Int. J. Hydrogen Energy, Vol. 11, No. 3, pp. 169-192, 1986. Printed in Great Britain. 0360-3199/86 $3.00+0.00 Pergamon Press Ltd. © 1986 International Association for Hydrogen Energy. MAGNESIUM AND MAGNESIUM ALLOY HYDRIDES P. SELVAM, B. VISWANATHAN, C. S. SWAMY and V. SRINIVASAN Department of Chemistry, Indian Institute of Technology, Madras 600 036, India (Received for publication 19 July 1985) Abstract--This review addresses the question of using magnesium and magnesium-based alloys as "rechargeable hydrogen storage" media. The effect of addition of organic compounds as well as other metals (both transition and non-transition elements) on the sorption characteristics of magnesium is considered in detail. The current status of information in this field is reviewed with a view to evaluating the potential of these materials for use as hydrogen storage media in vehicular applications. INTRODUCTION Hydrogen is an attractive and very efficient fuel as well as an excellent medium for energy storage. Given the necessary primary energy (coal, nuclear or solar), hydrogen could become a fuel of inexhaustible nature. According to energy researchers, hydrogen could be one of the possible secondary energy carders (fuel) at the end of the petroleum era [1,2]. As a result of diminishing fossil fuel (petroleum, natural gas and coal) supplies, hydrogen technology can be practically advan- tageous and it can be universally employed for house- hold, industrial and automotive purposes in a similar way to petroleum technology. It was the general con- sensus of the International Association for Hydrogen Energy (IAHE) and the World Hydrogen Energy Con- ferences (WHEC) that among the energy options avail- able, hydrogen, being unique and an environmentally compatible fuel, would start replacing the fossil fuels [3]. Therefore, WHEC call for a new urgency to the situation and suggest that this option should be con- sidered soon for the following reasons. • The acid rains caused by the pollution generated by fossil fuels make the lakes, tanks etc. unsuitable habitats for fish or aquatic plants; • there is evidence that the mean temperature of the world is increasing with increase of CO2 (the main combustion product of fossil fuels) build-up in the atmosphere, which will play havoc with the earth's climate; • a third factor is the ever-growing air pollution. The combustion products CO, CO2, SO2, soot and ash are dumped into the environment, the atmosphere, water and land, thus speeding up the degradation and poisoning of the biosphere. (a) Hydrogen Hydrogen is a clean fuel and the use of it as fuel holds no serious problems. Hydrogen is present in water in practically inexhaustible quantities and it can be gen- erated from readily available and abundant raw mat- erials and energy sources. Burning of hydrogen gives rise to water, a non-depletable material source and it is essentially non-polluting. Hence it is the only system where energy and environment are not adversaries. It produces more energy per unit weight than any other fuel (Table 1) [4]. Hydrogen has been well known as a fuel for internal combustion engines for a long time. In view of the problems of the energy supply and environ- ment protection, the question of the use of hydrogen as an alternative or additional fuel to gasoline has been thoroughly discussed [5]. The hydrogen energy system satisfies all the prerequisites such as abundant avail- ability at low cost, convenient distribution, versatility of usage and societal compatibility. The fact that hydro- gen is renewable and compatible with all the primary energy sources makes it the permanent energy system. It can be readily converted to mechanical, electrical or thermal energy with higher efficiencies than the fossil fuels. Best of all, hydrogen gas can easily substitute for natural gas, diesel and gasoline. So it ideally com- plements the other clean and versatile secondary energy Table 1. Energy densities of various fuels [4] Fuel Energy density heat of combustion (higher heating value) [MJ kg-l] Hydrogen Methane Ethane Propane Gasoline Natural gas Kerosene Crude oil Benzene Coal Ethanol Methanol Ammonia Wood 141.90 55.55 51.92 50.39 47.27 47.21 46.00 45.55 42.29 31.38 29.70 22.69 20.54 17.12 169