WO2012145531A2

WO2012145531A2 - Ion conductive multilayer structure

Info

Publication number: WO2012145531A2
Application number: PCT/US2012/034285
Authority: WO
Inventors: Chonglin Chen; Jian Liu; Gregory Roy COLLINS
Original assignee: Broard Of Regents Of The University Of Texas System
Priority date: 2011-04-21
Filing date: 2012-04-19
Publication date: 2012-10-26
Also published as: WO2012145531A3; WO2012145531A8

Abstract

A novel ion conductive material may be formed as a thin-film multilayered structure. The thin-film multilayered structure may be formed using multiple ion conductive sublayers of an electronic insulator and an ionic conductor. A nanometer-range thickness of the electronic insulator may be selected to provide desirable ion conductive properties that may be different from bulk material properties due to quantum size effects. The thin-film multilayered structure may behave in aggregate as an electronic insulator while behaving as an ion conductor at relatively low temperatures. The thin-film multilayered structure may be used as an electrolyte in a solid-oxide fuel cell (SOFC) for conduction of oxygen ions at relatively low temperatures, for example, less than about 600°C.

Description

ION CONDUCTIVE MULTILAYER STRUCTURE CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. Provisional Patent Application No. 61/477,959, filed on April 21, 2011, entitled ' 'MULTILA YERED ELECTROLYTE FOR SOLID- OXIDE FUEL CELLS", which is incorporated by reference herein in its entirety.

BACKGROUND

Field of the Disclosure

[0002] This specification relates to the field of ion conductive electrolyte materials, and more particularly to a multilayer structure for ion conductivity.

Description of the Related Art

[0003] Ion conductive materials are used in applications where conductivity of ionic species through a solid state phase is desired. The ionic conductivity may be specific to a particular ionic species for a given ion conductive material. The ionic conductivity may also be selective for ionic species in contrast to electronic conduction. Ion conductive materials may be used as membranes to facilitate ion exchange processes and/or reactions, such as an electrolyte in an electrochemical cell.

[0004] In one illustrative example, a solid-oxide fuel cell (SOFC) uses an ion conductive electrolyte, or SOFC electrolyte, to selectively conduct oxygen ions in order to generate electrical current. A desirable SOFC electrolyte may be selected based on a high ionic conductivity and a low electronic conductivity for efficient operation of the SOFC. SOFCs are often operated at high temperatures up to about 1100°C in order to attain desired ion conductive properties of the SOFC electrolyte. A high operating temperature of an SOFC is energetically disadvantageous and may be associated with a range of deleterious material properties that can shorten the lifetime of SOFC components.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is a block diagram of a prior art SOFC; [0006] FIG. 2 is a block diagram of selected elements of an embodiment of a novel and patentably distinct ion conductive material;

[0007] FIG. 3 is an image of a multilayer thin film;

[0008] FIG. 4 is a block diagram of selected elements of an embodiment of a novel SOFC stack; and

[0009] FIG. 5 is a flowchart disclosing an exemplary method of forming a novel ion conductive material.

DESCRIPTION OF THE EMBODIMENT(S)

[0010] The present disclosure pertains to a novel ion conductive material that is suitable for various applications where improved ionic conductivity is desired through a solid state structure, such as a membrane. In certain embodiments, an improvement in the ionic conductivity may be manifest as a lower temperature for a given ionic conductivity. Exemplary embodiments disclosed herein are described in the context of an electrolyte suitable for conducting oxygen ions in an SOFC. It will be understood that the SOFC embodiments are intended as descriptive, yet non-limiting, examples and that the novel ion conductive material disclosed herein may be used to advantageously conduct various types of ions in a number of different applications, as desired, including electrochemical electrolytes, ion exchange membranes, purification membranes, decontamination membranes, and ion exchange chromatography, among others. In various embodiments, the novel ion conductive material described herein may be configured to transport any of a number of ion species, including: O^", O^2" (oxygen anion); H⁺ (proton); OH^" (hydroxide ion); single-charged monoatomic ions, such as Na⁺,K⁺, CI^"; double-charged monoatomic ions, such as Ca²⁺, Mg²⁺; polyatomic inorganic ions; and organic ions.

[0011] Throughout this disclosure, a hyphenated form of a reference numeral refers to a specific instance of an element and the un-hyphenated form of the reference numeral refers to the element generically or collectively. Thus, for example, widget 12-1 refers to an instance of a widget class, which may be referred to collectively as widgets 12 and any one of which may be referred to generically as a widget 12.

[0012] As shown in FIG. 1, a prior art SOFC employs an electrochemical reduction- oxidation (redox) reaction of a gaseous fuel with oxygen to generate electrical current and water exothermically. The gaseous fuel may be a vaporized hydrocarbon, hydrogen, or other fuel that is oxidized at an anode separated by the SOFC electrolyte from a cathode, where reduction of oxygen from a supplied source, such as air, occurs. Due to the ion conductive property of the SOFC electrolyte, oxygen ions are drawn from the cathode by diffusion to react with the fuel at the anode, which provides a source of the voltage potential across the anode and the cathode. Electronic current flowing through an external circuit across the anode and the cathode provides electrical energy output and a pathway for the redox reaction to be sustained in the SOFC.

[0013] In the prior art SOFC of FIG. 1, desired properties of the anode material and the cathode material include high porosity, high electrical conductivity and high ionic conductivity, which favor the redox reaction. In contrast, desired properties of the SOFC electrolyte include high ionic conductivity but low electrical conductivity, in order to prevent current leakage that would inhibit transport phenomena associated with the redox reaction, as described above. The SOFC electrolyte should also be non-porous to prevent mixing of the fuel (i.e., organic compounds) and oxidant gas feeds. Many conventional SOFC electrolyte materials exhibit desirable ion conduction only at relatively high temperatures, that is, well above 600°C and up to about 1100°C. As shown in FIG. 1 , a conventional SOFC may be formed with a solid-state sandwich comprising three individual layers: the anode, the cathode and the SOFC electrolyte.

[0014] The anode of an SOFC may be composed of Ni (nickel), Cu (copper), Co (cobalt), Ru (ruthinium), which may be dispersed with a ceramic particulate, such as YSZ (yttria-stabilized zirconia), or Ce0₂ (ceria), to form a porous cermet. In certain instances, the anode may further be doped with a noble metal, such as Mo (molybdenum), Au (gold), Ru, and Li (lithium), among others. An alloy composition, such as Ni-Cu, Ni-Co, Cu-Co, may also be used for a metallic fraction of the anode. Among the known anode materials for use with SOFCs are perovskites, specifically titanates and chromates, which may be doped with Sr (strontium), La (lanthanum), Mn (manganese), Ga (gallium), Gd (gadolinium), Y (yttrium), Ni (niobium), Fe (iron), Co, Ni, and Cu.

[0015] Materials that have been used for the cathode in an SOFC include perovskite oxides, of the form ABO₃, where A may be Ba (barium), La, Sr and/or Ga; while B may be Co, Fe, Mn and/or Mg.

[0016] In prior art SOFC configurations, as shown in FIG. 1, the SOFC electrolyte has been formed as a single layer of ceramic material. Conventional SOFC electrolytes have been formed using specialized ceramic powders which are sintered to achieve a desired uniform microstructure of perovskite and/or fluorite structure types. Among the known fluorite structures for SOFC electrolytes are Gd-doped ceria (Gd:Ce0₂ or GCO), Sm (samarium)-stabilized ceria (Sm:Ce0₂ or SCO), and yttria- stabilized zirconia (Y:Zr0₂ or YSZ). In particular, YSZ in bulk form is a good ionic conductor with very good electronic insulating properties and has been used for SOFC electrolyte applications. A commonly used formulation in bulk form is YSZ having 8 mol % Y₂0₃ content. As with various other known SOFC electrolytes, ion conduction of bulk YSZ to a degree feasible for SOFC operation (i.e., high oxide ion conductivity sufficient to efficiently sustain the redox reaction) has been observed only at relatively high temperatures.

[0017] Due to the high operating temperature associated with certain conventional SOFC electrolytes, such as bulk YSZ, unwanted chemical or physical degradation may occur at an interlayer region, such as at a boundary region where the anode meets the SOFC electrolyte. For example, interdiffusion between the individual SOFC components may be promoted (i.e., energetically favorable) at higher temperatures, which may result in an undesired change in chemical composition, along with a corresponding undesired change in properties, such as reduced ion conductivity. Such degradation may substantially shorten the lifetime and/or long term stability of the entire SOFC, and therefore, may represent a significant negative factor for the economic viability of the SOFC. Therefore, reduction in the operating temperature may represent an important factor in sustaining a long lifetime for the SOFC components, which may have a major positive impact in the overall economic viability of such an SOFC.

[0018] Turning now to FIG. 2, selected elements of an embodiment of a novel and patentably distinct ion conductive material 200 are shown. Ion conductive material 200 is comprised of a multilayer of alternating phases: insulating phase 202 and conducting phase 204. Insulating phase 202 is a material that is a good electronic insulator, while conducting phase 204 is a material that is a good electronic conductor. Since insulating phase 202 is thoroughly interspersed between layers of conducting phase 204, the overall electrical conduction of ion conductive material 200 may be sufficiently low such that, in aggregate, ion conductive material 200 may be considered to be an insulating film or membrane.

[0019] In terms of ionic conductivity, both insulating phase 202 and conducting phase 204 may exhibit very good ionic conductivity. The ion conduction property of insulating phase 202 and/or conducting phase 204 may result from their respective material composition and may be a function of temperature, environment, or dependent on a particular ion species that is diffused or conducted through ion conductive material 200. The ion conduction property of insulating phase 202 and/or conducting phase 204 may also be a function of layer thickness 212 and/or 214, respectively, as will now be described in further detail.

[0020] As shown in FIG. 2, ion conductive material 200 may be formed using a deposition process to deposit, or grow, insulating phase 202 and conducting phase 204, in an alternating manner. The deposition process used to manufacture ion conductive material 200 may be any of a number of known thin-film deposition processes, such as pulsed laser deposition (PLD), RF/plasma deposition (sputtering), molecular beam epitaxy (MBE), cathodic arc deposition (arc-PVD), or electron beam evaporation, among other types of physical vapor deposition (PVD). Depending on the desired composition of insulating phase 202 and/or conducting phase 204, other thin-film deposition processes, such as chemical vapor deposition (CVD) or other types of chemical deposition may be used to form ion conductive material 200. It is noted that a deposition process used to grow insulating phase 202 and/or conducting phase 204 may result in a highly non-porous composite form of ion conductive material 200. It is further noted that a deposition process used to grow insulating phase 202 and/or conducting phase 204 may result in substantially pure individual layers of a desired chemical composition that have not been adulterated or contaminated in an undesired manner.

[0021] As a result of ion conductive material 200 being formed with a suitable deposition process, layer thickness 212 and/or 214 may be controllably dimensioned to be extremely small, down to a range of about 5-10 nanometers (nm). Due to the observation that material properties in the nanometer scale may be governed by quantum mechanics (also known as the quantum size effect), substantially different electronic and/or ionic transport properties may be observed and exploited, even when the underlying mechanisms are not yet well understood. One factor that may govern quantum size effects in nanoscale films is the vastly increased ratio of surface area to volume, similar to that of nanoparticles. For example, ion conductive behavior of insulating phase 202 may be drastically and unexpectedly different when deposited as a nanofilm than was previously observed for the same material in bulk form, which may impart significantly favorable properties to ion conductive material 200. In certain instances, a temperature dependence of ion conductive behavior may be observed to be different in a deposited nanofilm as compared to the bulk material form. It is therefore noted that different values fnr a nhvsical dimension of a nanoscale thin-film (e.g., thickness) may drastically change and/or govern material properties, in particular, electronic and/or ion transport properties.

[0022] Thus, in one embodiment of ion conductive material 200, layer thickness 212 of insulating phase 202 may be about 10 nm, while layer thickness 214 of conducting phase 204 may be about 100 nm. When insulating phase 202 is formed as a first fluorite-type ceramic material, ion conduction of insulating phase 202 having about 10 nm layer thickness 212 may be significantly improved in comparison to bulk properties of that same material (e.g., for dimensions greater than about 1 μιη). The improvement in ion conduction may be manifested as a much higher ion conductivity at a lower temperature, or more generally, as higher ion conductivity versus temperature. When conducting phase 204 is formed as a second fluorite-type ceramic material exhibiting substantially higher ion conduction than insulating phase 202, the overall ion conduction of ion conductive material 200 may be significantly improved in comparison to a bulk material comprised of the first fluorite-type material. When insulating phase 202 retains low electronic conductivity, even at layer thickness 212 of about 10 nm, then ion conductive material 200 may behave in aggregate as an insulator, even though conducting phase 204 is a good electronic conductor. It is noted that similar properties, or analogous changes in properties, may be observed with perovskite-type ceramic materials formed in the nanometer scale for insulating phase 202 and/or conducting phase 204.

[0023] As shown in FIG. 2, ion conductive material 200 is depicted having two (2) alternating layers of insulating phase 202 and conductive phase 204. As used herein, sublayer 210 of ion conductive material 200 may refer to a single instance of insulating phase 202 and conductive phase 204. In a simplest configuration, ion conductive material 200 may be configured with two sublayers 210, as shown in FIG. 2. As will be described in further detail below, configurations of ion conductive material 200 having different numbers of sublayers 210, as well as configurations terminating on two sides with an instance of insulating phase 202, may be implemented in different embodiments, as desired. The number of alternating layers as well as a relative thicknesses of each layer may be varied or 'tuned' or 'modulated' to achieve desired aggregate properties of ion conductive material 200, in particular, with respect to desired ion conductivity and/or electronic conductivity. [0024] Referring now to FIG. 3, a transmission-electron microscopy image of an embodiment of an ion conductive multilayer film having alternating layers of barium titanate and strontium titanate is shown. In FIG. 3, the non-porous multilayer film is shown having 12 sublayers that are less than about 100 nm in thickness and is illustrative of various types of ceramic thin-films that may represent ion conductive materials, as described herein.

[0025] Turning now to FIG. 4, one embodiment of selected elements of SOFC stack 400 employing novel and non-obvious SOFC electrolyte 410 is depicted. In various embodiments, SOFC electrolyte 410 may be formed with ion conductive material 200 (see FIG. 2) and may represent a configuration suitable for use as an SOFC electrolyte at a relatively low temperature. As shown, SOFC 400 includes anode 406 and cathode 408. In one embodiment, anode 406 and cathode 408 are formed using a new double -perovskite oxide including Pr (praseodymium), namely PrBaCo₂0₅₊₀ or PBCO. In another embodiment, cathode 408 is formed using PBCO while anode 406 is formed using Ni/YSZ cermet. SOFC electrolyte 410 is shown comprising four (4) sublayers 420 of insulating phase 402 and conducting phase 404, along with terminating insulating phase 402-1 at anode 406. In one embodiment of SOFC electrolyte 410, insulating phase 402 may be formed from YSZ having about 10 nm thickness, while conducting phase 404 may be formed from GCO having about 100 nm thickness (not drawn to scale in FIG. 4) in which a ratio of Gd:Ce is about 0.25. In such a configuration, SOFC stack 400 may be configured to sustain the redox reaction at temperatures as low as 400°C. It is noted that, in particular embodiments, SOFC stack 400 may be configured with six (6), eight (8), twelve (12), sixteen (16) or more sublayers 420, among other desired configurations.

[0026] As shown in FIGS. 2 and 4, respective composite material layers of ion conductive material 200 and SOFC electrolyte 410 are shown having uniform thickness for descriptive clarity. In different embodiments, a uniformity of respective material layers of ion conductive material 200 and SOFC electrolyte 410 may vary with an acceptable variance, for example, such as within about 1 nm. In some embodiments, a desired thickness of a particular type of material layer may vary across ion conductive material 200 and SOFC electrolyte 410. For example, different instances of insulating phase 402 may be grown to different thickness, such as 10 nm, 8, nm, 5 nm, 20 nm, etc., within a single instance of SOFC electrolyte 410. [0027] Turning now to FIG. 5, an embodiment of novel method 500 for producing an ion conductive material is illustrated in flow chart form. In one embodiment, method 500 is performed using thin-film deposition equipment, as described above. It is noted that certain operations described in method 500 may be optional or may be rearranged in different embodiments. Method 500 may begin by growing (operation 502) a first material layer. The first material layer may be a nanoscale film, such as insulating phase 202 (see FIG. 2). Then, a second material layer may be grown (operation 504). A deposition process, as described previously herein, may be employed to grow material layers in operations 502 and 504. An individual layer thickness, which may be in the nanoscale range, of material layers formed in operations 502 and 504 may be tuned for desired properties, in particular, for desired ion conductivity and/or for desired electronic conductivity.

[0028] Next in method 500, a decision may be made whether a desired number of sublayers has been formed (operation 506). When the result of operation 506 is NO, method 500 may loop back to operation 502 and operation 504 for growing additional material layers. When the result of operation 506 is YES, a terminal layer may be grown (operation 508). The terminal layer may be another instance of the first or second material layers, or may be a different material than the first and/or second material layers. It is noted that, in certain embodiments, operation 508 may be omitted from method 500. In various embodiments, additional operations for growing material layers of desired composition and thickness may be employed in conjunction with method 500 to form a desired structure, such as an SOFC electrolyte.

[0029] While the subject of this specification has been described in connection with one or more exemplary embodiments, it is not intended to limit the claims to the particular forms set forth. On the contrary, the appended claims are intended to cover such alternatives, modifications and equivalents as may be included within their spirit and scope.

Claims

WHAT IS CLAIMED IS:

1. A thin-film ion conductive structure, comprising:

N number of sublayers respectively formed with an insulating layer adjacent to a conductive layer, wherein N is greater than or equal to 2; wherein the insulating layer is a non-porous electronic insulator and the conductive layer is a non-porous electronic conductor; and wherein the thin-film ion conductive material behaves in aggregate as an electronic insulator across the N sublayers.

2. The thin-film ion conductive structure of claim 1, wherein the insulating layer includes yttria-stabilized zirconia (YSZ) and the conductive layer includes gadolinium doped ceria (GCO).

3. The thin-film ion conductive structure of claim 2, wherein the YSZ includes about 8 mol % Y₂O₃, and wherein a ratio of gadolinium to cerium is about 0.25.

4. The thin-film ion conductive structure of claim 1, further comprising: a terminal layer formed adjacent to a conductive layer of the Nth sublayer, the terminal layer comprising a material identical to the insulating layer.

5. The thin-film ion conductive structure of claim 1, further comprising: a cathode layer formed adjacent to the insulating layer of a first sublayer; and an anode layer formed adjacent to one of: a terminal layer and a conductive layer of the Nth sublayer.

6. The thin-film ion conductive structure of claim 5, wherein the cathode layer includes PrBaCo₂0₅₊₀, and wherein the anode layer includes a nickel/YSZ cermet.

7. The thin-film ion conductive structure of claim 1, wherein a thickness of at least some of the insulating layers is selected to be ion conductive at less than about 600°C.

8. The thin-film ion conductive structure of claim 7, wherein the thickness is about 10 nm.

9. The thin-film ion conductive structure of claim 1, wherein a thickness of at least some of the conducting layers is about 100 nm.

10. The thin-film ion conductive structure of claim 1, wherein insulating layers are conductive to oxide ions at less than about 600°C.

11. The thin-film ion conductive structure of claim 1, wherein N is a value between 8 and 12.

12. The thin-film ion conductive structure of claim 1, wherein the N sublayers are formed using a physical vapor deposition process.

13. A method of fabricating a thin-film ion conductive material: depositing a plurality of sublayers, wherein a sublayer includes: a first layer of a first material that is an electronic insulator; and a second layer of a second material that is an electronic conductor; wherein the first layer and the second layer are non-porous to organic compounds; and wherein the first layer is deposited at a thickness that enables high oxygen ion conductivity at a temperature less than about 600°C.

The method of claim 13, wherein the first material is yttria-stabilized zirconia (YSZ).

The method of claim 13, wherein the second material is gadolinium doped ceria (GCO).

16. The method of claim 13, wherein the thickness of the first layer is about 10 nm; and wherein a thickness of the second layer is about 100 nm.

17. The method of claim 13, further comprising: forming an anode layer and a cathode layer at opposite surfaces of the thin-film ion conductive material.

18. The method of claim 13, further comprising: forming a terminal layer consisting of the first material adjacent to the second layer of a terminal sublayer.

19. A solid-oxide fuel cell (SOFC) electrolyte, comprising:

N number of sublayers formed with an insulating layer adjacent to a conductive layer, wherein N is a value between 2 and 18; wherein each insulating layer is: a non-porous electronic insulator; ion conductive to oxide ions at less than about 600°C; and about 10 nm thick; wherein each conductive layer is: a non-porous electronic conductor; ion conductive to oxide ions at less than about 600°C; and about 100 nm thick; wherein the SOFC electrolyte behaves as an electronic insulator in aggregate across the N sublayers.

20. The SOFC electrolyte of claim 19, wherein each insulating layer consists of yttria-stabilized zirconia (YSZ) having about 8 mol % Y₂0₃; and wherein each conductive layer consists of gadolinium doped ceria (GCO) having a ratio of eadolinium to cerium of about 0.25.