Practical considerations electrode material

Ceria, particularly when doped with Gd203 or SmzOs," has received some attention for direct hydrocarbon conversion in SOFC. Dating back to Steele and co-workers,interesting properties have been demonstrated for ceria-based anodes in direct utilization of methane. Later work suggested that the performance of ceria-based anodes in hydrocarbons could be improved by the addition of precious-metal catalysts, at dopant levels,but the performance of these cells was still too low for practical considerations. The problem with doped ceria is likely that its electronic conductivity is not sufficient. In general, the electrode material should have a conductivity greater than 1 S/cm in order to be practical since a conductivity of 1 S/cm would lead to a cell resistance of 0.1 Q cm for an electrode thickness of 1 mm, even... 

Film electrodes are generally fabricated from conducting or semiconducting materials, which may be deposited as a result of either a physical or a chemical process (or some combination) onto a suitable substrate, which is typically an insulator. Key factors governing the desired thickness of the film are the electrical resistivity (p) or conductivity (k = 1/p) of the film material, which is a practical consideration in almost all cases, and the optical transparency or reflectance of the material if optical transmission or reflection is also desired. The optimum film thickness for an application involving both electrical and optical considerations will require a trade-off, since a decrease in resistivity (usually desirable) normally is also accompanied by a decrease in light transmission (undesirable for an optically transparent electrode). 

Both disk-shaped and cylinder electrodes can be fashioned with metal microwires and carbon fibers. Two practical considerations that must be kept in mind when working with small wires and fibers are their resistivity and their delicacy. The former consideration demands that only short sections of the microwire or fiber be used for electrode construction, while the latter mainly represents a significant source of frustration for the novice electrode maker. A 5-/ m-radius platinum wire at times seems to break just because you ve looked at it With a little practice, however, handling these small materials becomes straightforward. 

Materials that have a nonzero second-order susceptibility will produce light at twice the incident frequency. The magnitude of this effect is small, and has been a practical consideration only since the advent of lasers. If the symmetry of a crystal or other medium is such that it has a center of inversion, no SHG effect will be observed. However, surfaces by their very nature break this inversion symmetry. Hence, an SHG signal may arise at the electrode-solution interface even though both bulk phases may be considered centrosymmetric [66], The magnitude of the SHG signal is sensitive to surface conditions (e.g., electrode potential, ionic or molecular adsorption, etc.). Surface spectroscopy is also feasible since the SHG signal will be enhanced if either the incident frequency (to) or SHG (2co) corresponds to an electronic absorption of a surface species [66]. 

The main obstacle to creating liquid junction solar cells is photocorrosion of semiconductor electrodes, which reduces considerably their lifetime. In order to prevent, for example, anodic photocorrosion, a well-reversible redox couple is introduced into an electrolyte solution, so that the reaction of oxidation of the red component competes for photoholes with the reaction of photodecomposition of the electrode material (see Section IV.2). With the aid of this method, photocorrosion has been practically prevented in certain types of photocells and the duration of their continuous operation has been increased up to about one year. Yet, there are other, more subtle mechanisms of electrode degra-dation, which has hitherto prevented the lifetime of photoelectrochemical cells from becoming comparable with the 20-year lifetime of solid-state solar cells. 

In contrast to the Li-ion battery, the practical considerations of aprotic electrolytes used in Li-air battery are not limited to thermal stability, ionic transport, inertness towards electrode materials, and practical electrochemical window, but also include the reversibility issue of the formation of active materials that involves the subtle electrochemical reactions between the aprotic electrolytes and the reduced O2... 

In coulometry these exchange membranes are often used to prevent the electrolyte around the counter electrode from entering the titration compartment (see coulometry, Section 3.5). However, with membrane electrodes the ion-exchange activity is confined to the membrane surfaces in direct contact with the solutions on both sides, whilst the internal region must remain impermeable to the solution and its ions, which excludes a diffusion potential nevertheless, the material must facilitate some ionic charge transport internally in order to permit measurement of the total potential across the membrane. The specific way in which all these requirements are fulfilled in practice depends on the type of membrane electrode under consideration. 

In the previous section we considered the conditions under which mechanical resonances would occur in a TSM resonator. In considering only the mechanical properties of the crystal, however, we neglected consideration of how these resonances would actually be excited or detected. The device uses a piezoelectric substrate material in which the electric field generated between electrodes couples to mechanical displacement. This allows electrical excitation and detection of mechanical resonances. In constructing a practical sensor, changes in resonant frequency of the device are measured electrically. The electrical characteristics of the resonator can be described in terms of an equivalent-circuit model that describes the impedance (ratio of applied voltage to current) or admittance (reciprocal of impedance) over a range of frequencies near resonance. 

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