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engine burns fuel just like the gasoline engine in your car. Since hydrogen contains no carbon, however, the only product of its combustion with oxygen gas is water. If we burn the hydrogen in the atmosphere, which also contains nitrogen, some of the so-called "NOx" pollutant gases may be produced as a result of the heat of the reaction. Engines burning hydrogen are generally slightly more efficient than those burning gasoline.

2b. Hydrogen fuel cell

A perhaps more popular, and still more efficient option is the fuel cell. A fuel cell consists of something like an anode which we pass hydrogen gas over, and a cathode which oxygen gas is passed over, separated by a semi-permeable membrane. (If you don't know what anode and cathode mean, just hang with me.) At the anode, the hydrogen, which normally consists of a proton being orbited by an electron, is split apart into bare protons and bare electrons.

The protons are able to pass through the membrane and head toward the cathode, but the electrons are blocked and must travel around it, through an electrical circuit that will contain the vehicle engine. When the electrons finally arrive at the cathode they recombine with the protons and the oxygen to form water, the only waste product of this system. There are several variations on this theme, but you have the basic idea.

The biggest benefit of the fuel cell is efficiency. Much less energy is lost as heat. While the typical gasoline engine might convert 20% of the liberated energy into mechanical work (running the engine), for a fuel cell we could theoretically get something more like 80%. (In practice the number has been smaller, but still much higher than what we get with gasoline engines.) While each fuel cell can only produce a small voltage or current, they can be "stacked" to increase that limit to whatever we need.

On the down side, fuel cells can be quite expensive. Many of them, for example, use platinum catalysts, and platinum is currently over $1000/ounce. They are also often very sensitive to carbon monoxide poisoning, which can degrade their performance over time.

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For more information on the chemistry of hydrogen storage and fuel cells, see the book "Introduction to Hydrogen Technology" by Roman J. Press and others. Be warned, however, that the first edition has many typos.

Learn more about this author, David Shane.
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