Atmospheric electrochemistry

IV. ATMOSPHERIC ELECTROCHEMISTRY EFFECTS OF THE ELECTRICAL DOUBLE LAYER OF THE EARTH AND ACID RAINS... 

Yasuda I and Hishinuma M. Eattice expansion of acceptor-doped lanthanum chromites under high-temperature reducing atmospheres. Electrochemistry (Tokyo) 2000 68 526-530. 

L. R. Jordan, A. K. Shukla, T. Behrsing, et al. Effects of diffusion-layer morphology on the performance of polymer electrolyte fuel cells operating at atmospheric pressure. Journal of Applied Electrochemistry 30 (2000) 641-646. 

The electrosynthesis of hydride complexes directly from molecular hydrogen at atmospheric pressure by reduction of Mo(II) and W(II) tertiary phosphine precursors in moderate yield has been described as also the electrosynthesis of trihydride complexes of these metals by reduction of M(IV) dihydride precursors [101,102]. Hydrogen evolution at the active site of molybdenum nitrogenases [103] is intimately linked with biological nitrogen fixation and the electrochemistry of certain well-defined mononuclear molybdenum and tungsten hydrido species has been discussed in this context [104,105]. 

This effect is called the relaxation effect. Second, in the presence of the ionic atmosphere, a viscous drag is enhanced than in its absence because the atmosphere moves in an opposite direction to the moving ion. This retarding effect is called the electrophoretic effect. In Eq. (7.1), the Ah°°-term corresponds to the relaxation effect, while the E-term corresponds to the electrophoretic effect. For details, see textbooks of physical chemistry or electrochemistry. 

Controlled atmospheres are employed in electrochemistry whenever oxygen, water, or other constituents of the air may interfere with the reaction under study. The problem is basically twofold to supply and contain an atmosphere of suitable composition, and to exclude air. For our present purposes it is sufficient to confine the discussion to inert atmospheres that is, gases devoid of oxygen and water (and sometimes nitrogen). The use of high-purity argon, helium, or nitrogen is most common. 

Figure 19.2 Typical dry box suitable for high-quality electrochemistry. The purification train, recirculation unit, and automatic pressure control system are housed in the unit beneath the dry box. [Courtesy of Vacuum/Atmospheres Corporation, Hawthorne, CA.]...

Fig. 15.19. Quantum efficiency versus potential at various p-type semiconductors in a DMF-0.1 A4TBAP solution containing 5% water under a C02 atmosphere. Monochromatic light of 600 nm was used for p-Si, p-lnP, p-GaAs, and p-CdTe, while light of 400 nm was used for p-GaP. Scan rate 0.1 V/s. (Reprinted from I. Taniguchi, Electrochemical and Photoelectrochemical Reduction of Carbon Dioxide, in Modem Aspects of Electrochemistry, J. O M. Bockris, R. White, and B. E. Conway, eds., No. 20, Fig. 7, p. 357, Plenum, 1989.)...

Since the advantage of using nonaqueous systems in electrochemistry lies in their wide electrochemical windows and low reactivity toward active electrodes, it is crucial to minimize atmospheric contaminants such as 02, H20, N2, C02, as well as possible protic contaminants such as alcoholic and acidic precursors of these solvents. In aprotic media, these contaminants may be electrochemically active on electrode surfaces, even at the ppm level. In particular, when the electrolytes comprise metallic cations (e.g., Li, Mg, Na), the reduction of all the above-mentioned atmospheric contaminants at low potentials may form surface films as the insoluble products precipitate on the electrode surfaces. In such cases, the metal-solution interface becomes much more complicated than their original design. Electron transfer, for instance, takes place through electrode-solution rate limiting interphase. Hence, the commonly distributed solvents and salts for usual R D in chemistry, even in an analytical grade, may not be sufficient for use as received in electrochemical systems. 

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