Pt-alloy catalyst

Advancing from carbon-supported Pt to carbon-supported Pt alloy catalysts, to enhance the performance per milligram Pt by three- to four-fold [Mukeijee and Srinivasan, 1993 Mukeijee et al., 1995]. 

Colmenares L, Wang H, Jusys Z, Jiang L, Yan S, Sun GQ, Behm RJ. 2006. Ethanol oxidation on novel, carbon supported Pt alloy catalysts— Model studies under defined diffusion conditions. Electrochim Acta 52 221-233. 

Freund, A., Lang, J., Lehmann T., and Starz, K.A., Improved Pt alloy catalysts for fuel cells, Catal. Today, 27, 279, 1996. 

Pt-alloy catalysts, 40 132-133 Pt microcrystal particle size on soot, 40 ... 

Oxygen reduction mass activities of dealloyed ternary Pt alloy catalysts as cathodes in MEAs at 80°C, 150 kPa Oj. (R. Srivastava et al., Angewandte Chemie International Edition (2007), 46,8988. Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission.)... 

Mathias et al. showed that a 30% Ft alloy on a corrosion-resistant support gave an initial performance 50 mV below that of a standard 50% Pt/C catalyst. Although the Pt alloy catalyst showed much greater stability at 1.2 V, this large discrepancy in activity is unacceptable. Therefore, fhe challenge is to develop Pf (and Pf alloy) cafalysfs wifh high stabilify and similar activities to that shown by state-of-the-art Pt/C catalysts. 

How Pt alloy catalysts achieve CO tolerance has been much debated. Two mechanisms have been proposed ... 

Figure 6.20. Experimental linear sweep voltammogram of carbon-supported high surface area nanoparticle electrocatalyst in oxygen-saturated perchloric acid electrolyte (room temperature). Solid curve pure Pt dashed curve Pt50Co50 alloy electrocatalyst. Inset a blow up of the kinetically controlled ORR regime. Inset b comparison of the specific (Pt surface area normalized) current density of the Pt and the Pt alloy catalyst for ORR at 0.9 V.

While the stability of the monolayer Pt alloy catalyst concept was initially unclear and therefore threatened to make the monolayer catalyst concept a questionable longer term solution, a very recent discovery seems to lend support to the claim that Pt monolayer catalyst could be made into stable catalyst structures Zhang et al. [94] reported the stabilizing effect of Au clusters when deposited on top of Pt catalysts. The presence of Au clusters resulted in a stable ORR and surface area profile of the catalysts over the course of about 30,000 potential cycles. X-ray absorption studies provided evidence that the presence of the Au clusters modified the Pt oxidation potentials in such a way as to shift the Pt surface oxidation towards higher electrode potentials. 

Although carbon-supported Pt and Pt-alloy catalysts show increased ORR electrocatalytic activities, the activities of most... 

So far, various studies focused on developing catalyst materials with improved ORR activity, but only few reported the stability and durability of ORR catalysts. The study of accelerated durability tests (ADT) in conjunction with electron microprobe analysis (BMPA), LEED, and XRD techniques on Pt-based al-loys ° observed hd metal dissolution, diffusion of 3bulk oxides on the surface, and migration and agglomeration of Pt. Yu et al. compared the durability and activity of PtCo/C with Pt/C catalysts. Throngh determination of the electrochemically active sniiace area, mass, and specific activities with respeet to the potential cycles, they found the overall cell performance of PtCo/C is higher than that of Pt/C. They also concluded that the observed dissolution of Co has no severe impact on the cell performanee or membrane conductance. Additionally, Popov et al studied the stabihty of Pt M/C for X = 1,3 and M = V, Fe, Ni, Co. ADT analyses revealed that Pt/C has the lowest activity when eompared to Pt-alloy catalysts, and that the metal dissolntion is lower for a Pt M ratio of 3 1 than compared to a 1 1 ratio. Also, Pt-Ni showed a lower dissolution rate than the other considered Pt-M alloys. 

Iwasa N, Takezawa N (2003). New supported Pd and Pt alloy catalysts for steam reforming and dehydrogenation of methanol. Top Catal, 22, 215... 

Active Metal Surface Area m2/gm Pt Pt catalysts O Pt alloy catalysts... 

The vulnerability of Pt and Pt alloy catalysts to poisoning by trace contaminants at operation temperatures typical for a PEFC is well documented and is of clear concern in the design of a power system based on a PEFC stack. Sources of contaminants include both fuel and air feed streams as well as processes derived from chemical instability of cell component(s). As to the feed streams, polishing of anode feed streams generated by fuel processing upstream the cell should leave very low levels of CO to be dealt with effectively within the cell (see Sect. 8.3.7.1), whereas any traces of sulfur or ammonia have to be perfectly eliminated upstream the anode... 

A cathode potential of 0.7 V is required to reach an ORR current density of 1 A per cross-sectional square centimeter of a PEFC air cathode at 80 °C, corresponding to a cell voltage loss of 0.5 V. This is, by far, the largest loss in a hydrogen/air PEFC. Furthermore, such cathode performance requires Pt or Pt alloy catalyst of significant cost. The demonstrated 2006 PEFC cathode technology of 0.2 g Pt/kWpeak, translates, at the level of Pt... 

Further enhancement of the specific activity of PEFC air cathode catalysts has been achieved by moving on from carbon-supported Pt to carbon-supported Pt alloy catalysts [41, 78-81]. The gain in activity per unit mass of Pt in moving over from Pt to PtCo alloy cathode catalysts is demonstrated in Fig. 43 (explanation for this enhancement in activity has been given above). With the rise by factor... 

The possible complete replacement of Pt or Pt alloy catalysts employed in PEFC cathodes by alternatives, which do not require any precious metal, is an appropriate final topic for this section. Some nonprecious metal ORR electrocatalysts, for example, carbon-supported macrocyclics of the type FeTMPP or CoTMPP [92], or even carbon-supported iron complexes derived from iron acetate and ammonia [93], have been examined as alternative cathode catalysts for PEFCs. However, their specific ORR activity in the best cases is significantly lower than that of Pt catalysts in the acidic PFSA medium [93], Their longterm stability also seems to be significantly inferior to that of Pt electrocatalysts in the PFSA electrolyte environment [92], As explained in Sect. 8.3.5.1, the key barrier to compensation of low specific catalytic activity of inexpensive catalysts by a much higher catalyst loading, is the limited mass and/or charge transport rate through composite catalyst layers thicker than 10 pm. 

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. 

UTCFC has modified the carbothermal synthesis process (U.S. Patent 4,677,092, US 4,806,515, US 5,013,618, US 4,880,711, US 4,373,014, etc.) to prepare 40 wt% ternary Pt alloy catalysts. Various high-concentration Pt catalyst systems were synthesized and the electrochemical surface area (EGA) and electrochemical activity values compared to commercially available catalysts (see Table 3). The UTCFC catalysts showed EGA and activity values comparable to the commercial catalysts. A rotating disk electrode technique for catalyst activity measurements has been developed and is currently being debugged at UTCFC. 

Various binary and ternary Pt-alloy catalysts synthesized by spray pyrolysis and improved performance demonstrated in terms of lower g Pt/kW at 0.8 V - up to 40% reduction of Pt loading demonstrated for ternary Pt-alloy catalysts compared to supported Pt catalysts. 

SD is routinely used to deposit thin films and has proven benefits from economies of scale in the metallization of plastics. The technique has already been used to create enhanced and unique MEAs for H2 -air proton exchange membrane fuel cell (PEMFC) systems. In this project, JPL is pursuing the use of SD to create DMFC membrane electrode assembly structures with highly electro-active catalyst layers that will reduce the amount and cost of the Pt-alloy catalyst at the fuel cell anode. 

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