- Email: [email protected]
Hydrogen storage properties of Laves phase Ti1−xZrx(Mn0.5Cr0.5)2 alloys
RARE METALS
.
Vol.2 5 , Spec. Issue Oct 2006, p .218
Hydrogen storage properties of Laves phase Ti,- .Zr, (M Q , ~ C ~alloys ~.~)~ GUO Xiuinei, WU Erdong, and WANG Sucheng Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Scienres, Shenyang 110016, China (Received 2006-06-30) Abstract: The activation behaviors and hydrogen storage properties of the Laves phase Ti1 - - x Zr, ( Mno.5 Cro.5)2( x = 0, 0.1 , 0 . 2 , 0 . 3 2 , 0.5) alloys were investigated by the pressure-composition-temperature (PC-T) measurements. All the studied alloys show the single C14-type Laves phase structure based on the XRD data. Except for the alloys with very low Zr content of x = 0 and x = 0 . 1 , all these alloys can be fully activated. The P-C isotherms of the activated alloys show that, the introduction of Zr induces the decrease of the equilibrium pressures and the steeper plateaus. As the x increases, the maximum hydrogen absorption also increases, whereas the desorption of hydrogen decreases. These two effects result in a maximum reversible hydrogen storage capacity of H I M = 3 . 0 3 for the alloy at x = 0 . 3 2 . Furthermore, the well-defined plateau associated with the smallest hysteresis also appears at z = 0 . 3 2 .
Key words: Laves phase; hydrogen storage alloys; activation behavior; P-C isotherms
[ This work
1.
was
supported by the National Natural Science Foundation of China ( N o . 50371086). ]
Introduction
As a good candidate for energy, hydrogen has attracted more and more attention because of its clean and unexhausted characteristics. There are many works on studying the hydrogen storage materials to find good carriers for storing hydrogen conveniently and economically. Intermetallic compound is one of the promising materials for hydrogen storage. Most of the developed hydrogen storage intermetallic belongs to the AB5 and AB2 types. LaNi, is a typical representative of the AB5 type hydrogen storage alloys with a hexagonal CaCu5 structure. The M i s is easily activated and has a hydrogen storage capability of about I . 5 w t . % without being contaminated by impurities. However, the limiting hydrogen storage capacity and high cost of LaNi, can not satisfy aLl practical applications. The Ti and Zr based AB2 type Laves phase alloys have been studied extensively in recent years because of their higher hydrogen storage Corresponding author : 'XU Erdong
E-mail : ewu @ imr .ac .cn
capacity (about 1. 5wt.% - 1. 8 w t . % ) . Among them, the Ti-Mn based alloys show the properties of easy activation, good hydridingdehydriding kinetics, high hydrogen storage capability and relatively low cost [ 1 ] . Whereas, the major problems of Ti-Mn based alloys are the too high equilibrium pressure and serious hysteresis during hydriding and dehydriding . For examples, a desorption pressure as high as 1.29 MPa was reported by Liu et al . [ 2 ] for the good performance TiMnz-based alloy, and a large hysteresis was observed by Klyamkin et a1 . [ 3 ] for hydrogenation of TiMnz alloys. Multi-component substitution is an effective method to improve the overall performance of the AB,-type Laves phase hydrogen storage alloys[ 1-71. The partial replacement of Mn by Cr in the TiMn2-based alloys can improve the hysteresis in the hydrogen storage process. As reported by Liu et a l . [ 21, TiCn hydride has almost no hysteresis even at 197 K , but the introduction of Cr in the alloys can shorten the pla-
Guo X.M .et a2 . , Hydrogen storage of Laves phase Til-xZrz(Mq.SCr,,.,)2 alloys
teau region. Zr is an effective alloying element for increase the hydrogen storage capacity and decrease the plateau pressure in hydrogen absorption process. In this work, we added the Mn and Cr in the B side of the ABp type Laves phase alloys and constrained the Mn/Cr = 1. Ti is the main component in the A side with a partial substitution by Zr. We focus on the activation behaviors and hydrogen storage properties of the Til-.Zr,(Mno.sCro.s)2(x = 0 , 0.1, 0.2, 0.32, 0 . 5 ) alloys to find out the useful alloy for hydrogen energy storage that has moderate equilibrium pressure and small hysteresis with a large hydrogen storage capacity.
2. Experimental The samples were prepared by arc melting technique under an argon atmosphere in a water-cooled copper crucible. The purity of the constituent elements was as follows: 99. 9 w t . % f o r T i , 9 9 . 9 w t . % forZr, 9 9 . 9 w t . % for Mn, and 99.98 wt. % for Cr. An excess amount of Mn was added to compensate weight loss during melting. The alloy ingots were turned and remelted several times in order to improve the homogeneity, then sealed in a quartz tube full of argon and annealed at 1173 K for 72 h . The phase analysis and structure determination were performed by X-ray powder diffraction (XRD) using Cu Ka radiation on a Dlmax 2400 diffractometer. The XRD patterns were measured in steps of 0.04" ( 28 ) from 20" to 100" with a constant scanning rate of 4 (')*min-'. The activation behaviors and P-C isotherms of the samples were measured using a Sievertstype hydrogenator. The sample was smashed mechanically in order to expose more fresh and clean surfaces for hydrogen absorption. About 1 g of sample was put into the container of the apparatus. High purity hydrogen gas ( > 99.99%) was used for the measurements. The activation process was carried out at room temperature by adding hydrogen pressure in the hydrogenator. A critical pressure ( up to 3 MPa) was required for a sample to react with
219
hydrogen. After activation, the P-C isotherms were measured. The amount of hydrogen absorption was determined by a volumetric method. The P-C isotherm illustrates the relationship of the hydrogen absorption content and the corresponding equilibrium pressure.
3.
Results and discussion
3.1. XRD analysis of alloys The XRD patterns of the Ti, - - z Zr, ( M Q . ~ Cro.5)2(x= 0 , 0.1, 0.2, 0.32, 0.5) alloys are exhibited in Fig. 1. The calculated lattice parametels for each alloy are listed in Table l . The crystal structures of the alloys were characterized fmm XRD analysis as C14-type Laves phase (space group P6Jmmc ; No. 194). As shown in Fig. 1, no impurities are detectable in the XRD patterns, and the main peaks of the XRD patterns have shifted to the small angles with the increase of the Zr content x in the alloys.
1
x=o.5
3 3 4
3 3 8 3
m
a
2 0 I( )
Fig.1.
XRD patterns of Til-,Zr,(Mno.sCro.s)z.
Table 1. Unit cell parameters of Til-,Zrx (Mq,.s Cro.s)zcalculated from XRD data
' 0 0.1 0.2 0.32 0.5
28/(o) main+
Of
43.64 43.44 43.24 43.00 42.64
alnm
clnm
c/a
wnm3
0.48566 0.79662 1.6403 0.1627235
0.48726 0.48932 0.4s)uo 0.49627
0.80093 1.6437 0.1646790 0.80498 1.6451 O.lf69197 0.80959 1.6445 0.1699232 0.81363 1.6395 0.1735346
220
RARE METALS, Vol. 2 5 , Spec. Issue , Oct 2006
Fig. 2 presents the varying tendencies of the lattice parameters a and c with x , indicating that the a and c both increase linearly with the increase of x . This is due to the fact that the radius of Zr atom is larger than that of Ti atom. Furthermore, as shown in Fig. 3, the values of c l a keep nearly constant with x . Therefore, the unit cell volumes of the alloys expand isotopically with increase of x in the alloys. The line-dependent increases of a and c with Zr contents were also shown in the Huang and co-workers’ investigation of Ti, - I Zr,Mnl .4 alloys[ 1 ] . However, in their work, the ratios of c l a increase with Zr contents, resulting in anisotropic expansion of the unit cell volumes in the alloys. This may be attributed to the deficiency of Mn in the alloy.
t
1“”
Fig.2. Relationship between x and the crystal lattice parameters of Til_,Zr,(Mno.sCro.s)2. im
I
X
Fig.3.
Relatiomhip between x and c l a .
3.2. Activation behavior Activation is the first necessary step for the
application of hydrogen storage alloy. The activation characteristics of these alloys were evaluated by P-C isotherms at 293 K . The experiments show that except for the alloys at x = 0 and x = 0. 1, all the Til-*Zrz( M Q , ~ C ~alloys ~.~)~ could react with hydrogen at room temperature and the pressures up to 3 MPa. Generally, the activation conditions become rigorous with the decrease of Zr content, eventually as the x reduces to 0.1, the alloy can not absorb hydrogen even under the conditions of 673 K and 4 MPa. This phenomenon can be attributed to two factors. First, Zr atom is larger than that of Ti, so that the adding of Zr in the alloy will expand the unit-cell and interstitial of the alloy, and result in a larger space for the accommodation of hydrogen. Second, Zr has stronger affinity with hydrogen than Ti, thus the alloys with higher Zr content will react more easily with hydrogen. Bittner and Oesterreicher reported that no hydrogen absorption was observed in Tio.eZro.2Mn2 at room temperature and the pressure up to 7 MPa[6]. However, in this study, the T&.,ZrO., (Mm.5Cro.5)? could absorb hydrogen at room temperature and 3 MPa. A slightly larger radius of Cr atom in the 50% substituted alloy appears to cause about 2 . 9 % increase of the unit-cell volume, and leading to easier hydrogen absorption property of the alloy. The activation processes of the alloys Ti, - I Z ~ , ( M Q . ~ C ~x ~=. 0~ .)2~, (0 . 3 2 , 0 . 5 ) have been repeated five times to ensure a full activation of the samples. Several typical P-C isotherms during activation are illustrated in Fig. 4 . Activation performance of these alloys could be revealed from the diagrams. The activation curves in Fig. 4 indicate that the alloys could be fully activated after two cycles. As found from the activation diagrams, there is a decrease of absorption plateau pressure between the first cycle and the second one, and then the plateau pressures stay almost the same value during the further cycles. However, the desorption pressures did not change significantly during all activation processes. In comparison with the activation process of the LaNi5[ 8 1 , which demonstrates a large drop in the absorption plat-
Guo X.M .et al. , Hydrogen storage of Laves phase T i ~ - z Z r ~ ( ~ . 5 C r oalloys .5)2
HIM
Fig.4.
221
HIM
HIM
Activation curves of Til-,Zr,(Mq,.sCro.s)2: (a) x =0.2; (b) x =0.32; (c)
eau pressure after the first cycle, and the Laves phase Til -%ZrL. (Mno.5Cro.s)2alloys showed only a slight decrease between the first two cycles, indicating that the Til - Z~,MII,,~Cro.s)2alloys are less affected by the activation processes.
3.3. P-Cisotherms of activated alloys The fully activated samples have been evacuated at 473 K for 30 min to release most of the residual hydrogen. Thereafter, their hydrogen storage properties were characterized by the P-C isotherms measurements. The measured isotherms of the alloys at 293 K are exhibited in Fig. 5 . The derived data of hydrogen storage properties are listed in Table 2. As shown in Fig. 5 , along with the increase of Zr content x , the plateau pressures of the alloys have disuinctly decreased and the plateaus have become steeper. The hydrogen storage characteristics of the alloys are correlated with their lattice parameters. According to the interstitial size effect proposed by Lundin [ 91 , the increase of the unit cell volume leads to an increase of the interstitial size and then a decrease of the plateau pressure and hysteresis. Liu et a l . [ 2 ] have derived the same conclusions in their study. As mentioned above, adding Zr in the alloy expands the unit-cell volume, results in enlarging interstitial size, and easier activation. Therefore, the plateau pressure should decrease with x increasing. The steeper plateaus can be explained by a local environment model proposed by Northwood and coworkers[ lo]. Their argument is that the interstitial site surrounded by Zr and B-side atoms such as 2Zr2B has stronger affinity to hydrogen
1: =0.5.
than the corresponding 2Ti2B site, and 2Zr2B site is larger than 2Ti2B site as well. Therefore, hydrogen atoms would preferentially occupy the 2Zr2B sites, then the sites containing less Zr. As a result, the equilibrium pressures gradually increase with increasing hydrogen contents in the alloy, leading to a non-horizontal plateau in P-C isotherm. Consequently, the plateau slope will become steeper in the alloy with more Zr substitution. The hysteresis between hydrogenation and dehydrogenation is an important factor for the practical application of the hydrogen storage alloy. The extent of the hysteresis can be described by a hysteresis coefficient defined as HI = In( P h / P d e s ) (at H / M = 1.5). As listed in Table 2, with the increase of x , the H I value reduces to a minimum at x = 0.32 and then increases again. This phenomenon appears to be inconsistent with the proposed relation on hysteresis by Lundin et al . [ 9 1 . However, the smallest hysteresis in this study is still larger
-.-
d . 2
-0-~0.32 '
-A-~=0.5
222
RARE METALS, Vol. 2 5 , Spec. Issue, Oct 2006
Table 2.
Hydrogen storage properties of Til - =Zr+(Mn,&ro.5 ‘latea’ pressure’Mpa ( H I M = 1.5)
X
0.2 0.32 0.5
p a
P d
2.314 0.296 0.071
0.185 0.028
1.098
Hysteresis coefficient Hl
Maximum hydrogen storage capacity
0.7455 0.4724 0.9527
than that of Laves phase Ti-Zr-Mn-Cr-V alloys [ 2 ] , It is likely that the introduction of V has improved the hysteresis in the alloys. The hydrogen storage capacity is also closely related to the interstitial size and the change of the chemical affinity of alloy to hydrogen[ 51 . As mentioned above, Zr component in the alloy provides larger interstitial size and stronger affinity for hydrogen to reside. However, these favors in hydrogen absorption associated with Zr alloying will, inevitably make hydrogen difficult to be released from the alloy. Hence, as exhibited in Fig. 5 , along with increasing x , the maximum hydrogen storage capacities increase, but the hydrogen desorption capacities decrease as well. These two opposite effects are balanced at x = 0.32, forming a hydride of Tiom Zro 32 ( Mn, Cro 5)2H3 03 with a maximum reversible hydrogen storage capacity of H I M = 3.03 (see Table 2 ) . This value appears to be higher than that of to best performing Tio 6Zro,Mn2H2 hydride reported by Bitter et a l . [ 6 ]. Besides of the highest reversible hydrogen absorption, the Tio Zro 32 ( Mno Cro I2 alloy also shows the moderate equilibrium pressure and the lowest hysteresis during hydriding-dehydriding process among all the Ti, *Zrr(Mno 5Cro ) * alloys.
4.
)2
Conclusions
The activation behaviors and hydrogen storage properties of the Til - Zr, ( Mn,.5Cro.5 >z ( x = 0, 0.1. 0 . 2 . 0.32, 0.5) alloys were investigated. Most of these alloys can be activated at room temperature and the pressures up to 3 MPa . However, the alloys at x = 0 and x = 0.1 can not react with hydrogen even under the conditions of 673 K and 4 MPa. The equilibrium
HIM
Reversible hydrogen storage capacity HIM
2.58
2.43 3.03 3.00
3.24 3.42
pressures of the P-C isotherms are affected significantly by Zr contents in the alloys. The P-C isotherms show that along with the increase of x in the alloys, the equilibrium pressures decrease and the plateaus become steeper, and the hydrogen absorption capacities increase, but the desorption capacities decrease. The alloy at x = 0.32 exhibits the maximum reversible hydrogen storage capacity ( H I M = 3 . 03 1. Based on the hysteresis coefficient data, the alloy at x = 0 . 3 2 also has the lowest hysteresis. Overall, the Ti0.68Zr0.32( Mno.5Cro.5)z,used as a hydrogen storage alloy, shows the best performance with a large reversible hydrogen storage capacity, well-defined plateau and the lowest hysteresis.
References Huang T . Z . , Wu Z. , Yu X.B., et a l . , Hydrogen absorption-desorption behavior of zirconiumsubstituting Ti-Mn based hydrogen storage alloys. Intermetallic, 2004, 12: 91. Liu B. H . , Kim D. M . , Lee K . Y . , et a l . , Hydrogen storage properties of TiMnz-based alloys. J . Alloys C o m p . , 1996, 240: 214. Klyarnkin S .N . , Verbetsky V .N . , and Demidov V . A. , Thermodynamics of hydride formation and decomposition for TiMnz-Hz system at pressure up to 2000 atm. J . Less-Common M e t . , 1994, 205: L1. Au M . , Pourarian F . , Sankar S . G . , et a1 . , TiMnz-based alloys as high hydrogen storage materials. Mater. Sci. Engi., 1995, B33: 53. Bobet J . L . , and Darriet B . , Relationship between hydrogen sorption properties and crystallography for TiMnz based alloys. Int . J . Hydrogen Energy, 2000, 25: 767. Oesterreicher H . , and Bitter H . , Studies of hydrogen formation in Ti, - .Zr,MnZ. Mat. Res .
BdZ., 1978, 13: 83. Jacob I . , Stern A . , Moran A . , et a l . , Hydro-
Guo X
.M
.et d . , Hydrogen storage of Laves phase Til-,Zr, (Mn0.sCra.d~alloys
gen absorption in ( Zr,Til - I ) Bz ( B = Cr , Mn ) and the phenomenological model for the absorption capacity in pseudo-binary Laves-phase compounds. J. Less-Cornhn M e t . , 1980, 73:
369, [8] Kisi E. H.,Buckley C. E . , and Gray E.M . , The hydrogen activation of LaNis. J. Alloys Comp., 1992, 185: 369.
223
[ 9 ] Lundin C . E . , Lynch F.E., and Magee C.B.,
A correlation between the interstitial hole sizes in
[lo]
intermetallic compounds and the thermodynamic properties of the hydrides formed from those compounds. J . LeJs-Common M e t . , 1977, 56: 19. h e y D.G., and Northwood D.O., Storing energy in metal hydrides: a review of the physical metallurgy. J. Mater. Sci., 1983, 18: 321.
.
Vol.2 5 , Spec. Issue Oct 2006, p .218
Hydrogen storage properties of Laves phase Ti,- .Zr, (M Q , ~ C ~alloys ~.~)~ GUO Xiuinei, WU Erdong, and WANG Sucheng Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Scienres, Shenyang 110016, China (Received 2006-06-30) Abstract: The activation behaviors and hydrogen storage properties of the Laves phase Ti1 - - x Zr, ( Mno.5 Cro.5)2( x = 0, 0.1 , 0 . 2 , 0 . 3 2 , 0.5) alloys were investigated by the pressure-composition-temperature (PC-T) measurements. All the studied alloys show the single C14-type Laves phase structure based on the XRD data. Except for the alloys with very low Zr content of x = 0 and x = 0 . 1 , all these alloys can be fully activated. The P-C isotherms of the activated alloys show that, the introduction of Zr induces the decrease of the equilibrium pressures and the steeper plateaus. As the x increases, the maximum hydrogen absorption also increases, whereas the desorption of hydrogen decreases. These two effects result in a maximum reversible hydrogen storage capacity of H I M = 3 . 0 3 for the alloy at x = 0 . 3 2 . Furthermore, the well-defined plateau associated with the smallest hysteresis also appears at z = 0 . 3 2 .
Key words: Laves phase; hydrogen storage alloys; activation behavior; P-C isotherms
[ This work
1.
was
supported by the National Natural Science Foundation of China ( N o . 50371086). ]
Introduction
As a good candidate for energy, hydrogen has attracted more and more attention because of its clean and unexhausted characteristics. There are many works on studying the hydrogen storage materials to find good carriers for storing hydrogen conveniently and economically. Intermetallic compound is one of the promising materials for hydrogen storage. Most of the developed hydrogen storage intermetallic belongs to the AB5 and AB2 types. LaNi, is a typical representative of the AB5 type hydrogen storage alloys with a hexagonal CaCu5 structure. The M i s is easily activated and has a hydrogen storage capability of about I . 5 w t . % without being contaminated by impurities. However, the limiting hydrogen storage capacity and high cost of LaNi, can not satisfy aLl practical applications. The Ti and Zr based AB2 type Laves phase alloys have been studied extensively in recent years because of their higher hydrogen storage Corresponding author : 'XU Erdong
E-mail : ewu @ imr .ac .cn
capacity (about 1. 5wt.% - 1. 8 w t . % ) . Among them, the Ti-Mn based alloys show the properties of easy activation, good hydridingdehydriding kinetics, high hydrogen storage capability and relatively low cost [ 1 ] . Whereas, the major problems of Ti-Mn based alloys are the too high equilibrium pressure and serious hysteresis during hydriding and dehydriding . For examples, a desorption pressure as high as 1.29 MPa was reported by Liu et al . [ 2 ] for the good performance TiMnz-based alloy, and a large hysteresis was observed by Klyamkin et a1 . [ 3 ] for hydrogenation of TiMnz alloys. Multi-component substitution is an effective method to improve the overall performance of the AB,-type Laves phase hydrogen storage alloys[ 1-71. The partial replacement of Mn by Cr in the TiMn2-based alloys can improve the hysteresis in the hydrogen storage process. As reported by Liu et a l . [ 21, TiCn hydride has almost no hysteresis even at 197 K , but the introduction of Cr in the alloys can shorten the pla-
Guo X.M .et a2 . , Hydrogen storage of Laves phase Til-xZrz(Mq.SCr,,.,)2 alloys
teau region. Zr is an effective alloying element for increase the hydrogen storage capacity and decrease the plateau pressure in hydrogen absorption process. In this work, we added the Mn and Cr in the B side of the ABp type Laves phase alloys and constrained the Mn/Cr = 1. Ti is the main component in the A side with a partial substitution by Zr. We focus on the activation behaviors and hydrogen storage properties of the Til-.Zr,(Mno.sCro.s)2(x = 0 , 0.1, 0.2, 0.32, 0 . 5 ) alloys to find out the useful alloy for hydrogen energy storage that has moderate equilibrium pressure and small hysteresis with a large hydrogen storage capacity.
2. Experimental The samples were prepared by arc melting technique under an argon atmosphere in a water-cooled copper crucible. The purity of the constituent elements was as follows: 99. 9 w t . % f o r T i , 9 9 . 9 w t . % forZr, 9 9 . 9 w t . % for Mn, and 99.98 wt. % for Cr. An excess amount of Mn was added to compensate weight loss during melting. The alloy ingots were turned and remelted several times in order to improve the homogeneity, then sealed in a quartz tube full of argon and annealed at 1173 K for 72 h . The phase analysis and structure determination were performed by X-ray powder diffraction (XRD) using Cu Ka radiation on a Dlmax 2400 diffractometer. The XRD patterns were measured in steps of 0.04" ( 28 ) from 20" to 100" with a constant scanning rate of 4 (')*min-'. The activation behaviors and P-C isotherms of the samples were measured using a Sievertstype hydrogenator. The sample was smashed mechanically in order to expose more fresh and clean surfaces for hydrogen absorption. About 1 g of sample was put into the container of the apparatus. High purity hydrogen gas ( > 99.99%) was used for the measurements. The activation process was carried out at room temperature by adding hydrogen pressure in the hydrogenator. A critical pressure ( up to 3 MPa) was required for a sample to react with
219
hydrogen. After activation, the P-C isotherms were measured. The amount of hydrogen absorption was determined by a volumetric method. The P-C isotherm illustrates the relationship of the hydrogen absorption content and the corresponding equilibrium pressure.
3.
Results and discussion
3.1. XRD analysis of alloys The XRD patterns of the Ti, - - z Zr, ( M Q . ~ Cro.5)2(x= 0 , 0.1, 0.2, 0.32, 0.5) alloys are exhibited in Fig. 1. The calculated lattice parametels for each alloy are listed in Table l . The crystal structures of the alloys were characterized fmm XRD analysis as C14-type Laves phase (space group P6Jmmc ; No. 194). As shown in Fig. 1, no impurities are detectable in the XRD patterns, and the main peaks of the XRD patterns have shifted to the small angles with the increase of the Zr content x in the alloys.
1
x=o.5
3 3 4
3 3 8 3
m
a
2 0 I( )
Fig.1.
XRD patterns of Til-,Zr,(Mno.sCro.s)z.
Table 1. Unit cell parameters of Til-,Zrx (Mq,.s Cro.s)zcalculated from XRD data
' 0 0.1 0.2 0.32 0.5
28/(o) main+
Of
43.64 43.44 43.24 43.00 42.64
alnm
clnm
c/a
wnm3
0.48566 0.79662 1.6403 0.1627235
0.48726 0.48932 0.4s)uo 0.49627
0.80093 1.6437 0.1646790 0.80498 1.6451 O.lf69197 0.80959 1.6445 0.1699232 0.81363 1.6395 0.1735346
220
RARE METALS, Vol. 2 5 , Spec. Issue , Oct 2006
Fig. 2 presents the varying tendencies of the lattice parameters a and c with x , indicating that the a and c both increase linearly with the increase of x . This is due to the fact that the radius of Zr atom is larger than that of Ti atom. Furthermore, as shown in Fig. 3, the values of c l a keep nearly constant with x . Therefore, the unit cell volumes of the alloys expand isotopically with increase of x in the alloys. The line-dependent increases of a and c with Zr contents were also shown in the Huang and co-workers’ investigation of Ti, - I Zr,Mnl .4 alloys[ 1 ] . However, in their work, the ratios of c l a increase with Zr contents, resulting in anisotropic expansion of the unit cell volumes in the alloys. This may be attributed to the deficiency of Mn in the alloy.
t
1“”
Fig.2. Relationship between x and the crystal lattice parameters of Til_,Zr,(Mno.sCro.s)2. im
I
X
Fig.3.
Relatiomhip between x and c l a .
3.2. Activation behavior Activation is the first necessary step for the
application of hydrogen storage alloy. The activation characteristics of these alloys were evaluated by P-C isotherms at 293 K . The experiments show that except for the alloys at x = 0 and x = 0. 1, all the Til-*Zrz( M Q , ~ C ~alloys ~.~)~ could react with hydrogen at room temperature and the pressures up to 3 MPa. Generally, the activation conditions become rigorous with the decrease of Zr content, eventually as the x reduces to 0.1, the alloy can not absorb hydrogen even under the conditions of 673 K and 4 MPa. This phenomenon can be attributed to two factors. First, Zr atom is larger than that of Ti, so that the adding of Zr in the alloy will expand the unit-cell and interstitial of the alloy, and result in a larger space for the accommodation of hydrogen. Second, Zr has stronger affinity with hydrogen than Ti, thus the alloys with higher Zr content will react more easily with hydrogen. Bittner and Oesterreicher reported that no hydrogen absorption was observed in Tio.eZro.2Mn2 at room temperature and the pressure up to 7 MPa[6]. However, in this study, the T&.,ZrO., (Mm.5Cro.5)? could absorb hydrogen at room temperature and 3 MPa. A slightly larger radius of Cr atom in the 50% substituted alloy appears to cause about 2 . 9 % increase of the unit-cell volume, and leading to easier hydrogen absorption property of the alloy. The activation processes of the alloys Ti, - I Z ~ , ( M Q . ~ C ~x ~=. 0~ .)2~, (0 . 3 2 , 0 . 5 ) have been repeated five times to ensure a full activation of the samples. Several typical P-C isotherms during activation are illustrated in Fig. 4 . Activation performance of these alloys could be revealed from the diagrams. The activation curves in Fig. 4 indicate that the alloys could be fully activated after two cycles. As found from the activation diagrams, there is a decrease of absorption plateau pressure between the first cycle and the second one, and then the plateau pressures stay almost the same value during the further cycles. However, the desorption pressures did not change significantly during all activation processes. In comparison with the activation process of the LaNi5[ 8 1 , which demonstrates a large drop in the absorption plat-
Guo X.M .et al. , Hydrogen storage of Laves phase T i ~ - z Z r ~ ( ~ . 5 C r oalloys .5)2
HIM
Fig.4.
221
HIM
HIM
Activation curves of Til-,Zr,(Mq,.sCro.s)2: (a) x =0.2; (b) x =0.32; (c)
eau pressure after the first cycle, and the Laves phase Til -%ZrL. (Mno.5Cro.s)2alloys showed only a slight decrease between the first two cycles, indicating that the Til - Z~,MII,,~Cro.s)2alloys are less affected by the activation processes.
3.3. P-Cisotherms of activated alloys The fully activated samples have been evacuated at 473 K for 30 min to release most of the residual hydrogen. Thereafter, their hydrogen storage properties were characterized by the P-C isotherms measurements. The measured isotherms of the alloys at 293 K are exhibited in Fig. 5 . The derived data of hydrogen storage properties are listed in Table 2. As shown in Fig. 5 , along with the increase of Zr content x , the plateau pressures of the alloys have disuinctly decreased and the plateaus have become steeper. The hydrogen storage characteristics of the alloys are correlated with their lattice parameters. According to the interstitial size effect proposed by Lundin [ 91 , the increase of the unit cell volume leads to an increase of the interstitial size and then a decrease of the plateau pressure and hysteresis. Liu et a l . [ 2 ] have derived the same conclusions in their study. As mentioned above, adding Zr in the alloy expands the unit-cell volume, results in enlarging interstitial size, and easier activation. Therefore, the plateau pressure should decrease with x increasing. The steeper plateaus can be explained by a local environment model proposed by Northwood and coworkers[ lo]. Their argument is that the interstitial site surrounded by Zr and B-side atoms such as 2Zr2B has stronger affinity to hydrogen
1: =0.5.
than the corresponding 2Ti2B site, and 2Zr2B site is larger than 2Ti2B site as well. Therefore, hydrogen atoms would preferentially occupy the 2Zr2B sites, then the sites containing less Zr. As a result, the equilibrium pressures gradually increase with increasing hydrogen contents in the alloy, leading to a non-horizontal plateau in P-C isotherm. Consequently, the plateau slope will become steeper in the alloy with more Zr substitution. The hysteresis between hydrogenation and dehydrogenation is an important factor for the practical application of the hydrogen storage alloy. The extent of the hysteresis can be described by a hysteresis coefficient defined as HI = In( P h / P d e s ) (at H / M = 1.5). As listed in Table 2, with the increase of x , the H I value reduces to a minimum at x = 0.32 and then increases again. This phenomenon appears to be inconsistent with the proposed relation on hysteresis by Lundin et al . [ 9 1 . However, the smallest hysteresis in this study is still larger
-.-
d . 2
-0-~0.32 '
-A-~=0.5
222
RARE METALS, Vol. 2 5 , Spec. Issue, Oct 2006
Table 2.
Hydrogen storage properties of Til - =Zr+(Mn,&ro.5 ‘latea’ pressure’Mpa ( H I M = 1.5)
X
0.2 0.32 0.5
p a
P d
2.314 0.296 0.071
0.185 0.028
1.098
Hysteresis coefficient Hl
Maximum hydrogen storage capacity
0.7455 0.4724 0.9527
than that of Laves phase Ti-Zr-Mn-Cr-V alloys [ 2 ] , It is likely that the introduction of V has improved the hysteresis in the alloys. The hydrogen storage capacity is also closely related to the interstitial size and the change of the chemical affinity of alloy to hydrogen[ 51 . As mentioned above, Zr component in the alloy provides larger interstitial size and stronger affinity for hydrogen to reside. However, these favors in hydrogen absorption associated with Zr alloying will, inevitably make hydrogen difficult to be released from the alloy. Hence, as exhibited in Fig. 5 , along with increasing x , the maximum hydrogen storage capacities increase, but the hydrogen desorption capacities decrease as well. These two opposite effects are balanced at x = 0.32, forming a hydride of Tiom Zro 32 ( Mn, Cro 5)2H3 03 with a maximum reversible hydrogen storage capacity of H I M = 3.03 (see Table 2 ) . This value appears to be higher than that of to best performing Tio 6Zro,Mn2H2 hydride reported by Bitter et a l . [ 6 ]. Besides of the highest reversible hydrogen absorption, the Tio Zro 32 ( Mno Cro I2 alloy also shows the moderate equilibrium pressure and the lowest hysteresis during hydriding-dehydriding process among all the Ti, *Zrr(Mno 5Cro ) * alloys.
4.
)2
Conclusions
The activation behaviors and hydrogen storage properties of the Til - Zr, ( Mn,.5Cro.5 >z ( x = 0, 0.1. 0 . 2 . 0.32, 0.5) alloys were investigated. Most of these alloys can be activated at room temperature and the pressures up to 3 MPa . However, the alloys at x = 0 and x = 0.1 can not react with hydrogen even under the conditions of 673 K and 4 MPa. The equilibrium
HIM
Reversible hydrogen storage capacity HIM
2.58
2.43 3.03 3.00
3.24 3.42
pressures of the P-C isotherms are affected significantly by Zr contents in the alloys. The P-C isotherms show that along with the increase of x in the alloys, the equilibrium pressures decrease and the plateaus become steeper, and the hydrogen absorption capacities increase, but the desorption capacities decrease. The alloy at x = 0.32 exhibits the maximum reversible hydrogen storage capacity ( H I M = 3 . 03 1. Based on the hysteresis coefficient data, the alloy at x = 0 . 3 2 also has the lowest hysteresis. Overall, the Ti0.68Zr0.32( Mno.5Cro.5)z,used as a hydrogen storage alloy, shows the best performance with a large reversible hydrogen storage capacity, well-defined plateau and the lowest hysteresis.
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