Conversion Electrodes

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Figure 4.6 Daly collector system consisting of the conversion electrode and a scintillator (a) together with a photomultiplier (b), this detector is used in the ICP-MS from VC Instruments. (Reproduced by permission of CV Instruments Ltd.)...

NO in the equilibrium NO mixture at a constantly controlled high temperature diffuses toward the second half of the cavity, where it is oxidized electrochemically to NO2 by the use of an NO conversion electrode (Pt-Rh). Thus, almost pure NO2 can be obtained if the NO conversion efficiency is high. 

Figure 7.5 Schematic diagram of an off-axis detector for a mass spectrometer. The conversion dynode is maintained at a high potential (up to 10-20 kV) thus accelerating the ions to high velocities to improve the secondary emission efficiency (the diagram is drawn to illustrate the airangement for positive ions). The secondary elections are then accelerated to the first dynode (maintained at 2 kV) and the SEM then amphfies the secondary electron current as usual. For negative ions the conversion electrode is maintained at a high positive potential and secondary positive ions are accelerated on to the first dynode. This example portrays a quadrupole analyzer with a discrete dynode SEM. Reproduced from Photomultiplier Tubes Basics and Applications (3rd Edn), Hamamatsu Corporation, with permission.

FIGURE 4.4 Properties of lithium cells with metal oxide conversion electrodes, (a) The voltage vs. composition profile for various cells cycled between 0.01 V and 3 V (vs. Li/Li ) at a rate of C/5 (1 lithium ion per metal atom in 5 h). 

Irrespective of the question whether the material acts as anode or cathode, the following simplified and general reaction can be used to describe the overall process in a conversion electrode... 

An interesting feature of the system is the very low polarization compared to other conversion electrodes (Fig. 3.8, inset). This has been attributed to the mass transport kinetics where the mobility of the ionic species plays an important role. Hydride ions are supposed to move faster than 0 and F and it is believed thatthere are lower kinetic barriers in the case of hydride systems. But as the role of anion diffusion in the rate-determining step has not been elucidated in detail, the statement remains to be verified. 

Metal fluorides are an interesting class of compounds which may be used for conversion electrodes. Due to the highly ionic nature of their metal-fluoride bond, they should theoretically exhibit a much higher output voltage and lithium storage capacity than any other systems. However, many of the metal fluorides have a limited electrochemical activity with lithium due to their poor electronic conductivity brought about by their large band gap. 

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