In single-species ionic conductors, the Nernst equation can describe the cell potential. However, the Nernst equation is not valid for multi-species ionic conductor membranes. In this case, the cell potential for multi-species systems considering the transport properties of the migratory species can be calculated using the Godman-Hodgkin-Katz equation (aka GHK) . The GHK equation describes the cell potential in terms of transport parameters of permeates (here O "# , Na ! , and electronic defects [e $ ]/[h • ]) and internal EMFs given in terms of chemical potentials of neutral species (µ 0 ! and µ 2, ). The GHK equation can be rewritten in terms of the chemical potential of neutral species at the electrodes, transport properties of the electrolyte and electrode polarization resistance which in other terms are individual Nernst voltages and transport parameters. Therefore, the cell potential (experimentally measured quantity) in terms of the different chemical potential of mobile species across the mixed sodium-ion (NZGO) and oxygen-ion (YSZ) conductor is given by: