High-Surface-Area Electrode Structures

Although the apphcation of in-situ XRD to studies of electrode surfaces and stmctures continues to be investigated by the electrochemistfy group at Southampton, the studies are currently more focused on examining the practical high-surface-area electfode materials that are used as electrocatalysts in fuel cells and as electrode materials in batteries, rather than on the structure of the adsorbed layers described above. 

Carbon-supported metal nanoparticles are often employed as electrocatalysts in low-temperature, proton-exchange membrane, fuel cells. In the Southampton group, in-situ XRD has been used as a probe for both the composition and particle size as a function 

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Methods for the removal of low concentrations of species from effluent which involve either oxidation or adsorption will clearly require a cheap but high-surface-area electrode structure, and several carbon bed and carbon fibre electrode cells have been described. The latter are prepared from 5—15 fxm diameter carbon fibre which has a specific area of 260 m g and hence permits a high throughput of effluent. Such a cell has been used for treatment of paper mill effluent and a 70% reduction of BOD with a 95% removal of highly toxic chlorinated phenols has been claimed. 

A clear advantage of alkaline electrolysers is the use of nickel-based electrodes, thus avoiding the use of precious metals. Catalytic research is aimed at the development of more active anodes and cathodes, primarily the development of high surface area, stable structures. Nickel-cobalt spinel electrodes for oxygen evolution and high surface area nickel and nickel cobalt electrodes for hydrogen evolution have been shown at the laboratory scale to lead to a decrease in electrolyzer cell voltage [47]. More active electrodes can lead to more compact electrolysers with lower overall systems cost. 

This study clearly demonstrates that by using the differencing technique, sufficient sensitivity is achieved to permit the study of electrochemically induced structural changes in thin surface films. The results obtained indicate that it should be possible to study films of about 100-A thickness fairly easily with a small, fixed anode X-ray source, with such a system probably requiring less than 24 hours of data acquisition time. The study of adsorbed monolayers clearly will be more difficult. Such investigations will benefit from the use of high-surface-area electrodes, e.g., platinum black, and/or the use of the Laue geometry. 

In addition to bilayered electrodes with a functional layer and a support layer, electrodes have also been produced with multilayered or graded structures in which the composition, microstructure, or both are varied either continuously or in a series of steps across the electrode thickness to improve the cell performance compared to that of a single- or bilayered electrode. For example, triple-layer electrodes commonly utilize a functional layer with high surface area and small particle size, a second functional layer (e.g., reference [26]) or diffusion layer with high porosity and coarse structure, and a current collector layer with coarse porosity and only the electronically conductive phase (e.g., reference [27]) to improve the contact with the interconnect. 

O2 and H2 dissociation kinetics are better at higher temperatures (>400 °C), low-cost electrode structures of high surface area Ni and oxides such as spinels or perovskites to replace the very effective, but costly, Pt catalysts have been sought. 

The second critical problem is the chemical instability of Li which deposits during the cycling of secondary cells. Electrodeposited Li has such a high surface area that it is not stable in many solutions in which flat Li foil is stable (45c). Howeyer, the pessimistic opinions of (45b and c) haye not inhibited the authors of (45a) from claiming a patent for a nonaqueous battery using chalcogenide electrodes, the specific structure of which is the main feature of the SB (46). 

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