How do you store and transport the hydrogen?
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Hydrogen-filling stations are already open in several countries including the United States, Iceland, Japan and Germany. See FuelCells.org - Worldwide Hydrogen Fueling Stations to find a hydrogen station near you.
At this moment, the problem with putting pure-hydrogen vehicles on the road is the storage/transportation problem. Hydrogen is a bulky gas, and it is not nearly as easy to work with as gasoline. Compressing the gas requires energy, and compressed hydrogen contains far less energy than the same volume of gasoline. However, solutions to the hydrogen storage problem are surfacing.
For example, hydrogen can be stored in a solid form in a chemical called sodium borohydride, and this technology has appeared in the news recently because Chrysler is testing it. This chemical is created from borax (a common ingredient in some detergents). As sodium borohydride releases its hydrogen, it turns back into borax so it can be recycled.
Once the storage problem is solved and standardized, then a network of hydrogen stations and the transportation infrastructure will have to develop around it. The main barrier to this might be the technological sorting-out process. Stations will not develop quickly until there is a storage technology that clearly dominates the marketplace. For instance, if all hydrogen-powered cars from all manufacturers used sodium borohydride, then a station network could develop quickly; that sort of standardization is unlikely to happen rapidly, if history is any guide.
There might also be a technological breakthrough that could rapidly change the playing field. For example, if someone could develop an inexpensive rechargeable battery with high capacity and a quick recharge time, electric cars would not need fuel cells and there would be no need for hydrogen on the road. Cars would recharge using electricity directly.