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

Abstract An SOFC-anode material, Sr 2 MgMoO 6 −  δ , is investigated for both isovalent and aliovalent substitution effects at its redox-active cation site. Isovalent W VI -for-Mo VI substitution has little effect on crystal and redox chemistry of the phase, whereas aliovalent Nb V -for-Mo VI 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 Sr 2 Mg(Mo,Nb)O 6 −  δ with increasing Nb content. The two systems, Sr 2 Mg(Mo,W)O 6 −  δ and Sr 2 Mg(Mo,Nb)O 6 −  δ , are found stable (up to 1000 °C or higher) in both reductive (5% H 2 /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 Sr 2 Mg(Mo,W)O 6 −  δ both Mo and W show variable oxidation states whereas in Sr 2 Mg(Mo,Nb)O 6 −  δ Nb is more redox-active than Mo.