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Effect of alloy composition on hydrogen diffusion in the AB5-type hydrogen storage alloys
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JOURNAL OF
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ELSEVIER
Journal of ElectroanalyticalChemistry398 (1995) 37-41
Effect of alloy composition on hydrogen diffusion in the AB5-type hydrogen storage alloys Chiaki Iwakura a,*, Takafumi Oura a, Hiroshi Inoue a, Masao Matsuoka b, Yoshifumi Yamarnoto b a Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 593, Japan b Department of Chemistry, Faculty of Science and Engineering, Ritsumeikan University,Kusatsu, Shiga 525, Japan
Received 19 April 1995; in revisedform 10 July 1995
Abstract Disc-type and powder-type electrodes of hydrogen storage alloys having the compositions MmNi4.zAlo.sMo.3(Mm = misch metal; M = Cr, Mn, Fe, Co, Ni) were used for the evaluation of diffusion coefficients of hydrogen by the potential-step method and the electrochemical measurement of the pressure-composition isotherm respectively. The Einstein diffusion coefficient, being independent of hydrogen concentration, was measured as a function of the atomic radius of foreign metals(M). The diffusion kinetics of the hydrogen atom in the alloys was discussed based on the thermodynamic stability of the resulting hydrides and electrostatic interaction between foreign metals and hydrogen. Keywords: Hydrogendiffusion; Diffusion coefficient;Hydrogenstorage alloy; Nickel-hydrogenbattery
1. Introduction Hydrogen storage alloys have been used as negative electrodes for nickel-hydrogen rechargeable batteries because of their reversible absorption-desorption properties of hydrogen [1-3]. During charging and discharging, diffusion of hydrogen atoms in the alloy occurs together with charge-transfer reactions on the alloy surfaces. Since diffusion can be a rate-determining step in the charge-discharge processes, it must be a key factor for improving the properties of the negative electrode for high-rate capability. There are a large number of reports for evaluating the diffusion coefficient of hydrogen in metals such as Pd and Ni and alloys such as TiFe, Mg2Ni and LaNi 5 using quasi-elastic neutron scattering and nuclear magnetic resonance methods [4]. In addition, the diffusion coefficient of hydrogen was determined from galvanostatic or potentiostatic permeation of hydrogen through wire-type and foiltype electrodes of hydrogen storage metals such as Pd and Ni [5,6]. Recently, the time-scales of electrochemical absorption and desorption of hydrogen in relation to dimen-
* Corresponding author. 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0022-0728(95)04253-9
sions and geometries of host metal hydride electrodes have also been reported [7]. In contrast, there are few papers describing an electrochemical evaluation of the diffusion coefficient of hydrogen in hydrogen storage alloys [8], although the evaluation of the diffusion coefficient under the same conditions as used in the battery as a negative electrode is important. In the present work, the diffusion coefficient of hydrogen in AB5-type hydrogen storage alloys, substituted partially by various foreign metals, was evaluated using the potential-step method and the influence of foreign metals on the diffusion coefficient was investigated.
2. Experimental Hydrogen storage alloys having compositions MmNi4. 2Al0.sM0. 3 (M = Cr, Mn, Fe, Co, Ni) were prepared by the arc melting method in an atmosphere of argon gas. The composition of the misch metal (Mm) was 24.19% La, 55.24% Ce, 5.43% Pr, 15.08% Nd, 0.06% Sm. Two types of negative electrode were prepared in this work. One was a powder-type electrode for electrochemical measurement of the pressure-composition isotherm and the other was a disc-type electrode for measurement of the diffusion coef-
38
C. lwakura et al. /Journal of Electroanalytical Chemistry 398 (1995) 37-41
ficient of hydrogen in the alloys. The former was prepared by loading the alloy powders (particle size 106-125/zm ~b) mixed with polyvinyl alcohol solution in a porous nickel substrate in the manner reported previously [9,10]. The latter was prepared as follows. The as-prepared hydrogen storage alloys were sliced with a diamond cutter, resulting in disc-like alloy samples having a thickness of about 1 mm and a diameter of about 5 mm. The alloy discs were soldered to a nickel wire and covered with epoxy resin on one side of the surface. The resulting alloy disc electrodes were polished with emery papers (#400, #800), buffed with a 0.1 /zm alumina suspension and then washed in an ultrasonic bath for about 30 s prior to the electrochemical measurements. Crystallographic and electrochemical measurements were carried out as reported previously [9,10]. A Hg IHgO 16 M KOH electrode was employed as reference electrode and all electrode potentials are given with respect to this reference electrode. The electrolyte was a deaerated 6 M KOH solution. The powder-type electrode was charged for 2.5 h at 20 mA and discharged to - 0 . 5 V vs. Hg IHgO at the same current. In contrast, the disc-type electrodes were charged for 20 min at 1 mA and discharged to - 0 . 5 V vs. HglHgO at the same current. After each charging, the circuit was opened for 10 min. Such a cycle was repeated using a galvanostatic charge-discharge unit (Hoknto Denko, HJ-201B) and the potential of the negative electrode was recorded by a multichannel recorder (Yokogawa, 4176). The concentration of hydrogen absorbed in the alloys was evaluated by electrochemical measurements of pressure-composition isotherms. After ten charge-discharge cycles under the above described conditions, the powdertype electrodes were charged at 5 mA for various times and then the circuit was opened. After that, the equilibrium potential Eoq was measured using a potentiometer (Nichia, HT-1D2). The following equation holds between the equilibrium hydrogen pressure Pus and the Eeq value at 30°C in 6 M KOH solution [11]:
the potential step using a voltammetric analyzer (BAS, CV-50W). Pressure-composition isotherms for hydrogen absorption and desorption were measured by Sieverts' method at various temperatures. The enthalpy change AH of hydride formation was calculated by means of van't Hoff's equation In PH2 = A H / R T -
(2)
AS/R
where R and T are the gas constant and absolute temperature respectively.
3. Results and discussion
Most of the adsorbed hydrogen atoms (Had) on the surface of the hydrogen storage alloys, which are produced by the Volmer reaction H 2 0 + e----> Had + O H -
(3)
dissolve in the alloys to produce a solid solution (a-phase). Further increase in hydrogen concentration in the a-phase leads to formation of a hydride (/3-phase). It is well known that in the range of the a-phase, the concentration n of hydrogen atoms in the alloys, which is represented as an atomic ratio ( H / M ) of hydrogen to constituent metals, depends on the square-root of the PH2 value, as represented by the following equation: (4)
n = KsPH: 1/2 + K o
where K s and K 0 indicate the Sieverts' constant and the concentration of hydrogen atoms adsorbed on the alloy surfaces [13] respectively. A typical Sieverts' plot, based on a pressure-composition isotherm, for the MmNi4.sA10. 5 electrode at 30°C is shown in Fig. 1. The negative deviation from a straight line in the range of hydrogen concentration greater than about 0.08 is ascribable to the formation of hydride. By applying the data in Fig. 1 to Eq. (1), the potential Ead _. at which the formation of the a-phase is completed, and
Eeq(V vs. Hg [HgO) = - 0 . 9 2 9 6 - 0.0301 log PH2 (1) Since the Eoq value changes with temperature, this equation was corrected with E ° ( H 2 0 [ H 2 ) - E ° ( H g O I H g ) , log a(H20) and log T ( H 2 ) , where a(H20) and T ( H 2 ) are the activity of water and fugacity coefficient of H E at various temperatures in 6 M KOH solution, according to Ref. [12]. The diffusion coefficient of hydrogen in the alloys was measured by the potential-step method. After the disc-type electrodes had been activated by charge-discharge cycling ten times, known quantities of hydrogen were galvanostatically charged at 1 mA. After the potential of the disc-type electrodes reached the equilibrium potential, an overvoltage of + 200 mV was applied to these electrodes. Current-time transient curves were recorded immediately after
2.0 .,"
1.6
•
•
.6
E 1.2, ¢= 0.8 n
0.4 j ~ 0-0
#
0"
I
0.03
I
0.06 0.09 n /(H/M)
I
0.12
0.15
Fig. 1. Typical Sievens' plot of the pressure-composition isotherm (hydrogen absorption) for the MmNi4.sA10.s electrode at 30°C.
C. lwakura et aL /Journal of Electroanalytical Chemistry 398 (1995) 37-41 2.0
Table 1 Radius r, parameters K s and K 0 of Eq. (4), and potential range in a-phase at 30 °C for the MmNi4.2A10.sM0.3 electrodes
1.5 M Cr Mn Fe Co Ni
r a (,~)
Ks (atm a/z)
K0 (H/M)
Ead ~ a (V(vs.aglagO))
Ea - a + 0 (V(vs.aglHgO))
1.276 1.268 1.260 1.252 1.244
0.094 0.116 0.074 0.052 0.044
0.012 0.011 0.015 0.015 0.013
- 0.852 -0.847 - 0.858 - 0.863 - 0.869
- 0.911 -0.899 - 0.923 - 0.924 - 0.925
I \
,\
E ~ 1.0
II I--
n --0.02185 --0.02410
=0.02613
0.5
0 a
39
Ref. [14].
0
I
I
I
[
I
5
10
15
20
25
30
t/s
the potential E,~ ._, ,~+, at which the phase transition from a-phase to /3-phase begins, were evaluated to be - 0 . 8 6 9 and - 0 . 9 2 5 V vs. Hg [HgO respectively, as shown in Table 1. Sieverts' plots in the case of alloys in which various foreign metals are substituted for a part of the nickel are shown in Fig. 2. The values of K s, K 0, Ead _. and E,~ _~,~+~ for each alloy electrode are summarized in Table 1. The K s value decreased in the order Mn, Cr, Fe, Co, Ni and the values of Ead __,,~ and E,~ ... ,~+/3 shifted to the negative direction in the same order, indicating that the a-phase shifts to the lower hydrogen concentrations. During the discharge, adsorbed hydrogen atoms on the MmNia.2Al0.sM0. 3 alloys are electrochemically oxidized as follows: Had + OH- --->H 2 0 + e -
(5)
With decreasing surface concentration of the adsorbed hydrogen atoms, the hydrogen atoms in the alloys diffuse towards the surface at a velocity proportional to their concentration gradient. Therefore, either the hydrogen diffusion or the electrooxidation controls the discharge reaction. The time-course of oxidation current for the hydrogen storage metal was theoretically analyzed using the potentiostatic polarization method reported by Krapivnyi [15]. The following equation is valid for long periods of time when the discharge reaction is controlled by hydrogen diffusion in the metal [15]: In I =
In(2FAcDH/tS)
-
(Tr2DH/at~ 2)t
(6)
Fig. 3. The time-course of logarithmic oxidation current for the MmNi4.sAI0. 5 electrode with various hydrogen concentrations at 30°C.
where c, A, D H and 6 are hydrogen concentration (mol cm-3), surface area (cm2), Fick's diffusion coefficient (cm 2 s-1) and thickness of diffusion layer (cm) respectively. This equation was applied to the hydrogen diffusion in hydrogen storage alloys used in this work. As a typical example, the logarithmic oxidation current In I after the potential step for the MmNi4.zA10.sM03 electrode with various hydrogen concentrations is shown as a function of time in Fig. 3. The value of In I immediately after the potential step sharply decreases with decrease in concentration of adsorbed hydrogen on the electrode surface due to the electrooxidation. The value of In I thereafter decreases linearly according to Eq. (6) because the rate-determining step changes from electrooxidation to hydrogen diffusion in the alloy. The values of In I decrease linearly between about 20 s and 30 s and increase with increasing hydrogen concentration, as shown in Fig. 3. In this work, Fick's diffusion coefficient D H for hydrogen was calculated using the value of In I between 20 s and 30 s in Eq. (6). The D~ values for the MmNi4.EA10.sMo.3 electrodes are shown in Fig. 4 as a function of hydrogen concentration. The D n value decreased with increase in hydrogen concentration in all cases, because the hydrogen diffusion is interfered with due to the increase of interaction be-
2.0 3.5 1.6
3.0 2.5
1.2
Co
2.0
e
1.5
Cr
cf
io 08
© 0.4
1.0
Mn
0.5 0 0
0.03
0.06
0.09
0.12
0.15
n / (H/M)
Fig. 2. Sieverts' plots of the pressure-composition isotherms (hydrogen absorption) for MmNi4,2AIo.sMo.3(M = Cr, Mn, Fe, Co, Ni) electrodes at 30°C.
0
i 0
0.02
0.04
0.06
0.08
n / (H/M)
Fig. 4. Diffusion coefficients as a function of hydrogen concentration in the MmNi4.2Al0.sM0. 3 electrodes at 30°C.
C. lwakura et al. / Journal of Electroanalytical Chemistry 398 (1995) 37-41
40
tween the absorbed hydrogen atoms and the constituent metals with increasing hydrogen concentration. The Einstein diffusion coefficient D *, being independent of the hydrogen concentration, becomes a measure for comparison of diffusion behavior of hydrogen in the MmNia.2A10.sM0.3 electrodes. The following relationship between D* and D u applies [16,17]:
where /z° is the standard potential, RT ln(n/(1 - n ) ) is the ideal solution configurational term and Wn_ n is the partial interaction Gibbs energy of dissolved hydrogen, respectively. By differentiating both sides of Eq. (8) with respect to n, the following equation can be derived: RT
dn
-
n(1 - n)
+W._.
(9)
By substituting Eq. (9) in Eq. (7), D n is given by D H =
(1 D* - 1 -
+ n
WH_H ) n RT
(10)
Since the n value is much less than 1, this equation can be simplified to DH=D*
1+
RT
(11)
n
The D* value was evaluated by extrapolating the D H value to infinite dilution of hydrogen. The results are summarized in Table 2, together with the crystallographic and thermodynamic data. The crystallographic data of MmNi4.2Alo.5Mo.3 alloys indicated that the unit cell volume increased with increasing radius of foreign metals. An increase in the unit cell volume of the alloys stabilized the resulting metal hydride, as can be seen from the A H value. However, the A H value for the MmNia.2Alo.sMno. 3 alloy was somewhat smaller than that predicted from the
Table 2 Electronegativity X for element M, unit cell volume V, Einstein diffusion coefficient D * and activation energy E a for diffusion of hydrogen for the MmNi4.2A10.5 M0. 3 electrodes
M
Cr Mn Fe Co Ni
X a
1.66 1.55 1.83 1.88 1.91
a Ref. [20].
-16 -18
~
F e ~ . ~
Fe
°
o
-20
(7)
where tz H is the chemical potential of the hydrogen dissolved in the alloy at concentration n. In addition, P~H is represented by the following equation [18,19]: n (8) IXn = I~°n + RT l n l _ n + WH_Hn
-
~
-22
n d/z n D H = D * R T dn
d lz H
-14
V
AH
D * × 10 s
Ea
(,~3)
(kj (mol H 2 ) - 1)
(cm 2 s - l )
(kj m o l - t)
86.9 86.8 86.4 85.9 85.8
-31.1 - 32.6 - 30.7 - 29.0 - 27.9
2.18 1.62 2.55 2.99 3.24
24.4 26.3 21.0 19.9 19.1
-24 3.15 3.2 3.25 3.3 3.35 3.4 3.45 3.5 3.55 1000 T "1/ K-1 Fig. 5. Arrhenius plots of the D* electrodes.
value for the MmNi4.2Alo.5Mo.3
unit cell volume. In the case of the MmNi4.2Al0.sMn0. 3 alloy, the contribution of electronegativity to the increase of stability of the resulting hydride seems to be quite large, suggesting that not only the crystallographic parameter but also the electrostatic interaction is responsible for the thermodynamic stability of absorbed hydrogen. The D * value increases with an increase of the AH value, that is in the order Mn < Cr < Fe < Co < Ni. Therefore, it is concluded that the D * value is influenced by the stabilization of hydrogen atoms in the alloys substituted by various foreign metals, as might be expected. The D* value for each MmNia.2A10.sM0. 3 alloy was evaluated at various temperatures. The results are shown in Fig. 5. The logarithmic D * value was linearly dependent on the reciprocal of absolute temperature. The following relationship holds between the D* value and the activation energy E a for diffusion of H in the alloy: In D* = - E a / R T + In D O
(12)
where D O represents a frequency factor. The D * value was evaluated from the data in Fig. 5 and is summarized in Table 2, which indicates that the D * value increases with a decrease of the E a value.
4. Conclusions
Electrochemical investigations of the series of hydrogen storage alloys MmNia.2Alo.sMo.3 (M = Cr, Mn, Fe, Co, Ni), in which nickel is partially substituted by foreign metals, disclosed that the enthalpy change of hydride formation and the Einstein diffusion coefficient of hydrogen increase with decreasing atomic radius of the foreign metals. Crystallographic analyses indicated that a decrease in unit cell volume, except for the Mn-substituted alloy, destabilized the resulting hydrides due to the decrease in atomic radius of the foreign metals. In the case of the Mn-substituted alloy not only the crystallographic features but also the electrostatic interaction influenced the thermodynamic stability of the resulting hydride.
C. lwakura et al. /Journal of Electroanalytical Chemistry 398 (1995) 37-41
Acknowledgements This w o r k was partly D e v e l o p m e n t a l Scientific G r a n t - i n - A i d for Scientific the Ministry of Education,
supported by G r a n t - i n - A i d for Research no. 0 5 5 5 5 1 7 2 and R e s e a r c h (C) no.07651001 f r o m Science and Culture o f Japan.
References [1] C. lwakura and M. Matsuoka, Prog. in Batteries & Battery Materials, 10 (1991) 81. [2] C. lwakura, M. Matsuoka, T. Sakai and H. Ishikawa, Kagaku to Kogyo (Osaka), 65 (1991) 305, 346. [3] T. Sakai, M. Matsuoka and C. Iwakura, in K.A. Gshneidner, Jr. and L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Elsevier Science, Amsterdam, in press. [4] D. Richter, R. Hempelmann and R.C. Bowman, Jr., in L. Schlapbach (Ed.), Topics in Applied Physics, Vol. 67, Hydrogen in Intermetallic Compounds II, Springer, Berlin, 1992, Chapter 3. [5] C. Wagner and G. Heller, Z. Phys. Chem., Ser. B, 46 (1940) 242.
41
[6] M.D. Archer and N.C. Grant, Proc. Roy. Soc. London, Ser. A, 395 (1984) 165. [7] B.E. Conway and J. Wojtowicz, J. Electroanal. Chem., 326 (1992) 277. [8] B. Klein, A. Redeker and H. Ziichner, Z. Phys. Chem., 181 (1993) 95. [9] M. Matsuoka, K. Asai, Y. Fukumoto and C. Iwakura, Electrochim. Acta, 38 (1993) 659. [10] M. Matsuoka, T. Kohno and C. lwakura, Electrochim. Acta, 38 (1993) 787. [11] C. Iwakura, T. Asaoka, H. Yoneyama, T. Sakai, H. Ishikawa, K. Oguro, Nippon Kagaku Kaishi, (1988) 1482. [12] J. Balej, Int. J. Hydrogen Energy, 10 (1985) 365. [13] S. Kishimoto, M. Inoue, N. Yoshida and T.B. Flanagan, J. Chem. Soc., Faraday Trans. 1, 82 (1986) 2175. [14] L. Pauling, The Nature of the Chemical Bond, Cornell University Press, New York, 1960. [15] N.G. Krapivnyi, l~lektrokhimiya, 18 (1982) 1174. [16] H. Zi~chner and N. Boes, Ber. Bunsenges. Phys. Chem., 76 (1972) 783. [17] S. Tanaka, J.D. Clewley and T.B. Flanagan, J. Phys. Chem., 81 (1977) 1684. [18] H. Brodowsky, Ber. Bunsenges. Phys. Chem., 76 (1972) 740. [19] W. Auer and H.J. Grabke, Ber. Bunsenges. Phys. Chem., 78 (1974) 58. [20] A.L. Allred, J. Inorg. Nucl. Chem., 17 (1961) 215.
.
~" \
:
, 7~ ,:.
~
TM
JOURNAL OF
,
ELSEVIER
Journal of ElectroanalyticalChemistry398 (1995) 37-41
Effect of alloy composition on hydrogen diffusion in the AB5-type hydrogen storage alloys Chiaki Iwakura a,*, Takafumi Oura a, Hiroshi Inoue a, Masao Matsuoka b, Yoshifumi Yamarnoto b a Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 593, Japan b Department of Chemistry, Faculty of Science and Engineering, Ritsumeikan University,Kusatsu, Shiga 525, Japan
Received 19 April 1995; in revisedform 10 July 1995
Abstract Disc-type and powder-type electrodes of hydrogen storage alloys having the compositions MmNi4.zAlo.sMo.3(Mm = misch metal; M = Cr, Mn, Fe, Co, Ni) were used for the evaluation of diffusion coefficients of hydrogen by the potential-step method and the electrochemical measurement of the pressure-composition isotherm respectively. The Einstein diffusion coefficient, being independent of hydrogen concentration, was measured as a function of the atomic radius of foreign metals(M). The diffusion kinetics of the hydrogen atom in the alloys was discussed based on the thermodynamic stability of the resulting hydrides and electrostatic interaction between foreign metals and hydrogen. Keywords: Hydrogendiffusion; Diffusion coefficient;Hydrogenstorage alloy; Nickel-hydrogenbattery
1. Introduction Hydrogen storage alloys have been used as negative electrodes for nickel-hydrogen rechargeable batteries because of their reversible absorption-desorption properties of hydrogen [1-3]. During charging and discharging, diffusion of hydrogen atoms in the alloy occurs together with charge-transfer reactions on the alloy surfaces. Since diffusion can be a rate-determining step in the charge-discharge processes, it must be a key factor for improving the properties of the negative electrode for high-rate capability. There are a large number of reports for evaluating the diffusion coefficient of hydrogen in metals such as Pd and Ni and alloys such as TiFe, Mg2Ni and LaNi 5 using quasi-elastic neutron scattering and nuclear magnetic resonance methods [4]. In addition, the diffusion coefficient of hydrogen was determined from galvanostatic or potentiostatic permeation of hydrogen through wire-type and foiltype electrodes of hydrogen storage metals such as Pd and Ni [5,6]. Recently, the time-scales of electrochemical absorption and desorption of hydrogen in relation to dimen-
* Corresponding author. 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0022-0728(95)04253-9
sions and geometries of host metal hydride electrodes have also been reported [7]. In contrast, there are few papers describing an electrochemical evaluation of the diffusion coefficient of hydrogen in hydrogen storage alloys [8], although the evaluation of the diffusion coefficient under the same conditions as used in the battery as a negative electrode is important. In the present work, the diffusion coefficient of hydrogen in AB5-type hydrogen storage alloys, substituted partially by various foreign metals, was evaluated using the potential-step method and the influence of foreign metals on the diffusion coefficient was investigated.
2. Experimental Hydrogen storage alloys having compositions MmNi4. 2Al0.sM0. 3 (M = Cr, Mn, Fe, Co, Ni) were prepared by the arc melting method in an atmosphere of argon gas. The composition of the misch metal (Mm) was 24.19% La, 55.24% Ce, 5.43% Pr, 15.08% Nd, 0.06% Sm. Two types of negative electrode were prepared in this work. One was a powder-type electrode for electrochemical measurement of the pressure-composition isotherm and the other was a disc-type electrode for measurement of the diffusion coef-
38
C. lwakura et al. /Journal of Electroanalytical Chemistry 398 (1995) 37-41
ficient of hydrogen in the alloys. The former was prepared by loading the alloy powders (particle size 106-125/zm ~b) mixed with polyvinyl alcohol solution in a porous nickel substrate in the manner reported previously [9,10]. The latter was prepared as follows. The as-prepared hydrogen storage alloys were sliced with a diamond cutter, resulting in disc-like alloy samples having a thickness of about 1 mm and a diameter of about 5 mm. The alloy discs were soldered to a nickel wire and covered with epoxy resin on one side of the surface. The resulting alloy disc electrodes were polished with emery papers (#400, #800), buffed with a 0.1 /zm alumina suspension and then washed in an ultrasonic bath for about 30 s prior to the electrochemical measurements. Crystallographic and electrochemical measurements were carried out as reported previously [9,10]. A Hg IHgO 16 M KOH electrode was employed as reference electrode and all electrode potentials are given with respect to this reference electrode. The electrolyte was a deaerated 6 M KOH solution. The powder-type electrode was charged for 2.5 h at 20 mA and discharged to - 0 . 5 V vs. Hg IHgO at the same current. In contrast, the disc-type electrodes were charged for 20 min at 1 mA and discharged to - 0 . 5 V vs. HglHgO at the same current. After each charging, the circuit was opened for 10 min. Such a cycle was repeated using a galvanostatic charge-discharge unit (Hoknto Denko, HJ-201B) and the potential of the negative electrode was recorded by a multichannel recorder (Yokogawa, 4176). The concentration of hydrogen absorbed in the alloys was evaluated by electrochemical measurements of pressure-composition isotherms. After ten charge-discharge cycles under the above described conditions, the powdertype electrodes were charged at 5 mA for various times and then the circuit was opened. After that, the equilibrium potential Eoq was measured using a potentiometer (Nichia, HT-1D2). The following equation holds between the equilibrium hydrogen pressure Pus and the Eeq value at 30°C in 6 M KOH solution [11]:
the potential step using a voltammetric analyzer (BAS, CV-50W). Pressure-composition isotherms for hydrogen absorption and desorption were measured by Sieverts' method at various temperatures. The enthalpy change AH of hydride formation was calculated by means of van't Hoff's equation In PH2 = A H / R T -
(2)
AS/R
where R and T are the gas constant and absolute temperature respectively.
3. Results and discussion
Most of the adsorbed hydrogen atoms (Had) on the surface of the hydrogen storage alloys, which are produced by the Volmer reaction H 2 0 + e----> Had + O H -
(3)
dissolve in the alloys to produce a solid solution (a-phase). Further increase in hydrogen concentration in the a-phase leads to formation of a hydride (/3-phase). It is well known that in the range of the a-phase, the concentration n of hydrogen atoms in the alloys, which is represented as an atomic ratio ( H / M ) of hydrogen to constituent metals, depends on the square-root of the PH2 value, as represented by the following equation: (4)
n = KsPH: 1/2 + K o
where K s and K 0 indicate the Sieverts' constant and the concentration of hydrogen atoms adsorbed on the alloy surfaces [13] respectively. A typical Sieverts' plot, based on a pressure-composition isotherm, for the MmNi4.sA10. 5 electrode at 30°C is shown in Fig. 1. The negative deviation from a straight line in the range of hydrogen concentration greater than about 0.08 is ascribable to the formation of hydride. By applying the data in Fig. 1 to Eq. (1), the potential Ead _. at which the formation of the a-phase is completed, and
Eeq(V vs. Hg [HgO) = - 0 . 9 2 9 6 - 0.0301 log PH2 (1) Since the Eoq value changes with temperature, this equation was corrected with E ° ( H 2 0 [ H 2 ) - E ° ( H g O I H g ) , log a(H20) and log T ( H 2 ) , where a(H20) and T ( H 2 ) are the activity of water and fugacity coefficient of H E at various temperatures in 6 M KOH solution, according to Ref. [12]. The diffusion coefficient of hydrogen in the alloys was measured by the potential-step method. After the disc-type electrodes had been activated by charge-discharge cycling ten times, known quantities of hydrogen were galvanostatically charged at 1 mA. After the potential of the disc-type electrodes reached the equilibrium potential, an overvoltage of + 200 mV was applied to these electrodes. Current-time transient curves were recorded immediately after
2.0 .,"
1.6
•
•
.6
E 1.2, ¢= 0.8 n
0.4 j ~ 0-0
#
0"
I
0.03
I
0.06 0.09 n /(H/M)
I
0.12
0.15
Fig. 1. Typical Sievens' plot of the pressure-composition isotherm (hydrogen absorption) for the MmNi4.sA10.s electrode at 30°C.
C. lwakura et aL /Journal of Electroanalytical Chemistry 398 (1995) 37-41 2.0
Table 1 Radius r, parameters K s and K 0 of Eq. (4), and potential range in a-phase at 30 °C for the MmNi4.2A10.sM0.3 electrodes
1.5 M Cr Mn Fe Co Ni
r a (,~)
Ks (atm a/z)
K0 (H/M)
Ead ~ a (V(vs.aglagO))
Ea - a + 0 (V(vs.aglHgO))
1.276 1.268 1.260 1.252 1.244
0.094 0.116 0.074 0.052 0.044
0.012 0.011 0.015 0.015 0.013
- 0.852 -0.847 - 0.858 - 0.863 - 0.869
- 0.911 -0.899 - 0.923 - 0.924 - 0.925
I \
,\
E ~ 1.0
II I--
n --0.02185 --0.02410
=0.02613
0.5
0 a
39
Ref. [14].
0
I
I
I
[
I
5
10
15
20
25
30
t/s
the potential E,~ ._, ,~+, at which the phase transition from a-phase to /3-phase begins, were evaluated to be - 0 . 8 6 9 and - 0 . 9 2 5 V vs. Hg [HgO respectively, as shown in Table 1. Sieverts' plots in the case of alloys in which various foreign metals are substituted for a part of the nickel are shown in Fig. 2. The values of K s, K 0, Ead _. and E,~ _~,~+~ for each alloy electrode are summarized in Table 1. The K s value decreased in the order Mn, Cr, Fe, Co, Ni and the values of Ead __,,~ and E,~ ... ,~+/3 shifted to the negative direction in the same order, indicating that the a-phase shifts to the lower hydrogen concentrations. During the discharge, adsorbed hydrogen atoms on the MmNia.2Al0.sM0. 3 alloys are electrochemically oxidized as follows: Had + OH- --->H 2 0 + e -
(5)
With decreasing surface concentration of the adsorbed hydrogen atoms, the hydrogen atoms in the alloys diffuse towards the surface at a velocity proportional to their concentration gradient. Therefore, either the hydrogen diffusion or the electrooxidation controls the discharge reaction. The time-course of oxidation current for the hydrogen storage metal was theoretically analyzed using the potentiostatic polarization method reported by Krapivnyi [15]. The following equation is valid for long periods of time when the discharge reaction is controlled by hydrogen diffusion in the metal [15]: In I =
In(2FAcDH/tS)
-
(Tr2DH/at~ 2)t
(6)
Fig. 3. The time-course of logarithmic oxidation current for the MmNi4.sAI0. 5 electrode with various hydrogen concentrations at 30°C.
where c, A, D H and 6 are hydrogen concentration (mol cm-3), surface area (cm2), Fick's diffusion coefficient (cm 2 s-1) and thickness of diffusion layer (cm) respectively. This equation was applied to the hydrogen diffusion in hydrogen storage alloys used in this work. As a typical example, the logarithmic oxidation current In I after the potential step for the MmNi4.zA10.sM03 electrode with various hydrogen concentrations is shown as a function of time in Fig. 3. The value of In I immediately after the potential step sharply decreases with decrease in concentration of adsorbed hydrogen on the electrode surface due to the electrooxidation. The value of In I thereafter decreases linearly according to Eq. (6) because the rate-determining step changes from electrooxidation to hydrogen diffusion in the alloy. The values of In I decrease linearly between about 20 s and 30 s and increase with increasing hydrogen concentration, as shown in Fig. 3. In this work, Fick's diffusion coefficient D H for hydrogen was calculated using the value of In I between 20 s and 30 s in Eq. (6). The D~ values for the MmNi4.EA10.sMo.3 electrodes are shown in Fig. 4 as a function of hydrogen concentration. The D n value decreased with increase in hydrogen concentration in all cases, because the hydrogen diffusion is interfered with due to the increase of interaction be-
2.0 3.5 1.6
3.0 2.5
1.2
Co
2.0
e
1.5
Cr
cf
io 08
© 0.4
1.0
Mn
0.5 0 0
0.03
0.06
0.09
0.12
0.15
n / (H/M)
Fig. 2. Sieverts' plots of the pressure-composition isotherms (hydrogen absorption) for MmNi4,2AIo.sMo.3(M = Cr, Mn, Fe, Co, Ni) electrodes at 30°C.
0
i 0
0.02
0.04
0.06
0.08
n / (H/M)
Fig. 4. Diffusion coefficients as a function of hydrogen concentration in the MmNi4.2Al0.sM0. 3 electrodes at 30°C.
C. lwakura et al. / Journal of Electroanalytical Chemistry 398 (1995) 37-41
40
tween the absorbed hydrogen atoms and the constituent metals with increasing hydrogen concentration. The Einstein diffusion coefficient D *, being independent of the hydrogen concentration, becomes a measure for comparison of diffusion behavior of hydrogen in the MmNia.2A10.sM0.3 electrodes. The following relationship between D* and D u applies [16,17]:
where /z° is the standard potential, RT ln(n/(1 - n ) ) is the ideal solution configurational term and Wn_ n is the partial interaction Gibbs energy of dissolved hydrogen, respectively. By differentiating both sides of Eq. (8) with respect to n, the following equation can be derived: RT
dn
-
n(1 - n)
+W._.
(9)
By substituting Eq. (9) in Eq. (7), D n is given by D H =
(1 D* - 1 -
+ n
WH_H ) n RT
(10)
Since the n value is much less than 1, this equation can be simplified to DH=D*
1+
RT
(11)
n
The D* value was evaluated by extrapolating the D H value to infinite dilution of hydrogen. The results are summarized in Table 2, together with the crystallographic and thermodynamic data. The crystallographic data of MmNi4.2Alo.5Mo.3 alloys indicated that the unit cell volume increased with increasing radius of foreign metals. An increase in the unit cell volume of the alloys stabilized the resulting metal hydride, as can be seen from the A H value. However, the A H value for the MmNia.2Alo.sMno. 3 alloy was somewhat smaller than that predicted from the
Table 2 Electronegativity X for element M, unit cell volume V, Einstein diffusion coefficient D * and activation energy E a for diffusion of hydrogen for the MmNi4.2A10.5 M0. 3 electrodes
M
Cr Mn Fe Co Ni
X a
1.66 1.55 1.83 1.88 1.91
a Ref. [20].
-16 -18
~
F e ~ . ~
Fe
°
o
-20
(7)
where tz H is the chemical potential of the hydrogen dissolved in the alloy at concentration n. In addition, P~H is represented by the following equation [18,19]: n (8) IXn = I~°n + RT l n l _ n + WH_Hn
-
~
-22
n d/z n D H = D * R T dn
d lz H
-14
V
AH
D * × 10 s
Ea
(,~3)
(kj (mol H 2 ) - 1)
(cm 2 s - l )
(kj m o l - t)
86.9 86.8 86.4 85.9 85.8
-31.1 - 32.6 - 30.7 - 29.0 - 27.9
2.18 1.62 2.55 2.99 3.24
24.4 26.3 21.0 19.9 19.1
-24 3.15 3.2 3.25 3.3 3.35 3.4 3.45 3.5 3.55 1000 T "1/ K-1 Fig. 5. Arrhenius plots of the D* electrodes.
value for the MmNi4.2Alo.5Mo.3
unit cell volume. In the case of the MmNi4.2Al0.sMn0. 3 alloy, the contribution of electronegativity to the increase of stability of the resulting hydride seems to be quite large, suggesting that not only the crystallographic parameter but also the electrostatic interaction is responsible for the thermodynamic stability of absorbed hydrogen. The D * value increases with an increase of the AH value, that is in the order Mn < Cr < Fe < Co < Ni. Therefore, it is concluded that the D * value is influenced by the stabilization of hydrogen atoms in the alloys substituted by various foreign metals, as might be expected. The D* value for each MmNia.2A10.sM0. 3 alloy was evaluated at various temperatures. The results are shown in Fig. 5. The logarithmic D * value was linearly dependent on the reciprocal of absolute temperature. The following relationship holds between the D* value and the activation energy E a for diffusion of H in the alloy: In D* = - E a / R T + In D O
(12)
where D O represents a frequency factor. The D * value was evaluated from the data in Fig. 5 and is summarized in Table 2, which indicates that the D * value increases with a decrease of the E a value.
4. Conclusions
Electrochemical investigations of the series of hydrogen storage alloys MmNia.2Alo.sMo.3 (M = Cr, Mn, Fe, Co, Ni), in which nickel is partially substituted by foreign metals, disclosed that the enthalpy change of hydride formation and the Einstein diffusion coefficient of hydrogen increase with decreasing atomic radius of the foreign metals. Crystallographic analyses indicated that a decrease in unit cell volume, except for the Mn-substituted alloy, destabilized the resulting hydrides due to the decrease in atomic radius of the foreign metals. In the case of the Mn-substituted alloy not only the crystallographic features but also the electrostatic interaction influenced the thermodynamic stability of the resulting hydride.
C. lwakura et al. /Journal of Electroanalytical Chemistry 398 (1995) 37-41
Acknowledgements This w o r k was partly D e v e l o p m e n t a l Scientific G r a n t - i n - A i d for Scientific the Ministry of Education,
supported by G r a n t - i n - A i d for Research no. 0 5 5 5 5 1 7 2 and R e s e a r c h (C) no.07651001 f r o m Science and Culture o f Japan.
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41
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