Hydrogen gas (H2) is an attractive fuel carrier due to its high specific enthalpy; moreover, it is a clean source of energy because in the combustion reaction with oxygen (O2) it produces water as the only byproduct. The microbial electrolysis cell (MEC) is a promising technology for producing H2 from simple or complex organics present in wastewater and solid wastes. Methanogens and non-archaeal methane (CH4)-producing microorganisms (NAMPMs) often grow in the MECs and lead to rapid conversion of produced H2 to CH4. Moreover, non-archaeal methane production (NAMP) catalyzed by nitrogenase of photosynthetic bacteria was always overlooked. Thus, suppression of CH4 production is required to enhance H2 yield and production rate. This review comprehensively addresses the principles and current state-of-the-art technologies for suppressing methanogenesis and NAMP in MECs. Noteworthy, specific strategies aimed at the inhibition of methanogenic enzymes and nitrogenase could be a more direct approach than physical and chemical strategies for repressing the growth of methanogenic archaea. In-depth studies on the multiomics of CH4 metabolism can possibly provide insights into sustainable and efficient approaches for suppressing metabolic pathways of methanogenesis and NAMP. The main objective of this review is to highlight key concepts, directions, and challenges related to boosting H2 generation by suppressing CH4 production in MECs. Finally, perspectives are briefly outlined to guide and advance the future direction of MECs for production of high-purity H2 based on genetic and metabolic engineering and on the interspecific interactions.
Keywords: high-purity hydrogen generation; microbial electrolysis cells; non-archaeal methane production; suppressing methanogenesis; waste treatment technology.