Oxygen, chemical potential mobility

Chemical effects also occur in crystalline oxides that is, impurity atoms diffuse at varying rates through oxides. In all cases, cation diffusion is much faster than oxygen diffusion, but similar cations, such as Ca and Mg, can behave very differently in different oxides. This is due to differences in chemical potential and activity, and mobility differences. 

Since the state of a crystal in equilibrium is uniquely defined, the kind and number of its SE s are fully determined. It is therefore the aim of crystal thermodynamics, and particularly of point defect thermodynamics, to calculate the kind and number of all SE s as a function of the chosen independent thermodynamic variables. Several questions arise. Since SE s are not equivalent to the chemical components of a crystalline system, is it expedient to introduce virtual chemical potentials, and how are they related to the component potentials If immobile SE s exist (e.g., the oxygen ions in dense packed oxides), can their virtual chemical potentials be defined only on the basis of local equilibration of the other mobile SE s Since mobile SE s can move in a crystal, what are the internal forces that act upon them to make them drift if thermodynamic potential differences are applied externally Can one use the gradients of the virtual chemical potentials of the SE s for this purpose ... 

Under open circuit conditions, the electric current /= J) Zj-F-Jj vanishes. As long as tQ2- = 1 this means thaty o2- = To2- 7o2- - 0. or equally V /02- = Zj-F-Vtp = 0. This is true since oxygen ions are the mobile majority species with a constant chemical potential independent of any variation in the oxygen potential. It follows that the electrical potential in the oxide electrolyte of a galvanic cell is constant under open circuit conditions, despite the different oxygen potentials at the two electrodes. 

As discussed in table 10.1, the mobile species within a fuel cell are ions, which necessitate the electrolyte being an ionic conductor and electronic insulator. If the oxygen ions are the only charge carriers, the electron motive force (EMF) of the cell is determined from the chemical potential of oxygen (i.e., oxygen activity), which is expressed by the Nernst equation as... 

From a macroscopic standpoint, there is a difference in the chemical potential of the constituents in botli phases in contact. Owing to kinetics (low ionic mobility in the solid) and thermodynamics (solubility product), migration of cations towards the liquid phase and dissolution of the oxide do not occur. The difference in the chemical potential of oxygen does decrease through adsorption of water and dissociation of the adsorbed molecules, which explains the presence of hydroxyl groups on the surface. 

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