Bio-hydrogen production (BHP) produced from renewable bio-resources is an attractive route for green energy production, due to its compelling advantages of relative high efficiency, cost-effectiveness, and lower ecological impact. This study reviewed different BHP pathways, and the most important enzymes involved in these pathways, to identify technological gaps and effective approaches for process intensification in industrial applications. Among the various approaches reviewed in this study, a particular focus was set on the latest methods of chemicals/metal addition for improving hydrogen generation during dark fermentation (DF) processes; the up-to-date findings of different chemicals/metal addition methods have been quantitatively evaluated and thoroughly compared in this paper. A new efficiency evaluation criterion is also proposed, allowing different BHP processes to be compared with greater simplicity and validity.

In this paper, we aim to focus on the utilization of carbon dioxide (CO₂) as a feedstock for synthetic applications in chemical and fuel industries, through catalytic carboxylation and reduction reactions. Thermal catalytic conversion as well as non-thermal plasma assisted and photocatalytic conversion CO₂ into fuels such as methane, methanol, and long-chain hydrocarbons using different catalyst especially nano catalyst are also thoroughly compared and reviewed in this paper. Among currently available technical routes, the thermal catalytic conversion of CO₂ via the Fischer-Tropsch (FT) synthesis using nanoscale catalyst offers the most feasible, practical and mature alternative for industrial-scale applications on the short term basis, converting gigantic quantity of CO₂ captured from power plants into hydrocarbons.