Efficient and CO2-tolerant oxygen transport membranes prepared from high-valence B-site substituted cobalt-free SrFeO3−δ
Introduction
Oxygen non-stoichiometric and mixed ionic-electronic conducting (MIEC) oxides have attracted increasing attention for various high-temperature applications in energy and chemical industries, e.g., oxygen transport membranes (OTMs), membrane reactors, oxygen sensors and cathode materials for solid-oxide fuel cells (SOFCs) [1], [2], [3], [4], [5], [6]. Cobalt-based MIEC materials, for example, Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), are characterized as highly efficient catalysts for oxygen reduction reactions because of their favorable oxygen anion diffusion coefficients and high surface oxygen exchange kinetics [7], [8], [9]. Extremely low area-specific resistances of 0.021 and 0.08 Ω cm2 for BSCF electrodes at 600 °C in the SOFC single cell configuration or symmetrical cell configuration, and an oxygen permeation flux as high as 1.28 mL cm−2 min−1 at 850 °C with a BSCF-based OTM (thickness: 1.5 mm) have been reported [8], [10]. However, the phase transition from a cubic structure to a hexagonal structure or other mixed phases, and the formation of carbonates over the material surface or even inside the oxide bulk in the presence of CO2 have raised concerns [7], [11], [12]. These processes could gradually deteriorate the electro-catalytic activity, and slow down oxygen anion bulk diffusion and oxygen surface exchange kinetics.
To date, only very limited numbers of MIEC materials with high oxygen permeation flux or low area-specific resistance can retain original phase structure and maintain chemical stability, especially under long-term operation, high current polarization and harsh working conditions, for instance, a CO2-containing atmosphere. The instabilities could result from the presence of alkaline-earth metal ions, easily reducible transition metal ions and high concentration of oxygen vacancies in these oxygen non-stoichiometric MIEC materials [13]. To develop materials with high structural stability and favorable CO2 tolerance, several strategies have been suggested, e.g., avoidance of the use of alkaline-earth metal ions (especially Ba2+) and easily reducible transition-metal ions (e.g., Co3+) [14], [15], [16]. Although great efforts have been made towards the development of MIEC materials for OTMs, the tradeoff between activity and stability is inevitably encountered.
During the past decades, strategies such as applying dual-phase and dual-layer membranes have been proposed to alleviate aforementioned issues [17], [18], [19], [20], [21], [22]. Several combinations have been proposed, e.g., membranes with external or internal short-circuiting electrodes, dual-phase membranes comprised of a spinel oxide and an ionic conductor, and dual-layer membranes of SrFe0.8Nb0.2O3−δ/Ba0.5Sr0.5Co0.8Fe0.2O3−δ or CexGd1−xO2−δ/LaxSr1−xCoyFe1−yO3−δ [23], [24], [25], [26], [27], [28]. Although considerable efforts towards applications in CO2-containing atmosphere have been made, rather complex processes for the fabrication, the high price of the short-circuiting electrode, and concerns regarding the chemical compatibility and interfacial ion diffusion between two phases/layers, as well as the quite low oxygen permeability have not been completely resolved.
A compromise among all the strategies discussed above must be made to achieve a proper stability-activity balance. To this end, recent research has focused on another facile approach involving partial substitution with less CO2-reactive cations in the A and/or B sites. Fe-based materials are more stable towards chemical and structural stability and Fe elements in this composition are cost effective and accessible. Ideally, LnFeO3−δ (Ln=Lanthanides) should exhibit the most favorable tolerance under CO2-containing atmospheres; however, the oxygen permeation fluxes are relatively low [29]. The higher valence state and smaller size of La3+ in comparison with Sr2+ result in a lower oxygen vacancy concentration and a reduced lattice constant, thus leading to an extremely low oxygen flux. Therefore, SrFeO3−δ (SF) as the parent material could be a possible alternative [30], [31]. Compared with SrCoO3−δ, BaCoO3−δ and BaFeO3−δ as membrane materials, only a few publications are mainly on the SF-based materials [32]. Most recently, transition metal cations with a more stable oxidation state, e.g., Cr, Sc, Ti, Nb, Mo, Ga and Zr, have been widely applied to minimize the thermal and chemical expansion and stabilize the phase structure with facile oxygen diffusion (disordered oxygen vacancy) [14], [33], [34], [35], [36], [37], [38], [39], [40]. However, the incorporation of highly charged cations was observed to significantly decrease the oxygen permeation flux especially when the substituting level is higher than a certain value, e.g., 0.2 [40], [41], [42]. For example, it has been reported that the oxygen permeability of BaFeO3−δ could be improved by doping with Nb5+; however, a decreased oxygen permeability was observed when the substituting level was as high as 0.2 [43]. Therefore, proper-level substitution of highly charged cations into parent materials should be a feasible approach to obtain membranes with moderate oxygen permeation flux and enhanced structural and chemical stability.
Here a new series of materials, SrFe0.8M0.2O3−δ (M=Zr, Mo, and W), possessing relatively high phase structure stability and chemical stability when compared to the un-doped SF in the presence of CO2, have been developed. Furthermore, the relatively stable oxygen permeation fluxes when compared to the un-doped SF under the operation conditions with either He or CO2-containing He as the sweep gas on the permeate side were obtained. These findings suggest that SrFe0.8M0.2O3−δ (M=Zr, Mo, and W) materials are promising for applications in membrane technologies.
Section snippets
Material preparation and membrane fabrication
SrFe0.8M0.2O3−δ (M=Zr, Mo, and W) oxides were synthesized using a modified solid-state reaction method involving mechano-activation [25]. Stoichiometric SrCO3, MxOy and Fe2O3 (all analytic grades) were ground thoroughly using high-energy ball milling in an acetone medium at a rotational rate of 400 rpm for 60 min. The slurry was dried into powders and pressed into pellets. Then, the pellets were calcined at 1000 °C in air for 10 h to form the primary product, which was further ground, pressed and
Results and discussion
To verify the possibility of doping/substituting potential ions into SF, XRD measurements together with Rietveld refinement analysis of SrFe0.8M0.2O3−δ (M=Zr, Mo, and W) were performed, and the results are shown in Fig. 1. The XRD patterns of SrFe0.8M0.2O3−δ (M=Zr, Mo, and W) can be indexed mainly as a cubic structure with space group Pm-3m, except for a very weak peak, belonging to the pyrochlore phase or single oxide phase, which appears near the plane (110) for all the compositions. It
Conclusions
Cobalt-free SrFe0.8M0.2O3−δ (M=Zr, Mo, and W) perovskite membranes with improved CO2 tolerance and favorable oxygen permeation fluxes were developed. The relatively stable oxygen permeability was observed when compared to the un-doped SF membrane and the phase structure could be retained when the membrane was exposed to CO2-containing atmospheres. The substitution of transition metal ions with high-valence and redox-inactive transition metal ions leads to the enhanced CO2 tolerance and
Acknowledgments
This work was financially supported by the Key Projects in Nature Science Foundation of Jiangsu Province under Contract no. BK2011030 and by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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