Electrode Polarization and Related Phenomena

In Chapter 2, we approached alternating-current electrode polarization impedance from the phenomenological point of view, which parallels the historical development of this subject. Before we embark upon descriptions of electrochemical cells, ion-specific electrodes, and potentiometric techniques, it is necessary to discuss some of the electrochemical processes that occur at the interface between a solid electrode surface and a contacting electrolyte. 

By choice, the treatment here will be brief and incomplete. The processes which take place are complex and depend upon many factors. Two recent books have been entirely devoted to investigations of this subject (Adams, 1969 Newman, 1973). In the present chapter, we simply identify several phenomena and indicate how they relate to electrode use. A detailed discussion is well beyond the scope of this text, and the reader who requires more extensive information is referred to the texts cited above. 

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Signal distortion in electrophysiological recording systems may arise from a variety of sources series ac polarization impedance developed at electrode-electrolyte interfaces fluid interfaces in glass microelectrode systems and associated fluctuations of tip potentials and related phenomena stimulus-isolation units residual and stray capacitances not corrected for by compensating preamplifiers contact problems thermal noise and noise generated by other sources. 

The time dependence of the dielectric properties of a material (expressed by e or CT ) under study can have different molecular origins. Resonance phenomena are due to atomic or molecular vibrations and can be analyzed by optical spectroscopy. The discussion of these processes is out of the scope of this chapter. Relaxation phenomena are related to molecular fluctuations of dipoles due to molecules or parts of them in a potential landscape. Moreover, drift motion of mobile charge carriers (electrons, ions, or charged defects) causes conductive contributions to the dielectric response. Moreover, the blocking of carriers at internal and external interfaces introduces further time-dependent processes which are known as Maxwell/Wagner/Sillars (Wagner 1914 Sillars 1937) or electrode polarization (see, for instance, Serghei et al. 2009). 

Polarization curves. The rate at which the anodic or the cathodic process takes place depends on the potential ( ). The corrosion behaviour of the reinforcement can be described by means of polarization curves that relate the potential and the anodic or cathodic current density. Unfortunately, determination of polarization curves is much more complicated for metals (steel) in concrete than in aqueous solutions, and often curves can only be determined indirectly, using solutions that simulate the solution in the pores of cement paste. This is only partly due to the difficulty encountered in inserting reference electrodes into the concrete and positioning them in such a way as to minimize errors of measurement. The main problem is that diffusion phenomena in the cement paste are slow (Chapter 2). So when determining polarization curves, pH and ionic composition of the electrolyte near the surface of the reinforcement may actually be altered. 

The response of the fuel cell is determined by the electrochemical processes and associated kinetics at the electrode and electrode interface. The electrochemical processes depend on the mass and charge transfer between the bulk electrolyte solution and electrode surface. The rates at which these transfers occur are determined by the number of localized phenomena and largely depend on the materials involved. These processes are presented in this chapter and the relations between the fuel cell potential and current density are given in terms of BV and Tafel equations. The key losses in the fuel cell include the activation losses, ohmic losses, mass transport losses, and losses owing to reactant crossover and internal currents that are discussed in this chapter. The fuel cell polarization curve is presented and is discussed for low-temperature and high-temperature fuel cells such as PEMFC and SOFC, respectively. 

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