Electronic conductivity Direct conversion

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... 

Direct contact techniques do not work with electrolytic solutions. One cannot, for example, measure the resistance of salt water in a cell with an ordinary ohmmeter. This is due to the fact that the mobile charge carriers in an electrolytic solution are ions, not electrons. The conversion of ionic to electronic conduction at the electrode interface can only take place through an electrochemi-... 

Well-established anode materials are Ni cermets such as Ni/YSZ composites. The presence of the second phase increases the contact area and prevents the catalytically active Ni particles from aggregating. The use of the composite becomes problematic if hydrocarbons are to be directly converted Ni catalyzes cracking, and the resulting carbon deposition deactivates the fuel cells. Therefore either pure H2 has to be used or the fuel has to be externally reformed. A third way is internal conversion of CHV with H20 to synthesis gas. The necessary steam addition, however, reduces the overall efficiency. Another problem of Ni cermets, if they are to be used at lower temperatures, is a potential oxidation of the Ni. Alternatives are Cu/Ce02 cermets in which Cu essentially provides the electronic conductivity and Ce02 the catalytic activity. Note that an efficient current collecting property of the electrode presupposes a metal concentration above the percolation threshold. 

An interesting application of proton-electron conducting membranes has recently been reported by Li et al. [2.76]. These authors studied the conversion of CH4 first to C2H4 and its subsequent direct catalytic aromatization to benzene and other valuable aromatic hydrocarbons. Their reactor configuration is shown schematically in Figure 2.3. The two distinct additional features of their work are the use of an active catalyst for the reaction itself, (Mo/H-ZSM5), and the use of asymmetric membranes with a thin (10-30 pm)... 

Photovoltaic and photoconductive effects result from direct conversion of incident photons into conducting electrons within a material. The two effects differ in the method of sensing the photoexcited electrons electrically. Detectors based on these effects are called photon detectors, because they convert photons directly into conducting electrons no intermediate process is involved, such as the heating of the material by absorption of photons in a thermal detector which causes a change of a measurable electrical property. 

The band edges are flattened when the anode is illuminated, the Fermi level rises, and the electrode potential shifts in the negative direction. As a result, a potential difference which amounts to about 0.6 to 0.8 V develops between the semiconductor and metal electrode. When the external circuit is closed over some load R, the electrons produced by illumination in the conduction band of the semiconductor electrode will flow through the external circuit to the metal electrode, where they are consumed in the cathodic reaction. Holes from the valence band of the semiconductor electrode at the same time are directly absorbed by the anodic reaction. Therefore, a steady electrical current arises in the system, and the energy of this current can be utilized in the external circuit. In such devices, the solar-to-electrical energy conversion efficiency is as high as 5 to 10%. Unfortunately, their operating life is restricted by the low corrosion resistance of semiconductor electrodes. 

During reduction, electrons travel/rom the power pack, through the electrode, transfer across the electrode-solution interface and enter into the electroactive species in solution. Conversely, during oxidation, electrons move in the opposite direction, and are conducted away from the electroactive material in solution and across the electrode-solution interface as soon as the electron-transfer reaction occurs. (Incidentally, these different directions of electron movement explains why an oxidative current has the opposite sign to a reductive current, cf. Section 1.2.)... 

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