Platinum Metals Review - Volume 53, Issue 2, 2009

Crossover and performance of different 1 M low molecular weight organic fuels with a platinum-ruthenium (60:40) catalyst in a unit fuel cell were studied at different temperatures. Large, negatively charged or complicated molecules were found to have the lowest crossover rates through the Nafion^®115 membrane, and methanol had the highest permeability at all temperatures. The smallest molecule, formic acid, dissociates in water, resulting in a less severe crossover problem. In a PtRu-catalysed fuel cell, compounds with only one carbon atom exhibit superior performance compared to molecules having a carbon chain; with methanol and formaldehyde producing power densities up to five times higher than those achieved with molecules having a longer carbon chain. However, it should be noted that PtRu does not catalyse the breaking of the C–C bond; therefore, larger molecules can only be oxidised to derivative products. However, larger organic molecules show a lower rate of crossover through the Nafion^®membrane, which enables more concentrated solutions to be used to decrease the volume of the fuel. With the addition of a third metal to the PtRu-based catalyst, higher molecular weight molecules are good candidates for energy sources in a fuel cell.

Under the Platinum Development Initiative, platinum-based alloys were being developed for high-temperature and special applications requiring good corrosion and oxidation resistance. Work on ternary alloys had previously identified that the best of these alloys for both mechanical properties and oxidation resistance were Pt-Al-Cr and Pt-Al-Ru, as reported in Part I of the present series (1). This paper (Part II) describes transmission electron microscopy work undertaken on a range of ternary alloys to understand the strengthening mechanisms of the alloys, ascertain the nature of the ~ Pt3Al precipitates and deduce the misfits between the precipitates and the matrix. A binary Pt-Al alloy was also studied in an attempt to resolve the two different versions of the phase diagram. A modified D0’ctetragonal version of one of the lower temperature forms of ~ Pt3Al was discovered. The transformation producing the D0’cstructure was confirmed as displacive, since there were distinct bands in the precipitates, which were identified as twins at higher magnifications. The dislocation interactions were also studied, and found to be more complex than nickel-based superalloys, mostly due to the different precipitate types.

Ultrasonic properties of two platinum group metals, osmium and ruthenium, are presented for use in characterisation of their materials. The angle-dependent ultrasonic velocity has been computed for the determination of anisotropic behaviour of these metals. For the evaluation of ultrasonic velocity, attenuation and acoustic coupling constants, the higher-order elastic constants of Os and Ru have been calculated using the Lennard-Jones potential. The nature of the angle-dependent ultrasonic velocity is found to be similar to those of molybdenum-ruthenium-rhodium-palladium alloys, Group III nitrides and lave-phase compounds such as TiCr2, ZrCr2and HfCr2. The results of this investigation are discussed in correlation with other known thermophysical properties.