Hideo Naohara

Core-Protected Platinum Monolayer Shell High-Stability Electrocatalysts for Fuel-Cell Cathodes

More than skin deep: Platinum monolayers can act as shells for palladium nanoparticles to lead to electrocatalysts with high activities and an ultralow platinum content, but high platinum utilization. The stability derives from the core protecting the shell from dissolution. In fuel-cell tests, no loss of platinum was observed in 200?000 potential cycles, whereas loss of palladium was significant.

Highly stable Pt monolayer on PdAu nanoparticle electrocatalysts for the oxygen reduction reaction

Stability is one of the main requirements for commercializing fuel cell electrocatalysts for automotive applications. Platinum is the best-known catalyst for oxygen reduction in cathodes, but it undergoes dissolution during potential changes while driving electric vehicles, thus hampering commercial adoption. Here we report a new class of highly stable, active electrocatalysts comprising platinum monolayers on palladium-gold alloy nanoparticles. In fuel-cell tests, this electrocatalyst with its ultra-low platinum content showed minimal degradation in activity over 100,000 cycles between potentials 0.6 and 1.0 V. Under more severe conditions with a potential range of 0.6-1.4 V, again we registered no marked losses in platinum and gold despite the dissolution of palladium. These data coupled with theoretical analyses demonstrated that adding a small amount of gold to palladium and forming highly uniform nanoparticle cores make the platinum monolayer electrocatalyst significantly tolerant and very promising for the automotive application of fuel cells.

Electrocatalytic reactivity for oxygen reduction at epitaxially grown Pd thin layers of various thickness on Au(111) and Au(100)

Abstract Electrochemical reduction of oxygen was studied at the epitaxially grown ultra thin Pd layers of various thicknesses on Au(111) and Au(100) single crystal surfaces in 50 mM HClO 4 solution saturated by oxygen. The oxygen reduction at the Pd thin layers on the gold substrate started at more positive potential than that at the gold single crystal surfaces even only a submonolayer of Pd was deposited. The potentials where cathodic current of oxygen reduction started to increase were dependent on the potentials where the oxide of the ultra thin Pd layers was reduced.

Thickness dependent electrochemical reactivity of epitaxially electrodeposited palladium thin layers on Au(111) and Au(100) surfaces

Abstract Electrochemical characteristics of ultra thin Pd epitaxial layers deposited electrochemically on Au(111) and Au(100) surfaces were found to be strongly dependent on the surface structure and the thickness of the Pd thin layers. The electrochemical characterizations demonstrate that the Pd/Au(111) and Pd/Au(100) surfaces behave essentially like Pd(111) and Pd(100) surfaces, respectively. Formation and reduction of oxide on the Pd sub-monolayer surfaces took place at more and less positive potentials, respectively, than those on the surfaces of the Pd multilayer. The Pd/Au(100) electrode shows a much higher electrocatalytic activity for the oxidation of formaldehyde than that at the Pd/Au(111) electrode. Furthermore, the highest activity for the electro-oxidation of formaldehyde was observed at the Pd/Au(100) electrode when the thickness of the Pd thin layers was less than a monolayer. This behavior was discussed in terms of the potential- and thickness-dependent oxide formation–reduction on the Pd thin layers.

Epitaxial growth of a palladium layer on an Au(100) electrode

The electrochemical deposition process of palladium from a PdCl 4 2- complex on an Au(100) substrate was investigated using in situ scanning tunneling microscopy (STM). The reactant, i.e. the PdCl 4 2- complex, was found to adsorb on the Au(100) surface with an ordered structure in the potential region where neither a cathodic nor an anodic current flowed. The electrochemical deposition of palladium on the Au(100) proceeded at potentials more negative than 1.0 V. The electrodeposition of the first palladium layer started not only on the terrace but also on gold islands which were formed as a result of the lifting process of the reconstructed Au(100) surface and then proceeded two-dimensionally. The following palladium layer was found to grow mainly from the deposited palladium region on top of the gold islands. The shape of the gold island could be identified even after four palladium layers were deposited. The bulk and surface structures of the deposited palladium layer were characterized by X-ray diffraction (XRD) and the copper underpotential deposition (upd) reaction, respectively, and the formation of the Pd(100) phase with a (1 × 1) surface structure was confirmed.

Effects of adsorbed CO on hydrogen ionization and CO oxidation reactions at Pt single-crystal electrodes in acidic solution

CO was adsorbed on the three low index planes of Pt single-crystal electrodes, and the hydrogen ionization and the dissolved CO oxidation reactions on the CO-covered electrodes were investigated in acid solution. The oxidation wave of CO adsorbed at 50 mV vs. a reversible hydrogen electrode had two peaks; a small prepeak at around 0.5 V and a main oxidation peak at around 0.7 V. Fourier transform IR measurement and the number of electrons per site for adsorbed CO indicated that these oxidation peaks could not be assigned individually to the linear and the bridged CO species. These peaks reflect the difference in the kinetic stability among the adsorbed CO. The stripping of the prepeak CO (CO1(a)) initiated both the hydrogen ionization and the dissolved CO oxidation reactions to a value close to the diffusion-limited values. These reactions required only a very small number of free sites which are provided in the present case by the removal of the kinetically unstable CO1(a). CO1(a) was absent in the adsorbed CO at 0.4 V. The voltammograms for the dissolved CO oxidation on the CO-covered electrodes revealed the presence of other two adsorbed states, COII(a) and COIII(a), where COII(a) was most stable and gave the main peak of the oxidation wave of the adsorbed CO. COIII(a) was very sensitive to the experimental conditions and was responsible for the hump that appeared in the dissolved CO oxidation.

Electrochemical deposition of palladium on an Au(111) electrode: effects of adsorbed hydrogen for a growth mode

Abstract Electrochemical deposition of palladium on an Au(111) electrode was investigated using in situ scanning tunneling microscopy (STM) and electrochemical quartz crystal microbalance (EQCM). STM images clearly showed that palladium deposition was proceeded two-dimensionally even in the relatively large overpotential region up to ∼+0.3 V (vs. RHE). Many nuclei were created, however, in the potential region where hydrogen adsorption took place, i.e. more negative than +0.3 V. EQCM measurement showed that the surface mass was steadily increased during the potential scan as far as the cathodic current flowed even in the potential region where hydrogen adsorption took place. The abrupt surface mass decrease and increase were observed, however, when the potential was stepped from +0.4 V (hydrogen desorbed state) to +0.1 V (hydrogen adsorbed state) and from +0.1 to +0.4 V, respectively, showing the desorption and adsorption of PdC1 4 2− complex from the electrode surface upon hydrogen adsorption and desorption, respectively. These results support the model that the PdCl 4 2− complex plays an important role to inhibit the three-dimensional growth and facilitate the two-dimensional growth.

Analysis of irreversible oxidation wave of adsorbed CO at Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) electrodes

The oxidation wave of CO preadsorbed at 50 mV on Pt(1 1 1), (1 0 0) and (1 1 0) electrodes in phosphate buffer solution of pH 3 was observed as a function of the sweep rate. The sweep rate dependence of the peak current and peak potential, as well as the form of the wave, were examined on the basis of the Gilman mechanism that the electron transfer from a complex consisting of CO and oxygen containing species is the rate-determining step. An electron transfer step from CO itself was excluded. The peak current and peak potential analyses and the wave simulation gave the same value for Δf, the change in the interaction energy during the formation of the activated complex from the reactants. Δf was sweep-rate and surface-structure dependent. The nature of Δf was discussed.

Electrochemical Epitaxial Growth, Structure, and Electrocatalytic Properties of Noble Metal Thin Films on Au(111) and Au(100)

It is well-known that the physical and chemical properties of an ultra thin metal layer on a foreign substrate are different from those of the bulk metal.1,2 The establishment of the preparation method of the ultra thin metal layer with an ordered structure and the understanding of the origin of its unique physical and chemical properties are very important both for fundamental science and industrial applications. The epitaxial growth of a well-defined thin layer of metals has been achieved by vapor deposition, molecular beam epitaxy (MBE), and metalorganic chemical vapor deposition (MOCVD) under condition.3,4 Compared to the metal deposition by these techniques in vacuum, electrochemical metal deposition is economical and easy because expensive vacuum equipments are not necessary for the electrochemical deposition. Unfortunately, however, the quality of the electrodeposited metal layers is usually low. Recent development of electrochemistry of single crystal electrode and in situ surface characterization techniques such as scanning tunneling microscopy (STM) and surface X-ray scattering (SXS) of atomic resolution make the growth of metal layer with an ordered structure under electrochemical control possible.5