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Metallic hydrides; fundamental properties and applications
J. Phys. Chem. Solids Vol. 55. No.
1994
Elsetier Science Ltd Printed in Great Britain OOZZ-3697/94 $7.00 + 0.00
00223697(94)00159-6
METALLIC
12, pp. 1461-1470,
HYDRIDES; FUNDAMENTAL AND APPLICATIONS
PROPERTIES
G. G. LIBOWITZ G. G. Libowitz, Inc., P.O. Box 392, Morristown, NJ 07963, U.S.A. Abstract-Various types of metal hydrides are defined. The metallic hydrides which are usually formed by direct reaction of hydrogen gas with transition metals (Groups III-VIII, including rare earths and actinides) and their alloys and intermetallic compounds, are of most interest to materials scientists. Many of the metallic hydrides may also be formed by electrochemical reactions. Properties of these hydrides are discussed with emphasis on thermodynamics and phase relations. Several applications of metallic hydrides such as hydrogen storage, rechargeable batteries, hydrogen compressors, heat storage and heat pumps, isotope separation, powder metallurgy, sensors and activators, and hydrogen purification are briefly described. Keywords:
A. intermetallic compounds. A. metals, D. thermodynamic properties.
1. INTRODUCTION The term “metal hydrides” covers a very wide variety of materials, but it may be defined as compounds containing a metal-to-hydrogen bond. Metal hydrides may be divided into three groups, depending upon the nature of the metal-hydrogen bond, ionic, covalent or metallic. For the solid binary metal hydrides, the particular group is usually indicated by the position of the metal in the periodic table as shown in Fig. 1. Metals to the right of Group VIII form hydrides in which the M-H bond is covalent. They cannot be formed by direct reaction of metal and hydrogen gas, but rather by complex chemical reactions [l]. Beryllium also fits into this category. The alkali and alkaline earth metals form ionic hydrides. Hydrogen may be considered as the first member of the halogen group, and it exists as the Hion in these hydrides. Thus, NaH, for example, may be considered an alkali hydride. The properties of the alkali and alkaline earth hydrides are similar to those of the corresponding halides. Mg hydride has some properties of both ionic and covalent hydrides. As opposed to the ionic case, hydrogen enters the metal lattice as protons in the transition metals (Groups III-VIII including the rare earths and actinides). The presence of hydrogen has significant effects on the band structure of the metal [2-4] and introduces hydrogen induced states below the d (or f) band. The result is the formation of definite hydride phases with structures usually different from that of
the parent metal. Since these hydrides generally have metallic conductivities as well as other metallic properties, they are usually referred to as metallic hydrides to differentiate them from the other metal hydrides. However, some of the rare earth hydrides become semiconductors at high hydrogen contents [5]. The metallic hydrides can always be formed by direct reaction of the metal with hydrogen gas according to eqn (1): (2/n)M + H, + (2/n)MH,.
(1)
In some cases, the metallic hydride can be formed electrochemically according to the following reactions: (l/n)M + H+ + e- P (l/n)MH,,
(2)
in acid media, and (l/n)M + Hz0 +e-
P (l/n)MH,
+ OH-,
(3)
in alkaline media. Equation (1) is usually a spontaneous, exothermic, easily reversible reaction, i.e. when the metal is exposed to hydrogen gas, the hydride is formed with generation of heat. Heating the hydride drives the reaction in the reverse direction. Hence, metallic hydrides are sometimes called rechargeable or reversible metal hydrides. Equation (3) is the basis for the recently developed metal hydride battery and will be discussed in more detail in Section 3.
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G. G. LIBOWITZ
1462 IA
IIA
Fig. 1. Metals which form solid binary metal hydrides. ( ) Ionic hydrides; (8) covalent hydrides; and (0) metallic hydrides. *Denotes metals requiring hydrogen pressures greater than one atm (0.1 MPa) to form hydrides.
The Group III metals and the rare earths form fluorite-type dihydrides and trihydrides which are usually hexagonal. Eu forms only a dihydride which is actually ionic and similar in structure and properties to the alkaline earth hydrides. This is because of Hund’s rule which stabilizes the 4f shell so that only the two 6s electrons are available for bonding. Ytterbium dihydride appears to be ionic, but Yb also forms complex higher hydrides under higher hydrogen pressures [6]. The actinide metals form hydrides of different stoichiometrics. U and Pa form trihydrides having the /?-W structure; thorium forms a fluorite type dihydride and a higher hydride, Th,H,, , having a complex b.c.c. structure, and Np through Bk form dihydrides and trihydrides similar to the rare earths. The Group IV metals form dihydrides and a stabilized b-phase (b.c.c.) which may be viewed as a monohydride. The Group V metals form a group of nonstoichiometric “monohydrides” having a distorted b.c.c. metal structure with hydrogen atoms ordered in various configurations over tetrahedral or octahedral sites in the b.c.c. lattice. These phases and their complex phase diagrams are discussed in more detail by Schober and Wenzl [7]. V and Nb also form fluorite-type dihydrides. Except for Pd which forms a hydride with the NaCl-structure, the other hydride-forming metals in Groups VI-VIII (see Fig. 1) require high hydrogen pressures to form hydrides according to eqn (1). For example, the formation of NiH requires hydrogen pressures in excess of 0.6 GPa (6000 atm) [8] while the formation of molybdenum hydride requires hydrogen pressures of 23.5 GPa [9]. These high pressure hy-
drides are not of interest for hydride applications. All the metallic hydrides are nonstoichiometric, frequently exhibiting wide variations from the stoichiometric compositions. To avoid confusion, it should be mentioned here that there is extensive literature [lo] on what is referred to as “transition metal hydrides” which are different from the metallic hydrides discussed above. These are covalent hydrides in which the metalhydrogen bond is stabilized by a ligand, and they can range from simple carbonyls to complex organo-metallic compounds. As can be seen from Fig. 1, there are only about 25 transition metals that can easily form hydrides by eqn (1). (The high pressure hydrides and the higher actinides have been excluded.) Properties of metallic hydrides can be varied by forming solid solutions among these metals. Generally, the addition of a non-hydride-forming metal or Group VI-VIII metal to a Group III-V metal will decrease the maximum hydrogen content as well as the thermal stability of the resulting hydride. The properties of hydrides formed by solid solution alloys between two hydrideforming metals may sometimes have properties intermediate between the two corresponding binary metallic hydrides. However, more often, the properties of the alloy hydride cannot be easily deduced from the properties of the corresponding binary metallic hydrides. 1.l. Hydrides of inter-metallic compounds Because of the energy crises in the early 1970’s and the concept of a “hydrogen economy” [ 1l] there was a strong interest in developing new metallic hydrides
Metallic hydrides
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Table 1. Hydrides of intermetallic compounds Examples
Approximate no.
LaNi,H, ; YCo, H, ZrV,I&; BrFc,H,., CeNi, H, ; YFe, H4 s
20 15
Type Haucke phase (AB,)
Laves phase (AB,) PuNi,-type (AB,) Ce, N&-type (A2B,) (CsCl-CrB)-type (AB) Th,Mn,,-type (A6Bz~) MoSi,-type (A,B) Ti,Ni-type (A2B)
Y,Ni,H,; Th,Fe,H,, TiFeH,; ZrNiH, Y, Fe,, Hz,., Zr,PdH,, ; Hf, CuH,,, Ti,NiH; Hf,CoH,,,
for storing hydrogen and emphasis was placed on developing hydrides of intermetallic compounds which resulted in approximately 200 such hydrides. Every intermetallic compound that forms a hydride contains at least one metal which is a rare earth, actinide, or Group III, IV, or V hydride-former. The intermetallic compounds that form hydrides may generally be grouped in terms of their structure as shown in Table 1. which also includes the approximate number of hydrides in each group. Only those groups which are of most interest for applications will be discussed briefly here. A more detailed description of the properties of hydrides of intermetallic compounds may be obtained from a two-volume work edited by Schlapbach [12]. The Haucke-phases have a hexagonal structure and contain LaNi,; the hydride of this intermetallic compound has probably received more attention than any other. This group of hydrides was discovered at the Philips Eindhoven Laboratory [13] in 1970. The A in AB, is usually a Group III metal (including rare earths and Th), and B is usually a Group VIII metal. The Laves phase (AB2) hydrides have a complex cubic or hexagonal structure. The first Laves phase hydrides were discovered in 1966 [14], and about 75 such hydrides are known at the present time. A is usually a Group III, rare earth, or Group IV metal and B is usually a Group VIII metal, but it can also be a Group II. IV, VI or VII metal. The third group of interest is the CsCl-CrB structure type (AB). This includes TiFe, discovered in 1974 by Reilly and Wi.swall [ 151. The hydride of TiFe is probably the second most-studied intermetallic compound hydride (after LaNi,). This group also includes the hydride of ZrNi which was the first established hydride of an intermetallic compound in 1958 [16]. The A element in the AB group is a Group IV or rare earth metal and the B element is a Group VIII metal. Most intermetallic compounds retain their structure on hydride formation except for a discontinuous increase in lattice parameter, and sometimes a slight distortion of the structure. However, some compounds in the (AB) group undergo a structural change on hydride formation. For example, ZrCo changes from the cubic CsCl structure
25 15 15 15 6 5
to the orthorhombic CrB structure, while EuPd which has the CrB structure changes to the CsCl structure on hydriding. Others, such as TiFe retain their CsCl structure on hydriding, while ZrNi which originally has the CrB structure retains that structure on hydriding. The intermetallic compounds referred to in Table 1 (about 175) are just those that can be fit into a structure group. There are many other intermetallic compounds that form hydrides which cannot be so categorized. For example, in the Zr-Ni system, in addition to ZrNi, the compound Zr,Ni which has the CuAl, structure, also forms a hydride ZrrNiH,, [17] and the compound Zr,Ni,, which has a complex orthorhombic structure forms a hydride, Zr,Ni,,H,,
[181. Most of the 200 or so hydrides of intermetallic compounds can have their properties modified by substituting additional metals for those in the intermetallic compound to form ternary (or higher) hydrides. For example, if 10% of the Fe in TiFe is replaced by Ni to form TiFe,,Ni,,, the thermal stability of the hydride formed by the intermetallic compound is increased by about an order of magnitude [19]. Multiple substitutions also can be made. For example, in a recently investigated battery electrode material [20] based on LaNi,, 14% of the Ni was replaced by Co, and 16% by Al, while the La was replaced by mischmetal (Mm) which is an alloy of rare earth metals derived from the naturally occurring ore. The resulting electrode had the formula MmNi,, CO~.,A~~.~. Mm contains approximately 25-30% La, 45-55% Ce, 3-7% Pr, and l&20% Nd.
2. THERMODYNAMIC
PROPERTIES
The experimental technique most frequently utilized to obtain thermodynamic properties of metallic hydrides is measurement of equilibrium hydrogen pressures as a function of temperature and hydrogen content of the hydride. Pressure-composition isotherms such as those shown in Fig. 2 are obtained from such measurements. As hydrogen dissolves in the metal, the equilibrium hydrogen pressure increases until the solubility limit, x, is reached (for
G. G. LIBOWITZ
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x Hydrogento metal ratio (H/M) -
I I
I
n
S
Fig. 2. Pressure-composition isotherms for a metalhydrogen system. temperature, T,). The addition of more hydrogen results in formation of the nonstoichiometric hydride phase, MH,. Since there is now an additional phase in the system, the number of degrees of freedom decreases in accordance with the phase rule, and the hydrogen pressure remains constant across the range of hydrogen content from x to n, giving rise to a pressure plateau. As hydrogen is added to the system across the plateau (x to n), hydrogen saturated metal is converted to nonstoichiometric hydride, MH,. After the metal phase has been completely converted to hydride, further addition of hydrogen above the composition, n, results in an increase in hydrogen pressure as hydrogen dissolves in the hydride phase and the composition approaches the stoichiometric value, s. With increase in temperature, the solubility of hydrogen in the metal increases and the homogeneity range of the hydride phase also widens. Thus, phase diagrams may be deduced from such measurements. Assuming the thermodynamic activities of a solid to be unity, the integrated van? Hoff equation for eqn (1) may be written [21]: R In PH2= (AH/T)
- AS,
(4)
where PH2is the plateau pressure, and AH and AS are the enthalpy and entropy of hydride formation. Therefore, a plot of In PH2 vs reciprocal temperature should yield a straight line whose slope will yield a value for AH and intercept a value for AS. Actually, the thermodynamic properties obtained in this manner represent the formation of the nonstoichiometric hydride from the hydrogen-saturated metal and eqn (1) should actually be written: [2/(n - x)lMH,(s.s.)
+ H,(g) P ]2/(n -
As Seen from Fig. 2, however, as the temperature increases, the value of x increases and the value of n decreases. Nevertheless, straight lines are obtained in In P vs l/T plots over wide temperature ranges as illustrated, for example, for the GdH,_ 6 system in Fig. 3 which shows the results of two separate studies [22,23]. The AH values thus obtained agree with calorimetric measurements. In a detailed thermodynamic analysis, Rudman [24] showed that for hydrides which form exothermally, the enthalpies of hydride formation obtained from van’t Hoff plots will remain reasonably constant with temperature and agree with those obtained from calorimetric measurements. It should be mentioned here that the interactions of intermetallic compounds with hydrogen are treated as pseudo-binary systems; i.e. M in eqns (1) and (5) may represent an intermetallic compound as well as a single metal. This sometimes gives rise to problems as will be discussed in Section 3. Van’t Hoff plots are an indication of the thermal stability of a metallic hydride; several such plots are shown in Fig. 4. Generally, the Group III and rare earth hydrides are the most stable and the hydrides of intermetallic compounds are the least stable. Many metals and intermetallic compounds may form more than one hydride. This results in multiple plateaux in the pressure-composition isotherms as shown in Fig. 5 for the Nb-H system which forms a monohydride (b.c.c.) and a dihydride. Consequently, two (or more) van? Hoff plots are obtained as shown for the Nb-H system in Fig. 4. The equation for formation of the monohydride (i.e. the stabilized b.c.c. phase) is the same as eqn (S), but the second plateau
x)lMK. (5)
IO4
I
I
0.85
I 0.90
I 0.95
I 1.00
I 1.05
1.10
1.15
1000/2-K
Fig. 3. Van? Hoff plot for the gadolinium hydrogen system (Gd-GdH,): (0) Sturdy and Mulford, Ref. 22; and (0) Libowitz and Pack, Ref. 23.
Metallic hydrides
1465
H/M Atomratio
Fig. 6. Pressure
lo4 II
11 1
1.5
I
I
I
I
2
2.5
3
3.5
lOOO/T”K Fig. 4. Van? Hoff plots for formation of various metallic hydrides. The compositions shown are the stoichiometric values, but in almost every case, the hydride is hydrogen deficient (see Fig. 2 and eqn 5).
in Fig. 5 represents the formation of a nonstoichiometric (hydrogen deficient) dihydride from nonstoichiometric (hydrogen excess) monohydride:
3. APPLICATIONS
3.1. Practical problems Before describing specific applications, it should be pointed out that the above discussion dealt with the
41~
,
,
I
,
,
I
I
,
(
I
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
H/M Atom ratio
Fig. 5. Pressure composition isotherms for the niobium hydrogen system illustrating the formation of two hydrides. B is a monohydride with a distorted b.c.c. structure, and y is a dihydride with the fluorite structure.
ideal situation. Actually, there are several properties that give rise to problems in the utilization of metallic hydrides. A few of them will be mentioned here. It was stated previously that eqn (1) is a spontaneous reaction. This is sometimes true, but usually the metal requires activation before absorbing hydrogen. This is generally attributed to an oxide layer on the surface of the metal which inhibits catalytic dissociation of the H, molecule and also prevents its entry into the metal. Activation usually can be effected by heating the metal in a vacuum, admitting hydrogen gas at pressures in excess of the dissociation pressure and cooling. Because of the expansion of the metal on hydride formation (the volume of a metallic hydride is always greater than that of the metal) cracking occurs thus exposing new metal surface to the hydrogen gas. Sometimes it is necessary to repeat this process more than once in order to fully activate the metal. In many metal-hydrogen systems it has been observed that the apparent equilibrium pressure on absorption of hydrogen is higher than the corresponding pressure on desorption as illustrated in Fig. 6. Such behavior obviously violates the phase rule and is, therefore, indicative of a nonequilibrium situation. However, such isotherms are quite reproducible. This phenomenon is referred to as hysteresis. Examples of this in real systems are shown in Fig. 7 for the vanadium-hydrogen system and an alloyhydrogen system consisting of 80% Nb and 20% V. There have been many theories proposed to explain hysteresis [25]. However, it is generally accepted that it is the result of lattice strain and plastic deformation due to expansion of the lattice on hydride formation. Because the plateaux of absorption isotherms are usually more sloped with less well-defined end points relative to the desorption isotherm plateaux (as illustrated in Fig. 7) it has usually been assumed that the
G. G. LIBOWITZ
1466
favored. Reaction (6) requires rearrangement of metal atoms which is unlikely to occur at low temperatures; whereas the hydride formation reaction (6) involves little motion of metal atoms. However, at elevated temperatures, reaction (7) is more likely to occur, e.g. at 300°C [27]. Even at lower temperatures, disproportionation occurs after many hydrogen absorption-desorption cycles. Factors affecting disproportionation are discussed by Sandrock et al.
LO-
2.0 -
2
l.O-
i3 g
0.5 -
i?! a
PI.
8 0.2-
3.2. Hydrogen storage
3 3
O.l-
0.05 -
0.021 1 1 1 1 1 1 1 ’ 1 ’ 1 1 1.4 1.6 1.8 2.0 0.8 1.0 1.2 HIM
Fig. 7. Pressure composition isotherms showing hysteresis for the reaction in the V-H and Nb,,,V,,r-H system. (0, 0) absorption and desorption isotherms for the VH-VH,_, system; and (m, 0) absorption and desorption isotherms for the Nb,,8V,,H-Nb,,,V,2H,_6 system. [From: Lynch J. F., Libowitz G. G. and Maeland A. J. J. Less-Common Met. 103, 117 (1984).] desorption
isotherm
represents
the true equilibrium.
Also, it is frequently more difficult to attain reproducible experimental points on the absorption isotherms. However, Flanagan and Clewley [26] proposed that the dislocation energies causing the plastic deformation were approximately the same for hydride formation and decomposition. If so, neither the absorption nor the desorption isotherms represent the true equilibrium; rather it must lie somewhere in between. Although hydrides of intermetallic compounds are treated as pseudo-binary systems, the formation of most (if not all) hydrides of intermetallic compounds is metastable with respect to disproportionation of the intermetallic compound itself. For example, the free energy of the reaction: LaNi, + Hz + LaH, + 5Ni,
The first major application of metallic hydrides came with the advent of nuclear reactor technology. Hydrogen is an effective neutron moderator (i.e. it slows down neutrons with minimum absorption) and since metal hydrides can store hydrogen very efficiently as shown in Table 2, metal hydrides have been used as moderators in nuclear reactors [29]. As seen in Table 2, the number of hydrogen atoms per unit volume in most metallic hydrides is higher than in liquid hydrogen and, in some cases, even greater than in solid hydrogen. Therefore, the volume requirements for storing hydrogen as a hydride is much smaller than storage as liquid hydrogen or compressed gas. The aforementioned “hydrogen economy” [l I] which required the storage of hydrogen as a fuel for vehicles or for load leveling applications resulted in the development of many new metallic hydrides. The use of metallic hydrides is also an unusually safe method of storing hydrogen because they are generally quite stable below their dissociation temperature. Also, since the reverse of eqn (1) is an endothermic reaction, the self-cooling effect will suppress any loss of hydrogen if a leak develops in the storage system. However, metallic hydrides were found to be too heavy for use in automobiles (although for fleet vehicles, they are still under consideration), and their utilization in load leveling applications proved uneconomical. Nevertheless, metallic hydrides are used to store
hydrogen
pure
laboratory
for specialty hydrogen
such as
others
[30].
(6)
is - 104 kJ mol-’ H, as compared to AGlg8= 0 for the hydride formation reaction:
Table 2. Hydrogen densities in some hydrogen containing compounds Compound
LaNi, + 3H, e LaNi, H,.
applications, among
(7)
Since the free energy of reaction (6) is considerably more negative than reaction (7), disproportionation of LaNi, would occur rather than hydride formation. However, reaction (7) occurs because it is kinetically
Liquid hydrogen (20°K) Solid hydrogen (4.2”K) MgH, TiH, VH, PdH,, FeTiH, LaNi,H,,
No. of H atoms cm-3 x IO** 4.2 5.3 5.9 9.2 10.4 4.7 6.0 6.1
Metallic hydrides
Cl.,2.1 2.8
2.9
3.0
3.1
3.2
3.3
3.4
1OOWT”K 8. Van’t Hoff plots for LaNi,H,_, and Fig. V, ssTiO IOFe,,, H, 6 illustrating the use of metallic hydrides for hydrogen compression.
3.3. Hydrogen compressors As indicated by the van? Hoff equation (eqn 4) and Fig, 4, the hydrogen pressure in equilibrium with a metallic hydride increases exponentially with temperature and, therefore, metallic hydrides can be used for hydrogen compression. Hydrogen gas is absorbed at low temperature and pressure and desorbed at high temperature and pressure. As can be seen from eqn (4), the higher the value of AH, the higher the slope and therefore the greater the rise in pressure over a given temperature range. This is illustrated in Fig. 8 which shows van? Hoff plots for hydrides of LaNi, (AH = 30.9 kJ mol-’ HZ) and a solid solution alloy of and iron, V0,s5Ti0,r0 Fe,,, vanadium, titanium, (AH = 42.9 kJ mol-’ H,). Although, both hydrides have a dissociation pressure of =0.2 MPa at room temperature. the latter hydride, having the higher value of AH reaches a pressure 2.5 that of LaNi, hydride at 95°C. The advantages of hydride compressors over mechanical hydrogen compressors are that they are vibration-free and quiet, and can be operated with low grade waste heat or solar energy [30].
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alloys, but the concept was extended to utilize pairs of metallic hydrides in a heat pump [32]. A metal-hydride heat pump consists of two hydrides with dissimilar thermal stabilities in sealed containers which are arranged so that hydrogen gas can flow freely from one hydride to the other. This type of heat pump can be used for heat amplification, temperature upgrade, and cooling or refrigeration. The principle behind the operation of the hydride heat pump can best be understood from van? Hoff plots of two hydrides as shown in Fig. 9. For the case of air conditioning, cooling would be a result of the dissociation of the less stable hydride (an endothermic reaction) as hydrogen flows from the fully hydrided M,H, at point A to the initially hydrogen depleted M,. at point B to form the more-stable hydride M,H,. By holding M,H, at T, (usually the ambient temperature), P2 > P, and hydrogen will continue to flow from A to B so that the low temperature, T,, will be maintained. When the hydrogen content of MzH,, has been exhausted, the hydride is recharged by allowing it to return to the ambient temperature, T,,, , and heating the hydride, M, H, to T,, (e.g. using solar energy, hot water, etc.) and maintaining it at that temperature so that hydrogen will flow back to M2H,. (from point C to point D. since P4 > P3). By operating an identical two-hydride system in tandem, continuous cooling can be obtained. The same arrangement may be used for refrigeration using hydrides of different thermal stabilities. For example, M2H, would be less stable than the hydride used for air conditioning, i.e. one which dissociates near 0°C. For heat amplification, T, could be the temperature of cold water or the outside ambient temperature on
3.4. Metal hydride heat pumps The relatively high heats of formation of metallic hydrides plus the ease of reversibility of eqn (1) led to the proposal that they may be used to store thermal energy in a home [31]. This proved to be uneconomical because of the high cost of the hydride
1ITK Fig. 9. Van’t Hoff plots illustrating the operation of a metal hydride heat pump for cooling (air conditioning or refrigeration) and for heat amplification.
G. G. LIBOWITZ
1468
cool days, e.g. 15°C and T,, could be the temperature of hot water, about 85°C. Thus, space heating would be obtained at point B when hydrogen flows from point A, and also at point D during recharging of the less stable hydride. The use of heat pumps for temperature upgrade is illustrated in Fig. 10. In this case, waste heat at a temperature T,,, is used to discharge the less stable hydride, M2Hy, at point A while hydrogen is absorbed by the more stable hydride, M, H,, at point B to generate the usable higher temperature, r,. The hydride Mz H, is regenerated by allowing it to cool to room temperature, T,, and using the waste heat at T,,, to cause hydrogen flow from point C to point D. 3.5. Hydride batteries The use of metallic hydrides in rechargeable batteries is based on eqn (3) where the hydride acts as the negative electrode. The positive electrode is nickel hydroxide undergoing the reaction: Ni(OH2) + OH- $ NiOOH + H,O + ee, to form the oxyhydroxide of nickel. The overall cell reaction is then:
(l/n)M + Ni(OH), Eharp (l/n)MH,
+ NiOOH.
discharge
The impetus for the development of the metal hydride battery was replacement of the nickel-
cadmium battery, since Cd is a toxic metal. However, the metal hydride battery also has higher capacities and longer run times than the nickel-cadmium battery. Most metal hydride electrodes are based on the AB, Haucke phase or the Laves phase intermetallics (see Table 1). 3.6. Isotope separation The use of metallic hydrides for the separation of deuterium and tritium from hydrogen is based on the fact that the thermal stabilities of the deuterides and tritides are usually different from those of the protides. Normally, the protide of a metal is more stable than the corresponding deuteride or tritide. However, there are some metal hydrides that exhibit a small inverse isotope effect in which the hydride involving the heavier isotope is more stable. For the case of vanadium dihydride, this inverse isotope effect may be quite large (e.g. at 40°C the dissociation pressure of the deuteride is almost one-third that of the hydride) [33]. This is due to the much more negative values of AH for formation of the deuteride with respect to those of the dihydride (i.e. - 50.2 kJ mol-’ H, as compared to - 40.1 kJ mol-’ H,). A scheme for isotope separation has been devised [34] based on the fact that there is a crossover temperature at which the protide becomes more stable [34]. 3.7. Hydrogen separation and purljication Most hydride-forming metals and alloys will selectively absorb hydrogen. Consequently, they can be used to remove hydrogen from mixtures of gases. The hydrogen that is subsequently desorbed from the hydride is essentially free of impurities. The most serious problem utilizing this technique for separation is poisoning of the hydride alloy by other gases present in the hydrogen gas that will retard the rate of hydrogen absorption as well as decrease the total storage capacity for hydrogen. Hydrogen gas has been successfully recovered from ammonia purge gas streams on a large scale [35]. The purge gas contained 20% N,, 12% methane, 5% argon and about 3% ammonia and the hydride-forming alloy used was LaNi,. 3.8. Power metallurgy
l/Z-K
Fig. 10.Van’t Hoff plots illustrating the operation of a metal hydride heat pump for temperature upgrade.
The rapid expansion of metals on hydride formation will usually produce either powdered hydride or friable particles that can be easily made into powder by further comminution. Subsequent removal of the hydrogen by heating in a vacuum will result in finely powdered metals. This process has been used to prepare powdered metals of Ti, Th, Zr, etc. for further compaction into desired shapes [36]. More recently, this technique has been used to pre-
Metallic hydrides pare powdered alloys in the production of permanent magnet materials such as SmCo, and Nd2Fe,,B [37]. It has also been proposed for the preparation of metallic glass alloy powders [38].
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reaction, poisoning of the hydride by impurities in the hydrogen gas, and sloping isotherms. These factors are discussed in Ref. [30] as well as more detailed discussions on some of the applications.
3.9. Sensors and activators Because of the exponential rise in hydrogen pressure with temperature (e.g. see Fig. 7), metallic hydrides may be used as temperature sensors and activators. One device [39] which had been used for many years to detect fires in aircraft engines consists of a capillary tube, containing a metal hydride, which is inserted in the engine. The other end of the capillary tube is connected to a pressure sensitive switch. When the tube is in the vicinity of a fire, the sudden rise in temperature causes rapid release of hydrogen gas from the hydride thus increasing the pressure in the tube and tripping the pressure sensitive switch that, in turn, sets off an alarm. 3.10. Heat engines A simple extension of the hydride compressor concept leads to the design of a heat engine based on metallic hydrides. The high pressure hydrogen gas produced by heating the metallic hydride may be expanded through a turbine to generate electricity. The hydrogen gas is then re-absorbed in the hydride in preparation for the next cycle, thus resulting in an engine that converts thermal to electrical energy [40]. The expanding high pressure hydrogen gas also may be used to generate mechanical energy by driving pistons [41] or performing work by moving an impermeable diaphragm [42]. 3. I 1. Hydrogen getters Metallic hydrides may be used as getters to remove all traces of hydrogen gas from special vacuum tubes, for example. Also, with the advent of thermonuclear fusion technology, including weapons systems, it is important to remove and collect tritium gas from some systems for safety reasons because of the radioactivity of the isotope. A hydrogen getter material should be able to react rapidly with the gas and also have high thermal stability (i.e. low dissociation pressure). Most getter materials that are presently in use are alloys of zirconium, e.g. ZrNi, ZrV,, ZrAl,, with dissociation pressures ranging from 10m5to less than IO-’ MPa at room temperature. Finally, it should be pointed out that in the discussions of the use of metallic hydrides for various applications, only idealized situations were considered. In addition to some of the practical problems discussed above, there are many other factors that must be taken into consideration such as heat transfer problems which is the major factor affecting rates of
REFERENCES 1. Siegel B. and Libowitz G. G., in Me&l Hydrides (Edited by W. M. Mueller, J. P. Blackledge and G. G. Libowitz), p. 546. Academic Press, New York (1968). 2. Switendick A. C., in Hydrogen in Metals (Edited by G. Alefeld and J. Volkl), Vol. 1. p. 101. Springer-Verlag, Berlin (1978). D. A., in Metal Hydrides (Edited 3. Papaconstantopoulos by G. Bambakidis). p. 215. Plenum Press, New York (1980). Fukai Y., The Metal Hvdrogen System: Basic Bulk Properties, p. 300. Springer-Verlag, Berlin (I 992). Libowitz G. G., Ber. Bunsenges, Phys. Chem. 76, 837 (1972). Iwasieczko W., Drulis H. and Frak R. M., J. Alloys Comp. 180, 265 (1992). Schober T. and Wenzl H., in Hydrogen in Metals (Edited by G. Alefeld and J. Volkl), Vol. 2, p. II. Springer-Verlag, Berlin (1978). 8. Baranowski B.. in Ref. 7, Vol. 2, p. 158. 9. Belash I. T.. Antonov V. E. and Ponyatovskii E. G., Proc. Acad. Sci., USSR (Engl. Transl.) 235, 665 (1977); Ponyatovskii, E. G., Antonov. V. E. and Belash, I. T.. Inorg. Mater. (Engl. Transl.) 14, 1227 (1978). 10. see, for example, Muetterties E. L., Transition Metal Hvdrides. Marcel Dekker. Inc.. New York (1971): or btalytic Transition Metal Hydrides, (Edited by D. W. Slocum and W. R. Moser), Vol. 415. Annals of N.Y. Academy of Science (1983). Il. Gregory D. P., Sci. Am. 228, 1 (1973); Winsche W. E., Hoffman K. C. and Salzano F. J., Science 180, 1325 (1973). L.. Hvdrogen in Intermetallic Compounds. 12. Schlapbach Vols 1 and 2. Springer-Verlag, Berlin (1988) and-(1992). 13. van Vucht J. H. N., Kuijpers F. A. and Bruning H. C. A. M., Philips Rex Rpts 25, 133 (1970). E. A., Electrochem. Technol. 14. Pebler A. and Gulbransen 4.211 (1966): Trans. Met. Sot. AIME 289. 1593 (1967). 15. Reilly J, J. and Wiswall R. H., Inorg. ehem. i3, 218 (1974). 16. Libowitz G. G., Hayes H. F. and Gibb T. R. P., J. Phys. Chem. 62, 76 (1958). 17. Chikdene A.,. Baudry A., Boyer P., Miraglia S., Fruchart D. and Soubeyroux J. L.. Z. Phvs. 18
19
20
21
22. 23. 24. 25. 26.
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G. G. LIBOWITZ
27. Cohen R. L., West K. W. and Wernick J. H., J. LessCommon Mer. 73, 273 (1980). 28. Sandrock G, D., Goode11 P. D., Huston E. L. and Golben P. M., Z. Phys. Chem. N.F. 164, 1285 (1989). 29. Mueller W. M. in Ref. 1, pp. 23-38. 30. Sandrock G., Suda S. and Schlapbach L., in Ref. 12, Vol. 2, p. 197. 31. Libowitz G. G., Proc. 9th Intersoc. Energy Conversion Engng Conf, p. 332. Am. Inst. Chem. Eng., New York (1974). 32. Gruen D. M., Mendelsohn M. H. and Sheft I., Solar Energy 21, 153 (1978). 33. Wiswall R. H. and Reilly J. J., Inorg. Chem. 11, 1691 (1972). 34. Levinson S., Libowitz G. G. and Maeland A. J., U.S. Patent 4,659,554, April 21 (1987).
35. Sheridan J. J., Eisenberg F. G., Greskovich E. J., Sandrock G. D. and Huston E. L., J. Less-Common Met. 89, 447 (1983). 36. Blackledge J. P., in Ref. 1, p. 12. 37. Harris I. R. and McGuiness P. J., J. Less-Common Met. 174, 1273 (1991). 38. Maeland A. J. and Libowitz G. G., Mater. Len. 1, 3 (1982). 39. Warren D. E., Faughnan K. A., Fellows R. A., Godden J. W. and Seek B. M., J. Less-Common Met. 104, 375 (1984). 40. Powell J. R., Salzano F. J., Yu W. S. and Milau J., Science 193, 314 (1976). 41. Nomura K., Ishido Y. and Ono S., Energy Conversion 19, 49 (1978). 42. Northrup C. J. M. and Heckes A. A., J. Less-Common Met. 74, 419 (1980).
1994
Elsetier Science Ltd Printed in Great Britain OOZZ-3697/94 $7.00 + 0.00
00223697(94)00159-6
METALLIC
12, pp. 1461-1470,
HYDRIDES; FUNDAMENTAL AND APPLICATIONS
PROPERTIES
G. G. LIBOWITZ G. G. Libowitz, Inc., P.O. Box 392, Morristown, NJ 07963, U.S.A. Abstract-Various types of metal hydrides are defined. The metallic hydrides which are usually formed by direct reaction of hydrogen gas with transition metals (Groups III-VIII, including rare earths and actinides) and their alloys and intermetallic compounds, are of most interest to materials scientists. Many of the metallic hydrides may also be formed by electrochemical reactions. Properties of these hydrides are discussed with emphasis on thermodynamics and phase relations. Several applications of metallic hydrides such as hydrogen storage, rechargeable batteries, hydrogen compressors, heat storage and heat pumps, isotope separation, powder metallurgy, sensors and activators, and hydrogen purification are briefly described. Keywords:
A. intermetallic compounds. A. metals, D. thermodynamic properties.
1. INTRODUCTION The term “metal hydrides” covers a very wide variety of materials, but it may be defined as compounds containing a metal-to-hydrogen bond. Metal hydrides may be divided into three groups, depending upon the nature of the metal-hydrogen bond, ionic, covalent or metallic. For the solid binary metal hydrides, the particular group is usually indicated by the position of the metal in the periodic table as shown in Fig. 1. Metals to the right of Group VIII form hydrides in which the M-H bond is covalent. They cannot be formed by direct reaction of metal and hydrogen gas, but rather by complex chemical reactions [l]. Beryllium also fits into this category. The alkali and alkaline earth metals form ionic hydrides. Hydrogen may be considered as the first member of the halogen group, and it exists as the Hion in these hydrides. Thus, NaH, for example, may be considered an alkali hydride. The properties of the alkali and alkaline earth hydrides are similar to those of the corresponding halides. Mg hydride has some properties of both ionic and covalent hydrides. As opposed to the ionic case, hydrogen enters the metal lattice as protons in the transition metals (Groups III-VIII including the rare earths and actinides). The presence of hydrogen has significant effects on the band structure of the metal [2-4] and introduces hydrogen induced states below the d (or f) band. The result is the formation of definite hydride phases with structures usually different from that of
the parent metal. Since these hydrides generally have metallic conductivities as well as other metallic properties, they are usually referred to as metallic hydrides to differentiate them from the other metal hydrides. However, some of the rare earth hydrides become semiconductors at high hydrogen contents [5]. The metallic hydrides can always be formed by direct reaction of the metal with hydrogen gas according to eqn (1): (2/n)M + H, + (2/n)MH,.
(1)
In some cases, the metallic hydride can be formed electrochemically according to the following reactions: (l/n)M + H+ + e- P (l/n)MH,,
(2)
in acid media, and (l/n)M + Hz0 +e-
P (l/n)MH,
+ OH-,
(3)
in alkaline media. Equation (1) is usually a spontaneous, exothermic, easily reversible reaction, i.e. when the metal is exposed to hydrogen gas, the hydride is formed with generation of heat. Heating the hydride drives the reaction in the reverse direction. Hence, metallic hydrides are sometimes called rechargeable or reversible metal hydrides. Equation (3) is the basis for the recently developed metal hydride battery and will be discussed in more detail in Section 3.
1461
G. G. LIBOWITZ
1462 IA
IIA
Fig. 1. Metals which form solid binary metal hydrides. ( ) Ionic hydrides; (8) covalent hydrides; and (0) metallic hydrides. *Denotes metals requiring hydrogen pressures greater than one atm (0.1 MPa) to form hydrides.
The Group III metals and the rare earths form fluorite-type dihydrides and trihydrides which are usually hexagonal. Eu forms only a dihydride which is actually ionic and similar in structure and properties to the alkaline earth hydrides. This is because of Hund’s rule which stabilizes the 4f shell so that only the two 6s electrons are available for bonding. Ytterbium dihydride appears to be ionic, but Yb also forms complex higher hydrides under higher hydrogen pressures [6]. The actinide metals form hydrides of different stoichiometrics. U and Pa form trihydrides having the /?-W structure; thorium forms a fluorite type dihydride and a higher hydride, Th,H,, , having a complex b.c.c. structure, and Np through Bk form dihydrides and trihydrides similar to the rare earths. The Group IV metals form dihydrides and a stabilized b-phase (b.c.c.) which may be viewed as a monohydride. The Group V metals form a group of nonstoichiometric “monohydrides” having a distorted b.c.c. metal structure with hydrogen atoms ordered in various configurations over tetrahedral or octahedral sites in the b.c.c. lattice. These phases and their complex phase diagrams are discussed in more detail by Schober and Wenzl [7]. V and Nb also form fluorite-type dihydrides. Except for Pd which forms a hydride with the NaCl-structure, the other hydride-forming metals in Groups VI-VIII (see Fig. 1) require high hydrogen pressures to form hydrides according to eqn (1). For example, the formation of NiH requires hydrogen pressures in excess of 0.6 GPa (6000 atm) [8] while the formation of molybdenum hydride requires hydrogen pressures of 23.5 GPa [9]. These high pressure hy-
drides are not of interest for hydride applications. All the metallic hydrides are nonstoichiometric, frequently exhibiting wide variations from the stoichiometric compositions. To avoid confusion, it should be mentioned here that there is extensive literature [lo] on what is referred to as “transition metal hydrides” which are different from the metallic hydrides discussed above. These are covalent hydrides in which the metalhydrogen bond is stabilized by a ligand, and they can range from simple carbonyls to complex organo-metallic compounds. As can be seen from Fig. 1, there are only about 25 transition metals that can easily form hydrides by eqn (1). (The high pressure hydrides and the higher actinides have been excluded.) Properties of metallic hydrides can be varied by forming solid solutions among these metals. Generally, the addition of a non-hydride-forming metal or Group VI-VIII metal to a Group III-V metal will decrease the maximum hydrogen content as well as the thermal stability of the resulting hydride. The properties of hydrides formed by solid solution alloys between two hydrideforming metals may sometimes have properties intermediate between the two corresponding binary metallic hydrides. However, more often, the properties of the alloy hydride cannot be easily deduced from the properties of the corresponding binary metallic hydrides. 1.l. Hydrides of inter-metallic compounds Because of the energy crises in the early 1970’s and the concept of a “hydrogen economy” [ 1l] there was a strong interest in developing new metallic hydrides
Metallic hydrides
1463
Table 1. Hydrides of intermetallic compounds Examples
Approximate no.
LaNi,H, ; YCo, H, ZrV,I&; BrFc,H,., CeNi, H, ; YFe, H4 s
20 15
Type Haucke phase (AB,)
Laves phase (AB,) PuNi,-type (AB,) Ce, N&-type (A2B,) (CsCl-CrB)-type (AB) Th,Mn,,-type (A6Bz~) MoSi,-type (A,B) Ti,Ni-type (A2B)
Y,Ni,H,; Th,Fe,H,, TiFeH,; ZrNiH, Y, Fe,, Hz,., Zr,PdH,, ; Hf, CuH,,, Ti,NiH; Hf,CoH,,,
for storing hydrogen and emphasis was placed on developing hydrides of intermetallic compounds which resulted in approximately 200 such hydrides. Every intermetallic compound that forms a hydride contains at least one metal which is a rare earth, actinide, or Group III, IV, or V hydride-former. The intermetallic compounds that form hydrides may generally be grouped in terms of their structure as shown in Table 1. which also includes the approximate number of hydrides in each group. Only those groups which are of most interest for applications will be discussed briefly here. A more detailed description of the properties of hydrides of intermetallic compounds may be obtained from a two-volume work edited by Schlapbach [12]. The Haucke-phases have a hexagonal structure and contain LaNi,; the hydride of this intermetallic compound has probably received more attention than any other. This group of hydrides was discovered at the Philips Eindhoven Laboratory [13] in 1970. The A in AB, is usually a Group III metal (including rare earths and Th), and B is usually a Group VIII metal. The Laves phase (AB2) hydrides have a complex cubic or hexagonal structure. The first Laves phase hydrides were discovered in 1966 [14], and about 75 such hydrides are known at the present time. A is usually a Group III, rare earth, or Group IV metal and B is usually a Group VIII metal, but it can also be a Group II. IV, VI or VII metal. The third group of interest is the CsCl-CrB structure type (AB). This includes TiFe, discovered in 1974 by Reilly and Wi.swall [ 151. The hydride of TiFe is probably the second most-studied intermetallic compound hydride (after LaNi,). This group also includes the hydride of ZrNi which was the first established hydride of an intermetallic compound in 1958 [16]. The A element in the AB group is a Group IV or rare earth metal and the B element is a Group VIII metal. Most intermetallic compounds retain their structure on hydride formation except for a discontinuous increase in lattice parameter, and sometimes a slight distortion of the structure. However, some compounds in the (AB) group undergo a structural change on hydride formation. For example, ZrCo changes from the cubic CsCl structure
25 15 15 15 6 5
to the orthorhombic CrB structure, while EuPd which has the CrB structure changes to the CsCl structure on hydriding. Others, such as TiFe retain their CsCl structure on hydriding, while ZrNi which originally has the CrB structure retains that structure on hydriding. The intermetallic compounds referred to in Table 1 (about 175) are just those that can be fit into a structure group. There are many other intermetallic compounds that form hydrides which cannot be so categorized. For example, in the Zr-Ni system, in addition to ZrNi, the compound Zr,Ni which has the CuAl, structure, also forms a hydride ZrrNiH,, [17] and the compound Zr,Ni,, which has a complex orthorhombic structure forms a hydride, Zr,Ni,,H,,
[181. Most of the 200 or so hydrides of intermetallic compounds can have their properties modified by substituting additional metals for those in the intermetallic compound to form ternary (or higher) hydrides. For example, if 10% of the Fe in TiFe is replaced by Ni to form TiFe,,Ni,,, the thermal stability of the hydride formed by the intermetallic compound is increased by about an order of magnitude [19]. Multiple substitutions also can be made. For example, in a recently investigated battery electrode material [20] based on LaNi,, 14% of the Ni was replaced by Co, and 16% by Al, while the La was replaced by mischmetal (Mm) which is an alloy of rare earth metals derived from the naturally occurring ore. The resulting electrode had the formula MmNi,, CO~.,A~~.~. Mm contains approximately 25-30% La, 45-55% Ce, 3-7% Pr, and l&20% Nd.
2. THERMODYNAMIC
PROPERTIES
The experimental technique most frequently utilized to obtain thermodynamic properties of metallic hydrides is measurement of equilibrium hydrogen pressures as a function of temperature and hydrogen content of the hydride. Pressure-composition isotherms such as those shown in Fig. 2 are obtained from such measurements. As hydrogen dissolves in the metal, the equilibrium hydrogen pressure increases until the solubility limit, x, is reached (for
G. G. LIBOWITZ
1464
x Hydrogento metal ratio (H/M) -
I I
I
n
S
Fig. 2. Pressure-composition isotherms for a metalhydrogen system. temperature, T,). The addition of more hydrogen results in formation of the nonstoichiometric hydride phase, MH,. Since there is now an additional phase in the system, the number of degrees of freedom decreases in accordance with the phase rule, and the hydrogen pressure remains constant across the range of hydrogen content from x to n, giving rise to a pressure plateau. As hydrogen is added to the system across the plateau (x to n), hydrogen saturated metal is converted to nonstoichiometric hydride, MH,. After the metal phase has been completely converted to hydride, further addition of hydrogen above the composition, n, results in an increase in hydrogen pressure as hydrogen dissolves in the hydride phase and the composition approaches the stoichiometric value, s. With increase in temperature, the solubility of hydrogen in the metal increases and the homogeneity range of the hydride phase also widens. Thus, phase diagrams may be deduced from such measurements. Assuming the thermodynamic activities of a solid to be unity, the integrated van? Hoff equation for eqn (1) may be written [21]: R In PH2= (AH/T)
- AS,
(4)
where PH2is the plateau pressure, and AH and AS are the enthalpy and entropy of hydride formation. Therefore, a plot of In PH2 vs reciprocal temperature should yield a straight line whose slope will yield a value for AH and intercept a value for AS. Actually, the thermodynamic properties obtained in this manner represent the formation of the nonstoichiometric hydride from the hydrogen-saturated metal and eqn (1) should actually be written: [2/(n - x)lMH,(s.s.)
+ H,(g) P ]2/(n -
As Seen from Fig. 2, however, as the temperature increases, the value of x increases and the value of n decreases. Nevertheless, straight lines are obtained in In P vs l/T plots over wide temperature ranges as illustrated, for example, for the GdH,_ 6 system in Fig. 3 which shows the results of two separate studies [22,23]. The AH values thus obtained agree with calorimetric measurements. In a detailed thermodynamic analysis, Rudman [24] showed that for hydrides which form exothermally, the enthalpies of hydride formation obtained from van’t Hoff plots will remain reasonably constant with temperature and agree with those obtained from calorimetric measurements. It should be mentioned here that the interactions of intermetallic compounds with hydrogen are treated as pseudo-binary systems; i.e. M in eqns (1) and (5) may represent an intermetallic compound as well as a single metal. This sometimes gives rise to problems as will be discussed in Section 3. Van’t Hoff plots are an indication of the thermal stability of a metallic hydride; several such plots are shown in Fig. 4. Generally, the Group III and rare earth hydrides are the most stable and the hydrides of intermetallic compounds are the least stable. Many metals and intermetallic compounds may form more than one hydride. This results in multiple plateaux in the pressure-composition isotherms as shown in Fig. 5 for the Nb-H system which forms a monohydride (b.c.c.) and a dihydride. Consequently, two (or more) van? Hoff plots are obtained as shown for the Nb-H system in Fig. 4. The equation for formation of the monohydride (i.e. the stabilized b.c.c. phase) is the same as eqn (S), but the second plateau
x)lMK. (5)
IO4
I
I
0.85
I 0.90
I 0.95
I 1.00
I 1.05
1.10
1.15
1000/2-K
Fig. 3. Van? Hoff plot for the gadolinium hydrogen system (Gd-GdH,): (0) Sturdy and Mulford, Ref. 22; and (0) Libowitz and Pack, Ref. 23.
Metallic hydrides
1465
H/M Atomratio
Fig. 6. Pressure
lo4 II
11 1
1.5
I
I
I
I
2
2.5
3
3.5
lOOO/T”K Fig. 4. Van? Hoff plots for formation of various metallic hydrides. The compositions shown are the stoichiometric values, but in almost every case, the hydride is hydrogen deficient (see Fig. 2 and eqn 5).
in Fig. 5 represents the formation of a nonstoichiometric (hydrogen deficient) dihydride from nonstoichiometric (hydrogen excess) monohydride:
3. APPLICATIONS
3.1. Practical problems Before describing specific applications, it should be pointed out that the above discussion dealt with the
41~
,
,
I
,
,
I
I
,
(
I
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
H/M Atom ratio
Fig. 5. Pressure composition isotherms for the niobium hydrogen system illustrating the formation of two hydrides. B is a monohydride with a distorted b.c.c. structure, and y is a dihydride with the fluorite structure.
ideal situation. Actually, there are several properties that give rise to problems in the utilization of metallic hydrides. A few of them will be mentioned here. It was stated previously that eqn (1) is a spontaneous reaction. This is sometimes true, but usually the metal requires activation before absorbing hydrogen. This is generally attributed to an oxide layer on the surface of the metal which inhibits catalytic dissociation of the H, molecule and also prevents its entry into the metal. Activation usually can be effected by heating the metal in a vacuum, admitting hydrogen gas at pressures in excess of the dissociation pressure and cooling. Because of the expansion of the metal on hydride formation (the volume of a metallic hydride is always greater than that of the metal) cracking occurs thus exposing new metal surface to the hydrogen gas. Sometimes it is necessary to repeat this process more than once in order to fully activate the metal. In many metal-hydrogen systems it has been observed that the apparent equilibrium pressure on absorption of hydrogen is higher than the corresponding pressure on desorption as illustrated in Fig. 6. Such behavior obviously violates the phase rule and is, therefore, indicative of a nonequilibrium situation. However, such isotherms are quite reproducible. This phenomenon is referred to as hysteresis. Examples of this in real systems are shown in Fig. 7 for the vanadium-hydrogen system and an alloyhydrogen system consisting of 80% Nb and 20% V. There have been many theories proposed to explain hysteresis [25]. However, it is generally accepted that it is the result of lattice strain and plastic deformation due to expansion of the lattice on hydride formation. Because the plateaux of absorption isotherms are usually more sloped with less well-defined end points relative to the desorption isotherm plateaux (as illustrated in Fig. 7) it has usually been assumed that the
G. G. LIBOWITZ
1466
favored. Reaction (6) requires rearrangement of metal atoms which is unlikely to occur at low temperatures; whereas the hydride formation reaction (6) involves little motion of metal atoms. However, at elevated temperatures, reaction (7) is more likely to occur, e.g. at 300°C [27]. Even at lower temperatures, disproportionation occurs after many hydrogen absorption-desorption cycles. Factors affecting disproportionation are discussed by Sandrock et al.
LO-
2.0 -
2
l.O-
i3 g
0.5 -
i?! a
PI.
8 0.2-
3.2. Hydrogen storage
3 3
O.l-
0.05 -
0.021 1 1 1 1 1 1 1 ’ 1 ’ 1 1 1.4 1.6 1.8 2.0 0.8 1.0 1.2 HIM
Fig. 7. Pressure composition isotherms showing hysteresis for the reaction in the V-H and Nb,,,V,,r-H system. (0, 0) absorption and desorption isotherms for the VH-VH,_, system; and (m, 0) absorption and desorption isotherms for the Nb,,8V,,H-Nb,,,V,2H,_6 system. [From: Lynch J. F., Libowitz G. G. and Maeland A. J. J. Less-Common Met. 103, 117 (1984).] desorption
isotherm
represents
the true equilibrium.
Also, it is frequently more difficult to attain reproducible experimental points on the absorption isotherms. However, Flanagan and Clewley [26] proposed that the dislocation energies causing the plastic deformation were approximately the same for hydride formation and decomposition. If so, neither the absorption nor the desorption isotherms represent the true equilibrium; rather it must lie somewhere in between. Although hydrides of intermetallic compounds are treated as pseudo-binary systems, the formation of most (if not all) hydrides of intermetallic compounds is metastable with respect to disproportionation of the intermetallic compound itself. For example, the free energy of the reaction: LaNi, + Hz + LaH, + 5Ni,
The first major application of metallic hydrides came with the advent of nuclear reactor technology. Hydrogen is an effective neutron moderator (i.e. it slows down neutrons with minimum absorption) and since metal hydrides can store hydrogen very efficiently as shown in Table 2, metal hydrides have been used as moderators in nuclear reactors [29]. As seen in Table 2, the number of hydrogen atoms per unit volume in most metallic hydrides is higher than in liquid hydrogen and, in some cases, even greater than in solid hydrogen. Therefore, the volume requirements for storing hydrogen as a hydride is much smaller than storage as liquid hydrogen or compressed gas. The aforementioned “hydrogen economy” [l I] which required the storage of hydrogen as a fuel for vehicles or for load leveling applications resulted in the development of many new metallic hydrides. The use of metallic hydrides is also an unusually safe method of storing hydrogen because they are generally quite stable below their dissociation temperature. Also, since the reverse of eqn (1) is an endothermic reaction, the self-cooling effect will suppress any loss of hydrogen if a leak develops in the storage system. However, metallic hydrides were found to be too heavy for use in automobiles (although for fleet vehicles, they are still under consideration), and their utilization in load leveling applications proved uneconomical. Nevertheless, metallic hydrides are used to store
hydrogen
pure
laboratory
for specialty hydrogen
such as
others
[30].
(6)
is - 104 kJ mol-’ H, as compared to AGlg8= 0 for the hydride formation reaction:
Table 2. Hydrogen densities in some hydrogen containing compounds Compound
LaNi, + 3H, e LaNi, H,.
applications, among
(7)
Since the free energy of reaction (6) is considerably more negative than reaction (7), disproportionation of LaNi, would occur rather than hydride formation. However, reaction (7) occurs because it is kinetically
Liquid hydrogen (20°K) Solid hydrogen (4.2”K) MgH, TiH, VH, PdH,, FeTiH, LaNi,H,,
No. of H atoms cm-3 x IO** 4.2 5.3 5.9 9.2 10.4 4.7 6.0 6.1
Metallic hydrides
Cl.,2.1 2.8
2.9
3.0
3.1
3.2
3.3
3.4
1OOWT”K 8. Van’t Hoff plots for LaNi,H,_, and Fig. V, ssTiO IOFe,,, H, 6 illustrating the use of metallic hydrides for hydrogen compression.
3.3. Hydrogen compressors As indicated by the van? Hoff equation (eqn 4) and Fig, 4, the hydrogen pressure in equilibrium with a metallic hydride increases exponentially with temperature and, therefore, metallic hydrides can be used for hydrogen compression. Hydrogen gas is absorbed at low temperature and pressure and desorbed at high temperature and pressure. As can be seen from eqn (4), the higher the value of AH, the higher the slope and therefore the greater the rise in pressure over a given temperature range. This is illustrated in Fig. 8 which shows van? Hoff plots for hydrides of LaNi, (AH = 30.9 kJ mol-’ HZ) and a solid solution alloy of and iron, V0,s5Ti0,r0 Fe,,, vanadium, titanium, (AH = 42.9 kJ mol-’ H,). Although, both hydrides have a dissociation pressure of =0.2 MPa at room temperature. the latter hydride, having the higher value of AH reaches a pressure 2.5 that of LaNi, hydride at 95°C. The advantages of hydride compressors over mechanical hydrogen compressors are that they are vibration-free and quiet, and can be operated with low grade waste heat or solar energy [30].
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alloys, but the concept was extended to utilize pairs of metallic hydrides in a heat pump [32]. A metal-hydride heat pump consists of two hydrides with dissimilar thermal stabilities in sealed containers which are arranged so that hydrogen gas can flow freely from one hydride to the other. This type of heat pump can be used for heat amplification, temperature upgrade, and cooling or refrigeration. The principle behind the operation of the hydride heat pump can best be understood from van? Hoff plots of two hydrides as shown in Fig. 9. For the case of air conditioning, cooling would be a result of the dissociation of the less stable hydride (an endothermic reaction) as hydrogen flows from the fully hydrided M,H, at point A to the initially hydrogen depleted M,. at point B to form the more-stable hydride M,H,. By holding M,H, at T, (usually the ambient temperature), P2 > P, and hydrogen will continue to flow from A to B so that the low temperature, T,, will be maintained. When the hydrogen content of MzH,, has been exhausted, the hydride is recharged by allowing it to return to the ambient temperature, T,,, , and heating the hydride, M, H, to T,, (e.g. using solar energy, hot water, etc.) and maintaining it at that temperature so that hydrogen will flow back to M2H,. (from point C to point D. since P4 > P3). By operating an identical two-hydride system in tandem, continuous cooling can be obtained. The same arrangement may be used for refrigeration using hydrides of different thermal stabilities. For example, M2H, would be less stable than the hydride used for air conditioning, i.e. one which dissociates near 0°C. For heat amplification, T, could be the temperature of cold water or the outside ambient temperature on
3.4. Metal hydride heat pumps The relatively high heats of formation of metallic hydrides plus the ease of reversibility of eqn (1) led to the proposal that they may be used to store thermal energy in a home [31]. This proved to be uneconomical because of the high cost of the hydride
1ITK Fig. 9. Van’t Hoff plots illustrating the operation of a metal hydride heat pump for cooling (air conditioning or refrigeration) and for heat amplification.
G. G. LIBOWITZ
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cool days, e.g. 15°C and T,, could be the temperature of hot water, about 85°C. Thus, space heating would be obtained at point B when hydrogen flows from point A, and also at point D during recharging of the less stable hydride. The use of heat pumps for temperature upgrade is illustrated in Fig. 10. In this case, waste heat at a temperature T,,, is used to discharge the less stable hydride, M2Hy, at point A while hydrogen is absorbed by the more stable hydride, M, H,, at point B to generate the usable higher temperature, r,. The hydride Mz H, is regenerated by allowing it to cool to room temperature, T,, and using the waste heat at T,,, to cause hydrogen flow from point C to point D. 3.5. Hydride batteries The use of metallic hydrides in rechargeable batteries is based on eqn (3) where the hydride acts as the negative electrode. The positive electrode is nickel hydroxide undergoing the reaction: Ni(OH2) + OH- $ NiOOH + H,O + ee, to form the oxyhydroxide of nickel. The overall cell reaction is then:
(l/n)M + Ni(OH), Eharp (l/n)MH,
+ NiOOH.
discharge
The impetus for the development of the metal hydride battery was replacement of the nickel-
cadmium battery, since Cd is a toxic metal. However, the metal hydride battery also has higher capacities and longer run times than the nickel-cadmium battery. Most metal hydride electrodes are based on the AB, Haucke phase or the Laves phase intermetallics (see Table 1). 3.6. Isotope separation The use of metallic hydrides for the separation of deuterium and tritium from hydrogen is based on the fact that the thermal stabilities of the deuterides and tritides are usually different from those of the protides. Normally, the protide of a metal is more stable than the corresponding deuteride or tritide. However, there are some metal hydrides that exhibit a small inverse isotope effect in which the hydride involving the heavier isotope is more stable. For the case of vanadium dihydride, this inverse isotope effect may be quite large (e.g. at 40°C the dissociation pressure of the deuteride is almost one-third that of the hydride) [33]. This is due to the much more negative values of AH for formation of the deuteride with respect to those of the dihydride (i.e. - 50.2 kJ mol-’ H, as compared to - 40.1 kJ mol-’ H,). A scheme for isotope separation has been devised [34] based on the fact that there is a crossover temperature at which the protide becomes more stable [34]. 3.7. Hydrogen separation and purljication Most hydride-forming metals and alloys will selectively absorb hydrogen. Consequently, they can be used to remove hydrogen from mixtures of gases. The hydrogen that is subsequently desorbed from the hydride is essentially free of impurities. The most serious problem utilizing this technique for separation is poisoning of the hydride alloy by other gases present in the hydrogen gas that will retard the rate of hydrogen absorption as well as decrease the total storage capacity for hydrogen. Hydrogen gas has been successfully recovered from ammonia purge gas streams on a large scale [35]. The purge gas contained 20% N,, 12% methane, 5% argon and about 3% ammonia and the hydride-forming alloy used was LaNi,. 3.8. Power metallurgy
l/Z-K
Fig. 10.Van’t Hoff plots illustrating the operation of a metal hydride heat pump for temperature upgrade.
The rapid expansion of metals on hydride formation will usually produce either powdered hydride or friable particles that can be easily made into powder by further comminution. Subsequent removal of the hydrogen by heating in a vacuum will result in finely powdered metals. This process has been used to prepare powdered metals of Ti, Th, Zr, etc. for further compaction into desired shapes [36]. More recently, this technique has been used to pre-
Metallic hydrides pare powdered alloys in the production of permanent magnet materials such as SmCo, and Nd2Fe,,B [37]. It has also been proposed for the preparation of metallic glass alloy powders [38].
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reaction, poisoning of the hydride by impurities in the hydrogen gas, and sloping isotherms. These factors are discussed in Ref. [30] as well as more detailed discussions on some of the applications.
3.9. Sensors and activators Because of the exponential rise in hydrogen pressure with temperature (e.g. see Fig. 7), metallic hydrides may be used as temperature sensors and activators. One device [39] which had been used for many years to detect fires in aircraft engines consists of a capillary tube, containing a metal hydride, which is inserted in the engine. The other end of the capillary tube is connected to a pressure sensitive switch. When the tube is in the vicinity of a fire, the sudden rise in temperature causes rapid release of hydrogen gas from the hydride thus increasing the pressure in the tube and tripping the pressure sensitive switch that, in turn, sets off an alarm. 3.10. Heat engines A simple extension of the hydride compressor concept leads to the design of a heat engine based on metallic hydrides. The high pressure hydrogen gas produced by heating the metallic hydride may be expanded through a turbine to generate electricity. The hydrogen gas is then re-absorbed in the hydride in preparation for the next cycle, thus resulting in an engine that converts thermal to electrical energy [40]. The expanding high pressure hydrogen gas also may be used to generate mechanical energy by driving pistons [41] or performing work by moving an impermeable diaphragm [42]. 3. I 1. Hydrogen getters Metallic hydrides may be used as getters to remove all traces of hydrogen gas from special vacuum tubes, for example. Also, with the advent of thermonuclear fusion technology, including weapons systems, it is important to remove and collect tritium gas from some systems for safety reasons because of the radioactivity of the isotope. A hydrogen getter material should be able to react rapidly with the gas and also have high thermal stability (i.e. low dissociation pressure). Most getter materials that are presently in use are alloys of zirconium, e.g. ZrNi, ZrV,, ZrAl,, with dissociation pressures ranging from 10m5to less than IO-’ MPa at room temperature. Finally, it should be pointed out that in the discussions of the use of metallic hydrides for various applications, only idealized situations were considered. In addition to some of the practical problems discussed above, there are many other factors that must be taken into consideration such as heat transfer problems which is the major factor affecting rates of
REFERENCES 1. Siegel B. and Libowitz G. G., in Me&l Hydrides (Edited by W. M. Mueller, J. P. Blackledge and G. G. Libowitz), p. 546. Academic Press, New York (1968). 2. Switendick A. C., in Hydrogen in Metals (Edited by G. Alefeld and J. Volkl), Vol. 1. p. 101. Springer-Verlag, Berlin (1978). D. A., in Metal Hydrides (Edited 3. Papaconstantopoulos by G. Bambakidis). p. 215. Plenum Press, New York (1980). Fukai Y., The Metal Hvdrogen System: Basic Bulk Properties, p. 300. Springer-Verlag, Berlin (I 992). Libowitz G. G., Ber. Bunsenges, Phys. Chem. 76, 837 (1972). Iwasieczko W., Drulis H. and Frak R. M., J. Alloys Comp. 180, 265 (1992). Schober T. and Wenzl H., in Hydrogen in Metals (Edited by G. Alefeld and J. Volkl), Vol. 2, p. II. Springer-Verlag, Berlin (1978). 8. Baranowski B.. in Ref. 7, Vol. 2, p. 158. 9. Belash I. T.. Antonov V. E. and Ponyatovskii E. G., Proc. Acad. Sci., USSR (Engl. Transl.) 235, 665 (1977); Ponyatovskii, E. G., Antonov. V. E. and Belash, I. T.. Inorg. Mater. (Engl. Transl.) 14, 1227 (1978). 10. see, for example, Muetterties E. L., Transition Metal Hvdrides. Marcel Dekker. Inc.. New York (1971): or btalytic Transition Metal Hydrides, (Edited by D. W. Slocum and W. R. Moser), Vol. 415. Annals of N.Y. Academy of Science (1983). Il. Gregory D. P., Sci. Am. 228, 1 (1973); Winsche W. E., Hoffman K. C. and Salzano F. J., Science 180, 1325 (1973). L.. Hvdrogen in Intermetallic Compounds. 12. Schlapbach Vols 1 and 2. Springer-Verlag, Berlin (1988) and-(1992). 13. van Vucht J. H. N., Kuijpers F. A. and Bruning H. C. A. M., Philips Rex Rpts 25, 133 (1970). E. A., Electrochem. Technol. 14. Pebler A. and Gulbransen 4.211 (1966): Trans. Met. Sot. AIME 289. 1593 (1967). 15. Reilly J, J. and Wiswall R. H., Inorg. ehem. i3, 218 (1974). 16. Libowitz G. G., Hayes H. F. and Gibb T. R. P., J. Phys. Chem. 62, 76 (1958). 17. Chikdene A.,. Baudry A., Boyer P., Miraglia S., Fruchart D. and Soubeyroux J. L.. Z. Phvs. 18
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35. Sheridan J. J., Eisenberg F. G., Greskovich E. J., Sandrock G. D. and Huston E. L., J. Less-Common Met. 89, 447 (1983). 36. Blackledge J. P., in Ref. 1, p. 12. 37. Harris I. R. and McGuiness P. J., J. Less-Common Met. 174, 1273 (1991). 38. Maeland A. J. and Libowitz G. G., Mater. Len. 1, 3 (1982). 39. Warren D. E., Faughnan K. A., Fellows R. A., Godden J. W. and Seek B. M., J. Less-Common Met. 104, 375 (1984). 40. Powell J. R., Salzano F. J., Yu W. S. and Milau J., Science 193, 314 (1976). 41. Nomura K., Ishido Y. and Ono S., Energy Conversion 19, 49 (1978). 42. Northrup C. J. M. and Heckes A. A., J. Less-Common Met. 74, 419 (1980).