Mixed-conducting perovskite membranes for oxygen separation - Towards the development of a supported thin-film membrane
Prof. Dr. Ir. H. Verweij
Dr. H.J.M. Bouwmeester
Cobalt based perovskite-type oxygen separation membranes are currently receiving extensive scientific and technological interest due to their wide variety of possible applications in oxygen production, power generation processes and selective hydrocarbon oxidation. A major challenge for large-scale technical applications is to increase the oxygen flux through these membranes and/or to reduce operating temperatures. A logical approach to increase the oxygen flux at a given temperature is the reduction of the membrane thickness to micrometer dimensions by depositing a dense thin-film membrane on a porous support. This approach requires extensive knowledge on oxygen transport properties and ceramic processing of the membrane materials. In this study, the oxygen permeation and transport parameters of dense La1‑xSrxCoO3‑d membranes are investigated. The thickness of the membranes under investigation varies from millimeter to micrometer dimensions. In addition, the preparation of the thin-film membrane is studied, which includes the development of a suitable membrane support and the deposition of a dense, micrometer thick film on top of these supports.
In chapter 2, oxygen transport through dense, La1‑xSrxCoO3‑d membranes (x = 0.2, 0.5 and 0.7) in the thickness range of 0.5-2mm, is investigated using oxygen permeation measurements. The results indicate a clear influence of surface exchange on the oxygen permeation rate. However, the oxygen fluxes remain predominantly controlled by bulk oxygen diffusion through the membrane. The calculated characteristic membrane thickness Lc, below which oxygen transport is predominantly rate limited by surface exchange, varies approximately between 70 and 230 mm. From the oxygen pressure dependence of oxygen permeation it follows that the ionic conductivity of the different compositions La1‑xSrxCoO3‑d is unfavourably affected with lowering oxygen partial pressure and temperature, which is attributed to vacancy trapping effects associated with the ordering of oxygen vacancies.
The results of the permeation study are in excellent agreement with observations from conductivity relaxation experiments, which are described in chapter 3. Here, the chemical diffusion coefficient and oxygen transfer coefficients of the same compositions in the series La1‑xSrxCoO3‑d are studied using the conductivity relaxation technique. The chemical diffusivity and oxygen surface exchange in the perovskites under study appears to be highly correlated. The general trend displayed is that both parameters decrease with decreasing pO2 below about 10-2 bar at all temperatures. This is attributed to ordering of induced vacancies at low enough oxygen partial pressures. The observation that the correlation between both parameters extends even to the lowest pO2 values in this study, suggests a key role of the concentration of mobile oxygen vacancies, rather than of the extent of oxygen nonstoichiometry, in determining the rate of both processes. The characteristic thickness Lc shows only a weak dependence on oxygen partial pressure temperature and composition and is found to vary between 50 and 150 mm.
Chapters 4 and 5 focus on the development of a supported thin film membrane. In chapter 4, the processing of La1‑xSrxCoO3‑d powders into porous supports is described. Two approaches are followed. The first employs pressing of pre-sintered powders, homogenised by sieving and classified through sedimentation. Tuning of the pre-sintering temperature of the powder yields 30% porous materials with an average pore diameter of 0.8 mm. The surface roughness of these supports is rather high but the surface morphology can be improved by applying a perovskite coating. The second approach is based on compact formation by pressure filtration of commercial powder dispersed in isopropanol. Sintering of the substrates at 1020°C resulted in 30% porous materials with an average pore diameter of 0.3 mm. Although a minor amount of defects is present due to inhomogeneities in the powder, the morphology of these substrates is considered to be superior to that of the substrates prepared via pressing. Therefore, it is concluded that the substrates prepared via pressure filtration are most suitable for supporting thin film La1‑xSrxCoO3‑d membranes. In the final section of this chapter, the application of centrifugal casting in the development of tubular perovskite substrates is studied. Results of exploratory experiments indicate a great potential of this technique in the preparation of high-quality dense and porous La1‑xSrxCoO3‑d tubes. The deposition of La0.5Sr0.5CoO3-d films with a thickness of 5‑30 mm onto porous substrates using pulsed laser deposition is described in chapter 5. Initial deposition experiments on porous a‑Al2O3 supports are unsuccessful as crack formation occurs during cooling after the deposition process due to thermal mismatch. When La0.5Sr0.5CoO3-d supports are used, dense and crack-free films are obtained.
The permeation measurements of the supported thin-film membranes of La0.5Sr0.5CoO3‑d are presented in chapter 6. The data show that the oxygen fluxes are significantly higher than those measured for pressed disc membranes. Minor leakage is observed, amounting up to 5% of the total oxygen flux, which is caused by support inhomogeneities, which could not be covered by the laser deposition process. In the experimental range of thickness 7.5-20 mm, the oxygen flux is found to be independent of membrane thickness. A characteristic membrane thickness is calculated of about 50 mm, which value nicely agrees with that extracted from data of conductivity relaxation experiments if it is assumed that the porous support structure removes surface exchange limitations at the interface between porous support and thin film membrane. Mass transport through the porous support structure contributes significantly to the overall permeation resistance at 1000°C, but its influence becomes negligible at temperatures around 700°C.
In chapter 7, the influence of substitution of cobalt in La0.5Sr0.5CoO3‑d with 1 to 8 mole % of either copper or nickel on the oxygen permeation rate and transport parameters is studied. Oxygen permeation data indicate that nickel and copper doping results in a significant increase in the permeation rate. The largest increase in flux is measured for the lowest dopant concentration, amounting to a factor 1.4 for the nickel doped and 1.7 for the copper-doped sample. Conductivity relaxation data indicate that doping increases both the surface transfer and the bulk diffusion rates.
The main results achieved in this work are summarised in chapter 8, which also includes recommendations for future research.