Thermodynamic study of mixed anionic solid solutions using gradient solid electrolytes

Abstract

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 ΔG^E = 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.