Direct alcohol fuel cells (DAFCs) are attracting increasing interest as power sources for portable applications due to some unquestionable advantages over analogous devices fed with hydrogen.1 Alcohols, such as methanol, ethanol, ethylene glycol, and glycerol, exhibit high volumetric energy density, and their storage and transport are much easier as compared to hydrogen. On the other hand, the oxidation kinetics of any alcohol are much slower and still H2-fueled polymer electrolyte fuel cells (PEMFCs) exhibit superior electrical performance as compared to DAFCs with comparable electroactive surface areas.2,3 Increasing research efforts are therefore being carried out to design and develop more efficient anode electrocatalysts for DAFCs.

Palladium-Based Electrocatalysts for Alcohol Oxidation in Half Cells and in Direct Alcohol Fuel Cells

The modification of the electronic and chemical properties of Pt(111) surfaces by subsurface 3d transition metals was studied using density-functional theory. In each case investigated, the Pt surface d-band was broadened and lowered in energy by interactions with the subsurface 3d metals, resulting in weaker dissociative adsorption energies of hydrogen and oxygen on these surfaces. The magnitude of the decrease in adsorption energy was largest for the early 3d transition metals and smallest for the late 3d transition metals. In some cases, dissociative adsorption was calculated to be endothermic. The surfaces investigated in this study had no lateral strain in them, demonstrating that strain is not a necessary factor in the modification of bimetallic surface properties. The implications of these findings are discussed in the context of catalyst design, particularly for fuel cell electrocatalysts.

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Modification of the surface electronic and chemical properties of Pt(111) by subsurface 3d transition metals

The high cost of low temperature fuel cells is to a large part dictated by the high loading of Pt required to catalyse the oxygen reduction reaction (ORR). Arguably the most viable route to decrease the Pt loading, and to hence commercialise these devices, is to improve the ORR activity of Pt by alloying it with other metals. In this perspective paper we provide an overview of the fundamentals underlying the reduction of oxygen on platinum and its alloys. We also report the ORR activity of Pt5La for the first time, which shows a 3.5- to 4.5-fold improvement in activity over Pt in the range 0.9 to 0.87 V, respectively. We employ angle resolved X-ray photoelectron spectroscopy and density functional theory calculations to understand the activity of Pt5La.

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Understanding the electrocatalysis of oxygen reduction on platinum and its alloys

Synthesis and catalytic properties of metal nanoparticles: Size, shape, support, composition, and oxidation state effects

The nature and properties of surface-enhanced vibrational spectra of molecules adsorbed on surfaces that can enhance the absorption and the emission of electromagnetic radiation are discussed. Examples of surface-enhanced resonant Raman scattering, surface-enhanced Raman scattering in the near-infrared and surface-enhanced infrared are given. The rough surfaces used are metal island films coated with a nanometric organic film using the Langmuir-Blodgett technique or deposited by vacuum evaporation.

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Surface enhanced vibrational spectroscopy

The electrocatalytic activity for H2 oxidation in the presence of 100 ppm CO has been investigated on a
 series of binary Pt alloy electrocatalysts with non-precious metals of various compositions. Regardless of
 the composition, Pt–Fe, Pt–Ni, Pt–Co and Pt–Mo alloys have been found to exhibit excellent CO tolerance in H2
 oxidation, similar to that of the Pt–Ru alloy. At these CO-tolerant electrodes, the equilibrium coverage of CO
 was suppressed to values less than ca. 0.6. Based on X-ray photoelectron spectroscopy (XPS) data, it was found that
 the surfaces of all non-precious metal alloys are composed of a thin Pt layer with an electronic structure different
 from that of pure Pt, indicating an increased 5d vacancy of Pt in the layers of the CO-tolerant alloys.
 The CO coverage, particularly with multi-bonding, was lowered due to decreased electron donation from the
 Pt band to the 2π* orbital of CO. A weakening of bond strength between the Pt skin layer and CO was also indicated
 by in situ FTIR, suggesting that the H2 oxidation sites are not blocked by CO due to its enhanced mobility.
 Thus, the mechanism of CO tolerance described above
 at the Pt skin on alloy
 surfaces is proposed as a “detoxification mechanism”.

CO Tolerance of Pt alloy electrocatalysts for polymer electrolyte fuel cells and the detoxification mechanism

To clarify the CO-tolerant mechanism at Pt-based alloy anode catalysts, surface-enhanced infrared reflection-absorption spectroscopy with the attenuated total reflection technique (ATR-SEIRAS), coupled with CV measurement, was used to observe the oxidation process of adsorbed CO on a typical Pt-Fe (Pt-Fe = 0.27/0.73) alloy. The alloy electrode exhibits a lower saturated coverage of CO ({theta}{sub co} = 0.55) than that of pure Pt ({theta}{sub co} = 1.0). The dominating linear CO is observed around 2,000 cm{sup {minus}1} when the equilibrium adlayer of CO covers the alloy electrode; however, linear and bridged CO and also COOH were found at the pure Pt electrode at the same CO coverage in the non-steady-state. On the basis of previous results that a Pt skin is formed during the repetitive potential cycling due to the dissolution of Fe on the alloy surface and the skin exhibits less electronic density in the d band, it can be explained that the lowered linear CO coverage and almost no bridged CO are obtained as the result of the lowered back-donation of d electrons from the Pt skin to adsorbates on the alloy surface. The wavenumber shift of the linear CO stretching to a lower value at the alloy, whichmore » is not simple predicted by the lowering of the back-donation of the electron, is ascribed to the weakening of the C -Pt bond. As a presumable effect of the electronic structure change at the Pt skin, the dissociation-oxidation of adsorbed water as well as a formation of adsorbed HOOH species are clearly observed beyond 0.6 V in the electrolyte solution without CO, which is different from that at the pure Pt electrode. Carbonate species can also be detected around 1,300--1,450 cm{sup {minus}1}, which are possibly produced by the surface reaction of CO{sub 2} with water.« less

Oxidation of CO on a Pt-Fe alloy electrode studied by surface enhanced infrared reflection--absorption spectroscopy

Methanol oxidation has been investigated at a sputtered Pt film electrode by using in situ Fourier transform infrared spectroscopy with the attenuated total reflection technique, which can identify directly adsorbed species on the electrode surface. Linear CO, bridged CO, formyl species, and formic acid-related species have been clearly identified during the electro-oxidation of methanol in the intact cyclic voltammogram between 0.05 and 1.0 V, where the initial potential is applied at 0.05 or 1.0 V. The formyl species, linear CO, and bridged CO adsorb on the Pt surface at a low potential, for example, 0.05 V. A potential-induced conversion between the linear CO, the bridged CO, and the formic acid-related species is observed during the methanol oxidation. It was clear that an oxidation via formic acid, at least up to formic acid, is the predominant route for the methanol oxidation in a potential range from 0.2 V to ca. 0.6 V and the oxidation is prohibited by the high coverage of Pt sites with CO. In a h...

Attenuated Total Reflection−Fourier Transform Infrared Study of Methanol Oxidation on Sputtered Pt Film Electrode

Fourier transform infrared reflection absorption spectroscopy with the attenuated total reflection technique (ATR-FTIR), coupled with cyclic voltammetry (CV) measurement, is used to observe the oxidation process of adsorbed CO at Pt film sputtered on a silicon prism. The interesting bipolar shape of the linearly bonded CO band is observed at high coverage of CO, although no CO band is included in the reference spectrum. This asymmetric shape is ascribed to Fano resonance. In addition to a linear CO and a bridged CO, a new absorption band, presumably assigned to a carboxyl radical, was detected. This band may be formed by a heterogeneous reaction between adsorbed CO and H2O on the Pt surface in the hydrogen adsorption potential region. The adsorbed carboxyl radical was oxidized at a less positive potential than the adsorbed CO, which can be ascribed to a presumable origin for the pre-peak that appeared in a CV reading prior to the oxidation of such a linear or bridged CO. This oxidation led to the rearrang...

Oxidation of carbon monoxide at a platinum film electrode studied by Fourier transform infrared spectroscopy with attenuated total reflection technique

Yimin Zhu

Papers

CO Tolerance of Pt alloy electrocatalysts for polymer electrolyte fuel cells and the detoxification mechanism

Oxidation of CO on a Pt-Fe alloy electrode studied by surface enhanced infrared reflection--absorption spectroscopy

Attenuated Total Reflection−Fourier Transform Infrared Study of Methanol Oxidation on Sputtered Pt Film Electrode

Oxidation of carbon monoxide at a platinum film electrode studied by Fourier transform infrared spectroscopy with attenuated total reflection technique

Mechanism of CO Tolerance at Pt-Alloy Anode Catalysts for Polymer Electrolyte Fuel Cells