A. J. Maeland, Institute for Energy Technology, Kjeller, Norway
To store large quantities of hydrogen as compressed gas, liquid or solid requires energy intensive processes and raises questions of safety in the public mind. The storage of hydrogen in the form of metal hydrides is an attractive, less energy intensive, highly volume efficient and safe alternative. The weight efficiency of hydrogen storage in “conventional metal hydrides”, i.e. those formed reversibly by reaction with metal or alloy and hydrogen gas, is, however, low due to the weight of the metal or alloy and the limitation of the hydrogen to metal ratio imposed by the chemistry of the hydrides. Suitable “conventional metal hydride” storage materials developed in the past 35 years or so have therefore storage capacities less than about 2.5 wt. percent hydrogen. New approaches are needed and will be discussed, including complex metal hydrides (e.g. alanates), irreversible metal hydrides, non-classical metal hydrides (dihydrogen complexes) and other materials containing hydrogen.
Summary: Investigation and development of reversible metal hydrides as storage materials began in the late 1960’s. However, conventional metal hydrides are limited to hydrogen capacities less than 3 wt.% and fall short of today’s requirement for vehicular use. Other solutions are needed; possible candidates include complex hydrides, irreversible metal hydrides, dihydrogen complexes, and others.
