Galvanic cells mobile phases

Another kinetic aspect is observed if a component other than the electroactive species is predominantly mobile. The electroactive species are in this case made available to the electrolyte by the motion of the other components in the opposite direction. In a binary compound this does not make a difference to the electrode performance. But in the case of a compound with more than two components the composition is changed to values which are not expected from a thermodynamic point of view for the variation of the concentration of the electroactive species. Other phases are formed which may provide a lower or higher activity of the electroactive species than that expected thermodynamically. This has an influence both on the current and the cell voltage. Upon discharging and charging a galvanic cell, the composition of the electrode at the interface with the electrolyte may follow very different compositional pathways (Weppner, 1985). 

GITT also provides very comprehensive information about the kinetic parameters of the electrode by analysis of the electrical current. The current 1, which is driven through the galvanic cell by an external current or voltage source, determines the number of electroactive species added to (or taken away from) the electrode and discharged at the electrode/ electrolyte interface. A chemical diffusion process occurs within the electrode and the current corresponds to the motion of mobile ionic species within the electrode just inside the phase boundary with the electrolyte (at x = 0)... 

The properties of the electrodes are at the center of scientific interest. They qualitatively and quantitatively control the electrochemical reactions in galvanic cells. The properties of electrolytes are controlled by the concentrations of ions, their mobilities, and the interactions between ionic particles of opposite charge as well as their interactions with other constituents of the respective phases (e.g., solvent, membrane matrix, solid matrix). This latter area will, however, not be dealt with in any detail in this chapter. F.mpha.sis will be given to interfacial processes. Since the basic laws of electrochemistry were developed in systems with liqnid electrolytes, these relations will be derived and presented in this introduction for such systems. 

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