Assessment of bismuth oxide-based electrolytes for composite gas separation membranes

Abstract

Oxide + salt composites can be used in CO2 and NOx separation membranes, where high oxide-ion conductivity is crucial to improve performance. Pursuing this goal, the stability of three different bismuth oxide-based electrolytes (Cu + V, Y and Yb-doped) against molten alkali carbonates (Li, Na, K) or nitrates (Na, K) was tested firing them in the 450–550 °C temperature range, and with endurance tests up to 100 h. A well-known ceria-based composite was used as reference (CGO - Ce0.9Gd0.1O1.95). Oxides and composites were studied by X-ray diffraction, scanning electron microscopy and impedance spectroscopy (in air, 140–650 °C temperature range). Bi2Cu0.10V0.90O5.35 easily reacts with molten salts. Bi0.75Y0.25O1.5 and Bi0.75Yb0.25O1.5 have higher stability against molten carbonates and complete stability against molten nitrates. The Y-doped oxide stability against the molten carbonates was enhanced changing the molten salt composition (Y2O3 additions) and using lower firing temperatures. Above all, composites based on Y or Yb-doped Bi2O3 with molten alkali nitrates showed impressive 6× or 3× higher electrical conductivity at 290 °C, in air (4.88 × 10−2 and 2.41 × 10−2 S cm−1, respectively) than CGO-based composites (7.72 × 10−3 S cm−1), qualifying as promising materials for NOx separation membranes.