Compatibility Analysis of Chromites in Dual-Phase Composites for Oxygen Transport Membranes
Authors: Martinez Aguilera Lev , Kiebach Ragnar , Haugen Astri Bjørnetun 
Published: February 2020

An oxygen transport membrane (OTM) is a promising way of supplying oxygen from air to high temperature processes, such as gasification.[1] Several studies have demonstrated that mixed ionic-electronic conducting (MIEC) perovskites with composition AxSr1-xCoyFe1-yO3-d (A=La, Ba) have high oxygen-ionic and electronic conductivity, which are the basic requirements for an OTM material. However, these perovskites have a limited pO2 operation range, suffering irreversible chemical decomposition at pO2 values below 10-12. [2]
YCrO3-based perovskites show high electronic conductivity and thermo-chemical stability both at high temperatures (~1000 ºC) and in reducing environments (pO2 ~10-20 atm). [3,4] Hence, chromites in combination with highly stable ionic-conductors (e.g. ceria or zirconia-based materials) as dual-phase composites are alternative OTM materials. However, their chemical compatibility and stability together with ion-conductors in reducing conditions, as well as their oxygen permeation in different atmospheres, needs to be further investigated.
This study analyses the stability of dual-phase composites consisting of 30 vol.% perovskite Y0.8Ca0.2Cr0.8Co0.2O3 (YCCC) and 70 vol.% ionic conductors Ce0.9Gd0.1O1.95 (CGO10) or (Sc2O3)0.1(Y2O3)0.01(ZrO2)0.89 (10Sc1YSZ) as OTM materials. These dual-phase composites were sintered in air between 1200 ºC and 1450 ºC to analyze the effect of temperature on the composites, and stability tests were made by exposing the composites to humidified 5% H2/N2 at 850 ºC. SEM-EDS, XRD and thermal analyses were performed to characterize the composites and identify possible secondary phases that could detriment their performance. Oxygen flux tests were performed using 1 mm thick pellets in air/N2, air/CO2 and air/H2-3%H2O gradients at temperatures up to 950 ºC.
In 10Sc1YSZ-containing samples, densification was inhibited in the studied temperature range, which might be related to evaporation of transition metals from the perovskite B site, Ca mobility and possible reactions with Zr. Metal evaporation was slightly intensified after reducing conditions, resulting in relative densities <80%. Chromium depletion from the composite and formation of Cr2O3 crystals was observed in samples sintered at 1450 ºC.
On the other hand, CGO10-based composite reached ~ 90% of relative density at 1200 ºC, though CaCrO4 formation, probably related to Ca excess in the perovskite, was identified. Nevertheless, higher stability towards reducing conditions was observed. Initial oxygen permeation tests performed in dense CGO10-YCCC composite pellets show fluxes up to 0.4 Nml·cm-2·min-1 at 950ºC in air/N2 gradient. The CGO10-YCCC dual-phase composite is therefore a promising material, which is now under further optimization and development as a tubular, multilayer OTM.
[1] Julbe, A. et. al. Catalysis Today, 104 (2005), 102-113
[2] Niedrig, C. et al. Journal of The Electrochemical Society, 160 (2) F135-F140 (2013).
[3] Gupta, S. et al. Materials Science and Engineering, 90 (2015), 1–36.
[4] Suvorov, S.A. et al. Refractories and Industrial Ceramics, 45-2 (2004), 94-99.