O2 chemical potential of nonstoichiometric ceria, CeO2−x, determined by a solid electrochemical method

Abstract

The chemical potential of oxygen in nonstoichiometric ceria, CeO2−x, was measured versus composition x (10−4 ≤x ≤10−2) and temperature (823≤T≤1073 K) using a novel solid electrochemical cell, utilizing yttrium stabilized zirconia. The composition of CeO2−x was varied by coulometric titration. The O2 chemical potential was determined from the electromotive force of the cell with air used as the reference gas. With this new experimental setup, it is possible to carry out long coulometric titrations at very low oxygen partial pressures (limited by 10−30 atm at 823 K or by 10−13 atm at 1473 K). For x between 10−4 and 10−2, the O2 partial pressure in equilibrium with CeO2−x was found to change from 10−8 to 10−18 atm at 1073 K and from 10−16 to 10−30 atm at 823 K. The oxygen vacancies in CeO2−x were determined to be V″0 in this (x,T) range. The charge on aliovalent cations impurities was found to dominate the negative (effective) charge concentration for x≲10−3. The oxygen partial molar enthalpy ΔH̄(O2) is 870 kJ/mol (9 eV) for x≳3×10−3 decreasing for smaller x to a value of 670 kJ/mol (7 eV) for x=10−4.