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