Oxygen permeation performance of BaBiO3−δ ceramic membranes

https://doi.org/10.1016/j.memsci.2009.08.019Get rights and content

Abstract

In this work, perovskite BaBiO3−δ disk membranes were synthesized with the molar ratio (z) of BiO1.5 to BaO between 0.5 and 3 at varying sintering temperatures. Disk membranes with z > 1.33, associated with a lower amount of Bi-rich perovskite phase, showed mechanically weak properties while membranes with z  1 showed superior stability at temperatures in excess of 800 °C. The best performance was obtained for the z = 0.86 disk membrane, reaching oxygen fluxes of 1.2 ml min−1 cm−2 at 950 °C. This was attributed to the higher sintering temperature and the formation of oxygen deficient phase of BaBiO3−δ perovskite. For gas testing temperatures above 800 °C, it was found that the oxygen permeation was limited by both bulk diffusion and surface kinetics as oxygen flux did not increase proportionally to the inverse of membrane thickness reduction. Further analysis showed that the activation energy for oxygen ionic transport changed at 800 °C, however the z = 1 sample displayed the opposite trend from other compositions, indicating the formation of more oxygen vacancies in the crystal lattice. Mechanically stable disk membranes exposed to thermal cycling tests resulted in crystal structure instability of the pure perovskite (z = 1) and loss of oxygen vacancies while the z < 1 sample was relatively stable. Conversely, z > 1 sample showed superior thermal cycling and crystal structure stability.

Introduction

Dense metal oxide membranes belong to a separate class of inorganic membrane materials that rely on both ionic and electrical conductivity, the so-called mixed ionic–electronic conducting (MIEC) membranes. These membranes have demonstrated significant potential for delivering tonnage oxygen for clean energy delivery, in processes such as coal gasification or oxy-fuel combustion coal power plants [1]. A large number of MIEC compounds have been investigated in the literature, mainly based on perovskites (ABO3−δ) and fluorites (AδB1−δO2−δ and A2δB2−2δO3) or dual-phases by the introduction of metal or ceramic elements. Among these ceramic structures which exhibit MIEC properties, perovskite materials have been intensively studied due to their high (oxygen) ionic and electronic conductivity as well as their favorable phase stability [2], [3].

MIEC compounds can be fabricated into dense ceramic membranes. MIEC membranes require a driving force for the transport of oxygen, which is supplied by the oxygen partial pressure gradient from the feed and permeate side of the membrane. As this is an ionic process, the electrons involved in the electrochemical oxidation and reduction of oxygen ions and oxygen molecules, respectively, are transported in opposite directions, ensuring overall electrical neutrality [2]. The rate of oxygen transport is limited by surface exchange and/or bulk diffusion steps. Surface exchange kinetics, whereby the oxygen molecule is broken down into its ionic form, is controlled by the material and temperature [4], [5], [6]. Bulk-diffusion kinetics are regimented by oxygen vacancies in the material structure, temperature and membrane thickness [3]. When oxygen permeation is bulk-diffusion limited, significant improvement of oxygen flux can be obtained by reducing the thickness of the membrane, while catalytic surface modification enhances oxygen flux when surface exchange is the limiting step [7], [8]. In industrial applications, oxygen transport limitations must be reduced in order to optimize membrane performance.

MIEC membranes are generally derived from perovskite structures, such as ABO3−δ which contains an equivalent number of A-site and B-site cations as well as oxygen vacancies. The ionic conduction occurs by oxygen ions ‘hopping’ from one vacant site to a neighboring vacant site, while the electronic conduction occurs via Bn+–O–B(n+1)+ conduction pairs. In most instances electronic conductivity is more dominant than ionic conductivity [2], [3]. Of the many perovskite formulations reported in the literature, barium bismuth oxides demonstrated high conductivity (electronic and ionic) and high oxygen vacancy concentration. However, most studies were focused on the structure and conductivity of BaBiO3 [9], [10], [11], [12], [13], [14] which contains bismuth ions with different valence states (+3 and +5) [11], [15]. Klinkova et al. [11] developed detailed phase diagrams and elucidated the structure of BaO:BiO1.5 using thermal gravimetric analysis, differential thermal analysis, X-ray diffraction and transmission electron microscopy. They reported that oxygen losses within the perovskite were enhanced with increasing temperature from 600 to 1000 °C through the formation of oxygen deficient compounds of BaBiO2.88, BaBiO2.83, BaBiO2.75 and BaBiO2.55. Takahashi et al. [14] studied the perovskite composition of (BaO)x(Bi2O3)1−x (x = 0.67) and reported that the conductivity was predominantly electronic. Accordingly, barium bismuth perovskite compounds can give enhanced electrical conductivity and oxygen vacancy formation.

Nevertheless, there are very limited studies reported in the literature on the oxygen permeation of barium bismuth oxides. In this work, we investigate the performance of BaBiO3−δ perovskite membranes based on BaO–Bi2O3 formulation. The molar ratio (z) of BiO1.5 to BaO was varied and the structure of the materials was characterized by X-ray diffraction to evaluate the effect of different perovskite phases on oxygen permeation. The morphology of the samples was analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy. In addition, the perovskite materials were exposed to thermal cycling in air and long term stability testing at an elevated temperature under a nitrogen atmosphere.

Section snippets

Synthesis and preparation

Powders with varying BiO1.5 (as Bi2O3) to BaO molar ratios of z = 0.5–3 were prepared by solid-state reactions at high temperatures. Stoichiometric quantities of Ba(NO3)2 (99.0+%, All-tech) and Bi2O3 (99.9+%, Sigma–Aldrich) were ball-milled with ethanol in a planetary mill (Pulverisette 5, Fritsch) at 500 rpm for 3 h. As-mixed powders were initially calcined at 600 °C for 3 h in order to decompose Ba(NO3)2 into BaO, followed by a further increase in temperature to 700 °C for 9 h. As-calcined powders

Results and discussion

Fig. 2(a) and (b) shows typical SEM images of barium bismuth oxide disk membranes z = 0.86 sintered at 1000 and 1080 °C, respectively. The lower sintering temperature resulted in the formation of porous structure. However, the sintering effect is clearly evidenced when the sintering temperature was raised to 1080 °C, delivering a dense surface with large grains of approximately 50 μm in size. EDX results for two representative samples presented in Table 1 indicated the formation of a homogenous

Conclusions

In this work, perovskite disk membranes were synthesized with the molar ratio (z) of BiO1.5 to BaO between 0.5 and 3 at varying sintering temperatures. All XRD patterns predominantly exhibited peaks characteristic of BaBiO3 perovskite structures, though z = 3 sample showed a rhombohedral phase. Powders prepared with z = 1.17–3 revealed low intensity peaks at 2θ = 40.2 and 50.5° which were associated with Bi-rich perovskite phase and lack of mechanical stability for testing membranes in excess of 800 

Acknowledgments

The authors would like to thank the Australian Research Council for financial support on this research project. Jaka Sunarso also acknowledges Ph.D. scholarship (UQIRA and UQILAS) provided by the University of Queensland. The authors would like to thank Dr. Kevin Jack for his assistance in the XRD analysis.

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