning over two decades, using Scopus data. The review highlights the growing push to integrate hydrogen production into larger bioenergy systems, focusing on improving the core processes and removing obstacles. Techniques like codigestion and two-stage anaerobic digestion can increase production yields. The review also highlights the possibility of combining waste treatment with energy production to address environmental issues. Incorporating dark- and photo-fermentation processes in commercial hydrogen production shows greater potential for commercial use, potentially increasing yield. The study concluded by identifying challenges for the various modes of production and ways to overcome them. It also provided pertinent information on potential future research directions.