Significance of interfaces in solid-state cells with porous electrodes of mixed ionic-electronic conductors

Solid-state cells based on different electrolyte materials and identical electrodes of a porous mixed ionic-electronic conductor (MIEC) are studied using impedance spectroscopy. It is found that impedance spectra of these cells are dramatically influenced by the properties of the electrolytes, implying that the electrolyte-electrode interfaces, including the triple-phase boundaries (TPBs), are important reaction sites in these cells and can not be ignored, in general, in modelling the electrode kinetics of porous MIEC electrodes. Further, the observed effect of electrical state of an MIEC electrode (or an applied voltage across the cell) on cell impedance conclusively confirms that the nature of the rate-limiting step(s) of the electrode reactions occurring in such a cell is electrochemical; it is not true, in general, that the electrode kinetics of such a cell are determined merely by pure chemical processes such as 'surface exchange' and oxygen diffusion.