Recent advances on the use of promoters in biochemical hydrogen production: A comprehensive review

•Different categories of promoters are devoted to BioH2 production.•Fe-based additives are the most utilized promoters for BioH2 production.•The promoter can interact with the substrate, bacterial cells, or enzymes.•Promoters can enhance hydrogen production through different mechanisms. Biochemical routes for hydrogen generation represent great opportunities to generate hydrogen from renewable and economical bio-resources like biomass and wastes. In this context, several processes were devoted for this purpose such as dark fermentation, photo-fermentation, and direct/indirect photolysis. Enhancement of the biohydrogen production yield is a key challenge to boost the economic feasibility and shifting it towards large-scale application. In this regard, the use of promoters represents a promising approach for the enhancement of the biohydrogen generation in terms of both the hydrogen production rate (HPR) and the hydrogen yield (HY). This review provides a comprehensive survey on the various types of promoters used for different biochemical hydrogen production processes. The promoters were classified based on their chemical nature into different categories. In addition, they were assessed in terms of their type, dose effect and their impact on the hydrogen yield and the hydrogen production rate. The possible mechanisms through which different promoters enhance the hydrogen production were discussed. Finally, the review highlights the possible approaches to enhance the biohydrogen production in terms of promoters’ types and characteristics, which can help to grasp the limitations and bottlenecks of biohydrogen production and enhance its economic feasibility.