Pt electrocatalysts

Since catalyst performance for the HOR is strongly dependent on the total active surface area, supported catalysts have been developed to maximize the catalyst surface area. This issue is discussed in more detail in Section 9.8. 

In a PEMFC, when operating with pure hydrogen at practical current densities, the anode potential is typically less than -l- O.IV (vs. RHE). 

In addition, CO was found to be a poisoning adsorbate during the oxidation of methanol and other small organic molecules. In the 

For many years it has been well known that CO electrooxidation on platinum is a structure-sensitive reaction. Studies with singlecrystal electrodes have shown that the kinetic parameters depend not only on the surface composition of the catalyst but also on the symmetry of the surface and that the presence of steps and defects alters significandy the reaction rate. As a consequence, the surface structure of the nanoparticles should also affect the performance for the oxidation of CO. Understanding how the different variables affect CO oxidation on Pt nanoparticles dispersed on carbon requires the control of the platinum surface in a similar way as has been achieved for single-crystal electrodes. In this sense, the influence of the surface site distribution on CO oxidation using nanoparticles of well-defined 

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Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

The concept of a promoter can also be extended to the case of substances which enhance the performance of an electrocatalyst by accelerating the rate of an electrocatalytic reaction. This can be quite important for the performance, e.g., of low temperature (polymer electrolyte membrane, PEM) fuel cells where poisoning of the anodic Pt electrocatalyst (reaction 1.7) by trace amounts of strongly adsorbed CO poses a serious problem. Such a promoter which when added to the Pt electrocatalyst would accelerate the desired reaction (1.5 or 1.7) could be termed an electrocatalytic promoter, or electropromoter, but this concept will not be dealt with in the present book, where the term promoter will always be used for substances which enhance the performance of a catalyst. 

Specific Activity (SA) and Mass Activity (MA) of Pt Electrocatalysts Supported on Different Carbon Powders Characterized by Specific Surface Area (S) and Particle Size (d)... 

Mukeijee S, McBreen J. 1989. Effect of particle size on the electrocatalysis by carbon-supported Pt electrocatalysts an in situ XAS investigation. J Electroanal Chem 448 163-171. 

Wang H, Wingender Ch, Baltmschat H, Lopez M, Reetz MT. 2001b. Methanol oxidation on Pt, PtRu, and colloidal Pt electrocatalysts A DEMS study of product formation. J Electroanal Chem 509 163-169. 

Friedrich KA, Henglein F, Slimming U, Unkauf W. 2001. In-situ vibrational spectroscopy on Pt electrocatalysts. Electrochim Acta 47 689-694. 

Markovic N, GasteigerH, Ross PN. 1997a. Kinetics of oxygen reduction on ViQikl) electrodes Implications for the crystalhte size effect with supported Pt electrocatalysts. J Electrochem Soc 144 1591-1597. 

Fig. 14.12 Effect of pore morphology of the carbon support (CMK-3 and WMC) on the activity of Pt electrocatalyst (Reprinted from [148] with permission from Elsevier).

Lee, K., Zhang, J., Wang, H., and Wilkinson, D. P. Progress in the synthesis of carbon nanotube- and nanofiber-supported Pt electrocatalysts for PEM fuel cell catalysis. Journal of Applied Electrochemistry 2006 36 507-522. 

This review will focus on the applications of XAS in the characterization of low temperature fuel cell catalysts, in particular carbon supported Pt electrocatalysts, Pt containing alloys for use as anode and... 

A particular Pt-skin single-crystal surface, the Pt3Ni (111) face, was reported to exhibit an extraordinary ORR activity after annealing and formation of the Pt skin structure [87]. This facet exceeded the activity of the Pt(lll) single-crystal surface by a factor of 10 x, while it was found to be about 90-fold more active than a state-of-the-art high surface area carbon-supported Pt electrocatalyst. The enhancement... 

The construction materials of each sensor part will influence its operating characteristics, as illustrated in the following examples. Choosing a Au rather than a Pt electrocatalyst for the sensing electrode allows for selective determination of in the... 

Phosphoric-acid fuel cell (PAFC) — In PAFCs the -> electrolyte consists of concentrated phosphoric acid (85-100%) retained in a silicon carbide matrix while the -> porous electrodes contain a mixture of Pt electrocatalyst (or its alloys) (-> electrocatalysis) supported on -> carbon black and a polymeric binder forming an integral structure. A porous carbon paper substrate serves as a structural support for the electrocatalyst layer and as the current collector. The operating temperature is maintained between 150 to 220 °C. At lower temperatures, phosphoric acid tends to be a poor ionic conductor and poisoning of the electrocatalyst at the anode by CO becomes severe. 

The CO and CyHn catalytically react with the 02 at the surface of the Pt electrocatalyst. When the 02 content is below stoichiometric the electrode surface is depleted of 02 causing an increase in the (P02)ref/(P02)exhaust generating... 

As with the technologies considered earlier, the main deterrent is cost. Today s fuel cell demonstration cars and buses are custom-made prototypes that cost about 1 million apiece.41 Economies of scale in mass manufacture would bring this cost to a more reasonable 6,000-10,000 range. This translates to about 125 per kilowatt of engine power, which is about four times as high as the 30 per kilowatt cost of a comparable gasoline-powered internal combustion engine.41 A major cost component in the PEM fuel cell is the noble metal (usually Pt) electrocatalyst. Efforts are underway in many laboratories to find less expensive substitutes (see for example, Refs. 42-44). 

As indicated above, the Ap technique has been applied to several other phenomena involving Pt-based electrocatalysts. The first report of Ap applied to operating Pt electrocatalysts was based on Hads at anodic potentials. The nature of Ha on Pt, and its contribution to the effective double layer, had long been a matter of debate. " Ap analysis of Pt Lmn XANES showed the H to be highly delocalized, and hopping between one-fold and three-fold (fee) sites on the Pt surface. While prior research had pointed to such activity, the realistic extent in respect to potential was murky due to the nature of the analytical techniques (e.g., IR spectroscopy, UHV studies, etc.) employed. The study by Teliska et al., ... 

Figure 22 Electrode potential dependence of shifts for CO (circles) and CN (squares) chemisorbed on a 10-nm Pt electrocatalyst in an electrochemical environment. (From Refs. 3 and 4.)... 

Development of supported Pt electrocatalysts came as a result of intensive research on fundamental and applied aspects of electrocatalysis [especially for kinetically difficult oxygen reduction reaction (ORR)] fueled by attempts at commercialization of medium-temperature phosphoric acid fuel cells (PAFCs) in the late 1960s and early 1970s. Dispersion of metal crystallites in a conductive carbon support resulted in significant improvements in all three polarization zones (activation, ohmic, and... 

One factor that may be important, but not systematically investigated, is the influence of the Pt electrocatalyst-support interactions on the electrocatalytic activity for O2 reduction. In Figure 14, an attempt to incorporate the pHzpc as a qualitative measure of the importance of carbon surface chemistry and metal-support interaction on the electrocatalytic activity of Pt is reported. The trend of the data in Figure 14 suggests that the specific activity for oxygen reduction increases as the pHzpc of the surface becomes more basic this effect may be related to the parallel increase of the particle size with the pHzpc of the catalyst. At this stage, one... 

In conclusion, even if the CH3OH oxidation occurs for supported electrocatalysts in a similar way as on smooth Pt electrodes, the surface characteristics of the catalysts, besides Pt dispersion, may play a significant role in the reduction of the overpotential for this process. The acid-base functional groups influence the oxidation mechanism as they establish the level of metal-support interaction and the surface adsorption behavior. Thus, the optimization of such parameters can significantly improve the activity of Pt electrocatalysts for CH3OH oxidation. 

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. 

Figure 22 shows the oxidation of methanol on a submonolayer of Pt on Ru, Ptd gRuio/C (3.9 pg cm" Pt), and commercial PtRu/C (10 pg cm" Pt) electrocatalysts. The Pt-mass specific activity (current) of the monolayer-level electrocatalyst is several times higher than that of a commercial sample. 

Figure 22. Oxidation of methanol on a submonolayer of Pt on Ru, i.e., Pt3.9Ruio/C (3.9 jag cm 2 Pt) and comercial PtRu/C (10 pg cm Pt) electrocatalysts in 0.5 M CH3OH + 0.1 M H2SO4 sweep rate of 50 mV s room temperature. The currents presented are normalized by Pt mass.

Demonstrate the performance of combined features derived from the best performing low-Pt electrocatalyst and the optimal MEA structure in a short stack fuel cell. 

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