Thermodynamic study of mixed anionic solid solutions using gradient solid electrolytes

The thermodynamic properties of K2CO3 -K2SO4 solid solutions with hexagonal structure have been measured using a solid-state cell, incorporating a composite solid electrolyte with step-changes in composition. The cell with the configuration Pt, CO'2 + O'2 || K2CO3 | K2(CO3)x (SO4)1-X || CO''2 + O''2, Pt X=1 X=X was investigated in the temperature range of 925 to 1165 K. The composite gradient solid electrolyte consisted of pure K2CO3 at one extremity and the solid solution under study at the other. The Nernstian response of the cell to changes impartial pressures of CO2 and O2 at the electrodes and temperature was demonstrated. The activity of K2CO3 in the solid solution was measured by three techniques. All three methods gave identical results, indicating unit transport number for K+ ions and negligible diffusion potential due to concentration gradients of carbonate and sulfate ions. The activity of K2CO3 exhibits positive deviation from Raoult's law. The excess Gibbs energy of mixing of the solid solution can be represented using a subregular solution model ΔGE = X(1-X)[5030X + 4715(1- X)] Jmol-1. By combining this information with the phase diagram, mixing properties of the liquid phase were obtained