Electrocatalytic reactivity for oxygen reduction at epitaxially grown Pd thin layers of various thickness on Au(111) and Au(100)
Introduction
The electrochemical reduction of oxygen plays important roles in many processes such as electrochemical energy conversion/storage, metal corrosion and electrocatalysis. It is especially important for the realization of highly efficient fuel cells, because the overpotential for oxygen reduction on noble metals such as Au and Pt was relatively large compared to that for hydrogen oxidation. It is also important in relation to the oxygen reduction in respiratory chain. Thus, the mechanism of the electrochemical reduction of molecular oxygen at wide varieties of electrode materials has been extensively studied [1]. Most of these studies were, however, carried out using polycrystalline electrodes. Because the electrochemical activity should be drastically affected by the surface structure of the electrode, it is essential to investigate the process at atomically well-defined electrode surfaces. Since Clavilier et al. reported that the single crystal surface of noble metals can be prepared by a flame annealing and quenching technique [2], [3] the oxygen reduction reaction has been investigated using well-defined surfaces such as Au [4], [5], [6], [7] and Pt [8], [9], [10] single crystal electrodes.
It is also known that the electrocatalytic activities on an electrode surface can be modified and controlled by the deposition of the small amount of foreign metals. Motoo and Watanabe propose the idea of an ‘adatom’ in this respect [11], [12]. The electrochemical deposition of various noble metals such as Ru, Rh and Pd on a Pt electrode were carried out to obtain a highly active electrode [13], [14], [15]. Recently, Kolb et al. investigated the adsorption and absorption of hydrogen and electrochemical oxidation of formic acid on ultra thin Pd layers on single crystalline gold electrodes and found that the activities of the electrode depended on the thickness of the Pd layers [16], [17]. Well-defined ultra thin Pt and Pd layers were successfully formed on gold single crystal substrates using electrochemical atomic layer epitaxial growth (ECALE) method in our group [18], [19], [20], [21]. We also found that the electrochemical formation and reduction of palladium oxide as well as electrocatalytic activity for the oxidation of formaldehyde on these surfaces were strongly dependent on the surface structures and the thickness of the ultra thin Pd layers [22].
In this study, reactivity for electrochemical reduction of oxygen is investigated at the epitaxially grown Pd thin layers of various thicknesses on Au(111) and Au(100) surfaces and the effect of the Pd surface oxide on the activity for oxygen reduction is discussed.
Section snippets
Experimental
Gold single crystals were prepared by the Clavilier’s method [2], [3]. The gold single crystal was cut to expose a certain face ((111) or (100)), polished and, then annealed for ca. 4 h at 800°C in an electric furnace in air. The electrode was annealed by a hydrogen oxygen flame for a few seconds and then quenched in argon saturated Milli-Q water before each measurement.
Electrolyte solutions were prepared using HClO4 (Suprapure reagent grade, Wako Pure Chemicals), K2PdCl4 (Reagent grade, Wako
Results and discussion
Fig. 1a and a’ show the voltammograms (sweep rate: 50 mV/s) of an Au(111) electrode in 50 mM HClO4 solution saturated with oxygen and argon, respectively. The voltammogram of Au(111) obtained in the argon saturated solution was the same as that previously reported [19], [23], [24], [25] indicating that a well-defined Au(111) surface was exposed. In the oxygen saturated solution, when the potential was swept from +0.9 V to negative direction, the cathodic current started to flow gradually from
Conclusions
Epitaxially grown ultra thin Pd layers on the Au(111) and Au(100) substrates showed a very high electrocatalytic activity for the reduction of oxygen. The thickness and structure dependencies of the ultra thin Pd layers were clearly observed in the voltammograms obtained. At the palladium monolayer, oxygen reduction proceeded in relatively positive potential region than that at the multi-layers of palladium at the both substrates. The oxide reduction of the Pd layers seemed to be important for
Acknowledgements
This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Area of ‘Electrochemistry of Ordered Interfaces’ (No. 09237101) from the Ministry of Education, Science, Sports and Culture, Japan. H.N. acknowledges the Japan Society for the Promotion of Science for the Research Fellowship. S.Y. acknowledges a grant from Iketani Science and Technology Foundation.
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