Pt based materials

Very few electrode materials have been shown to be capable of adsorbing methanol in acidic media, and of these only Pt-based materials display a high enough sta-bihty and activity to be attractive as catalysts. The overall reaction mechanism for methanol oxidation is (Eq. 9-34) ... 

This scenario calls for a deep characterization of Pt based materials in terms of how many alloyed phases and segregated phases are present in the mixture as well as their composition. The more accurately the bulk of the Pt based materials is characterized the more precise the characteristics of surface are because the bulk and surface should not be so dissimilar, above the micrometric dimension, in terms of phases and composition. However, the same should not be strictly expected for nanoscale dimensions. Platinum nanoparticle based materials may not form a true alloy but a surface composition much dissimilar from the core given the equalized quantity of both the bulk and surface free energy. To complicate the picture even more, the electrochemical results often depend on the technique used to evaluate the activity of the catalysts. 

The ORR catalysts are used in either acidic or basic electrolyte solutions, and when they are used in either acidic or alkaline fuel cells in the presence of high oxidizing oxygen, electrochemical stabilities of both the catalysts and their support materials are very important for their practiced applications. In the presence of O2, the potential of electrode coated with a catalyst (or the catalyst s potential) will be higher than 1.2 V vs RHE. This potential is higher than the oxidation potentials of almost all metal and carbon materials, or in other words, almost all metal and carbon materials are thermodynamically unstable in the sense of electrochemistry. However, due to the slow kinetics of the oxidation process, or the oxidation product is less soluble, the carbon and Pt-based materials can still be used as ORR catalysts for fuel cells even at acidic or basic environment and high temperatures such as 70-80 °C. 

Platinum and Pt-based materials are currently the best electrocatalysts for these reactions. The price and the hmited reserves of Pt are the prime obstacles to adequately developing this major field. Extensive research efforts have been devoted to the development of highly active and cost-effective electrocatalysts. This chapter describes recent advances in electrocatalysts for anodic reactions in low-temperature fuel cells that use methanol and ethanol as fuels in acidic media. Special attention is focused on the effort to decrease Pt content in the catalysts. Electrocatalysts employed in alkaline fuel cells are not discussed as they have been adequately covered in Chap. 5. 

In a low-temperature fuel cell, hydrogen gas is oxidized into protons, electrons, and other by-products when other fuels are used at the anode. At the cathode of the fuel cell, the oxygen is reduced, leading to formation of water. Both the anodic and cathodic reactions require electrocatalysts to reduce the overpotentials and increase reaction rates. In the state-of-the-art low-temperature fuel cells, Pt-based materials are used as the electrocatalysts for both the reactions however, the high cost and limited resources of this precious metal are hindering the commercialization of fuel cells. Recent efforts have focused on the discovery of electrocatalysts with little or no Pt for oxygen reduction reaction (ORR) [1-3]. 

High cost considering the cost for large-scale applications (e.g., automobile), Pt-based materials and other noble metals are far too costly for wide adoption. 

Normally, the kinetics of ORR and OER occurring at the cathode of fuel cells, including direct methanol fuel cells (DMFCs) is very slow. In order to speed up the ORR kinetics to reach a practical usable level in a fuel cell, ORR catalyst is needed at the air cathode. Platinum (Pt)-based materials are the most practical catalysts used in PEM technology. These Pt-based catalysts are too expensive to make fuel cells commercially viable, and hence extensive research over the past several decades has been focused on development of alternative catalysts. These alternative electrocatalysts include noble metals and allo37S, carbon materials, quinone and its derivatives, transition metal macrocyclic compounds, transition metal chalcogenides, transition metal carbides and transition metal oxides. In this chapter, we focus on both noble and nonnoble electrocatalysts being used in air cathodes and the kinetics and mechanisms O2 reduction/oxidation reaction (both ORR and OER), catal37zed by them. 

In the quest to prepare efficient, durable, inexpensive catalysts as alternatives to Pt and Pt-based materials, non-Pt catalysts may be a feasible way to permanently resolve the cost issue in PEM fuel cell commercialization. Although there is still a long way to go in making them comparable with Pt-based catalysts, significant progress in non-Pt catalysts has been reported in recent years. 

Pt is the most efficient catalyst for ORR and has been extensively studied as single crystal surfaces, polycrystalline, alloys, and NP materials. To reduce the cost and enhance the efficiency, Pt has been obtained in highly dispersed form on different high surface area conductive supports. Among various supports, carbon is the most investigated one. Details of work carried out on the ORR on Pt-based materials prior to 2008 or so have been comprehensively reviewed by Antolini et al. [119]. 

CO Adsorption Microcalorimetry on Pt-Based Materials Literature Survey... 

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