EMF of bielectrolyte solid state cells with different mobile ions in each phase

Abstract

An expression for the emf of an isothermal solid state bielectrolyte cell is developed starting from the basic equations of transport for each solid electrolyte. If the two different migrating species react to form a compound at well defined activity at the interface between the two solid electrolytes, interfacial chemical potentials and the emf are unambiguously defined. The interfacial chemical potentials are determined by the transport properties of the two solid electrolytes. The emf of the bielectrolyte cell will be less than that calculated from the Gibb's energy change for the virtual cell reaction when the interfacial or electrode chemical potentials lie outside the electrolytic conduction domain (t_ion>0.99) of the solid electrolytes. Using estimated values for electonic and hole conductivities in Na β-alumina, the emf of a bielectrolyte combination consisting of β-alumina and (CaO)ZrO₂ is computed for different chemical potentials of sodium and oxygen at the electrodes. The chemical potential gradient for sodium across β-alumina is found to be very small because the conductivity of β-alumina is much larger than that of (CaO)ZrO₂. When two cationic or anionic conductors with different mobile species are coupled together, the chemical potentials at junction of the solid electrolytes and hence the emf are usually ill defined. A definite relationship between ionic fluxes in each electrolyte is necessary to mathematically define the interfacial chemical potentials.