Hydrogen Storage in Magnesium-Based Alloys
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ackground Following the U.S. oil crisis of 1974, research into alternative energy-storage and distribution Systems was vigorously pursued. The controlled oxidation of hy drogen to form water was proposed as a clean energy System, creating a need for light and safe hydrogen-storage media. Extensive research was done on intermetallic alloys, which can störe hydrogen at densities of about 1500 cm 3 -H 2 gas/ cm 3 -hydride, higher than the storage density achieved in liquid hydrogen (784 cmVcm 3 at -273°C) or in pressure tanks (-200 cm 3 /cm 3 at 200 atm). The interest in metal hydrides accelerated fol lowing the development of portable elec tronic devices (video cameras, cellular phones, laptop Computers, tools, etc.), which created a consumer market for compact, rechargeable batteries. Initially, nickel-cadmium batteries fulfilled this need, but their relatively low energy den sity and the toxicity of cadmium helped to drive the development of higher-energydensity, less toxic, rechargeable batteries. 40
This resulted in the development of the nickel-metal hydride (Ni-MH) battery and its commercialization by the end of 1990. Experience has shown that the best materials for the negative electrode of small Ni-MH batteries are alloys of the type (RE)Ni5. Here (RE) Stands for a rareearth element, such as lanthanum, or a combination of rare-earth elements, such as are found in the natural ore misch metal. These alloys can reversibly störe six hydrogen atoms, forming the hydride (RE)Ni 5 H 6 . The use of (RE)Ni5 alloys in Ni-MH batteries has been reviewed by Sakai and co-workers.1 Because rare-earth metals have high density and are rela tively expensive, other alloys, including m a g n e s i u m - b a s e d alloys, are b e i n g studied for use as battery electrodes. Re search on metal hydrides before 1992 can be found in the books edited by Schlapbach.2 Magnesium and magnesium alloys are also being studied for the reversible stor age of hydrogen gas. The basic material properties one must consider in the application of metal hydrides to hydrogengas storage are ■ the pressure-temperature (P-T) relationship for reversible hydride formation, ■ the effective hydrogen-storage capacity, ■ the kinetics of hydrogen storage and delivery, ■ the lifetime of the storage material (usually, storage capacity decreases with cyclic Operation), and i ■ the chemical stability of the hydrided and dehydrided material in the presence of traces of contaminant gases (e.g., CO and C0 2 ). Other material parameters of interest, especially for the design of high-capacity energy-storage devices, are (1) the cost and availability of the intermetallic, (2) the volume change in the intermetal
lic during hydride formation, (3) decrepitation of the intermetallic during cyclic hydrogen charging/discharging Opera tion, (4) thermal conductivity of the ma terial (to transport the heat of hydride formation/decomposition), and (5) the need to activate the material into accepting hydrogen reversibly. Figure 1 shows the hydrogen-gas absorption/desorptio
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