Effects of synthesis methods on oxygen permeability of BaCe_0.15Fe_0.85O_3−δ ceramic membranes

Abstract

Dense BaCe_0.15Fe_0.85O_3−δ (BCF1585) ceramic membranes synthesized by the solid-state reaction (SSR) method and EDTA-citric acid (EC) process were investigated by X-ray powder diffraction, total conductivity, oxygen permeation, etc. XRD results revealed the perovskite structure of the powders prepared by the EC process was easier to be developed than that of prepared by SSR method. Membranes derived from EC had higher density, pure phase structure and fewer defects comparing to those derived from SSR method. However, membranes derived from SSR method had higher oxygen permeability. Thickness experiments revealed that the oxygen permeation fluxes of the membranes synthesized by both methods are all jointly controlled by surface exchange and bulk diffusion in the range of 0.7–2.0 mm. The long-term oxygen permeation operation revealed that the membranes derived from both methods exhibit good oxygen permeation stability.

Introduction

Oxygen permeable ceramic membranes with mixed ionic and electronic conductivity are receiving considerable attention due to their potential applications in oxygen separation from air [1], [2], [3], partial oxidation of natural gas in membrane reactors [4], [5], [6], [7], [8], [9] and as SOFC cathode materials [10]. For industrial application of mixed conducting membranes, the membrane materials should have high oxygen permeability, high oxygen permeation stability and excellent structural stability under reducing environments.

Oxygen permeable membranes with high oxygen permeability are usually in perovskite structure and contain Co^2+/3+ in the materials. However, Co^2+/3+ are easy to be reduced in reducing atmosphere and result in the failure of membrane structure in operation. Recently, we have developed a series of cobalt-free oxygen permeable membranes (BaCe_xFe_1−xO_3−δ) [11] with perovskite structure. The membranes possess not only considerable oxygen permeability, but also high structural stability under reducing environments at elevated temperatures. The materials can sustain their perovskite structure under 10% H₂–Ar mixed gas at 900 °C only with 5% cerium in the B-site. At the same time, the membranes have oxygen permeation fluxes of over 0.5 ml/cm² min under air/He gradient at 900 °C for 1.0 mm thickness membranes if the amount of cerium doped in B-site is no more than 15%.

There are many methods to prepare perovskite powders, such as solid-state reaction (SSR) method, EDTA-citric acid (EC) process, glycine-nitrate combustion process (GNP) and chemical co-precipitation method, etc. Powders synthesized by different processes have different particle sizes, sintering activities and chemical compositions as expected. These factors will lead to different microstructures of as obtained ceramic membranes, thus result in different oxygen permeation fluxes. Kharton et al. [12] reported that SrCo_0.6Fe_0.25Cu_0.15O_3−δ synthesized by SSR method had larger particle size and higher oxygen permeability than that of synthesized by cellulose precursor process. Tan et al. [13] reported that oxygen permeability of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ synthesized by SSR method was higher than that of synthesized by EC process. Both of them attributed this effect to the higher oxygen ions transport in the bulk than along the grain boundary. Furthermore, it was reported that mixed conducting oxides synthesized by different methods had different phase compositions, for example, the phase composition of SrFeCo_0.5O_y is largely determined by the synthesis method [14], [15].

In this paper, we will continue our previous investigation and focus on the effects of synthesis methods on ceramic microstructure, phase composition, total conductivity and oxygen permeation of BaCe_0.15Fe_0.85O_3−δ membranes.