In my first blogpost I mentioned that liquid hydrogen has 2.67 times the energy density of gasoline. Further, its only combustion byproduct is water vapor. Taken together, these make it a rather intriguing fuel in my book. So, let’s take a closer look.
First of all, pure hydrogen does not exist in large quantities on our planet. It is always combined with other elements in the form of compounds, the most common of which is water. The water must undergo electrolysis to extract the hydrogen. Since outside energy is required to do this, hydrogen is not a primary source of energy, but simply a medium for storing energy produced by other means.
No matter. Batteries are used exactly the same way.
The major drawback of pure hydrogen is that it must either be stored under extremely high pressure (typically 10,000 psi) or in cryogenic flasks (boiling point = 21ºK) to take advantage of its high energy density. Both of these add weight and complexity to this otherwise simple fuel.
Also, there is inherent danger in using either approach in an automotive application. A ruptured cylinder could produce an enormous fire ball or shoot off like a rocket. And a splash of cryogenic liquid can burn flesh as badly as any flame.
A promising alternative, however, is to store hydrogen in the form of a metal-hydride. Since hydrogen atoms are tiny and have only a single electron in their 1S orbital, they have a great affinity for many metals. These metals can suck up hydrogen like a sponge sucks up water.
Palladium, for instance, will hold 900 times its own volume in hydrogen. That is a volumetric storage density that exceeds liquid hydrogen all by itself! The palladium adds significant weight, of course, and the pure metal is very expensive. Nonetheless, the hydrogen is easily recovered by simply heating the material.
Palladium is just one example. Much research is being done on other metal complexes to find ones that have both a low cost and low weight. Promising candidates include combinations of lithium, boron and aluminum.
Keep an eye on this. My hunch is that hydrogen will find a significant role as the portable fuel of the future – a future that will ultimately be powered by nuclear means.