Use of defect equilibrium diagrams to understand minority species transport in solid electrolytes

One of the functions of an electrolyte is to act as a selective filter, allowing the transport of ionic species, but not electronic species. Leakage of electronic species through the electrolyte in an electrochemical cell causes a reduction in the output voltage, as well as leading to self-discharge and capacity loss. The parameter that describes the fraction of the total current through an electrochemical system that is carried by a particular species is its transference number. There are several ways in which this quantity can be determined experimentally, and one of the most common is known as the Hebb–Wagner method, which involves the measurement of the steady state current through a selectively polarized electrochemical cell. The transference number is not a constant for a given material, but instead, is dependent upon the chemical potentials of the constituent components within it. Thus, it is dependent upon the local composition and will not be uniform throughout an electrolyte in a galvanic cell in which the electrodes have different values of the chemical potentials. One can understand the results of Hebb–Wagner experiments using a Defect Equilibrium Diagram as a thinking tool. A straightforward method for its development in the general case of a binary electrolyte and the relationship between defect concentrations and electric potential are presented. Gradients in the concentrations of electrons and holes lead to diffusion, and thus externally measurable charge transport. The form of the dependence of the minority species currents upon the applied voltage depends upon the direction of cell polarization, and the potential of the reference electrode plays an important role in the determination of their respective magnitudes. A double cell arrangement can be used to help obtain meaningful experimental results.