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%H2mass, where water can be recycled from polymer electrolyte membrane fuel cells or 8.33%H2mass based on the water/carbohydrate slurry; volumetric density of carbohydrate is >100kg of H2/m3. 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 (c) 2012 John Wiley & Sons, Ltd.