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. 2019 Aug 24;19:955–964. doi: 10.1016/j.isci.2019.08.032

Chemical Environment-Induced Mixed Conductivity of Titanate as a Highly Stable Oxygen Transport Membrane

Guanghu He 1,2, Wenyuan Liang 1,3, Chih-Long Tsai 2, Xiaoliang Xia 1, Stefan Baumann 2, Heqing Jiang 1,3,5,∗, Wilhelm Albert Meulenberg 2,4
PMCID: PMC6742913  PMID: 31518903

Summary

Coupling of two oxygen-involved reactions at the opposite sides of an oxygen transport membrane (OTM) has demonstrated great potential for process intensification. However, the current cobalt- or iron-containing OTMs suffer from poor reduction tolerance, which are incompetent for membrane reactor working in low oxygen partial pressure (pO2). Here, we report for the first time a both Co- and Fe-free SrMg0.15Zr0.05Ti0.8O3−δ (SMZ-Ti) membrane that exhibits both superior reduction tolerance for 100 h in 20 vol.% H2/Ar and environment-induced mixed conductivity due to the modest reduction of Ti4+ to Ti3+ in low pO2. We further demonstrate that SMZ-Ti is ideally suited for membrane reactor where water splitting is coupled with methane reforming at the opposite sides to simultaneously obtain hydrogen and synthesis gas. These results extend the scope of mixed conducting materials to include titanates and open up new avenues for the design of chemically stable membrane materials for high-performance membrane reactors.

Subject Areas: Chemical Reaction, Membranes, Chemical Reactions in Materials Science

Graphical Abstract

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Highlights

  • •

    A new both Co- and Fe-free titanate-based oxygen transport membrane is developed

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    The membrane exhibits superior reduction tolerance in 20 vol.% H2/Ar

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    The membrane shows an environment-induced mixed conductivity

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    The material is well suited for membrane reactor for coupling two reactions

Chemical Reaction; Membranes; Chemical Reactions in Materials Science

Introduction

Process intensification based on catalytic membrane reactors (CMRs) combing catalytic reactions and separation processes in one single unit presents one of the most important trends in today's chemical engineering and process technology (Morejudo et al., 2016, Tou et al., 2017). As one of the typical inorganic membranes for CMRs, dense oxygen transport membranes (OTMs) with mixed ionic-electronic conductivity (Wang et al., 2005a, Wang et al., 2005b) exhibit high oxygen ion permeability and infinite selectivity at elevated temperatures because of mobile oxygen vacancies and electronic defects. These features enable an OTM to simultaneously combine oxygen-related chemical reactions at two opposite sides of the membrane, leading to an integral coupling of reaction-separation-reaction processes and, hence, benefit with regard to energy consumption, capital cost, and catalytic performance. These benefits of OTM reactor have been demonstrated in our previous works (Jiang et al., 2008, Jiang et al., 2009a, Jiang et al., 2009b, Jiang et al., 2010a, Jiang et al., 2010b). For example, water splitting was coupled with partial oxidation of methane (POM) using a BaCoxFeyZr1-x-yO3−δ membrane (Jiang et al., 2008). At one side of the membrane, hydrogen is obtained from water splitting; meanwhile, at the other side, POM reaction occurs to produce synthesis gas with a H2/CO ratio of around 2, which is proper for the subsequent Fischer-Tropsch or methanol production. In addition, some other researchers combine two oxygen-involved reactions at the opposite sides of OTM reactors for two synthesis gases (i.e., H2/N2 and H2/CO) production for ammonia and liquid fuel (Li et al., 2016), large-scale hydrogen production (Fang et al., 2016, Li et al., 2017), or CO2 capture and utilization (Kathiraser et al., 2013, Zhang et al., 2014), further underscoring the promise of OTM reactors for coupling two reactions on the opposite sides.

In spite of these advantages, distinct skepticism currently remains about the applicability of such reactors owing to the poor reduction tolerance of the existing OTM materials whose two sides are usually subjected to an oxygen-containing species (H2O, NOx) and a reductive gas (CH4, C2H6), respectively. Conventional “first-generation” OTMs are based on cobalt perovskite oxides (e.g., Ba0.5Sr0.5Co0.8Fe0.2O3−δ) that exhibit high oxygen permeability and work well for air separation at relatively high oxygen partial pressure (pO2) (Tan et al., 2008, Thursfield and Metcalfe, 2007), but the performance of Co-based membranes drops rapidly owing to the fast and deep reduction of cobalt ions followed by the eventual collapse of perovskite structure under low pO2 environments in which two reactions are coupled at opposite sides of the membranes (Bouwmeester, 2003, Ovenstone et al., 2008). “Second-generation” OTMs, based on iron-containing oxides such as Ba0.5Sr0.5Fe0.8Zn0.2O3-δ, BaCexFe1-xO3-δ, and dual phase Ce0.9Gd0.1O2−δ – NiFe2O4, exhibit relatively higher stability in low pO2 than Co-based membrane (Luo et al., 2011, Wang et al., 2005a, Wang et al., 2005b, Zhu et al., 2006). Regrettably, performance degradation of Fe-based membranes cannot be avoided since the damage of Fe-based material is also inevitable (Neagu et al., 2013) when being used as reactor for coupling two chemical reactions under low pO2. Hence, there is an urgent need to develop new membrane materials that allow the deployment of OTM reactors exhibiting desirable oxygen permeability as well as high chemical stability under low pO2 atmospheres such as H2O, CH4, and H2.

In this regard, titanates (e.g., SrTiO3) are very intriguing materials because of the following considerations. (1) Titanates have excellent chemical and redox stability at high temperatures (Calle-Vallejo et al., 2010). (2) Under low pO2 atmospheres, titanates show n-type conduction behaviors because of the reduction of Ti from 4+ to 3+ (Balachandran and Eror, 1981, Singh et al., 2013). Such reduction is compensated by the release of lattice oxygen from titanates to fulfill the electric neutrality and thereby forming oxygen vacancies (Balachandran and Eror, 1981, De Souza, 2015), enabling these titanates to possess mixed oxygen ionic-electronic conductivity for the applications in electrode for solid oxide fuel cells (SOFCs) (Ruiz-Morales et al., 2006) and electrocatalyst for metal-air batteries (Chen et al., 2015). (3) Furthermore, the ionic radii of Ti4+ and Ti3+ are comparable, which can avoid the large thermal and chemical expansions of titanates and ensure their mechanical and structural integrity under high temperature and reducing atmospheres. (4) The deep reduction of TiOx to metallic state in H2 is thermodynamically unfavorable (Neagu et al., 2013), suggesting superior chemical stability of titanates compared with cobalt- or iron-containing oxides (Calle-Vallejo et al., 2010). (5) Finally, B-site doping of Mg acceptor in titanates is helpful in creating more oxygen vacancies and lowering the sintering temperature (Li et al., 2014). Hence, the facts of the mixed ionic-electronic conductivity of titanates induced by low pO2 environment and their superior chemical stability provide an opportunity to develop a new-generation titanate-based OTM reactor that is very suitable for coupling two oxygen-related reactions in low pO2 environments.

Here, we first report a novel chemical environment-responsive mixed conducting SrMg0.15Zr0.05Ti0.8O3−δ (SMZ-Ti) oxygen transport membrane containing neither cobalt nor iron. With high valency and excellent anti-reduction properties, the Zr doping at the Ti-site of SMZ-Ti was used to dismiss the mismatch between cations for stabilizing the cubic structure. When being exposed to air as shown in Scheme 1, perovskite SMZ-Ti shows very poor mixed conductivity because the change of Ti valence from 4+ to 3+ is highly restricted under high pO2 environment, and thus the Ti-based membrane shows negligible oxygen permeability under high pO2 environment. Once both sides of the SMZ-Ti membrane are subjected to steam and methane, respectively, these low pO2 environments induce the release of lattice oxygen (O∗) from SMZ-Ti, forming oxygen vacancies in the lattice, which are subsequently compensated by the modest reduction of Ti4+ to Ti3+ (Ti4+ + e → Ti3+). These oxygen vacancies and electronic defects induced by the chemical environment enable the dense SMZ-Ti membrane to show a mixed oxygen ionic-electronic conductivity and oxygen permeability, thus allowing the permeation of oxygen produced from water dissociation (H2O → O∗ + H2) on the membrane surface to the other side where it is consumed by POM to produce synthesis gas (CH4 + O∗ → CO + 2 H2). Simultaneously, pure hydrogen as the product of water splitting is obtained. Apart from this chemical environment-induced mixed conductivity, SMZ-Ti also exhibits superior chemical stability under these low pO2 atmospheres because the titanium ions (Ti4+/Ti3+) will not be deeply reduced to metallic state (Ti0), contrasting with the well-known cobalt- or iron-containing membranes. Therefore, these features of SMZ-Ti dovetail exactly with OTM reactors to couple reaction-separation-reaction process, extend the scope of mixed conducting materials to include titanates, and open up new avenues for the design of promising chemically stable membrane materials for use in high-performance membrane reactors under harsh reaction conditions.

Scheme 1.

Scheme 1

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The Exploitation of SMZ-Ti Mixed Conductivity Induced by Low pO2 Environment and the Coupling of Water Splitting with Partial Oxidation of Methane Using an SMZ-Ti Membrane

Results and Discussion

Chemical Stability in Low pO2 Atmospheres

As mentioned earlier, OTM reactors for practical application must exhibit excellent chemical resistance to reducing gases. When coupling two reactions in an OTM reactor, H2 either is formed at the membrane surface by light hydrocarbon conversion process or exits on both sides of membrane. To evaluate the chemical stability against reducing atmosphere, SMZ-Ti membranes were treated in H2 atmosphere, comparing with five typical Co- or Fe-containing OTMs, including BaFe0.4Zr0.2Co0.4O3−δ (BFZ-Co) (Jiang et al., 2010a, Jiang et al., 2010b), Ba0.98Ce0.05Fe0.95O3-δ (BC-Fe) (Li et al., 2016), Sm0.15Ce0.85O1.925 – Sm0.6Sr0.4Al0.3Fe0.7O3−δ (SDC−SSAFe) (Fang et al., 2016, Li et al., 2017), La0.9Ca0.1FeO3−δ (LC-Fe) (Wu et al., 2015), and SrTi0.75Fe0.25O3−δ (ST-Fe) (Schulze-Küppers et al., 2015). Figures 1 and S1 depict the X-ray diffraction (XRD) patterns of SMZ-Ti, BFZ-Co, BC-Fe, SDC – SSAFe, LC-Fe, and ST-Fe membranes before and after H2 treatment. As shown in Figure 1A, the as-synthesized SMZ-Ti membrane reveals a highly crystalline character, indexed as the cubic perovskite phase with space group Pm-3m. A weak peak at around 43° (2θ) could be assigned to MgO (Tkach et al., 2004). The structural evolution of SrMgxZr0.05Ti0.95-xO3-δ with varying Mg contents was investigated (Figure S2), and SrMg0.15Zr0.05Ti0.8O3-δ was selected for the following studies. After annealing in a 20 vol.% H2/Ar atmosphere at 900°C for 24 h, the peaks in XRD pattern in Figure 1B of SMZ-Ti oxide associated with the cubic structure were still maintained, and no new phases were found even when the H2-exposure period was extended to 100 h. In contrast, the cubic perovskite structures of the conventional BFZ-Co, BC-Fe, LC-Fe, and ST-Fe membrane materials were decomposed seriously after exposure to 20 vol.% H2/Ar atmosphere for 24 h (Figures 1B and S1). Similarly, after the same atmosphere exposure, a few impurity phases corresponding to Sr4Fe6O13 (JCPDS no. 78-2403) and Fe metallic (JCPDS no. 87-0721) were also observed in the XRD pattern of the SDC−SSAFe membrane, indicating that this membrane material is still chemically unstable in the low oxygen partial pressure atmosphere. Thus, these chemical stability tests reveal that SMZ-Ti shows superior reduction-tolerant ability compared with the conventional Co- or Fe-containing membranes, which will assure the long-term operating durability of the SMZ-Ti membrane used as a membrane reactor working in reducing atmospheres.

Figure 1.

Figure 1

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XRD Analyses of the Samples

(A) As-synthesized SMZ-Ti, BFZ-Co, BC-Fe, and SDC-SSAFe membrane materials at 950°C (see also Figure S2).

(B) H2-exposed SMZ-Ti, BFZ-Co, BC-Fe, and SDC-SSAFe membrane materials at 900°C (see also Figure S1).

Oxygen Permeability of SMZ-Ti under Different Working Conditions

Besides the superior chemical stability in low pO2, another core requirement for new-generation OTMs is that the membrane should also possess a desirable oxygen permeability in low pO2 environment. Therefore, the oxygen permeation flux of the SMZ-Ti membrane was investigated under four working conditions with different pO2. The order of these working conditions for permeation measurement was Cond. I, Cond. II, Cond. III, and Cond. IV. As shown in Figure 2A, when one side of the membrane was fed with air while helium was swept on the other side (Cond. I), the oxygen permeation flux of the doped perovskite SMZ-Ti was about 0.02 cm3 min−1 cm−2 at 990°C. This value is approximately one order of magnitude higher than that of undoped SrTiO3 membrane exposed to air/argon pO2 gradient at 1000°C (Schulze-Küppers et al., 2015), indicating the positive influence of Mg and/or Zr doping on oxygen permeability of SrTiO3. This observation can be supported by the increased electrical conductivity and reduced activation energy for oxygen transport through Mg-doped SrTiO3 as compared with undoped SrTiO3 (Inoue et al., 1991, McColm and Irvine, 2001). However, the oxygen permeation flux of such titanate-based membrane is still too low under high pO2 because of the presence of air. Once the helium sweep gas was changed to diluted hydrogen (Cond. II), the flux increased dramatically to over 0.1 cm3 min−1 cm−2; continuously climbed up to 0.21 cm3 min−1 cm−2 when both sides of the SMZ-Ti membrane were subjected to steam and methane, respectively (Cond. III); and even reached a value of 0.56 cm3 min−1 cm−2 after introducing some CO2 into the CH4 stream (Cond. IV) to form much lower pO2 at the methane side owing to the DRM reaction (CH4+CO2→2CO+2H2). These distinct differences in oxygen permeation flux of the SMZ-Ti membrane exposed to the above-mentioned four conditions show that pO2 is strongly associated with oxygen permeability of SMZ-Ti.

Figure 2.

Figure 2

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Oxygen Permeability of SMZ-Ti Membrane

Oxygen permeation flux (A) and pO2 at the two sides (B) of SMZ-Ti membranes at 990°C under four different operating conditions (see also Figure S3 and Table S1).

(Cond. I) FAir = 60 cm3 min−1; FHe = 20 cm3 min−1.

(Cond. II) FAir = 15 cm3 min−1; FH2 = 5 cm3 min−1 diluted by 15 cm3 min−1 of helium.

(Cond. III) FH2O = 30 cm3 min−1 carried by 10 cm3 min−1 of helium; FCH4 = 3 cm3 min−1 diluted by 13 cm3 min−1 of helium, 4 cm3 min−1 of N2 as internal standard gas.

(Cond. IV) FH2O = 30 cm3 min−1 carried by 10 cm3 min−1 of helium; FCH4 = 3 cm3 min−1, FCO2 = 1.5 cm3 min−1 diluted by 11.5 cm3 min−1 of helium, 4 cm3 min−1 of N2 as internal standard gas.

To gain insight into the relationship between the pO2 and oxygen permeability of SMZ-Ti, we performed equilibrium pO2 calculation based on the gas components on the opposite sites of the SMZ-Ti membrane at 1 atm and 990°C using Gibbs free energy minimization algorithm on HSC Chemistry 5.0 software (Table S1), the same method as previous studies on the thermodynamic equilibrium for H2O splitting and DRM (Furler et al., 2012, Gardner et al., 2013). As shown in Figure 2B, the pO2 at the two sides of the SMZ-Ti membrane decreases from 10−0.7/10−3.2 atm at Cond. I to 10−9.5/10−18.4 atm at Cond. IV, corresponding to the gradual increase in oxygen permeation flux presented in Figure 2A. Compared with the pO2 at permeate side under Cond. I, a much lower pO2 of 10−15.7 atm under Cond. II is obtained because of the hydrogen gas, which facilitates the formation of electronic defects and oxygen vacancies in SMZ-Ti due to the reduction of Ti4+ to Ti3+, as well as a larger gradient of pO2 across the membrane as driving force, and thereby yields a higher permeation flux of 0.13 cm3 min−1 cm−2. This environment-responsive mixed conductivity of SMZ-Ti is further verified by the increase of oxygen permeation flux of the SMZ-Ti membrane when BOTH sides of the membrane are exposed to H2O and CH4 or H2O and CH4−CO2 atmospheres with lower pO2 (Cond. III and Cond. IV), respectively, even though the pO2 gradient across the membrane in either cases is slightly smaller than that for air/He-H2 gradient (Cond. II). Basically, the pO2 calculated by HSC Chemistry represents an overall average equilibrium amount of oxygen in a whole system. However, if a series of chemical reactions takes place sequentially in the system, the involved components will be unevenly distributed and thus lead to a pO2 gradient throughout the catalyst bed (Chen et al., 2018). In the case of Cond. IV in the present work, a series of catalytic reforming and syngas oxidation reactions occurs at the permeate side of the SMZ-Ti membrane (Figure S3). The CO and H2 produced from dry reforming of CH4 with CO2 at the top zone of catalyst bed can reach the near SMZ-Ti surface and further consume the permeated oxygen species, thus obtaining the highest environment-induced mixed conductivity and oxygen permeation flux. It is necessary to point out that the thickness of the SMZ-Ti membrane used here is 0.7 mm, which means that the permeation flux can be improved by shaping the material into hollow fiber configuration membrane with thin dense layer and larger effective area. Also, the rate-controlling step in this H2O splitting-oxygen separation-catalytic reforming process will be determined in the future, and further enhanced oxygen permeation flux can be expected. Obviously, the oxygen permeation flux of the SMZ-Ti membrane is obviously increasing with decreasing pO2, even though the driving force between feed and permeate sides decreases in particular from Cond. II to Cond. III, which likely results from the chemical environment-induced mixed conductivity of SMZ-Ti. This feature finely matches the working environment of OTM reactor for coupling two reactions.

Electrical Conductivity of SMZ-Ti under Low pO2 Atmospheres

The environment-induced mixed conductivity of the SMZ-Ti membrane can be also validated from the point of the electrical conductivity under different pO2 atmospheres. For this purpose, electrochemical impedance spectra measurements were performed to investigate the electrical conduction behavior of SMZ-Ti. The Nyquist plots of AC impedance measurements of SMZ-Ti under different pO2 at 900°C are shown in Figure 3A. The oxygen partial pressure was controlled by means of Ar-H2-H2O gas mixtures, which are similar to water splitting condition as shown in Figure 1. The typical stabilization time for each impedance measurement condition was over 50 h. A model circuit was used for simulating the impedance data as shown in the inset of Figure 3A, where R is a resistance, L is an inductance, and CPE is a constant phase element. From the simulation results, R1 can be assigned as total resistance across the measured sample, L1 is the inductance from the system setup, R2/CPE2 represents the contact resistance in between sample and electrode and R3/CPE3 is the electrochemical reaction for gas exchange in between sample and atmosphere (Irvine et al., 1990). Based on the Nyquist plots and the model circuit, the electrical conductivity of SMZ-Ti increases gradually with decreasing pO2 according to the H2/H2O mole ratio, showing an n-type conduction behavior. In particular, the resistance of SMZ-Ti under dry 3 vol.% H2/Ar atmosphere was approximately 10 Ω and thereby the electrical conductivity in this case was ∼0.1 S/cm. This value is basically in agreement with the conductivity of SrTiO3 in previous studies (Balachandran and Eror, 1981, Inoue et al., 1991).

Figure 3.

Figure 3

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Electrical Conductivity and XPS Characterization of SMZ-Ti Sample

(A) AC impedance spectroscopy of Pt/SMZ-Ti/Pt as a function of oxygen partial pressure at 900°C, the pO2 was also calculated using HSC Chemistry 5.0 software.

(B) Ti 2p XPS spectra from the fractured surface of the SMZ-Ti sample before and after AC impedance measurement.

After nearly 300 h of the impedance measurement at low pO2 atmospheres, the chemical state of Ti ions in the spent SMZ-Ti sample was studied by X-ray photoelectron spectroscopy (XPS) analysis to compare with the results of the sample before impedance measurement. As shown in Figure 3B, the peaks of the fresh SMZ-Ti sample located at binding energies 458.7 eV (Ti 2p3/2) and 464.4 eV (Ti 2p1/2) are ascribed to Ti4+ in perovskite lattice (Bharti et al., 2016). The Ti 2p shoulder peaks at binding energy 457.7 eV (Ti 2p3/2) and 463.4 eV (Ti 2p1/2) are corresponding to Ti3+ (Wang et al., 2011), which is related to the intrinsic excitation of electrons from the valence bands of Ti4+ to the conduction bands by Mg2+ acceptor doping and thereby resulting in the reduction to Ti3+ to fulfill the electric neutrality criteria (Singh et al., 2013). After impedance measurement, the percentage of Ti3+ in Ti cations for the used SMZ-Ti sample increased significantly compared with the as-prepared sample, revealing the chemical reduction of Ti4+ ions in 3% H2-Ar at 900°C. Also, it can be inferred that such environment-induced Ti4+ reduction will be further enhanced by decreasing the pO2 (e.g. 50 vol.% H2). In addition, no XPS peaks of Ti2+, Ti+, and metallic Ti0 species were observed in the used sample after this long-term exposure to low pO2 atmospheres, indicating that no deep reduction occurred for Ti4+/Ti3+ ions to cause cubic structure distortion. Accordingly, SMZ-Ti has superior reduction tolerance, which is again confirmed by the XRD patterns of the SMZ-Ti sample before and after impedance measurement (Figure S4).

The impedance spectra indicate that the electrical conductivity of SMZ-Ti increases by decreasing the pO2, showing an n-type conduction behavior. Such behavior is related to the modest reduction of Ti4+ to Ti3+ in low pO2 according to the XPS studies of SMZ-Ti sample before and after impedance measurement. Furthermore, this n-type conduction behavior of SMZ-Ti is in accordance with the observed oxygen permeation fluxes of the SMZ-Ti membrane under different working conditions as shown in Figure 2, which again confirms that lower pO2 environment can induce the modest reduction of Ti4+ to Ti3+ in SMZ-Ti, allowing the SMZ-Ti membrane to possess higher mixed conductivity and better oxygen permeability.

Membrane Performance under Reaction Condition

After confirming that the SMZ-Ti membrane possesses superior chemical stability and good oxygen permeation flux under low pO2, we then proceeded to evaluate SMZ-Ti membrane performance under harsh chemical reaction conditions. For this purpose, coupling of water splitting with dry reforming of methane is an ideal model process because it provides a good platform to investigate the tolerance of the SMZ-Ti membrane to harsh chemical environment, including CH4, H2, CO2, and CO, especially CO2, which is another main problem of many perovskite-type OTMs suffering from CO2 erosion via carbonate formation (Yi et al., 2010, Zhang et al., 2017).

Following a 100-h operation at varying conditions, two sides of the SMZ-Ti membrane was then subjected to H2O splitting and dry reforming of CH4 (mole ratio of CH4/CO2 = 2), respectively, and continuously operated at constant condition for another 100 h, as shown in Figure 4A. Throughout this period, CH4 conversion remained at about 72% without any fluctuations, CO selectivity was high (96%–97%), and at the opposite side the H2 production rate stayed roughly at 1.1 cm3 min−1 cm−2. These results indicate that SMZ-Ti was operated steadily when its two sides were subjected to the reaction conditions of water splitting coupled with dry reforming of methane. Noted that the CH4 conversion in this case exceeds the equilibrium value of CH4 conversion according to DRM reaction (CH4 + CO2 → 2CO + 2H2) calculated by HSC Chemistry 5.0 (Figure S5), suggesting that POM also takes place with DRM. The composition of the effluent stream at the permeate side as function of time was provided in Table S2. A typical gas composition of ∼48% He, ∼16.3% N2, ∼3.6% CH4, ∼17.2% H2, ∼14.9% CO, and a small amount of CO2 implies that the possible H2O and/or CO2 formed at the membrane surface were finally converted with unreacted methane to syngas via reforming processes. The carbon balance at this side of the membrane maintained at over 96% within the investigation period. However, it is clear that the mole balance of carbon increased slowly with operating time, suggesting that slight carbon deposition may take place in the catalyst bed. This is confirmed by the thermogravimetric analysis of the Ni/Al2O3 catalyst after an approximately 200-h operation (Figure S6).

Figure 4.

Figure 4

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Membrane Performance of SMZ-Ti under Reaction Conditions

(A) Stable operation of the SMZ-Ti membrane reactor at 990°C. H2O side: FH2O = 30 cm3 min−1, FHe = 10 cm3 min−1; CH4-CO2 side: Ftotal = 20 cm3 min−1 (FCH4 = 3 cm3 min−1, FCO2 = 1.5 cm3 min−1, FN2 = 4 cm3 min−1, FHe = 11.5 cm3 min−1).

(B and C) SEM-EDXS images of the H2O side of the SMZ-Ti membrane after about 200 h operation.

(D and E) SEM-EDXS images of the CH4-CO2 side of the SMZ-Ti membrane after about 200 h operation.

(F and G) SEM-EDXS images of the cross section of the BSCF membrane after treatment at 900°C for 0.5 h in 2.5 vol.% H2 – 75 vol.% H2O-22.5% He atmosphere.

See also Table S2, Figures S5–S10.

Both sides of the spent SMZ-Ti membrane were studied by scanning electron microscope (SEM)-energy-dispersive X-ray spectroscopy (EDXS). The same as the as-prepared SMZ-Ti membrane (Figure S7), the spent membrane was still intact and did not show any physical damage (Figures 4B and 4D). What is more, the Ti element (small white spots) at the two sides of the spent membrane was evenly distributed (Figures 4C and 4E). In contrast to the conventional cobalt-containing Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) membrane treated under simulated H2O splitting condition for only 0.5 h, many small particles of 2–5 μm were observed on the cross section of the membrane (Figure 4F). EDXS results reveal that these particles mainly consisted of cobalt (Figure 4G), indicating that the cobalt ions in BSCF were reduced and exsolved from the perovskite lattice under such low pO2 atmosphere. This is similar to the previous observations of the cobalt-based oxygen transport membrane (Jiang et al., 2010a, Jiang et al., 2010b). The slight carbon enrichment at the CH4-CO2 side of the spent membrane (Figure S8) is probably due to a small amount of coke deposition and/or carbonate formation on the membrane surface. To examine the CO2 tolerance of SMZ-Ti, the SMZ-Ti sample was subjected to annealing in 7.5 vol.% CO2/He at 990°C for 24 h. The cubic structure of CO2-annealed SMZ-Ti remained unchanged, and no obvious carbonate formation was observed (Figure S9). This observation is in contrast with that for BSCF after exposure to CO2 under the same condition. These results clearly demonstrate that SMZ-Ti had better CO2 tolerance than BSCF. Compared with the as-prepared SMZ-Ti membrane, no obvious difference in the XRD patterns of the spent membrane except the strontium silicate diffraction peaks (due to commercial glass sealant used in this work) was found (Figure S10). Thus, all these results confirm that SMZ-Ti is a reduction-tolerant, CO2-stable and high-permeability oxygen transport membrane, holding great promise for a new-generation membrane used as membrane reactors.

Conclusions

We report the first novel chemical environment-induced mixed conducting SMZ-Ti oxygen transport membrane (OTM) containing neither cobalt nor iron. Contrary to the well-known cobalt- or iron-containing membrane suffering from poor reduction tolerance, our results demonstrate that the novel SMZ-Ti membrane exhibits both excellent chemical stability for 100 h in 20 vol.% H2/Ar and environment-induced mixed ionic-electronic conductivity due to the modest reduction of Ti4+ to Ti3+ in low pO2. These features dovetail exactly with OTM reactors to couple two reactions under low pO2, which was also highlighted by coupling water splitting with methane reforming at the opposite sides to simultaneously obtain pure hydrogen and synthesis gas. Our findings of a new class of mixed ionic-electronic conductor can extend the limited choice of OTM materials to include titanates and open up new avenues for the design of promising chemically stable membrane materials for use in high-performance membrane reactors toward green and sustainable chemistry.

Limitations of the Study

In this study, the thickness of the SMZ-Ti membrane used is 700 μm. So, hollow fiber configuration SMZ-Ti membranes with thin dense layer and larger effective area will be developed for further improving the permeation flux.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (21676284, 21506237), the International Partnership Program of the Chinese Academy of Sciences (Grant No. 153937KYSB20180048) and the Grant of DICP & QIBEBT UN201708. G.H. gratefully thanks the support via “Youth Innovation Promotion Association Chinese Academy of Sciences Grant 2018245.” The authors thank Dr. Ivanova Mariya for her support on analyzing the AC impedance spectra. Support in XPS analysis by Dr. Heinrich Hartmann is acknowledged.

Author Contributions

H.J. and G.H. conceived and designed the experiments. G.H. conducted the experiments and summarized the data. W.L. conducted the sample preparation. C.-L.T. performed the modeling of impedance spectra. X.X. undertook the treatment of BSCF sample. S.B. and W.A.M. assisted with the impedance measurement and sample annealing. H.J. supervised the whole work. H.J. and G.H. wrote the manuscript.

Declaration of Interests

The authors declare that they have no competing interests.

Published: September 27, 2019

Footnotes

Supplemental Information can be found online at https://doi.org/10.1016/j.isci.2019.08.032.

Supplemental Information

Document S1. Transparent Methods, Figures S1–S10, and Tables S1 and S2
mmc1.pdf (1.7MB, pdf)

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Associated Data

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Supplementary Materials

Document S1. Transparent Methods, Figures S1–S10, and Tables S1 and S2
mmc1.pdf (1.7MB, pdf)

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