while the vehicle is in an enclosed space could result in an explosion hazard as hydrogen gas builds up in the space. There is also always the possibility that the storage tank could be damaged in an accident with catastrophic consequences; however they would probably be the strongest part of a vehicle.

1b. Liquid hydrogen

We can avoid the problems of high pressure gas entirely by storing hydrogen as a liquid. This is one of the fuels used by the space shuttle. To get the same amount of energy stored in one gallon of gasoline we would need 1.27 gallons of liquid hydrogen; however, the hydrogen would only weight 1/8 as much.

All the problems with this method are the result of the fact that liquid hydrogen is really, really cold. It's -423 degrees Fahrenheit, to be exact. So special, very well insulated tanks would be needed to hold it both in your car and at the refueling station. Furthermore, since you could never insulate the tank perfectly, there would always be some boil-off, meaning you would be slowly losing fuel, even if your car was just sitting in the driveway.

1c. Metal hydride storage

A third option is to store the hydrogen chemically, in solids. For example, lithium borohydride (LiBH4) is a solid at room temperature that contains, in each molecule, four atoms of hydrogen. The hydrogen can be released by heating the material, or by reacting it with another chemical, such as water.

The good news is that these solids can pack the hydrogen extremely tightly. Many of them actually offer a higher hydrogen density than liquid hydrogen. They also eliminate the dangers of high pressures, or the expense of constant boil-off.

On the downside, metal hydrides do add additional weight to the fuel tank. (For example, lithium borohydride contains lithium and boron that is never used.) Refueling may also be difficult, because high temperatures and high hydrogen pressures are often required to drive the chemical reaction backwards and reproduce the original material after the hydrogen has been used. (It may also take a long time.) If small amounts of impurities are introduced each refueling cycle, this can also degrade the performance of the material over time.

2.USING HYDROGEN

After we figure out how to store the hydrogen, there are essentially two ways we can use it. In similar fashion to your current vehicle, we can burn it in a hydrogen combustion engine. Alternatively, we can use it in a fuel cell.

2a. Hydrogen combustion engine

The hydrogen combustion

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