But hydrogen-powered cars cannot exist without hydrogen filling stations, and vice versa.

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"Fueling is particularly difficult because the amount of money that would have to be put in to get hydrogen generated at filling stations is very costly and would require cooperation from everyone from the automaker to the government to the fueling station owners, " See said.

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One promising idea is to extract the hydrogen from natural gas in large reformers located at filling stations, and then store it on board vehicles in solids known as metal hydrides.

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Hydrogen fuel cell cars need hydrogen and, even in California, there are only around 25 filling stations in the whole of the state.

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When hydrogen is sent into the solid, the reaction creates enough heat to make a filling station very uncomfortable.

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Delivered to a local filling station, such liquids can be readily reformed on site into pure hydrogen for powering fuel cells.

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Such materials could be used to make hydrogen cartridges that would slot into fuel-cell cars, making refuelling as simple as pulling into a filling station and swapping an empty cartridge for a full one.

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