Cathode electrocatalysts, characterization techniques

From the above discussion it becomes apparent that some conflicting experimental evidence exists on hydrocarbon adsorption and on surface intermediates. This arises primarily from the use of electrocatalysts of varying histories and pretreatments. It should be stressed that many adsorption studies were performed on anodically pretreated platinum. The removal of surfaces oxides from such electrodes may have not been always accomplished when the surface was cathodically reduced in some experiments, as outlined in Section IV,D. Obviously, different surface species could exist on bare or on oxygen-covered electrocatalysts. Characterization of surface structure and activity and of adsorbed species using modern spectroscopic techniques would provide useful information for fuel cell and selective electrocatalytic oxidations and reductions. 

Electrocatalysts advocated for methanol oxidation at the anode and oxygen reduction at the cathode in DMFCs are required to possess well-controlled structure, dispersion, and compositional homogeneity [46 9]. The electrocatalytic activities of both anode and cathode catalysts are generally dependent on numerous factors such as particle size and particle size distribution [50-54], morphology of the catalyst, catalyst composition and in particular its surface composition [55,56], oxidation state of Pt and second metal, and microstructure of the electrocatalysts [49,57,58]. With the frequently attempted surface manipulation strategies for nanosized electrocatalysts to increase their catalytic efficiencies toward MOR and ORR, rigorous characterization techniques which can provide information about nanoscale properties are critically required. For example, parameters such as particle size and variation in surface composition have strong influence on catalytic efficiency. Further, if the nanoparticles are comprised of two or more metals, both the composition and the actual distribution will... 

The elemental composition, oxidation state, and coordination environment of species on surfaces can be determined by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. Both techniques have a penetration depth of 5-20 atomic layers. Especially XPS is commonly used in characterization of electrocatalysts. One common example is the identification and quantification of surface functional groups such as nitrogen species found on carbon-based catalysts.26-29 Secondary Ion Mass spectrometry (SIMS) and Ion Scattering Spectroscopy are alternatives which are more surface sensitive. They can provide information about the surface composition as well as the chemical bonding information from molecular clusters and have been used in characterization of cathode electrodes.30,31 They can also be used for depth profiling purposes. The quantification of the information, however, is rather difficult.32... 

Synchotron based techniques, such as surface X-ray scattering (SXS) and X-ray absorption spectroscopy (XAS), have found increased use in characterization of electrocatalysts during electrochemical reactions.37 These techniques, which can be used for characterization of surface structures, require intricate cell designs that can provide realistic electrochemical conditions while acquiring spectra. Several examples of the use of XAS and EXAFS in non-precious metal cathode catalysts can be found in the literature.38 2... 

The urgent need to develop more efficient fuel cell anodes and cathodes has brought the electrochemical, catalytic, and surface science communities closer than ever before and has made electrocatalysis a rapidly growing field both in experimental new findings and in theoretical understanding. It is very likely that the rapid advances in catalyst and electrocatalyst nanoparticle preparation and characterization [10], together with the utilization of new powerful in situ techniques, such as electrochemical nuclear magnetic resonance (NMR) [216] in conjunction with... 

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