The hydrogen economy presents an appealing energy future but its implementation must solve numerous problems ranging from low-cost sustainable production, high-density storage, costly infrastructure, to eliminating safety concern. The use of renewable carbohydrate as a high-density hydrogen carrier and energy source for hydrogen production is possible due to emerging cell-free synthetic biology technology—cell-free synthetic pathway biotransformation (SyPaB). Assembly of numerous enzymes and co-enzymes <em>in vitro</em> can create complicated set of biological reactions or pathways that microorganisms or catalysts cannot complete, for example, C<sub>6</sub>H<sub>10</sub>O<sub>5</sub> (aq) + 7 H<sub>2</sub>O (l) à 12 H<sub>2</sub> (g) + 6 CO<sub>2</sub> (g) (PLoS One 2007, 2:e456). Thanks to 100% selectivity of enzymes, modest reaction conditions, and high-purity of generated hydrogen, carbohydrate is a promising hydrogen carrier for end users. Gravimetric density of carbohydrate is 14.8 H<sub>2</sub> mass% if water can be recycled from proton exchange membrane fuel cells or 8.33% H<sub>2</sub> mass% without water recycling. Renewable carbohydrate can be isolated from plant biomass or would be produced from a combination of solar electricity/hydrogen and carbon dioxide fixation mediated by high-efficiency artificial photosynthesis mediated by SyPaB. The construction of this carbon-neutral carbohydrate economy would address numerous sustainability challenges, such as electricity and hydrogen storage, CO<sub>2</sub> fixation and long-term storage, water conservation, transportation fuel production, plus feed and food production.
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