Cathode electrocatalysts catalytic activities

A similar catalytic activity with a monomeric porphyrin of iridium has been observed when adsorbed on a graphite electrode.381-383 It is believed that the active catalyst on the surface is a dimeric species formed by electrochemical oxidation at the beginning of the cathodic scan, since cofacial bisporphyrins of iridium are known to be efficient electrocatalysts for the tetraelectronic reduction of 02. In addition, some polymeric porphyrin coatings on electrode surfaces have been also reported to be active electroactive catalysts for H20 production, especially with adequately thick films or with a polypyrrole matrix.384-387... 

As discussed earlier, it is generally observed that reductant oxidation occurs under kinetic control at least over the potential range of interest to electroless deposition. This indicates that the kinetics, or more specifically, the equivalent partial current densities for this reaction, should be the same for any catalytically active feature. On the other hand, it is well established that the O2 electroreduction reaction may proceed under conditions of diffusion control at a few hundred millivolts potential cathodic of the EIX value for this reaction even for relatively smooth electrocatalysts. This is particularly true for the classic Pd initiation catalyst used for electroless deposition, and is probably also likely for freshly-electrolessly-deposited catalysts such as Ni-P, Co-P and Cu. Thus, when O2 reduction becomes diffusion controlled at a large feature, i.e., one whose dimensions exceed the O2 diffusion layer thickness, the transport of O2 occurs under planar diffusion conditions (except for feature edges). 

In a fuel cell, the electrocatalysts generate electrical power by reducing the oxygen at the cathode and oxidizing the fuel at the anode [1], Pt and Pt alloys are the most commonly used electrocatalysts in PEFCs due to their high catalytic activity and chemical stability [99-103]. 

Beyond human-made Pt- or noble-metal-based ORR electrocatalysts, there exist very active biomimetic carbon-nitrogen-iron ORR electrocatalysts that show great potential for use in PEMFC cathodes, even rivaling the catalytic activity of pure Pt. 

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. 

Zeis R, Mathur A, Fritz G, Lee J, Erlebacher J (2007) Platinum-plated nanoporous gold an efficient, low Pt loading electrocatalyst for PEM fuel cells. J Power Sources 165 65-72 Wu H, Wexler D, Wang G (2009) PtjNi alloy nanoparticles as cathode catalyst for PEM fuel cells with enhanced catalytic activity. J Alloy Compd 488 195-198... 

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... 

We have reviewed the family of dealloyed Pt-based nanoparticle electrocatalysts for the electroreduction of oxygen at PEMFC cathodes, which were synthesized by selective dissolution of less-noble atoms from Pt alloy nanoparticle precursors. The dealloyed PtCua catalyst showed a promising improvement factor of 4-6 times on the Pt-mass ORR activity compared to a state-of-the-art Pt catalyst. The highly active dealloyed Pt catalysts can be implemented inside a realistic MEA of PEMFCs, where an in situ voltammetric dealloying procedure was used to constructed catalytically active nanoparticles. The core-shell structural character of the dealloyed nanoparticles was cmifirmed by advanced STEM and elemental line profile analysis. The lattice-contracted transition-metal-rich core resulted in a compressive lattice strain in the Pt-rich shell, which, in turn, favorably modified the chemisorption energies and resulted in improved ORR kinetics. 

Direct methanol fuel cells (DMFCs) are attracting much more attention for their potential as clean and mobile power sources for the near future [1-8], Generally, platinum (Pt)- or platinum-alloy-hased nanocluster-impregnated carbon supports are the best electrocatalysts for anodic and cathodic fuel cell reactions. These materials are veiy expensive, and thus there is a need to minimize catalyst loading without sacrificing electro-catalytic activity. Because the catalytic reaction is performed by fuel gas or fuel solution, one way to maximize catalyst utilization is to enhance the external Pt surface area per unit mass of Pt. The most efficient way to achieve this goal is to reduce the size of the Pt clusters. 

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... 

The rare noble metal Pt is the most commonly used metal in fuel cell catalysts. A Pt-based catalyst supported on carbon has both high intrinsic activity and great stability [7-10]. It is generally accepted that the catalytic activity of a Pt-based catalyst is highly dependent on the dispersion and the size distribution of the Pt crystallites [11, 12]. Therefore, much attention has been foeused upon the preparation of highly dispersed Pt catalysts. Nanosized platinum particles dispersed on high-surfaee-area carbon substrates have been used as an electrocatalyst for both anode and cathode. 

The sluggish kinetics of the cathode ORR means that more active electrocatalysts are definitely necessary to overcome the large overpotential on the cathode. Although the current carbon-supported Pt catalysts have the most active catalytic activity toward the ORR, they are still not fully satisfactory in terms of activity and stability. In recent years, one focus of catalyst studies has been to further develop more active and durable catalysts. 

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