Upload
others
View
8
Download
0
Embed Size (px)
Citation preview
INTRODUCTION
ELECTROSORPTION OF HYDROGEN
INTO PALLADIUM-PLATINUM-RHODIUM ALLOYS
M. Łukaszewski a, M. Grdeń a, A. Czerwiński a, b
a Warsaw University, Department of Chemistry, Pasteura 1, 02-093 Warsaw, Poland
b Industrial Chemistry Research Institute, Rydygiera 8, 01-793 Warsaw, Poland
We present here the results of cyclic voltammetric and
chronoamperometric studies on hydrogen electrosorption in Pd-Pt-
Rh ternary alloys.
Pd-Pt-Rh alloys (thickness 0.20-0.60 mm) were deposited
on Au wires (diameter 0.5 mm) at a constant potential from a bath
containing PdCl2, H2PtCl6, RhCl3 and HCl. Bulk compositions
(expressed in atomic percentages) of the alloys were determined
using EDAX analyzer (EDR-286) coupled with a LEO 435VP
scanning electron microscope.
All cyclic voltammetric and chronoamperometric
experiments were performed at room temperature in 0.5 M H2SO4
solution deoxygenated with Ar. All potentials are recalculated with
respect to the SHE.
RESULTS AND DISCUSSION
Voltammograms for Pd-Pt-Rh alloys resemble CV curves typical
of other noble metal/alloy electrodes. The potential regions of
hydrogen sorption/desorption, double layer charging and surface
oxidation/oxides reduction can be distinguished.
The amount of hydrogen absorbed in Pd-Pt-Rh alloys is lower
than for pure Pd and decreases with the increase in the bulk content
of both Pt and Rh.
In the case of Pd-Pt-Rh alloys the region of a-b transition is
shifted negatively in comparison with Pd indicating lower stability
of the b-phase.
The hysteresis between absorption and desorption is observed in
chronoamperometric experiments. This effect is smaller for the Pd-
Pt-Rh system than for Pd.
Maximum time needed for hydrogen absorption/desorption is
observed for potentials corresponding to the a-b transition region.
The slow process of phase transition seems to control the rate of
hydrogen absorption/desorption into/from thin Pd-based electrodes.
Due to electrochemical dissolution of Rh and Pd during potential
cycling in the oxygen region the composition of both alloy surface
and bulk is altered, modifying hydrogen electrosorption properties.
A new bulk phase is generated possessing absorption
characteristics different from those typical of a fresh alloy. The
existence of two separate phases is reflected in two pairs of
hydrogen absorption/desorption signals, two transition regions on
the hydrogen absorption capacity vs. potential dependence and two
maxima of time needed for hydrogen absorption/desorption.
1 2
Cyclic voltammetry of fresh alloys
Chronoamperometry of fresh alloys
Influence of potential cycling on hydrogen
electrosorption in Pd-Pt-Rh alloys
Fig. 5. Cyclic voltammograms for a Pd-Pt-Rh alloy containing in the bulk 81% Pd, 6%
Pt and 13% Rh recorded in the hydrogen region (0.01 V s-1) after indicated number of
potential cycles in the oxygen region (0.1 V s-1). Insert: cyclic voltammograms
recorded in the oxygen region (0.1 V s-1) before and after the procedure of potential
cycling. Arrows indicate changes in currents observed during potential cycling.
-0.004
-0.002
0
0.002
0.004
-0.1 0 0.1 0.2 0.3 0.4
Potential / V vs. SHE
Cu
rre
nt
/ A
initial state
after 30 cycles
after 55 cycles
after 110 cycles
after 165 cycles
-0.003
-0.002
-0.001
0
0.001
0.3 0.5 0.7 0.9 1.1 1.3
Potential / V vs. SHE
Cu
rre
nt
/ A
1st cycle
165th cycle
Diminution of oxides reduction
peak and its positive shift
indicate the decrease in real
surface area and surface
enrichment with Pt
at the expense of Pd and Rh
Dissolution of Pd and Rh leads to the decrease in alloy layer thickness
together with composition changes, which reduce the total amount of absorbed hydrogen
The decrease in real surface area causes
lowering of hydrogen adsorption currents
An additional pair of peaks grows
during potential cycling. Since the charge
connected with these signals is higher than
charges expected for any surface processes,
they seem to originate form bulk processes
of hydrogen absorption/desorption into/from
a new alloy phase
Fig. 6. Hydrogen oxidation currents recorded after hydrogen electrosorption at
various potentials in an electrochemically aged Pd-Pt-Rh alloy (subjected to 165
cycles in the oxygen region) containing in the bulk 82% Pd, 11% Pt and 7% Rh; scan
rate 0.01 V s-1. Insert: cyclic voltammogram in the full hydrogen-oxygen potential
range; scan rate 0.01 V s-1.
-0.0005
0
0.0005
0.001
0.0015
-0.1 0 0.1 0.2 0.3 0.4 0.5
Potential / V vs. SHE
Cu
rre
nt
/ A
-0.002
-0.001
0
0.001
-0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.3
Potential / V vs. SHE
Cu
rre
nt
/ A
two pairs of oxidation peaks
due to hydrogen absorption/desorption
into/from two separate alloy phases
SUMMARY
Hydrogen electrosorption in thin Pd-Pt-Rh alloys
has been studied in acidic solution using cyclic
voltammetry and chronoamperometry. The influence of
electrode potential on the amount of electrosorbed
hydrogen has been examined. Three potential regions are
distinguished corresponding to the existence of a- and b-
phases as well as the a-b phase transition. The maximum
amount of absorbed hydrogen depends on the bulk
composition being relatively high for Pd-rich alloys,
although smaller than for Pd. The b-phase in the Pd-Pt-
Rh system is less stable than in Pd, which is reflected in a
lower potential of the a-b transition. The phase transition
seems to control the rate of the processes of hydrogen
absorption/desorption into/from thin Pd-based electrodes.
The hysteresis effect is observed in chronoamperometric
absorption and desorption experiments. Potential cycling
of Pd-Pt-Rh alloys through the oxygen region leads to
significant changes in hydrogen absorption properties
including the appearance of an additional bulk phase.
Fig. 7. Hydrogen charges normalized to maximum values (Q/Qmax) vs. potential for an
electrochemically aged Pd-Pt-Rh alloy obtained in chronoamperometric absorption
and desorption experiments. For comparison data for a fresh alloy and Pd are shown.
Insert: time needed for a steady-state saturation of the electrochemically aged Pd-Pt-
Rh alloy with hydrogen and time needed for the oxidation of hydrogen electrosorbed at
-0.09 V vs. potential of absorption and desorption, respectively.
0
0.2
0.4
0.6
0.8
1
-0.1 -0.05 0 0.05 0.1 0.15 0.2
Potential / V vs. SHE
Q / Q
ma
x
0
2
4
6
8
10
12
-0.15 -0.1 -0.05 0 0.05 0.1 0.15
Potential / V vs. SHE
Tim
e / s
absorption
desorptionPd
fresh alloy:
81% Pd
6% Pt
13% Rh
aged alloy: 82% Pd
11% Pt
7% Rh
Two maxima of time needed for hydrogen absorption
and desorption correspond to two transition regions
on the hydrogen capacity vs. potential dependence
Fig. 3. Chronoamperograms recorded for hydrogen electrosorption in a Pd-Pt-Rh
alloy at various potentials after pretreatment at 0.36 V. Insert: hydrogen oxidation at
various potentials after electrosorption at -0.09 V.
-0.03
-0.025
-0.02
-0.015
-0.01
-0.005
0
0 2 4 6 8 10 12 14 16 18
Time / s
Cu
rre
nt
/ A
-0.14 V
-0.09 V
-0.06 V
-0.03 V
0 V
0.03 V0
0,02
0,04
0,06
0 1 2 3 4 5 6 7 8
Time / s
Cu
rre
nt
/ A
0.36 V
0.11 V
0.06 V
0.03 V
0 V
-0.04 V
Absorption potentials Desorption potentials
hydrogen evolution current
current due to hydrogen electrosorption
After a rapid drop of current
due to a decrease in hydrogen
concentration in the b-phase
a region of a lower slope or
even a plateau is observed
reflecting a low rate of the
phase transition, followed
by fast oxidation of hydrogen
remaining as the a-phase
Fig. 4. Hydrogen absorption capacities, H/(Pd+Pt+Rh) vs. potential for Pd and a Pd-
Pt-Rh alloy obtained in chronoamperometric absorption and desorption experiments.
Insert: time needed for a steady-state saturation of the electrode with hydrogen and
time needed for the oxidation of hydrogen electrosorbed at -0.09 V vs. potential of
absorption and desorption, respectively.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
-0.1 -0.05 0 0.05 0.1 0.15 0.2
Potential / V vs. SHE
H / (
Pd
+P
t+R
h)
Pd, absorption
Pd, desorption
Pd-Pt-Rh, absorption
Pd-Pt-Rh, desorption
0
5
10
15
20
25
30
35
40
45
-0.15 -0.1 -0.05 0 0.05 0.1 0.15
Potential / V vs. SHE
Tim
e / s
Pd
alloy:
81% Pd
6% Pt
13% Rh
hysteresis
The maxima of time needed
for hydrogen absorption
and desorption correspond
to the potential regions
of a-b phase transition
Fig. 2. Hydrogen absorption capacities, H/(Pd+Pt+Rh) vs. potential for Pd and Pd-
Pt-Rh alloys of different bulk compositions. The amount of electrosorbed hydrogen was
calculated from the charge of hydrogen oxidation peaks on voltammograms recorded
after electrode polarization at a given potential; scan rate 0.01 V s-1. Insert: maximum
absorption capacities vs. global bulk content of Pt and Rh.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
-0.15 -0.05 0.05 0.15 0.25
Potential / V vs. SHE
H / (
Pd
+P
t+R
h)
Pd
87% Pd - 5% Pt - 8% Rh
87% Pd - 8% Pt - 5% Rh
81% Pd - 6% Pt - 13% Rh
78% Pd - 3% Pt - 19% Rh
63% Pd - 32% Pt - 5% Rh
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 5 10 15 20 25 30 35 40 45
(Pt + Rh) / % at.
H / (
Pd
+P
t+R
h)
a-phase
b-phase
a-b transition
Pd alloying with non-absorbing elements
(Pt and Rh) results in a decrease in alloy
ability to absorb hydrogen
Fig. 1. Hydrogen oxidation currents recorded after hydrogen electrosorption at
various potentials in a Pd-Pt-Rh alloy containing in the bulk 81% Pd, 6% Pt and 13%
Rh; scan rate 0.01 V s-1. Insert: cyclic voltammogram in the full hydrogen-oxygen
potential range; scan rate 0.01 V s-1.
-0.001
0
0.001
0.002
0.003
0.004
-0.1 0 0.1 0.2 0.3 0.4
Potential / V vs. SHE
Cu
rre
nt
/ A
oxidation of hydrogen
mainly absorbed
as the b-phase
oxidation of adsorbed hydrogen
and hydrogen absorbed as the a-phase
hydrogen evolution current
hydrogen desorption
surface oxidation
reduction of surface oxides
hydrogen absorption/adsorption
-0.003
-0.002
-0.001
0
0.001
0.002
0.003
-0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.3
Potential / V vs. SHE
Cu
rre
nt
/ A
double layer charging