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

Structural and Conducting Features of Niobium-Doped Lanthanum Tungstate, La₂₇(W_1–<i>x</i>Nb_<i>x</i>)₅O_55.55−δ

The most studied ceramic proton conductors are those based on the perovskite structure. However, these materials have some practical drawbacks including their poor tolerance to carbonation. Hence, proton-conducting ceramic materials with fluorite structure are currently under investigation. One of the most studied materials is the lanthanum tungstate, “Ln₆WO₁₂”. Here, we report a new series of compounds La₂₇W_{5–5<i>x</i>}Nb_5<i>x</i>O_{55.5–5<i>x</i>/2}□_{8.5+5<i>x</i>/2} obtained by niobium doping to optimize/increase the amount of oxygen vacancies. The limiting composition has been established as La₂₇NbW₄O_55.0□_9.0 with an astonishing 14% of oxygen vacancies. The materials have been studied by Rietveld analysis of high-resolution laboratory X-ray powder diffraction data and electron microscopy. Thermal analysis measurements in a wet atmosphere indirectly confirm the increase of oxygen vacancies as the amount of incorporated protons increases with the niobium content. A thorough electrical characterization has been carried out including overall conductivity measurements in wet and dried atmospheres, conductivity as a function of the oxygen partial pressure, and electronic contribution by the Hebb–Wagner polarization method. The data collected suggest that the proton conductivity is dominant below 600 °C. However, above 800 °C the conductivity values are almost independent of the water partial pressure which indicates that the oxide ion is the main charge carrier. The highest conductivity value was measured for La₂₇NbW₄O₅₅, i.e., 0.01 S·cm^–1 at 800 °C compared to 0.004 S·cm^–1 for the nonsubstituted material La₂₇W₅O_55.5. At temperatures below 800 °C, these materials are nearly pure ionic conductors with transport numbers higher than 0.98, while at higher temperatures these compounds are mixed ionic–electronic conductors displaying both n- and p-type electronic contributions