Impedance interpretation mass transfer

Proper cell design is essential to reduce the uncertainty of the interpretation. Reference electrodes can be used to isolate the impedance of cell components, well-defined convective systems can be employed to queintify the role of mass transfer, and electrode configurations can be selected to minimize the role of current and potential distributions across the surface of the electrodes. 

Use of well-defined convective systems allows the influence of mass transfer to be treated explicitly eind quantitatively thus the interpretation of impedance data in terms of interfacial processes can be emphasized. Several experimental systems are commonly employed. 

The uncertainty associated with the interpretation of the impedance response can be reduced by using an electrode for which the current and potential distribution is uniform. There are two types of distributions that can be used to guide electrode design. As described in Section 5.6.1, the primary distribution accounts for the influence of Ohmic resistance and mass-transfer-limited distributions account for the role of convective diffusion. The secondary distributions account for the role of kinetic resistance which tends to reduce the nonuniformity seen for a primary distribution. Thus, if the primary distribution is uniform, the secondary... 

Experimental systems used for electrochemical measurements should be selected to take maximum advantage of well-imderstood phenomena such as mass transfer so as to focus attention on the less-understood phenomena such as electrode kinetics. For example, the study of electrochemical reactions in stagnant environments should be avoided because concentration and temperature gradients give rise to natural convection, which has an effect on mass transfer that is difficult to characterize. It is better to engage in such experimental investigations in systems for which mass transfer is well defined. To simplify interpretation of the impedance data, the electrode should be uniformly accessible to mass transfer. 

Electrochemistry in general and the EIS in particular are often used to analyze both bulk sample conduction mechanisms and interfacial processes, where electron transfer, mass transport, and adsorption are often present. EIS analysis has often treated the bulk and interfacial processes separately [4]. The analysis is achieved on the basis of selective responses of bulk and interfacial processes to sampling AC frequencies. The features appearing in the impedance AC frequency spectmm can be described according to the theory of impedance relaxations. Again, as in the case of any other spectroscopy method, the subject of the EIS analysis is the detection and interpretation of these spectrum features. 

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