Energies 2022, 15, 7783. https://doi.org/10.3390/en15207783 www.mdpi.com/journal/energies Review Biohydrogen—A Green Fuel for Sustainable Energy Solutions Fariha Kanwal and Angel A. J. Torriero * School of Life and Environmental Sciences, Faculty of Science Engineering & Built Environment, Deakin University, Burwood, VIC 3125, Australia * Correspondence: angel.torriero@deakin.edu.au; Tel.: +61-3-9244-6897 Abstract: Energy plays a crucial role in the sustainable development of modern nations. Today, hydrogen is considered the most promising alternative fuel as it can be generated from clean and green sources. Moreover, it is an efficient energy carrier because hydrogen burning only generates water as a byproduct. Currently, it is generated from natural gas. However, it can be produced using other methods, i.e., physicochemical, thermal, and biological. The biological method is considered more environmentally friendly and pollution free. This paper aims to provide an updated review of biohydrogen production via photofermentation, dark fermentation, and microbial electrolysis cells using different waste materials as feedstocks. Besides, the role of nanotechnology in enhancing biohydrogen production is examined. Under anaerobic conditions, hydrogen is produced during the conversion of organic substrate into organic acids using fermentative bacteria and during the conversion of organic acids into hydrogen and carbon dioxide using photofermentative bacteria. Different factors that enhance the biohydrogen production of these organisms, either combined or sequentially, using dark and photofermentation processes, are examined, and the effect of each factor on biohydrogen production efficiency is reported. A comparison of hydrogen production efficiency between dark fermentation, photofermentation, and two-stage processes is also presented. Keywords: energy; photofermentation; dark fermentation; microorganisms; biohydrogen; microbial electrolysis cell 1. Introduction 1.1. Global Energy Scenario and Global Warming All the social, physical, and economic activities of human life are sustained by energy. The continual energy supply needed for increasing global demand creates a substantial challenge for our societies. According to the IEO reference case [1], the world energy need is expected to rise by 50% from 2018 to 2050. This energy requirement has been satisfied primarily by exploiting massive fossil fuels [1]. Carbon-rich energy carriers (fossil fuels) are produced in two steps. The first step consists of photosynthesis, while the second consists of decomposing organic matter, which has been compacted for millions of years under high pressure and temperature. The ability of fossil fuels to act as efficient energy carriers and their easy transferability into different types of energy has made them the motor of the industrial revolution [2]. However, the other side of the picture shows that burning these fossil fuels creates a major drawback in the form of greenhouse gas emissions, contributing to global warming and climate change. Global warming is the central issue of today’s world because it continuously raises the Earth’s temperature and drastically affects agriculture and food security [3,4]. The developing regions of the world are principally affected. The expected results may include social and political uncertainty, mass migration, and military conflicts. Sea level rise and ocean acidification are the other significant problems [5,6]. In addition, Citation: Kanwal, F.; Torriero, A.A.J. Biohydrogen—A Green Fuel for Sustainable Energy Solutions. Energies 2022, 15, 7783. https:// doi.org/10.3390/en15207783 Academic Editors: Konstantina Tsigkou and Panagiota Tsafrakidou Received: 27 September 2022 Accepted: 18 October 2022 Published: 20 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/license s/by/4.0/).