# Hydriding Reaction ofLaNi5: Correlations

Authors: P. Millet; C. Lebouin; R. Ngameni; A. Ranjbari; M. Guymont
Journal: Research Letters in Physical Chemistry
 (2008)

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## Abstract

This research work concerns the hydriding reaction ofLaNi5 during the first hydriding cycles (activation process). Step-by-step sorption isotherms (Δ[H/M]≈0.03) were measured at 298 K, in the composition range 0<H/M<2.0, at the beginning (first hydriding cycle, where hysteresis is maximum) and at the end (tenth hydriding cycle, where hysteresis is minimum) of the activation process, offering the possibility to correlate thermodynamic states (pressure-composition data points) to sorption kinetics. Using pneumatochemical impedance spectroscopy (PIS), experimental impedance diagrams were obtained for each data point of the isotherms. Microscopic rate parameters such as surface resistance and hydrogen diffusion coefficient were obtained as a function of composition, by fitting appropriate model equations to experimental impedances. It is found that the high-frequency pneumatochemical resistance significantly decreases during activation. This is correlated with the surface increase of the solid-gas interface area. The hydrogen diffusion coefficient is found to be larger at the beginning of the activation process and lower on a fully activated sample.


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## Body

## 1. Introduction

Metal-hydride systems are increasingly studied for hydrogen-storage applications. In
particular, many intermetallic compounds (IMCs) such as the Haucke (AB5)
phases offer several  interesting features, in terms of specific energy, volumetric
energy, and kinetics. Practical applications require the determination of both thermodynamic
and kinetic properties which are, in general, measured separately. Pneumatochemical
impedance spectroscopy (PIS) [1] now offers the possibility of analyzing hydriding/dehydriding
reaction mechanisms, raw kinetic data being measured using a classical Sieverts
experimental setup [2]. Using PIS, kinetic information is not only a measure of
a global reaction rate (limited by any rate-determining step): the different steps
involved in the sorption reaction are put into evidence and the values of individual
rate parameters are inferred. Results obtained so far on various IMCs indicate
that most hydriding mechanisms involve two microscopic reaction steps: (i) the
surface chemisorption of molecular hydrogen which is dissociated into two
surface ad-atoms; (ii) the diffusion-controlled transport of hydrogen atoms to
bulk regions where they can either accumulate (in solid-solution domains) or
precipitate to form metal hydrides (in two-phase domains). PIS is therefore an in situ spectroscopic tool which can
be used to analyze surface and bulk processes in solid-gas systems.Activation is the term referring to the first hydriding cycles of a brittle hydrogen
absorbing IMC, during which the sample is gradually pulverized into a powder of
fine particles. This pulverization is induced by the precipitation of a hydride
phase of larger lattice parameters than the host IMC. This in turn generates
internal strain and stresses and leads to the formation of a few micrometers
thick particles, with large concentration of defects. During activation,
thermodynamic properties (in particular pressure-composition equilibrium
isotherms) and sorption kinetics are significantly modified. Whereas it is
known from the  literature that the absorption pressure plateau significantly
decreases during the first hydriding cycles [3]; it is still unclear how  individual
rate parameters (surface resistance and hydrogen diffusion coefficient) are related
to the thermodynamic states of the sample. The purpose of the work presented
here is therefore to correlate thermodynamic states and sorption kinetics
measured during the activation of LaNi5.


## 2. Experimental Section

The basic principles  and details about
pneumatochemical impedance spectroscopy have been previously reported [1]. A
thorough description of the experimental setup and experimental requirements for
measuring pneumatochemical impedances is described in [4]. In the present work, 2.9 g of LaNi5 powder
(Alfa Aesar GmbH, Karlsruhe, Germany) were
placed in the reaction chamber of the PIS setup. Because the sample is prone to
surface oxidation [5], it was loaded in an inert atmosphere of argon using a
glove box. The setup can be gas-purged down to secondary vacuum levels (1×10−6 mbar) using a turbomolecular pumping station (BOC Edwards Co., Crawley, UK) to
remove any residual impurity. Alphagaz
grade 1 H2 (Air Liquide Co., Paris, France) was used in the experiments.


## 3. Results and Discussion

### 3.1. Experimental Results

Two {pressure-composition} isotherms, measured at 298 K on
LaNi5-H2 (g) up to the composition point H/M ≈ 2, are plotted in Figure 1(a) (the maximum
composition at 298 K, not represented here, is H/M ≈ 5.8). The first isotherm was recorded during
the first hydriding cycle and the second one was recorded at the end of the
activation process, during the 10th hydriding cycle. At low-hydrogen content
(0.0 < H/M < 0.5), a solid solution of hydrogen (α-LaNi5)
is formed. As the hydrogen content increases, a nonstoechiometric hydride phase
(LaNi5H6-x or β-LaNi5 phase) begins to be formed.
The system becomes monovariant and a pressure plateau develops. This is the
two-phase domain which extends
over the 0.5 < H/M < 5.8 composition range. At higher hydrogen contents,
a solid solution of hydrogen in β-LaNi5 is formed. In this
research work, experiments plotted in Figure 1 were intentionally stopped at
H/M ≈ 2.0 to avoid lengthy and unnecessary
measurements over the whole composition range.(a) Equilibrium pressure; (b)RHF; (c) DH; (d) dP/dCH as a function of composition H/M measured on LaNi5 at 298 K. (Data points labelled 1, 2, 3 in (a) refer to the impedance diagrams of Figure 2.)
(a)(b)(c)(d)The main characteristic associated
with metal-hydride systems is the presence of a large hysteresis. The
difference between absorption and desorption plateau pressures is significantly
large (ca. 1400 mbar) for the first
isotherm (Figure 1(a)). The amplitude of hysteresis decreases during
activation, and a final value of ca. 400 mbar, observed during the 10th hydriding cycle (Figure 1(a)), is typical of
an activated material. Hysteresis also exists at phase boundaries, the
composition at which the first hydride nucleus is formed during absorption
being different from the composition at which the last hydride nucleus
disappears during desorption (Figure 1(a)).Each data point of Figure1(a)
corresponds to one gas transfer experiment during which the transient pressure
measured in the reaction chamber and the transient hydrogen mass flow entering
the reaction chamber were synchronously sampled, yielding one impedance diagram
per experiment. Typical
impedance diagrams measured during activation in two-phase domains are plotted
in Figure 2. Impedance spectra numbers (1, 2, 3) of Figure 2 are referring to
the gas transfer experiments (1, 2, 3) of Figure 1(a). They are expressed in
Pa⋅mol−1⋅s and not in Pa⋅mol−1⋅s⋅cm2, that is,
they are not corrected for the solid-gas interface area. As analyzed elsewhere
[1], in two-phase domains, a typical semicircle is observed in the high-frequency
(HF) domain along the real axis (see, e.g., diagram 1 in Figure 2). This is the
signature of the parallel connection of a capacitance (associated with the dead
volume of the reaction chamber) and a resistance in Pa⋅mol−1⋅s. In single-phase domains (SPDs), the diameter of this HF semicircle is equal to the value
of the surface resistance Rs (the chemisorption step of the sorption
process) but in two-phase domains (TPDs), this is the sum (Rs+RPT),
RPT being the resistance associated with the phase transformation
process [6]. The HF semicircle is then followed at lower frequencies by a
characteristic medium frequency (MF) region related to hydrogen diffusion and
ends up at low frequencies (LF) in a capacitive shape (a semi-infinite line
along the imaginary axis).Figure 2
Experimental (o) and model (–) impedance diagrams. 1, 2, 3 refer to data points of Figure1(a).
(1) First hydriding cycle, absorption, H/M ≈ 1.5,
(2) First hydriding cycle, desorption, H/M ≈ 1.4,
(3) 10th hydriding cycle, absorption, H/M ≈ 1.5.Each experimental impedance
diagram of Figure1(a) was fitted using model equations applicable to single-
and two-phase domains, as described in [6]. From the fits, values of the HF
resistance (RHF), the hydrogen diffusion coefficient (DH),
and the slope of the isotherm at the measurement point (dP/dCH) were
obtained. All results are plotted as a function of the composition H/M in Figures
1(b), 1(c), and 1(d), respectively. In Figure 1(b),  the true solid-gas
interface area has been taken into account and RHF values are
expressed in Pa⋅mol−1⋅s⋅cm2.


### 3.2. Shape of Impedance Diagrams during Activation

The first impedance diagram of Figure2 (curve 1) was measured on the unactivated
sample, on the pressure plateau in the two-phase domain (data point 1 in Figure 1(a)). A large HF
resistance of ca.4×1011⁢Pa⋅mol−1⋅s is observed. Once the scan is reversed, the first
impedance diagram measured on the desorption pressure plateau (point 2 in Figure 1(a)) is also
plotted in Figure 2 (curve 2). An overall lower HF resistance is measured but
two consecutive semicircles are observed, suggesting a bimodal distribution of
particle size. In other experiments (not reported here), more than two HF
semicircles were observed. It can be concluded that fragmentation of initial
particles leads to particles of heterogeneous sizes. Finally, the impedance
diagram measured in two-phase domains on the activated sample (point 3 in Figure 1(a)) is also
plotted in Figure 2 (curve 3). The impedance is significantly lower for each
frequency, indicating that the sorption kinetics will be higher on the activated
sample.


### 3.3. High-Frequency Resistance

It is observed from the whole set of experimental impedance diagrams (not represented here) that
the high-frequency resistanceRHF (expressed in Pa⋅mol−1⋅s)
decreases exponentially with the number of hydriding cycles, down to a
constant value at the end of the activation process. This does not mean
that the kinetics of the surface step (in SPDs) or the phase transformation
process (in TPDs) are varying during activation: mostly HF resistances are a function of the
operating temperature which is constant here. The reduction of RHF must be correlated with the increasing solid-gas (and interphase) interface
areas resulting from the sample pulverization. To take into account the
modification of the solid-gas interface area during activation, RHF values plotted in Figure 1(b) are expressed in Pa⋅mol−1⋅s⋅cm2.
Real surfaces areas were estimated from mean particle size values obtained from
microscope observations. Assuming that particles are spherical, taking a mean
particle radius of 50 μm for the first hydriding cycle and a
density of 8.2 for LaNi5 [7], a total surface of 212 cm2 was used. Taking
a mean particle radius of 5⁢μm for the tenth hydriding cycle, a total
surface of 2120 cm2 was used. When HF resistances are expressed per
cm2 of solid-gas interface, they are mostly constant in TPDs (Figure
1(b)), although their values spread over a decade of units (this comes from
experimental uncertainties and the estimation of the true interface area). In
SPDs, HF resistances are lower than those in TPDs where RPT values
(the resistance associated with the phase transformation process) are included,
as discussed in [6]. From the data of Figure 1(b), RPT is found to
remain almost constant during activation.From these results, a limitation of PIS analysis should be pointed out (especially
when powdered samples are used, as LaNi5 considered here): the mean
particle radius (at the measurement point) must be known to normalize impedance
diagrams. This is because different triplet values {r;D;dP/dC} lead to similar
impedance diagrams. If not, it is difficult to analyze quantitatively the
hydriding kinetics.


### 3.4. Hydrogen Diffusion Coefficient

Concerning hydrogen diffusion coefficients, the most striking result is that values are
spread over several (three) decades of values. General trends are that values
measured on unactivated samples are larger than those measured on fully
activated samples and that values measured in solid-solution domains are larger
than those measured in two-phase domains. At 298 K,DH values measured
on an unactivated sample are within the range 1×10−9–1×10−8 cm2⋅s−1, whereas those measured on an activated sample are
spread over the 1×10−11–1×10−10 cm2⋅s−1 range. It is concluded that large concentrations of defects (point defects,
dislocations), appearing in activated samples because of the precipitation of
the β-LaNi5H hydride phase, contribute to a reduction of the kinetics of the diffusion step but
a relationship between kinetics and microstructure still needs to be
established to clarify the point. Anyway, concerning the overall kinetics, this
effect is largely counterbalanced by the large increase of the solid-gas
interface area.


### 3.5. Slope of the Isotherm

The slopes of the isotherms obtained from the automated fitting procedure are plotted in Figure1(d) as a function
of hydrogen composition. Although a certain discrepancy is observed, there is a
qualitative agreement with the experimental isotherms of Figure 1(a), the slope
being larger in solid-solution domains than in two-phase domains, except at the
foot of the isotherms where hydrogen trapping effects prevail. From a
quantitative viewpoint, care must be taken when comparing the data of Figure 1(d)
to the isotherms of Figure 1(a). This is because in Figure 1(a), the hydrogen
content is expressed in H/M units whereas in Figure 1(d), the hydrogen content
is expressed in concentration (CH in mol⋅cm−3).


## 3.1. Experimental Results

Two {pressure-composition} isotherms, measured at 298 K on
LaNi5-H2 (g) up to the composition point H/M ≈ 2, are plotted in Figure 1(a) (the maximum
composition at 298 K, not represented here, is H/M ≈ 5.8). The first isotherm was recorded during
the first hydriding cycle and the second one was recorded at the end of the
activation process, during the 10th hydriding cycle. At low-hydrogen content
(0.0 < H/M < 0.5), a solid solution of hydrogen (α-LaNi5)
is formed. As the hydrogen content increases, a nonstoechiometric hydride phase
(LaNi5H6-x or β-LaNi5 phase) begins to be formed.
The system becomes monovariant and a pressure plateau develops. This is the
two-phase domain which extends
over the 0.5 < H/M < 5.8 composition range. At higher hydrogen contents,
a solid solution of hydrogen in β-LaNi5 is formed. In this
research work, experiments plotted in Figure 1 were intentionally stopped at
H/M ≈ 2.0 to avoid lengthy and unnecessary
measurements over the whole composition range.(a) Equilibrium pressure; (b)RHF; (c) DH; (d) dP/dCH as a function of composition H/M measured on LaNi5 at 298 K. (Data points labelled 1, 2, 3 in (a) refer to the impedance diagrams of Figure 2.)
(a)(b)(c)(d)The main characteristic associated
with metal-hydride systems is the presence of a large hysteresis. The
difference between absorption and desorption plateau pressures is significantly
large (ca. 1400 mbar) for the first
isotherm (Figure 1(a)). The amplitude of hysteresis decreases during
activation, and a final value of ca. 400 mbar, observed during the 10th hydriding cycle (Figure 1(a)), is typical of
an activated material. Hysteresis also exists at phase boundaries, the
composition at which the first hydride nucleus is formed during absorption
being different from the composition at which the last hydride nucleus
disappears during desorption (Figure 1(a)).Each data point of Figure1(a)
corresponds to one gas transfer experiment during which the transient pressure
measured in the reaction chamber and the transient hydrogen mass flow entering
the reaction chamber were synchronously sampled, yielding one impedance diagram
per experiment. Typical
impedance diagrams measured during activation in two-phase domains are plotted
in Figure 2. Impedance spectra numbers (1, 2, 3) of Figure 2 are referring to
the gas transfer experiments (1, 2, 3) of Figure 1(a). They are expressed in
Pa⋅mol−1⋅s and not in Pa⋅mol−1⋅s⋅cm2, that is,
they are not corrected for the solid-gas interface area. As analyzed elsewhere
[1], in two-phase domains, a typical semicircle is observed in the high-frequency
(HF) domain along the real axis (see, e.g., diagram 1 in Figure 2). This is the
signature of the parallel connection of a capacitance (associated with the dead
volume of the reaction chamber) and a resistance in Pa⋅mol−1⋅s. In single-phase domains (SPDs), the diameter of this HF semicircle is equal to the value
of the surface resistance Rs (the chemisorption step of the sorption
process) but in two-phase domains (TPDs), this is the sum (Rs+RPT),
RPT being the resistance associated with the phase transformation
process [6]. The HF semicircle is then followed at lower frequencies by a
characteristic medium frequency (MF) region related to hydrogen diffusion and
ends up at low frequencies (LF) in a capacitive shape (a semi-infinite line
along the imaginary axis).Figure 2
Experimental (o) and model (–) impedance diagrams. 1, 2, 3 refer to data points of Figure1(a).
(1) First hydriding cycle, absorption, H/M ≈ 1.5,
(2) First hydriding cycle, desorption, H/M ≈ 1.4,
(3) 10th hydriding cycle, absorption, H/M ≈ 1.5.Each experimental impedance
diagram of Figure1(a) was fitted using model equations applicable to single-
and two-phase domains, as described in [6]. From the fits, values of the HF
resistance (RHF), the hydrogen diffusion coefficient (DH),
and the slope of the isotherm at the measurement point (dP/dCH) were
obtained. All results are plotted as a function of the composition H/M in Figures
1(b), 1(c), and 1(d), respectively. In Figure 1(b),  the true solid-gas
interface area has been taken into account and RHF values are
expressed in Pa⋅mol−1⋅s⋅cm2.


## 3.2. Shape of Impedance Diagrams during Activation

The first impedance diagram of Figure2 (curve 1) was measured on the unactivated
sample, on the pressure plateau in the two-phase domain (data point 1 in Figure 1(a)). A large HF
resistance of ca.4×1011⁢Pa⋅mol−1⋅s is observed. Once the scan is reversed, the first
impedance diagram measured on the desorption pressure plateau (point 2 in Figure 1(a)) is also
plotted in Figure 2 (curve 2). An overall lower HF resistance is measured but
two consecutive semicircles are observed, suggesting a bimodal distribution of
particle size. In other experiments (not reported here), more than two HF
semicircles were observed. It can be concluded that fragmentation of initial
particles leads to particles of heterogeneous sizes. Finally, the impedance
diagram measured in two-phase domains on the activated sample (point 3 in Figure 1(a)) is also
plotted in Figure 2 (curve 3). The impedance is significantly lower for each
frequency, indicating that the sorption kinetics will be higher on the activated
sample.


## 3.3. High-Frequency Resistance

It is observed from the whole set of experimental impedance diagrams (not represented here) that
the high-frequency resistanceRHF (expressed in Pa⋅mol−1⋅s)
decreases exponentially with the number of hydriding cycles, down to a
constant value at the end of the activation process. This does not mean
that the kinetics of the surface step (in SPDs) or the phase transformation
process (in TPDs) are varying during activation: mostly HF resistances are a function of the
operating temperature which is constant here. The reduction of RHF must be correlated with the increasing solid-gas (and interphase) interface
areas resulting from the sample pulverization. To take into account the
modification of the solid-gas interface area during activation, RHF values plotted in Figure 1(b) are expressed in Pa⋅mol−1⋅s⋅cm2.
Real surfaces areas were estimated from mean particle size values obtained from
microscope observations. Assuming that particles are spherical, taking a mean
particle radius of 50 μm for the first hydriding cycle and a
density of 8.2 for LaNi5 [7], a total surface of 212 cm2 was used. Taking
a mean particle radius of 5⁢μm for the tenth hydriding cycle, a total
surface of 2120 cm2 was used. When HF resistances are expressed per
cm2 of solid-gas interface, they are mostly constant in TPDs (Figure
1(b)), although their values spread over a decade of units (this comes from
experimental uncertainties and the estimation of the true interface area). In
SPDs, HF resistances are lower than those in TPDs where RPT values
(the resistance associated with the phase transformation process) are included,
as discussed in [6]. From the data of Figure 1(b), RPT is found to
remain almost constant during activation.From these results, a limitation of PIS analysis should be pointed out (especially
when powdered samples are used, as LaNi5 considered here): the mean
particle radius (at the measurement point) must be known to normalize impedance
diagrams. This is because different triplet values {r;D;dP/dC} lead to similar
impedance diagrams. If not, it is difficult to analyze quantitatively the
hydriding kinetics.


## 3.4. Hydrogen Diffusion Coefficient

Concerning hydrogen diffusion coefficients, the most striking result is that values are
spread over several (three) decades of values. General trends are that values
measured on unactivated samples are larger than those measured on fully
activated samples and that values measured in solid-solution domains are larger
than those measured in two-phase domains. At 298 K,DH values measured
on an unactivated sample are within the range 1×10−9–1×10−8 cm2⋅s−1, whereas those measured on an activated sample are
spread over the 1×10−11–1×10−10 cm2⋅s−1 range. It is concluded that large concentrations of defects (point defects,
dislocations), appearing in activated samples because of the precipitation of
the β-LaNi5H hydride phase, contribute to a reduction of the kinetics of the diffusion step but
a relationship between kinetics and microstructure still needs to be
established to clarify the point. Anyway, concerning the overall kinetics, this
effect is largely counterbalanced by the large increase of the solid-gas
interface area.


## 3.5. Slope of the Isotherm

The slopes of the isotherms obtained from the automated fitting procedure are plotted in Figure1(d) as a function
of hydrogen composition. Although a certain discrepancy is observed, there is a
qualitative agreement with the experimental isotherms of Figure 1(a), the slope
being larger in solid-solution domains than in two-phase domains, except at the
foot of the isotherms where hydrogen trapping effects prevail. From a
quantitative viewpoint, care must be taken when comparing the data of Figure 1(d)
to the isotherms of Figure 1(a). This is because in Figure 1(a), the hydrogen
content is expressed in H/M units whereas in Figure 1(d), the hydrogen content
is expressed in concentration (CH in mol⋅cm−3).


## 4. Conclusions

The hydriding reaction of LaNi5 has been analyzed during the activation
process using pneumatochemical impedance spectroscopy. Experimental impedance
diagrams have been measured during the first and the tenth hydriding cycle.
Using model equations, individual rate parameters (surface resistance and
hydrogen diffusion coefficient) have been obtained as a function of composition
H/M. Impedance diagrams show that the overall absorption kinetics largely
increases during activation. This is mostly due to the increase of the
solid-gas interface areas. When the solid-gas surface area is taken into
account, it is shown that the high-frequency resistance related to surface
hydrogen chemisorption and hydride precipitation remains mostly constant. To a
lesser extent, the hydrogen diffusion coefficient decreases during activation.
This is attributed to the formation of defects. Results obtained along
hysteresis loops will be presented in a subsequent communication.



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*Source: 346545-2008-04-29.xml*