Non-Cottrell behaviour of potentiostatic processes in mixed conductors with comparable ionic and electronic conductivities

A theoretical investigation of potentiostatic polarization processes in mixed conductors with arbitrary electronic transport numbers has been performed. As the potentiostatic polarization is accompanied by a chemical diffusion process inside the mixed conductor located between blocking electrodes, Pick's second law of diffusion has been solved for asymmetric electrochemical cells in order to obtain concentration profiles of the mobile species, current-time relationships and the impedance of the asymmetric cell. Ohmic interfacial resistances as well as the ohmic bulk resistance owing to the total conductivity of the mixed conductor are considered. The theoretical results reveal that chemical diffusion processes in mixed conductors with comparable ionic and electronic conductivities show non-Cottrell behaviour resulting in non-Cottrell type current-time relationships. These relations allow the exact description of the short-time behaviour of potentiostatic diffusion processes occurring during chronoamperometric measurements. In addition, the non-Cottrell behaviour of potentiostaic processes in mixed conductors with arbitrary electronic transport numbers employed in symmetric electrochemical cells is discussed.