There's 3 options:
1. Store the Hydrogen as a compressed gas.
2. Store the Hydrogen as a liquid.
3. Store the Hydrogen chemically bonded to a chemical.
3. The 3rd option is simply the only way to go. There are materials call Hydrides that absorb Hydrogen like a sponge absorbs water. Typically, the tanks are filled with granulated Hydrides, and Hydrogen is pressurized into the material. Hydrides have many advantages over liquid & gas. One is that the density of the Hydrogen stored in the Hydride can be GREATER than that of liquid Hydrogen. This translates directly into smaller and fewer storage tanks. Once the Hydride is "charged" with Hydrogen, the Hydrogen becomes chemically bonded to the chemical. Even opening the tank, or cutting it in half will not release the Hydrogen gas. In addition, you could even fire incendiary bullets through the tank and the Hydride would only smolder like a cigarette. It is in fact, a safer storage system than your Gasoline tank is.
Then how do you get the Hydrogen back out? To release the Hydrogen gas from the Hydride, it simply needs to be heated. This is either done electrically, using the waste exhaust heat, or using the waste radiator coolant heat..
Our vehicle starts on Gasoline and runs for about 5 minutes to heat everything up ( including the Hydride ). The time it takes to heat up the Hydride is about as long as it takes the heater in your car to warm up and blow out hot air. As soon at the Hydride is sufficiently warm, Hydrogen is released from the tanks and the on-board computer detects the presence of Hydrogen pressure. The fuel system then seamlessly switches over to Hydrogen and remains in that mode until no more Hydrogen is released from the tanks. Leaving the Gasoline fuel system intact also enables you to run on Gasoline should you ever run out of Hydrogen. .
The only exhaust products from using Hydrogen as a fuel are steam ( water vapor ) and a tiny amount of Nitrogen Oxides. It's about as clean burning as you can get.
And that's all great, but where do we get this cheap source of hydrogen? Currently the possibilities are from reforming fossil fuels, typically natural gas, which is inefficient as a significant part of the energy contained in hydrocarbons is thrown away in doing this, and there's no net reduction in greeenhouse gasses (if that's a concern), or by hydrolosis of water using electricty, which again is not a particularly efficient process overall (taking generating and transmission losses into account). If natural gas is going to be the source, it would appear to me to be even more efficient just to burn it directly in the engines, something that is already done.
Yah, but the dead weight of the tank is a lot greater for a hydride storage system than a liquid or carbon-fiber compressed gas storage tank.
Early H2 vehicles will have to have Honda Insight-like ruthless weight control, if they're going to have the performance Americans tend to expect from their cars.