Metal-electrolyte interface polarization effects

When a metal is in contact with an electrolyte solution, a dc potential occurs which is the result of two processes. These are (1) the passage of metallic ions into solution from the metal, and (2) the recombination of metal ions in the solution with free electrons in the metal to form metal atoms. After a metal electrode is introduced into an electrolyte, equilibrium is eventually established and a constant electrode potential is established (for constant environmental conditions). At equilibrium, a dipole layer of charge (electrical double layer) exists at the metal-electrolyte interface. There is a surface layer of charge near the metal electrode and a layer of charge of opposite sign associated with the surrounding solution. Although diffuse, this dipole layer produces an effective electrical capacitance (Cp) which accounts for the low-frequency behavior of the electrode polarization impedance as discussed in Chapters 2, 3, and 4. 

Some period of anodic polarization of a structure is allowed at which a total effect of metal loss is not observed. This results from a certain degree of reversibility of electrode processes occurring on the metal-electrolytic environment phase interface. On the basis of laboratory and field measurements, the following criterion of corrosion hazard to an industrial structure has been assumed due to electrolytic corrosion caused by stray currents ... 

Unfortunately, unlike the situation at the Hg/electrolyte solution interface, independent variation of metal surface charge at metals cannot be made at the metal/vacuum Interface, but the electron work function 9 of the metal can be determined accurately iii situ (e.g., by means of the u.v. photoelectric effect), and changes of due to adsorption of polar and nonpolar molecules can be measured, giving the net surface dipole potential X of the adsorbed film of atoms or molecules. For low coverages (0), X is usually linear in 0 but increases less rapidly than linearly as 0 - 1, due to mutual depolarization amongst similarly oriented surface dipoles ... 

During the last few years a new application of sohd electrolytes has emerged. It was found that the catalytic activity and selectivity of the gas-exposed electrode surface of metal electrodes in solid electrolyte cells is altered dramatically and reversibly upon polarizing the metal/solid electrolyte interface. The induced steady-state change in catalytic rate can be up to 9000% higher than the normal (open-circuit) catalytic rate and up to 3 x 10 higher than the steady-state rate of ion supply. " This new effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) has been already demonstrated for more than... 

In this Chapter, the progress recently made in the field of electrochemical promotion (EP) of catalytic gas reactions is reviewed. The phenomenon consists of electrochemical polarization of metal or metal oxide electrodes interfaced with solid electrolytes which result in a pronounced increase in the catalytic reaction rate. The effect is also termed non-Faradaic electrochemical modification of catalytic activity (NEMCA effect), since the rate increase may exceed the ionic current by several orders of magnitude. The promotion is not limited to the electrochemically polarized interface between catalyst and solid electrolyte, but extends to the entire catalyst surface exposed to the reactive gas. In fact, one of the major challenges in the field of electrochemical promotion is to elucidate the exact mechanism by which the promoting effect propagates from one interface to the other. 

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