High-Yield Production of Biohydrogen from Carbohydrates and Water Based on In Vitro Synthetic (Enzymatic) Pathways

Abstract

Distributed production of green and low-cost hydrogen from renewable energy sources is necessary to develop the hydrogen economy. Carbohydrates, such as cellulose, hemicellulose, starch, sucrose, glucose, and xylose, are abundant renewable bioresources and can provide the source of hydrogen. In this chapter, in vitro synthetic (enzymatic) pathways that overcome the limiting yields of hydrogen-producing microorganisms are discussed. These in vitro synthetic pathways produce hydrogen with theoretical yields from polymeric and monomeric hexoses or xylose with water of 2 mol of hydrogen per carbon molecule of carbohydrate. In the past years, hydrogen production rate of in vitro synthetic enzymatic pathways has been improved to 150 mmol/L/h by 750-fold through systematic optimization. All of the thermostable enzymes used in the pathways have been recombinantly produced in E. coli, and some of them are immobilized for enhanced stability and simple recycling. Redox enzymes are being engineered to work on low-cost and highly stable biomimetic coenzymes. It is expected that low-cost green hydrogen can be produced at $2.00/kg hydrogen in small-sized atmospheric pressure bioreactors in the future.