Mo-Substituted Lanthanum Tungstate La_{28–<i>y</i>}W_4+<i>y</i>O_54+δ: A Competitive Mixed Electron–Proton Conductor for Gas Separation Membrane Applications

Molybdenum substituted lanthanum tungstate, La_{28–<i>y</i>}(W_1–<i>x</i>Mo_<i>x</i>)_4+<i>y</i>O_54+δ (<i>x</i> = 0–1, <i>y</i> = 0.923), was investigated seeking for an enhancement of the n-type electronic conductivity for its use as a mixed electron–proton conductor in hydrogen gas separation membrane applications. The materials were synthesized by the freeze-drying precursor method, and they were single phase after firing between 1300 and 1500 °C for <i>x</i> ≤ 0.8. The crystal structure changed from cubic (<i>x</i> ≤ 0.4) to rhombohedral (<i>x</i> ≥ 0.6) with increasing the molybdenum content. Transmission electron microscopy (TEM) investigations revealed an ordering of the oxygen vacancies with increasing Mo-content, giving rise to superstructure domains. The dependency of the conductivity with the oxygen and water partial pressure showed that these materials are good mixed electron–proton conductors under wet reducing conditions for <i>x</i> ≤ 0.4. The conductivity of the materials with <i>x</i> ≥ 0.6 was dominated by electrons, and they are expected to be less chemically stable due to the lower redox stability of Mo⁶⁺. The total conductivities in humidified H₂ were 0.016 S/cm for <i>x</i> = 0.2 and 0.043 S/cm for <i>x</i> = 0.4 at 900 °C, and they were stable under these conditions for more than 60 h. The ambipolar proton–electron conductivity was estimated to be ∼1.6 × 10^–3 S/cm for <i>x</i> = 0.4 at temperatures as low as 600 °C, which makes this family of materials very interesting and competitive candidates for applications such as hydrogen gas separation membranes at lower temperatures than state-of-the-art materials