Chemistry Department, Lomonosov Moscow State University, Moscow 119899, RussianFederation
S.V.MITROKHIN AND V.N.VERBETSKY
USE OF METAL HYDRIDES
AS SOURCES OF HYDROGEN FOR VARIOUS PRACTICAL APPLICATIONS
1. INTRODUCTION
Chemistry of hydrogen and its compounds, particularly metal hydrides, is an intensively developed field of chemistry. First of all it is connected with the unique properties of hydrides which are widely used in various practical applications, such as neutron radiation shields, rocket fuel components, catalysers of hydrogenation, purification of hydrogen and production of fine metal powders. Investigations in the field of metal hydrides are especially important in view of the environmental problems and development of new non-fossil fuels
Practical application of hydrides is closely connected with the solution of several basic problems of peculiarities of hydrogen interaction with intermetallic compounds and alloys of different structure types, determination of thermodynamic, kinetic and structural data, necessary for the development of new hydrides and for estimation of hydride stability.
2.1. Interaction with hydrogen, sorption characteristics, thermodynamics of rare-earth and transition metal alloys.
The development of principles of metal-hydride technology requires a detailed study of hydrogen interaction with metals and alloys in wide ranges of pressure and temperature, investigation of factors controlling the kinetics and thermodynamics of hydride formation, analysis of crystal structure of hydrides. A physicochemical investigation of metal-hydrogen systems using X-ray, neutron diffraction, DTA, microscopy and calorimetry has been done for the solution of these problems
Thermodynamic investigation of the reaction of hydrogen with intermetallic compounds (IMC) of different structure types (RT, RT2, RT5 where R - rare-earth, Ti, Zr; T - transition metal) using methods of physicochemical analysis has allowed to develop the knowledge of the mechanism of hydrogen interaction with metal matrix, the hysteresis phenomenon, to propose the models for calculation of hydride formation enthalpies [1-6]. Calorimetric measurements of RT5-H2 and RT2-H2 systems have disclosed the existence of intermediate hydride phases in some systems and state a pronounced dependence of absorption (desorption) reaction enthalpy on temperature.
It has been stated that the thermodynamic aspect of the hydrogen interaction with intermetallic
2. BASIC RESEARCH
compounds consists in the fact that the absorption of hydrogen can proceed in several different paths and that the reversible absorption-desorption reaction is not the only possibility. Actually for a number of IMC, especially at high temperatures and large hydrogen pressure, the reaction of hydrogenolysis (disproportionation) of metallic matrix due to hydrogen influence becomes thermodynamically preferable [7-8]. In some cases the hydrogenolysis reaction is preceded by a stage of intermetallic hydride amorphisation with the formation of metastable products with high hydrogen content.
The investigation of hydrogen absorption kinetics by alloys of LaNi5, Ti(Zr)Cr2, FeTi type allows to propose the following mechanism. At the initial stage the reaction rate is described by a "contracting sphere" model and limiting stage is the rate of hydride phase nucleation. At the final stage of reaction the limiting process is the rate of hydrogen diffusion through the hydride layers [9-11].
A great importance for the development of the conception of hydride structure and of the metal-hydrogen bonding nature play the works on neutron diffraction investigation and description of hydride crystal structure [12] and magnetic measurements [13-15] and X-ray emission spectroscopy [16, 17]. In these works the crystal, magnetic and electron structure of several intermetallic hydrides have been described for the first time.
Rather detailed study of the interaction of hydrogen with multicomponent alloys of RT5 type has been performed in [18]. The enthalpies of reaction, the influence of different constituents of the alloys on the properties of hydrides have been stated.
The investigation of hydrogen interaction with alloys of Ti-V-M (M - Al, Fe, Co, Ni) systems [19, 20] provided for the first time to obtain and to discuss the properties of ternary hydrides and to determine the character of hydriding reactions of alloys belonging to 2- and 3-phase regions of metallic system state diagrams.
Basing on the results of these studies the model for "design" of compositions of alloys with pre-set hydride dissociation pressures have been proposed. Table I show the characteristics of three groups of alloys which reversibly absorb considerable amounts of hydrogen [21-24]. The P-C-isotherms for some selected systems are shown in Fig. 1-3.
2.2. Hydrogen interaction with magnesium alloys.
A great deal of consideration has been given to interaction with hydrogen of some magnesium IMC and alloys, mainly due to their large hydrogen absorption capacity (up to 7.6 mass.% H2) and to the fact that these compounds seem to be promising for preparing high-temperature hydrogen storage systems. The main obstacle for magnesium hydride application is the extremely low rate of interaction with hydrogen. Most effective for overcoming this appear the methods of magnesium alloying with rare-earth metals, calcium and nickel. To determine the hydride formation mechanism and to optimise the conditions of synthesis the interaction of hydrogen with IMC, solid solutions and multiphase alloys in the Mg-Ln(La, Ce, Er, Yb), Mg-Ca-M(Al, Zn, Cu, Ce, Ni), Mg-Ln(La, Ce, Sc, Y, Mn)-M(Al, Ni) systems has been investigated [25-38]. The role of phase composition and alloy microstructure in hydriding reactions has been stated. On the base of kinetic measurements the values of activation energy of hydriding-dehydriding reactions in multicomponent magnesium-based systems have been calculated. It has been shown that doping of magnesium by small amounts of nickel and rare-earth metals allows to increase the rate of magnesium hydride nucleation by several times: 90% of transformation takes place in the period of 1-2 minutes at 300oC (Fig.4). The model for reaction of MgH2 formation in the presence of rare-earth metals
has been proposed. It is based on the consideration that lanthanide hydride inclusions act as active conductors of dissociated hydrogen to the magnesium surface and also as magnesium hydride nucleation centres (Fig.5) [38].
2.3. Metal hydrides with high density.
The investigation of hydrogen interaction with alloys of Ti-Ta; Zr-Ta; Ti-W; Ti-Ta-W; Zr-Sc; Hf-Sc systems was carried out using X-ray and DTA methods [39]. The peculiarities of hydride formation were studied and phase diagrams of investigated systems were proposed. All alloys of Ti-Ta system, representing a continuous solid solution, contrary to pure metals, react with hydrogen with a high rate at P < 30 atm and T ~ 500-550 forming two types of hydrides: fct TiH2-type structure (Ta < 55 at.%) and bct TaH-type structure (Ta > 55 at.%) while P-alloys of titanium-tantalum-tungsten system interact with hydrogen at ambient temperature and P = 20 atm after a short induction period. The phase diagrams of investigated systems are presented in Fig.6,7. The dependencies of roentgen density (pH) and NH on hydride composition are presented in Fig.8,9. The results show that the density of Ti-containing materials can be increased by 1.5-2 times, while preserving of high hydrogen density in volume unit. So, it is possible to conclude that multicomponent hydrides are rather perspective for neutron radiation shielding.
3. APPLICATIONS
The complex basic research allowed to propose a series of hydrogen absorbing materials with easily tailored properties. These results were applied for the development of (i) new hydriding materials [40] and production technology [41] of alloys with wide range of working temperatures and pressure, (ii) systems of hydrogen storage and transport, (iii) technology of production of alloys and (iv) technology of production of highly disperse metal powders. Table 1 shows the main properties of various alloys already recommended or applied in different metal-hydride devices
The realisation of metal-hydride method of hydrogen accumulation is implemented mainly in two directions. First is the development of diminutive storage systems for R&D laboratories and for apparatuses consuming small amounts of hydrogen. Laboratory accumulator is usually a cylindrical vessel filled with alloys on base of LaNi5, MmNi5 or (Ti,Zr)(Mn,Cr)2 with a capacity of 300-1000 l of hydrogen. The work pressure in such accumulators is about 0.5-5.0 atm in the temperature range 20-100oC [42].
Second is the development of storage systems for vehicular applications and energy devices. Accumulators for vehicular and energy applications can store about 2-20 STPm3 hydrogen and are rather complex devices, equipped with heat-exchanger with hot (cold) water or gas inlets and outlets [43]. The perspective trend of the storage system development is creation module systems and maintenance of needed hydrogen capacity by a certain number of modules [24]. The characteristics of such module are presented in Table 2.
Another example of vehicular application is the set of hydrogen storage modules which was used for petrol-hydrogen powered truck ZIL-130 (Fig. 10). Each of these modules can store up to 7 m3 of hydrogen (35 kg of Ti-V-Fe alloy [40]). Field tests of this truck showed that the application of hydrogen decreases the petrol consumption by 15% and by 10 times the toxic concentration in exhaust gases.
TABLE 1. Characteristics of hydrogen storage alloys.
Alloy type Dissociation pressure at room temperatures Absorption-desorption temperature range, °C Reversible mass.% H2
Ti-V-M (M=Al,Fe,Co,Ni) V - 5-10% M - 5-45% << 1 200 - 450 3 -3.8
TiFe - 3-5%Ce, V, Mn 0.5-8 20 - 100 1.7 - 1.8
V
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