David Thompsett

Electrocatalysts for fuel cells

Abstract A brief description of the six main types of fuel cell which are currently under research and development is given. The discussion focuses on recent developments in the polymer electrolyte-based proton exchange membrane fuel cell with description of the limitations imposed by current electrocatalysts and the benefits offered by the development of improved materials.

Carbon Monoxide Electro‐oxidation Properties of Carbon‐Supported PtSn Catalysts Prepared Using Surface Organometallic Chemistry

A series of platinum-tin catalysts supported on carbon have been prepared from organometallic precursors using surface organometallic chemistry (SOMC). The catalysts were characterized using chemisorption, transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry. The addition of tin to Pt/C suppresses chemisorption of both hydrogen and carbon monoxide, with a rapid decrease on addition of just a small amount of tin, leveling off to give a plateau at higher loading. TEM, EDX, and XPS provide evidence that the platinum and tin occur together on the support and that on exposure to air the catalysts consist mainly of metallic platinum in association with tin oxide. A catalyst prepared using SOMC and another of similar loading prepared using conventional precipitation were compared for the electro-oxidation of adsorbed carbon monoxide using cyclic voltammetry. The catalyst prepared using SOMC showed enhanced activity with a larger decrease in the onset potential of carbon monoxide oxidation compared to Pt/C. Comparison of a range of carbon support platinum-tin catalysts with different loading prepared using SOMC showed a decrease in the onset potential for the catalyst at low loading with no further significant decrease in potential on addition of further tin. The peak intensities, however, changed significantly with an increase in loading.

In situ X-ray absorption spectroscopy and X-ray diffraction of fuel cell electrocatalysts

The utility of in situ X-ray absorption spectroscopy (XAS) in determining structural parameters, through analysis of the extended X-ray absorption fine structure (EXAFS), and electronic perturbations, through a white line analysis of the X-ray absorption near edge structure (XANES), is demonstrated for Pt/C, PtRu/C and PtMo/C fuel cell electrodes. The results provide verification that the enhancement of CO tolerance of the alloy catalysts occurs via an intrinsic mechanism for the PtRu alloy, whilst a promotion mechanism is in operation for the PtMo alloy. Preliminary results of an in situ powder X-ray diffraction (XRD) method which utilises synchrotron radiation (SR) and a curved image plate detector are also presented, using Pd/C as an example. The lattice expansion upon formation of the β-hydride is clearly observed.

The Proton Exchange Membrane Fuel Cell Performance of a Carbon Supported PtMo Catalyst Operating on Reformate

The proton exchange membrane fuel cell performance of carbon-supported Pt, PtRu, and PtMo catalysts on reformate gas mixtures was investigated. The catalysts were tested as low Pt loaded electrodes in membrane electrode assemblies at 80°C using H 2 ; 10, 40, 100 ppm CO in H 2 ; 25% CO 2 in H 2 , and 40 ppm CO/25% CO 2 in H 2 reformate gas mixtures as the fuel stream. The PtMo catalyst showed better CO tolerance than the PtRu catalyst at 100 ppm, but showed poorer CO 2 tolerance to both Pt and PtRu. With 40 ppm CO/25% CO2 in H 2 . PtMo showed inferior overall reformate tolerance to PtRu.

Chasing changing nanoparticles with time-resolved pair distribution function methods.

When materials are reduced to the nanoscale, their structure and reactivity can deviate greatly from the bulk or extended surface case. Using the archetypal example of supported Pt nanoparticles (ca. 2 nm diameter, 1 wt % Pt on Al(2)O(3)) catalyzing CO oxidation to CO(2) during cyclic redox operation, we show that high energy X-ray total scattering, used with subsecond time resolution, can yield detailed, valuable insights into the dynamic behavior of nanoscale systems. This approach reveals how these nanoparticles respond to their environment and the nature of active sites being formed and consumed within the catalytic process. Specific insight is gained into the structure of the highly active Pt surface oxide that formed on the nanoparticles during catalysis.

Controlled modification of carbon supported platinum electrocatalysts by Mo

A carbon-supported platinum electrocatalyst was modified with molybdenum using surface organometallic chemistry. The catalyst was characterized using transmission electron microscopy, cyclic voltammetry (CV), polarization studies, and in situ fluorescence extended X-ray absorption fine structure (EXAFS) studies. The CV and polarization studies show that CO oxidation starts at low overpotentials, similar to those of a conventially prepared PtCoMo/C electrocatalyst. EXAFS at the Mo K edge recorded at 0.65 V show that the Mo exists as an oxide species associated with the Pt surface with a Mo-O distance of 1.75 A. At 0.05 V this oxide is reduced with the formation of a metal-metal bond between the Mo and Pt, with a bond distance of 2.63 A. ©2001 The Electrochemical Society. All rights reserved.

Effect of Ru surface composition on the CO tolerance of Ru modified carbon supported Pt catalysts

A series of ruthenium modified carbon supported catalysts have been prepared by surface organometallic chemistry (SOMC) with the following nominal Ru∶Pt surface ratios, (1∶4)RuPt/C, (1∶2)RuPt/C, (3∶4)RuPt/C and (1∶1)RuPt/C. The catalysts were characterised using X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), cyclic voltammetry (CV), and half-cell polarisation studies. The XRD measurements showed that a bulk PtRu alloy was not formed following SOMC modification. However, the EXAFS measurements indicated that a surface alloy is formed upon electrochemical reduction of the Ru modified catalysts. The CV studies show that the electrooxidation of CO on the Ru modified Pt/C catalysts occurs at lower potentials than on the unmodified Pt/C catalysts, but at higher potentials than on an alloyed PtRu/C with a bulk composition of 1∶1 Pt∶Ru. Half cell polarisation measurements in 100 ppm CO in H2 show that the CO tolerance of the SOMC RuPt/C catalysts approached that of the conventional PtRu/C alloy catalyst. The results therefore indicate that a bulk alloy phase is not an essential factor in the improvement in CO tolerance of PtRu/C catalysts over that of Pt/C.

The origin of high activity but low CO2 selectivity on binary PtSn in the direct ethanol fuel cell

The most active binary PtSn catalyst for direct ethanol fuel cell applications has been studied at 20 °C and 60 °C, using variable temperature electrochemical in situ FTIR. In comparison with Pt, binary PtSn inhibits ethanol dissociation to CO(a), but promotes partial oxidation to acetaldehyde and acetic acid. Increasing the temperature from 20 °C to 60 °C facilitates both ethanol dissociation to CO(a) and then further oxidation to CO2, leading to an increased selectivity towards CO2; however, acetaldehyde and acetic acid are still the main products. Potential-dependent phase diagrams for surface oxidants of OH(a) formation on Pt(111), Pt(211) and Sn modified Pt(111) and Pt(211) surfaces have been determined using density functional theory (DFT) calculations. It is shown that Sn promotes the formation of OH(a) with a lower onset potential on the Pt(111) surface, whereas an increase in the onset potential is found upon modification of the (211) surface. In addition, Sn inhibits the Pt(211) step edge with respect to ethanol C-C bond breaking compared with that found on the pure Pt, which reduces the formation of CO(a). Sn was also found to facilitate ethanol dehydrogenation and partial oxidation to acetaldehyde and acetic acid which, combined with the more facile OH(a) formation on the Pt(111) surface, gives us a clear understanding of the experimentally determined results. This combined electrochemical in situ FTIR and DFT study provides, for the first time, an insight into the long-term puzzling features of the high activity but low CO2 production found on binary PtSn ethanol fuel cell catalysts.

Promotion of Direct Methanol Electro‐oxidation by Ru Terraces on Pt by using a Reversed Spillover Mechanism

By examining Pt‐core–Ru‐shell nanocatalysts of different compositions for the electro‐oxidation of methanol, a volcano activity response is revealed according to Ru coverage. This activity profile can be accounted for by a bifunctional mechanism of spilling over the hydroxy species from Ru to Pt in close proximity with supplemental electronic and structural promotions. At high surface coverage of Ru on Pt, it is revealed that a new ‘direct’ pathway of Ru terraces on Pt sites in close vicinity can provide synergetic catalysis. Pt sites activate the methoxy surface species, which migrate to the Ru terrace to react with its surface oxygenates, from water dissociation, for accelerated CO2 formation through a ‘reversed’ spillover mechanism. This non‐CO electro‐oxidation route to CO2 on a Ru surface requires a lower potential to take place than the corresponding process on a Pt surface.

Experimental and theoretical studies of fuel cell catalysts: Density functional theory calculations of H2 dissociation and CO chemisorption on fuel cell metal dimers

Abstract Gradient-corrected density functional theory (GC-DFT) calculations have been performed for molecular hydrogen, carbon monoxide and the metal dimers PtPt, PtNi, and PtRu. The dissociative adsorption of molecular hydrogen on these metal dimers has been modelled. Derived potential energy surfaces for the dissociation of molecular hydrogen on these dimers are presented. Furthermore, the interaction with carbon monoxide has been studied. Here we find that Pt2 binds CO significantly stronger than PtNi and PtRu. We present equilibrium geometries, calculated binding energies, Mulliken charge distributions, and orbital energies. Our results correlate well with our experimental studies of the hydrogen electro-oxidation reaction in a proton-exchange fuel cell where the fuel to the anode ( Pt Pt alloy on carbon electrode) is hydrogen, prepared by reforming of methane, which contains trace amounts of carbon monoxide.