- Email: [email protected]
Embrittlement of LaNi5-type alloy electrodes during the cathodic evolution of hydrogen
.5,er*,orhim,eo “em “al. 27. NO Rimed in Orcal Bnrain.
12. pp
17234727.
1982.
M13%4@4/62/12172345 0 1982. Pcqamen
so3.00/0 Press Ltd.
EMBRITTLEMENT OF LaNi,-TYPE ALLOY ELECTRODES DURING THE CATHODIC EVOLUTION OF HYDROGEN TOHRU
KITAMURA,
CHIAKI
IWAKURA and
HIDEO
TAMURA
Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565, Japan (Received 5 April 1982) Abstract-In general, the hydrogen storage alloy electrodes such as LaNi, were liable to sustain damage owing lo hydrogen embrittlement during cathodization. However, sucha damage wasfound to be not serious as far as the hydrogen fugacity, being connected to the cathodic overpotential, was less than the equilibrium hydrogen pressure. Thus, the surface damage can be prevented by controlling the current density and temperature.
INTRODUCTION Up to the present, a lot of works on the physicochemical properties of hydrogen storage alloys have been done from both fun&mental and practical aspects. However, there seem to have been only a few publications on their electrochemical properties; The published works were mainly concerned with the utilization of hydrogen storage alloys as anodes in fuel cells[l]. In our previous works[2], some hydrogen storage alloys such as LaNi,, L&o, and MmNi, (Mm: rare earth mischmetal) were found to have a high electrocatalytic activity for hydrogen evolution, almost comparable to Pt and Pd electrodes. Also the linear relation between the heat of hydride formation and the activation energy for hydrogen evolution was clarified[3], and this shows the possibility of bridging the electrochemical observation with the chemical properties for these alloys. It is therefore of great interest to investigate the utility of such hydrogen storage alloys as cathode materials for electrolytic processes. From these view points, some IaNi,-type alloy electrodes were prepared and their surface damage owing to the hydrogen embrittlement during cathodization was investigated in the present work.
EXPERIMENTAL The test electrodes were prepared by the following procedure. First, the stoichiometric mixtures of La, Mm (rare earth mischmetal containing Ce, La and Nd as main ingredients), Co and Ni were pressed to form a pellet at 5 tcmL2 and then melted in an arc furnace under an argon atmosphere. The samples were melted repeatedly to ensure homogeneity. Next, the resulting alloy ingots were cut with a diamond blade and then polished mechanically with both fine emery papers and polishing alumina into mirror image. Finally, they were mounted into glass holders with epoxy resin and sealed with trifluoro-chloroethylene resin (Daifloil #200, Daikin Ind. Ltd.) so that 0.25 cmw2 of the llat surface was remained uncovered. The all-Pyrex glass cell with a teflon stopper was used as an electrolytic 1723
cell. The counter electrode was a platinum sheet with large surface area. A mercury (II) oxide electrode (Hg/HgO, 1 M KOH) was used as a reference electrode. Electrolysis was carried out in pre-electrolyzed 1 M KOH under galvanostatic conditions by using a dc power supply (HP-5-IA, Nichia Keiki Ltd.). The surface morphology of the test electrodes was examined with SEM (S-450, Hitachi Ltd.). In order to determine an equilibrium hydrogen pressure, PH2, the differential thermal analysis was carried out for LaNi,Pd by using Hydrogen Atmosphere High Pressure Differential Thermal Analyzer (Shinku Riko Co. Ltd.). RESULTS
AND DISCUSSION
Hydrogen storage alloys such as LaNi, and MmNi, are characterized by their easy activation process for the reaction with gaseous hydrogen, their large absorbability and diffusionability of hydrogen. One of the major problems for the practical application of such alloys as hydrogen storage materials is their pulverization in the course of the hydriclingdehydriding cycles, due to the lattice change or phase transition. When the cathodic evolution of hydrogen takes place at electrodes of these alloys, hydrogen atoms adsorbed on the electrode surface must smoothly diffuse into the bulk and then form hydrides, similarly to chemical reaction in gassolid phases. This means that th& electrode surface is damaged owing to the hydrogen embrittlement during the cathodic evolution of hydrogen. Since the net surface area of the electrode increases as a result of the surface damage, the electrode potentials can be expected to increase gradually when cathodized galvanostaticaily. The increase of the electrode potentials can therefore be a measure of the degree of the surface damage. Figure 1 shows some typical potential-time curves of LaNi5, LaCo, and MmNi, electrodes under at 30°C. Evidently, the rapid increase of 1 mAcm_’ the potentials is observed for LaNi, and L.&o, electrodes, whereas any appreciable change in potential is not observed for MmNi, electrode over the whole period of measurements. So, it can be presumed
I724
TOHRU
Fig. 1. Variation
of
KITAMURA,CHIAKI
IWAKURAANDHIDEO
TAMURA
potential with time under 1 mAcm-* B: LaCo,,
that the severe surface damage was caused by the formation of hydrides on both LaNi, and L&o, electrodes but not on the MmNi, electrode under these experimental conditions. The contrastive behaviours, of these electrodes are explained on the basis of the difference of PH,. The idealized isotherms for the chemical reaction of these alloys with hydrogen are illustrated in Fig. 2, in which the logarithmic equilibrium pressure of hydrogen, logPy,, are plotted against the hydrogen-to-metal ratio m the hydride, H/M, at each temperature. The u phase, consisting of a hydride with low hydrogen content, is formed in region A. There exists a solid solution region of 01phase and B phase, consisting of a hydride with much higher hydrogen content, in region B. In this region, the equilibrium pressure for the reaction MH(a) + xH, = MH(fl remains constant in a wide range of H/M, and the pulverization of alloy particles occurs because of the drastic change of lattice parameters. After c1 phase is fully saturated with hydrogen, only j? phase is present as shown in region C. Two single phases, denoted as regions A and C, generally obey the “Sieve&s law”; ie H/M is proportional to the square root of the external hydrogen pressure. The hydriding of the practical alloys proceeds in region B at almost the same pressure. In the case of the electrochemical reaction, the hydrogen embrittlement is known to be caused by the diffusion of hydrogen from the surface to the bulk of a
at 30°C for various electrodes. C: MmNi,.
A: LaNi,,
hydrogen evolving electrode. The diffusion of hydrogen into the bulk of the electrode is also known to be controlled by the internal hydrogen fugacity, fn,, which corresponds to the hydrogen pressure in cavities of the electrode materials and depends on the cathodic over-potential, v. As described previously, hydrogen storage alloys generally have their own values of PM, in the moderate temperature range. It can be speculated that the H/M ratio begins to increase and at the same time the jl phase formation may occur iffn, (being dependent on 11)z+ &, (eigenvalue for each alloy). The comparisons between fn, and PHI for LaNi,, L&o, and MmNi, under 1 mAcm-’ at 30°C are given in Table 1. The dependence of fH2 on tl varies in accordance with the mechanism of hydrogen evolution reaction[4]. In this work, the following equation was adopted to calculate the values of f”,. This was proposed for the hydrogen evolution reaction at Ni Table 1. Eqruubrium pressures, PH,, and hydrogen fugacities, /u,. under I mAcn~-~ at 30°C for LaCo,, LaNis and MmNi, Laces
01
MmNi,
LaNiS
6.2 x lo-’
3.1
6.4
5.7
19 7.6
, 2 00
2 20
2 40
260
280
3 JO
T' x IO3 /K-l Hydrogen-
to-metal
ratlo
H/M
Fig. 2. Pressure vs composition isotherms for an idealized alloy-hydrogen system. Tj -z r, < T3.
Fig. 3. Equilibrium hydrogen pressures US reciprocal temperatures for LaNi,H, and LaNi,PdH,. o: LaNi,H,, l : LaNi,PdH,.
Embrittlement of LaNi,-type alloy electrodes
Fig. 4. Surface morphology
electrode
in alkaline
and Genshaw[S].
solution
LaNi,
(C)
MmNi,
and MmNi, electrodes before and after 10 h of cathodic polarizationunder 1 n~Acm_~ at 30°C.
of LaCo 5, LaNi,
by Atrens,
f~, = exp(F~PRV.
Cb)
1725
Mezzanotte (1)
It could be reasonable because the large parts of surface atoms are Ni in the QWEof LaNi,-type electrodes and moreover the probable reaction mechanism proposed in the previous paper[2] was the same among LaNi,, MmNi, and Ni electrodes. According to Shamsul Huq and Rosenberg[6], the intrinsic chemical properties of
the surface atoms dominate the mechanism of the hydrogen evolution reaction, while the crystalline properties intluenee the reaction rate. The values of v were measured by cathodizing the alloy electrodes under 1 mAcm_’ at 30°C in 1 M KOH. And the values of Pn, were calculated, quoting the thermal values of the references[7,8]. As can be seen from Table 1, the values of fH1 surpass those of PHq under the given conditions for the LaCo, and L~NI, electrodes, whereas the reverse is true for the MmNi,
1726
TOHRU KITAMURA, CHIAKI IWAKURA AND HIDEO TAMURA
Fig. 5. Surface morphology
of LaNi,Pd
electrode before and after 10 h of cathodic polarization 1 mAcm_’ at 30°C.
electrode. Therefore, the former two electrodes have the possibility to sustain damage but the latter electrode might remain stable under the cathodization conditions. The surface morphologies of these electrodes before and after cathodiaation are shown in Fig. 4. These results are in accord with the above expectations: Both LaCo, and LaNi, electrodes were proved to be damaged as a result of the formation of jl phase with the higher hydrogen content after 10 h of cathodization [Fig. 4(a), (b)]. On the other hand, MmNi, electrode was quite stable even after 10 h of cathodization [Fig. 4(c)]. In order to obtain further proof, LaNi, Pd, a pseudobinary alloy, was prepared. This alloy was reported to possess the higher PH, value than LaNi, by van Mal, Buschow and Miedema[9]. Some plots of the logarithmic equilibrium pressure of hydrogen, In Pql, against the reciprocal temperature, l/T, are shown m Fig. 3 for LaNi.+Pd as well as LaNi,. These were determined from the DTA data in a hydrogen atmosphere. As can be seen from Fig. 3, this system obeys the following van? Hoff relation very closely over a wide temperature range. RTh
PH2 = AH -TAS
(2)
where AH is the heat of hydride formation and AS is the formation entropy of the hydride. The values of AH and AS were then determined to be - 28.3 kJ mol- ’ and 106.0 J mall ’ K- 1 respectively for L.aNi,Pd. The Pu2 value calculated from (2) is 4.5 atm at 30°C. Thefn, value determined from the measured q value at 30°C was 2.5 atm for the same alloy. Therefore one can
10-c
under
predict that the LaNi,Pd electrode is not damaged by the cathodization under 1 mAcm_‘. As shown in Fig. 5, no appreciable change in the surface morphology of LaNi, Pd electrode was observed after 10 h of cathodization. As can be seen from (1) and (2), the fu, value decreases but the PH, value increases with increasing temperature, since AH < 0. It is therefore presumed that the higher the temperature is, the more stable the electrode is against hydrogen embrittlement, and vice oema. Some examples of the l;i, (under 1 mA cm- ‘) and PH, values at different temperatures are given in Table 2. Accordingly, LaNi, electrode is expected to be unstable at 10 and 30°C but be stable at 50 and 70°C. These were also proved as shown in Fig. 6. Severe damage is observed at 10°C as well as at 30°C. whereas only slight damage is seen at 50 and 70°C. Table 2. Equilibrium pressures, PH,, and hydrogen fugacities, fH,, under 1 mAor_’ at various temperatures for LaNi, T (“C) PH, (atm) fH, (atm)
10
30
50
IO
1.3
3.1
6.5
13
5.7
2.1
I.5
1.3
The degree of the surface damage is also dependent on the cathodic current density at a constant temperature, since fH,, related to q by (I), increases with increasing i. Some values of fH, and PH,
5OT
70-c
Fig. 6. Surface morphology of LaNi, electrode after 10 h of cathodic polarization under 1 rrrA~~rn_~at
various temperatures.
Embrittlement
Fig. 7. Surface morphology
of LaNi,-type
LaNi, i(mAcm-‘)
0.01
0.1
1.0
PH, (atm)
3.1
3.1
3.1
1.1
1.1
5.7
MmNi, i(mAcm-‘) PH, (atm) ftl, (atm)
1.0 19 7.6
I727
of LaNi, and MmNis electrodes after 10 h of cathodic polarization various current densities at 30°C.
Table 3. Equilibrium pressures, PH*, and hydrogen fugacities, f&, under various current densities at 30°C
fH, (atm)
alloy electrodes
10 19
under
CONCLUSION The main conclusions to be drawn from this work are as follows. (1) The hydrogen storage alloy electrodes such as LaNiS are generally liable to sustain damage owing to hydrogen embrittlement during cathodization. (2) The hydrogen embrittlement of these electrodes corresponds to the formation of j3 phase which has higher hydrogen content. (3) The surface damage of such alloy electrodes can be prevented by controlling the temperature and the current density so that& (dependent on q) -z PH,.
52
under various current densities at 30°C are given in Table 3. From these data, it is expected that LaNiS electrode is not damaged under 0.01 and 0.1 mAcm_’ but is damaged under 1 mAcm_’ of galvanostatic polarization. Furthermore, MmNi, electrode is expected to be damaged under 10 mAcm_’ at 30°C. Results of the cathodic polarization of these electrodes under various current densities are shown in Fig. 7. Evidently, the validity of the above presumption can be seen from Fig. 7. Only slight damages are observed after 10 h of polarization under 0.01 and 0.1 mA cm - ’ for LaNi, electrode and severe damage is seen under 10 mA cm- ’ for MmNi, electrode. These are in sharp contrast to those shown in Fig. 4.
REFERENCES 1. G. Bronoel, J. Sarradin, A. Percheron-Guegan and J. C. Achard, Mar. Res. Bull. 13, 1265 (1978). 2. T. Kitamura, C. Iwakura and H. Tamura, Chem. Lert. %5 (1981). 3. T. Kitamura, C. Iwakura and H. Tamura, Chem. Lat. 1755 (1981). 4. J. G’M. Bockris and P. K. Subramanyan, Elecrrochim. Actn 16, 2169 (1971). 5. A. Atrens, D. Mezzanotte and M. Genshaw, Corros. Sci. 20, 673 (1980). 6. A. K. M. Shamsul Huq and J. Rosenberg, .I. electrochem. Sot. 111, 270 (1964). 7. Y. Osumi, A. Kato, H. Suzuki and M. Nakane, J. Iesscommon Metals 66, 67 (1979). 8. F. A. Kuijpers, J. less-common Met& 27, 27 (1972). 9. H. H. van Mal, K. H. J. Busehow and A. R. Miedema, J. less-common Metals 35, 65 (1974).
12. pp
17234727.
1982.
M13%4@4/62/12172345 0 1982. Pcqamen
so3.00/0 Press Ltd.
EMBRITTLEMENT OF LaNi,-TYPE ALLOY ELECTRODES DURING THE CATHODIC EVOLUTION OF HYDROGEN TOHRU
KITAMURA,
CHIAKI
IWAKURA and
HIDEO
TAMURA
Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565, Japan (Received 5 April 1982) Abstract-In general, the hydrogen storage alloy electrodes such as LaNi, were liable to sustain damage owing lo hydrogen embrittlement during cathodization. However, sucha damage wasfound to be not serious as far as the hydrogen fugacity, being connected to the cathodic overpotential, was less than the equilibrium hydrogen pressure. Thus, the surface damage can be prevented by controlling the current density and temperature.
INTRODUCTION Up to the present, a lot of works on the physicochemical properties of hydrogen storage alloys have been done from both fun&mental and practical aspects. However, there seem to have been only a few publications on their electrochemical properties; The published works were mainly concerned with the utilization of hydrogen storage alloys as anodes in fuel cells[l]. In our previous works[2], some hydrogen storage alloys such as LaNi,, L&o, and MmNi, (Mm: rare earth mischmetal) were found to have a high electrocatalytic activity for hydrogen evolution, almost comparable to Pt and Pd electrodes. Also the linear relation between the heat of hydride formation and the activation energy for hydrogen evolution was clarified[3], and this shows the possibility of bridging the electrochemical observation with the chemical properties for these alloys. It is therefore of great interest to investigate the utility of such hydrogen storage alloys as cathode materials for electrolytic processes. From these view points, some IaNi,-type alloy electrodes were prepared and their surface damage owing to the hydrogen embrittlement during cathodization was investigated in the present work.
EXPERIMENTAL The test electrodes were prepared by the following procedure. First, the stoichiometric mixtures of La, Mm (rare earth mischmetal containing Ce, La and Nd as main ingredients), Co and Ni were pressed to form a pellet at 5 tcmL2 and then melted in an arc furnace under an argon atmosphere. The samples were melted repeatedly to ensure homogeneity. Next, the resulting alloy ingots were cut with a diamond blade and then polished mechanically with both fine emery papers and polishing alumina into mirror image. Finally, they were mounted into glass holders with epoxy resin and sealed with trifluoro-chloroethylene resin (Daifloil #200, Daikin Ind. Ltd.) so that 0.25 cmw2 of the llat surface was remained uncovered. The all-Pyrex glass cell with a teflon stopper was used as an electrolytic 1723
cell. The counter electrode was a platinum sheet with large surface area. A mercury (II) oxide electrode (Hg/HgO, 1 M KOH) was used as a reference electrode. Electrolysis was carried out in pre-electrolyzed 1 M KOH under galvanostatic conditions by using a dc power supply (HP-5-IA, Nichia Keiki Ltd.). The surface morphology of the test electrodes was examined with SEM (S-450, Hitachi Ltd.). In order to determine an equilibrium hydrogen pressure, PH2, the differential thermal analysis was carried out for LaNi,Pd by using Hydrogen Atmosphere High Pressure Differential Thermal Analyzer (Shinku Riko Co. Ltd.). RESULTS
AND DISCUSSION
Hydrogen storage alloys such as LaNi, and MmNi, are characterized by their easy activation process for the reaction with gaseous hydrogen, their large absorbability and diffusionability of hydrogen. One of the major problems for the practical application of such alloys as hydrogen storage materials is their pulverization in the course of the hydriclingdehydriding cycles, due to the lattice change or phase transition. When the cathodic evolution of hydrogen takes place at electrodes of these alloys, hydrogen atoms adsorbed on the electrode surface must smoothly diffuse into the bulk and then form hydrides, similarly to chemical reaction in gassolid phases. This means that th& electrode surface is damaged owing to the hydrogen embrittlement during the cathodic evolution of hydrogen. Since the net surface area of the electrode increases as a result of the surface damage, the electrode potentials can be expected to increase gradually when cathodized galvanostaticaily. The increase of the electrode potentials can therefore be a measure of the degree of the surface damage. Figure 1 shows some typical potential-time curves of LaNi5, LaCo, and MmNi, electrodes under at 30°C. Evidently, the rapid increase of 1 mAcm_’ the potentials is observed for LaNi, and L.&o, electrodes, whereas any appreciable change in potential is not observed for MmNi, electrode over the whole period of measurements. So, it can be presumed
I724
TOHRU
Fig. 1. Variation
of
KITAMURA,CHIAKI
IWAKURAANDHIDEO
TAMURA
potential with time under 1 mAcm-* B: LaCo,,
that the severe surface damage was caused by the formation of hydrides on both LaNi, and L&o, electrodes but not on the MmNi, electrode under these experimental conditions. The contrastive behaviours, of these electrodes are explained on the basis of the difference of PH,. The idealized isotherms for the chemical reaction of these alloys with hydrogen are illustrated in Fig. 2, in which the logarithmic equilibrium pressure of hydrogen, logPy,, are plotted against the hydrogen-to-metal ratio m the hydride, H/M, at each temperature. The u phase, consisting of a hydride with low hydrogen content, is formed in region A. There exists a solid solution region of 01phase and B phase, consisting of a hydride with much higher hydrogen content, in region B. In this region, the equilibrium pressure for the reaction MH(a) + xH, = MH(fl remains constant in a wide range of H/M, and the pulverization of alloy particles occurs because of the drastic change of lattice parameters. After c1 phase is fully saturated with hydrogen, only j? phase is present as shown in region C. Two single phases, denoted as regions A and C, generally obey the “Sieve&s law”; ie H/M is proportional to the square root of the external hydrogen pressure. The hydriding of the practical alloys proceeds in region B at almost the same pressure. In the case of the electrochemical reaction, the hydrogen embrittlement is known to be caused by the diffusion of hydrogen from the surface to the bulk of a
at 30°C for various electrodes. C: MmNi,.
A: LaNi,,
hydrogen evolving electrode. The diffusion of hydrogen into the bulk of the electrode is also known to be controlled by the internal hydrogen fugacity, fn,, which corresponds to the hydrogen pressure in cavities of the electrode materials and depends on the cathodic over-potential, v. As described previously, hydrogen storage alloys generally have their own values of PM, in the moderate temperature range. It can be speculated that the H/M ratio begins to increase and at the same time the jl phase formation may occur iffn, (being dependent on 11)z+ &, (eigenvalue for each alloy). The comparisons between fn, and PHI for LaNi,, L&o, and MmNi, under 1 mAcm-’ at 30°C are given in Table 1. The dependence of fH2 on tl varies in accordance with the mechanism of hydrogen evolution reaction[4]. In this work, the following equation was adopted to calculate the values of f”,. This was proposed for the hydrogen evolution reaction at Ni Table 1. Eqruubrium pressures, PH,, and hydrogen fugacities, /u,. under I mAcn~-~ at 30°C for LaCo,, LaNis and MmNi, Laces
01
MmNi,
LaNiS
6.2 x lo-’
3.1
6.4
5.7
19 7.6
, 2 00
2 20
2 40
260
280
3 JO
T' x IO3 /K-l Hydrogen-
to-metal
ratlo
H/M
Fig. 2. Pressure vs composition isotherms for an idealized alloy-hydrogen system. Tj -z r, < T3.
Fig. 3. Equilibrium hydrogen pressures US reciprocal temperatures for LaNi,H, and LaNi,PdH,. o: LaNi,H,, l : LaNi,PdH,.
Embrittlement of LaNi,-type alloy electrodes
Fig. 4. Surface morphology
electrode
in alkaline
and Genshaw[S].
solution
LaNi,
(C)
MmNi,
and MmNi, electrodes before and after 10 h of cathodic polarizationunder 1 n~Acm_~ at 30°C.
of LaCo 5, LaNi,
by Atrens,
f~, = exp(F~PRV.
Cb)
1725
Mezzanotte (1)
It could be reasonable because the large parts of surface atoms are Ni in the QWEof LaNi,-type electrodes and moreover the probable reaction mechanism proposed in the previous paper[2] was the same among LaNi,, MmNi, and Ni electrodes. According to Shamsul Huq and Rosenberg[6], the intrinsic chemical properties of
the surface atoms dominate the mechanism of the hydrogen evolution reaction, while the crystalline properties intluenee the reaction rate. The values of v were measured by cathodizing the alloy electrodes under 1 mAcm_’ at 30°C in 1 M KOH. And the values of Pn, were calculated, quoting the thermal values of the references[7,8]. As can be seen from Table 1, the values of fH1 surpass those of PHq under the given conditions for the LaCo, and L~NI, electrodes, whereas the reverse is true for the MmNi,
1726
TOHRU KITAMURA, CHIAKI IWAKURA AND HIDEO TAMURA
Fig. 5. Surface morphology
of LaNi,Pd
electrode before and after 10 h of cathodic polarization 1 mAcm_’ at 30°C.
electrode. Therefore, the former two electrodes have the possibility to sustain damage but the latter electrode might remain stable under the cathodization conditions. The surface morphologies of these electrodes before and after cathodiaation are shown in Fig. 4. These results are in accord with the above expectations: Both LaCo, and LaNi, electrodes were proved to be damaged as a result of the formation of jl phase with the higher hydrogen content after 10 h of cathodization [Fig. 4(a), (b)]. On the other hand, MmNi, electrode was quite stable even after 10 h of cathodization [Fig. 4(c)]. In order to obtain further proof, LaNi, Pd, a pseudobinary alloy, was prepared. This alloy was reported to possess the higher PH, value than LaNi, by van Mal, Buschow and Miedema[9]. Some plots of the logarithmic equilibrium pressure of hydrogen, In Pql, against the reciprocal temperature, l/T, are shown m Fig. 3 for LaNi.+Pd as well as LaNi,. These were determined from the DTA data in a hydrogen atmosphere. As can be seen from Fig. 3, this system obeys the following van? Hoff relation very closely over a wide temperature range. RTh
PH2 = AH -TAS
(2)
where AH is the heat of hydride formation and AS is the formation entropy of the hydride. The values of AH and AS were then determined to be - 28.3 kJ mol- ’ and 106.0 J mall ’ K- 1 respectively for L.aNi,Pd. The Pu2 value calculated from (2) is 4.5 atm at 30°C. Thefn, value determined from the measured q value at 30°C was 2.5 atm for the same alloy. Therefore one can
10-c
under
predict that the LaNi,Pd electrode is not damaged by the cathodization under 1 mAcm_‘. As shown in Fig. 5, no appreciable change in the surface morphology of LaNi, Pd electrode was observed after 10 h of cathodization. As can be seen from (1) and (2), the fu, value decreases but the PH, value increases with increasing temperature, since AH < 0. It is therefore presumed that the higher the temperature is, the more stable the electrode is against hydrogen embrittlement, and vice oema. Some examples of the l;i, (under 1 mA cm- ‘) and PH, values at different temperatures are given in Table 2. Accordingly, LaNi, electrode is expected to be unstable at 10 and 30°C but be stable at 50 and 70°C. These were also proved as shown in Fig. 6. Severe damage is observed at 10°C as well as at 30°C. whereas only slight damage is seen at 50 and 70°C. Table 2. Equilibrium pressures, PH,, and hydrogen fugacities, fH,, under 1 mAor_’ at various temperatures for LaNi, T (“C) PH, (atm) fH, (atm)
10
30
50
IO
1.3
3.1
6.5
13
5.7
2.1
I.5
1.3
The degree of the surface damage is also dependent on the cathodic current density at a constant temperature, since fH,, related to q by (I), increases with increasing i. Some values of fH, and PH,
5OT
70-c
Fig. 6. Surface morphology of LaNi, electrode after 10 h of cathodic polarization under 1 rrrA~~rn_~at
various temperatures.
Embrittlement
Fig. 7. Surface morphology
of LaNi,-type
LaNi, i(mAcm-‘)
0.01
0.1
1.0
PH, (atm)
3.1
3.1
3.1
1.1
1.1
5.7
MmNi, i(mAcm-‘) PH, (atm) ftl, (atm)
1.0 19 7.6
I727
of LaNi, and MmNis electrodes after 10 h of cathodic polarization various current densities at 30°C.
Table 3. Equilibrium pressures, PH*, and hydrogen fugacities, f&, under various current densities at 30°C
fH, (atm)
alloy electrodes
10 19
under
CONCLUSION The main conclusions to be drawn from this work are as follows. (1) The hydrogen storage alloy electrodes such as LaNiS are generally liable to sustain damage owing to hydrogen embrittlement during cathodization. (2) The hydrogen embrittlement of these electrodes corresponds to the formation of j3 phase which has higher hydrogen content. (3) The surface damage of such alloy electrodes can be prevented by controlling the temperature and the current density so that& (dependent on q) -z PH,.
52
under various current densities at 30°C are given in Table 3. From these data, it is expected that LaNiS electrode is not damaged under 0.01 and 0.1 mAcm_’ but is damaged under 1 mAcm_’ of galvanostatic polarization. Furthermore, MmNi, electrode is expected to be damaged under 10 mAcm_’ at 30°C. Results of the cathodic polarization of these electrodes under various current densities are shown in Fig. 7. Evidently, the validity of the above presumption can be seen from Fig. 7. Only slight damages are observed after 10 h of polarization under 0.01 and 0.1 mA cm - ’ for LaNi, electrode and severe damage is seen under 10 mA cm- ’ for MmNi, electrode. These are in sharp contrast to those shown in Fig. 4.
REFERENCES 1. G. Bronoel, J. Sarradin, A. Percheron-Guegan and J. C. Achard, Mar. Res. Bull. 13, 1265 (1978). 2. T. Kitamura, C. Iwakura and H. Tamura, Chem. Lert. %5 (1981). 3. T. Kitamura, C. Iwakura and H. Tamura, Chem. Lat. 1755 (1981). 4. J. G’M. Bockris and P. K. Subramanyan, Elecrrochim. Actn 16, 2169 (1971). 5. A. Atrens, D. Mezzanotte and M. Genshaw, Corros. Sci. 20, 673 (1980). 6. A. K. M. Shamsul Huq and J. Rosenberg, .I. electrochem. Sot. 111, 270 (1964). 7. Y. Osumi, A. Kato, H. Suzuki and M. Nakane, J. Iesscommon Metals 66, 67 (1979). 8. F. A. Kuijpers, J. less-common Met& 27, 27 (1972). 9. H. H. van Mal, K. H. J. Busehow and A. R. Miedema, J. less-common Metals 35, 65 (1974).