Platinum electrocatalyst stability

Phosphoric acid concentrated to 100% is used as an electrolyte in this cell that operates between 150°C and 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor and carbon monoxide poisoning of the platinum electrocatalyst in the anode becomes severe. The relative stability of concentrated phosphoric acid is high compared to other common acids. Consequently, the PAFC is capable of operating at the high end of the acid temperature range (100°C-220°C). In addition, the use of concentrated acid (100%) minimizes the water vapor pressure, so water management in the cell is not difficult. The matrix universally used to retain the acid is silicon carbide and the electrocatalyst in both the anode and the cathode is platinum. 

Li, F. H., Li, F, Song, j. X., Song, J. F, Han, D. X., and Niu, L. (2009]. Green synthesis of highly stable platinum nanoparticles stabilized by amino-terminated ionic liquid and its electrocatalysts for dioxygen reduction and methanol oxidation. Electrochem. Common., 11, pp. 351-354. 

Stabilization of Platinum Electrocatalysts Using Gold Clusters... 

Zhang J, Sasaki K, Sutter E, Adzic RR. 2007b. Stabilization of platinum oxygen reduction electrocatalysts using gold clusters. Science 315 220-222. 

Zhang, J., K. Sasaki, E. Sutter, and R. R. Adzic. Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters. Sci-... 

Sasaki, K. et al., Ultra-low platinum content fuel cell anode electrocatalyst with a longterm performance stability, Electrochim. Acta, 49, 3873, 2004. 

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. 

Besides activity, durability of metal electrode nano-catalysts in acid medium has become one of the most important challenges of low-temperature fuel cell technologies. It has been reported that platinum electrode surface area loss significantly shortens the lifetime of fuel cells. In recent years, platinum-based alloys, used as cathode electrocatalysts, have been found to possess enhanced stability compared to pure Pt. The phenomenon is quite unusual, because alloy metals, such as Fe, Co and Ni, generally exhibit greater chemical and electrochemical activities than pure Pt. Some studies have revealed that the surface stmcture of these alloys differs considerably from that in the bulk A pure Pt-skin is formed in the outmost layer of the alloys due to surface segrega-... 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

A typical example includes the yttria-stabilized-zirconia-based high-temperature potentiometric oxygen sensor which is widely used in automotive applications. Platinum thick films are applied, forming both the cathode and anode of the sensor. The thick electrode has a porous structure which provides a larger electrode surface area compared to non-porous structures. For current measurement, a porous electrode is desirable since it leads to a larger current output. If the metallic film serves as the electrocatalyst, a porous structure is also desirable, for it provides more catalytic active sites. On the other hand, electrodes formed by the thick-film technique do not have an exact, identical... 

Noble metals applied as electrocatalysts for the oxygen reduction have been largely utilized because of their high electrocatalytic activity and stability. Investigations are concentrated on platinum, palladium, silver and gold. The application of noble metal catalysts is limited by two fundamental disadvantages high cost and low availability. Thus, it is important to construct cathodes with small amounts of the noble metal which are obtained, for example, by dispersed platinum on an appropriate support. 

Size- and shape-controlled cubo-octahedral platinum(o) nanoparticles of 4nm average size stabilized by sodium polyacrylate showing 111 and 100 surfaces were used to prepare Vulcan supported electrocatalysts. Cyclic voltammetric CO oxidation studies carried out by the thin film rotating disc method show two different sites of CO oxidation (Fig. 2.19). This can be assigned to differences in the activity of the crystal surfaces and is in agreement with single crystal studies. TEM results after cyclic voltammetric characterization show a complete absence of agglomerations. 

In the above reactions, the oxidation process takes place in the anode electrode where the methanol is oxidized to carbon dioxide, protons, and electrons. In the reduction process, the protons combine with oxygen to form water and the electrons are transferred to produce the power. Figure 9-1 is a reaction scheme describing the probable methanol electrooxidation process (steps i-viii) within a DMFC anode [1]. Only Pt-based electrocatalysts show the necessary reactivity and stability in the acidic environment of the DMFC to be of practical use [2], This is the complete explanation of the anodic reactions at the anode electrode. The electrodes perform well due to the presence of a ruthenium catalyst added to the platinum anode (electrode). Addition of ruthenium catalyst enhances the reactivity of methanol in fuel cell at lower temperatures [3]. The ruthenium catalyst oxidizes carbon monoxide to carbon dioxide, which in return helps methanol reactivity with platinum at lower temperatures [4]. Because of this conversion, carbon dioxide is present in greater quantity around the anode electrode [5]. 

The current state of the art carbon supported electrocatalyst is made using variants of the colloidal approach. A common approach is to dissolve the metal salt solution in an appropriate solvent followed by reduction to form a colloid. A wide variety of recipes using reducing agents, organic stabilizers, or shell-removing approaches have also been developed in recent years. The patents most frequently referred to in this field are from United Technologies by Petrow and Allen.Sols of the metal are obtained for instance by an initial formation of a metastable platinum-sulfito complex, which is inert at ambient temperature but decomposes and produces the small Pt-crystallites at temperatures in excess of 60°C. Thus a relatively well-defined crystal size between 2 to 6 nm can be obtained. 

Zhang S, Shao Y, Yin G, Lin Y. Stabilization of platinum nanoparticle electrocatalysts for oxygen reduction using poly(diallyldimethylammonium chloride). J Mater Chem 2009 19(42) 7995. 

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