Platinum metal alloy membranes

FIGURE 12.5 Progressive poisoning from 10,40, and 100 ppm CO on pure Pt and PIq jRuq j alloy anodes. Increased CO tolerance is shown by the PIq jRuq5 alloy anodes. The MEAs are based on catalyzed substrates bonded to Nafion 115. The single cell is operated at 80°C, 308/308 kPa, 1.3/2 stoichiometry with full internal membrane humidification. (From Ralph, T. R. and Hogarth, M. R, Platinum Metal Rev., 46,117, 2002. With permission.)... 

Electrodes were fabricated with catalyst layers containing platinum-ruthenium alloys and platinum-ruthenium oxide. Membrane electrode assemblies were fabricated with such cells, and the performance was evaluated in a full cell configuration. Although ruthenium oxide is a proton conductor and is expected to enhance the rate of proton transport from the interface during methanol oxidation, no noticeable improvement in activity of the catalyst layer was observed by addition of ruthenium oxide. The role of other metal oxides such as tungsten oxide will be investigated next year, along with evaluation of non-noble metal catalysts based on nickel, titanium, and zirconium. 

Burkhanov, G. S., Gorina, N. B., Kolchugina, N. B., Roshan, N. R., Slovetsky, D. I., Chistov, E. M. (2011). PaUadium-hased alloy membranes for separation of high purity hydrogen from hydrogen-containing gas mixtures. Platinum Metals Review, 55(1), 3—12. 

In the area of membranes, DOE research strategies include studies of hydrophilic additives, non-aqueous proton conductors, and phase segregation control - both in polymers and two-polymer composites. The DOE s catalysts strategies include lowering platinum group metals (PGM) content, developing affordable platinum-based alloys, and developing non-platinum catalysts. 

At present there are no alternative cathode electrocatalysts to platinum. Some platinum alloy electrocatalysts prepared on traditional carbon black supports offer a 25 mV performance gain compared with Pt electrocatalysts. However, only the more stable Pt-based metal alloys, such as PtCr, PtZr, or PtTi, can be used in PEMFC, due to dissolution of the base metal by the perfluorinated sulfonic acid in the electrocatalyst layer and membrane [26]. The focus of the continued search for the elusive electrocatalyst for oxygen reduction in acid environment should be on development of materials with required stability and greater activity than Pt. 

F.A. Lewis, K. Kandasamy, B. Baranowski, The uphill diffusion of hydrogen—strain-gradient-induced effects in palladium alloy membranes. Platinum Metals Review, 32... 

Knapton A. G., Palladium alloys for hydrogen diffusion membranes, Platinum Metals Rev., 21 (1977) 44-50. 

The electrolyte is a perfluorosulfonic acid ionomer, commercially available under the trade name of Nafion . It is in the form of a membrane about 0.17 mm (0.007 in) thick, and the electrodes are bonded directly onto the surface. The elec trodes contain veiy finely divided platinum or platinum alloys supported on carbon powder or fibers. The bipolar plates are made of graphite or metal. 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

The catalysts at the anode can be made less sensitive to CO poisoning by alloying platinum with other metals such as ruthenium, antimony or tin[N.M. Markovic and P.N. Ross, New Flectro catalysts for fuel cells CATTECH 4 (2001) 110]. There is a clear demand for better and cheaper catalysts. Another way to circumvent the CO problem is to use proton-exchange membranes that operate at higher temperatures, where CO desorbs. Such membranes have been developed, but are not at present commercially available. 

Since Pt dissolution is favored by high electrode potential, relative humidity, and temperature, the possibility to limit the risk of electrocatalyst aging is based on the use of Pt-alloy catalyst instead of pure platinum, at least for the cathode, which is characterized by higher potential with respect to anode, and by adoption of operative conditions not too severe in terms of humidity and temperature. While this last point requires interventions on the membrane structure, the study of catalyst materials has evidenced that a minor tendency to sintering can be obtained by the addition of non-noble metals, such as Ni, Cr, or Co, to the Pt cathode catalyst [59, 60], suggesting a possible pathway for future work. On the other hand also the potential application of non-platinum catalysts is under study, in particular transition metal complexes with structures based on porphyrines and related derivatives have been proposed to substitute noble metals [61], but their activity performance is still far from those of Pt-based catalysts. 

Polymer electrolyte fuel cells, also sometimes called SPEFC (solid polymer electrolyte fuel cells) or PEMFC (polymer electrolyte membrane fuel cell) use a proton exchange membrane as the electrolyte. PEEC are low-temperature fuel cells, generally operating between 40 and 90 °C and therefore need noble metal electrocatalysts (platinum or platinum alloys on anode and cathode). Characteristics of PEEC are the high power density and fast dynamics. A prominent application area is therefore the power train of automobiles, where quick start-up is required. 

Metal membranes that are composed principally of palladium alloys are used to purify hydrogen. Although tubular and foil membrane units are commercially available, several types of composite membranes are currently under development to minimize the use of palladium, a precious, platinum-group metal. 

Min et al. [35] experimented on high-catalyst loading with 60% carbon and 40% Teflon backing claimed to be the most efficient electrode for direct methanol/proton exchange membrane fuel cell (PEMFC). The catalysts used were platinum and ruthenium which formed an alloy at an atomic ratio 1 1. The formation of the alloy was seen in XRD as there were no pure metal peaks found. The alloy formation of Pt and Ru promotes oxidation of methanol at lower temperatures. The 60% carbon backing makes it evident that the lower the percentage of carbon increases the efficiency. 

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