Renewable hydrogen production by dark-fermentation: Current status, challenges and perspectives.

Global urbanization has resulted in amplified energy and material consumption with simultaneous waste generation. Current energy demand is mostly fulfilled by finite fossil reserves, which has critical impact on the environment and thus, there is a need for carbon-neutral energy. In this view, biohydrogen (bio-H) is considered suitable due to its potential as a green and dependable carbon-neutral energy source in the emerging 'Hydrogen Economy'. Bio-H production by dark fermentation of biowaste/biomass/wastewater is gaining significant attention. However, bio-Hproduction still holds critical challenges towards scale-up with reference to process limitations and economic viabilities. This review illustrates the status of dark-fermentation process in the context of process sustainability and achieving commercial success. The review also provides an insight on various process integrations for maximum resource recovery including closed loop biorefinery approach towards the accomplishment of carbon neutral H production.