State-of-the-art of biochar amended dark fermentative hydrogen production: A sustainable coupling of decarbonization pathways towards low carbon future

Dark fermentation (DF) is well regarded and appraised as an efficient and green route for producing hydrogen (H2). Low yield of hydrogen is an inherent issue of the DF process restricting its further progress towards commercial margins. Biochar, the carbonaceous solid end product of biomass pyrolysis process is a potential additive to amend the DF processes for intensification of hydrogen production. In this article, the biochar amended DF systems have been holistically reviewed and the current state-of-the-art of the integrated technology is presented. Different physical and chemical properties of the biochar namely, surface area and porosity, elemental and compound composition, surface chemical/functional groups, pH, micronutrient content and electroconductivity majorly influence the hydrogen production in DF systems. Amendment of different biochar have remarkably facilitated 20.72–328.51% enhancement of the hydrogen yield (HY) and 26.05–575.41% enhancement of hydrogen production rate (HPR), validating the immense potential of the biochar for amelioration of hydrogenesis performance of DF systems. Formation of stable biofilm of hydrogen producing bacteria (HPB), regulation of the system pH, facilitation of better electron transfer and stimulation of the hydrogenase enzymes are the main factors responsible for enhancement of hydrogenesis by biochar. Doping of biochar by metal/non-metal substance and combining with inorganic nanoparticles also exhibited promising enhancement of HY from DF. Clear understanding of the impacts of biochar on the metabolic pathways of hydrogen production, manifestation of underlying enhancement mechanisms and knowledge of existing barriers is necessary to fully exploit the benefits of the biochar amendment strategy in DF, which are presented in this article. Based on the present state-of-the-art, a closed-loop operation ensuring enhanced resource utilization, carbon circularity and low carbon footprint has been proposed for future development towards large scale clean hydrogen production.