A solid-state electrochemical device for studying thermodynamic properties of non-stoichiometric oxides: Application to UO2+x

•Set-up combines coulometric titration and e.m.f. measurements, provides relationship between oxygen activity and oxygen content in non-stoichiometric oxides.•The oxygen activity in UO2+x is controlled during oxidation or reduction experiments, for temperatures ranging from 1093 to 1273 K, for values of x < 10−3.•A model is developed to interpret deviation from stoichiometry changes in terms of predominant point defects.•The role of cation impurities is estimated.