Metal hydrides, such as lithium aluminum hydride, are powerful reducing agents and potential replacements for fossil fuels. Nickel and palladium hydrides use interstitial bonding and could be used for vehicular hydrogen storage. Challenges include back pressure and substrate expansion.
Traditional hydrides are simple compounds in which hydrogen has a negative charge. They often contain one or more positive metal ions, such as lithium aluminum hydride (LiAlH4). These substances are bases and are powerful reducing agents that can be dangerous to handle. However, in the search for suitable replacements for fossil fuels, metal hydrides are considered likely candidates. This may be especially true for transition metal hydrides.
Some of the more common traditional metal hydrides are those of sodium, calcium and nickel. These substances are classified as alkali, alkaline earth and transition metal hydrides, respectively. For an alkali or alkaline earth metal hydride, the chemical bond is most commonly of the covalent, ionic, and mixed ionic varieties. Nickel hydride, used in the manufacture of vehicle batteries, is formed by combining elements under high pressure. This metal hydride exhibits a different type of chemical bonding, which is thought to be essential for the hydrogen storage process.
Nickel hydride resembles to some extent the hydride of its fellow transition metal, palladium. These two elements join with hydrogen through a variety of metallic bonds called “interstitial bonding”. In this type of bond, the larger atoms have smaller atoms, in this case hydrogen, sandwiched 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. While palladium is prohibitively expensive, it could in theory be used and would present a safer and more efficient means of transporting vehicular hydrogen than pressurized tanks of gas.
Palladium atoms are nearly 5.5 times larger than those of hydrogen. Nickel atoms are 4.6 times larger than hydrogen. This compares to a 2.1-fold ratio for iron and carbon, which interstitially bond to form carbon steel. Whatever the relationship between atomic size ratio and ease of diffusive insertion, this correlation in bonding with that of carbon steel indicates that both nickel and palladium hydrides are alloys.
If hydrides are to be considered serious contenders for use, some challenges must be faced: an example of this can be seen in fuel storage. For one, when hydrogen gas is diffused into a metal, it quickly creates back pressure that slows further diffusion. Doping the primary metal with another metallic element can reduce this tendency. Another problem is that with each repeated cycle, the metal hydride substrate expands and contracts. Substrate chunks can break down into smaller particles, producing fine particles that become a source of difficulty if left unfiltered. Finally, the hydrides must outpace competitors, which include possibly liquefied hydrogen and liquid boron-hydrogen complexes.
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