Traditional hydrides are simple compounds in which hydrogen bears a negative charge. They often contain one or more positive, metal ions — as in, for example, lithium aluminum hydride (LiAlH4). These substances are bases and are powerful reducing agents that can be dangerous to handle. Nevertheless, in the search for suitable replacements for fossil fuels, metal hydrides are considered likely candidates. This may be particularly true for transition metal hydrides.
Some of the more common traditional metal hydrides are those of sodium, calcium and nickel. These substances are categorized respectively as the hydrides of alkali, alkaline earth and transition metals. For an alkali or an alkaline earth metal hydride, chemical bonding is most commonly of the covalent, ionic and mixed ionic varieties. Nickel hydride, used in the manufacture of vehicular batteries, is formed through combining the elements under high pressure. This metallic hydride exhibits a different kind of chemical bond, which is believed to be essential to the hydrogen storage process.
Nickel hydride resembles to some degree the hydride of its fellow transition metal, palladium. These two elements unite with hydrogen through a variety of metallic bond called "interstitial bonding." In this type of bonding, larger atoms have smaller atoms — in this case hydrogen — inserted between them. Not requiring the stringent conditions needed for nickel, palladium hydride forms at room temperature and atmospheric pressure, storing up to 900 times its volume in hydrogen. Although palladium is prohibitively expensive, it could theoretically be used and would present a safer, more efficient means of carrying vehicular hydrogen than pressurized tanks of gas.
Palladium atoms are nearly 5.5 times as large as those of hydrogen. Nickel atoms are 4.6 times larger than hydrogen. This compares to a ratio of 2.1 times for iron and carbon, which bond interstitially to form carbon steel. Whatever relationship atomic size ratio bears to the ease of diffusive insertion, this correlation in bonding to that of carbon steel indicates both nickel and palladium hydrides are alloys of sorts.
If hydrides are to be considered serious contenders for use, some challenges must be met — one example of this can be seen in fuel storage. For one, as hydrogen gas is diffused into a metal, it quickly builds up a back-pressure that slows the further diffusion. Doping the primary metal with another metallic element may lessen this tendency. Another problem is that with each repeated cycle, the hydride metal substrate expands and contracts. Substrate pieces can break down into smaller particles, producing fines that become a source of difficulty unless filtered out. Finally, hydrides must out-perform the contenders, which include possibly liquified hydrogen and liquid boron-hydrogen complexes.