WO2011008275A2 - Systems and methods for enhancing the surface exchange of oxygen - Google Patents

Abstract

Systems and methods related to the enhancement of the surface exchange of oxygen are provided. The invention involves, in some embodiments, affecting and/or introducing a strain (e.g., a tensile or compressive strain) to a material constructed and arranged for the surface exchange of oxygen. The introduction and/or affecting of the strain into such a material may enhance the rate at which oxygen reacts in a surface exchange reaction at the surface of the material, which can enhance the rate at which the oxygen (including oxygen itself, or an oxidized or reduced form of oxygen such as O²) permeates the material. For example, in some embodiments, the material constructed and arranged for the surface exchange of oxygen may comprise an oxide, and, after applying a strain to the material, the rate of a surface exchange reaction between the oxygen and the material (e.g., an oxygen reduction reaction to produce, for example, O²) may increase, relative to the rate that would be observed using an unstrained material. By enhancing the rate at which the surface exchange reaction takes place, a relatively large amount of O² might be present at the material surface, which can lead to an enhancement in the rate of transport of the oxygen (optionally in an ionic form such as O²) through the material.

Description

SYSTEMS AND METHODS FOR ENHANCING THE SURFACE EXCHANGE

OF OXYGEN

GOVERNMENT SPONSORSHIP

This invention was made with government support under Grant No. NSF-CBET-

0844526 awarded by the National Science Foundation. The government has certain rights in the invention.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application Serial

No. 61/225,259, filed July 14, 2009, entitled "Systems and Methods Related to Strained Materials," by Ia O', et α/., which is incorporated herein by reference in its entirety for all purposes.

FIELD OF INVENTION

Systems and methods for enhancing the surface exchange of oxygen are generally described.

BACKGROUND

Surface exchange refers to the process by which a species is oxidized or reduced at an interface between two media. Surface exchange of oxygen can play an important role in a variety of processes. For example, the rate of surface exchange of oxygen at a fuel cell cathode can affect the amount of power produced by the fuel cell. As another example, the surface exchange of oxygen at an oxygen-separation membrane surface can determine how quickly oxygen can be separated from another species. The ability to increase the rate of surface exchange reactions can enhance the performance of such systems. For example, increasing the surface exchange rate of oxygen at a fuel cell cathode can increase the amount of power produced by a fuel cell. Accordingly, compositions, systems, and methods for controlling the rate of surface exchange reactions are generally desirable.

SUMMARY OF THE INVENTION

The invention described herein generally relates to systems and methods for enhancing, controlling, and/or otherwise affecting the surface exchange of oxygen. The subject matter of the present invention involves, in some cases, interrelated products, alternative solutions to a particular problem, and/or a plurality of different uses of one or more systems and/or articles.

In one aspect, an article is described. In some embodiments, the article can comprise a material constructed and arranged for the surface exchange of oxygen, wherein the electrical surface exchange coefficient of the material under a strain with respect to oxygen is greater than the electrical surface exchange coefficient of an unstrained material with respect to oxygen under essentially identical conditions.

In another aspect, a method is described. The method can comprise, in some cases, introducing a strain to a material constructed and arranged for the surface exchange of oxygen, and exposing the material to oxygen such that, upon contacting the material, the oxygen undergoes a surface exchange reaction with the material at a rate greater than the rate of the surface exchange reaction of the oxygen with an unstrained material under essentially identical conditions.

Other advantages and novel features of the present invention will become apparent from the following detailed description of various non-limiting embodiments of the invention when considered in conjunction with the accompanying figures. In cases where the present specification and a document incorporated by reference include conflicting and/or inconsistent disclosure, the present specification shall control. If two or more documents incorporated by reference include conflicting and/or inconsistent disclosure with respect to each other, then the document having the later effective date shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. In the figures: FIGS. IA- II include, according to one set of embodiments, exemplary schematic diagrams illustrating the strain of a material constructed and arranged for the surface exchange of oxygen;

FIGS. 2A-2C include exemplary schematic diagrams illustrating the strain of a material constructed and arranged for the surface exchange of oxygen, according to some embodiments;

FIG. 3 includes exemplary schematic diagrams illustrating the introduction of strain via a change in composition, according to one set of embodiments;

FIGS. 4A-4C include (A-B) exemplary diffraction plots and (C) an exemplary schematic diagram illustrating the cube-on-cube alignment for GDC on YSZ and the 45° in-plane rotation of LSC on GDC, according to some embodiments;

FIGS. 5A-5F include, according to one set of embodiments, (A) optical images of various sizes of deposited LSC, (B) a schematic illustration of an EIS measurement setup, (C) an exemplary plot of EIS data, (D) exemplary plots of k^chem and k^q values as a function of the partial pressure of O₂, (E) an exemplary plot of calculated VSC values as a function of the partial pressure of O₂, and (F) exemplary plots of <5as a function of the partial pressure of O₂;

FIGS. 6A-6C include (A-B) AES analysis results of a standard LSC pellet and an LSCπo_nm film and (C) an X-ray diffraction pattern of an LSC pellet, according to some embodiments;

FIGS. 7A-7B include exemplary AFM measurements of (A) LSC_45nm and (B) LSCπo_nm dense films, according to some embodiments;

FIG. 8 includes, according to some embodiments, a schematic illustration of the crystallographic relationship between the rhombohedral structure and cubic perovskite structure;

FIGS. 9A-9F include (A) a schematic illustration of the normal diffraction measurements of a thin film sample, (B) a schematic illustration of the off-normal diffraction measurements of a thin film sample, (C) a schematic illustration outlining the off-normal X-ray diffraction configurations of the { 110 } _pc planes from a (001)_pc oriented film, (D) a schematic outlining the off-normal X-ray diffraction configurations of the {111 }p_C planes in a (01 l)_pc oriented film, (E) a schematic outlining the relationship with the [001] pole figure, and (F) a schematic outlining the relationship with the [Oi l] pole figure; FIG. 10 includes exemplary off-normal X-ray diffraction results of { 111 }cubic reflections from a (OOl)cubic and a (01 l)cubic LSC parallel to the film surface, according to some embodiments;

FIGS. 1 IA-I IF include, according to one set of embodiments, (A) a schematic of an electron back scattering diffraction system and (B-F) exemplary experimental EBSD results;

FIGS. 12A-12B include exemplary X-ray diffraction results from (A) GDC (002)_cub,_c and YSZ (002)cubic reflections and (B) LSC (002)_cublc reflection, according to some embodiments;

FIGS. 13A-13B include, according to one set of embodiments, (A) middle frequency (MF) feature in EIS as a function of the partial pressure of O₂ and (B) a plot of MF resistance (R_MF) as a function of the partial pressure of O₂;

FIGS. 14A-14B include (A) a plot of ASC as a function of the partial pressure of O₂ and (B) a plot of k^tr as a function of 1000/T, according to some embodiments;

FIG. 15 includes an exemplary plot of the thermodynamic enhancement factor as a function of the partial pressure of O₂, according to one set of embodiments;

FIGS. 16A-16G include, according to some embodiments, (A-F) exemplary schematic diagrams illustrating oxygen incorporation into a material and (G) an exemplary plot of free energy as a function of distance;

FIGS. 17A-17B include exemplary RHEED patterns, according to one set of embodiments;

FIGS. 18A-18C include, according to some embodiments, (A) a schematic illustration of a sample testing configuration and (B-C) schematic illustrations outlining an equivalent circuit and corresponding Nyquist plot for the experimental system in FIG. 18 A;

FIGS. 19A-19B include exemplary plots of A^ as a function of the partial pressure of θ₂, according to one set of embodiments;

FIGS. 20A-20B include, according to some embodiments, exemplary plots of k^chem as a function of the partial pressure of O₂;

FIGS. 21A-21B include exemplary plots of VSC as a function of the partial pressure of O₂, according to some embodiments; and

FIGS. 22A-22B include exemplary schematic diagrams outlining the relationship between the LSCn₃ and LSC_2I4 lattices. DETAILED DESCRIPTION

Systems and methods related to the enhancement of the surface exchange of oxygen are provided. The invention involves, in some embodiments, affecting and/or introducing a strain (e.g., a tensile or compressive strain) to a material constructed and arranged for the surface exchange of oxygen. The introduction and/or affecting of the strain into such a material may enhance the rate at which oxygen reacts in a surface exchange reaction at the surface of the material, which can enhance the rate at which the oxygen (including oxygen itself, or an oxidized or reduced form of oxygen such as O^2") permeates the material. For example, in some embodiments, the material constructed and arranged for the surface exchange of oxygen may comprise an oxide, and, after applying a strain to the material, the rate of a surface exchange reaction between the oxygen and the material (e.g., an oxygen reduction reaction to produce, for example, O^2") may increase, relative to the rate that would be observed using an unstrained material. By enhancing the rate at which the surface exchange reaction takes place, a relatively large amount of O^2" might be present at the material surface, which can lead to an enhancement in the rate of transport of the oxygen (optionally in an ionic form such as O^2") through the material.

The term "surface exchange" is given its ordinary meaning in the art, and is used to refer to the process by which a species is oxidized or reduced at an interface between two media. For example, the surface exchange of oxygen can refer to a process by which oxygen is oxidized and/or reduced at the interface between two media. In some embodiments, the surface exchange of oxygen can include a process by which molecular oxygen is reacted to form an oxidized or reduced species and/or to a process by which an oxidized or reduced oxygen species is reacted to form molecular oxygen. In some embodiments, surface exchange of oxygen can involve the reduction of molecular oxygen at an interface between two media. For example, molecular oxygen can be transported to the interface between the atmosphere and a material (e.g., a membrane, a cathode, etc.), where it is reduced to O^2". In some cases, the surface exchange of oxygen can include a process by which a reduced oxygen species (e.g., O^2") is oxidized at an interface between two media. For example, O^2" can be transported through a material and oxidized to form O₂ at the interface between the material and the ambient atmosphere. One of ordinary skill in the art would be capable of identifying other types of surface exchange reactions that could be used in association with the systems and methods described herein.

In some embodiments, the ability of oxygen (or an oxidized or reduced form of oxygen) to participate in a surface exchange reaction is either made possible at all, or improved after exposure of one or more materials constructed and arranged for the surface exchange of oxygen to the conditions described. For example, a material (or multiple materials) may exist in which, inherently, prior to the technique of the invention, surface exchange of oxygen upon contacting the material(s) may be effectively impossible or may be possible only to a small degree. After exposure to the appropriate conditions, the ability of oxygen to participate in a surface exchange reaction upon interacting with the material(s) improves to the point that oxygen surface exchange can be carried out in a measurable manner. Specifically, in one set of embodiments, a strain may be introduced to a material (or multiple materials) constructed and arranged for the surface exchange of oxygen, and, prior to the introduction of the strain, oxygen can participate in a surface exchange reaction only at a first, relatively low, rate.

However, after the introduction of a strain to the material(s), in accordance with the invention, the surface exchange reaction involving oxygen can occur at a second rate at least about 2 times, at least about 5 times, at least about 10 times, at least about 25 times, at least about 50 times, at least about 100 times, at least about 250 times, at least about 500 times, at least about 1000 times, at least about 10,000 times, at least about

100,000 times, between about 2 times and about 1,000,000 times, between about 2 times and about 100,000 times, between about 5 times and about 1,000,000 times, between about 5 times and about 100,000 times, between about 10 times and about

1,000,000 times, between about 10 times and about 100,000 times, between about 100 times and about 1 ,000,000 times, between about 100 times and about 100,000 times, between about 1000 times and about 1,000,000 times, or between about 1000 times and about 10,000 times greater than the first rate.

Those of ordinary skill in the art will understand, from this disclosure, the meaning of "constructed and arranged for the surface exchange of oxygen." Generally, such materials, interfaces, and/or devices are defined as those that make use of the oxidation and/or reduction of oxygen. Examples are contained throughout this disclosure, and include fuel cell cathodes, oxygen-separation membranes, hydrolysis electrodes, and the like. In some embodiments, the material constructed and arranged for the surface exchange of oxygen comprises an oxide. Upon introducing a strain to the material constructed and arranged for the surface exchange of oxygen (e.g., comprising an oxide), the rate of the surface exchange reaction of oxygen with the material may be greater than the rate of the surface exchange reaction of oxygen with an unstrained material under essentially identical conditions. "Essentially identical conditions," in this context, means conditions that are similar or identical, other than the presence of the strain. For example, otherwise identical conditions may refer to a material that is identical and an environment is identical (e.g., identical temperature, pressure, other processing conditions, etc.), but where no strain is introduced to the material.

The material constructed and arranged for the surface exchange of oxygen can comprise a variety of suitable oxides. In some embodiments, the material can comprise a perovskite material. In some instances, the oxide can have the formula ABO₃, where A comprises at least one of La, Sr, Ca, Ba, Li, Na, K, Cs, Ce, Pr, Nd, Sm, Pm, Eu, Gd, Pm, Tb, Dy, Ho, Tm, and Yb; B comprises a transition metal (e.g., Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, and Uub); and O represents oxygen. In some embodiments, the material may comprise a perovskite-like structure. For example the material may comprise an oxide with the formula A₂BO₄, where A and B are defined as above, and O represents oxygen. In some cases, the material can comprise an oxide with the formula Di_-xE_xBO_y, wherein D and E are different and are each chosen from among La, Sr, Ca, Ba, Li, Na, K, Cs, Ce, Pr, Nd, Sm, Pm, Eu, Gd, Pm, Tb, Dy, Ho, Tm, and Yb; B comprises a transition metal (e.g., Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, and Uub); and O represents oxygen. In some instances, x in the formula Di_-xE_xBO_y can be between about O and about 0.5, between about 0.1 and about 0.4, or between about 0.15 and about 0.25. In some cases, y in the formula Di_-xE_xBO_y can be between about 3 and about 5 (e.g., y can be about 3, about 4, or about 5). Specific examples of oxides that may be used include, but are not limited to, Lai_-xSr_xCoθ₃, K₂NiF₄-type La₂Ni0_4+dei_ta, Lai_-xSr_xMnO₃, and the like, and, in some cases, x can lie within any of the ranges defined above (e.g., between about 0 and about 0.5, between about 0.1 and about 0.4, or between about 0.15 and about 0.25). The material can comprise, in some cases, (Lai-_bSr_b)₂CoO₄ wherein, for example, b can be between about 0.2 and about 0.8, between about 0.4 and about 0.6, or between about 0.45 and about 0.55.

In some embodiments, the material may comprise La_{0 8}Sr₀₂Coθ3-d_eita (e.g., La_{0 8}Sro ₂CoO₃ which can be, for example, (001)_pc-oriented) or (La_{0 5}Sr₀ s^CoCM-_dei_ta- In some cases, delta in the above formulas can be less than about 0.5, less than about 0.1 , less than about 0.01, less than about 0.001, less than about 10^"4, less than about 10^'5, substantially zero, between about 10^"5 and about 0.5, between about 10^"4 and about 0.5, between about 0.001 and about 0.5, between about 0.01 and about 0.5, between about 0.1 and about 0.5, between about 10^"5 and about 0.1, between about 10^"4 and about 0.1, between about 0.001 and about 0.1, between about 0.01 and about 0.1, between about 0 and about 0.5, between about 0 and about 0.1, between about 0 and about 0.01, between about 0 and about 0.01, between about 0 and about 0.001, between about 0 and about 10^"4, or between about 0 and about 10^"5.

In some embodiments, the material may comprise a fluorite. The material constructed and arranged for the surface exchange of oxygen may comprise, in some embodiments, an oxygen reduction catalyst. The material constructed and arranged for the surface exchange of oxygen may comprise, in some instances, a mixed species of electrons and oxygen.

The material constructed and arranged for the surface exchange of oxygen to which a strain is introduced may be in the form of a film, in some instances. In some embodiments, the average thickness of the film is less than about 10 microns, less than about 1 micron, less than about 100 nm, less than about 10 nm, or thinner. The average thickness of the film can be, in some instances, between about 1 nm and about

10 microns, between about 1 nm and about 1 micron, between 1 nm and about 100 nm, between about 10 nm and about 10 microns, between about 10 nm and about 1 micron, or between 10 nm and about 100 nm. In some cases, the films of material may be part of an electrode (e.g., a cathode in an electrochemical cell such as a fuel cell). In addition, the films of material may be part of a membrane (e.g., a gas separation membrane). In some instances, the films may be part of particles that make up an electrode or membrane (e.g., the films may be part of particles that are agglomerated to form an electrode such as the cathode of a fuel cell).

In some cases, the material constructed and arranged for the surface exchange of oxygen can be proximate a substrate (e.g., a growth substrate). For example, in some cases, the material constructed and arranged for the surface exchange of oxygen can be in direct physical contact with a substrate such as a growth substrate.

In some instances, the material constructed and arranged for the surface exchange of oxygen to which strain is introduced may comprise a crystalline material. In some cases, the material constructed and arranged for the surface exchange of oxygen may be polycrystalline. In other cases, the material constructed and arranged for the surface exchange of oxygen may be a single-crystal material, which may optionally incorporate one or more defects. For example, the material constructed and arranged for the surface exchange of oxygen may comprise, in some embodiments, an epitaxially grown material (e.g., an epitaxial layer on a growth substrate).

Strain (e.g., tensile strain or compressive strain) may be introduced to the material constructed and arranged for the surface exchange of oxygen using any suitable method. Strain can be introduced via mechanical, thermal, geometric, composition, and/or chemical means, or combinations thereof. In some embodiments, introducing a strain comprises forming a layer of the material constructed and arranged for the surface exchange of oxygen over a substrate. In some cases, the lattice parameter of the material constructed and arranged for the surface exchange of oxygen and the lattice parameter of the substrate are different. The difference in the lattice parameter may lead to a strain in the material constructed and arranged for the surface exchange of oxygen formed over the substrate. For example, in some embodiments, forming a layer of material constructed and arranged for the surface exchange of oxygen over a substrate comprises epitaxially growing a layer of the material constructed and arranged for the surface exchange of oxygen on the substrate. The differences in the lattice parameters of the epitaxial film and the substrate will cause a strain to be introduced into the epitaxial film.

Producing a change in lattice parameter via epitaxial growth of a material on a substrate is shown schematically in FIG. IA. In FIG. IA, substrate material 1 10 and material 112 (constructed and arranged for the surface exchange of oxygen) are illustrated in their bulk forms in section 1 14. The distance between unit cells in the crystal lattice (i.e., the lattice parameter) of bulk substrate material 110 is illustrated as distance 114. In addition, the distance between unit cells in the crystal lattice of material 1 12 in its bulk form is illustrated as lattice parameter 1 16. Geometric strain can be induced by depositing (e.g., via epitaxial thin film deposition) material 1 12 on substrate 110. When material 1 12 is grown on substrate 1 10, as shown in section 120, material 112 can be strained such that its lattice parameter is increased to be substantially equal to dimension 114. In the set of embodiments illustrated in FIG. IA, the substrate material 110 has a larger lattice parameter than material 112, which results in a tensile strain within material 112. It should be understood, however, that in other embodiments, the substrate material 110 can have a smaller lattice parameter relative to material 112, producing a compressive strain in material 112.

While the substrate can be substantially planar in some cases (e.g., FIG. IA), in other cases, the substrate may be any other suitable shape. For example, the substrate may be a cylinder (e.g., a fiber, a hollow cylinder, etc.) and the material constructed and arranged for the surface exchange of oxygen may be formed as a layer over the cylinder. In some instances, the substrate may comprise a core (e.g., a spherical core), and the material constructed and arranged for the surface exchange of oxygen may be formed over the core. An exemplary embodiment including a core shell arrangement is shown in FIG. IB. In such cases, the lattice parameter of the core and the lattice parameter of the shell material can be different. Strain may be introduced when the material constructed and arranged for the surface exchange of oxygen is formed over a core (e.g., where the material constructed and arranged for the surface exchange of oxygen partially encapsulates the core or where the material constructed and arranged for the surface exchange of oxygen fully encapsulates the core, as is the case in FIG. IB). Strain can be produced, for example, due to a mismatch in the lattice parameters of the two materials. In some cases, strain can be produced (in place of or in addition to the lattice mismatch strain) due to the geometry of the core. For example, subsequent layers of material grown on a sphere define relatively large surface areas, which can result in relatively large strains within the outermost layers.

In some cases, introducing a strain comprises heating the material constructed and arranged for the surface exchange of oxygen and the substrate, wherein the substrate has a first thermal expansion coefficient, and the material constructed and arranged for the surface exchange of oxygen has a second thermal expansion coefficient that is different from the first thermal expansion coefficient. FIG. 1 C illustrates one set of embodiments in which a core and a shell are heated. A thermal strain can be produced when the core and the shell expand to different extents. Thermal strain may also be introduced when substantially planar substrates, or any other shapes of substrates, are used. Introducing a strain to a material constructed and arranged for the surface exchange of oxygen may comprise introducing a mechanical strain, in some

embodiments. FIGS. 2A-2C include schematic diagrams of systems in which a mechanical strain is introduced. FIG. 2A includes a schematic illustration of a system in which substrate material 110 and material 112 (constructed and arranged for the surface exchange of oxygen) are not subject to a mechanical strain. Introducing a mechanical strain may comprise applying a physical force (i.e., a stress) to a material, which can produce a compressive or tensile strain in the film and/or the substrate. For example, in FIG. 2B, a mechanical force (i.e., stress) is applied to the ends of the materials in the directions of arrows 124, thereby producing a tensile strain in substrate material 110 and oxygen surface exchange material 1 12. While a tensile strain is illustrated in FIG. 2B, it should be understood that, in some cases, a compressive stress and strain can be used. In some embodiments, an asymmetric strain can be introduced in which moments are imparted onto the two ends causing a tensile strain on the top film and compressive strain on the bottom film, or vice versa. For example, in FIG. 2C, a mechanical force is applied in the direction of arrows 126, thereby producing a tensile strain in material 112 and a compressive strain in substrate material 110.

Introducing a strain to a material may comprise, in some embodiments, introducing a compositional change in a material constructed and arranged for the surface exchange of oxygen. For example, a dopant may be added to a material constructed and arranged for the surface exchange of oxygen. The dopant may, in some cases, replace an atom in a crystal lattice. The dopant atom may have a different ionic radius relative to the atom that originally occupied the lattice site, producing a change in the lattice parameter of the material. In some instances, the dopant may be positioned within an interstitial space of a lattice. A change in composition of a material grown on a fixed lattice substrate can produce a strain (or a change in strain) of the material.

FIG. 3 schematically illustrates a set of embodiments in which a strain is introduced via a change in composition.

In some embodiments, introducing a strain comprises introducing a chemical strain, which may comprise, for example, adding atoms or vacancies to a material one wishes to strain. For example, a material comprising an oxide can be strained when the material is exposed to an oxygen-rich environment (which may introduce excess oxygen atoms) or an oxygen-reducing environment (which may introduce oxygen vacancies). The increase or decrease in oxygen content can induce a change in the lattice parameter of the material comprising the oxide and create a chemical strain on the material. Once the chemical change is created and quenched, the material can maintain the increased or decreased oxygen content (and associated strain), allowing for a more energetic oxide for catalytic reactions. Such a method may be useful, for example, in tailoring

low-temperature oxide catalysts, such as those used for catalyst reactors, electrodes, and the like.

Any suitable amount of strain can be applied to a material constructed and arranged for the surface exchange of oxygen, in accordance with the embodiments described herein. In some embodiments, the absolute value of the strain of the material (e.g., the first material constructed and arranged for the surface exchange of oxygen and/or second (or additional) materials constructed and arranged for the surface exchange of oxygen, as discussed in more detail below), measured relative to an unstrained sample of the material at room temperature, is at least about 0.01%, at least about 0.05%, at least about 0.1%, at least about 0.5%, at least about 1%, at least about 2%, between about 0.01% and about 5%, between about 0.05% and about 5%, between about 0.1% and about 5%, between about 0.01% and about 2%, between about 0.05% and about 2%, between about 0.1% and about 2%, between about 0.01% and about 1%, between about 0.05% and about 1%, or between about 0.1% and about 1%. One of ordinary skill in the art will be capable of calculating a strain based upon the geometry of the material. In general, strain is calculated as: strain(%) = ±—t x 100% = ±^-A _x 100% [ 1 ] where / is the final length of the measured dimension in the strained state, and L is the length of the measured dimension in an unstrained state. The lengths used to calculate strain may be measured, for example, by using X-ray diffraction to measure lattice parameters.

In some cases, multiple materials constructed and arranged for the surface exchange of oxygen (e.g., 2, 3, 4, or more) can be used. For example, in some embodiments, an article can comprise a first material constructed and arranged for the surface exchange of oxygen and a second material constructed and arranged for the surface exchange of oxygen positioned proximate the first material. In the set of embodiments illustrated in FIG. ID, a second material 1 13 constructed and arranged for the surface exchange of oxygen is positioned proximate first material 1 12 constructed and arranged for the surface exchange of oxygen. As used herein, two materials (e.g., layers of materials) are "proximate" when they are either physically connected (i.e., one is layered upon the other) or when they are sufficiently close to retain their desired combined functionality. For example, first and second materials constructed and arranged for the surface exchange of oxygen might be proximate each other as long as their positions are sufficiently close that any material between the two materials does not significantly impede the surface exchange of oxygen, relative to the level of oxygen surface exchange that would be observed in the absence of the intermediate material. In some embodiments, two materials can be proximate when they are positioned in direct contact with each other. In some instances, two materials can be proximate while one or more other materials are positioned between them. Accordingly, while second material 1 13 is shown as being positioned directly on first material 112 in FIG. ID, it should be understood that, in other embodiments, first material 112 and second material 113 can remain proximate while one or more other layers is positioned between them.

The second material constructed and arranged for the surface exchange of oxygen can be of any suitable form. In some cases (e.g., in FIG. ID), the second material constructed and arranged for oxygen exchange can comprise a layer that partially or substantially completely covers the first material constructed and arranged for oxygen exchange. In some cases, the first and second materials can be arranged in alternating layers (e.g., alternating series of 3, 4, 5, 6, or more layers). In FIG. IE, for example, the first and second materials are arranged as alternating series of eight layers on substrate 110. In some embodiments (e.g., when the materials constructed and arranged for the surface exchange of oxygen are part of an electrode for use in a fuel cell), substrate 110 can comprise an electrolyte. In addition, an optional counter-electrode 122 can be included, electrically isolated from materials 112 and 113 by substrate electrolyte 1 10.

The second material constructed and arranged for oxygen exchange can, in some embodiments, comprise a plurality of islands of material proximate the first material constructed and arranged for oxygen exchange, as illustrated, for example, in FIG. 1 F. In some embodiments, both of the first and second materials constructed and arranged for oxygen exchange can comprise a plurality of islands arranged proximate each other, as illustrated in FIG. IG. As mentioned above, more than two materials constructed and arranged for the surface exchange of oxygen can be used, in some embodiments. For example,

FIGS. IH- II include schematic illustrations of articles in which third material 123 constructed and arranged for the surface exchange of oxygen is positioned proximate first material 112 and second material 113. In the set of embodiments illustrated in

FIG. IH, materials 112, 113, and 123 are arranged as a plurality of islands. In the set of embodiments illustrated in FIG. II, materials 112, 113, and 123 are arranged as an alternating series of layers.

Not wishing to be bound by any particular theory, the second material constructed and arranged for oxygen exchange might further enhance the surface exchange of oxygen by introducing additional strain into the system. For example, the second material might have a lattice parameter that is different from the first material constructed and arranged for oxygen exchange, and therefore, a stress might be introduced into the first and/or second material upon positioning the second material proximate the first material (e.g., depositing the second material on the first material). Not wishing to be bound by any particular theory, the presence of a second material might introduce strain and cation segregation (such as Sr enrichment) at an interface, which might lead to increasing oxygen vacancies near the interface and/or create active sites for surface oxygen exchange. In cases where any of the forms of LSC described herein are used as a material constructed and arranged for the surface exchange of oxygen, the presence of low-spin Co³⁺ associated with the second material might lead to higher binding of O₂ molecules on Co³⁺-O₅ pyramidal sites (reduced barrier for surface oxygen exchange), relative to LSC materials with relatively high spin Co³⁺.

The second material constructed and arranged for the surface exchange of oxygen can include any of the properties (e.g., sizes (e.g., average thicknesses), materials, etc.) described herein with respect to the first material constructed and arranged for the surface exchange of oxygen. The second material constructed and arranged for the surface exchange of oxygen can be thin, relative to the first material, in some

embodiments. For example, in some cases, the average thickness of the first material constructed and arranged for the surface exchange of oxygen (e.g., the material closer to the substrate on which the materials are positioned) can be at least about 10 times, at least about 100 times, at least about 1000 times, between about 5 times and about 1000 times, between about 5 times and about 100 times, or between about 5 times and about 50 times the average thickness of the second material (e.g., a film of the second material, the average thicknesses of a plurality of islands of the second material) constructed and arranged for the surface exchange of oxygen. In some embodiments, the average thickness of the second material constructed and arranged for the surface exchange of oxygen can be less than about 100 run, less than about 10 nm, less than about 1 run, between about 0.001 nm and about 10 nm, between about 0.01 nm and about 10 nm, between about 0.1 nm and about 10 nm, between about 0.001 nm and about 1 nm, between about 0.01 nm and about 1 nm, or between about 0.1 nm and about 1 nm

In some embodiments, one of the materials constructed and arranged for the surface exchange of oxygen (e.g., the first material, which might be closer to the substrate on which the materials are deposited than the second material) can be

Lai_-xSr_xCoO₃ (e.g., La₀₆Sr₀₄CoO₃ or La_{0 8}Sr_{O 2}CoO₃)) and, in some such embodiments, the second material can be (Lai._bSr_t,)₂CoO₄ (e.g., (La_{0 5}Sr_O s)₂CoO₄). x and b in the formulas Lai_-xSr_xCoO₃ and (Lai._bSr_t,)₂CoO₄ can be the same or different. In some embodiments, x in the formula La^_xSr_xCoO₃ can between about 0 and about 0.6, between about 0.1 and about 0.3, between about 0.15 and about 0.25, between about 0.3 and about 0.5, or between about 0.35 and about 0.45. In some embodiments, b in the formula (Lai._bSr_t,)₂CoO₄ can between about 0.2 and about 0.8, between about 0.4 and about 0.6, or between about 0.45 and about 0.55. In some cases, the second material can comprise an oxide comprising Co with relatively low-spin Co³⁺.

In instances in which multiple materials constructed and arranged for the surface exchange of oxygen are used, the absolute value of the average strain of the materials, measured relative to an unstrained sample of the materials at room temperature, can be at least about 0.01%, at least about 0.05%, at least about 0.1%, at least about 0.5%, at least about 1%, at least about 2%, between about 0.01% and about 5%, between about 0.05% and about 5%, between about 0.1% and about 5%, between about 0.01% and about 2%, between about 0.05% and about 2%, between about 0.1% and about 2%, between about 0.01% and about 1%, between about 0.05% and about 1%, or between about 0.1% and about 1%. The average strain imparted to n materials is calculated using the following weighted average formula:

Average Strain

wherein /, is the average thickness of material /, t_maι is the sum of the average thickness of all of the materials constructed and arranged for the surface exchange of oxygen, and strain, is the strain imparted to the i'^h material constructed and arranged for the surface exchange of oxygen. For example, in cases in which two materials constructed and arranged for the surface exchange of oxygen are used, the average strain imparted to the two materials would be calculated as:

Average Strain, two materials =

wherein /_/ is the average thickness of the first material constructed and arranged for the surface exchange of oxygen, t₂ is the average thickness of the second material constructed and arranged for the surface exchange of oxygen, strain_/ is the strain imparted to the first material, and strain is the strain imparted to the second material.

A surface exchange coefficient (e.g., an electrical surface exchange coefficient or a chemical exchange surface coefficient) of the strained material (or combination of strained materials) may be, in some instances, larger than the surface exchange coefficient of an unstrained material (or combination of unstrained materials) under essentially identical conditions, in some embodiments. For example, in some cases, a surface exchange coefficient (e.g., an electrical surface exchange coefficient) of the strained material (or combination of strained materials) is at least about 2 times, at least about 5 times, at least about 10 times, at least about 100 times, at least about 1000 times, at least about 10,000 times, at least about 100,000 times, between about 2 times and about 1,000,000 times, between about 2 times and about 100,000 times, between about 5 times and about 1,000,000 times, between about 5 times and about 100,000 times, between about 10 times and about 1 ,000,000 times, between about 10 times and about 100,000 times, between about 100 times and about 1,000,000 times, between about 100 times and about 100,000 times, between about 1000 times and about

1,000,000 times, or between about 1000 times and about 10,000 times greater than the surface exchange coefficient of an unstrained material (or combination of unstrained materials) under essentially identical conditions.

In some embodiments, the surface exchange coefficient may be measured as an electrical surface exchange coefficient. The electrical surface exchange coefficient is calculated as: k" = RT/4F²R_LFA_maleπalc_o [2] where R is the universal gas constant (8.314 J mol^" K^" ), T is the absolute temperature, F is the Faraday's constant (96,500 C mol^"1), R_LF is the LF real resistance, A_mαterιαι is the area of the material over which the species (e.g., oxygen) is interacting, and c₀ is the lattice concentration of the species in the material (e.g., oxygen in any of the forms of LSC described herein).

While the embodiments described herein are predominantly drawn to instances in which the material comprises an oxide, and the species comprises oxygen (which can participate in a surface exchange reaction in which it is reduced to, for example, O^2"), it should be understood that the invention is not so limited. For example, in some embodiments, the species may comprise fluorine (e.g., an ion-exchange system in which F₂ gas and an F^" ion are employed), and, upon introduction of a strain, a surface exchange reaction of fluorine at a surface of the material may proceed at an enhanced rate. In some cases, the species may comprise chlorine (e.g., an ion-exchange system in which Cl₂ and a Cl^" ion are employed), and, upon introduction of a strain, a surface exchange reaction of chlorine at a surface of the material may proceed at an enhanced rate. In some embodiments, the material may comprise a proton exchange membrane, and the species may comprise hydrogen gas (i.e., the surface exchange reaction involves H₂ and H⁺). One or ordinary skill in the art will be capable of pairing compositions of materials and species such that a desired surface exchange rate and/or permeability are attained.

The systems and methods described herein may exhibit several advantages compared to traditional systems and methods. In some embodiments, large increases in surface exchange allow for improved performance at lower temperatures. For example many solid oxide fuel cells require temperatures above about 800 ⁰C in order to efficiently produce electricity. The ability to achieve similar performance at lower temperatures would allow for decreased cost, shortened start-up times, and lowered degradation of the fuel cell. Similar benefits may be realized for other systems and devices that employ permeable materials.

The systems and methods described herein may be useful in a variety of applications. For example, the systems and methods described herein can improve efficiency and/or conversion in portable and stationary power applications in consumer, industrial, and military settings, for example when used as part of an electrode (e.g., a cathode) in a fuel cell. In addition, the systems and methods described herein can improve the functionality and performance of membranes (e.g., oxide membranes) utilized for gas separation (e.g., oxygen separation), which can be used in sensors, chemical processing facilities, power generation, and the like. The highly active surfaces described herein can also be utilized for improving the efficiency and design of reaction chambers.

The following documents are incorporated herein by reference in their entirety for all purposes: U.S. Provisional Patent Application Serial No. 61/225,259, filed July 14, 2009, entitled "Systems and Methods Related to Strained Materials," by Ia O\ et al. All other patents, patent applications, and documents cited herein are also hereby incorporated by reference in their entirety for all purposes.

The following examples are intended to illustrate certain embodiments of the present invention, but do not exemplify the full scope of the invention.

EXAMPLE l

Conventional solid oxide fuel cells (SOFCs), such as those designed to integrate with gas turbines for power generation, typically operate at temperatures above 800 °C. The operation of SOFCs for auxiliary and distributed power applications can benefit from a reduction in SOFC operating temperatures (e.g., to 500 °C), where SOFC costs, start-up time, and degradation can be reduced. The voltage loss associated with oxygen reduction reaction (ORR) at the cathode may be one barrier to achieving acceptable SOFC efficiency at intermediate temperatures. Cathode performance may be improved, in some cases, by employing mixed ionic-electronic conductors such as perovskite Lai_-xSr_xCoθ₃ and K₂NiF₄-type La₂NiO_4+S with higher oxygen vacancy concentration and higher oxygen ion transport than, for example, conventional cathode material

Lai_-xSr_xMnO₃. Lack of fundamental understanding in the ORR mechanism at the molecular level can limit the design of new cathode materials with enhanced activity. Establishing scaling relationships between ORR activity and thin-film electrode geometries can aid in determining whether ORR kinetics is limited by surface oxygen exchange or oxygen ion transport. ORR activity may also be dependent on the crystallographic surfaces.

In this example, strain-induced enhancement of the ORR activity of oxide surfaces is described. Specifically, this example describes strain-induced activity enhancement of ORR in (001)_pc-oriented Lao ₈Sro₂Coθ3-δ (abbreviated as "LSC" for purposes of this example) films (subscript "pc" denotes the pseudocubic notation) epitaxially grown on 9.5 mol% Y₂θ₃-stabilized ZrO₂ (YSZ) (001) single-crystal, with an epitaxial buffer layer of 20 mol% gadolinium-doped ceria (GDC). As deposited LSC films with thicknesses of 45 nm and 130 nm were found to have in-plane tensile strains of 1.4% and 0.3% and normal compressive strains of -0.9% and -0.2% at room temperature, respectively. ORR kinetics of the LSC films were examined by

electrochemical impedance spectroscopy (EIS) at 520 ⁰C with P₀ ranging from 10^"4 to

1 atm. Surface oxygen exchange coefficients, k, and oxygen nonstoichiometry, δ, were found to increase with decreasing film thickness and were considerably greater than those of polycrystalline LSC thin films and LSC powder. Not wishing to be bound by any particular theory, the enhanced k of the (001)_pc LSC surface relative to LSC powder (by approximately one order of magnitude) might be due to increased oxygen vacancy concentrations induced by tensile strains in the films.

Pulsed laser deposition (PLD) was utilized to initially deposit a GDC film of about 5 nm on a single crystal of YSZ with a (001) orientation. GDC was used as an interlayer to prevent chemical reactions between LSC and YSZ. LSC thin-films with thicknesses of 45 nm and 130 nm were subsequently deposited on the GDC/YSZ (001) substrate. Elemental analysis of two representative LSC films using Rutherford backscattering spectroscopy (RBS) revealed an average bulk film composition of

15.0±0.5 at.% La_ave , 3.5±0.5 at.% Sr_ave ,17.5±1.0 at.% Co_ave and 64.0±5.0 at.% O_ave , which was very close to the nominal stoichiometry of LSC.

Auger electron spectroscopy (AES) analysis revealed that the surfaces of as- deposited LSC films were nearly stoichiometric, having comparable surface chemical compositions to that of fractured surfaces of an LSC pellet, as illustrated in

FIGS. 6A-6C. FIGS. 6A-6B include AES analysis results of standard LSC pellet and LSCπo_nm film with atomic ratios calculated using (A) manufacturer-provided relative sensitivity factor (RSF) and (B) calibrated RSF. Calibrated-RSF values were adjusted by normalizing result from standard pellet (La_{0 S}Sr₀₂CoO₃) as La 16 at.%, Sr 4 at.%, Co 20 at.%, and O 60 at.%. Using this calibrated RSF, the stoichiometric composition of

LSCπonm film was estimated as La 18.4 at.%, Sr 9.3 at.%, Co 21.5 at.% and O 50.8 at.%, that shows slight enrichment of La with 2.4 at.% excess and Sr with 5.3 at.% excess. FIG. 6C includes an X-ray diffraction pattern of an LSC pellet utilized as a standard for AES exhibiting correct phase in comparison with reference standards. LSC powder was prepared by polymerizable complexation method using stoichiometric amounts of La(NOs)₃, Sr(NO₃ )₂ and Co(NO₃)₃ dissolved in the mixture of citric acid, ethylene glycol and DI water. Esterification reaction was promoted at 100 ⁰C followed by charring at 400 ⁰C to burn out polymeric resin. Obtained black precursors were calcined at 600 ⁰C for the crystallization.

Atomic force microscopy (AFM) showed that as-deposited LSC films had surface root mean squared (RMS) roughness of 3-6 run (See FIGS. 7A-7B).

FIGS. 7A-7B include AFM measurements of (A) LSC₄₅Hm and (b) LSC_130nm dense films, revealing surface RMS roughness of about 3.4 nm and about 6.2 nm, respectively. AFM images with a 1 μm² analysis area are shown with a maximum height of 50 nm.

LSC has rhombohedral symmetry with the space group R3c , where the unit cell is defined typically in a hexagonal cell with a = 5.44 A, c = 13.11 A, and in a

rhombohedral cell with a=5.403 A and «=60.569°. To examine the crystallographic relationship between rhombohedral LSC and GDC or YSZ that have cubic symmetry, it can be convenient to redefine the rhombohedral LSC structure as a pseudocubic perovskite unit cell with lattice parameters of α_pc ~ 3.84 A (See FIG. 8, which includes a schematic illustration of the crystallographic relationship between the rhombohedral structure and cubic perovskite structure).

FIG. 4A includes out-of-plane X-ray diffraction data of 45 nm and 130 nm LSC film on GDC/YSZ (001). The small peak marked with an asterisk can be associated with the reflection from the film interfaces, not the film as no change was found for films of different thicknesses. (See G. Kim, S. Wang, A. J. Jacobson, Z. Yuan, W. Dormer, C. L. Chen, L. Reimus, P. Brodersen, C. A. Mims, Appl. Phys. Lett. 2006, 88, 024103)

Indexing of peaks was performed using pseudocubic crystallographic orientation for LSC with space group of Pm3m X-ray diffraction patterns of LSC thin-films with thicknesses of 45 nm and 130 nm grown on the GDC/YSZ (001) substrate, only revealed (00/) peaks from LSC, GDC and YSZ, as shown in FIG. 4A. Measured rocking-curve data of the LSC (002)_pc reflection of films of 45 nm and 130 nm in the inset of FIG. 4A, exhibited full-width-half-maximum (FWHM) of about 1.2 ° and about 1.0 °, respectively, which indicated good film crystallinity. As the GDC (002) (20 = 33.06°) and the LSC (01 l)_pc (20 = 33.28°) peaks overlap in the off-normal XRD, additional (φ-scaή) X-ray diffraction scans (see the schematic in FIG. 9) were collected to examine if LSC films were composed solely of the (001)_pc plane parallel to the film surface. FIGS. 9A-9B include schematic illustrations of the (A) normal and (B) off-normal diffraction measurements of the thin film sample. The normal scan was the same as traditional Bragg-Brentano constructions, while the off- normal scan was performed with a tilting χ-angle equivalent to the inter-planar angle of the crystal structure. FIGS. 9C-9D include the off-normal X-ray diffraction

configurations of the (C) { 110}_pc planes from the (001)_pc oriented film, and (D) the { 111 } _pc planes in the (011 )_pc oriented film. FIGS. 9E-9F include their relationships with (E) the [001] pole figure and (F) the [011] pole figure.

The off-normal ^-scan of the LSC { 111 }_pc reflections from the (001)_pc oriented film showed four strong peaks expected from the four-fold cubic symmetry, while no peak was observed for the LSC { 11 1 }_pc reflections from the (01 l)_pc oriented film. These observations indicated the absence of LSC (01 l)_pc parallel to the film surface and provided evidence that LSC films were single-crystal with LSC (001)_pc//YSZ (001) (See FIG. 10, which includes the off-normal X-ray diffraction results of { 111 } cubic reflections from (OOl)cubic and (01 l)cubic LSC parallel to the film surface).

Further evidence for the LSC (001)_pc oriented films was provided by electron back scattering diffraction (EBSD) results, which showed single color diffraction maps in three perpendicular directions, [001]_pc, [230]_pc and [320]_pc. FIGS. 1 IA-I IF include the electron back scattering diffraction (EBSD) results obtained from a LSC film with thickness of 45 nm and an area of 100 μm * 100 μm. FIG. 1 IA includes a schematic of EBSD measurement showing normal direction (ND), rolling direction (RD) and transverse direction (TD) of measurement. FIG. 1 IB includes the obtained Kikuchi pattern from the LSC film surface in ND, and 11C includes the calculated diffraction pattern assuming cubic LSC unit cell in ND. Uniform color figures in FIG. 1 ID (ND), FIG. 1 IE (RD), and FIG. 1 IF (TD) indicate single crystal character of the film without any grain boundaries. The insets in FIGS. 1 ID-I IF are the inverse-pole figures that show the orientation of the film in each direction. EBSD was performed using a Zeiss Supra55VP field emission scanning electron microscope (FESEM). EBSD orientation maps were acquired using HKL Nordlys II EBSD pattern collection systems with a spatial resolution of about 10 nm and an angular resolution of about 0.25°.

FIG. 4B includes in-plane X-ray diffraction patterns for the 130-nm LSC film, GDC buffer layer and YSZ (001) substrate. The LSC {110}_pc reflections had a 45° difference in the jangle relative to GDC and YSZ {220}_pc (FIG. 4B) reflections. This suggested that the LSC films were grown with a 45° in-plane rotation relative to the fluorite structure of GDC and YSZ, which have the cube-on-cube alignment, as shown in FIG. 4C, which includes a schematic showing the cube-on-cube alignment for GDC on YSZ and the 45° in-plane rotation of LSC on GDC.

As-deposited LSC (001)_pc films were found dilated in-plane and compressed in the direction normal to the film surface at room temperature, which is clearly shown in the high-resolution X-ray diffraction scans. FIGS. 12A-12B include X-ray diffraction results from (A) GDC (002)_cublc and YSZ (002)_cub,c reflections and (B) LSC (002)_cublc reflection. For LSC|₃o_nm, a peak shift of LSC (002)_cubic to a lower angle, indicating relaxation of the strain, was clearly observed; on the contrary, the GDC and YSZ peaks did not exhibit any noticeable shift. The constrained in-plane lattice parameter of LSC determined from the (01 l)_pc peak position and the constrained lattice parameter of LSC normal to the film surface determined from the (002)_pc peak positions are listed in Table 1. Assuming a Poisson ratio, v, of 0.25 for LSC, the relaxed cubic lattice parameter, α_pc, of the LSC films of 45 nm and 130 nm were equal to within experimental accuracy (3.86 A and 3.85 A, respectively). Not wishing to be bound by any theory, this may indicate that the different lattice parameters are a direct consequence of strain. The in-plane tensile strains and normal compressive strains of the constrained films relative to the relaxed LSC lattice parameters were then computed, as listed in Table 1. The in- plane and normal strains for the 45-nm film (in-plane 1.4 %; normal -0.93 %) were found to be greater than those of the 130-nm film (in-plane 0.3 % and normal -0.2 %). In addition, it is noted that these films had considerably larger pseudocubic lattice parameter and unit cell volume than LSC powder (See Table 2), which can be attributed to higher lvalues.

Table 1. Measured lattice parameters assuming pseudocubic LSC, cla ratios, space group, and relaxed lattice; from normal and off-normal X-ray diffraction data.

Constrained normal lattice parameters, c, were measured by using inter-planar distance of the (002) planes. Constrained in-plane lattice parameters, a, were calculated by combining c with inter-planar distance of the (011) planes obtained from in-plane diffraction data.

ORR kinetics of epitaxial LSC (001)_pc thin- films were examined using

electrochemical impedance spectroscopy (EIS). EIS measurements were conducted on patterned epitaxial LSC microelectrodes fabricated by photolithography and acid etching. An optical image of the dense, thin-film micropatterned LSC ranging in size from

25 microns to 200 microns is shown in FIG. 5A. The LSC in FIG. 5A was supported on a 5 nm GDC interlayer and single crystal YSZ (001) substrate. EIS data from LSC microelectrodes were collected using a microprobe station setup, where platinum (Pt) coated tungsten micro-needles were used to contact the LSC working electrode and porous Pt counter electrode on the bottom side of the YSZ substrate, as shown

schematically in FIG. 5B. The schematic in FIG. 5B shows the EIS measurement setup using Pt-coated tungsten micro-needles contacting LSC microelectrode and porous Pt counter electrode. The entire sample was contained in a heating stage to control temperature and P₀ . EIS data collected from LSC (001)_pc films of 45 nm and 130 nm at 520 ⁰C showed similar features, and representative data of the 130 run film measured as a function of P₀₂ (between 10^"4 atm and 1 atm at 520 °C)is plotted in FIG. 5C. The inset of

FIG. 5C shows the high-frequency intercept. A large increase in LF impedance was observed with changing P₀^ while HF was unchanged. Three distinctive features were noted, which are labeled as high-frequency (HF), middle-frequency (MF) and low- frequency (LF). The EIS data were fitted using a standard resistor (R_HF) for HF and resistors (R₁) in parallel with a constant phase elements (CPE₁) for MF and LF (R_HF- (R_MF/CPE_MF)-(R_LF/CPE_LF)). The CPE impedance can be written as Z =

where ω is the angular frequency, Q is the non-ideal "capacitance", and n is the non- ideality factor of CPE. The HF feature was found unchanged with P_Oi , and its magnitude and activation energy (about 1.15 eV) were comparable to those of oxygen ion conduction in YSZ reported previously. (See P. S. Manning, J. D. Sirman, R. A. DeSouza, J. A. Kilner, Solid State Ionics 1997, 100, 1)

FIGS. 13A-13B include (A) middle frequency (MF) feature in EIS for 130nm LSC film shown as a function of P_Oi at 520 °C and (B) a plot of MF resistance (RMF) showing substantially no trend with varying P₀^ for the 45 nm and 130 nm LSC films at

520 °C. The MF feature, which was found to have area-specific capacitances of about IxIO^"3 F cm^"2 and unaffected by P_0^ , may have been due to interfacial transport of oxygen ions between the LSC film and the GDC layer, as a comparable feature (with area-specific capacitance of 0.01-0.04 F cm^"2) was previously observed on

La_{0 5}Sr_{0 5}CoO₃ZSYSZ. (See Y. L. Yang, C. L. Chen, S. Y. Chen, C. W. Chu, A. J.

Jacobson, J. Electrochem. Soc. 2000, 147, 4001) In contrast, the LF feature changed significantly with P₀^ , with the real impedance decreasing with increasing P_α from 10^"4 atm to 1 atm. The fitted values of n for semi-circle CPE _LF were found to range from about 0.96 to about 1.0 over the entire examined P₀ range. Not wishing to be bound by any theory, this may indicate that the ORR kinetics on the LSC thin-film electrodes were limited by surface oxygen exchange. This is in agreement with the critical thickness L_c, estimate where L_c = D*/k*, where D* is tracer oxygen diffusivity and k* is tracer surface exchange coefficient, below which the ORR kinetics are limited by surface oxygen exchange but not by oxygen ion diffusion. LSC films of 45 nm and 130 nm are considerably smaller than the L_c of 1 micron found for LSC powder at 520 ⁰C.

Electrical and chemical surface oxygen exchange coefficients were calculated from real and imaginary impedance data as a function of P₀ , respectively. The electrical surface exchange coefficient (Id), which is comparable to k (See J. Maier, Solid State Ionics 1998, 112, 197, which is incorporated herein by reference in its entirety), was determined using the expression k^q = RT 14F² R_LFA_eleclrodec_o , where R is the universal gas constant (8.314 J mol^"1 K^"1), T is the absolute temperature, F is the

Faraday's constant (96,500 C mol^"1), R_LF is the LF real resistance, A_eι_ectrOde is the area of the microelectrode and C₀ is the lattice oxygen concentration in LSC. C₀ can be calculated as: c₀ = (3-δ)/V_m, where V_1n is the molar volume of LSC = 33.66 cm³ mol^"1. (See J.

Maier, Physical chemistry of ionic materials: ions and electrons in solids John Wiley,

Chichester, England ; Hoboken, NJ 2004, p. 537 and J. Fleig, J. Maier, J. Eur. Ceram.

Soc. 2004, 24, 1343, which are incorporated herein by reference in their entirety) In addition, the chemical surface exchange coefficient (k_Chem) was determined as

k_chem = l/τ , where / is the film thickness, and ris the reciprocal of the angular peak frequency of the LF semicircle. (See Y. L. Yang, C. L. Chen, S. Y. Chen, C. W. Chu, A.

J. Jacobson, J. Electrochem. Soc. 2000, 147, 4001 and G. T. Kim, S. Y. Wang, A. J.

Jacobson, Z. Yuan, C. L. Chen, J. Mater. Chem. 2007, 17, 1316, which are incorporated herein by reference in their entirety)

FIG. 5D includes plots of calculated Id and k_ct,_em values for LSC films with thicknesses of 45 nm (o) and 130 nm (D), each with 200 micron diameters, obtained between 10^"4 to 1 atm at 520 °C are shown in FIG. 5D. FIG. 5D also includes plots of extrapolated k (approximately equivalent to W ~ k ) and k_Chem values from previous studies of (▲ ) De Souza et al. , (Solid State Ionics 1999, 126, 153) (T) Carter et al, (

Solid State Ionics 1992, 53-6, 597) and (Δ)Van Der Haar et al. (L. M. van der Haar, M.

W. den Otter, M. Morskate, H. J. M. Bouwmeester, H. Verweij, J. Electrochem. Soc.

2002, 149, J41), for comparison. The P₀₂ dependence of A* (k^q∞ P£) was found to be m '45nm = 0.62 and m '_130nm = 0.65, while the P₀₂ dependence of k_chem (k^chem oc P₀" ) was m4$_nm— 0.66 and mi₃o_nm - 0.72. These values were in good agreement with those of k_cf,_em found for LSC powder, from which a dissociative adsorption step was proposed as rate- limiting for surface oxygen exchange (m in the range of about 0.43 to about 0.92) (See S. B. Adler, X. Y. Chen, J. R. Wilson, J. Catal. 2007, 245, 91)

FIG. 14A includes a plot of the area specific chemical capacitance (ASC) of LSC films of 45 nm (o) and 130 nm (o) as a function of P₀ at 520 ⁰C, and fitting curves of LSC 45nm ( ^v ) ' and LSC 1 ,3_nOnm ( ^v- - -) ' based on the method of Kawada et al. ( ^vSee T.

Kawada, J. Suzuki, M. Sase, A. Kaimai, K. Yashiro, Y. Nigara, J. Mizusaki, K.

Kawamura, H. Yugami, J. Electrochem. Soc. 2002, 149, E252). W and k_chem for the 45 nm film were higher than those for the 130 nm film, and W and k_cι_iem from both films were greater than those extrapolated (See FIG. 14) from results previously reported for LSC powder, as shown in FIG. 5D. Not wishing to be bound by any theory, the enhanced k⁹ and k_chem of LSC thin films might be attributed to strains in the epitaxial structure, which could induce changes in the thermodynamic and electronic properties of LSC films and lead to higher oxygen nonstoichiometry relative to LSC powder.

Volume-specific chemical capacitance (VSC), indicative of changes in the oxygen nonstoichiometry induced by changes in the electrical potential, was calculated from area-specific capacitance (ASC) (See FIG. 14B), which was obtained from EIS data using the expression: ASC = (l / A_eleclrode "j{(R_LF )'^"" QJ'" . (See J. Fleig, Solid State Ionics 2002, 150, 181, which is incorporated herein by reference) FIG. 14B includes a plot outlining the extrapolation of K_tr data from LSC powder data down to 520 °C, accomplished by drawing a linear extrapolation from available data points above 700 °C. The extrapolation was accomplished using the methods described in van der Haar et al. (see J. Electrochem. Soc. 2002, 149, J41). This extrapolation was repeated for the entire range of P₀^ reported and shown in FIG. 5D. FIG. 5E includes calculated VSC and thermodynamic factor, γ, of 45nm and 130 nm LSC microelectrodes having 200 micron diameters. In FIG. 5E, γ decreases with decreasing P₀ between 10^"4 atm to 1 atm at

520 ⁰C. VSC values of LSC films of 45 nm and 130 nm were found to increase with decreasing P₀ from 1 atm to 10^"4 atm at 520 ⁰C, with the 45 nm film having a higher

VSC than the 130 nm film. In addition, it should be noted that VSCs of both LSC films at 520 ⁰C were approximately twice that of the LSC powder at 10^"4 atm (VSC_p0Wder = 320 F cm^"3) while they were about 100 times greater at 1 atm (VSC_p0Wder = 3 F cm^"3). The

VSC values of LSC powder were calculated as: VSC = (AF²

+ 3RT/δ(3 - δ)J\ where a(x) is a factor that describes deviation from ideal solid solution, specifically how the enthalpy of oxygen in LSC changes with δ. (See T. Kawada, J. Suzuki, M. Sase, A. Kaimai, K. Yashiro, Y. Nigara, J. Mizusaki, K. Kawamura, H. Yugami, J. Electrochem. Soc. 2002, 149, E252) The a(x) value of LSC powder was obtained from a previous study of Mizusaki et al. while lvalues were extrapolated to 520 ⁰C from the

thermodynamic parameters of Mizusaki et al. (See J. Solid State Chem. 1989, 80, 102)

Moreover, the trend in magnitude of VSC for the LSC thin films was in agreement with its thermodynamic enhancement factors (χ), which describes the extent of the release and uptake of oxygen as a function of P_a . FIG. 15 includes a plot of the thermodynamic enhancement factor (χ) of the epitaxial LSC films with thicknesses of 45 nm (o), 130 nm (G) and La_{0 8}Sr_{0 2}CoO_3-δ powder (- - -) at 520 °C. χof the LSC films were calculated using the relationship: γ= k_Ch_en/k?- (See S. Kim, Y. L. Yang, A. J.

Jacobson, B. Abeles, Solid State Ionics 1999, 121, 31). The γ values from the 130-nm film were found to be about 1.3 times higher than those of the 45-nm film, γ of La_{0 8}Sr₀₂CoO_3-s powder was calculated as follows: (1) The relationship between δ and P₀ for La₀ sSr₀₂CoO_3-δ powder at 520 °C was calculated by using the thermodynamic parameters reported by Mizusaki et al. (See J. Mizusaki, Y. Mima, S. Yamauchi, K. Fueki, H. Tagawa, J. Solid State Chem. 1989, 80, 102) (2) γ Of La_{0 8}Sr₀₂CoO_3-5 powder was estimated from the relationship between P₀ and c_o, by using the expression of γ = dμ_o /2RTd\nc_o = 5InP₀₂ /291nc_o . (3) The oxygen concentration C₀, was defined by (3 - <5 )I V_m , where V_1n is the molar volume of La_{0 8}Sr₀₂CoO_3-δ (33.66 cm³ mor') and δ is the oxygen nonstiochiometry that is a function of P₀ . γ values of thin films are generally 2 orders of magnitude lower than those of LSC powder at high P₀ and they become comparable at 10^"4 atm (See FIG. 16). Not wishing to be bound by any theory, higher VSC and lower γ of the LSC films in this study, compared to LSC powder, indicate that it may be easier to change δm^' . the LSC films with electrical potential or

V

The lvalues in the LSC films were obtained following the procedure of Kawada et al. (See T. Kawada, J. Suzuki, M. Sase, A. Kaimai, K. Yashiro, Y. Nigara, J.

Mizusaki, K. Kawamura, H. Yugami, J. Electrochem. Soc. 2002, 149, E252, which is incoφorated herein by reference in its entirety) The expression

J= 3/2 - 1 /2^9 - \2RT I^AF² ICyscVJ - a{ Jt)] was substituted into

T[AS_Q (X) +

- <5))], which was used to fit the experimental VSCs of LSC films at different P₀^ . The data fitting first allowed estimation of the thermodynamic parameters including the standard partial molar enthalpy (in equilibrium with 1 atm of O₂) of oxygen AH_Q {X) , nonideality factor a(x), and standard partial molar entropy of oxygen in LSC As_o° {x) , of the LSC films at 520 °C (See Table 3). Δ/7_ø(x)and As_o° (x) found for LSC thin films were comparable to those of

LSC powder while a(x) for LSC thin films was about 3 times greater than that of LSC powder. (See J. Mizusaki, Y. Mima, S. Yamauchi, K. Fueki, H. Tagawa, J. Solid State

Chem. 1989, 80, 102) The a(x) parameter can be related to the density of states near the

Fermi Level, g(εβ, by using: a(x) = B/g(εf). (See M. H. R. Lankhorst, H. J. M.

Bouwmeester, H. Verweij, Solid State Ionics 1997, 96, 21 ; M. H. R. Lankhorst, H. J. M.

Bouwmeester, H. Verweij, Phys. Rev. Lett. 1996, 77, 2989; and M. H. R. Lankhorst, H. J. M. Bouwmeester, H. Verweij, J. Solid State Chem. 1997, 133, 555) Not wishing to be bound by any theory, higher a(x) values may suggest reduction in the g(εβ in the LSC thin films.

The c> values in the LSC films at 520 ⁰C were calculated subsequently using experimental VSCs and estimated a(x) from the calculation above. FIG. 5F includes plots of the oxygen nonstoichiometry δ, of 45 nm and 130 nm LSC films as a function of

P₀ at 520 ⁰C. Fitting curves of LSC_45nm ( ) and

( ) are shown using the method by Kawada et al. (T. Kawada, J. Suzuki, M. Sase, A. Kaimai, K. Yashiro, Y. Nigara, J. Mizusaki, K. Kawamura, H. Yugami, J. Electrochem. Soc. 2002, 149, E252). lvalues for LSC powder at 520 °C, which were calculated by using thermodynamic parameters of Mizusaki et al. (J. Mizusaki, Y. Mima, S. Yamauchi, K. Fueki, H. Tagawa, J. Solid State Chem. 1989, 80, 102), are shown for comparison. Estimated

thermodynamic parameters for LSC_45nm are Δ/?£(x) = -110 kJ mol^"1, a(x) = 1,000-1,500 kJ mol^"1, As_o° (x) = -92 J mol^"1 K^"1, and for LSCi_30nm are Ah₀° {x)= -1 10 kJ mol^"1, a(x) = 1,400-1,700 kJ mol^"1, As₀° (x) = -90—98 J mol^"1 K^"1. Interestingly, δ values of the LSC films were about 3 to 4 times higher than that of LSC powder extracted from the thermodynamic parameters of Mizusaki et al. (J. Solid State Chem. 1989, 80, 102) at 10^"4 atm while they were found to increase to about 100 times greater at P₀ greater than 0.2 atm. This is in contrast to the findings of polycrystalline LSC films reported by Kawada et al. (J Electrochem. Soc. 2002, 149, E252) where lvalues of polycrystalline

La_{0 6}Sr₀₄CoO₃ films at 600 °C were found to have considerably lower oxygen vacancy concentrations than LSC powder. (See J. Mizusaki, Y. Mima, S. Yamauchi, K. Fueki, H. Tagawa, J. Solid State Chem. 1989, 80, 102) Not wishing to be bound by any theory, it is hypothesized that higher lvalues of epitaxial LSC thin films estimated from EIS data may be due to residual strains in the films at 520 ⁰C. Using the volumetric chemical expansion coefficient (β_c = 0.112) induced oxygen nonstoichiometry changes for LSC reported previously (X. Y. Chen, J. S. Yu, S. B. Adler, Chem. Mat. 2005, 17, 4537) small uniaxial strains greater than 0.06% would be sufficient to induce the difference in the δ value between the 45-nm LSC film and LSC powder (J. Mizusaki, Y. Mima, S.

Yamauchi, K. Fueki, H. Tagawa, J. Solid State Chem. 1989, 80, 102) at 10^"4 atm shown in FIG. 5F. The findings of epitaxial LSC (001)_pc films in this example illustrate ways to achieve enhanced surface oxygen exchange and ORR activity on strained oxide surfaces, which can provide new strategies to develop highly active cathode materials for highly- efficient and intermediate-temperature SOFCs.

In summary, LSC (001)_pc films of 45 and 130 nm were grown epitaxially on a YSZ (001) substrate with a GDC buffer layer of about 5 nm. These epitaxial LSC films were found to have enhanced Id and k_chem up to one order of magnitude higher than LSC powder in the P₀₂ range from 10^"4 to 1 atm at 520 ⁰C. In addition, VSC of these epitaxial LSC films and estimated lvalues of these films from experimental VSC values were considerably greater than those of LSC powder. Not wishing to be bound by any theory, enhanced surface oxygen exchange kinetics on the epitaxial LSC (001)_pc surface may be due to strains in the films. For example, FIGS. 16A- 16G include schematic figures illustrating oxygen incorporation into a (001)_pc LSC film. FIG. 16A includes an illustration of the dissociation of O₂ molecule and incorporation into the oxygen vacancy sites of the LSC film surface. FIG. 16B includes an illustration of hopping to the neighboring oxygen vacancy. FIG. 16C includes an illustration of diffusion in the lattice structure. During the migration stage, incorporated oxygen can jump to the vacancy site from an initial state (FIG. 16D), an activated state (FIG. 16E) to a final state (FIG. 16F). This jump of oxygen to neighboring vacancy site has an activation energy barrier and this is related with the in-plane lattice parameter of LSC film (See FIG. 16G). The in- plane dilation of the film can make this jump by reducing required activation energy. This also might enhance the supply of the oxygen vacancy to the surface of the film resulting in better catalytic activity.

Experimental Procedures

Target Synthesis:

La_{0 8}Sr₀₂CoO₃ (LSC) was synthesized using solid-state methods from

stoichiometric mixtures OfLa₂O₃, SrCO₃, and Co₃O₄ (Alfa Aesar, USA) calcined at

1,000 ⁰C in air for 12 hours. Gd_{0 2}Ce₀₈O₂ (GDC) was prepared by Pechini method using Gd(NO₃)₃ and Ce(NO₃)₃ (Aldrich, USA) dissolved in de-ionized water with ethylene glycol, citric acid (Aldrich, USA). After esterification at 100 ⁰C, the resin was charred at 400 ⁰C and finally calcined at 800 ⁰C in air for 1 hour. Pulsed laser deposition (PLD) target pellets with 25 mm diameter were subsequently fabricated by uniaxial pressing at 50 MPa. The pellets were fully sintered at 1,100 ⁰C in air for 14 hours.

Film Growth:

Single crystal 9.5 mol% Y₂O₃-stabilized ZrO₂ (YSZ) wafers, (001) orientation and dimensions of 10x10x0.5 mm (Princeton Scientific, USA), were used as substrate. Prior to LSC and GDC deposition, a platinum ink (Pt) (#6082, BASF, USA) counter electrode was painted on one side of the YSZ and dried at 800 ⁰C in air for 1 hour. The YSZ wafer was affixed to the PLD substrate holder using a small amount of silver paint (Leitsilber 200, Ted Pella, USA) for thermal contact. PLD was performed using a KrF excimer laser at λ = 248 nm, 10 Hz pulse rate and 50 mJ pulse energy. The YSZ substrate was positioned about 5 cm from the target and deposition was done under P₀ of 10 mTorr at 680 °C. The GDC interlayer was deposited with 500 pulses while the LSC films were deposited with 15,000 and 30,000 pulses. The utilization of reflection high-energy electron diffraction (RHEED) enabled diagnostic in situ monitoring of the LSC film growth. FIG. 17A-17B include the typical RHEED pattern of the substrate (A) initially parallel to the [100] direction of the YSZ (001) substrate before deposition, and (B) diffracted from LSC (001) film in [1 10] azimuth after deposition of 45 nm thickness. Initially, the RHEED diffraction pattern was obtained from the (001) YSZ with the [100] orientation (FIG. 17A). At the end of LSC deposition, typical bidimensionally diffraction features corresponding to the growth construction from LSC film was obtained (FIG. 17B ).

Film Characterization and Electrodes Fabrication:

The composition of the epitaxial film was analyzed using Rutherford

backscattering spectroscopy (Evans Analytical, CA) using He²⁺ ion beam at 2.275 MeV with a detector angle of 160° and 1 12° for normal and shallow grazing angle, respectively. Thin-film X-ray diffraction was performed using a four-circle

diffractometer (Bruker D8, Karlsruhe, Germany). Measurements were performed in normal and off-normal configuration.

Positive photoresist (MicroChem, USA) was applied on the LSC surface and patterned using a mask aligner (Karl Suss, Germany, Λ=365 nm). The photoresist was developed (MicroChem, USA) and the metal oxide thin-films were etched in hydrochloric acid (HCl) to remove LSC film excess and create the circular

microelectrodes. Before electrochemical testing, microelectrode geometry and morphology was examined by optical microscopy (Carl Zeiss, Germany) and atomic force microscopy (AFM) (Veeco, USA). AFM measurements after acid-etching of the LSC film revealed thicknesses of 45 nm and 130 nm for 15,000 pulses and 30,000 pulses, respectively.

Electrochemical Impedance Spectroscopy:

The LSC microelectrode and porous Pt counter electrode were contacted by Pt- coated tungsten probes having a radius of 0.6 microns. EIS measurements of microelectrodes about 200 microns in diameter were performed using a microprobe station (Karl Suss, Germany) connected to a frequency response analyzer (Solartron 1260, USA) and dielectric interface (Solartron 1296, USA). Temperature was controlled at 520 °C with heating stage (Linkam TS 1500, UK) and data were collected between 1 MHz to 1 mHz using a voltage amplitude of 10 mV. EIS testing temperature was calibrated with a thermocouple contacting the thin-film surface and deviation of ±5 °C was observed. EIS experiments were completed under varying Ar to O₂ mixtures between P_0^ of 10^"4 atm and 1 atm. ZView software (Scribner Associates, USA) was used to construct the equivalent circuit and perform complex least squares fitting. Table 2 includes the calculated chemical expansion of unit cell (ΔF/F)_cAem using relaxed cubic unit cell volume and bulk LSC cubic volume from Table 1. The unit cell volume of LSC films were found to be greater than bulk LSC powder with an expansion of 2.45 % and 1.65 % for the films of 45 nm and 130 nm, respectively. This volume increase, known as chemical expansion was utilized to calculate the expected oxygen nonstoichiometry, δ, of the LSC film. Utilizing the volumetric chemical expansion coefficient of bulk LSC powder determined at temperatures greater than 600 ⁰C from a previous study, δ of 0.22 and 0.17 was estimated for as-deposited 45 nm and 130 nm LSC films at room temperature, respectively. It should be noted that lvalues calculated from electrochemical measurements at 520°C were lower than those estimated from the chemical expansion as-prepared LSC films at room temperature. Not wishing to be bound by any theory, this difference in nonstoichiometry may have been due to any of the following factors: 1) oxygen uptake into the LSC films during EIS measurements at 520 ⁰C as these films were deposited at oxygen partial pressure of 10 mTorr.

Considering the diffusivity of LSC, (D ~ 10^"12 cm² s^'1) and electrical surface exchange coefficient, tf, of ~10^"8 cm s^"1 from this example, the LSC film of 130 nm and 45 nm, should reach oxygen equilibrium in about 20 minutes. Given the extended time of EIS experiments performed at 520 °C (>4 hours), these LSC films have reached oxygen equilibrium; 2) the volumetric chemical expansion coefficient of LSC at room

temperature is considerably different from LSC powder determined at temperatures greater than 600 ⁰C; and 3) some changes in the unit cell volume of LSC thin films occur upon heating and EIS measurements at 520 ⁰C, which could lead to the chemical expansion of unit cell at 520 ⁰C smaller than those found at room temperature. Table 2. Calculated chemical expansion of unit cell(ΔF IV)_chem using relaxed cubic unit cell volume and bulk LSC cubic volume from Table 1.

LSCi_30n 1.65 % 0.168 δ is obtained by the derivation from Adler et al. (S. B.

Adler, Chem. Rev. 2004, 104, 4791)

Table 3. Calculated partial molar enthalpy, Ah_o° (x) , and entropy, Δs₀° (x) , of oxygen and non-ideality factor, a(x), for LSC bulk and polycrystalline and epitaxial thin-film. Detailed procedure to obtain parameters is explained in the experimental section and Kawada et al. (T. Kawada, J. Suzuki, M. Sase, A. Kaimai, K. Yashiro, Y. Nigara, J. Mizusaki, K. Kawamura, H. Yugami, J. Electrochem. Soc. 2002, 149, E252)

EXAMPLE 2

This example describes introducing a strain to multiple materials positioned proximate each other to enhance the surface exchange of oxygen. Specifically,

(La_{0 5} Sr_{0 5})₂CoO₄ (i.e., LSC_2I4) was deposited on La_{0 8}Sr₀₂CoO₃ and La₀₆Sr_{O 4}CoO₃ (i.e., 80-20 LSCn₃, and 60-40 LSCn₃, respectively) surfaces, and the surface exchange of oxygen was quantified.

First, PLD targets were synthesized. La_{0 8}Sr_{0 2}CoO₃ (LSCi π) was synthesized using solid-state reaction from stoichiometric mixtures Of La₂O₃, SrCO₃, Co₃O₄ (Alfa Aesar, USA) calcined at 1,000 °C in air for 12 hours. The (La_{0 5}Sr_{0 S})₂CoO₄ (LSC_2I4) and Gd_{0 2}Ce₀₈O₂ (GDC) powders were synthesized via the Pechini method.

La(NCb)₃ -6H₂O, Sr(NO₃)₂, Co(NO₃)₂-6H₂O, and separately Gd(NO₃)₃, and Ce(NO₃)₃ are dissolved into a de-ionized water, ethylene glycol, citric acid (Sigma-Aldrich, USA) mixture to synthesize LSC_2I4 and GDC respectively. After esterification at 100 °C, the resin was charred at 400 °C and then calcined at 800 ⁰C in air for 1 hour. Phase purity of the powders were confirmed from conventional Bragg-Brentano X-ray powder diffraction (XRD, Cu K alpha, 50 kV, 250 mA, Rigaku H3R). The pulsed laser deposition (PLD) targets with a 25 mm diameter were subsequently formed by uniaxial pressing at 50 MPa. The LSC_2I4, LSCn₃, and GDC targets were all sintered at 1350 ⁰C in air for 20 hours.

Next, LSCn₃ and LSC_2I4 were deposited via pulsed laser deposition PLD). Two single-crystal 9.5 mol% Y₂O₃-stabilized ZrO₂ (YSZ) substrates with (100) orientation, dimensions 5 x 5 x 0.5 mm and 10 x 5 x 0.5 mm, and one-sided polished (surface roughness <lnm) (Princeton Scientific, USA) were co-deposited for each film

configuration. Platinum ink (Pt, #6082, BASF, USA) counter electrodes were painted on the un-polished side of the YSZ and sintered at 800 °C in air for 1 hour. The oxide films were deposited by pulse laser deposition (PLD) using a KrF excimer laser with λ = 248 run, 10 Hz pulse rate, and about 50 mJ pulse energy under /O₂ of 10 mTorr with 500 pulses of GDC (about 5 nm) at 450 °C, followed by 15,000 pulses of LSC, ₁₃ (about 50 nm) at 675 °C. Reflection high-energy electron diffraction (RHEED) was utilized to provide in-situ monitoring of GDC/LSCi i₃/LSC₂i₄ film growth. Immediately after completing the LSCn₃ base film deposition a surface was is subsequently deposited; a reference sample adding an additional 25 pulses LSCn₃ (about 0.3 nm), and then the primary experimental samples consisting of about 0.3 nm (25 pulses), about 2 nm (150 pulses), about 12 nm (900 pulses), and about 35 nm (2700 pulses) of LSC_{2 I4}. After completing the final deposition, the samples were cooled to room temperature over the course of about 1 hr within the PLD chamber at 10 mTorr.

Oxide phase purity and orientation were investigated via thin-film HRXRD using a four-circle diffractometer (Panalytical, USA). Surface morphology was examined by optical microscopy (Carl Zeiss, Germany), atomic force microscopy (AFM, Veeco, USA), and scanning electron microscopy (SEM, Carl Zeiss, Germany). A 1% lattice mismatch between the LSCi ₁₃ films and the LSC_2H films was observed; the LSC_2J4 films were in tension while the LSCn₃ films were in compression. FIGS. 22A-22B include exemplary schematic diagrams outlining the relationship between the LSCn₃ and LSC_{2 I4} lattices. FIG. 22A includes a polyhedral model, while FIG. 22B includes a space-filling model

The LSC microelectrodes were fabricated by photolithography and the following process: OCG positive photoresist (Arch Chemical Co, USA) was applied on the film surface and patterned using a mask aligner (Karl Siiss, Germany, λ =365 nm). The photoresist was developed using Developer 934 1 :1 (Arch Chemical Co., USA) developer, and the films were subsequently etched in hydrochloric acid (HCl) to form circular microelectrodes with diameters of about 200 μm. The photoresist was removed with acetone. Electrochemical impedance spectroscopy (EIS) was collected using a microprobe station setup (Karl Siiss, Germany) connected to a frequency response analyzer (Solatron 1260, USA) and a dielectric interface (Solartron 1296, USA). Pt- coated tungsten carbide probes were utilized to contact the LSC electrode and porous Pt counter electrode. FIG. 18A includes a schematic illustration of the sample testing configuration. Temperature was controlled at 550 ⁰C using a heating stage (Linkam TS 1500, UK, temperature measured with second thermocouple on sample surface), and data were collected between 1 MHz to 1 mHz using a voltage amplitude of 10 mV. EIS experiments were conducted under Ar and O₂ mixtures in the range from P₀2 of 10^"4 to 1 atm. FIGS. 18B-18C include details of the equivalent circuit and corresponding Nyquist plot for this experimental system. ZView software (Scribner Associates, USA) was used to construct the equivalent circuit and perform complex least-square-fitting to extract the fitting parameters to describe the system.

EIS measurements allowed for the determination of electrical oxygen exchange coefficient (k^q) and chemical oxygen exchange coefficient (k^chem) values for the (100) surfaces Of La_{0 8}Sr₀₂CoO₃ (i.e., LSCm 80-20) and La_{0 6}Sr₀₄CoO₃ (i.e., LSCn₃ 60-40), with different coverages Of (La_{0 S}Sr_{0 S})₂CoO₄ (i.e., LSC_2I4), as shown in FIGS. 19A-19B and FIGS. 20A-20B, respectively. All of the films on which LSC_2I4 was added exhibited enhanced oxygen reduction reactions compared to the base film for both LSCn₃ 80-20 and LSC_{1 13} 60-40. The largest enhancement was observed on samples with relatively low coverage of LSC_2I4. For samples with a base film Of La_{0 8}Sr₀₂CoO₃, W

enhancements of up to 3 orders of magnitude were observed. For samples with a base film Of La_{O 6}Sr₀₄CoO₃, enhancements of up to 2 orders of magnitude were observed. Similar enhancement trends were observed for k^{c em} for both Lao sSro ₂CoO₃ and

La_{0 6}Sr_{O 4}CoO₃.

The volume specific capacitance (VSC) values of these films are shown in FIGS. 21 A-21B. The VSC values did not appear to be influenced by the addition of LSC_2I4. LSC 60-40 exhibited a larger VSC than LSC 80-20, which may have been due to the higher oxygen vacancy concentration with increasing Sr doping in the perovskite material.

While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any

combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the scope of the present invention.

The indefinite articles "a" and "an," as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean "at least one."

The phrase "and/or," as used herein in the specification and in the claims, should be understood to mean "either or both" of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the "and/or" clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to "A and/or B," when used in conjunction with open-ended language such as "comprising" can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

As used herein in the specification and in the claims, "or" should be understood to have the same meaning as "and/or" as defined above. For example, when separating items in a list, "or" or "and/or" shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as "only one of or "exactly one of," or, when used in the claims, "consisting of," will refer to the inclusion of exactly one element of a number or list of elements. In general, the term "or" as used herein shall only be interpreted as indicating exclusive alternatives (i.e. "one or the other but not both") when preceded by terms of exclusivity, such as "either," "one of," "only one of," or "exactly one of." "Consisting essentially of," when used in the claims, shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase "at least one," in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase "at least one" refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, "at least one of A and B" (or, equivalently, "at least one of A or B," or, equivalently "at least one of A and/or B") can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitional phrases such as "comprising," "including," "carrying," "having," "containing," "involving," "holding," and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases "consisting of and "consisting essentially of shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.

What is claimed is:

Claims

1. An article, comprising:

a material constructed and arranged for the surface exchange of oxygen, wherein the electrical surface exchange coefficient of the material under a strain with respect to oxygen is greater than the electrical surface exchange coefficient of an unstrained material with respect to oxygen under essentially identical conditions.

2. A method, comprising:

introducing a strain to a material constructed and arranged for the surface exchange of oxygen, and

exposing the material to oxygen such that, upon contacting the material, the oxygen undergoes a surface exchange reaction with the material at a rate greater than the rate of the surface exchange reaction of the oxygen with an unstrained material under essentially identical conditions.

3. An article or method as in any one of the preceding claims, wherein the material comprises an oxide.

4. An article or method as in any one of the preceding claims, wherein the material comprises an oxide with the formula Di_-xE_xBO_y, wherein

D and E are different and are each chosen from the list consisting of La, Sr, Ca, Ba, Li, Na, K, Cs, Ce, Pr, Nd, Sm, Pm, Eu, Gd, Pm, Tb, Dy, Ho, Tm, and Yb;

B comprises a transition metal; and

O represents oxygen.

5. An article or method as in any one of the preceding claims, wherein the material comprises Lai_-xSr_xCoO₃.

6. An article or method as in any one of the preceding claims, wherein the material comprises La_{0 8}Sr₀₂CoO₃.

7. An article or method as in any one of the preceding claims, wherein the material comprises a crystalline material.

8. An article or method as in any one of the preceding claims, wherein introducing a strain comprises forming a layer of the material over a substrate.

9. An article or method as in any one of the preceding claims, wherein the lattice parameter of the material and the lattice parameter of the substrate are different

10. An article or method as in any one of the preceding claims, wherein forming a layer of the material over a substrate comprises epitaxially growing a layer of the material on the substrate.

11. An article or method as in any one of the preceding claims, wherein introducing a strain comprises forming the material over a core, wherein the material at least partially encapsulates the core, and the lattice parameter of the core and the lattice parameter of the material are different.

12. An article or method as in any one of the preceding claims, wherein introducing a strain comprises heating the material and the substrate, wherein the substrate has a first thermal expansion coefficient, and the material has a second thermal expansion coefficient that is different from the first thermal expansion coefficient.

13. An article or method as in any one of the preceding claims, wherein introducing a strain comprises introducing a chemical strain.

14. An article or method as in any one of the preceding claims, wherein the electrical surface exchange coefficient of the strained material with respect to oxygen is at least about 2 times greater than the electrical surface exchange coefficient of an unstrained material with respect to oxygen under essentially identical conditions.

15. An article or method as in any one of the preceding claims, wherein the electrical surface exchange coefficient of the strained material with respect to oxygen is at least about 5 times greater than the electrical surface exchange coefficient of an unstrained material with respect to oxygen under essentially identical conditions.

16. An article or method as in any one of the preceding claims, wherein the electrical surface exchange coefficient of the strained material with respect to oxygen is at least about 10 times greater than the electrical surface exchange coefficient of an unstrained material with respect to oxygen under essentially identical conditions.

17. An article or method as in any one of the preceding claims, wherein the electrical surface exchange coefficient of the strained material with respect to oxygen is at least about 100 times greater than the electrical surface exchange coefficient of an unstrained material with respect to oxygen under essentially identical conditions.

18. An article or method as in any one of the preceding claims, wherein the electrical surface exchange coefficient of the strained material with respect to oxygen is at least about 1000 times greater than the electrical surface exchange coefficient of an unstrained material with respect to oxygen under essentially identical conditions.

19. An article or method as in any one of the preceding claims, wherein the absolute value of the strain of the material, measured relative to an unstrained sample of the material at room temperature, is at least about 0.01%.

20. An article or method as in any one of the preceding claims, wherein the absolute value of the strain of the material, measured relative to an unstrained sample of the material at room temperature, is at least about 0.05%.

21. An article or method as in any one of the preceding claims, wherein the absolute value of the strain of the material, measured relative to an unstrained sample of the material at room temperature, is at least about 0.1%.

22. An article or method as in any one of the preceding claims, wherein the absolute value of the strain of the material, measured relative to an unstrained sample of the material at room temperature, is at least about 0.5%.

23. An article or method as in any one of the preceding claims, wherein the absolute value of the strain of the material, measured relative to an unstrained sample of the material at room temperature, is at least about 1%.

24. An article or method as in any one of the preceding claims, wherein the material comprises a film with an average thickness of less than about 10 microns.

25. An article or method as in any one of the preceding claims, wherein the material comprises a film with an average thickness of less than about 1 micron.

26. An article or method as in any one of the preceding claims, wherein the material comprises a film with an average thickness of less than about 100 nm.

27. An article or method as in any one of the preceding claims, wherein the material comprises a film with an average thickness of less than about 10 nm.

28. An article or method as in any one of the preceding claims, wherein introducing a strain comprises changing the composition of the material.

29. An article or method as in any one of the preceding claims, wherein the strain is a tensile strain.

30. An article or method as in any one of the preceding claims, wherein the strain is a compressive strain.

31. An article or method as in any one of the preceding claims, wherein a second material constructed and arranged for the surface exchange of oxygen is positioned proximate the first material constructed and arranged for the surface exchange of oxygen.

32. An article or method as in any one of the preceding claims, wherein the first material constructed and arranged for the surface exchange of oxygen comprises Lai_-xSr_xCoθ₃, and the second material constructed and arranged for the surface exchange of oxygen comprises (La_1-bSr_b)₂CoO₄.