Hydrogen Storage

Journal of Materials Processing Technology 162–163 (2005) 169–177

An overview of advanced materials for hydrogen storage Elena David National Institute of Cryogenics and Isotopic Technologies, Rm. Valcea, O.P.4, P.O. Box 10, Code 240050, Romania

Abstract In a future sustainable energy system based on renewable energy, environmentally harmless energy carriers like hydrogen will be of crucial importance. One of the major impediments for the transition to a hydrogen-based energy system is the lack of satisfactory hydrogen storage alternatives. In the last years, the possible to store hydrogen in various materials was extensively studied. This paper is a preliminary study with the focus on advanced nanostructured materials such as solids of large surface area based on carbon structures, metals and different types of metal alloys, other intermetallic compounds, etc. as possibilities for hydrogen storage. The newest materials used for hydrogen storage are light metal alloys. We have so far focused in this review almost exclusively on experimental studies. Also there are presented the most important characteristics of these materials such as mechanical strength, porosity and affinity to hydrogen, and also the recent developments in the search for innovative materials with high hydrogen-storage capacity and our contribution in this field. © 2005 Elsevier B.V. All rights reserved. Keywords: Carbon structures; Metal alloys; Light metal hydrides; Hydrogen storage

1. Introduction The current interest in hydrogen is primarily due to environmental concerns of the harmful emissions from the fossil fuels used presently. Also, a demand for more efficient power sources has increased the interest in different kinds of new technologies, such as fuel cells using hydrogen or hydrocarbons as fuel [1,2]. It is widely believed that hydrogen will within a few years become the fuel that powers most vehicles and portable devices, i.e. hydrogen will become the means of storing and transporting energy. The reason is the depletion of oil and the relatively facile production of hydrogen from the various renewable sources of energy – hydroelectric, wind, solar, geothermal – with water being the only raw material needed. To release the energy, hydrogen can be burned in an efficient and clean way in a fuel cell to form water again, or made to drive an electrochemical cell as in the commonly used nickel hydride battery. While hydrogen has many obvious advantages, there remains a problem with storage and transportation. Pressurised hydrogen gas takes a great deal of volume compared with, for example, gasoline with equal energy content, about 30

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times bigger volume at 100 bar gas pressure. Condensed hydrogen is about 10 times denser, but is much too expensive to produce and maintain [3]. There are also obvious safety concerns with the use of pressurised or liquefied hydrogen in vehicles. The results obtained and presented by many studies show that three kinds of materials are competitive for to be used in hydrogen storage processes. These are materials based on carbon structures, metals and metal alloys. We will refer especially at these kinds of materials.

2. Materials based on carbon structures The best performance in hydrogen storage was achieved with materials based on carbon structures of highest effective porosity. The two forms of carbon that is the most known to us are diamond and graphite. In diamond each atom is fully coordinated symmetrically in space in all three dimensions. Graphite, on the other hand, is build up of a two-dimensional hexagonal sheet of carbon atoms, with long distance between each sheet. However, there are also other forms of carbon structures such as fullerenes and nanotubes, that are the newest advanced carbon structures, with special properties. Fig. 1 shows the structure of graphite, diamond, fullerenes and of single-wall carbon nanotube.

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Fig. 1. Structure of (a) graphite, (b) diamond, (c) fullerenes and (d) singlewall carbon nanotube.

Fullerenes are single or multiple layers of graphite wrapped together into very stable ball (Fig. 1c) or tube molecules – single-wall carbon nanotube (SWCT), Fig. 1d, respectively. 2.1. Fullerenes Fullerenes are a new class of carbon aromatic compounds with unusual structural, chemical and physical properties which, in turn, will lead to novel and unexpected applications. Fullerenes are synthesised carbon molecules usually shaped like a football, such as C60 and C70 and they are able to hydrogenate through the reaction [4]: C60 +xH2 O + xe− ↔ C60 Hx + xOH−

(1)

According to theoretical calculations, the most stable of these are C60 H24 , C60 H36 and C60 H48 , latter of which is equal to 6.3 wt.% of hydrogen adsorbed [4,5]. An experimental study made by Chen et al. shows that more than 6 wt.% of hydrogen can be adsorbed on fullerenes at 180 ◦ C and at about 25 bar [6]. Usually the bonds between C and H atoms are so strong that temperatures over 400 ◦ C are needed to desorb the hydrogen [5], but Chen et al. were able to do this at a temperature below 225 ◦ C [6]. Despite the quite high hydrogen-storing ability, the cyclic tests of fullerenes have shown poor properties of storing hydrogen [7]. In our laboratory, it was obtained a mixture of C60 and C70 fullerenes by electric arc discharge method in which carbon is vaporised between two graphite electrodes. In Fig. 2 is presented the image of the C70 fullerene surface obtained by transmission electron microscopy (TEM) and SEM image of single-wall carbon nanotubes (SWCTs). Our experimental study concerning hydrogen storage showed that more than 0.6 wt.% of hydrogen can be adsorbed on fullerenes at 200 ◦ C and at about 12 bar.

Fig. 2. (a)TEM image of C70 fullerene. Each white feature corresponds to a column of C70 molecules lying parallel to the electron beam. The arrow indicates that defects occur in clusters separating large perfect crystallites. These defects are the hosts for hydrogen storage. (b) SEM image of singlewall carbon nanotubes (SWCTs).

wall nanotubes (MWNT). A single-wall nanotube can be up to 100 times stronger than that of steel with the same weight. The Young’s modulus of SWNT is up to 1 TPa, which is 5 times greater than steel (230 GPa) while the density is only 1.3 g/cm3 [8]. That means that materials made of nanotubes are lighter and more durable. Another property of carbon nanotubes is their ability to quickly adsorb high densities of hydrogen at room temperature and atmospheric pressure [9]. An SEM image of SWCTs is shown in Fig. 2b. Nanotubes are manufacturing from pure carbon. Pure carbons only have two covalent bonds sp2 and sp3 , the former constitutes graphite and the latter constitutes diamond. sp2 is a strong bond within a plane but weak between planes. sp2 is composed of one s-orbital and two p-orbitals. When more sp2 bonds together, they form six-fold structures, like honey comb pattern, which is a plane structure, the same structure as graphite. Graphite is stacked layer by layer so it is only stable for one single sheet. Viewing these layers perpendicularly shows the honey comb patterns of graphite. Wrapping these patterns back on top of themselves, joining the edges and close one end while leave one end open, it forms a tube of graphite, which is known as nanotube [10]. In Fig. 3 it is presented schematically the nanotube forming. Carbon nanotubes were discovered in 1991 accidentally when synthesising fullerenes [11]. Hydrogen can be stored into the nanotubes by chemisorption or physisorption. The methods of trapping hydrogen are not known very accurately [11,12] but density functional calculations have shown some insights into the mechanisms

2.2. Carbon nanotubes Nanotubes are the strongest carbon fibres that are currently known, single-wall nanotube (SWNT; see Fig. 1c) and multi-

Fig. 3. The forming mechanism of nanotube.

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The density functional calculations have shown that theoretically in proper conditions a single-wall nanotube can adsorb over 14 wt.% [11,12] and a multi-wall nanotube about 7.7 wt.% of hydrogen [12]. Dillon and co-workers [14] reported the first experimental result of high hydrogen uptake by a nanotube. They estimated that hydrogen could achieve a density of 5–10 wt.%. Chen et al. [15] reported that alkalidoped nanotubes are able to store even 20 wt.% under ambient pressure, but are unstable or require elevated temperatures [15,16]. The result has shown to be in a great disagreement with other results and has been thought to be incorrect [17]. Recent results on hydrogen uptake of single-wall nanotubes are promising. At 0.67 bar and 327 ◦ C, about 7 wt.% of hydrogen have been adsorbed and desorbed with a good cycling stability [18]. Another result at ambient temperature and pressure shows that 3.3 wt.% hydrogen can be adsorbed and desorped reproducibly and 4.2 wt.% hydrogen with a slight heating [19]. The hydrogen amounts storaged depend of the number of layers Nl and of tube diameter. The effect of nanotube type (SWNT or MWNT) on the storage capacity for hydrogen has been investigated theoretically [20]. The calculated storage hydrogen mass as a function of number of layers and tube diameter is presented in Fig. 5. Hydrogen condenses as a monolayer at the surface of the nanotube or condenses in the cavity of the tubes. The condensed hydrogen has the same density as liquid hydrogen at −253 ◦ C. The arc discharge method produces a good-quality multiwall and single-wall carbon nanotubes. The studies performed by us within a project the system used for to obtain carbon structures of fullerenes was used for to produce carbon nanotubes. For this it was utilised two graphite electrodes to generate an arc by a high d.c. current. After arc discharging for a period of time, a carbon rod builds up at the cathode. Carbon nanotube bundles and amor-

Fig. 4. Hydrogen adsorption in a nanotube: (a) exterior adsorption with H/C coverage 1.0, (b) interior adsorption with coverage 1.0, (c) interior adsorption with coverage 1.2 and (d) interior adsorption with coverage 2.4 [12].

[11–13]. Calculations indicate that hydrogen can be adsorbed at the exterior of the tube wall by H C bonds with a H/C coverage 1.0 or inside the tube by H H bonds with a coverage up to 2.4 [11,12] as shown in Fig. 4 modified from [12]. The adsorption into the interior wall of the tube is also possible but not stable. The hydrogen relaxes inside the tube forming H H bonds [11]. The numbers in the figure tell the bond lengths in 10−10 m. Multi-wall nanotubes, in which two or more single tubes are rounded up each other with van der Waal’s attraction, can adsorb hydrogen between the singlewall nanotubes. The hydrogen causes the radius of the tubes to increase and thus makes a multi-wall nanotube less stable [12]. In nanotube bundles hydrogen can also be adsorbed in the middle of different tubes.

Fig. 5. Calculated storage hydrogen mass% as a function of the number of layers Nl and as a function of the tube diameter.

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Fig. 7. Pore structure of activated carbon – fibrous form. Fig. 6. Pore structure of activated carbon – granular form.

phous carbon both form at cross-section of the rod. Helium gas was present to increase the speed of carbon deposition. This method does not produce clean results due to the existence of these amorphous carbons. Some parameters that are critical in this process are: the pressure of the helium, the temperature and the d.c. current. Efficient cooling is necessary to form homogenous deposition of carbon nanotubes. Our studies will try to perfect the arc discharge method by using graphite electrodes that contain metal catalysts. Even though the price of the nanotubes is still high, they have a good potential in hydrogen storing and will remain as materials that are competitive for to be used in hydrogenstorage processes. 2.3. Activated carbons Bulky carbon with high surface area, the so-called activated carbon, is other carbon structure able to adsorb hydrogen in its microscopic pores. The main problems are that only some of the pores are small enough to catch the hydrogen atom and that high pressures must be applied in order to get the hydrogen into the pore. The pores in activated carbons are scattered over a wide range of size and shape. The pores are classified by their sizes usually into three groups: (i) macropores having average diameter more than 50 nm, (ii) mesopores with diameter 2–50 nm and (iii) micropores having average diameter less than 2 nm. These are further divided into supermicropores (0.7–2.0 nm) and ultramicropores of diameter less than 0.7 nm. The pore structure in woodderived carbons is a memory of the texture of the precursor wood and hence differ based on the type of wood/plant (see Figs. 6 and 7). Basically this consists of slit-like voids and macropores. In some wood carbon, these are of similar shape and size and are equally distributed, whereas in coal they are of very different shapes and sizes and are organised in circular fashion. These macropores are in turn connected to

mesopores and micropores. This is shown in Fig. 6. Fig. 7 includes the structure of activated carbon fibres which, in contrast to wood-based activated carbon, predominantly consists of micropores directly exposed to the surface of the fibres. About 5.2 wt.% of hydrogen adsorbed into the activated carbon has been achieved at cryogenic temperatures and in pressures of about 45–60 bar [20]. In ambient temperature and pressure of 60 bar, the storage capacity has been only approximately 0.5 wt.% [20,25]. In our recent studies, we developed chemically activated carbons (ACWPh, ACSPh) [23]. These were produced by mixing an chemical agent (phosphoric acid) with a young carbonaceous material obtaining from walnut shells (CW) and sawdust (CS) and carbonising the resultant mixture at 550 ◦ C under inert atmosphere. The resulting very porous carbon structure is filled with activation agent, which removed from carbon by washing. The characteristics of these materials are presented in Table 1. Such as can see from Table 1, activated carbon (ACWPh) has a pore volume of 1.49 cm3 /g, which is 37 times greater than graphite. About 2.2–2.8 wt.% H2 of hydrogen adsorbed into these activated carbons has been achieved at cryogenic temperatures and in pressures of about 12–15 bar. For microporous carbons the details of the pores (size and shape) apparently affect the specific H2 uptake to a large extent. In addition to general-purpose active carbons, advanced active carbons with specific control on pore structure have been developed over the past few decades for specific applications. Research and development efforts are continuing for more and more efficient applications of these materials. Carbon molecular sieves (CMS) are a special class of active carbons having small pore sizes with a sharp distribution in a range of micropores, as compared with other activated carbons. The pore size in CMSs is comparable to the size of adsorbate molecules, such as nitrogen and hydrogen [24].

Table 1 Texture analysis and H2 storage capacities at (−196 ◦ C) and 12 bar Nr. crt.

Material

Pore volume (cm3 /g)

SBET (m2 /g)

Density (cm3 /g)

Hardness (GN/m2 )

Median pore ˚ diameter (A)

H2 storage capacity (wt.%)

1 2 3

Graphite ACSPh ACWPh

0.04 0.85 1.49

7 1620 1850

2.26 0.47 0.43

30.15 16.06 14.78

– 3–300 3–200

0 2.2 2.8

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In addition, the temperature of adsorption governs the rate of adsorption of a gas because of the activated diffusion of adsorption molecules in micropores.

3. Metals and alloys for hydrogen storage Hydrogen is a highly reactive element and is known to form hydrides and solid solutions with thousands of metals and alloys. Most of the natural elements adsorb hydrogen under proper conditions. Metal hydrides are composed of metal atoms that constitute of a host lattice and hydrogen atoms that are trapped in interstitial sites, such as lattice defects. The trap site can be a vacancy or a line defect. In the case of a line defect, a string of hydrogen atoms may accumulate along the defect. Such a string increases the lattice stress, especially if two adjacent atoms recombine to form molecular hydrogen [25]. Since adsorption of hydrogen increases the size of lattices [25,26], the metal is usually ground to a powder in order to prevent the decrepitation of metal particles. There are two possible ways of hydriding a metal, direct dissociative chemisorption and electrochemical splitting of water. These reactions are: M + X/2 · H2 ↔ MHX

(2)

M + X/2 · H2 O + X/2 · e− ↔ MHx + X/2·OH−

(3)

where M represents the metal. In electrochemical splitting there has to be a catalyst, such as palladium, to break down the water. The necessary condition for hydrogen storage is that the thermodynamic and kinetic conditions are fulfilled. In that case, a metal exposed to hydrogen gas absorbs hydrogen until equilibrium is established. There are a number of reaction steps that kinetically may hinder a hydrogen-storing system to reach its thermodynamical equilibrium of hydrogen storage within a reasonable time. The reaction rate of a metal–hydrogen system is therefore a function of pressure and temperature. Metal and hydrogen usually form two different kinds of hydrides, ␣-phase and ␤-phase hydride. In ␣-phase there is only some hydrogen adsorbed and in ␤-phase the hydride is fully formed. A schematic of phase transition is presented in Fig. 8. When charging, hydrogen diffuses from the surface of the particle trough the ␤-phase to the phase-transition interface and forms additional ␤-phase hydride. When discharging, hydrogen from the phase-transition interface diffuses through the ␣-phase to the surface of the particle where it is recombined into the form of molecular hydrogen [27]. Hydrogen storage in metal hydrides is a complex process consisting of a lot of mechanistic steps and depends on important parameters. A metal surface has to be able to dissociate the hydrogen molecule and to allow hydrogen atoms to move easily in order to be able to store hydrogen. Metals differ in the ability to dissociate hydrogen, this ability being dependent on surface structure, morphology and purity. Normally

Fig. 8. Schematic of phase transition in metal hydrides.

the hydrides are divided into high-temperature hydrides and low-temperature hydrides depending on the temperature of absorption/desorption process. In the low-temperature hydrides, the hydrogen normally is bound through covalent bonding and the metal hydride consists of high-molecularweight material. In the high-temperature hydrides, the hydrogen is normally ionic bound, and the metal hydride consists of low-molecular-weight material. The hydrogen-storage capacities are higher for the high-temperature hydrides. The most common examples of hydrogen-storing alloys are Fe–Ti hydrides, La–Ni hydrides, Ti–Zr–V series of hydrides, etc. Metals can absorb hydrogen in atomic form and thereby act as hydrogen “sponges” [28]. Around 50 metallic elements of the periodic table can absorb hydrogen in great quantity and the possible choices of hydrogen storage materials are, therefore, enormous. Many scientific and engineering studies have been carried out of the absorption/desorption of hydrogen in metals and development of such storage devices. Daimler-Benz, for example, produced in the early 1980s a car fuelled by hydrogen where the storage tank was a chunk of Fe–Ti metal alloy [29]. The volume of this storage device is less than a factor of 2 greater than the equivalent gasoline tank, but the problem is that the hydride is 20 times heavier [30]. The only successful commercial large-scale application of metal hydrides as hydrogen storage so far is the metal hydride battery, which has supplied battery power to many small electrical appliances such as mobile phones and portable computers. Hydride systems based on existing metal hydrides cannot store large amounts of hydrogen and the development of new kinds of hydride materials is required. 3.1. Light metal alloys for hydrogen storage If metal hydrides are to become important energy carriers in mobile vehicles, the mass of the system needs to be reduced from today’s devices, such as FeTi. This puts strong constraints on the chemical elements which can be used. A

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very promising approach is to use magnesium. The hydride, MgH2 , can store up to 7.6 wt.% of hydrogen [28,29]. The automobile industry has set 5–5.6 wt.% as a target for efficient hydrogen storage. But there are three problems with pure Mg: (i) The rate at which hydrogen absorbs and desorbs is too low because diffusion of hydrogen atoms through the hydride is slow. Problem (ii) is that the hydrogen molecules do not readily dissociate at the surface of Mg to generate the hydrogen atoms that diffuse into the metal. Transition metals can catalyse this bond-breaking/formation even at the surface, but not main group elements [30]. Problem (iii) is that hydrogen atoms bind too strongly with the Mg atoms, i.e. the enthalpy of formation of the hydride is too large, so that the hydride needs to be heated to very high temperature, around 350 ◦ C, in order to release hydrogen gas at high enough pressure (over 1 atm) [28]. A useful metal hydride should release the hydrogen in the temperature range between 50 and 100 ◦ C. It is important to distinguish between problems (i) and (ii) although they both have to do with the kinetics of absorption and desorption processes. These problems could be solved efficiently by different means, but problem (iii) remains unsolved. Our studies performed till now were focused on these three problems. The first problem could be reduced by forming a composite of small Mg crystals agglomerated together as opposed to a large chunk of Mg crystal. The clusters formed by ball milling are micrometer sized such as seen in the scanning electron microscopy (SEM) and the transmission electron micrograph (TEM) images (Fig. 9). The size of the magnesium grains in this composite is 40–200 nm. Hydrogen absorption and desorption of this material was found to be reasonably fast. The dark regions represent crystalline domains, while the lighter regions are less dense and likely amorphous. The scanning electron microscopy (SEM), shown in Fig. 9b, reveals the crystalline structure of the particles. Another way to improving the kinetics is to mix in another chemical component, for example, by mixing in Ni. Channels with lower binding energy and smaller migration barriers for the hydrogen are apparently formed and maintained through out the hydride, thus enabling fast loading and unloading. This may occur because holes where the hydrogen atom sites are now surrounded by two types of atoms forming bonds of different strength with the hydrogen atom. The alloy hydride can also have different diffusivity because the alloy has a different lattice structure than pure magnesium. The problem is that this metal, Ni, is heavy and greatly reduces the weight percent of hydrogen in the hydride. The second problem, the dissociation of hydrogen molecules on the surface, could be solved by adding a small amount of a catalytically active metal on the surface of the Mg clusters, as shown in Fig. 9b. When hydrogen is bonded chemically in a solid structure, such as a metal hydride, it has to overcome certain activation barriers during the process. Near the surface it can adsorb in a weak van de Waal’s interaction with the surface electrons through a small polarisation of its electronic charge (ph-

Fig. 9. Transmission electron microscopy (TEM) micrograph of the powder showing the nanocrystalline structure of the particles (a), and a scanning electron microscopy (SEM) micrograph of a powder of Mg particles which have small Ni clusters attached on the surface in order to catalyse the break up of hydrogen molecules during adsorption (b).

ysisorption). If the kinetic energy is large enough for the H H bond to stretch slightly (one activation barrier), its molecular bond can shift partly towards the surface and become more tightly bound to the surface (chemisorption). If the kinetic energy is so high that the H H bond can be split (another activation barrier) and turned completely towards the surface, the hydrogen atoms can be dissociated and atomic chemisorbed to the surface, but only if there are two free sites at the surface such is shown in Fig. 10. Using Mg as example, the activation barrier for dissociation is approximately 0.5 eV [31]. Furthermore, the transfer of the dissociated hydrogen atom into the interior of the solid structure includes another barrier of approximately 0.4 eV and inside the metal the hydrogen atom can move by diffusion at yet another activation barrier of approximately 0.1 eV. The kinetic energy depends on temperature and the activation barriers therefore set a lower temperature limit below which reactions are too slow to be useful. The depth of the potential well determines the stability of the storage, i.e. the temperature range of operation and the possibility to maintain storage at lower pressure. The formation of a hydride can be illustrated by a p–c–T diagram, where the hydrogen pressure (p) is plotted as a function of atomic ratio of hydrogen (c) at a given temperature (T), such as presented in Fig. 11. The p–c–T diagram illustrates the formation of solid solution (␣-phase) at low H-pressures

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Fig. 10. Molecular processes involved when storing hydrogen.

and atomic ratio [32], a pressure plateau where there is a continuing transformation from the solid solution ␣-phase to the metal hydride ␤-phase at constant pressure [33] and the growth of the ␤-phase with increasing hydrogen pressures and atomic ratio. The p–c–T diagram may show hysteresis when coming from high to low hydrogen content (decomposition) as compared to low to high hydrogen content (formation). This occurs if, for example, strain that has been accumulated in the sample is relaxed by plastic deformation. In Fig. 11 the first plateau corresponds to the absorption of hydrogen into pure magnesium and the second plateau denotes the absorption of hydrogen into the Mg2 Ni alloy. The absorption/desorption on hydrogen into this alloy is very fast and it is clear that microstructure plays an important role in the performance of the material. To ensure that large volumes of hydrogen can be stored it is essential to use small granules of the base material to make a large surface area available. When the hydride is formed in alloy, it has been seen that a surface passivation layer may be formed, consisting of a

Fig. 11. Characteristics of p–c–T diagram for hydrogen absorption/desorption processes on MgNi alloy.

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combination of the metal oxides, hydroxides, carbon–oxygen compounds and water. This surface passivation layer acts as a diffusion barrier and decreases the numbers of dissociation sites for H2 , and thus reduce the hydrogen uptake rate. Poisoning effects can also occur, where gaseous impurities in the reacting atmosphere are inhibiting the hydrogen dissociation. Very strong (SO2 , H2 S and CH3 SH) and strong gaseous poisons (O2 , CO2 and CO) has to be avoided, in contrast to CH4 , C2 H4 and N2 that have no measurable effect on the hydrogen absorption kinetics. The third problem with pure Mg as a hydrogen-storage device is the most serious one, thanks to the high enthalpy of formation of the hydride. Some improvement has been shown to occur by adding Ni to Mg. The desorption temperature of the Mg2 Ni alloy is about 50–60 ◦ C lower than for pure Mg, but this is still far from low enough. The addition of the heavy transition metal greatly reduces the storage capacity, down to 3.8% from 7.6% in pure Mg. A better solution would be to add a light metal that has low affinity to hydrogen, for example, aluminium or another light main group element that competes with hydrogen for the valence electrons of Mg. This strategy has apparently not been tried and will be pursued in a new proposed project. 3.2. Alanates and other hydrides Some of the lightest elements in the periodic table, for example, lithium, boron, sodium and aluminium, form stable and ionic compounds with hydrogen [34]. The hydrogen content reaches values of up to 18 mass% for LiBH4 . However, such compounds desorb the hydrogen only at temperatures from 80 to 600 ◦ C, and the reversibility of the reaction is not yet clear for all systems. Bogdanovic and Schwickardi [35] showed in 1996 that the decomposition temperature of NaAlH4 can be lowered by doping the hydride with TiO2 . The same group showed the reversibility of the reaction for several desorption/absorption cycles. This is a good example of the potential of such hydrides, which were discovered more than 50 years ago. Several points have to be clarified. First is the high desorption temperature due to the poor kinetics of the system or due to the thermodynamic stability of the compound. The kinetics can be improved by applying an appropriate catalyst to the system and apparently also by ball milling and the introduction of defects. Second, the conditions for a reversible reaction, for example, formation and stabilisation of clusters of an intermetallic compound of the remaining metals on desorption, and third is the desorption reaction and the intermediate reaction products (decomposition in several steps). These questions can be answered by the role of light hydrides in an on-board fuel cell. The exhaust gas of a fuel cell is water vapour, which could be collected and reused for on board hydrogen production. The common experiment [34] – shown in many chemistry classes – where a small piece of

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sodium floating on water produces hydrogen, demonstrates such a process. Sodium is transformed to sodium hydroxide in this reaction. The reaction is not reversible, but sodium hydroxide could later be removed and reduced in a solar furnace back to sodium. Each sodium atom produces one hydrogen atom, so the corresponding gravimetric hydrogen density of the sodium reaction is slightly more than 4 mass%. Lithium used in the same way would deliver up to 14 mass% of hydrogen. The alkali metals as a hydrogen source are easy to handle, and a car could be refilled within a few minutes. To deliver the necessary 4 kg of hydrogen using the water produced in the fuel cell would take 28 kg of lithium. After using up all the hydrogen the tank would contain 99 kg of lithium hydroxide, ready to be recycled [35]. Besides the alloys described above, there are several other families of intermetallics having a capability of hydrogen absorption, none of which has attained commercial interest. These include, for example, A2 B, AB3 , A2 B7 , A3 B, etc. The A elements are usually Ti, Zr, Hf, Th or a lanthanide (atomic number 57–71). The B elements can be a variety of transition and non-transition metals such as V, Cr, Mn and Fe. Some of these have good hydrogen-storage capacities but do not have favorable absorption/desorption characteristics. One promising, but quite expensive group is alloys based on vanadium. Especially, a combination of V–Ti–Fe is an attractive alternative. For example, (V0.9 Ti0.1 )0.95 Fe0.05 has a maximum hydrogen-storage capacity of 3.7 wt.% and a reversible hydrogen-storage capacity of 1.8 wt.% and still with good absorption properties [36], but the system is too heavy.

4. Conclusions Research is focused on solid-state hydrogen storage using gas on solid adsorption in materials such as high surface area carbons, or absorption in the interstices of a metal. In solidstate storage, gas on solids and metal hydrides are the options which are safer technologies and they provide high storage capacity than physical storage systems. There is research going on determining the hydrogen adsorption/desorption properties of available materials based on carbon structures and absorption/desorption on metals and metal alloys. Basic concept on mechanisms of solid-state storage is essential in order to get more knowledge about high-performance storage materials such as storage capacities, rates of charge and discharge, thermal and mechanical effects of available materials, increasing capability of manufacturing of new storage materials, like metal alloys and high surface area activated carbons. Materials systems, such as fullerenes, carbon nanotubes, light metal alloys, are very promising new materials they have a good potential in hydrogen storage. Although the lightweight Mg-class hydrides show high hydrogen-storage capacities even after extensive cycling they cannot for the present be regarded as a solution for vehicular application

mainly because of the slow kinetics in the lower temperature range. There is reason for hope that one day much better hydrogen-storage materials will be discovered and developed, rather as we have seen a revolution in high-temperature superconducting materials or hard permanent magnets. We must bear in mind that to develop a sustainable future energy policy requires us to focus not only on the scientific and technical challenge, but also on vital adaptations by the socioeconomic system and a change in attitudes to energy.

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Further reading [21] K. Bohmhammel, J. Garche, L. J¨orissen, T. Schliermann, W. Sch¨utz, R. Str¨obel, V. Trapp, G. Wolf, J. Power Sources 84 (1999) 221– 224. [22] T. Bose, R. Chanine, E. Poirier, Int. J. Hydrogen Energy 26 (2001) 831–835.