INTRODUCTION

A constant demand for energy stimulates the search for new power sources. Among various ideas two proposals have recently been regarded in that context, namely the use of hydrogen and fuel cells. However, the utilization of fuel cells as well as

hydrogen (as an easily-available, high-energy, non-toxic energy carrier) is not commonly used due to the fact that numerous physical, chemical, technological and economical problems have to be solved [1]. The storage of hydrogen in the safe form of metal

hydrides seems to be very promising solution [2, 3].

Due to both good electrocatalytic properties and the ability to absorb hydrogen [4], Pd and its alloys are particularly important in the aspect of energy conversion and storage. Examination of Pd-Ru alloys properties should be extremely important due to

the fact that a Pd-Ru alloy with Pd bulk content ca. 99% is able to absorb ca. 15% more hydrogen than pure Pd [5].

Studies on properties of Pd-Ru alloys are important from both the fundamental and practical point of view. These systems can be considered as catalysts, membranes for hydrogen purification and hydrogen-metal model systems [6-9].

ELECTROCHEMICAL, MICROSCOPIC AND SPECTROSCOPIC

CHARACTERIZATION OF ELECTRODEPOSITED Pd-Ru ALLOYS

K. Hubkowska a, M. Łukaszewski 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

Pd-Ru alloys were deposited on a gold wire at constant potential (ranging from 0.3 to 0.44 V vs. RHE) from baths prepared by mixing in various proportions aqueous solutions of RuCl3 (0.32 M) and PdCl2 (0.11 M) with the addition of 1 M HCl. As the

substrates for the deposits a gold wire (99.99%, 0.5 mm diameter), a gold sheet (for XRD measurements), a etched gold-plated quartz crystal (AFM experiments) were used. The thickness of Pd-Ru layers was ca. 0.1–1.1 μm for the deposits on wires, ca. 2–5 μm

for the deposits on sheets and ca. 0.1–0.3 μm for EQCM electrodes. The surface geometric area of samples were ca. 0.1 cm2 for deposits on wires, ca. 1 cm2 for the deposits on sheets.

Alloy bulk composition was analyzed by atomic emission spectroscopy (AES) and by EDX method. Additionally, a method based on the potential of the α→β phase transition of absorbed hydrogen was also applied. Alloy surface composition was determined by

XPS. Surface morphology was examined using SEM and AFM. Structural characterization was done in XRD measurements.

All electrochemical experiments were carried out in 0.5 M H2SO4 solution, deoxygenated with an Ar stream. The solutions were prepared from analytical grade reagents (POCh, Poland) and water triply distilled and purified in a Millipore system. The

298 K temperature was controlled by a thermostat (Lauda RE 306). The Hg| Hg2SO4| 0.5 M H2SO4 was used as a reference electrode and Pt gauze as the auxiliary electrode. All potentials are recalculated with respect to the reversible hydrogen electrode in the

working solution (RHE). Electrochemical measurements were carried out using a CH Instrument potentiostat (model 604) or AUTOLAB potentiostat.

EXPERIMENTAL

RESULTS AND DISCUSSION

Warsaw University

Fig. 4. Deconvolution of XPS spectrum signals from Ru3d; 95.8% Pd-Ru

(AES) in the bulk, 90% Pd-Ru on the surface

Fig. 5. Correlation between alloy surface composition obtained from XPS

measurements and the potential of surface oxide reduction peak on CV

curves for fresh Pd-Ru deposits.

On CV curves the current signals due to hydrogen electrosorption and surface oxidation were present. A single peak

due to surface oxide reduction was observed, placed at potential lower than that for pure Pd (Fig. 6). According to the

literature [10], this difference indicates on surface alloy formation between Pd and Ru surface atoms. A correlation was

found between the potential of surface oxide reduction peak and alloy composition obtained from XPS spectra (Figs. 4, 5).

Fig. 6. CV curves for Pd-Ru alloys of various bulk compositions in the

range 0.5–1.5 V, scan rate 0.1 Vs-1.

Fig. 7. Influence of hydrogen absorption on the course of CVs in the oxide

region; (1) before exposure to hydrogen (2) after single hydrogen sorption

(3) after multiple hydrogen sorption.

Fig. 1. Influence of bath composition (a) and deposition potential (b) on the bulk composition of electrodeposited Pd-Ru alloys; in rectangles: deposition current intensity

(xx · 0.1 mA) / sample thickness (xx μm).

The bulk composition of the Pd-Ru deposits was controlled by changing either bath composition or deposition

potential (Fig. 1). For a constant deposition potential the increase in the concentration of Ru/Pd ratio in the bath resulted in

the decrease in Pd bulk concentration in the alloy. On the other hand, when the bath composition remained constant and

deposition potential was changed, Pd bulk concentration decreased with decreasing deposition potential.

a b

Fig. 2. (a) SEM image of the surface

of a Pd-Ru deposit (90% Pd in the

bulk); (b) AFM image of the surface

of a Pd-Ru sample (90% Pd in the

bulk) deposited on Au etched quartz

crystal.

SEM and AFM images (Fig. 2) showed that tight deposit fully covering the substrate were obtained. Neither

cracking nor flaking of the freshly deposited layers occurred.

a b

On XRD patterns (Fig. 3) of Pd-Ru samples two groups of lines could be distinguished, namely those

originating from Au substrate and those from the Pd-Ru deposit. The signals for the deposit were shifted into

higher scattering angles with respect to that for pure Pd. No lines from metallic Ru were found. These results

indicate the presence of a single alloy phase of fcc structure and a lattice parameter smaller than that of pure Pd.

Thus, the formation of a homogeneous solid solution during Pd and Ru electrodeposition has been confirmed.

Fig. 3. XRD pattern of a 97.4% Pd-Ru (AES) deposit. (a) full spectrum under vacuum and 1 atm. hydrogen, (b) evolution of (111) signal in different atmospheres; vac –

vacuum, H (1) – first exposure to hydrogen, He (1) – first exposure to helium, H (2) – second exposure to hydrogen, He (2) – second exposure to helium.

a b

This work was financially supported by NCN (National Center of Science), grant No. DEC-2011/01/N/ST4/02285. The SEM

images were obtained using the equipment purchased within CePT Project No. POIG.02.02.00-14-024/08-00.

ACKNOWLEDGEMENTS

[1] A. Czerwiński, Accumulators, batteries, cells (in Polish), Wydawnictwo Komunikacji i Łączności, Warszawa (2005).

[2] L. Schlapbach, A. Züttel, Nature 414 (2001) 353.

[3] G.W. Crabtree, M.S. Dresselhaus, M.V. Buchanan, Phys. Today 57 (2004) 39.

[4] F.A. Lewis, The Palladium/Hydrogen System, Academic Press, New York (1967).

[5] K. Hubkowska, U. Koss, M. Łukaszewski, A. Czerwiński, J. Electroanal. Chem. 704 (2013) 10.

[6] J. Bagchi, S.K. Bhattacharya, Transition Metal Chem. 32 (2007) 47.

[7] Z. Bai, L. Yang, J. Zhang , L. Li, J. Lv, C. Hu, J. Zhou, Catal. Commun. 11 (2010) 919.

[8] P. Kędzierzawski, A. Mikołajczuk, A. Borodziński, B. Mierzwa, L. Stobiński, ECS Transactions 28 (2010) 23.

[9] L. Sun, D. Cao, G. Wang, J. Appl. Electrochem. 38 (2008) 1415.

[10] R. Woods, in: A. J. Bard (Ed), Electroanalytical Chemistry, vol. 9. Marcel Dekker, New York, 1976, pp. 2-162.

REFERENCES

It was observed that the electrochemical properties of surface oxides and electrode surface roughness were affected

by the exposure to hydrogen (Fig. 7). After multiple hydrogen absorption on the surface of Pd-Ru alloys oxides were

formed more easily, they were more stable and the surface became less rough than before the exposure to hydrogen.