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Substitution effect of elements in Zr-based alloys with Laves phase for nickel-hydride battery
Journal of
AND COMPOUND5 ELSEVIER
Journal of Alloys and Compounds 231 (1995) 587-593
Substitution effect of elements in Zr-based alloys with Laves phase for nickel-hydride battery H. Nakano a, S. W a k a o b ~School of High-Technology for Human Welfare, Tokai University, 317 Nishino, Numazu, Shizuoka 410-03, Japan bTokai University, 3-10-22 Daita, Setagaya, Tokyo 155, Japan
Abstract
By examining the electrochemical and thermodynamic properties of Zr]_xA'x(VyNizMn,B'v)2÷ ~ ( 0 <~ x <~ 0.1; 0.10 ~
1. Introduction The chemical formula of a Laves phase intermetallic compound is expressed as A B 2. T h e r e are three types of Laves phase compounds, namely C15 (MgCu 2 type, cubic), C14 (MgZn 2 type, hexagonal) and C36 (MgNi 2 type, hexagonal). A m o n g these Laves phase compounds, Z r M n 2 ( H / M = 1.2), ZrCr 2 ( H / M = 1.3) and ZrV2 ( H / M = 1.8) contain much absorbed hydrogen and the hydrogen of these materials is in a tetrahedral site composed of A3B 1 or AEB 2. Many fundamental studies have been made on the hydrogen absorptiondesorption characteristics, thermodynamic properties, kinetics, diffusion mechanisms, etc. of these binary Laves phase alloys or the pseudobinary ones in which Z r is partially substituted with Ti and B site atoms are partially substituted with transition metals. Some alloys with non-stoichiometric composition, such as ABE+ ~ [1,2], have also been investigated, Several Laves phase alloys have large hydrogen 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 5 - 8 3 8 8 ( 9 5 ) 0 1 7 3 3 - X
absorption capacities and therefore have attracted interest as materials for negative electrodes in N i hydride batteries. Research on application to cells is in progress. Electrochemical properties of the C14-type Laves phase alloy have been studied in Refs. [3-5], for example, capacity as high as 3 7 0 m A h g -1 has been reported. Electrochemical properties of the C15-type alloy have also been examined [6-8] with a view to attaining higher capacity, improving the high rate capability for discharge and explaining the relation between the capacity and the occlusion site of hydrogen. In addition, discussion has been presented from the thermodynamics viewpoint of the fact that the cell volume depends upon the partial molar enthalpy change AH ° for hydride formation, i.e. the stability of the absorbed hydrogen. In this paper, for a series of 20 C15-type Z r system Laves phase alloys in which Z r is partially substituted with Ti or Nb and the B atom group is partially substituted with Ni, V, Co, Cr, Cu, Fe, etc., the c h a r g e -
Zr(VoloNio.54Mno.34Cuooz)2o Zr(Vo.loNio.6zMno.zs)z A Zr(VoAoNio.57Mno.28Feoos)z.l Zr(Vo.loNio.57Mno.28Coo.os)2 a Zr(Vo.~oNio.57Mno.28Cro.os)21 Zr(VoAoNio55Mno.3oCoo.os)21 Zr(VoAoNio.55Mno.3oCro.o~)z.1 Zr(VoAoNio.53Mno.32Coo.os)2.1 Zr(Vo.loNio.53Mno.3oCoo.os)2.2 Zr(VoAsNio.4~Mno.39Coo.os)2.2
Zro.95Tio.os(Vo.~oNio.49Mno.36Coo.os)2.o 5 Zro.gTioA(Vo.~oNio.49Mno.36Feo.os)2.o 5 Zro.gTio.~(VoloNio.49Mno36Cro.o5)2.o5 ZrogTio.~(VoaoNio.49Mno.36Cooos)zo 5 Zro.95Tio.os(VoaoNio.53Mno.3zCoo.os)2.1 Zro.gsTio.os(VoaoNio.svMno.zsCro.os)2.~ Zro.gTioa(Vo.~oNio.ssMno.3oCoo.os)2.t Zro.gTio.~(VoaoNio.53Mno.3zFeo.os)2.~ Zro.9Tio.l(Vo.loNio.49Mno.36Coo.os)2.1 Zro.gNbo.~(Vo.~oNio54Mno.3~Cooos)zl
X-1 X-2 X-3 X-4 X-5 X-6 X-7 X-8 X-9 X-10
Y-1 Y-2 Y-3 Y-4 Y-5 Y-6 Y-7 Y-8 Y9 Y-10
390 394 431 394 419 420 393 431 384 383
425 386 372 409 434 412 428 403 328 440
C - Ha ( m A h g -~)
368 378 390 370 408 393 368 403 367 383
380 348 334 406 355 397 416 359 319 403
17 b
332 360 368 350 397 365 357 386 364 382
351 339 328 400 352 387 405 339 313 337
55 b
312 350 350 338 391 358 352 362 350 381
327 328 322 394 342 381 399 328 310 330
110 b
D-Cap (mA h g - l )
303 333 338 326 385 345 347 361 335 380
301 314 318 388 336 367 382 326 300 320
220 b
0.823 0.881 0.867 0.881 0.94 0.878 0.943 0.90 0.913 0.99
0.79 0.902 0.952 0.96 0.946 0.92 0.92 0.908 0.940 0.794
RC
41.5 41.5 43.3 42.6 38.4 40.5 37.0 37.4 41.4 36.6
43.0 39.1 39.6 38.0 41.2 38.9 40.6 41.5 38.0 47.2
-AH° ( k J m o l H ~ ~)
124.9 127.3 123.7 126.4 124.6 118.5 118.8 121.2 124.2 125.7
118.1 120.7 123.6 124.8 113.4 126.0 125.1 125.4 121.0 130.2
-AS° ( J K - ~ m o l H ~ ~)
4.34 3.58 6.44 4.92 1.25 5.19 1.60 1.33 4.33 0.85
7.82 3.13 2.72 8.65 7.39 1.41 3.31 4.14 1.92 8.41
-AGO ( k J m o l H ~ 1)
87 73 64 75 100 81 92 94 81 97
91 100 100 100 75 100 100 93 94 69
Rc15 (%)
3.511 3.506 3.531 3.504 3.501 3.507 3.495 3.488 3.511 3.495
3.550 3.510 3.513 3.503 3.532 3.514 3.532 3.519 3.498 3.561
Vc15 (10ZA)
1.5239 1.5222 1.5226 1.5213 1.521 1.5205 1.5174 1.519 1.5195 1.517
1.528 1.5215 1.5226 1.522 1.5231 1.522 1.524 1.5232 1.5195 1.5258
r,v (A)
0.4037 0.3909 0.3825 0.3933 0.4111 0.4144 0.4079 0.3985 0.3870 0.3986
0.4294 0.4366 0.4328 0.4352 0.4245 0.4282 0.4176 0.4213 0.4081 0.3713
XB - XA
1.616 1.618 1.612 1.620 1.628 1.631 1.637 1.630 1.622 1.637
1.616 1.636 1.633 1.635 1.627 1.630 1.622 1.625 1.629 1.602
Xav
Hydrogen content absorbed into electrode (current density for charge, 17 m A g ~); ~current density for discharge (mA g-l) cut-off potential -0.60 V(Hg/Hg0), 25 °C; ~ratio of discharge capacities at current densities of 17 and 220 m A g-X.
Alloy
No.
Table 1 Charged hydrogen quantity (C - H), discharge capacity (D-Cap), existence ratio (Rc~5) of C15 type, unit cell volume (VcJs), average atomic radius (ray), difference in electronegativity (g~ - XA), partial molar enthalpy and entropy for hydrogenation at H / M = 036
/
~
~
=
.~
o~
H. Nakano, S. Wakao / Journal of Alloys and Compounds 231 (1995) 587-593
discharge characteristics and ties are examined. Factors molar enthalpy for hydride volume are also discussed in radii and electronegativities ments,
thermodynamic properinfluencing the partial formation and the cell a relation to the atomic of the constituent ele-
589
3. Results and discussion
3.1. Partial molar enthalpy for hydride formation As seen in Table 1, the absolute value of the partial molar enthalpy AH ° for hydride formation of the alloy lay in the range from 36.6 to 47.2 kJ m o l H f .
2. Experimental details
3.2. Electrochemical properties
Alloy specimens were prepared in amounts of 20-30g each by arc melting in an argon atmosphere within the following composition range: ! V yNzzMn,Bv)2+ " t Zrl_xAx( . (0~
The charging effÉciency (acquisition capacity/charging electricity quantity)remained almost 100% until the hydrogen quantity absorbed in the electrode (acquisition capacity) reached 2 0 0 m A h g -1. However, after this level exceeded, the efficiency decreased as the hydrogen gas generation increased. The maximum acquisition capacity in the perfectly charged state was approximately 330-440 mA h g-l, which is equivalent to H / M = 0.83-1.10 (Table 1). The discharge capacity at a current density of 1 7 m A g -1 was greater than 3 5 0 m A h g -1 except for alloys X-2, X-3 and X-9 (Table 1). Even at a considerably large current density of 220 mA g-i, all the alloy electrodes exhibited discharge capacities larger than 300 mA h g-l, in particular alloys X-4, X-7 and Y-5 exhibited capacities larger than 380 mA h g- 1. The ratio of the discharge capacity to the acquisition capacity was 82%-100% at 1 7 m A g -1. However, as the current density increased, the ratio decreased and the hydrogen quantity remaining in the alloy increased. It was noted that an alloy with a large remaining hydrogen quantity tended to have a large absolute value of AH °. As examples of alloys with good fatness in the discharge curve, X-10, Y-2 and X-6 were considered (Fig. 1). The high rate capability for discharge, R, of all the alloys except X-l, X-10, Y-l-Y-4 and Y-6 was more
,
,
,
,
~
, a 21
-o.6o -r" -rt~ > -0.80 ~> .~ E0J B a_ -1.0o
, o
200
Capacity (mAh g-')
) 400
Fig. 1. Discharge curves: 1, X-10; 2, Y-6; 3, Y-2 (17 mA g ~, 25 °C).
H. Nakano, S. Wakao / Journal of Alloys and Compounds 231 (1995) 587-593
590
, " o _ " ' a l ~ o - r ~ . _ , ~ o,,, u ~ xJ o
,
o.a
m o.4
0.0
i
i
35
40 45 50 IAH*I (kdrn°lH2"l) Fig. 2. Relation between absolute value of AH ° and high rate capability for discharge,
than 0.9. Although some deviation was seen as shown in Fig. 2, the high rate capability for discharge decreased as the absolute value of AH ° increased, From the above results it is concluded that an alloy with a AH ° value of 36-42 kJ molH~ 1 generally has a large discharge capacity and an excellent high rate capability for discharge. However, such electrochemical properties are liable to be influenced especially by the state of the surface in accordance with the preparation method of the alloy, such as solidification rate, heat treatment and surface oxidation. This being the case, some variations in the results might be due to the kinds of treatment methods used.
3.3. Substitution effect 3.3.1. Qualitative idea In general with binary metal-hydrides the ionicbond-like characteristics between the metal and the hydrogen are strengthened as the difference in electronegativity between the metal and the hydrogen increases. This explains why the absolute value of AH ° for hydride formation increases with the difference in electronegativity. Likewise, the characteristics of the ionic bond are strengthened when the difference in electronegativity between the alloy and the hydrogen is increased by means of element substitution in the
case of the Laves phase alloys. Thus the absorbed hydrogen is believed to be stabilized by alloying with elements which give larger differences in electronegativity. The series shown in Table 2 is an arrangement of the elements in accordance with the degree of Pauling electronegativity X as modified by Allred [10]. When an alloy is substituted by its anterior, the hydrogen is stabilized at its site. In the opposite case, when the alloy is substituted by its posterior, the hydrogen is destabilized. The series is convenient enough to think of the substitution effect. On the other hand, when the atomic radius of the A or B atom is changed, the geometrical dimensions of the layer structure among atoms are also changed. When the atomic radius of the A or B atom (or both) is decreased, the lattice is believed to be contracted. Conversely, when the atomic radius is increased, the lattice is thought to be expanded. This relation leads us to the following idea: when an atom in an alloy lattice is substituted by one with a smaller atomic radius, the lattice is contracted, and vice versa. Thus the stability of a hydride could be controlled by controlling its lattice geometry. The size order of the atomic radii is shown in Table 3 together with their numerical values [11]. The absolute value of AH ° is decreased by Z r ~ Ti, Mn ~ Ni, Cr---, Co, Fe, Ni and V ~ Ni substitutions. This fact satisfies the general rule referred to above. However, in some cases of substitution, namely N i ~ Co and Co ~ Fe, the absolute value of the actual AH ° decreased. As two general rules, an increase in AH ° was noticed as a result and discordance occurred in some cases. Discordance is also seen in the Zr(Wo.33 xNio.42MnoA7+xCoo.os)2.4 system C15-type V---~Mn substitution in the Laves phase alloy. The absolute value of the actual AH ° was decreased on this occasion, but an increase was noticed from the electronegativity and a decrease from the atomic radius as a result [12]. Although almost all the elements in the series of atomic radii and electronegativities comply with each other, inversion occurs in some cases, e.g. V and Mn. With the substitution of the elements for which inversion occurs, it is considered that the actual AH ° will be
Table 2 Series of electronegativities of elements x=
Zr < Ti < M n < N b < 1.33 1.54 1.55 1.60
V < Cr < Fe < C o < C u < 1.63 1.66 1.83 1.88 1.90
Ni H 1.91 2.1
Table 3 Series of atomic radii of elements r(,~) =
Zr > Nb > Ti > V > Mn > Cr > Cu > Fe > Co > Ni 1.771 1.625 1.614 1.491 1.428 1.423 1.413 1.411 1.385 1.377
H. Nakano, S. Wakao / Journal of Alloys and Compounds 231 (1995) 587-593
influenced by the stronger factor of either the effect of the atomic radius or that of the electronegativity.
3.3.2. Quantitative discussion It is necessary for these kinds of substitution effects to be dealt with quantitatively in order to explain their influence on the substitution quantity. To examine quantitatively the substitution effect (expansion-contraction .of the lattice or stability-instability of the hydroger 0 in terms of atomic radii or electronegativities, an average atomic radius ray and an average electronegativity Xav were introduced as follows:
i
,
I
~ 45 -~ "" ~" "r ~ 40
o O
o o..._ o
o
o 351.60
'
~ 1.62 Xav
0"~
'
' 1.64
(1)
i
Xav --
,
ai
Z
'
,-,
,
Fig. 4. Relation between average electronegativity Xav and absolute value of AH °.
(airi) rav --
so I
591
tendency was noted that the absolute value of AH ° decreased. The correlation is given by
(aix,)
(2)
Zi ai
IAH°I = 453.6 - 254.2Xav
(r = 0.91)
(4)
On the other hand, when both rav and Xav are considered, the correlation is further enhanced:
where r~ and Xi are the atomic radii and electronegativities corresponding to the individual constituent elements in the alloy respectively and a~ is the number of atoms. Plotted in Fig. 3 is the relation between AH ° and the average atomic radius rav. Although a considerable degree of deviation is seen, a correlation as shown in
IAn°l = 27.9 + 232.3rav - 209.8Xa v
(r = 0.93)
(5) 0
Between the absolute value of the enthalpy Ancalc
the following equation is obtained and a tendency was noticed that the absolute value of AH ° increased as rav increased:
calculated from Eq. (5) and the measured enthalpy AH ° a fairly good linear relation was seen as shown in Fig. 5. Since it is understood from the results in the previous subsection that AH ° of the alloys with excellent electrochemical characteristics lies in the range from - 3 6 to - 4 2 kJ molH~ 1, the ranges of ray and Xav calculated from the above equation are given as
IAH°I = -1061.0 + 723.7ray
1.516 ~< ray ~< 1.525
(6)
1.621 ~
(7)
(correlation coefficient r = 0.74)
(3)
Shown in Fig. 4 is the relation between AH ° and the average electronegativity X~v. As Xav increased, a
When the B atom in the AB2+ ~ alloy system is substituted by an element with a larger elec50
50
'
'
,~
'
0
-r
"5
E 45
~
""
°
0
0
o
"r <::] 4 0
35
¥ E ~.~45 ,..,,
O
--~
0
oO
c~i~ 0
1.516
I 1.520
<] 4C
t 1.524 rav
'
o
-
O
'
I 1.528
(~,)
Fig. 3. Relation between average atomic radius ra~ and absolute value of AH °.
ZoOO
35
35
I
t
40
45
IAH~xpl
50
(kJrnolH=-~)
Fig. 5. Relation between calculated and measured values of AH °.
592
H. Nakano, S. Wakao / Journal of Alloys and Compounds 231 (1995) 587-593
tronegativity, the hydrogen becomes unstable, because the difference in electronegativity between the hydrogen and the alloy decreases. On the other hand, if the bond strength of the alloy is shown by the difference in electronegativity between the A and B atoms, the alloy becomes stable, because the difference in electronegativity between the A and B atoms increases, Consequently, the result of Eq. (4) does not contradict that of the reversed stability presented by Miedema [13,14]. We consider that this result is fundamentally similar to the Miedema's rule, although the method of expressing it differs. However, our result did not correspond with Miedema's rule when part of the A atom was substituted by another element. Although it is necessary to consider the influences of the atomic radius and the difference in electronegativity between the A and B atoms, etc. in this case, it is difficult to explain these effects because of their complexity. It remains a problem to be solved. Also, we have an idea that these factors are interrelated, because the atomic radii and electronegativities of pure metals are correlated appreciably, though the improvement in the correlation in Eq. (5) can be explained as a statistical problem, 3.3.3. Factors which influence the cell v o l u m e
Plotted in Fig. 6 is the relation between the cell volume Vcl5 of C15 type and the absolute value of AH °. Although a considerable deviation is noticed, the following correlation is obtained: IAH°I = -360.6 + 1.1Vcl5
(r = 0.83)
(8)
The above equation indicates that the absolute value of AH ° increased as the cell volume increased; a similar relation is also seen in rare earth-nickel systems [15-17]. On the other hand, a tendency is noted between Vc~5 and ray that VC15 increases with increasing rav as described by the equation
50
l
,
r'~ -r -~ 4 6 "~
,
'
o ~ ~
o
o
o 7•"r < 40 ~ -~~8o% 35
3.48
o O o °° t
t
3.52
i
I
3.56
Vc15 (XlOaA) Fig. 6. Relation between cell volume Vc~5 and absolute value of AH°.
Vcl 5 = -568.9 + 604.7ray
(r = 0.86)
(9)
When the difference in electronegativity between A and B is increased, the bond strength between the two is increased and the lattice is considered to be contracted. Therefore, to examine this influence, the difference AXB A in electronegativity between A and B was introduced as defined by AXB-A=XB--XA (10) In this case the alloy composition AB2+,~ can be expressed as A3/t3+~)B~/(3+~) (B)2.0 on the assumption that the atomic ratio of the A and B groups is 1:2. Accordingly, XA and XB show the individual average electronegativities of these A and B atomic groups. Taking account of rav and this AXB_A, the cell volume is given as follows and the correlation is further improved: Vcl 5 = -596.5 + 628.9rav - 22.6AXB_A
(r = 0.89) (11)
From the above description it is seen that dealing quantitatively with the substitution effect by considering the atomic radius and/or electronegativity contributes to the design of alloys with excellent electrochemical characteristics, although the explanation is quite an approximate one.
4. Conclusions
By discussing how the substitution effect corresponds to the atomic radius and electronegativity after the examination of the electrochemical and thermodynamic properties of multicomponent alloys of the Z r - V - N i - M n system with C15-type Laves phase, the conclusions shown below were obtained. (1) AH ° exhibited a fairly good correlation with the average electronegativity and correlated also with the average atomic radius rav. By consideration of these two factors together, the correlation was further augmented. (2) The cell volume of C15 type correlated with AH °. Also, it has been pointed out that the cell volume depends upon the average atomic radius ray and the difference A yB_ A in electronegativity between A and B. (3) Many of these alloys whose absolute value of AH ° lies in the range from 36 to 42 kJ molH~ ~ have a discharge capacity greater than 3 8 0 m A h g 1 at a current density of 220 m A g-1 and a good high rate capability for discharge has been obtained. The values of rav and ,¥av in this range have been obtained as 1.516 ~
H. Nakano, S. Wakao / Journal of Alloys and Compounds 231 (1995) 587-593
References [1] D. Shaltiel, I. Jacob and D. Davidov, J. Less-Common Met., 53 (1977) 117. [2] EG. Ivey and D.O. Northwood, Z. Phys. Chem. N.F. 147 (1986) 194. [3] Y. Moriwaki, M. Ikoma, Ii Gamo, H. Kawano, N. Yanagihara and T. Iwaki, Kokai Tokkyo Kohou A, (1985) 241652 (in Japanese). [4] H. Miyamura, T. Sakai, N. Kuriyama, K. Oguro, A. Kato and H. Ishikawa, Electrochem. Soc. Proc., 92-5 (1992) 179. [5] J. Huot, E. Akiba, T. Ogura and Y. Ishido, Denki Kagaku, 61 (1993) 1424. [6] S. Wakao and H. Sawa, J Less-Common Met., 172-174 (1991) 1219. [7] S. Wakao and Y. Yonemura, J. Less-Common Met., 89 (1983) 481.
[8]
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H. Sawa and S. Wakao, Mater. Trans. JIM, 31 (1990) 487.
[9] H. Nakano and S. Wakao, Denki Kagaku, 62 (1994) 953. [10] A.L. Allred, J. Inorg. Nucl. Chem., 17 (1961) 215. [11] Nihon Kinzoku Gakkai, Hi Kagaku Ryouronteki Kinzoku Kagobutu, Maruzen, Tokyo, 1975, p. 296. [12] I. Wada, H. Nakano and S. Wakao, J. Adv. Sci., 5 (1993) 11. [13] A.R. Miedema, J. Less-Common Met., 32 (1973) 117. [14] H.H. van Mal, K.'H.J. Buschow and A.R. Miedema, J. LessCommon Met., 35 (1974) 65. [15] W. Luo, S. Luo, J.D. Clewley and T.B. Flanagan, J. Alloys Comp., 202 (1993) 147. [16] C. Lartigue, A. Percheron-Guegan, J. Achard and F. Tasset, J. Less-Common Met., 75 (1980) 23. [17] A.L. Shilov, M.E. Kost and N.T. Kuznetsov, J. Less-Common Met., 105 (1985) 221.
AND COMPOUND5 ELSEVIER
Journal of Alloys and Compounds 231 (1995) 587-593
Substitution effect of elements in Zr-based alloys with Laves phase for nickel-hydride battery H. Nakano a, S. W a k a o b ~School of High-Technology for Human Welfare, Tokai University, 317 Nishino, Numazu, Shizuoka 410-03, Japan bTokai University, 3-10-22 Daita, Setagaya, Tokyo 155, Japan
Abstract
By examining the electrochemical and thermodynamic properties of Zr]_xA'x(VyNizMn,B'v)2÷ ~ ( 0 <~ x <~ 0.1; 0.10 ~
1. Introduction The chemical formula of a Laves phase intermetallic compound is expressed as A B 2. T h e r e are three types of Laves phase compounds, namely C15 (MgCu 2 type, cubic), C14 (MgZn 2 type, hexagonal) and C36 (MgNi 2 type, hexagonal). A m o n g these Laves phase compounds, Z r M n 2 ( H / M = 1.2), ZrCr 2 ( H / M = 1.3) and ZrV2 ( H / M = 1.8) contain much absorbed hydrogen and the hydrogen of these materials is in a tetrahedral site composed of A3B 1 or AEB 2. Many fundamental studies have been made on the hydrogen absorptiondesorption characteristics, thermodynamic properties, kinetics, diffusion mechanisms, etc. of these binary Laves phase alloys or the pseudobinary ones in which Z r is partially substituted with Ti and B site atoms are partially substituted with transition metals. Some alloys with non-stoichiometric composition, such as ABE+ ~ [1,2], have also been investigated, Several Laves phase alloys have large hydrogen 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 5 - 8 3 8 8 ( 9 5 ) 0 1 7 3 3 - X
absorption capacities and therefore have attracted interest as materials for negative electrodes in N i hydride batteries. Research on application to cells is in progress. Electrochemical properties of the C14-type Laves phase alloy have been studied in Refs. [3-5], for example, capacity as high as 3 7 0 m A h g -1 has been reported. Electrochemical properties of the C15-type alloy have also been examined [6-8] with a view to attaining higher capacity, improving the high rate capability for discharge and explaining the relation between the capacity and the occlusion site of hydrogen. In addition, discussion has been presented from the thermodynamics viewpoint of the fact that the cell volume depends upon the partial molar enthalpy change AH ° for hydride formation, i.e. the stability of the absorbed hydrogen. In this paper, for a series of 20 C15-type Z r system Laves phase alloys in which Z r is partially substituted with Ti or Nb and the B atom group is partially substituted with Ni, V, Co, Cr, Cu, Fe, etc., the c h a r g e -
Zr(VoloNio.54Mno.34Cuooz)2o Zr(Vo.loNio.6zMno.zs)z A Zr(VoAoNio.57Mno.28Feoos)z.l Zr(Vo.loNio.57Mno.28Coo.os)2 a Zr(Vo.~oNio.57Mno.28Cro.os)21 Zr(VoAoNio55Mno.3oCoo.os)21 Zr(VoAoNio.55Mno.3oCro.o~)z.1 Zr(VoAoNio.53Mno.32Coo.os)2.1 Zr(Vo.loNio.53Mno.3oCoo.os)2.2 Zr(VoAsNio.4~Mno.39Coo.os)2.2
Zro.95Tio.os(Vo.~oNio.49Mno.36Coo.os)2.o 5 Zro.gTioA(Vo.~oNio.49Mno.36Feo.os)2.o 5 Zro.gTio.~(VoloNio.49Mno36Cro.o5)2.o5 ZrogTio.~(VoaoNio.49Mno.36Cooos)zo 5 Zro.95Tio.os(VoaoNio.53Mno.3zCoo.os)2.1 Zro.gsTio.os(VoaoNio.svMno.zsCro.os)2.~ Zro.gTioa(Vo.~oNio.ssMno.3oCoo.os)2.t Zro.gTio.~(VoaoNio.53Mno.3zFeo.os)2.~ Zro.9Tio.l(Vo.loNio.49Mno.36Coo.os)2.1 Zro.gNbo.~(Vo.~oNio54Mno.3~Cooos)zl
X-1 X-2 X-3 X-4 X-5 X-6 X-7 X-8 X-9 X-10
Y-1 Y-2 Y-3 Y-4 Y-5 Y-6 Y-7 Y-8 Y9 Y-10
390 394 431 394 419 420 393 431 384 383
425 386 372 409 434 412 428 403 328 440
C - Ha ( m A h g -~)
368 378 390 370 408 393 368 403 367 383
380 348 334 406 355 397 416 359 319 403
17 b
332 360 368 350 397 365 357 386 364 382
351 339 328 400 352 387 405 339 313 337
55 b
312 350 350 338 391 358 352 362 350 381
327 328 322 394 342 381 399 328 310 330
110 b
D-Cap (mA h g - l )
303 333 338 326 385 345 347 361 335 380
301 314 318 388 336 367 382 326 300 320
220 b
0.823 0.881 0.867 0.881 0.94 0.878 0.943 0.90 0.913 0.99
0.79 0.902 0.952 0.96 0.946 0.92 0.92 0.908 0.940 0.794
RC
41.5 41.5 43.3 42.6 38.4 40.5 37.0 37.4 41.4 36.6
43.0 39.1 39.6 38.0 41.2 38.9 40.6 41.5 38.0 47.2
-AH° ( k J m o l H ~ ~)
124.9 127.3 123.7 126.4 124.6 118.5 118.8 121.2 124.2 125.7
118.1 120.7 123.6 124.8 113.4 126.0 125.1 125.4 121.0 130.2
-AS° ( J K - ~ m o l H ~ ~)
4.34 3.58 6.44 4.92 1.25 5.19 1.60 1.33 4.33 0.85
7.82 3.13 2.72 8.65 7.39 1.41 3.31 4.14 1.92 8.41
-AGO ( k J m o l H ~ 1)
87 73 64 75 100 81 92 94 81 97
91 100 100 100 75 100 100 93 94 69
Rc15 (%)
3.511 3.506 3.531 3.504 3.501 3.507 3.495 3.488 3.511 3.495
3.550 3.510 3.513 3.503 3.532 3.514 3.532 3.519 3.498 3.561
Vc15 (10ZA)
1.5239 1.5222 1.5226 1.5213 1.521 1.5205 1.5174 1.519 1.5195 1.517
1.528 1.5215 1.5226 1.522 1.5231 1.522 1.524 1.5232 1.5195 1.5258
r,v (A)
0.4037 0.3909 0.3825 0.3933 0.4111 0.4144 0.4079 0.3985 0.3870 0.3986
0.4294 0.4366 0.4328 0.4352 0.4245 0.4282 0.4176 0.4213 0.4081 0.3713
XB - XA
1.616 1.618 1.612 1.620 1.628 1.631 1.637 1.630 1.622 1.637
1.616 1.636 1.633 1.635 1.627 1.630 1.622 1.625 1.629 1.602
Xav
Hydrogen content absorbed into electrode (current density for charge, 17 m A g ~); ~current density for discharge (mA g-l) cut-off potential -0.60 V(Hg/Hg0), 25 °C; ~ratio of discharge capacities at current densities of 17 and 220 m A g-X.
Alloy
No.
Table 1 Charged hydrogen quantity (C - H), discharge capacity (D-Cap), existence ratio (Rc~5) of C15 type, unit cell volume (VcJs), average atomic radius (ray), difference in electronegativity (g~ - XA), partial molar enthalpy and entropy for hydrogenation at H / M = 036
/
~
~
=
.~
o~
H. Nakano, S. Wakao / Journal of Alloys and Compounds 231 (1995) 587-593
discharge characteristics and ties are examined. Factors molar enthalpy for hydride volume are also discussed in radii and electronegativities ments,
thermodynamic properinfluencing the partial formation and the cell a relation to the atomic of the constituent ele-
589
3. Results and discussion
3.1. Partial molar enthalpy for hydride formation As seen in Table 1, the absolute value of the partial molar enthalpy AH ° for hydride formation of the alloy lay in the range from 36.6 to 47.2 kJ m o l H f .
2. Experimental details
3.2. Electrochemical properties
Alloy specimens were prepared in amounts of 20-30g each by arc melting in an argon atmosphere within the following composition range: ! V yNzzMn,Bv)2+ " t Zrl_xAx( . (0~
The charging effÉciency (acquisition capacity/charging electricity quantity)remained almost 100% until the hydrogen quantity absorbed in the electrode (acquisition capacity) reached 2 0 0 m A h g -1. However, after this level exceeded, the efficiency decreased as the hydrogen gas generation increased. The maximum acquisition capacity in the perfectly charged state was approximately 330-440 mA h g-l, which is equivalent to H / M = 0.83-1.10 (Table 1). The discharge capacity at a current density of 1 7 m A g -1 was greater than 3 5 0 m A h g -1 except for alloys X-2, X-3 and X-9 (Table 1). Even at a considerably large current density of 220 mA g-i, all the alloy electrodes exhibited discharge capacities larger than 300 mA h g-l, in particular alloys X-4, X-7 and Y-5 exhibited capacities larger than 380 mA h g- 1. The ratio of the discharge capacity to the acquisition capacity was 82%-100% at 1 7 m A g -1. However, as the current density increased, the ratio decreased and the hydrogen quantity remaining in the alloy increased. It was noted that an alloy with a large remaining hydrogen quantity tended to have a large absolute value of AH °. As examples of alloys with good fatness in the discharge curve, X-10, Y-2 and X-6 were considered (Fig. 1). The high rate capability for discharge, R, of all the alloys except X-l, X-10, Y-l-Y-4 and Y-6 was more
,
,
,
,
~
, a 21
-o.6o -r" -rt~ > -0.80 ~> .~ E0J B a_ -1.0o
, o
200
Capacity (mAh g-')
) 400
Fig. 1. Discharge curves: 1, X-10; 2, Y-6; 3, Y-2 (17 mA g ~, 25 °C).
H. Nakano, S. Wakao / Journal of Alloys and Compounds 231 (1995) 587-593
590
, " o _ " ' a l ~ o - r ~ . _ , ~ o,,, u ~ xJ o
,
o.a
m o.4
0.0
i
i
35
40 45 50 IAH*I (kdrn°lH2"l) Fig. 2. Relation between absolute value of AH ° and high rate capability for discharge,
than 0.9. Although some deviation was seen as shown in Fig. 2, the high rate capability for discharge decreased as the absolute value of AH ° increased, From the above results it is concluded that an alloy with a AH ° value of 36-42 kJ molH~ 1 generally has a large discharge capacity and an excellent high rate capability for discharge. However, such electrochemical properties are liable to be influenced especially by the state of the surface in accordance with the preparation method of the alloy, such as solidification rate, heat treatment and surface oxidation. This being the case, some variations in the results might be due to the kinds of treatment methods used.
3.3. Substitution effect 3.3.1. Qualitative idea In general with binary metal-hydrides the ionicbond-like characteristics between the metal and the hydrogen are strengthened as the difference in electronegativity between the metal and the hydrogen increases. This explains why the absolute value of AH ° for hydride formation increases with the difference in electronegativity. Likewise, the characteristics of the ionic bond are strengthened when the difference in electronegativity between the alloy and the hydrogen is increased by means of element substitution in the
case of the Laves phase alloys. Thus the absorbed hydrogen is believed to be stabilized by alloying with elements which give larger differences in electronegativity. The series shown in Table 2 is an arrangement of the elements in accordance with the degree of Pauling electronegativity X as modified by Allred [10]. When an alloy is substituted by its anterior, the hydrogen is stabilized at its site. In the opposite case, when the alloy is substituted by its posterior, the hydrogen is destabilized. The series is convenient enough to think of the substitution effect. On the other hand, when the atomic radius of the A or B atom is changed, the geometrical dimensions of the layer structure among atoms are also changed. When the atomic radius of the A or B atom (or both) is decreased, the lattice is believed to be contracted. Conversely, when the atomic radius is increased, the lattice is thought to be expanded. This relation leads us to the following idea: when an atom in an alloy lattice is substituted by one with a smaller atomic radius, the lattice is contracted, and vice versa. Thus the stability of a hydride could be controlled by controlling its lattice geometry. The size order of the atomic radii is shown in Table 3 together with their numerical values [11]. The absolute value of AH ° is decreased by Z r ~ Ti, Mn ~ Ni, Cr---, Co, Fe, Ni and V ~ Ni substitutions. This fact satisfies the general rule referred to above. However, in some cases of substitution, namely N i ~ Co and Co ~ Fe, the absolute value of the actual AH ° decreased. As two general rules, an increase in AH ° was noticed as a result and discordance occurred in some cases. Discordance is also seen in the Zr(Wo.33 xNio.42MnoA7+xCoo.os)2.4 system C15-type V---~Mn substitution in the Laves phase alloy. The absolute value of the actual AH ° was decreased on this occasion, but an increase was noticed from the electronegativity and a decrease from the atomic radius as a result [12]. Although almost all the elements in the series of atomic radii and electronegativities comply with each other, inversion occurs in some cases, e.g. V and Mn. With the substitution of the elements for which inversion occurs, it is considered that the actual AH ° will be
Table 2 Series of electronegativities of elements x=
Zr < Ti < M n < N b < 1.33 1.54 1.55 1.60
V < Cr < Fe < C o < C u < 1.63 1.66 1.83 1.88 1.90
Ni H 1.91 2.1
Table 3 Series of atomic radii of elements r(,~) =
Zr > Nb > Ti > V > Mn > Cr > Cu > Fe > Co > Ni 1.771 1.625 1.614 1.491 1.428 1.423 1.413 1.411 1.385 1.377
H. Nakano, S. Wakao / Journal of Alloys and Compounds 231 (1995) 587-593
influenced by the stronger factor of either the effect of the atomic radius or that of the electronegativity.
3.3.2. Quantitative discussion It is necessary for these kinds of substitution effects to be dealt with quantitatively in order to explain their influence on the substitution quantity. To examine quantitatively the substitution effect (expansion-contraction .of the lattice or stability-instability of the hydroger 0 in terms of atomic radii or electronegativities, an average atomic radius ray and an average electronegativity Xav were introduced as follows:
i
,
I
~ 45 -~ "" ~" "r ~ 40
o O
o o..._ o
o
o 351.60
'
~ 1.62 Xav
0"~
'
' 1.64
(1)
i
Xav --
,
ai
Z
'
,-,
,
Fig. 4. Relation between average electronegativity Xav and absolute value of AH °.
(airi) rav --
so I
591
tendency was noted that the absolute value of AH ° decreased. The correlation is given by
(aix,)
(2)
Zi ai
IAH°I = 453.6 - 254.2Xav
(r = 0.91)
(4)
On the other hand, when both rav and Xav are considered, the correlation is further enhanced:
where r~ and Xi are the atomic radii and electronegativities corresponding to the individual constituent elements in the alloy respectively and a~ is the number of atoms. Plotted in Fig. 3 is the relation between AH ° and the average atomic radius rav. Although a considerable degree of deviation is seen, a correlation as shown in
IAn°l = 27.9 + 232.3rav - 209.8Xa v
(r = 0.93)
(5) 0
Between the absolute value of the enthalpy Ancalc
the following equation is obtained and a tendency was noticed that the absolute value of AH ° increased as rav increased:
calculated from Eq. (5) and the measured enthalpy AH ° a fairly good linear relation was seen as shown in Fig. 5. Since it is understood from the results in the previous subsection that AH ° of the alloys with excellent electrochemical characteristics lies in the range from - 3 6 to - 4 2 kJ molH~ 1, the ranges of ray and Xav calculated from the above equation are given as
IAH°I = -1061.0 + 723.7ray
1.516 ~< ray ~< 1.525
(6)
1.621 ~
(7)
(correlation coefficient r = 0.74)
(3)
Shown in Fig. 4 is the relation between AH ° and the average electronegativity X~v. As Xav increased, a
When the B atom in the AB2+ ~ alloy system is substituted by an element with a larger elec50
50
'
'
,~
'
0
-r
"5
E 45
~
""
°
0
0
o
"r <::] 4 0
35
¥ E ~.~45 ,..,,
O
--~
0
oO
c~i~ 0
1.516
I 1.520
<] 4C
t 1.524 rav
'
o
-
O
'
I 1.528
(~,)
Fig. 3. Relation between average atomic radius ra~ and absolute value of AH °.
ZoOO
35
35
I
t
40
45
IAH~xpl
50
(kJrnolH=-~)
Fig. 5. Relation between calculated and measured values of AH °.
592
H. Nakano, S. Wakao / Journal of Alloys and Compounds 231 (1995) 587-593
tronegativity, the hydrogen becomes unstable, because the difference in electronegativity between the hydrogen and the alloy decreases. On the other hand, if the bond strength of the alloy is shown by the difference in electronegativity between the A and B atoms, the alloy becomes stable, because the difference in electronegativity between the A and B atoms increases, Consequently, the result of Eq. (4) does not contradict that of the reversed stability presented by Miedema [13,14]. We consider that this result is fundamentally similar to the Miedema's rule, although the method of expressing it differs. However, our result did not correspond with Miedema's rule when part of the A atom was substituted by another element. Although it is necessary to consider the influences of the atomic radius and the difference in electronegativity between the A and B atoms, etc. in this case, it is difficult to explain these effects because of their complexity. It remains a problem to be solved. Also, we have an idea that these factors are interrelated, because the atomic radii and electronegativities of pure metals are correlated appreciably, though the improvement in the correlation in Eq. (5) can be explained as a statistical problem, 3.3.3. Factors which influence the cell v o l u m e
Plotted in Fig. 6 is the relation between the cell volume Vcl5 of C15 type and the absolute value of AH °. Although a considerable deviation is noticed, the following correlation is obtained: IAH°I = -360.6 + 1.1Vcl5
(r = 0.83)
(8)
The above equation indicates that the absolute value of AH ° increased as the cell volume increased; a similar relation is also seen in rare earth-nickel systems [15-17]. On the other hand, a tendency is noted between Vc~5 and ray that VC15 increases with increasing rav as described by the equation
50
l
,
r'~ -r -~ 4 6 "~
,
'
o ~ ~
o
o
o 7•"r < 40 ~ -~~8o% 35
3.48
o O o °° t
t
3.52
i
I
3.56
Vc15 (XlOaA) Fig. 6. Relation between cell volume Vc~5 and absolute value of AH°.
Vcl 5 = -568.9 + 604.7ray
(r = 0.86)
(9)
When the difference in electronegativity between A and B is increased, the bond strength between the two is increased and the lattice is considered to be contracted. Therefore, to examine this influence, the difference AXB A in electronegativity between A and B was introduced as defined by AXB-A=XB--XA (10) In this case the alloy composition AB2+,~ can be expressed as A3/t3+~)B~/(3+~) (B)2.0 on the assumption that the atomic ratio of the A and B groups is 1:2. Accordingly, XA and XB show the individual average electronegativities of these A and B atomic groups. Taking account of rav and this AXB_A, the cell volume is given as follows and the correlation is further improved: Vcl 5 = -596.5 + 628.9rav - 22.6AXB_A
(r = 0.89) (11)
From the above description it is seen that dealing quantitatively with the substitution effect by considering the atomic radius and/or electronegativity contributes to the design of alloys with excellent electrochemical characteristics, although the explanation is quite an approximate one.
4. Conclusions
By discussing how the substitution effect corresponds to the atomic radius and electronegativity after the examination of the electrochemical and thermodynamic properties of multicomponent alloys of the Z r - V - N i - M n system with C15-type Laves phase, the conclusions shown below were obtained. (1) AH ° exhibited a fairly good correlation with the average electronegativity and correlated also with the average atomic radius rav. By consideration of these two factors together, the correlation was further augmented. (2) The cell volume of C15 type correlated with AH °. Also, it has been pointed out that the cell volume depends upon the average atomic radius ray and the difference A yB_ A in electronegativity between A and B. (3) Many of these alloys whose absolute value of AH ° lies in the range from 36 to 42 kJ molH~ ~ have a discharge capacity greater than 3 8 0 m A h g 1 at a current density of 220 m A g-1 and a good high rate capability for discharge has been obtained. The values of rav and ,¥av in this range have been obtained as 1.516 ~
H. Nakano, S. Wakao / Journal of Alloys and Compounds 231 (1995) 587-593
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H. Sawa and S. Wakao, Mater. Trans. JIM, 31 (1990) 487.
[9] H. Nakano and S. Wakao, Denki Kagaku, 62 (1994) 953. [10] A.L. Allred, J. Inorg. Nucl. Chem., 17 (1961) 215. [11] Nihon Kinzoku Gakkai, Hi Kagaku Ryouronteki Kinzoku Kagobutu, Maruzen, Tokyo, 1975, p. 296. [12] I. Wada, H. Nakano and S. Wakao, J. Adv. Sci., 5 (1993) 11. [13] A.R. Miedema, J. Less-Common Met., 32 (1973) 117. [14] H.H. van Mal, K.'H.J. Buschow and A.R. Miedema, J. LessCommon Met., 35 (1974) 65. [15] W. Luo, S. Luo, J.D. Clewley and T.B. Flanagan, J. Alloys Comp., 202 (1993) 147. [16] C. Lartigue, A. Percheron-Guegan, J. Achard and F. Tasset, J. Less-Common Met., 75 (1980) 23. [17] A.L. Shilov, M.E. Kost and N.T. Kuznetsov, J. Less-Common Met., 105 (1985) 221.