Ion-Conducting Ceramic Membranes and their Applications for Air Separation and Chemical Synthesis


Austin Environ Sci. 2017; 2(2): 1020.

Ion-Conducting Ceramic Membranes and their Applications for Air Separation and Chemical Synthesis

Zhang C¹, Tan X² and Liu S¹*

¹Department of Chemical Engineering, Curtin University, Australia

²Department of Chemical Engineering, Tianjin Polytechnic University, PR China

*Corresponding author: Shaomin Liu, Department of Chemical Engineering, Curtin University, Australia

Received: April 07, 2017; Accepted: April 10, 2017; Published: April 17, 2017


State of the art

According to recent scientific studies, we are currently suffering more unprecedented natural disasters caused by climate change (global warming), which is due to the emission of large amounts of greenhouse gases. How to reduce climate change is driving the political, social and technological agenda. In terms of greenhouse gas emissions, coal power generation makes up a large proportion of CO2 worldwide emissions. In order to minimize such emissions, a series of clean energy projects have been initiated for energy production with Carbon Capture and Storage (CCS). These projects can be classified into three categories: the Integrated Gasification Combined Cycle (IGCC) technology (pre-combustion), Oxyfuel, and post-combustion (requiring CO2 separation from the flue gases) [1]. Compared to the former two categories, post-combustion technology involving the CO2 separation stage is too expensive to justify economically, due to the vast amount of flue gas with diluted CO2. Comparatively, IGCC and Oxyfuel projects are more feasible. However, these two clean energy categories need oxygen as the feed gas. For instance, if pure oxygen instead of air is used in power plants, the major constituent of the waste gas produced during the combustion process would be CO2, which can be easily and economically captured. In the oxygen market, consumption has been dominated by conventional industries like metal manufacturing, chemicals, pharmaceuticals, petroleum, glass, cement, ceramics, pulp/paper manufacturing and others; the power generation is only sharing 4% [2]. However, in the near future, this market for energy will be massively expanded by the deployment of these clean energy projects. Current tonnage O2 production by a cryogenic process is very expensive and energy intensive. The addition of an extra cryogenic air separation unit in the coal gasification or Oxyfuel power plant is impractical because of its high capital investment and operational cost. Membrane technology is becoming more and more attractive for a possible energy-efficient gas separation method. Due to their lower selectivity, polymeric membranes have been ruled out for pure oxygen production purpose but they have applications in getting oxygen-enriched atmosphere. Oxygen and nitrogen have very much similar kinetic diameters (O2: 0.346 nm; N2: 0.364 nm), therefore it is difficult to perform the air separation using microporous molecular sieving membranes like zeolite/silica membranes. Fortunately, the oxygen selective membranes can be made from dense ceramic membranes. Dense Mixed Ionic and Electronic Conducting (MIEC) ceramic membranes can deliver 100% pure O2 under differential O2 partial pressure gradients without the requirement of external electric power, offering the potential to reduce the separation cost and improve the viability of these clean energy technologies. From the perspective of application, membranes must possess sufficiently high oxygen flux value and good structural stability to withstand practical conditions. In real applications, the oxygen permeation performance is dependent on several factors such as the material composition, the membrane thickness and operating conditions [3].

Among the reported ceramic materials with MIEC properties, perovskite and fluorite oxides have attracted most attention from researchers as they display high oxygen flux values, in particular the former membranes. The general formula of the perovskite is ABO3 in which the metal cations at A sites are larger than the B site cations (Figure 1a). The framework of perovskite ABO3 is very similar to ReO3, consisting of the corner-sharing BO6 octahedra with A cations located in twelve-coordinated interstices, as shown in (Figure 1a). Such an oxygen vacancy-disordered cubic perovskite structure is favourable for oxygen ion movement and electron conduction thus displaying the highest oxygen flux. In order to maintain the cubic perovskite structure with sufficient oxygen vacancy concentration, partial replacement (also called doping) of A or B cations in the ABO3 by other cations with different ionic radius, lower valence states and lower metal-oxygen bond energy is frequently performed. So far, a series of perovskite oxides with formula AyA'(1-y)BxB'(1-x)O(3-a) of have been reported to possess appreciably high oxygen permeation rates. In this formula, the A/A' belongs to the group of the alkaline-earth or rare-earth elements consisting of La, Sr, Ba, or Ca; and the B/B' is largely taken from the group of transitional metals consisting of Sc, Cr, Mn, Fe, Co, Ni, Cu, Ga, Zr, or Zn [4]. Actually, perovskite ABO3 has a very large flexibility to accommodate many other metal elements thus 90% of the metal elements in the periodic table have been attempted to be incorporated inside the perovskite structure to tune the properties to suit different applications. Among this series, typical examples are Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF) and La0.6Sr0.4 Co0.2Fe0.8O3-d (LSCF) developed from the parent material of SrCoO3 [5,6]. BSCF is featured by its higher oxygen flux values but lower stability; on the contrary, LSCF is characterised by its moderate flux but higher stability. If the membrane is considered for other purposes more than the pure oxygen production from air, it may require higher stability to tolerate the practical atmosphere. As such, we have to consider the different susceptibility of perovskite oxides to react with other gases like H2O, CO2, H2 and CH4. If these gases are present in a sufficiently high concentration, they quickly poison the perovskite membrane surface and fail the function of oxygen transport. To optimize the membrane for oxygen separation, in addition to the material selection, engineering considerations should also be given, which will be discussed from (Figure 1b).

Citation: Zhang C, Tan X and Liu S. Ion-Conducting Ceramic Membranes and their Applications for Air Separation and Chemical Synthesis. Austin Environ Sci. 2017; 2(2): 1020. ISSN:2573-3605