Among the various renewable energy sources, biohydrogen is gaining a lot of traction as it has very high efficiency of conversion to usable power with less pollutant generation. The various technologies available for the production of biohydrogen from lignocellulosic biomass such as direct biophotolysis, indirect biophotolysis, photo, and dark fermentations have some drawbacks (e.g., low yield and slower production rate, etc.), which limits their practical application. Among these, metabolic engineering is presently the most promising for the production of biohydrogen as it overcomes most of the limitations in other technologies. Microbial electrolysis is another recent technology that is progressing very rapidly. However, it is the dark fermentation approach, followed by photo fermentation, which seem closer to commercialization. Biohydrogen production from lignocellulosic biomass is particularly suitable for relatively small and decentralized systems and it can be considered as an important sustainable and renewable energy source. The comprehensive life cycle assessment (LCA) of biohydrogen production from lignocellulosic biomass and its comparison with other biofuels can be a tool for policy decisions. In this paper, we discuss the various possible approaches for producing biohydrogen from lignocellulosic biomass which is an globally available abundant resource. The main technological challenges are discussed in detail, followed by potential solutions.

Changes in the environment due to multiple factors, such as combustion of fossil fuels, heating, transportation, deforestation, etc., have led to more greenhouse gases in the atmosphere, which eventually led to a rise in global temperatures. Carbon dioxide (CO₂) is the major factor for the rapid rise in global temperature. One of the most encouraging technological advances to address global warming is to transform CO₂ into value-added commodities that offer a win–win strategy. In this regard, intensive research has been pursued around the world for development of feasible systems in product recovery or product synthesis from CO₂-rich industrial emissions. We envision that the biological CO₂ reduction or conversion process can be beneficial for developing carbon-neutral technologies. The integration of CO₂-emitting industrial technologies with CO₂-converting biological systems can be helpful in achieving sustainable value-added products with no or minimal loss of energy and materials that are assuring for improved economics. The CO₂-converting bioprocesses can be directly integrated with the processes emitting a high amount of CO₂. This symbiotic integration can make the whole process carbon neutral. Herein, this review highlights an insight on research activities of biological CO₂ mitigation using photo catalysts (algae and photo bacteria), an anaerobic biocatalyst (bacteria), gas fermentation, and an enzymatic catalyst. Perspectives and challenges of these technologies are discussed.