Dark fermentation is considered a promising strategy for clean energy production, where hydrogen can directly be produced from organic residues in the role of functional microbes. However, the low efficiency and poor stability of dark fermentation systems inhibit them from moving to industrial -scale applications. Biochar, the carbonaceous additive, has demonstrated great potential in improving the fermentation performance of the systems resulting from its outstanding physicochemical properties. This review critically summarizes the recent advances in adding biochar to improve the performance of dark fermentation systems. The production methods and physicochemical properties of biochar, as well as their influencing factors during pyrolysis, are first discussed. Then, the promoting effect of biochar on hydrogen production and its underlying mechanisms are explained in detail. Finally, the concluding remarks and perspectives are proposed. This review will deepen the understanding on the specific mechanisms by which biochar enhances hydrogen production in dark fermentation systems. It can also provide a reference for future study to build better biochar-added systems with higher efficiency and more stable environment.
