Investigation of Hydrogen Adsorption-Absorption
into Thin Palladium Films
I. Theory
C. Gabrielli,
a,
* P. P. Grand,
a,b
A. Lasia,
b,
*
,z
and H. Perrot
a,
*
a
CNRS, UPR 15, Physique des Liquides et Electrochimie, Associe ´a ` l’Universite ´ Pierre et Marie Curie,
75252 Paris Cedex 05, France
b
De ´partement de Chimie, Universite ´ de Sherbrooke, Sherbrooke, Que ´bec J1K 2R1, Canada
Models of insertion of hydrogen in metal films coating a gold electrode are presented in terms of faradaic impedance. They take
into account adsorption of the protons from the solution on the electrode surface two-step mechanism or direct absorption in the
metal one-step mechanism and diffusion in the metal. In addition, trapping of the hydrogen atoms and direct exchange of the
trapped hydrogen with the solution are considered. The main feature discussed here is the change of the charge-transfer resistance
with the thickness of the metal film. This surprising behavior is related to the exchange reaction of trapped hydrogen with the
solution. The impedance diagrams are calculated for a palladium film both in the underpotential deposited UPD domain and the
hydrogen evolution domain. In the UPD domain an impedance of the ion-blocking electrode is found. In the hydrogen evolution
domain two capacitive loops are found for a film whose thickness is less than 1 micrometer.
© 2004 The Electrochemical Society. DOI: 10.1149/1.1797033 All rights reserved.
Manuscript submitted October 8, 2003; revised manuscript received April 19, 2004. Available electronically October 27, 2004.
Hydrogen absorbing materials are important in hydrogen storage,
metal-hydride batteries, and hydrogen purification. These materials
are usually complex alloys optimized for their practical use. On the
other hand, palladium is a metal which can absorb a large quantity
of hydrogen. The palladium-hydrogen system has been extensively
studied in the gas phase, however, the electrochemical conditions
are more complex. In the practical applications, diffusion, solubility,
physical stability, and kinetics of hydrogen adsorption/absorption
processes determine the utility of hydrogen absorbing materials. As
it is difficult to study complex and often brittle alloys, palladium can
be used as a simple model to study fundamental properties of hy-
drogen absorbing materials and to better understand mechanisms
and kinetics of hydrogen adsorption and entry into metals. It is
possible to investigate these processes by electrochemical tech-
niques and to determine the thermodynamic and kinetic parameters
of hydrogen adsorption-absorption and evolution reactions in the
Pd/H system.
Despite a large number of studies of the Pd/H system in gaseous
phase and in solution for more than one century,
1
several phenom-
ena remain largely unknown. The literature evokes various models,
sometimes contradictory, for the adsorption-absorption processes.
Bagotskaya
2
and later Frumkin
3
have postulated that hydrogen at-
oms enter directly into a metal without going through an intermedi-
ate adsorbed state. This mechanism is called a one-step process in
the following. Bockris et al.
4
claimed that hydrogen absorption oc-
curs indirectly and involves two steps, adsorption of hydrogen on
the metal surface followed by a chemical absorption into the sub-
surface metal layer. This mechanism is called the two-step process.
Of course, at more negative potentials, a hydrogen evolution step
should be added to the total reaction mechanism.
5
The latter mecha-
nisms have already been analyzed in detail using electrochemical
impedance spectroscopy EIS.
6,7
Palladium metal and other metal
hydrides form two phases, - and -PdH, with a distinct phase
transition.
8,9
It has been shown that hydrogen evolution occurs only
after -phase formation.
10
Hydrogen absorption/diffusion in metals is related to the crystal
structure of the material and the possibility of trapping of hydrogen
atoms. Palladium is characterized by face-centered cubic fcc struc-
ture, which has two types of interstitial sites, octaedric and tet-
raedric, corresponding to minima of the potential energy. It is gen-
erally assumed that hydrogen diffuses in a perfect palladium crystal
by jumping from one octaedric site to another.
11
This diffusion can
be perturbed by structural defects of the metallic structure
12,13
where
hydrogen can be trapped.
14,15
These traps can be reversible or irre-
versible. They can be of various natures due to coalescence of voids,
where H
2
is formed, zones with strong stress defects, dislocations,
grain boundaries, dislocation-interstitial traps, impurities, or
precipitates.
16
The dislocations, grain boundaries, and interstitial at-
oms are supposed to be reversible traps whereas the precipitates
would be irreversible.
17
The dislocations, where hydrogen atoms
accumulate, increase the diffusion rate if they are mobile under the
effect of stresses.
12,18
Defects may also perturb the measurements of
the diffusion coefficient. For samples with traps, the diffusion coef-
ficient measured is only an apparent value.
19
The trapping/releasing
model in diffusion
14
has been used by Yang et al.
20
who considered
interstitial sites in the -phase as reversible trapping sites, and by
Buckley et al.
21
who included the trapping process in the transfer
function describing the response to a flux modulation on the en-
trance side of a palladium membrane.
On the other hand, hydrogen atoms can cause a disturbance of
the diffusion process because the hydrogen absorbed in the intersti-
tial position deforms the palladium lattice and induces stresses.
22
These stresses propagate in the whole volume of the specimen with
the velocity of sound.
23
Zoltowski has modified the equations of
impedance spectroscopy
24
and permeation transfer function
25
to take
into account the effect of stresses induced by hydrogen insertion. He
has shown that this effect increases the apparent diffusion coefficient
as compared with the system without stress. Other authors have also
taken into account stress in the interpretation of the experimental
data.
26-28
Examination of thin palladium layers should allow studying
problems related to hydrogen trapping and stress. Investigations of
the electrodeposition mechanism of palladium monolayers on gold
monocrystals have shown that adsorption and absorption of hydro-
gen on Pd/Au111 occurs in two separate steps for films thinner
than 10 monolayers.
29-32
Using cyclic voltammetry, it has also been
shown that the quantity of hydrogen inserted in thin palladium films
depends on the potential sweep rate and the thickness of the
film.
33,34
Moreover, for the thinnest films the hydrogen absorbed in
a sublayer located just under the surface of the electrode largely
contributes to the total amount of hydrogen in the metal.
The values of the diffusion coefficient depend very much on the
methods of sample preparation, the experimental techniques used,
phase composition, and the presence of stress, which lead to a large
dispersion of the values published in the literature.
35-37
In our recent
* Electrochemical Society Active Member.
z
E-mail: a.lasia@usherbrooke.ea
Journal of The Electrochemical Society, 151 11 A1925-A1936 2004
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A1925