Abstract:A special in situ PEM fuel cell has been developed to allow X-ray absorption measurements during real fuel cell operation. and electronic (atomic XAFS) structure of the anode catalyst, are monitored as a function of the current. In hydrogen, the NPt-Ru coordination number increases much slower than the NPt-Pt with increasing current, indicating a more reluctant reduction of the surface Pt atoms near the hydrous Ru oxide islands. In methanol, both O[H] and CO adsorption are separately visible with the ∆µ technique and reveal a drop in CO and an increase in OH coverage in the range of 65-90 mA/cm 2 . With increasing OH coverage, the Pt-O coordination number and the AXAFS intensity increase. The data allow the direct observation of the preignition and ignition regions for OH formation and CO oxidation, during the methanol fuel cell operation. It can be concluded that both a bifunctional mechanism and an electronic ligand effect are active in CO oxidation from a PtRu surface in a PEM fuel cell.

Commercially available carbon‐supported pure Pt and pure Ru catalysts were heat treated in order to modify their individual particle sizes, mixed in a mortar and tested for their suitability as anode catalysts in polymer electrolyte fuel cells (PEMFCs). The procedure chosen for mixing monometallic catalysts should bring more flexibility to the preparation of catalysts, which could thus be easily adjusted to the working conditions. The catalyst mixtures were structurally characterized using a combination of X‐ray diffraction (XRD) and transmission electron microscopy (TEM), and indicated an increase in particle size with heat treatment temperature, while the size distribution remained sufficiently narrow. The particle size effect on the resulting activity of the catalyst mixtures was investigated by current‐voltage measurements (U/i curves) in single cell fuel cells. A mixture of the as‐received platinum and ruthenium gave the best results, which was almost comparable to a commercial Pt‐Ru alloy catalyst purchased from E‐TEK inc. Only a minor decrease in cell performance was seen during long term operation. Following operation, the X‐ray diffraction patterns show reflections of the Pt fcc phase, but not the hcp Ru phase. This may be explained by leaching of the pure ruthenium catalysts and re‐decoration on Pt nanoparticles, or by formation of an amorphous Ru oxide or, less likely, Pt‐Ru alloy during operation. However, these structural changes do not seem to significantly affect the cell performance.

Low-temperature polymer electrolyte fuel cells (PEMFCs) are among the key technologies for future energy policies, as they offer an almost emission-free alternative to conventional energy sources and converters. [1] The main advantage of the fuel cell is the generation of electricity from hydrogen in an environmentally-friendly way without combustion, high temperatures, rotating parts and electrical generators. At present, the main drawback, however, is the high cost of the fuel cell technology, [2] and further research is required before fuel cells can compete with conventional energy sources in terms of cost efficiency and long-term performance.As long as pure hydrogen is used in the PEMFC, platinum is the most efficient catalyst at both the anode and the cathode side. For cost efficiency, however, it is intended to replace expensive pure hydrogen by methanol (direct methanol fuel cell ± DMFC) or reformate. In this case, the cell performance decreases considerably, since CO impurities from the fuel adsorb strongly on Pt surfaces poisoning the electrocatalytically active sites. [3] Consequently, one emphasis in fuel cell research is the development of less CO-sensitive catalysts. Over the past years different binary and ternary Pt systems, e.g. PtRu, [4±8] Pt-Mo, [9] Pt-Sn [10] and Pt-Ru-W, [11,12] were investigated and showed improved activity for the oxidation of H 2 /CO mixtures. However, in most cases the long-term stability of these catalysts has not been studied in detail. Recent investigations reported a significant drop in activity, apparently because many alloy catalysts are not stable under the acidic fuel cell working conditions. Often a fraction of the less-noble element, like non-alloyed Ru in Pt-Ru or especially Sn in Pt-Sn alloys, shows a tendency towards leaching into the electrolyte. In this background in-situ investigations seem indispensable to reveal and pursue structural changes of the catalyst during fuel cell operation.There are many different characterization methods that can be applied for in-situ investigations, e.g. in-situ FTIR, [13] V-ray diffraction, [14] and V-ray absorption spectroscopy. [15±17] In particular X-ray absorption spectroscopy (XAS) is extremely suitable for in-situ experiments, as it is highly versatile, and measurements can be carried out in different environments, different atmospheres and at various temperatures. [15] The feasibility of in-situ V-ray absorption measurements in a working fuel cell has been shown by Viswanathan et al., [18,19] and by our group [20] both in 2002. The experimental set-up and in-situ cell of the Smotkin group, however, were somewhat different from those developed by our group, which have been tested successfully at the beamlines BM 29 at ESRF, Grenoble, and V 1 at Hasylab, Hamburg, as described in. [20] In the work presented here, the development and optimization of an in-situ XAS fuel cell in transmission and fluorescence geometry is reported. Different carbon-supported and unsupported Pt-Ru electrocatalysts were investigated, ...