Electrocatalysts platinum metals

The second form consists of Pt metal but the iridium is present as iridium dioxide. Iridium metal may or may not be present, depending on the baking temperature (14). Titanium dioxide is present in amounts of only a few weight percent. The analysis of these coatings suggests that the platinum metal acts as a binder for the iridium oxide, which in turn acts as the electrocatalyst for chlorine discharge (14). In the case of thermally deposited platinum—iridium metal coatings, these may actually form an intermetallic. Both the electrocatalytic properties and wear rates are expected to differ for these two forms of platinum—iridium-coated anodes. 

Transition-metal -phthalocyanines as catalysts in acid medium. To prevent carbonate formation by the carbon dioxide in the air or that produced by oxidation of carbonaceous fuels, an acid electrolyte is necessary hence it is important to find electrocatalysts for an acid medium. Independently of Jasinski, we were soon able to show 3>4> that under certain conditions the reduction of oxygen in dilute sulfuric acid proceeded better with phthalocyanines on suitable substrates than with platinum metal. The purified phthalocyanines were dissolved in concentrated sulfuric acid and precipitated on to the carbon substrate by addition of water. This coated powder was made into porous electrodes bound with polyethylene and having a geometrical surface of 5 cm2 (cf. Section 2.2.2.1.). The results obtained with compact electrodes of this type are shown in Fig. 6. 

The thermally prepared oxides of the so-called rarer platinum metals are among the best electrocatalysts known for the oxygen gas evolution reaction from aqueous systems. Of these oxides, Ru02 exhibits the highest catalytic activity (at least in relatively short term tests) and has been investigated in most detail. Much of the published work on Ru02 has been stimulated by the success of Ru02-based anodes in chlor-alkali cells. 

Zhang, J. et al., Platinum and mixed platinum-metal monolayer fuel cell electrocatalysts design, activity and long-term performance stability, ECS Trans., 3, 31, 2006. 

Because of the irreversible and not well-understood change of the electrocatalyst surface above 1.0 V, early mechanistic studies were conducted under ill-defined conditions. Thus, while anodic evolution of Oj takes place always in the presence of oxygen-covered electrodes, the cathodic reaction proceeds on either oxygen-covered or oxygen free surfaces with different mechanisms (77,158). The electrochemical oxide path, proposed for oxide-covered platinum metals in alcaline electrolytes (759,160), has been criticized by Breiter (7), in view of the inhibition of oxygen reduction by the oxygen layers. Present evidence points to the peroxide-radical mechanism (77,... 

Apart from poisoning by adsorbing impurities, the working electrode potential can also contribute to suppress electrocatalytic activity. Platinum metals, for instance, passivate or form surface oxygen and oxide layers above 1 V (Section IV,D), which inhibit Oj reduction (779,257,252) and oxidation of carbonaceous reactants (7, 78, 253, 254) however, decomposition of hydrogen peroxide on platinum is accelerated by oxygen layers (255). Some electrocatalysts may corrode or dissolve, especially in acidic electrolytes, while reactants may contribute to dissolution. Thus, ethylene oxidation on palladium to acetaldehyde proceeds via a Pd-ethylene complex, which releases colloidal palladium in solution (28, 29). Equivalent to this is the surface roughening and the loss of Pt in gas phase ammonia oxidation (256, 257). 

J. P. Sauvage, J. P. Electrochemical Reduction of Carbon-Dioxide Mediated by Molecular Catalysts Coord. Chem. Rev. 1989, 93, 245. (c) Sullivan, B. P. Reduction of carbon dioxide with platinum metals electrocatalysts. A potentially important route for the future production of fuels... 

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. 

The Cr(2p), Co(2p), and Ni(2p) X-ray photoelectron spectra for the samples were also studied, and the oxidation states of Cr, Co, and Ni as well as their relative intensities were obtained. From these data it was found that the Pt-Co/C sample had the lowest overall oxidizing components among the binary- and ternary-alloy electrocatalysts. Surface atomic ratios for Cr Pt, Co Pt, and Ni Pt of the carbon supported electrocatalysts, obtained from their respective X-ray photoelectron spectra, are summarized in Table 10.5. The results indicate some surface enrichment of platinum metal in all the binary-alloy electrocatalysts, namely Pt-Cr/C, Pt-Co/C, and Pt-Ni/ C. However, a surface enrichment of base metals was found in the ternary-alloy electrocatalysts, as can be seen from Table 10.5. The results suggest a higher electrocatalytic activity towards the oxygen... 

Table 16.2. Physical characterization of PtRu alloy electrocatalysts [43]. (Reprinted from Ralph TR, Hogarth MP. Catalysis for low temperature fuel cells, part II the anode challenges. Plat Met Rev 2002 46(3) 117-35, 2002. With permission from Platinum Metals Review.)...

The anodic oxidation of fuels in low temperature cells, mainly on platinum metals, platinum metal alloys and alloys of platinum metals with other metals, is the subject of this chapter. Most oxidation studies were made on these metals because the efficiency of other electrocatalysts is too low. The mechanism for the oxidation of carbon monoxide, nlixtures of hydrogen and carbon monoxide, formic acid, methanol, higher alcohols, hydrocarbons, and hydrazine is discussed in separate sections. 

The reduction of molecular oxygen that is supplied either directly from containers or in a diluted form as air constitutes the reaction at the cathode in fuel cells. The use of air is preferable for economic reasons. Platinum metals and alloys of platinum metals are electrocatalysts for acid and alkaline electrolytes. Silver, silver alloys, nickel, carbon, and intermetallic compounds represent less expensive electrocatalysts for the oxygen electrode in alkaline solutions. In contrast to the hydrogen electrode, the overvoltage of the oxygen electrode is large at temperatures below 100 °C when a reasonable current is drawn. 

Carbon can be used [7,11,62,70—73] as an electrocatalyst for the O2 reduction in alkaline electrolytes (compare also section 5 in chapter VIII). The performance which is not so good as that of silver (see Fig. 79) appears adequate for certain purposes, for instance, in small zinc-air cells. Activation procedures [72,73] which are not of an electrochemical nature improve the performance of carbon oxygen electrodes. The performance rapidly becomes poor with decreasing pH below pH <14. In acid solution, the impregnation of carbon with platinum metals or other electrocatalysts is required. The data [73] in Table 8... 

The corrosion of electrodes in fuel cells leads to a decrease in performance with time. It may determine the life of the fuel cell. The rate of corrosion depends upon the electrode material, the electrolyte and the temperature. The presence of anions which form complexes with the respective electrocatalyst (for instance C for platinum metals) has to be avoided since complex formation tends to increase the rate of corrosion. In general, corrosion problems become more serious with increasing temperature. While the corrosion of electrocatalysts is discussed in this chapter, the degradation of electrode structures due to mechanical strain or to the disintegration of the bonding material is not dealt with. 

The preceding results demonstrate the corrosion of palladium and platinum at potentials of the oxygen layer and in the region of oxygen evolution. A similar situation may be expected for the other platinum metals. Since the potential of the anode of a fuel cell remains below those of the oxygen region, the corrosion of the electrocatalyst will be absent. However, corrosion of the platinum metals may be expected for the cathode. The precise assessment of this corrosion is difficult since it may be accompanied by other degradation effects. 

One factor contributing to the inefficiency of a fuel ceU is poor performance of the positive electrode. This accounts for overpotentials of 300—400 mV in low temperature fuel ceUs. An electrocatalyst that is capable of oxygen reduction at lower overpotentials would benefit the overall efficiency of the fuel ceU. Despite extensive efforts expended on electrocatalysis studies of oxygen reduction in fuel ceU electrolytes, platinum-based metals are stiU the best electrocatalysts for low temperature fuel ceUs. 

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]. 

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