1
INVESTIGATIONS ON SOLID STATE MATERIALS
FOR HYDRGEN STORAGE AND ITS UTILIZATION
SINGH S.K.
Department of Physics, DCR University of Science & Technology Murthal
(Sonepat)-131039, Haryana, India
Annotation. The rare earth pentanickelides LaNi5 and MmNi5 (Mm: Mischmetal typically
contains 49 %La, 32 %Ce, 13 %Nd, 4 %Pr and 2 % other rare earths) have been synthesized in the form
of amorphous and amorphous like phases. It has been found that the amorphous phases have higher
hydrogen storage capacity than their crystalline versions. The variation in the electronic behavior on
hydrogenation has been monitored by investigating the variation of resistively as a function of time. A
very curious result embodied in the observation that the thick (~150nm) and thin (~30nm) films of both
LaNi5 and MmNi5 have similar resistively variations, has been found in the present investigation. The
present result is in sharp contrast to the earlier known observations [1]. The present results suggest that
it is the surface characteristics and the variation of density of states (DOS) and not the film thicknesses
which eventually decide the electrical resistively variations on hydrogenation. The details of the influence
of surface and the density of states (M-H, H-H interactions) on the hydrogenation behavior have been
described and discussed. Yet another aspect which has been described in this communication related to
the application of hydrogen/hydride as alternate fuel.
Keywords: rare earth pentanickelides; amorphous phases; hydrogenation; thick and thin films;
resistively variations; surface characteristics.
1. Introduction
Rare earth pentanickelides correspond to one of the few hydrogen storage materials
capable of storing hydrogen at volumetric density higher than that of liquid hydrogen [2-4].
These materials can also absorb and desorbs hydrogen reversible and hence they represent
the key energy materials in the realm of “hydrogen – the future fuel” scenario. As is well known
by now, the hydrogen absorption characteristics crucially depend on the crystal structure and
the electronic band structure characteristics of these intermetallics [5-7]. In the light of this,
studies on hydrogenation and its correlation with structural and electronic behaviors of these
intermetallic represent a potentially important area of investigation. We have carried out studies
regarding the influence of the structural features on the hydrogenation behaviour. We have also
investigated the variation of the electronic characteristics originating from the hydrogen
absorption. In particular we have synthesized amorphous and amorphous like phases of rare
earth pentanickelides (e.g. LaNi5, MmNi5). The hydrogenation behaviour of these phases has
revealed that they have a higher hydrogen capacity than the corresponding crystalline versions.
As regards the monitoring of the variation of the electronic behaviour on hydrogenation, we
have employed thin films since decrepitation in these unlike in bulk is negligible. We have
found a very curious result in regard to the resistivity variation. The thick (~150nm) and thin
(~30nm) films have been found to exhibit similar resistivity variations with respect to time on
hydrogenation under specific conditions of the film surfaces. As for example a thick film of
LaNi5 (~150nm) when aged at ambient conditions shows resistivity variation behaviour similar
to that of a thin film (~30nm). This result is in sharp contrast with the earlier observations that
the thick and thin films invariably have distinctly different resistivity variation characteristics [1].
It has been shown by us that the mechanism put forward for the electrical resistivity variation
on hydrogenation is inadequate. I have been able to explain the resistivity variations
satisfactorily by taking into account the surface characteristics and the variation in the DOS.
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2. Experimental techniques and results
The rare earth pentanickelides i.e. RNi5 intermetallics (R=La, Mm) were prepared by
melting the stoichiometric mixtures in the quartz (silica) tube under the vacuum of ~10-6 torr.
This melting was carried out several times in order to procure proper stoichiometric alloy. The
RNi5 ingot so formed, were tested by X-ray diffraction technique employing Guinier focusing
camera in the transmission mode. The powder diffraction results conformed that the
synthesized materials corresponded to the known hexagonal system with CaCu5 structures.
Thin films of RNi5 intermetallics were prepared by vacuum thermal vapour deposition
technique under the vacuum of ~10-6 torr using suitable masks. These films were characterized
by transmission electron microscopy. Fig. 1 exhibits representative transmission electron
micrograph and diffraction pattern from the as deposited LaNi5 thin films. Similar results were
obtained from MmNi5 films. As is vividly apparent whereas the microstructure exhibits
featureless characteristic, the diffraction patterns show halos. From these results it is discerned
that the as deposited films are amorphous. The melt quenching (rapid cooling) of these RNi 5
intermetallics also produced amorphous like phases. The X-ray diffraction patterns of the as
quenched phases revealed that these phases were microcrystalline.
Fig.1 (a)
Fig.1. (b)
Fig.1. (a) Transmission electron micrograph of the as grown LaNi5 thin film, bringing put the
amorphosity of the film. (b) Electron diffraction pattern of the thin film of LaNi5 exhibiting halos
and hence revealing the amorphosity
With the known results that the amorphous phases have certain advantages over
crystalline forms in regard to hydrogen capacity we studied the influence of hydrogenation on
the structural as well as electronic properties of these amorphous and amorphous like phases.
The amorphous phases may have higher hydrogen capacity than their crystalline counterparts.
In order to check this conjecture low pressure hydrogen absorption was monitored. This was
accomplished by employing the electrolytic cell and placing the known quantity of RNi5 material
at cathode. Fig. 2 shows the hydrogen concentration versus time (t) curve of MmNi5. From the
figure it is evident that the as quenched (amorphous) form has higher hydrogen capacity than
the crystalline version.
It has now been known that the variation in physical properties of these intermetallic as
a result of hydrogenation can not be monitored easily since the material decrepitates on
hydrogen absorption. However, the problem of decrepitation can be avoided by taking the
material in the form of thin films. Keeping this fact in view, thin films of LaNi5 and MmNi5 were
prepared by thermal evaporation technique in vacuum. The electrical behaviour of these films
was then monitored by measuring the variation in the resistivity of the films on hydrogenation
as a function of time.
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The variation of resistivity as a function of time was monitored for the films of the two
thickness ranges i.e. a) thick films (100-150nm) and b) thin films (30-40nm). The resistivity
variation as a function of hydrogenation time for a large number of films corresponding to the
above two categories was investigated at the temperature of 25 oC.
HYDROGEN ABSORPTION
IN MmNi5 per gm
NORMAL
(AS GROWN)
TIME (Minutes)
Fig.2. Hydrogen absorption in as
syntheized (polycrystalline) MmNi5
and quenched MmNi5
Resistvity φx10-4  cm
VOLLUME OF HYDROGEN IN ml
QUENCHED
TIME (Minutes)
Fig.3. Resistvity variation of LaNi5 thik
film (~ 1500 Å) on hydrogenation
Representative curves revealing the resistivity variations with respect to hydrogenation
time for thick and thin films are shown in Fig. 3 and 4 respectively. As can be easily discerned
from the figures, the resistivity variation for thick and thin films are remarkable different from
each other. The effect of thickness will be to increase the resistivity of the alloy film itself
through enhanced surface scattering of electrons with decreasing thickness. But, the resistivity
variation of LaNi5 thick and thin film Fig. 3 and 4 on hydrogenation is different, particularly the
decrease (BC in Fig. 3) of resistivity in thin films is rather surprising. The hydrogenation
behaviour is known to be crucially dependent on the physical nature and surface structure of
the storage material.
The effect of surface structure on hydrogenation characteristics of thick and thin films
was verified by the films of MmNi5– a material which is less susceptible to surface oxidation
than the LaNi5. The important result coming from this was that regardless of the thickness, the
resistivity variation on hydrogenation has similar features. A representative figure bringing out
the ρ-t characteristics of MmNi5 thick (~150nm) and thin (~ 30nm) films is shown in Fig. 5, as
can be seen the trend of resistivity variation for both the thick and thin films is rather similar. In
both the cases the resistivity on hydrogenation increases to a saturation value. On exposure to
atmosphere i.e. on aging (~72 hours), the thin films of MmNi5 started exhibiting behaviour
similar to that of LaNi5 thin film (fig. 6). The differences in the behaviour apparently come
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through due to the contamination of the surface of the film. As expected, the thinner films would
undergo surface contamination rather rapidly and the thin LaNi5 films get contaminated almost
instantaneously. The MmNi5 films on the other hand are comparatively stable and deteriorate
only slowly. Yet, another interesting observation suggesting the role of the surface
characteristics was in regard to the ρ-t behaviour of thick films (~150nm) of LaNi5. After aging
through exposure to ambient atmosphere for 42 hours the LaNi5 thick film (Fig. 7) started
exhibiting ρ-t characteristics similar to that of LaNi5 thin film. The above results clearly point out
that the ρ-t behaviour does not depend on the thickness but it is related to the surface
characteristics. If the surface characteristics of the two types of films are similar, the ρ-t
characteristics of these films would also be similar.
3. Discussion
Resistvity φ (thin)x10-4  cm
Resistvity φx10-4  cm
Resistvity φx10-4  cm
Fig. 2 represents the hydrogen concentration versus time curve of MmNi5 when
activated electrolytically i.e. low pressure hydrogen absorption. Initially the hydrogen
concentration increases and attains saturation with time for crystalline MmNi5 as well as
quenched MmNi5. The higher absorption capacity in quenched forms may arise due to the
relaxation of the condition of a specific density of interstitial positions in the amorphous
versions.
Generally the hydrogen capacity is controlled by the density of voids and also the size
of the voids [8]. In quenched forms more number of voids is expected for hydrogen atom
occupation. From the above result, it is apparent that the hydrogen concentration is dependent
on the structural characteristics (amorphous/crystalline). In sharp contrast to the crystalline
versions, the amorphous like as quenched phases not only possess higher hydrogen capacity
but also have more ductility and less subject to pulverization [9].
TIME (Minutes)
Fig.4. Resistvity variation of LaNi5 thin
film (~ 300 Å) on hydrogenation
TIME (Minutes)
Fig.5. The φ-t characteristics of MmNi5
thick (~ 1500 Å) and thin (~ 300 Å)
films on hydrogenation
Resistvity φx10-4  cm
Resistvity φx10-4  cm
5
TIME (Minutes)
Fig.6. Resistvity variation on MmNi5
thin film on hydrogenation after
atmospheric aging (~72 hours)
TIME (Minutes)
Fig.7. Resistvity variation of LaNi5
thick film on hydrogenation after
atmospheric aging (~42 hours)
Earlier the present author reported resistivity variation of RNi5 films with time [6, 11].
Other workers have investigated exclusively the variation of resistivity of these films and put
forward a mechanism to make these variations intelligible [1]. They suggested that the
resistivity variation is dependent upon the thickness of the films. The increase of resistivity on
hydrogenation was explained by the formation of H- ion layer and the decrease by the
formation of highly contaminated metal hydride layer. However, they neither considered the
surface characteristics nor changes in the density of states (DOS) on hydrogenation. Both
these features are known to influence crucial hydrogenation characteristics. As a matter of fact
the present observations reveal that the mechanism put forward in reference [1] is quite
inadequate since it has been found that thick and thin films may exhibit similar characteristics
(Fig. 5). By taking into account the surface characteristics and the variation in the DOS, we
have been able to explain satisfactorily the variation of resistivity consequent to hydrogen
absorption.
Keeping in view the foregoing points, we have put forward an explanation by taking into
account both the expected surface characteristics and the variation of the density of state for
making observed ρ-t variation for thick as well as the thin films explicable.
The present results suggest that the thickness of the film does not decide the ρ-t
behaviour. Surface is known to play a crucial role in the hydrogen storage capacity and
consequently on the ρ-t behaviour of different films. The surface characteristics of the hydrogen
storage material especially RNi5 type materials particularly that of LaNi5 have been thoroughly
explored by several workers. According to surface mechanisms put forwarded that La in LaNi5
captures the “lattice dissolved” as well as atmospheric oxygen producing La2O3 and free
metallic nickel on the surface [7]. It is the high affinity of ‘La’ towards oxygen that there is
always free Ni at the surface which is capable of dissociating H2 molecule in atoms which
eventually diffuse into the lattice and get slowed down and become ineffective after a certain
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period when the hydrogen on the surface gets partially bonded. It is also known that repeated
hydrogenation leads to extraction of dissolved oxygen in the material, its eventual reactions on
the surface produces La2O3 and increases the free concentration of nickel on the surface.
Thus, it has been estimated that 25 hydrogenation cycles would produce up to 1 atomic
percent Ni (~100nm) on the LaNi5 surface.
In the case of LaNi5 films it is expected that the thicker films (~150nm) would have bulk
like behaviour, since the surface would get contaminated similar to the bulk. On the other hand
in the case of thin film (~30nm), the surface is expected to be severely contaminated through
oxide formation. The surface of the thin film would, therefore, have high coverage by oxide
formation. The higher oxide coverage, the greater would be the extent of free nickel on the
surface since oxide formation leads to the reaction
4LaNi5 + 3O2
→
2La2O3 + 2ONi
(1)
The availability of large nickel concentration would lead to the formation of large nickel
nuclei. The surface would therefore have lower density of La2O3/Ni channels which would be
available for hydrogen diffusion leading to absorption/desorptions (Fig. 8). In addition to this,
the density of interstitial voids in thin films is also likely to be lower than that of the bulk
counterparts.
In the light of this, the hydrogen concentration in thin films would be less. On the other
hand thick films (LaNi5) have higher hydrogen capacity due to less oxide contamination. So the
surface as well as the hydrogen capacity plays a dominant role in governing the ρ-t variations
of these films. On following we proceed first to outline the hydrogenation characteristic i.e. ρ-t
variation for thick film and thereafter for the thin films. In order to explain the basic differences
of the two thickness ranges of films we have considered the effects of surface structure on the
hydrogenation and resistivity.
Fig.3 shows the resistivity (f) vs time (t) curve of LaNi5 thick film on hydrogenation.
Initially the resistivity decreases (BC of Fig. 3) and then resistivity increases to CD and
eventually attains a saturation value DEK. The variation in the electronic and optical properties
of metals/intermetallics on hydrogen absorption becomes intelligible in terms of the changes in
the density of states (DOS) consequent to the hydrogen concentration. As already mentioned
the hydrogen absorption in the thicker film is analogous to the bulk material, the resistivity
variation of thick film on hydrogenation can easily be understood based on the variation of the
density of states.
The decrease of (BC of Fig.3) in LaNi5 thick film is thought to be due to the M-H
interaction in which hydrogen produces additional states Fig. 9b. Due to the additional
hydrogen potential the ‘d’ states get lowered on energy scale and the new structure
corresponds to a larger value of DOS at the Fermi level. This is in accordance with the known
results on the variation in DOS consequent to hydrogenation. In this case the resistivity
(conductivity) is expected to decrease (increase), this explains the relative drop manifested by
BC in Fig. 3. This corresponds to medium hydrogen concentration. As further hydrogenation
proceeds i.e. high hydrogen concentration is achieved H-H interaction onsets. Together with
the M-H interaction it affects the DOS drastically; these changes incorporate considerable
reduction in the DOS available for the conduction (Fig. 9 c). Now the lower states of the host
material are well separated as hybridized parabolic ‘sp’ band, and most of the states are also
occupied at the Fermi level. This causes the situation where the electronic conduction would
require intra-band transition. The conductivity (resistivity) would therefore get-reduced
(increased) drastically. Consequently the ‘f’ (CD of Fig. 3) increases and attains saturation with
time.
Fig. 4 shows the ρ-t behaviour of thin film of LaNi5 which is different from that of thick
film. From the figure, it is apparent that the resistivity decreases from B’ to C’ thereafter it
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attains a saturation C’F’. The decrease in the resistivity of LaNi5 thin film like thick film
originates due to M-H interactions in which the hydrogen atoms interact dominantly with metal
atoms in the intermetallic matrix. This corresponds to the situation of low hydrogen
concentration region (BC region of Fig.3). Due to the lower density of La2O3/Ni interfaces and
lower hydrogen concentration in the solid solution, there is a highly decreased probability of
interaction among the H atoms i.e. H-H interaction. So the resistivity in Fig. 4 (B’C’) decreases
initially and never increases thereafter. This is because unlike in the thicker films the H-H
interaction responsible for the variation in DOS leading to an increase in resistivity (region CD
in Fig.3) will not switch on in the thin film case. The slow rate of decrease of resistivity in thin
film with time may be due to the high oxide coverage, leaving lesser density of channels for
hydrogen diffusion. Since the density of channels is known to be the rate controlling factor, the
switching of M-H interaction in the film would be accordingly slow. After an optimum hydrogen
uptake, the resistivity decrease would attain a saturation value. When the thin film surface is
not significantly contaminated through oxidation as in fresh MmNi5 films, the thin film ρ-t
characteristics are similar to that of the MmNi5 thick film (Fig.5). Likewise when the thick film
surface gets significantly contaminated through aging as in LaNi5 film and the surface becomes
similar to that of a thin LaNi5 film; the ρ-t behaviour also becomes similar to that of the thin film
(Fig.7).
Fig.8. Expected surface characteristics of thick (fresh) and thin (fresh) LaNi5 films
(a) No hydrogenation
Fig.9.
(b) Occurrence of new states
due to M-H interaction
(c) Metal hydride
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4. Application of MmNi 4.5Al 0.5 hydride as fuel in I.C. engine
One of the aims of the studies of the synthesis of the rare earth based pentanickelides
was to use its hydride as a source of hydrogen for fuelling the I.C. engine in automotive area.
C
A
D
B
Fig.10. A hydride/hydrogen I. C. engine. The I.C. engine was a standard Villier’s 1.75 H.P.
(3000 RPM) cane crushing machine which usually runs on gasoline (the gasoline tank (D) is
abandoned in the present work). Carburetor (C) of the engine was modified and a cam induced
injector system (B) was fabricated for running the engine on hydride/ hydrogen (contained in
tank (A)).
The primary step in the automotive applications apparently would be to run a
hydrogen/hydride powered internal combustion engine. To meet this goal I have utilized RNi5H6
type hydride for running a Villiers internal combustion (1.75 HP; 3000 RPM; Fig.10).This is the
four strokes I.C Engine generally used for cane crushing, but can also be used for running a
mobike/scooter. The carburetor of this engine was modified on lines similar to those suggested
by White et al. [13]. The hydrogen injection into the main chamber was carried out by cam
induced fuel injection pump(Fig.10).The hydrogen injection into the chamber was carried out by
cam induced fuel injection pump. The hydrogen gas was supplied by MmNi4.5Al0.5 hydride which
partly dissociated to yield hydrogen at ambient condition. At later stages the dissociation was
obtained by the engine exhaust gases which raised the temperature up to ~ 50 oC. It was
possible to control the back-firing of the engine through controlled injection of hydrogen. The
I.C engine gave satisfactory performance for one hour with employed hydride of about 5 Kg. It
is proposed to utilize twin and improved hydride tanks to achieve a higher range.
5. Conclusion
It has been found that synthesis of the amorphous phases of LaNi5 and MmNi5 is
possible in thin film form by vacuum thermal vapour deposition and in bulk form by melt
quenching in liquid nitrogen. Interestingly the low pressure hydrogen storage capacity of
amorphous and amorphous like phases is higher than that of the corresponding crystalline
counterparts. It has been concluded that the curious resistivity variation in thick and thin films of
LaNi5 and MmNi5 with hydrogenation time is due to the surface structure characteristics. The
resistivity variation on hydrogenation would be same regardless of their thicknesses for the
films having similar surface characteristics. This has been explained on the basis of the effect
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of surface structure on hydrogenation and consequent changes in the density of states (M-H,
H-H interactions).The application of RNi5 hydrides in I.C engine has been described.
Acknowledgement
The author likes to thank Professors G. Spazzafumo, O.N.Srivastava FNA, L.M. Das
and Dr.Hank Barten for helpful discussions. The present work was carried out under the
AICTE, New Delhi sponsored research project on Hydrogen Energy.
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