Novel Molten Oxide Membrane for Ultrahigh Purity Oxygen Separation

Research Article www.acsami.org

Novel Molten Oxide Membrane for Ultrahigh Purity Oxygen Separation from Air Valery V. Belousov,* Igor V. Kulbakin, Sergey V. Fedorov, and Anton A. Klimashin Laboratory of Functional Ceramics, A.A. Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, 49 Leninskii Pr., 119334 Moscow, Russia ABSTRACT: We present a novel solid/liquid Co3O4−36 wt % Bi2O3 composite that can be used as molten oxide membrane, MOM (Belousov, V. V. Electrical and Mass Transport Processes in Molten Oxide Membranes. Ionics 22, 2016, 451−469), for ultrahigh purity oxygen separation from air. This membrane material consists of Co3O4 solid grains and intergranular liquid channels (mainly molten Bi2O3). The solid grains conduct electrons, and the intergranular liquid channels predominantly conduct oxygen ions. The liquid channels also provide the membrane material gas tightness and ductility. This last property allows us to deal successfully with the problem of thermal incompatibility. Oxygen and nitrogen permeation fluxes, oxygen ion transport number, and conductivity of the composite were measured by the gas flow, volumetric measurements of the faradaic efficiency, and four-probe dc techniques, accordingly. The membrane material showed the highest oxygen selectivity jO2/jN2 > 105 and sufficient oxygen permeability 2.5 × 10−8 mol cm−1 s−1 at 850 °C. In the range of membrane thicknesses 1.5−3.3 mm, the oxygen permeation rate was controlled by chemical diffusion. The ease of the MOM fabrication, combined with superior oxygen selectivity and competitive oxygen permeability, shows the promise of the membrane material for ultrahigh purity oxygen separation from air. KEYWORDS: MOM, MIEC, oxygen permeation, oxygen selectivity, oxygen separation

1. INTRODUCTION The rapidly expanding market of specialty gases is caused by an increasing number of possible applications. Such gases are of particularly high purity grades (99.999% and higher). Unique properties of specialty gases allow us to increase productivity and reduce costs for many industries (e.g., analytical, pharmaceutical, electronics, petrochemical, etc.).1 In particular, ultrahigh purity oxygen is required for pharmaceuticals and biotechnology as well as for the semiconductor and photovoltaic industries. The infinite oxygen separation factor of mixed ionic− electronic membranes is the major advantage over pressureand temperature-swing adsorption, cryogenic, and porous membranes technologies. Interesting transport properties of mixed-conducting materials were first discussed by Carl Wagner.2 He showed that the mixed ionic−electronic oxide material that was placed between two gases, differing in oxygen partial pressure, can operate as an oxygen separation membrane (Figure 1). Later, Teraoka et al.3 proposed the perovskiterelated mixed conductor LSCF as a membrane material. Bouwmeester and Burggraaf4 and Pena and Fierro5 compiled and reviewed the perovskite-related membrane materials (the reader is referred to these comprehensive reviews). Subsequently, Mazanec et al.6 suggested a concept of dual-phase oxygen-permeable composites. In the case when percolating pathways were formed for each component, the high mixed conductivity of oxygen ions and electrons was observed in solid © 2016 American Chemical Society

Figure 1. Ion and electron transfer through mixed-conducting membrane.

electrolyte/noble metal composites. The mixed-conducting cermets such as YSZ−Pd,7 samarium-doped CeO2−Pd8 and CeO2−Ag,9 and rare-earth-doped Bi2O3−Ag10,11 demonstrate an imposing oxygen permeation rate. Also, the ceramic composites δ-Bi2O3−In2O3 or NiO fabricated by in situ Bi2O3 melt crystallization method,12,13 Ce0.8Gd0.2O2−δ−CoFe2O4,14 and Ce0.8Sm0.2O2−δ−SrCO3Co3O4 show high oxygen permeation rates.15 The potential and practical benefits from the implementation of ceramic membranes in the production processes have been discussed elsewhere.16,17 Received: May 30, 2016 Accepted: August 2, 2016 Published: August 2, 2016 22324

DOI: 10.1021/acsami.6b06357 ACS Appl. Mater. Interfaces 2016, 8, 22324−22329

Research Article

ACS Applied Materials & Interfaces Recently, molten oxide membranes (MOMs) based on the ductile solid/liquid composites have been developed.18−24 These ductile MOM materials are designed as a replacement to the fragile ceramic membrane materials.25 The MOM material includes solid grains and liquid channels at the grain boundaries. Such material is called a liquid-channel−grainboundary structure (LGBS).18,19 The solid grains conduct electrons, and the liquid channels conduct oxygen ions. The liquid channels also provide the membrane material’s gas tightness and ductility. This last property allows us to deal successfully with the problem of thermal incompatibility. There are two types of MOMs. The first type is the LGBS materials based on Bi2O3 (Bi2O3−ZnO, Bi2O3−NiO, and Bi2O3− In2O3).22−24 The second type is the LGBS materials based on V2O5 (V2O5−BiVO4 and V2O5−ZrV2O7).19−21 Transport properties of these MOMs are reviewed in ref 26. Lately, a molten oxide fuel cell (MOFC) based on the TeO2−Bi2Te4O11 LGBS electrolyte has been developed.27 In this paper, we focus on the transport properties of a novel MOM based on the Co3O4−36 wt % Bi2O3 LGBS composite consisting of solid grains Co3O4 and intergranular liquid channels (molten Bi2O3 and Co3O4). The choice of these materials is due to the fact that molten Bi2O3 shows the highest known oxygen ionic conductivity ∼4 S cm−1 and solid Co3O4 has high electronic conductivity ∼35 S cm−1 at 850 °C.28,29 This electronic conductivity is more than 1 order of magnitude higher than that of In2O3, NiO, or ZnO included in the abovementioned MOM materials. The highly conductive Bi2O3 and Co3O4 materials provide the composite’s sufficient ambipolar conductivity of oxygen ions and electrons (if percolating pathways exist for each component) and, therefore, sufficient oxygen permeation rate. The liquid phase provides gas tightness and ductility. An ideal oxygen ion transport membrane requires gas tightness, ductility, and sufficient ambipolar conductivity and surface exchange reaction rate. The use of these two materials enabled us to largely fulfill this requirement.

Figure 2. Phase diagram of Bi2O3−Co3O4.30 the faradaic efficiency were used to calculate the oxygen ion transport number. The conductivity and transport number measurement details were described elsewhere.19 2.3. Oxygen Permeation. The measurement of oxygen permeation flux through the MOM was carried out by the gas-flow technique.19 A disc-shaped membrane (surface area 2.8 cm2, thickness 1.5, 2.0, 2.6, or 3.3 mm) was attached at the end of a quartz tube in such a way that one side of the disc was being exposed to a flow of air, whereas the other was being swept by helium. The electrochemical cell was placed into a furnace and heated to 790−850 °C. After eutectic melting (780 °C in Figure 2), the liquid phase on the sample surface ensured the gas-tight seal. After sealing, the air was being fed at a rate of 2 mL/min, while the other side was being swept by helium at 5−20 mL/min. We measured the oxygen and nitrogen concentrations in the sweeping gas flow using a gas chromatograph (Crystallux-4000M, Russia) after the steady state had been reached. Once the sweeping gas-flow rate had been changed, it resulted in a variation of oxygen partial pressure at the oxygen-lean side of the membrane.

3. RESULTS AND DISCUSSION 3.1. Phase Composition. Figure 3 shows the XRD pattern of the Co3O4−36 wt % Bi2O3 LGBS composite after cooling

2. EXPERIMENTAL SECTION 2.1. Materials. The gas-tight solid/liquid Co3O4−36 wt % Bi2O3 composite material was fabricated by the grain-boundary wetting method.23 The samples were synthesized in two steps. In a first step, we prepared the Co3O4−36 wt % Bi2O3 ceramic composite. For that we ground the mixture of a certain composition of Bi2O3 (99.9%) and Co3O4 (99.9%) powders in a ball mill (Fritsch, Pulverisette 5) and pressed it into cylinders (thickness 1−4 mm, diameter 25 mm) and parallelepipeds (5 × 5 × 40 mm3) using 2 GPa pressure at room temperature. After that, we sintered the samples at 700 °C (this temperature is below the eutectic point, 780 °C in Figure 2)30 in air for 10 h. In a second step, we heated the sintered ceramic composite to the two-phase area of the Bi2O3−Co3O4 phase diagram (temperature range 790−850 °C in Figure 2, solid Co3O4 + liquid). While the ceramic composite was being heated, the grain-boundary wetting by a chemically compatible liquid occurred at the eutectic point. The result was Co3O4−36 wt % Bi2O3 LGBS material, including solid grains of Co3O4 and intergranular liquid channels. 2.2. Characterization. Phase composition of the samples was examined by X-ray diffraction (Shimadzu XRD-6000, Cu Kα radiation) in the range of 2θ from 15° to 60°. The phase identification was carried out using the Joint Committee on Powder Diffraction Standards (JCPDS) files. The sample microstructure was investigated by analytical scanning electron microscopy (JSM-7401F, Japan) of the fracture face. We used the standard four-probe dc technique to measure the sample conductivity at 700−850 °C. The volumetric measurements of

Figure 3. XRD pattern of Co3O4−36 wt % Bi2O3 LGBS composite after cooling from 850 °C.

from 850 °C. In this pattern, the peaks related to the cubic spinel Co3O4 with space group Fd3m, a = b = c = 8.083 Å (JCPDS No. 42−1467), and cubic sillenite Bi88Co3O136 with space group I23, a = b = c = 10.184 Å, phases were identified. The sillenite phase is isostructural to the cubic γ-Bi2O3 with lattice parameter a = 10.250 Å (JCPDS No. 45−1344).31,32 3.2. Microstructure. During the Co3O4−36 wt % Bi2O3 ceramic composite heating into a two-phase area of the Bi2O3− 22325


Research Article

ACS Applied Materials & Interfaces Co3O4 phase diagram (solid Co3O4 + liquid in Figure 2), the grain-boundary wetting by a chemically compatible liquid and LGBS formation occurred. In Bi2O3-based ceramic composites at the eutectic point, the grain-boundary wetting transition has been demonstrated.33 Eventually, the LGBS material, including Co3O4 solid grains and liquid channels at the grain boundaries, is obtained. Microstructure of this LGBS material after cooling from 850 °C is presented in Figure 4. Two constituents with

Figure 4. BSEM image of the fracture face of Co3O4−36 wt % Bi2O3 LGBS composite after cooling from 850 °C.

different color contrast, namely, dark grains and light intergranular layers, are observed on the fracture face. The highlighted areas of grain and intergranular layer from the fracture face, as marked in the SEM images in Figure 5, were selected for EDX analysis. The dark grain (area 1) contains only Co and O, and the light intergranular layer (area 2) also includes Bi on a par with Co and O. On the basis of the Bi2O3− Co3O4 phase diagram and XRD and EDX analysis data, we can conclude that the dark grains are Co3O4 and light intergranular layers consist of Bi88Co3O136 and Co3O4 phases at room temperature. According to the Co3O4−Bi2O3 phase diagram (Figure 2), while the composite is being heated, the Bi88Co3O136 phase decomposes to δ-Bi2O3 and Co3O4 at 760 °C followed by the melting of intergranular layers at 780 °C. As a result, the LGBS material is obtained. The ductility of the LGBS material (Figure 6) is provided by the intergranular liquid channels. 3.3. Conductivity. The temperature dependence of conductivity of the Co3O4−36 wt % Bi2O3 composite is presented in Figure 7 (conductivities of Co3O4 and Bi2O3 for comparison). The contribution of Co3O4 electron conductivity is considerable.29According to the literature data,34,35 the nonlinear nature of the composite conductivity is associated with a normal-inverse spinel transition in Co3O4, which is accompanied by a change in electrical conductivity. Presumably it is connected with the partial change of Co3+ electronic spin states from the spin-paired to the spin-unpaired configuration. At 780 °C, the grain-boundary wetting transition and LGBS formation occurred (Figure 4). The formed liquid channels can provide the oxygen ion transport.36 This is confirmed by the measurements of oxygen ion transport number (Table 1). In a first approximation, ionic conductivity of the intergranular liquid channels, predominantly consisting of molten Bi2O3, could be described by a percolation model using hopping transport.37 In the temperature range 780−850 °C, the

Figure 5. EDX spectra of selected areas of Co3O4−36 wt % Bi2O3 LGBS composite after cooling from 850 °C.

Figure 6. Photograph of (a) initial Co3O4−36 wt % Bi2O3 ceramic composite sintered at 700 °C and (b) after its deformation at 810 °C.

Co3O4−36 wt % Bi2O3 LGBS material is a mixed ion− electronic conductor (MIEC). However, the contribution of ionic conductivity is not considerable (Table 1). The ionic conductivity could be increased by increasing the volume fraction of liquid, but it will significantly reduce the durability of the membrane material. The volume fraction of liquid cannot 22326


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where P′O2 and P″O2 are the oxygen partial pressures at oxygenrich and -lean sides of the membrane, jO2 is the oxygen permeation flux through the membrane, L is the membrane thickness, R is the universal gas constant, F is the Faraday constant, T is the temperature, and σamb is the ambipolar conductivity of oxygen ions and electrons: σiσe σamb = σi + σe (2) Here σe and σi are the electronic and ionic conductivities, respectively. Usually, chemical diffusion controls oxygen permeation if the membrane material is thick. In the event of the thin membranes, oxygen permeation is limited by both chemical diffusion and surface exchange (mixed controlled kinetics). While the membrane thickness is decreasing, the limited passing through the surfaces becomes rate determining. In this case, we can describe the oxygen permeation flux by eq 338

Figure 7. Temperature dependences of conductivities of Co3O4−36 wt % Bi2O3, Co3O4, and Bi2O3.

Table 1. Values of Oxygen Ion Transport Number (ti), Ionic (σi), and Ambipolar (σamb) Conductivities, and Calculated (jO2,theor) and Experimental (jO2,exp) Oxygen Permeation Fluxes (L = 2.0 mm) T °C

ti

790 810 830 850

0.105 0.080 0.060 0.045

σi S cm−1 0.72 0.78 0.82 0.88

σamb S cm−1 0.64 0.72 0.77 0.84

jO2,exp mol cm−2 s−1 1.2 1.6 1.7 1.9

× × × ×

−7

10 10−7 10−7 10−7

jO2,theor mol cm−2 s−1 2.0 2.7 2.8 3.0

× × × ×

−7

10 10−7 10−7 10−7

ln

jO = 2

′ PO 2

PO′ RT σamb ln 2 PO″2 16F (L + 2Lc) 2

(3)

where Lc is the membrane characteristic thickness. When the membrane becomes thinner than Lc, permeation due to the surface exchange reactions become slower than that by chemical diffusion. The oxygen permeation occurs in three stages (Figure 1): (1) At the air/membrane interface, the molecules of oxygen dissociate into oxygen anions (surface exchange kinetics). (2) Oxygen anions migrate through the bulk of membrane (chemical diffusion). (3) At the membrane/ oxygen interface oxygen anions combine back to oxygen molecules (surface exchange kinetics). Each stage can be a rate-limiting step of the entire permeation kinetics. In order to establish which one is, we measured the oxygen flux depending on the thickness of membrane (Figure 9). If oxygen permeation through the

″ PO 2

1.38 1.61 1.52 1.50

exceed 35%. It is caused by the demand for the mechanical strength of the LGBS membrane material. 3.4. Oxygen Permeation. Figure 8 presents the oxygen permeation flux through the Co3O4−36 wt % Bi2O3 LGBS as a

Figure 8. Dependence of oxygen permeation flux of Co3O4−36 wt % Bi2O3 LGBS composite on oxygen partial pressure gradient at 790− 850 °C, L = 2 mm. Figure 9. Thickness dependence of oxygen permeation flux of Co3O4−36 wt % Bi2O3 LGBS composite; log(P′O2/P″O2) = 0.5.

function of oxygen partial pressure logarithm difference ln(P′O2/ PO″ 2) between the sides of the membrane. Chemical diffusion or surface exchange reactions can control the overall oxygen permeation kinetics.20 In order to describe the oxygen permeation flux, when chemical diffusion limits the rate of oxygen transport, Wagner’s equation is used jO = 2

PO′ RT σamb ln 2 2 PO″2 16F L

membrane obeys eq 1, the oxygen flux should be inversely proportional to the membrane thickness. Oxygen permeation flux depending on the thickness of Co3O4−36 wt % Bi2O3 LGBS membrane is linear (Figure 9). Consequently, it is the chemical diffusion that controls the entire permeation kinetics. In Table 1 we present the computed (by eq 1) and experimental values of the oxygen permeation fluxes. Chemical

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4. CONCLUSIONS Here we report an innovative Co3O4−36 wt % Bi2O3 MOM material for high purity oxygen separation from air. The components of this composite material were selected due to their high electronic and ionic conductivities in order to achieve high ambipolar conductivity. This MOM is a new material, including electron-conducting solid grains and oxygen ionconducting intergranular liquid channels, in contrast with usual ceramic composite membranes. The liquid channels also provide the membrane material gas tightness and ductility. This last property allows us to deal successfully with the problem of thermal incompatibility. The ductile MOM materials are designed as a replacement for the fragile ceramic membrane materials. The gas-tight ductile solid/liquid Co3O4− 36 wt % Bi2O3 composite was fabricated using the grainboundary wetting technique. Transport properties (conductivity, oxygen ion transference number, and oxygen permeability and selectivity) and microstructure of the composite were studied. This composite (volume fraction of liquid ∼30%) has shown competitive oxygen permeability and superior oxygen selectivity. Due to the requirements for mechanical properties of the composite, the volume fraction of liquid should not exceed 35%. In the range of 1.5−3.3 mm composite thicknesses, used in this study, the oxygen permeation flux was described by Wagner’s equation. The oxygen ion transfer in intergranular liquid channels was the rate-limiting step. A surface exchange reaction rate was sufficient because of the presence of Co3O4. The Co3O4−36 wt % Bi2O3 MOM material shows promise for technological exploitation, since it exhibits high oxygen permeability and selectivity, resistance to oxygen electrochemical potential gradient, and it consists of chemically compatible components, and has a low cost of fabrication.

diffusion controls the oxygen permeation rate, because represented values (Table 1) are of the same order of magnitude. Thus, oxygen ion transport turned out to be the rate-limiting step of the mechanism of oxygen permeation through Co3O4−36 wt % Bi2O3 LGBS material. 3.5. Oxygen Selectivity. Chromatograms of the gas permeation fluxes are presented in Figure 10. A chromatogram

Figure 10. (a) Chromatogram of the gas permeation flux of the Co3O4−36 wt % Bi2O3 ceramic composite (the chromatogram confirms that this ceramic material is not gas-tight) and (b) chromatogram of gas permeation flux of Co3O4−36 wt % Bi2O3 LGBS composite (the chromatogram confirms that this LGBS material is gas-tight and oxygen-permeable).

in Figure 10 b confirms that the Co3O4−36 wt % Bi2O3 LGBS material is oxygen-permeable. As the nitrogen permeation flux (jN2) through this material is negligible (Figure 10 b), the oxygen selectivity jO2/jN2 tends to infinity. Assuming that the nitrogen concentration in the gas-permeation flux is below the detection limit of the gas chromatograph (10−8 g mL−1), we can evaluate the selectivity as jO2/jN2 > 105. This is possible, if the air nitrogen does not dissolve in the molten oxide providing gas tightness. 3.6. Performance. To evaluate the membrane material productivity, the oxygen permeability (j*O2) was calculated using formula 4 jO* = jO 2

2

L ln(PO′ 2 /PO″2)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +7-499-135-2060. Fax: +7-499-135-4513. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Russian Science Foundation, Project No.16-19-10608.

■

(4)

The value of oxygen permeability of Co3O4−36 wt % Bi2O3 LGBS membrane material, presented in Table 2, is comparable with the modern membrane materials.

REFERENCES

(1) Kerry, F. G. Industrial Gas Handbook: Gas Separation and Purification; CRS Press, Taylor & Francis Group: Boca Raton, FL, 2007.

Table 2. Values of Oxygen Permeability (j*O2) for Ceramic (CER), Cermet (CM), and Molten (MOM) Membranes membrane Pr0.6Sr0.4Co0.5Fe0.5O3−δ (CER) Ba0.5Sr0.5Co0.8Fe0.2O3−δ (CER) La0.6Ca0.4Co0.8Fe0.2O3−δ (CER) In2O3−55 wt % Bi2O3(CER) (Bi2O3)0.75(Er2O3)0.25− 40 vol % Ag (CM) In2O3−48 wt % Bi2O3 (MOM) Co3O4−36 wt % Bi2O3 (MOM)

T °C 900 900 900 800 680 850 850

jO2 mol cm−2 s−1 2.3 1.1 1.2 5.3 1.8 8.7 1.9

× × × × × × ×

−7

10 10−6 10−7 10−8 10−7 10−8 10−7

L mm 0.6 1.2 1.0 2.9 1.3 2.6 2.0

22328

P′O2 atm 0.21 0.21 0.20 0.21 1.00 0.21 0.21

P″O2 atm 0.001 0.0065 0.0053 0.054 0.000002 0.038 0.047

j*O2 mol cm−1 s−1 2.6 3.8 3.3 1.1 1.8 1.3 2.5

× × × × × × ×

−9

10 10−8 10−9 10−8 10−9 10−8 10−8

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Novel Molten Oxide Membrane for Ultrahigh Purity Oxygen Separation

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