SPECIAL ISSUE PAPER A new high-energy density hydrogen carrier— carbohydrate—might be better than methanol Yi-Heng Percival Zhang 1,4,5,6, * ,† , Jian-He Xu 2 and Jian-Jiang Zhong 3 1 Biological Systems Engineering Department, Laboratory of Biofuels and Carbohydrates, Virginia Tech, Blacksburg, VA 24061, USA 2 Laboratory of Biocatalysis and Bioprocessing, State Key Laboratory of Bioreactor Engineering, East China University of Science and Technology, Shanghai 200237, China 3 School of Life Sciences & Biotechnology, Key Laboratory of Microbial Metabolism (MOE), Shanghai Jiao-Tong University, Shanghai 200240, China 4 Institute for Critical Technology and Applied Science (ICTAS), Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA 5 DOE Bioenergy Science Center, Oak Ridge, TN 37831, USA 6 Gate Fuels Inc., Blacksburg, VA 24060, USA SUMMARY High-density hydrogen storage in the form of renewable carbohydrate becomes possible because cell-free synthetic enzymatic pathway biotransformation (SyPaB) can 100% selectively convert carbohydrate and water to high-purity hydrogen and carbon dioxide under modest reaction conditions (below water boiling temperature and atmospheric pressure). Gravimetric density of carbohydrate (polysaccharide) is 14.8% H 2 mass, where water can be recycled from polymer electrolyte membrane fuel cells or 8.33% H 2 mass based on the water/carbohydrate slurry; volumetric density of carbohydrate is >100 kg of H 2 /m 3 . Renewable carbohydrate would be more advantageous over methanol according to numerous criteria: substrate cost based on energy content (cost per gigajoule), energy conversion efficiency, catalyst cost and availability, sustainability, safety, toxicity, and applications. Huge potential markets of SyPaB from high-end applications (e.g., biohydrogenation for synthesis of chiral compounds and sugar batteries) to low-end applications (e.g., local satellite hydrogen generation stations, distributed electricity generators, and sugar fuel cell vehicles) would be motivation to solve the remaining obstacles soon. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS biomass; carbohydrate; cell-free synthetic pathway biotransformation (SyPaB); hydrogen carrier; hydrogen production; hydrogen storage; sugar fuel cell vehicle Correspondence *Yi-Heng Percival Zhang, 304 Seitz Hall, Biological Systems Engineering Department, Virginia Tech, Blacksburg, VA 24061, USA. † E-mail: ypzhang@vt.edu Received 8 September 2010; Revised 16 March 2011; Accepted 8 January 2012 1. INTRODUCTION Mobility usually represents civilization level [1–3]. The utilization of liquid fuels along with internal combustion engines (ICEs) has greatly enhanced the mobility of human beings because liquid fuels have high energy storage densities, they can be easily transported and conveniently stored, and ICEs have high power-to-weight ratio (e.g., watt output/gram engine) and low production costs based on cost- per-watt output [3,4]. But concerns of depleting crude oil, soaring prices of crude oil, climate change associated with net carbon dioxide emissions, uneven resource distribution, wealth transfer, national energy security, and air pollution are driving to seek for clean and sustainable alternative transportation fuels [5,6]. The next transportation revolution would mainly occur as a transition from ICEs to the hydrogen/electricity systems [6–8]. Because electricity storage densities in the batteries (e.g., ~0.14 MJ/kg of lead acid battery, ~0.46 MJ/kg of lithium battery) [9,10] are far less than those of available hydrogen means (e.g., 5.7 MJ/kg of 4% H 2 storage container), a majority of future transporta- tion vehicles would be based on the hydrogen/fuel cell/ motor systems [8,11,12]. In addition to energy storage density, the underlying premise of the hydrogen economy is that hydrogen fuel cells have much higher energy effi- ciencies (~50–70%) than internal combustion engines (~20–40%) that are restricted by the second law of thermodynamics. Thus, this transition from heat engines to fuel cells would decrease consumption of primary INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.2897 Copyright © 2012 John Wiley & Sons, Ltd.