Cu-doped ceria: Oxygen vacancy formation made easy

► Oxygen vacancies are extremely important for the properties of ceria. ► This is the first theoretical study on the reduction properties of Cu-doped bulk ceria. ► We find that the first oxygen vacancy forms almost spontaneously (Evac almost zero). ► We find that Cu also affects - facilitates - the formation of a second O vacancy in the material. ► In-depth analysis of the relative importance of three different origins of the lowering Evac.