States and challenges for high-value biohythane production from waste biomass by dark fermentation technology

► Hythane (H2 + CH4) has attracted growing attention as clean vehicle fuel. ► We review the progress of bioH2 and bioCH4 fermentation in the past ten years. ► We discuss the feasibility of biohythane output from bioH2 and bioCH4. ► This review is intended to provide a state-of-the-art insight to biohythane.