102
J. Phys. Chem. 1984, 88, 102-105
Hydrogen Storage Properties of CeNi,,
AI, Alloys
V. K. Sinhat and W. E. Wallace* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: March 24, 1983)
The CeNi5-,A1,-H2 system with 0.5 < x < 1 was studied as a function of composition, temperature (0-70 "C), and hydrogen pressure (0.01-50 atm). CeNi5, which is not a hydrogen absorber, becomes an excellent hydrogen storage material when 10-20% Ni is replaced by Al. Hydrogen densities of the CeNi5-,Al, alloys relative to liquid hydrogen are in the range of 1.1-1.3. A1 has a dramatic effect in stabilizing the hydrides of these alloys. The vapor pressure of the hydrides of CeNi5,Al, alloys at room temperature can be expressed as PHz(atm) = 1365.8e-955X.The enthalpy and entropy of dehydrogenation for these alloys are 17.7-18.6 kJ/(mol of H,) and 77 J/(mol K), respectively. The kinetics of the hydrogen sorption are extremely rapid, and the processes are nearly complete in 60-200 s. The increase in the enthalpy of dehydrogenation of the CeNi5-,Al, alloy with increasing A1 content is attributed to the electronic effect.
Introduction Metal hydrides have acquired considerable scientific and technological significance in recent While the subject matter of dramatic changes in electronic and magnetic properties of the host metal on hydrogenation has beeen of great concern to the scientists, the remarkably large hydrogen capacity and favorable vapor pressures of hydrides under mild conditions of temperature and pressure have attracted technologists. These materials are of significance for a variety of engineering applications, e.g., refrigerators, heat pumps, hydrogen storage (fuel for automobiles), getters, etc. LaNi, is an excellent hydrogen storage material except for its high cost and large density. In recent years considerably efforts4-* have been made to develop better hydrogen storage materials. CeNi5, which is isomorphous with LaNi5, does not absorb hydrogen. The reasons behind the divergent behaviors of LaNi5 and CeNi, are reasonably well understood. La exists in LaNi, as La3+, whereas Ce in CeNi, occurs as Ce4+. Because of the higher valence the lattice of CeNi, is contracted about 15% compared to LaNi5. The smaller lattice contains very small interstitial sites for hydrogen occupancy and hence the hypothetical CeNi5 hydride is too unstable to exist. The influence of size of the interstitial hole on the vapor pressure of the hydride has also been discussed e l s e ~ h e r e . ~ - ' ~ Pourarian and Wallace'' studied the CeNi,-H, system in which there was a partial replacement of Ni by Cu and observed that the vapor pressure of the hydride decreased to a reasonable value, e.g., N 11 atm for the hydride of CeNi, 5Cuz,5at room temperature (23 "C). Sinha and Wallace,I2 in a recent communication, reported the hydrogenation characteristics of quaternary alloys CeNi,,Cu,Al,. They observed that AI had a far more profound effect on stabilizing 'the hydride than Cu. Hydrogen storage properties of the ternary CeNi5-,Al, alloys are, therefore, a subject matter of great interest. There appear to be no published data on the CeNiS,A1,-H, system except the magnetic susceptibility, crystallographic, and hydrogen capacity data of the hydride of CeNi4Al.I 3 , l 4 In the present paper we report the pressure-temperaturecomposition data of the CeNi5-,A1,-H2 system with 0.5 x < 1. The study also includes kinetic information on the hydrogenation/dehydrogenation processes and the associated thermodynamic properties. As is noted below, the ENi5,Al, alloys have many features which are characteristic of an excellent hydrogen storage material. Experimental Section The CeNi,,Al, alloys with x = 0.5,0.6, and 1.0 were prepared by the same procedure as described in our previous communic a t i o n ~ . ' ~The ~ ' ~ purities of the starting materials were as follows: Ce, 99.9%; Ni, 99.99%; Al, 99.99%. After several meltings of +On leave from the National Institute of Foundry and Forge Technology, Ranchi 834003, Bihar State, India.
0022-3654/84/2088-0102$01.50/0
TABLE I: Crystallographic Data of the CeNi,-,Al, and Their Hydrides
alloy
a, A
c,
a
CeNi,., Alo.s 4.919 4.045 CeNi4.5~10.5H5.8 5.309 4.245 CeNi,., Alo.6 4.920 4.051 5 ~ 3 1 1 4.240 C e N i 4 . 4 "0.6 7.5' CeNi,Al 4.952 4.091 CeNi,AIH,., 5.240 4.217
Alloys
A
V , .k3
V
1
ya
hydrogen capacity, cm3 o f H J (g of alloy)
84.76 o.22 103.62 8 4 . 9 2 \ o.22 103.57
1.3
170
1.3
168
8 6 ' 8 8 \ 0.15
1.1
146
100.28
a Y is the hydrogen density defined as mass of hydrogen in hydride per unit volume relative t o mass per unit volume of liquid hydrogen.
the metals in a cold boat, the alloys were annealed at -900 "C for 3 h. The experimental techniques for recording the pressure-composition-temperature data were those described earlier.' Hydrogen dissociation pressure in the range of 50-1-atm pressure was measured by pressure gauges, and lower pressures were measured by a mercury manometer. Temperature of the sample zone was controlled to approximately k l "C and was measured by a precision thermometer. Hydrogen gas of purity 99.999% was used for the experiment. Samples for the hydrogenation experiment were in the form of coarse granules and weighed approximately 2 g. X-ray diffraction data of the host alloy and their hydrides were recorded on an X-ray diffractometer using Cu K a radiation from a single crystal monochromator. Before the X-ray diffraction data were recorded, the hydrides were poisoned with SOz according to the technique described earli(1) See, for example: "Proceedings of the International Symposium on the Properties and Applications of Metal Hydrides", published in J . Less-Common Met. 1980, 73, 74. (2) See, for example: "Proceedings of the International Symposium on the Electronic Structure and Properties of Hydrogen in Metals, Richmond, VA, March 4-6, 1982", in press. (3) Wallace, W. E. J . Less-Common Met. 1982, 88, 141. (4) Van Essen, R . M.; Buschow, K. H. J . Res. Bull. 1980, 15, 1149. (5) Pourarian, F.; Fujii, H.; Wallace, W. E.; Sinha, V. K.; Smith, H. Kevin. J . Phys. Chem. 1981, 85, 3105. (6) Sinha, V. K.; Pourarian, F.; Wallace, W. E. 1.Phys. Chem. 1982,86, 4952. (7) Sinha, V. K.; Pourarian, F.; Wallace, W. E. J . Less-Common Met. 1982, 87, 283. (8) Sinha, V. K.; Wallace, W. E. J. Less-Common Met., submitted. (9) Lundin, C. E.; Lynch, F. E.; Magee, C. B. J . Less-Common Met. 1977, 56. 19. ( I O ) Sinha, V. K.; Wallace, W. E. J . Less-Common Met. submitted. (1 1) Pourarian, F.; Wallace, W. E. J . Less-Common Met. 1982,87, 275. (12) Sinha, V. K.; Wallace, W. E., to be submitted for publication. (13) Takeshita, T.; Malik, S. K.; Wallace, W. E. J. Solid Slate Chem. 1978, 23, 271. (14) Malik, S. K.; Boltich, E. B.; Wallace, W. E. J . Solid State Chem. 1980, 33, 263. (15) Sinha, V. K.; Wallace, W. E., to be submitted for publication.
0 1984 American Chemical Society
Hydrogen Storage Properties of CeNi,-,Al, Alloys 100
,
The Journal of Physical Chemistry, Vol. 88, No. 1, 1984 103 I
look
I
A
x
i
tlid
0
0.01
0
I
2
3
o'c
4
0 01' 0
5
I
I
I
I
I
2
3
5
4 g otom H /mole a l l o y
g otom H / m o l e alloy
Figure 1.
Pressure-composition isotherms for the CeNi4,,A1,,,-H2 sys-
Figure 2.
I
Pressure-composition isotherms for the CeNi, ,AI, 6H2system
tem. I
er.5,32,15 This was to prevent loss of H2 during the diffraction studies. Results and Discussion Crystal Structure. The CeNi,,Al, alloys and their hydrides were all single-phase materials possessing the CaCu, structure. It was interesting to note that the hydrides had very sharp, well-defined peaks similar to those of the host alloys. The lattice constants were computed by using a least-squares-fit program. The results are listed in Table I. It is observed that the lattice constants or the unit-cell volumes of CeNi,,Al, alloys increase with increasing values of x. This result is expected because Al, having a larger atomic radius, substitutes for the Ni. Upon hydrogenation these alloys undergo a volume expansion ranging from 15% to 22%. The lattice constants or the unit-cell volumes of the hydrides of these ternaries are observed to decrease with increasing A1 content, in spite of the larger lattice constants of the host alloy. This is attributed to the smaller number of H atoms per unit cell in the Al-rich alloys. The volume expansion per H atom is essentially the same in all these alloys. Our results of lattice constants for the CeNi,Al alloy are in excellent agreement with those reported by Takeshita et al.13 However, we observe slightly larger lattice constants for its hydride. This discrepancy may be explained by the fact that these authors reported the hydrogen capacity for the alloy as 3.7 H/formula unit (fu), whereas we observe a larger capacity, e.g., 4.8 H/fu. The hydrogen density of the alloys relative to liquid hydrogen and the hydrogen capacity in cm3 of H2/(g of alloy) are also listed in the last two columns of Table I. It is observed that these Ce-Ni-A1 ternaries could store 1C-30% more hydrogen per unit volume than liquid hydrogen. Pressure-Composition Isotherms. The experimentally determined pressure-composition isotherms for the CeNi, ,Al, 5-H2 and the CeNi,,AIo6-H, systems are plotted in Figures 1 and 2, respectively. The pressure-composition data for the CeNi4A1-H2 system at room temperature (23 "C) are plotted in Figure 3. For ready reference the room-temperature pressure-composition data of the other two systems are also replotted in Figure 3. These data were recorded after charging and uncharging the samples with hydrogen several times. It is of great interest to note that CeNi,, which otherwise does not hydrogenate, becomes an excellent hydrogen storage material by replacing only 10% of Ni by Al. Consequently, the CeNi4 alloy absorbs approximately 5.8 H/fu and its hydride possesses a very favorable vapor pressure, e.g., -8.5 atm at room temperature. Replacing 12% of Ni by A1 brings down the vapor pressure of the hydride to -6.5 atm without reduced hydrogen capacity (Figure 2). The hydride of CeNi4Al, where 20% of Ni is replaced by Al, is very stable, exhibiting a vapor pressure of -0.09 atm at room temperature
x
0 01 0
CeNi,,Al,,-H,
(22°C)
CeNi,,AI,,-H,
123'CI
I i
1
I
I
1
I
2
3
4
g atom H /mole
5
OIIOY
Figure 3. Pressure-compositionisotherms for the CeNi5,A1,-H2 system at room temperature.
(Figure 3). The room-temperature dissociation pressure at the midpoints of the two-phase field (where 50% a and 50% fl are in equilibrium) is plotted as a function of A1 content of the host alloy in Figure 4. It is observed that there is an excellent linear fit of P H Ivs. x in CeNi5-xAl,. The equation of the straight line obtained by using least-squares technique was derived to be p H 2 (atm) = 16.6 - 1 6 . 5 2 ~with a correlation coefficient ( r ) of 0.9997. Since the above equation extrapolates the vapor pressure of binary CeNi, to be 16.6 atm, a much lower value than is shown by experiment, an exponential curve fitting was attempted, this despite the excellent linear fit. The equation was derived to be p H 2(atm) = 1365.8e-9,55Xwith a coefficient of determination ( r 2 )of 0.98. This latter equation gives a more realistic value of p H 2at lower values of x. The lattice constants of the alloys are also plotted as a function of x in Figure 4. The lattice constant data of CeNi, were taken from the literature.I6 As expected, there is a good linear fit of a (or c) vs. x. It is observed in Figure 4 that the lattice constants of the alloy increase only mildly with increasing Al, whereas the vapor pressure of the hydride decreases dramatically. The inverse effect of magnitude of lattice constants on vapor pressure of the hydride has also been discussed e l s e ~ h e r e . ~However, J~ if we compare our present results on the CeNi5-,A1,-H2 system with (16) Wernick,
J. H.; Geller, S. Acta Crysrallogr. 1959, 12, 662.
104
The Journal of Physical Chemistry, Vol. 88, No. 1 , 1984
Sinha and Wallace Time Lsec)
Absorption
'"I
0
120
60
240
180
_x-
0
I
Absorption
o
Absorptlon
A
~ e s o r p i ~ o n O"C
1
1
0;
200
O°C
x
22'C
1
1
400
Desorption
600
Time ( s e c i
Figure 5. Kinetics of absorption and desorption of hydrogen by CeNi4SA10 and its hydride, respectively. 0.5
0 x
in
1.0
CeNi,.,
Absorption T i m e
4.0
0
1.5
AI,
( s e d
120
60
I80
x-x-,-,-.-.-x_L.-x-.._o-o-o-L,-o-a-o-o-o-o
i
Figure 4. Plots of dissociation plateau pressures of the hydrides and the lattice constants of the CeNi5_xAl,alloys.
TABLE 11: Enthalpies and Entropies of Dehydrogenation for the CeNi,..,Al, Alloys
-CeN i4.
A I,,.
g-atom of H/ (mol of alloy)
IN,kJ/ (mol ot H 2 )
1.5 2.0 2.5 3.0 3.5
17.6 17.6 17.9 18.0 18.0 17.5 17.2
4.0 4.5
-H
CeNI4.4AL3.6
AS,J/
Ah', kJ/ (mol K ) (mol of H,) 73.6 75.0 77.2 78.4 79.5 78.8 78.5
19.3 18.9 18.9
18.4 18.1 17.7
-H* AS, J/ (mol K ) 77.3 77.0 77.7 77.1 77.2 76.7
E
x
Absorption
0
Absorption
A
Desorption
0
Desorption
O°C
22'C 0°C
22'C
z 2 cn
,
I
200 Desorption
1
I
400
I
I
600
Time ( s e c i
Figure 6. Kinetics of absorption and desorption of hydrogen by CeNi4,4A10,6 and its hydride, respectively.
those on the CeNi5-xCux-Hz system," an interesting result is obtained. CeNi3Cu2,for example, which has almost the same lattice constants as the CeNi,Al, has approximately a 150 times higher vapor pressure for the hydride. Also, only 10%replacement of Ni with A1 has a more profound effect in stabilizing the hydride than does 50% replacement of Ni with Cu, although the latter host alloy has larger lattice constants. These results clearly indicate that apart from the size effect the electronic factors are also extremely important in the stability of the hydride. Unlike monovalent Cu, Al, which is a trivalent metal, will more effectively act as an electron donor and raise the CeNi, band. The increased value of AE (or AH) will thus reduce the vapor pressure of the hydride, since the change in entropy (AS) is probably not significant. This result is, in fact, experimentally observed (see below), Thermodynamic Functions in the Two-Phase Field ( a p). The enthalpy ( A H ) and entropy (AS) in the two-phase field (a + p) of the CeNi5-,Alx-Hz system were computed from the van? Hoff plot of In p H 2vs. I / T , using a least-squares technique. The results showed a good linear fit of In pH2vs. 1 / T in all cases. The computed AH and AS values as a function of hydrogen concentration are listed in Table 11. It is observed in Table I1 that AH and A S remain almost constant as a function of concentration, as expected of the two-phase field, for both the systems. The mean AH and AS for the CeNi, 5Alo 5-H2 system were computed to be 17.7 kJ/(mol of H2) and 77.3 J/(mol K), respectively. The corresponding values for the CeNi, 4Alo 6-H2 system were computed as 18.6 kJ/(mol of H,) and 77.2 J/(mol K), respectively. Since the AS for the CeNi, sAlo5-Hz and CeNi44Alo6-H2 systems are the same, the observed lower plateau pressure occurs because of a larger AH. As discussed earlier, the larger AH (or AE)for the CeNi, ,A10 6-H2 system is expected, because higher electron
+
concentration of A1 will raise the energy band of CeNi,. Consequently, one would expect further increased value of AH for the CeNi4Al-H2 system. It is of great practical significance that the enthalpy of dehydrogenation for the CeNi4,,Alo,,and CeNi4,4A10,6 alloys is only the value for those of LaNi, or other HYSTOR al10ys.l~ Kinetics of Hydrogen Sorption. The kinetics of hydrogen absorption and desorption were observed to be extremely fast for the CeNi5,A1,-H2 system. The kinetic data were recorded after charging and uncharging samples with hydrogen several times in a manner similar to that described e l ~ e w h e r e . ~Results of hydrogen absorption at 0 and 22 "C and desorption a t 0 "C by CeNi4,5Alo,5 and its hydride, respectively, are plotted in Figure 5 . Experimental data for hydrogen absorption and desorption for the CeNi4,4Alo,6 alloy and its hydride, respectively, at 0 and 22 OC are plotted in Figure 6. It is observed in Figures 5 and 6 that the kinetics of hydrogen sorption by the CeNi,-,Al, alloys are extremely rapid. For example, at room temperature (22 "C) these alloys absorb approximately 7 0 4 0 % of hydrogen in less than 40 s, while absorption is nearly complete in approximately 70-80 s. It is noted that the absorption kinetics are faster at 0 "C. Similarly, the hydrides of these alloys desorb 69-70% of the hydrogen in less than 100 s while hydrogen release is nearly complete in 200-300 s. The desorption kinetics are faster at room temperature, as expected (Figure 6). Also, it is of interest to note that replacement of as much as 20% of Ni by A1 has no deleterious effect on reaction kinetics. In fact, the alloys of the present investigation needed no activation to higher temperatures even during the first hydrogen charging. (17) Huston, E. I,.; Sandrock, G. D. J . Less-Common Met. 1980, 74, 435.
J . Phys. Chem. 1984, 88, 105-107
ConcIusions The CeNi,,Al, alloys appear to be excellent hydrogen storage materials. The hydrogen capacity for these alloys is approximately 30% higher than liquid hydrogen and is quite close to that of the paradigm alloy LaNi,. The kinetics of hydrogen sorption by the CeNi,,Al, alloys and their hydries are extremely rapid. Because of the low endothermal nature of dehydrogenation, these alloys appear better for hydrogen storage than LaNi, or other HYSTOR alloys14 for which the AH of dehydrogenation is twice as high. Because of the replacement of Ni with Al, the CeNi,-,Al, alloys become lighter and less expensive. Over and above all, the dramatic effect of A1 in stabilizing the CeNi,,Al, hydrides can be very well exploited in adjusting the vapor pressure of the hydride
105
to any desired value. For example, from Figure 4 it is observed that the hydride of CeNi,,,Al, will exhibit a vapor pressure of 1.5 atm at r w m temperature. In specific engineering applications, such as multistage heat pumps or refrigerators where several hydrides of different vapor pressures are needed, the CeNi,-,Al, alloys are certainly superior to any of the hydrogen storage alloys developed so far. Also, by virtue of their large hydrogen capacity, the ternary Ce-Ni-A1 alloys are better than the Ce-Ni-Cu ternaries" or the Ce-Ni-Cu-A1 quaternary alloys.I2
Acknowledgment. This work was supported by a contract with the Koppers Co., Inc., Pittsburgh, PA. Registry No.
Hydrogen Sorption by the Hyperstoichiometric Ce,,,
HZ,1333-74-0; AI/Ce/Ni, 87803-88-1.
Ni2.5Cu2.5 Alloys
V. K. Sinhat and W. E. Wallace* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: March 24, 1983)
,
Hydrogenation characteristics of the Cel+,Ni2$ 2 ~alloys ~ with x = 0.1-0.2 were studied as a function of temperature (0-100 " C ) and hydrogen dissociation pressure (0.05-50 atm). HyperstoichiometricCe up to approximately 10%appears to substitute for the Ni and/or Cu, whereas additional excess Ce generates an extraneous phase. CeNi, which otherwise does not absorb hydrogen becomes an excellent hydrogen storage material in the presence of 10% excess Ce and 50% Ni replaced by Cu. The hydrogen capacity of this Cel lNi25C~2 alloy is quite high, e.g., -5.3 H/formula unit (fu), and the vapor pressure of its hydride is -4 atm at 22 "C. The mean enthalpy and entropy of dehydrogenation for the Cel ]Nil ,CuZ alloy are 20.6 kJ/(mol of H2) and 80.5 J/(mol K), respectively. These values are lower than those for the conventional hydrogen storage materials. The kinetics of hydrogen sorption for the Cel+,Ni2 sCu2 alloys are extremely fast.
,
Introduction A number of alloys and intermetallic compounds are capable of absorbing and subsequently releasing large quantities of hydrogen under mild conditions of temperature and pressure.' These metal-hydrogen systems have attracted considerable attention because of their possible use as hydrogen storage materials for energy-related applications. Ever since the early discovery of LaNiS2and TiFe3 as hydrogen storage materials, there has been a continuous search for better materials. In a series of papers from this laboratory it has been recently ~ h o w n that ~ - ~ the nonstoichiometric ZrMn2-based alloys in which Zr and/or Mn are substituted by Ti or other 3d-transition metals have features which make them attractive for hydrogen storage. CeNi,, having the same crystal structure (hexagonal CaCu, type) as LaNi,, does not hydrogenate, presumably because of the great instability of its (hypothetical) hydride. Since Ce and La have similar affinity for hydrogen, this latter effect may be primarily attributed to the smaller interstitial holes available for H atoms in the CeNis lattice. Lundin et a1.* have shown that shrinkage of the metal lattice produces an increased hydrogen vapor pressure. Pourarian and Wallaceg have recently investigated the CeNi5-,Cu,-H2 system. They observed that in the presence of Cu the lattice of the host alloy was enlarged and consequently the alloy could be hydrogenated at reasonable pressures. For example, the vapor pressure of the hydride of CeNi, 5 C ~ 2 was observed to be 11 atm at room temperature (23 "C). Further increase in the Cu content of the alloy decreased its hydrogen capacity without significantly reducing the vapor pressure of the hydride. A metal hydride having vapor pressure in the range of 2-3 atm has more applications value. Therefore, the present investigation was made on ,-HZ system with the hope that hyperstoithe Cel+xNi2,5C~2
,
-
'On leave from the National Institute of Foundry and Forge Technology, Ranchi 834003, Bihar State, India.
0022-3654/84/2088-0105$01 S O / O
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TABLE I: Crystallographic Data for Ce,,,Ni,
