Cathode electrocatalysts oxygen reduction reaction

The development of cheap and efficient electrocatalysts, especially the oxygen reduction electrocatalyst, is another task for the large-scale commer-ciahzation of PEMFCs. In any PEMFCs, there are two types of electrocatalysts one catalyzes the anodic hydrogen oxidation reaction and the other the cathodic oxygen reduction reaction. Both electrocatalysts rely heavily on the use of precious metals, especially the Pt-based electrocatalysts. The tasks in the development of electrocatalysts include the improvement of electrocatalytic activity, the reduction of the loading of precious metals, or even the replacement of precious metals with cheap metals, and the... 

In this section, we summarize the kinetic behavior of the oxygen reduction reaction (ORR), mainly on platinum electrodes since this metal is the most active electrocatalyst for this reaction in an acidic medium. The discussion will, however, be restricted to the characteristics of this reaction in DMFCs because of the possible presence in the cathode compartment of methanol, which can cross over the proton exchange membrane. 

In practice, in all fuel cells that involve the utilization of 02 from air (Section 13.4.5), the oxygen reduction reaction [Eqs. (13.4) and (13.24)] is always rate determining for terrestrial applications. One can seejust how important it is to attempt to develop electrocatalysts for the cathodes of fuel cells on which the enhanced current density is high and Tafel slope is low and the efficiency of energy conversion, therefore, maximal. The direct relation of the mechanism of oxygen reduction and the associated Tafel parameters to the economics of electricity production and transportation is thus clearly seen. 

Pt and Pt-bimetallic nanoparticle catalysts were employed in commercial prototype phosphoric acid fuel cells even in the mid-1970s [7], so in fact the concept of nanoparticle electrocatalysts is not new. Like many industrial catalysts, however, the catalysts are put into use long before their structure and properties are clearly understood, and that was certainly the case, for example, for the Pt-Co-Cr air cathode catalysts used at United Technologies [8]. In this chapter we review studies, primarily from our laboratory, of Pt and Pt-bimetallic nanoparticle electrocatalysts for the oxygen reduction reaction (ORR) and the electrochemical oxidation of H2 (HOR) and H2/CO mixtures. 

Pt-doped carbon aerogels have been used successfully in the preparation of cathode catalyst layers for oxygen reduction reaction (ORR) in PEMFC systems [83-86]. Thus, different Pt-doped carbon aerogels with a Pt content of around 20 wt% were prepared by impregnation [83]. Results obtained with these Pt catalysts were compared with others supported on carbon blacks Vulcan XC-72 and BP2000, which are commonly used as electrocatalysts. The accessibility of the electrolyte to Pt surface atoms was lower than expected for high-surface-area... 

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]. 

Electrocatalytic Reduction of Oxygen. Oxygen reduction reaction (ORR) occurs on the cathode side of low temperature fuel cells and heavily loaded Pt/C is the most common electrocatalyst. Replacement of ORR catalysts with less expensive materials would have higher technical impact than for anode catalysts. Transition metals loaded carbides and carbide-metal codeposited carbon have been investigated for ORR application. For example, 40 wt% Pt/WC electrocatalyst prepared with RDE electrode showed a cathodic current (-5 x 10 A) similar to that of 40 wt% Pt/C with 0.5 M H2SO4, 100 mv/s and 2000 rpm (160). Also, 40 wt% Pt/WC exhibited electrochemical stability after 100 cycles of cyclic voltammetry from 0 to 1.4 V (vs RHE), whereas the cathodic current of 40 wt% Pt/C disappeared after 100 cycles. 

De Jonghe s group studied the effects of nanoparticle infiltration on LSM cathodes to improve the catalytic activity of LSM at reduced temperatures [75]. Smo.eSro.rCoOs-j [samarium strontium cobaltite (SSC)] was chosen to infiltrate into LSM because it is an effective electrocatalyst for oxygen reduction reactions owing to its MIEC properties. The SSC solution was prepared using the nitrate salts... 

This chapter concentrates on the first principles studies of fiiel cell air-cathode electrocatalysts, i.e., oxygen reduction reaction (ORR) electrocatalysis. Recent progress in theoretical methodologies will be introduced and the current status of first principles studies of the ORR will be reviewed. The chapter is organized as follow first, the background theory and the oxygen reduction reaction will be introduced, and then theoretical applications in the study of the ORR will be... 

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