Isovalent and aliovalent substitution effects on redox chemistry of Sr2MgMoO6 − δ SOFC-anode material

An SOFC-anode material, Sr2MgMoO6 − δ, is investigated for both isovalent and aliovalent substitution effects at its redox-active cation site. Isovalent WVI-for-MoVI substitution has little effect on crystal and redox chemistry of the phase, whereas aliovalent NbV-for-MoVI substitution lowers the degree of cation order and, rather importantly, creates oxygen vacancies in the lattice. The increased oxygen-vacancy concentration should be a positive factor regarding the SOFC-anode performance, but a disadvantage is that electrical conductivity is slightly depressed in Sr2Mg(Mo,Nb)O6 − δ with increasing Nb content. The two systems, Sr2Mg(Mo,W)O6 − δ and Sr2Mg(Mo,Nb)O6 − δ, are found stable (up to 1000 °C or higher) in both reductive (5% H2/Ar) and oxidative (air) atmospheres, the range of oxygen-content variation upon such redox-cycling getting narrower with increasing substitution level. XANES data at the L edges of Mo, W and Nb reveal that in Sr2Mg(Mo,W)O6 − δ both Mo and W show variable oxidation states whereas in Sr2Mg(Mo,Nb)O6 − δ Nb is more redox-active than Mo.