Elucidation of subtle degradation mechanisms in composite CO2 separation membranes

Abstract

Composite membranes for CO2 separation based on CGO (Gd-doped ceria) and the eutectic mixture of Na and Li carbonates, with distinct salt content (around 20 and 40 vol%) and microstructures (starting oxide grain size in the 170–220 nm range), were prepared and tested up to 250 h, at 650 °C, under distinct atmospheres (CO2 gradient or fully immersed in CO2 or Ar), to elucidate degradation mechanisms. Substantial oxide grain growth (50–65%) was observed during endurance tests of membranes with higher and lower oxide content, respectively. These relative changes were unable to explain the observed stability, higher for membranes with lower salt content. Combined microstructural, structural and electrochemical analyses of samples revealed that the solubility of metal oxides in the molten phase plays a crucial role. While ceria is poorly soluble, Gd oxide dissolves easily under acidic conditions (high CO2, membrane feed side) but precipitates under low CO2 (sweep side). Grain growth with formation of regions with distinct Gd content (core-shell CGO grains) explain the degradation of the oxide scaffold. This information provides important guidelines on membrane design, pointing towards oxide scaffolds with large and well percolated grains, also moderate acidity of the molten phase, to improve membrane stability.