Thermodynamic Properties and Cyclic-Stability of the System Mg₂FeH₆

Abstract

The demand for energy has increased continuously since the beginning of the industrial revolution. At the moment, the supply of energy can still be satisfied from fossil sources, however, these are limited and cause environmental problems. In contrast, thermal solar energy based power stations offer the possibility of clean and less expensive energy. To make these more effective and independent of the variable input of the sun, thermal energy-storage systems are needed which operate at temperatures between 400 °C and 550 °C. Although at the present no economical thermal energy storage systems are available, magnesium and its alloys in the form of the hydrides are potential candidates. The energy density of these systems is much higher (1080 kJ/kg ) [1] than comparable systems such as latent heat storage systems (174 kJ/kg at 308 °C) [2]. For commercial use, high reversibility of the storage material must be guaranteed at a minimum cost. To enhance the efficiency of a thermo-electric power plant it is necessary to use as high a working temperature as possible. Over the years we have been trying to develop high temperature storage systems having a good cyclic stability and report here that the Mg₂FeH₆ system is eminently suitable for this task. To add iron to a Mg system is not a new idea: before the common hydride-phase of Mg and Fe was discovered, various groups used iron as a catalyst for the hydrogenation reaction or as a medium to hinder agglomeration. These investigations were, however, restricted to a limited number of hydrogenation and dehydrogenation cycles [3, 4, 5]. In these early investigations, the Mg₂FeH₆-phase was not identified and this was first explicitly synthesized and characterised in 1984 by Yvon and Didisheim et al. [6, 7] by using a sintering technique employing stoichiometric Mg / Fe pellets under high pressure 2–12 MPa and temperatures of 450 °C-520 °C for 2 to 10 days. The yield was about 50% and purification was necessary. In further studies by Konstanchuk et. al. [8, 9, 10], the conditions of formation and the kinetic behaviour of mechanically alloyed mixtures of Fe and Mg have been investigated. Mechanical alloying has also been reported by Mitov, Khrussanova et. al. [11] and Huot and Akiba et. al. [12] and the latter reported a yield of 84% [13]. Although the system has received some attention, the available data, especially for technical application, are not satisfactory.