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Hydrogen absorption in highly disordered Mg-based alloys
hr. J Hydro@v Energy, Vol. 12. No. 6, pp. 411415. Printed in Great Britain.
1987. 0
HYDROGEN
ABSORPTION
1987 Intematmnal
IN HIGHLY DISORDERED ALLOYS
0360-3199/87 $3.00 + 0.00 Pergamon Journals Ltd. Association for Hydrogen Energy.
Mg-BASED
K.C.HoNcandK.SApRu 4853 Gamber Drive, Troy, MI 48098, U.S.A. (Receivedforpublication
24 December 1986)
Abstract-Several highly disordered magnesium based films (Mg-Fe binary and Mg-Fe-Al ternary) have been prepared by sputtering. The hydrogen storage capacities of these films have been determined and compared to the corresponding calculated value. Hydrogen absorption and desorption kinetics of these films were studied. The data indicate that the hydrogen sorption rates of these disordered films are much faster than for pure Mg films or Mg powders. The kinetic data are compared to our kinetic models and show that during the early hydride formation process in these films the rate-determining step is the dissociation of H, to H-atom on the surface, while during the early desorption stage the rate-determining step is the diffusion of H-atoms.
INTRODUCTION Magnesium can store up to 7.6 wt% hydrogen and has been mentioned as a candidate for hydrogen storage medium [l--4]. However, this metal by itself does not react readily with hydrogen to form the hydride and is very sensitive to trace amounts of impurities existing in the hydrogen stream. Moreover, the hydride of magnesium is relatively stable. In order to improve the properties and therefore the usefulness of Mg, some researchers have reported the use of catalysts such as Ni, Cu and Al [5-81, while others have worked on Mgcontaining alloys or intermetallic compounds such as MgzNi and MgzCu [9-141. Iron, while being a low-cost metal, is also a good hydrogenation catalyst. Very little work on the addition of Fe to Mg-based materials has been reported. This may be due to the lack of mutual solubility between Mg and Fe. We report here some data on hydrogen absorption in thin film Mg-Fe binary and Mg-Fe-Al ternary systems [ 151.
EXPERIMENTAL
analysis. The structure of the sample was determined by X-ray diffraction. Each sample was transferred through air to a stainless steel reactor for testing.
Apparatus
As shown in Fig. 1, the testing system was a conventional Sievert’s type apparatus using Edward’s 63M diffstak vacuum equipment. The whole system was made of 316 stainless steel. To reduce hydrogen leak under high pressure to a minimum, the joints in the system were all VCR fittings. The volumes in major testing zones a, b, c shown in Fig. 1 were all of the order of 20 cm3. Several larger cylinders were also attached to the system. All volumes were calibrated within + 0.2 cm3 precision. When the temperature of reactor was different from ambient temperature, the testing zone was divided into hot and cold zones. The volumes of hot and cold zones were determined at various temperatures (0-4OO”C). The pressure of the system was measured by Heise pressure gauges, model 701B (O-3000 torr and O-2000 psi, with ? 0.1% precision). Activation
Samplepreparation
Thin film samples were prepared by co-sputtering techniques using a Veeco Sputtering system under 10-3-10-4 torr argon atmosphere. The substrate used was stainless steel sheet (0.05 mm thick). Prior to film deposition, the chamber was evacuated better than 10m6 torr and the target was pre-sputtered for at least one hour. Two sets of samples were made: Mg-Fe binary and Mg-Fe-Al ternary. The thickness and net weight of the samples was of the order of 10 pm and RIO-200 mg respectively. The chemical composition of the films was examined by EDS (Energy Dispersive Spectroscopy) 411
The activation procedure of the samples studied was as follows. The system with a sample inside the reactor was evacuated to 10-4-10-5 torr. Next, the sample was heated at 200°C for one hour. High purity hydrogen (99.999%) of 40-50 atm pressure was introduced at 200°C and the sample was heated up to 400°C and left there for l-2 hours. The temperature of the sample was then reduced to 300°C where it was left over night. Finally, the sample was cooled to 200°C and the hydrogen pressure was released. The above procedure was repeated for several cycles until the hydrogen storage capacity of the samples was steady.
412
K. C. HONG AND K. SAPRU O-2000 psi H.P. 0
To H, Cylinder
To High Vacuum and Vent Systems
Furnace
Fig. 1. Testing apparatus.
Capacityandkineticsmeasurements: The hydrogen storage capacities of the samples studied were determined by desorption method. The charged sample was heated from ambient temperature (at zero pressure) up to 450°C. The absorption/ desorption kinetics of the samples were studied at 200°C and/or 250°C. For the desorption measurements, the initial pressure of the sample zone (i.e. zone a in Fig. 1) was its equilibrium pressure, while the other portion of the closed system (i.e. zones b and c in Fig. 1) was under vacuum. When the valve “V” between reactor and the rest of the system was opened, the hydrogen pressure vs time was recorded. From this the amount of the hydrogen desorbed was calculated. The absorption kinetics was measured in a similar manner except the initial pressure in zones b and c was about 1 atm higher than the equilibrium pressure existing in the sample section. Blank substrates tested under similar conditions stored negligible amounts of hydrogen. Hence, no correction due to the substrate was applied.
6.0 \ \ 7.0 3
Mgl-,FexHz(l-,) \ \
6.0 -
‘\ \ \
5.0 -
\ \ \
4.0 -
\ \ ‘0
3.0 -
0
0
‘\
0
\O \ \
2.oG
\ \ 1.0 -
0 0
I 0.2
I
I
0.4
0.6
\
\
\
0.8
\
\
\
\, 1.0
NW
Fig. 2. Hydrogen storage capacity in the Mg,_,Fe system.
RESULTS AND DISCUSSION The X-ray diffraction pattern indicated that the Mg-Fe binary films were all of multiphase structure which consisted of crystalline Mg, a-Fe and amorphous phase. The hydrogen storage capacity of this binary
HYDROGEN
ABSORPTION
Table 1. Summary of the hydrogen absorption in Mg-Fe-Al Sample No.
H-110 H-127 H-123 H-125 H-145 H-132
Fe 0.31 0.39 0.35 0.30 0.22 0.26
0:: 0.54 0.58 0.60 0.62 0.64
ternary films Structure*
H wt% Stored Expt-1 Calculatedt
Composition*
Chemical
413
IN Mg-BASED ALLOYS
Al 0.18 0.07 0.07 0.10 0.16 0.10
3.2 3.5 4.1 5.0 3.4 3.5
2.95 2.94 3.26 3.52 3.92 3.91
multiphase multiphase multiphase amorphous multiphase multiphase
* by EDS. t Assuming that only Mg can absorb hydrogen, i.e. Mg,(Fe,Al),,H,,. $ by X-ray diffraction.
Mg,_,Fe,(O shown region
< x < 0.65) is between 2.5 to 3.8 wt%. As in Fig. 2, the optimum composition is in the 0.4 < x < 0.6. The decrease in the storage
capacity in the Mg-rich films is not expected and has been attributed to the MgO formation. The Auger depth profile of these samples indicated that the degree of oxidation of films increases with the increase in magnesium concentration and reaches a maximum for the pure Mg films. The amount of oxide was significant, up to lO-20% of the total film weight. X-ray diffraction patterns also confirmed the oxide formation. The oxide not only results in a decrease in the hydrogen storage capacity, but also makes activation difficult. It is interesting to note from Fig. 2 that in the region x(Fe)>0.4, the hydrogen stored in the sample is higher than the calculated value (the dashed curve in the figure) based on the formula Mgi_,FeH2(i_,) in which we assume that only magnesium can absorb hydrogen in this binary system. From the Auger depth profile, the films
100
with x(Fe)>.0.35 had a very thin oxide layer of about 25A on the surface and a thick iron-rich layer next to it. This iron-rich layer protected bulk magnesium from oxidation and also led to easy activation. In their studies on the Mg*Nii_,Fe, ternary, Lupu et al. [14] found that the Mossbauer spectra in the high iron alloy (x = 0.25 to 0.37) revealed magnetic hypertine splitting. These results suggest that the electronic structure of iron was modified by magnesium. We speculate that in the highly disordered Mgl_,Fe,(0.5cx<0.65) film magnesium atoms may also modify the electronic structure of iron such that new bonding sites are generated to absorb more hydrogen. Further work is needed to confirm this. The hydrogen sorption kinetics of the Mgi_,Fe, films studied is much faster than that of pure magnesium. These samples can reversibly absorb and desorb hydrogen even at temperature as low as 180°C. For the Mg-Fe-Al ternary alloys, the experimental results are summarized in Table 1. The structures of
r
01’ 0
’ 18
’
’ 26
’
’ 34
’
’
’
42
’ 50
’
’ 58
’
’ 66
’
’
’
74
20
Fig. 3. The hydrogen
storage capacity of these alloys
’
82
414
K. C. HONG AND K. SAPRU
these films in general are multiphase consisting of crystalline Mg, a-Fe and amorphous phase. However, in one case the sample Mgo.s~eo.soAlo,lo was completely amorphous as indicated by X-ray diffraction shown in Fig. 3. The hydrogen storage capacity of these alloys vary from 3.2% to 5.0 wt%, with the x(Mg) from 0.5 to 0.65. In Table 1 we also include the calculated H wt% of each sample, again assuming that only magnesium can absorb hydrogen. It is worthwhile to note that the hydrogen stored in those samples with x(Mg) <0.6 is higher than that of the calculated values, especially for the sample with an amorphous structure. This phenomenon is similar to that in the Mg-Fe binary system and might also be due to the modification of electronic structure of Fe and/or Al by Mg atoms. The hydride of the Mg-Fe-Al ternary alloys studied, like the Mg-Fe binary, has much better sorption kinetics 10 1.0 as compared to that of MgH2. As shown in Fig. 4, the sample H-125 (MgO.&eo.scAlo.lo) can desorb about Time (Hr) 80% of the stored hydrogen in 20 minutes at 300°C and the sample H-99 (Mgo,45Feo.sAlo.os) can release more than 50% of hydrogen in one hour at 200°C. The film or Fig. 4. Hydrogen desorption kinetics of samples H-125 powder of magnesium hydride desorbed insignificant (Mgo.&eo.&% ,o) and H-99 (Mg0.45Fe0.sA10.& amounts of hydrogen at temperatures below 250°C. The fast sorption kinetics in Mg-Fe-Al over Mg has been attributed to enhancement of iron in the surface layer. A typical Auger depth profile given in Fig. 5 indicated that
looI 80 -
Mg
4
6
Sputter Time (Min.)
Fig. 5. Auger Spectroscopy depth profile of the sample H-99 (MgO,,,FeO,~oAIO.oS) at 115 kminlrate.
HYDROGEN
415
ABSORPTION IN Mg-BASED ALLOYS 24
1
I
Time(Min.)
Fig. 6. The plots of P vs t for the desorption kinetics of H-132 (Mg, MFe0.26A10.01)at 250°C (X0 = 2.45 wt%). 2.30 tl 0
10
20
30
40
50
60
Time(Mln.)
the surface layer in the Mg-Fe-Al
sample was rich in Fe. This iron-rich layer served as a catalyst and therefore improved the hydrogen absorptionidesorption kinetics. In Fig. 6, we plot P vs t for the desorption kinetics of Mga&0,26A10.~l and in Fig. 7 we plot log P vs t and Pl/2 vs t for the absorDtion kinetics of the same samole. The linear plots in both cases, according to our‘ kinetic
Fig. 7. The plots of P% vs t and log P vs t for the absorption kinetics of H-132 sample at 250°C (X0 = 0).
models [16], indicate that during the early desorption stage the diffusion of the absorbed H-atoms is the rate-determining step, while in the early absorption process, the dissociation of hydrogen molecule to Hatom on the surface is the rate-determining step.
CONCLUSION We have measured the hydrogen absorption in the Mg-Fe binary and Mg-Fe-Al ternary films. The hydrogen storage capacity for the samples (especially the film having an amorphous structure) with x(Mg) ~0.6 is higher than the calculated value. The hydrogen sorption kinetics of the hydrides of these samples are much better than that of MgH2. The kinetic data indicate that the dissociation of Hz to H-atom on the surface is the rate-determining step for the early absorption process, while the bulk diffusion of hydrogen is the rate-determining step for the early desorption process.
REFERENCES 1. B. Vigeholm, J. Kjoller, B. Larsen and A. S. Pederson, Int. J. Hydrogen Energy 8,809 (1983). 2. R. K. Lyon and N. J. Elizabeth, U.S. Patent No. 3479165 (1969).
3. J. F. Stampfer, C. E. Holley and J. F. Suttle, J. Am. Chem. Sot.-82,3504 (1960). 4. C. M. Stanger. J. Inore. Nucl. Chem. 39. 221 (1977). 5. K. Nomuri, E. Akib”a and S. Ono, ht. J.‘ Hydrogen Energy 6,295 (1981). 6. J. J. Reilly and R. H. Wiswall. Jr, Inorg. Chem. 6, 2220 (1967). 7. M. H. Mintz, Z. Gavra and Z. Hadari, J. Inorg. NW/. Chem. 40,765 (1978). 8. B . Bogdanovic, Znt. J. Hydrogen Energy 9,937 (1984). 9. J. J. Reilly and R. H. Wiswall, Jr, Inorg. Chem. 7, 2254 (1968).
10. B. Darriet, M. Pezat, A. Hbika and P. Hagenmuller, Inc. J. Hydrogen Energy 5,173 (1980). 11. I. Higashi, N. Shiotani, M. Uda, T. Mizoguchi and H. Katoh, Solid State Chem. 36, 225 (1981). 12. G. Bruzzone, G. Costa, M. Ferrettl and G. L. Oleese, Int. J. Hvdroaen Enerav 8.459 (1983). 13. J. P. Darnandery, 6’: Darrier and M. Pezat. ht. J. Hydrogen Energy 8,705 (1983). 14. D. Lupu, A. Biris, E. Indrea, N. Aldea and R. V. Bucur, Int. J. Hydrogen Energy 8, 797 (1983). 15. S. R. Ovshinsky, K. Sapru, K. Dee and K. C. Hong, U.S. Patent No. 4,431,561 (1984). 16. K. C. Hong and K. Sapru, Proc. Int. Symp. Hydrogen Systems, Beijing, China, Vol. 1, p. 403. Pergamon Press (1985).
1987. 0
HYDROGEN
ABSORPTION
1987 Intematmnal
IN HIGHLY DISORDERED ALLOYS
0360-3199/87 $3.00 + 0.00 Pergamon Journals Ltd. Association for Hydrogen Energy.
Mg-BASED
K.C.HoNcandK.SApRu 4853 Gamber Drive, Troy, MI 48098, U.S.A. (Receivedforpublication
24 December 1986)
Abstract-Several highly disordered magnesium based films (Mg-Fe binary and Mg-Fe-Al ternary) have been prepared by sputtering. The hydrogen storage capacities of these films have been determined and compared to the corresponding calculated value. Hydrogen absorption and desorption kinetics of these films were studied. The data indicate that the hydrogen sorption rates of these disordered films are much faster than for pure Mg films or Mg powders. The kinetic data are compared to our kinetic models and show that during the early hydride formation process in these films the rate-determining step is the dissociation of H, to H-atom on the surface, while during the early desorption stage the rate-determining step is the diffusion of H-atoms.
INTRODUCTION Magnesium can store up to 7.6 wt% hydrogen and has been mentioned as a candidate for hydrogen storage medium [l--4]. However, this metal by itself does not react readily with hydrogen to form the hydride and is very sensitive to trace amounts of impurities existing in the hydrogen stream. Moreover, the hydride of magnesium is relatively stable. In order to improve the properties and therefore the usefulness of Mg, some researchers have reported the use of catalysts such as Ni, Cu and Al [5-81, while others have worked on Mgcontaining alloys or intermetallic compounds such as MgzNi and MgzCu [9-141. Iron, while being a low-cost metal, is also a good hydrogenation catalyst. Very little work on the addition of Fe to Mg-based materials has been reported. This may be due to the lack of mutual solubility between Mg and Fe. We report here some data on hydrogen absorption in thin film Mg-Fe binary and Mg-Fe-Al ternary systems [ 151.
EXPERIMENTAL
analysis. The structure of the sample was determined by X-ray diffraction. Each sample was transferred through air to a stainless steel reactor for testing.
Apparatus
As shown in Fig. 1, the testing system was a conventional Sievert’s type apparatus using Edward’s 63M diffstak vacuum equipment. The whole system was made of 316 stainless steel. To reduce hydrogen leak under high pressure to a minimum, the joints in the system were all VCR fittings. The volumes in major testing zones a, b, c shown in Fig. 1 were all of the order of 20 cm3. Several larger cylinders were also attached to the system. All volumes were calibrated within + 0.2 cm3 precision. When the temperature of reactor was different from ambient temperature, the testing zone was divided into hot and cold zones. The volumes of hot and cold zones were determined at various temperatures (0-4OO”C). The pressure of the system was measured by Heise pressure gauges, model 701B (O-3000 torr and O-2000 psi, with ? 0.1% precision). Activation
Samplepreparation
Thin film samples were prepared by co-sputtering techniques using a Veeco Sputtering system under 10-3-10-4 torr argon atmosphere. The substrate used was stainless steel sheet (0.05 mm thick). Prior to film deposition, the chamber was evacuated better than 10m6 torr and the target was pre-sputtered for at least one hour. Two sets of samples were made: Mg-Fe binary and Mg-Fe-Al ternary. The thickness and net weight of the samples was of the order of 10 pm and RIO-200 mg respectively. The chemical composition of the films was examined by EDS (Energy Dispersive Spectroscopy) 411
The activation procedure of the samples studied was as follows. The system with a sample inside the reactor was evacuated to 10-4-10-5 torr. Next, the sample was heated at 200°C for one hour. High purity hydrogen (99.999%) of 40-50 atm pressure was introduced at 200°C and the sample was heated up to 400°C and left there for l-2 hours. The temperature of the sample was then reduced to 300°C where it was left over night. Finally, the sample was cooled to 200°C and the hydrogen pressure was released. The above procedure was repeated for several cycles until the hydrogen storage capacity of the samples was steady.
412
K. C. HONG AND K. SAPRU O-2000 psi H.P. 0
To H, Cylinder
To High Vacuum and Vent Systems
Furnace
Fig. 1. Testing apparatus.
Capacityandkineticsmeasurements: The hydrogen storage capacities of the samples studied were determined by desorption method. The charged sample was heated from ambient temperature (at zero pressure) up to 450°C. The absorption/ desorption kinetics of the samples were studied at 200°C and/or 250°C. For the desorption measurements, the initial pressure of the sample zone (i.e. zone a in Fig. 1) was its equilibrium pressure, while the other portion of the closed system (i.e. zones b and c in Fig. 1) was under vacuum. When the valve “V” between reactor and the rest of the system was opened, the hydrogen pressure vs time was recorded. From this the amount of the hydrogen desorbed was calculated. The absorption kinetics was measured in a similar manner except the initial pressure in zones b and c was about 1 atm higher than the equilibrium pressure existing in the sample section. Blank substrates tested under similar conditions stored negligible amounts of hydrogen. Hence, no correction due to the substrate was applied.
6.0 \ \ 7.0 3
Mgl-,FexHz(l-,) \ \
6.0 -
‘\ \ \
5.0 -
\ \ \
4.0 -
\ \ ‘0
3.0 -
0
0
‘\
0
\O \ \
2.oG
\ \ 1.0 -
0 0
I 0.2
I
I
0.4
0.6
\
\
\
0.8
\
\
\
\, 1.0
NW
Fig. 2. Hydrogen storage capacity in the Mg,_,Fe system.
RESULTS AND DISCUSSION The X-ray diffraction pattern indicated that the Mg-Fe binary films were all of multiphase structure which consisted of crystalline Mg, a-Fe and amorphous phase. The hydrogen storage capacity of this binary
HYDROGEN
ABSORPTION
Table 1. Summary of the hydrogen absorption in Mg-Fe-Al Sample No.
H-110 H-127 H-123 H-125 H-145 H-132
Fe 0.31 0.39 0.35 0.30 0.22 0.26
0:: 0.54 0.58 0.60 0.62 0.64
ternary films Structure*
H wt% Stored Expt-1 Calculatedt
Composition*
Chemical
413
IN Mg-BASED ALLOYS
Al 0.18 0.07 0.07 0.10 0.16 0.10
3.2 3.5 4.1 5.0 3.4 3.5
2.95 2.94 3.26 3.52 3.92 3.91
multiphase multiphase multiphase amorphous multiphase multiphase
* by EDS. t Assuming that only Mg can absorb hydrogen, i.e. Mg,(Fe,Al),,H,,. $ by X-ray diffraction.
Mg,_,Fe,(O shown region
< x < 0.65) is between 2.5 to 3.8 wt%. As in Fig. 2, the optimum composition is in the 0.4 < x < 0.6. The decrease in the storage
capacity in the Mg-rich films is not expected and has been attributed to the MgO formation. The Auger depth profile of these samples indicated that the degree of oxidation of films increases with the increase in magnesium concentration and reaches a maximum for the pure Mg films. The amount of oxide was significant, up to lO-20% of the total film weight. X-ray diffraction patterns also confirmed the oxide formation. The oxide not only results in a decrease in the hydrogen storage capacity, but also makes activation difficult. It is interesting to note from Fig. 2 that in the region x(Fe)>0.4, the hydrogen stored in the sample is higher than the calculated value (the dashed curve in the figure) based on the formula Mgi_,FeH2(i_,) in which we assume that only magnesium can absorb hydrogen in this binary system. From the Auger depth profile, the films
100
with x(Fe)>.0.35 had a very thin oxide layer of about 25A on the surface and a thick iron-rich layer next to it. This iron-rich layer protected bulk magnesium from oxidation and also led to easy activation. In their studies on the Mg*Nii_,Fe, ternary, Lupu et al. [14] found that the Mossbauer spectra in the high iron alloy (x = 0.25 to 0.37) revealed magnetic hypertine splitting. These results suggest that the electronic structure of iron was modified by magnesium. We speculate that in the highly disordered Mgl_,Fe,(0.5cx<0.65) film magnesium atoms may also modify the electronic structure of iron such that new bonding sites are generated to absorb more hydrogen. Further work is needed to confirm this. The hydrogen sorption kinetics of the Mgi_,Fe, films studied is much faster than that of pure magnesium. These samples can reversibly absorb and desorb hydrogen even at temperature as low as 180°C. For the Mg-Fe-Al ternary alloys, the experimental results are summarized in Table 1. The structures of
r
01’ 0
’ 18
’
’ 26
’
’ 34
’
’
’
42
’ 50
’
’ 58
’
’ 66
’
’
’
74
20
Fig. 3. The hydrogen
storage capacity of these alloys
’
82
414
K. C. HONG AND K. SAPRU
these films in general are multiphase consisting of crystalline Mg, a-Fe and amorphous phase. However, in one case the sample Mgo.s~eo.soAlo,lo was completely amorphous as indicated by X-ray diffraction shown in Fig. 3. The hydrogen storage capacity of these alloys vary from 3.2% to 5.0 wt%, with the x(Mg) from 0.5 to 0.65. In Table 1 we also include the calculated H wt% of each sample, again assuming that only magnesium can absorb hydrogen. It is worthwhile to note that the hydrogen stored in those samples with x(Mg) <0.6 is higher than that of the calculated values, especially for the sample with an amorphous structure. This phenomenon is similar to that in the Mg-Fe binary system and might also be due to the modification of electronic structure of Fe and/or Al by Mg atoms. The hydride of the Mg-Fe-Al ternary alloys studied, like the Mg-Fe binary, has much better sorption kinetics 10 1.0 as compared to that of MgH2. As shown in Fig. 4, the sample H-125 (MgO.&eo.scAlo.lo) can desorb about Time (Hr) 80% of the stored hydrogen in 20 minutes at 300°C and the sample H-99 (Mgo,45Feo.sAlo.os) can release more than 50% of hydrogen in one hour at 200°C. The film or Fig. 4. Hydrogen desorption kinetics of samples H-125 powder of magnesium hydride desorbed insignificant (Mgo.&eo.&% ,o) and H-99 (Mg0.45Fe0.sA10.& amounts of hydrogen at temperatures below 250°C. The fast sorption kinetics in Mg-Fe-Al over Mg has been attributed to enhancement of iron in the surface layer. A typical Auger depth profile given in Fig. 5 indicated that
looI 80 -
Mg
4
6
Sputter Time (Min.)
Fig. 5. Auger Spectroscopy depth profile of the sample H-99 (MgO,,,FeO,~oAIO.oS) at 115 kminlrate.
HYDROGEN
415
ABSORPTION IN Mg-BASED ALLOYS 24
1
I
Time(Min.)
Fig. 6. The plots of P vs t for the desorption kinetics of H-132 (Mg, MFe0.26A10.01)at 250°C (X0 = 2.45 wt%). 2.30 tl 0
10
20
30
40
50
60
Time(Mln.)
the surface layer in the Mg-Fe-Al
sample was rich in Fe. This iron-rich layer served as a catalyst and therefore improved the hydrogen absorptionidesorption kinetics. In Fig. 6, we plot P vs t for the desorption kinetics of Mga&0,26A10.~l and in Fig. 7 we plot log P vs t and Pl/2 vs t for the absorDtion kinetics of the same samole. The linear plots in both cases, according to our‘ kinetic
Fig. 7. The plots of P% vs t and log P vs t for the absorption kinetics of H-132 sample at 250°C (X0 = 0).
models [16], indicate that during the early desorption stage the diffusion of the absorbed H-atoms is the rate-determining step, while in the early absorption process, the dissociation of hydrogen molecule to Hatom on the surface is the rate-determining step.
CONCLUSION We have measured the hydrogen absorption in the Mg-Fe binary and Mg-Fe-Al ternary films. The hydrogen storage capacity for the samples (especially the film having an amorphous structure) with x(Mg) ~0.6 is higher than the calculated value. The hydrogen sorption kinetics of the hydrides of these samples are much better than that of MgH2. The kinetic data indicate that the dissociation of Hz to H-atom on the surface is the rate-determining step for the early absorption process, while the bulk diffusion of hydrogen is the rate-determining step for the early desorption process.
REFERENCES 1. B. Vigeholm, J. Kjoller, B. Larsen and A. S. Pederson, Int. J. Hydrogen Energy 8,809 (1983). 2. R. K. Lyon and N. J. Elizabeth, U.S. Patent No. 3479165 (1969).
3. J. F. Stampfer, C. E. Holley and J. F. Suttle, J. Am. Chem. Sot.-82,3504 (1960). 4. C. M. Stanger. J. Inore. Nucl. Chem. 39. 221 (1977). 5. K. Nomuri, E. Akib”a and S. Ono, ht. J.‘ Hydrogen Energy 6,295 (1981). 6. J. J. Reilly and R. H. Wiswall. Jr, Inorg. Chem. 6, 2220 (1967). 7. M. H. Mintz, Z. Gavra and Z. Hadari, J. Inorg. NW/. Chem. 40,765 (1978). 8. B . Bogdanovic, Znt. J. Hydrogen Energy 9,937 (1984). 9. J. J. Reilly and R. H. Wiswall, Jr, Inorg. Chem. 7, 2254 (1968).
10. B. Darriet, M. Pezat, A. Hbika and P. Hagenmuller, Inc. J. Hydrogen Energy 5,173 (1980). 11. I. Higashi, N. Shiotani, M. Uda, T. Mizoguchi and H. Katoh, Solid State Chem. 36, 225 (1981). 12. G. Bruzzone, G. Costa, M. Ferrettl and G. L. Oleese, Int. J. Hvdroaen Enerav 8.459 (1983). 13. J. P. Darnandery, 6’: Darrier and M. Pezat. ht. J. Hydrogen Energy 8,705 (1983). 14. D. Lupu, A. Biris, E. Indrea, N. Aldea and R. V. Bucur, Int. J. Hydrogen Energy 8, 797 (1983). 15. S. R. Ovshinsky, K. Sapru, K. Dee and K. C. Hong, U.S. Patent No. 4,431,561 (1984). 16. K. C. Hong and K. Sapru, Proc. Int. Symp. Hydrogen Systems, Beijing, China, Vol. 1, p. 403. Pergamon Press (1985).