PROTONIC CERAMIC ELECTROCHEMICAL CELLS (PCECS) COMPRISING A CONTACT MATERIAL FOR AN OXYGEN ELECTRODE, AND RELATED PCEC STACKS AND METHODS OF PRODUCING HYDROGEN GAS

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No. 63/730,052, filed Dec. 10, 2024, the disclosure of which is hereby incorporated herein in its entirety by this reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contact No. DE-AC07-05-ID14517 awarded by the United States Department of Energy. The government has certain rights in the invention.

TECHNICAL FIELD

This disclosure relates generally to protonic ceramic electrochemical cells (PCECs), and more particularly to PCECs comprising a contact material for an oxygen electrode, the contact material comprising a chemical formula of LaM_xN_1−xO_3−δ, where M and N are independently selected from a transition metal; x is a real number in a range of 0≤x≤1; and δ is an oxygen deficiency.

BACKGROUND

Solid-oxide electrolysis cells (SOECs) are one type of electrochemical cell that has been employed to produce hydrogen from water (e.g., steam) when operated in an electrolysis cell mode and to produce power when operated in a fuel cell mode. However, SOECs can suffer from material degradation and material incompatibilities at a high operating temperature (e.g., above about 600° C., above about 700° C.) used to operate the SOECs.

Protonic ceramic electrochemical cells (PCECs) may be operated at relatively lower temperatures compared to SOECs since a proton-conducting electrolyte material of the PCECs generally exhibits a lower ionic diffusion activation energy than an oxygen ion-conducting electrolyte material (e.g., yttria-stabilized zirconia (YSZ), gadolinia-doped ceria (GDC)) of the SOECs.

PCECs may be used for power generation in a fuel cell mode, and hydrogen production in an electrolysis cell mode. In order to realize commercial success, PCECs have been combined in stacks to make them scalable. However, conventional PCEC stack technologies suffer from poor electrical contact between the PCECs within the PCEC stack. This reduces the scalability of the PCEC stack, and limits the utilization and commercialization of the PCEC stack.

Furthermore, the electrochemical activity and stability of a contact material for oxygen electrodes of the PCEC stacks play an important role in the overall PCEC performance and energy efficiency. Currently, the contact materials for the oxygen electrodes of SOECs have been utilized as the contact materials for the oxygen electrodes of PCECs. However, such contact materials suffer from high cost and poor stability under the operating conditions of PCECs (e.g., high steam concentration).

SUMMARY

In a first aspect, a protonic ceramic electrochemical cell (PCEC) is disclosed. The PCEC includes an oxygen electrode configured to produce oxygen gas from steam and a hydrogen electrode configured to produce hydrogen gas from the steam. The oxygen electrode includes a first side and a second side opposite to the first side. The PCEC also includes a proton-conducting ceramic electrolyte between the hydrogen electrode and the first side of the oxygen electrode. The PCEC further includes a contact material adjacent to the second side of the oxygen electrode. The contact material includes a chemical formula LaM_xN_1−xO_3−δ, where each of M and N is independently a transition metal; x is a real number in a range of 0≤x≤1; and δ is an oxygen deficiency.

In a second aspect, a protonic ceramic electrochemical cell (PCEC) stack is disclosed. The PCEC stack includes at least one PCEC and at least one additional PCEC adjacent to the at least one PCEC. The at least one PCEC includes an oxygen electrode, a hydrogen electrode, a proton-conducting ceramic electrolyte between the hydrogen electrode and one side of the oxygen electrode, and a contact material adjacent to other side of the oxygen electrode opposite to the one side of the oxygen electrode. The at least one additional PCEC includes an additional oxygen electrode, an additional hydrogen electrode, an additional proton-conducting ceramic electrolyte between the additional hydrogen electrode and one side of the additional oxygen electrode, and an additional contact material adjacent to other side of the additional oxygen electrode opposite to the one side of the additional oxygen electrode. The PCEC stack further includes an interconnect material disposed between the contact material of the at least one PCEC and the additional hydrogen electrode of the at least one additional PCEC. At least one of the contact material and the additional contact material includes a chemical formula LaM_xN_1−xO_3−δ where M and N are independently selected from a transition metal; x is a real number in a range of 0≤x≤1; and δ is an oxygen deficiency.

In a third aspect, a method of producing hydrogen gas is disclosed. The method comprises feeding steam into a protonic ceramic electrochemical cell (PCEC). The PCEC comprises an oxygen electrode configured to produce oxygen gas from the steam, a hydrogen electrode configured to produce hydrogen gas from the steam, and a proton-conducting ceramic electrolyte between the hydrogen electrode and one side of the oxygen electrode. The PCEC further comprises a contact material in direct contact with another side of the oxygen electrode. The contact material comprises a chemical formula of LaM_xN_1−xO_3−δ wherein each of M and N is independently a transition metal; x is a real number in a range of 0≤x≤1; and δ is an oxygen deficiency. The method further comprises applying a potential difference between the oxygen electrode and the hydrogen electrode of the PCEC to produce the hydrogen gas from the steam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified isomeric view of a protonic ceramic electrochemical cell (PCEC), in accordance with embodiments of the disclosure;

FIG. 2 is a simplified schematic diagram showing the PCEC of FIG. 1 operated in an electrolysis mode to produce H₂ gas from steam;

FIG. 3 is a simplified isomeric view of a PCEC stack, in accordance with embodiments of the disclosure;

FIG. 4 is a graphical result showing the comparative electrical conductivities of various potential contact materials at a temperature range of from about 400° C. to about 1000° C.;

FIG. 5 is a graphical result showing the effect of steam concentration on the electrical conductivities of an LNC55 contact material and an LNC73 contact material at a temperature of about 600° C.;

FIG. 6 is a graphical result showing the durability of the electrical conductivities of the LNC55 contact material and the LNC73 contact material at a temperature of about 600° C. at different time periods;

FIG. 7 is a graphical result showing the thermal expansion coefficient values of various potential contact materials in comparison to the thermal expansion coefficient value of a PNC73 oxygen electrode at a temperature of about 600° C.;

FIG. 8 is a graph showing the comparative thermal expansion coefficient values of the LNC73 contact material, the LNC55 contact material, and the PNC73 oxygen electrode at a temperature range of from about room temperature to about 1000° C.;

FIG. 9 is a graph showing the comparative current densities of the PCECs at different voltages and at different operating temperature s when LNC55, LNC73 or silver (Ag) was used as the contact material for the oxygen electrode of PCECs; and

FIG. 10 is a graph showing the comparative polarization resistance of the PCECs at different voltages and at different operating temperature s when LNC55, LNC73 or silver (Ag) was used as the contact material for the oxygen electrode of PCECs.

DETAILED DESCRIPTION

A protonic ceramic electrochemical cell (PCEC) of the disclosure utilizes a contact material for the oxygen electrode that exhibits high electrical conductivity, high stability, and improved bonding with an oxygen electrode under the operating conditions of the PCEC (e.g., a steam concentration of at least about 50% by volume in air, an operating temperature of from about 400° C. to about 700° C.). The contact material for the oxygen electrode (also referred herein as “contact material”) exhibits substantially the same thermal expansion coefficient value as that of the oxygen electrode. The contact material may be substantially free of alkaline earth metal, therefore exhibiting an enhanced stability and durability under the operating conditions of the PCEC compared to conventional contact materials comprising alkaline earth metal (e.g., Sr). Furthermore, the contact material according to embodiments of the disclosure may be substantially free of precious metal (e.g., Ag, Au, Pt) and therefore significantly more cost-effective compared to conventional contact material comprising a precious metal, while providing the PCEC with a comparable electrochemical performance and cell durability as the conventional PCEC including the precious metal-based contact material.

The following description provides specific details, such as material compositions, device and/or system configuration, and operating conditions (e.g., temperatures) in order to provide a thorough description of embodiments of the disclosure. However, a person of ordinary skill in the art will understand that the embodiments of the disclosure may be practiced without necessarily employing these specific details. Indeed, the embodiments of the disclosure may be practiced in conjunction with conventional systems and methods employed in the industry. In addition, only those components and acts necessary to understand the embodiments of the disclosure are described in detail below. A person of ordinary skill in the art will understand that some components (e.g., temperature detectors) are inherently disclosed herein and that adding various conventional components and acts would be in accord with the disclosure.

Further, the illustrations presented herein are not actual views of any protonic ceramic electrochemical cell (PCEC) or any PCEC stack, or any component thereof, but are merely idealized representations, which are employed to describe embodiments of the present disclosure.

As used herein, the singular forms following “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

As used herein, “and/or” includes any and all combinations of one or more of the associated listed items.

As used herein, the term “may” with respect to a material, structure, feature, or method act indicates that such is contemplated for use in implementation of an embodiment of the disclosure, and such term is used in preference to the more restrictive term “is” so as to avoid any implication that other compatible materials, structures, features, and methods usable in combination therewith should or must be excluded.

As used herein, any relational term, such as “first,” “second,” “top,” “bottom,” “upper,” “lower,” “above,” “beneath,” “side,” “upward,” “downward,” etc., is used for clarity and convenience in understanding the disclosure and accompanying drawings, and does not connote or depend on any specific preference or order, except where the context clearly indicates otherwise. For example, these terms may refer to an orientation of elements (e.g., components) of any PCEC when utilized in a conventional manner. Furthermore, these terms may refer to an orientation of elements of any PCEC as illustrated in the drawings.

As used herein, the term “configured” refers to a size, shape, material composition, material distribution, orientation, and arrangement of one or more of at least one structure and at least one apparatus facilitating operation of one or more of the structure and the apparatus in a pre-determined way.

As used herein, the term “substantially” in reference to a given parameter, property, or condition means and includes to a degree that one skilled in the art would understand that the given parameter, property, or condition is met with a small degree of variance, such as within acceptable manufacturing tolerances. By way of example, depending on the particular parameter, property, or condition that is substantially met, the parameter, property, or condition may be at least 90.0% met, at least 95.0% met, at least 99.0% met, or even at least 99.9% met.

As used herein, the term “about” in reference to a numerical value for a particular parameter is inclusive of the numerical value and a degree of variance from the numerical value that one of ordinary skill in the art would understand is within acceptable tolerances for the particular parameter. For example, “about” or “approximately” in reference to a numerical value may include additional numerical values within a range of from 90.0 percent to 108.0 percent of the numerical value, such as within a range of from 95.0 percent to 105.0 percent of the numerical value, within a range of from 97.5 percent to 102.5 percent of the numerical value, within a range of from 99.0 percent to 101.0 percent of the numerical value, within a range of from 99.5 percent to 100.5 percent of the numerical value, or within a range of from 99.9 percent to 100.1 percent of the numerical value.

As used herein, the term “compatible” means that a material does not undesirably react, decompose, or absorb another material, and also that the material does not undesirably impair the chemical and/or mechanical properties of the another material.

As used herein, the term “protonic ceramic electrochemical cell” or “PCEC” means and includes an electrochemical cell that converts water (e.g., steam) to oxygen gas and hydrogen gas while operation in an electrolysis mode, and utilizes a ceramic electrolyte as a proton conductor between an anode and a cathode of the electrochemical cell.

As used herein, the term “oxygen electrode” means and includes an electrode of the PCEC configured to produce oxygen gas from steam when the PCEC is operated in an electrolysis mode.

As used herein, the term “hydrogen electrode” means and includes an electrode of the PCEC configured to produce hydrogen gas from steam when the PCEC is operated in an electrolysis mode.

As used herein, the term “alkaline earth metal” means one or more of six (6) elements found in Group 2 (also known as Group IIA) of the periodic table, which includes beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).

As used herein, the term “lanthanide element” means one or more of fifteen (15) metallic elements in the periodic table with atomic numbers of 57 through 71, which includes lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

As used herein, the term “transition metal” means and includes a metallic element in the d-block of the periodic table (groups 3-12) that have incompletely filled d-subshells.

FIG. 1 is a simplified isomeric view of a PCEC 100 in accordance with embodiments of the disclosure. The protonic ceramic electrochemical cell (PCEC) 100 includes, when operated in an electrolysis mode, an oxygen electrode 102 configured to produce oxygen gas from steam and a hydrogen electrode 106 electrode configured to produce hydrogen gas from the steam. The oxygen electrode 102 has a first side and a second side opposite to the first side. The PCEC also includes a proton-conducting ceramic electrolyte 104 between the hydrogen electrode 106 and the first side of the oxygen electrode 102. The PCEC further includes a contact material 103 (e.g., current collector) adjacent to the second side of the oxygen electrode 102.

FIG. 2 is a simplified schematic diagram showing the PCEC 100 operated in an electrolysis mode to produce H₂ gas 170 from steam 150 (e.g., gaseous H₂O). The steam 150 is directed into an electrochemical apparatus 140 containing the PCEC 100 therein. The steam 150 is directed to interact with the oxygen electrode 102 of the PCEC 100. A potential difference (e.g., voltage) is applied between the oxygen electrode 102 (serving as an anode) and the hydrogen electrode 106 (serving as a cathode), so that oxidation of the steam 150 takes place at the oxygen electrode 102 to generate oxygen (O₂) gas, protons (H⁺), and electrons (e⁻) according to the following equation (1):

The generated O₂ gas may exit the PCEC 100 as an O₂ gas stream 160. The generated protons permeate (e.g., diffuse) across the proton-conducting ceramic electrolyte 104 to the hydrogen electrode 106, while the generated electrons are directed to a power source 120 through an external circuitry.

At the hydrogen electrode 106, the protons exiting the proton-conducting ceramic electrolyte 104 react with the electrons received from the power source to form H₂ gas, according to the following equation (2):

The generated H₂ gas may exit the PCEC 100 as the H₂ gas stream 170.

The PCEC 100 may also be operated (e.g., reversibly operated) in a fuel cell mode to generate electricity from H₂ gas 170 (e.g., at least a portion of the H₂ gas produced when the PCEC 100 is operated in the electrolysis mode). When the PCEC 100 is operated in a fuel cell mode, the H₂ gas 170 (e.g., at least a portion of the H₂ gas produced when the PCEC 100 is operated in the electrolysis mode) is directed into the PCEC 100 to interact with the hydrogen electrode 106 (also known as a “fuel electrode” when the PCEC 100 is operated in the fuel cell mode). The hydrogen electrode 106 serves as an anode and the oxygen electrode 102 serves as a cathode, so that an oxidation of the hydrogen gas 170 takes place at the hydrogen electrode 106 to generate protons (H⁺) and electrons (e⁻) according to the following equation (3), which is the reverse reaction of equation (2) above:

The generated H⁺ permeate (e.g., diffuse) across the proton-conducting ceramic electrolyte 104 to the oxygen electrode 102, and the generated e are directed to the power source through the external circuitry 120. At the oxygen electrode 102, the generated H⁺ exiting the proton-conducting ceramic electrolyte 104 react with e received from the power source, and O₂ gas 160 (e.g., at least a portion of the O₂ gas produced when the PCEC 100 is operated in the electrolysis mode) is directed into PCEC 100 to generate electricity and produce H₂O, according to the following equation (4), which is the reverse reaction of equation (1) above:

The operating conditions of the PCEC 100 may at least partially depend on the material compositions of the oxygen electrode 102, the hydrogen electrode 106, and the proton-conducting ceramic electrolyte 104. In some embodiments, the PCEC 100 is operated at a temperature within a range of from about 400° C. to about 700° C. (e.g., from about 400° C. to about 600° C.; from about 500° C. to about 600° C.). In some embodiments, the PCEC 100 is operated at a steam?concentration of at least about 50% by volume in air.

The oxygen electrode 102 may include any material composition suitable for electrochemically generating oxygen gas from steam, when the PCEC 100 is operated in the electrolysis mode. In some embodiments, the oxygen electrode 102 is formed of and includes a perovskite material (e.g., a layered perovskite material) compatible with the material compositions of the proton-conducting ceramic electrolyte 104 and the hydrogen electrode 106 under the operating conditions (e.g., temperature, pressure, current density, etc.) of the PCEC 100. The layered perovskite may be formulated to conduct hydrogen ions (H⁺) (i.e., protons), oxygen ions (O²⁻), and electrons (e⁻). The layered perovskite of the oxygen electrode 102 may facilitate the production of H₂ gas from steam (e.g., through water splitting reaction (WSR)) when the PCEC 100 is operated in an electrolysis mode at a temperature within the range of from about 400° C. to about 700° C., and may also facilitate electricity generation from H₂ gas (e.g., the oxygen reduction reaction (ORR)) when the PCEC 100 is operated in fuel cell mode at a temperature within the range of from about 400° C. to about 700° C. (e.g., from about 400° C. to about 600° C.).

The oxygen electrode 102 may be formed of and include a perovskite material having the chemical formula of DBO_3−δ, where D represents one or more lanthanide elements; B represents one or more of Co, Fe, Ni, Cu, Zn, Mn, Cr, and Nd; and δ is an oxygen deficiency. In some embodiments, the oxygen electrode includes a perovskite material having the chemical formula of DAB₂O_5+δ, where D represents one or more lanthanide elements; A represents one or more of Sr, Ca, and Ba; B represents one or more of Co, Fe, Ni, Cu, Zn, Mn, Cr, and Nd; and δ is an oxygen deficiency. The oxygen electrode may be configured as a layered perovskite material.

In some embodiments, the oxygen electrode 102 includes a material of chemical formula (Pr_1−mLn_m)(Ba_nSr_1−n)(Co_pTn_1−p)O_5+δ, where Ln is selected from La, Nd, Ce, Pm, Sm, Er, Gd, Dy, Ho, and Yb; Tn is selected from Fe, Ni, Cu, Zn, Mn, Cr, and Nd; m is a real number in a range of 0≤m≤1; n is a real number in a range of 0≤n≤1; p is a real number in a range of 0≤p≤1; and δ is an oxygen deficiency. In some embodiments, the oxygen electrode includes Pr_0.5La_0.5BaCoO_5+δ, where δ is an oxygen deficiency.

In some embodiments, the oxygen electrode 102 includes a material of chemical formula (Pr_1−mLn_m)(Co_pTn_1−p)O_3−δ, where Ln is selected from La, Nd, Ce, Pm, Sm, Er, Gd, Dy, Ho, and Yb; Tn is selected from Fe, Ni, Cu, Zn, Mn, Cr, or Nd; m is a real number in a range of 0≤m≤1; p is a real number in a range of 0≤p≤1; and δ is an oxygen deficiency.

In some embodiments, the oxygen electrode 102 includes a praseodymium cobalt nickelate material of chemical formula PrNi_fCo_1−fO_3−δ, wherein f is a real number in a range of 0≤f≤1; and δ is an oxygen deficiency. As non-limiting examples, the oxygen electrode may include PrNi_0.7Co_0.3O_3−δ (PNC73), PrNi_0.5Co_0.5O_3−δ (PNC55), or a combination thereof.

In some embodiments, the oxygen electrode 102 includes a compound containing at least one transition metal (e.g., Pr, La) and having a perovskite structure. As non-liming examples, the oxygen electrode 102 may include lanthanum strontium cobalt ferrite (LSCF, La_1−aSr_aFe_1−bCo_bO_3−δ, where 0.2≤a≤0.8; 0.1≤b≤0.9; and δ is an oxygen deficiency), lanthanum strontium manganite (LSM, La_1−cSr_cMnO_3−δ, where 0.2≤c≤0.8; and δ is an oxygen deficiency), and lanthanum strontium cobaltite (LSC, La_1−dSr_dCoO_3−δ, where 0.2≤d≤0.8; and 6 is an oxygen deficiency).

The hydrogen electrode 106 may include any material composition suitable for electrochemically generating hydrogen gas from steam, when the PCEC 100 is operated in an electrolysis mode. By way of non-limiting examples, the hydrogen electrode 106 may comprise a cermet material including at least one metal (e.g., Ni) and at least one perovskite. The at least one perovskite may include a yttrium- and ytterbium-doped barium-cerate-zirconate (BCZYYb), a yttrium- and ytterbium-doped barium-strontium-niobate (BSNYYb), a doped barium-zirconate, a doped barium-cerate, a doped barium zirconate-cerate, a barium-yttrium-stannate, a barium-calcium-niobate, or any combination thereof. In some embodiments, the hydrogen electrode 106 comprises a nickel/perovskite cermet (Ni-perovskite) material including, but not limited to, Ni—BSNYYb, Ni—BaCeO₃, Ni—BaZrO₃, Ni—Ba₂(YSn)O_5.5, Ni—Ba₃(CaNb₂)O₉. In some embodiments, the hydrogen electrode 106 comprises Ni—BCZYYb. Non-limiting examples of Ni—BCZYYb include Ni—BaCe_0.4Zr_0.4Y_0.1Yb_0.1O_3−δ, Ni—BaCe_0.5Zr_0.3Y_0.1Yb_0.1O_3−δ, or Ni—BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ where δ is an oxygen deficiency.

The proton-conducting ceramic electrolyte 104 may be formed of and include at least one electrolyte material compatible with the material compositions of the oxygen electrode 102, and the hydrogen electrode 106 under the operating conditions (e.g., steam concentration, temperature, pressure, current density, etc.) of the PCEC 100. The proton-conducting ceramic electrolyte 104 may be formulated to remain substantially adhered (e.g., laminated) to the oxygen electrode 102 and the hydrogen electrode 106 at relatively high current densities, such as at current densities greater than or equal to about 0.1 amperes per square centimeter (A/cm²) (e.g., greater than or equal to about 0.5 A/cm², greater than or equal to about 1.0 A/cm², greater than or equal to about 2.0 A/cm², greater than or equal to about 3.0 A/cm², greater than or equal to about 4.0 A/cm², etc.).

In some embodiments, the proton-conducting ceramic electrolyte 104 includes a perovskite material having an ionic conductivity (e.g., H⁺ conductivity) greater than or equal to about 10⁻² S/cm (e.g., within a range of from about 1×10⁻² S/cm to about 1 S/cm) at one or more temperatures within a range of from about 400° C. to about 700° C.

By way of non-limiting example, the proton-conducting ceramic electrolyte 104 may comprise a yttrium- and ytterbium-doped barium-cerate-zirconate (BCZYYb) such as BaCe_fZr_0.8−rY_0.2−sYb_sO_3−δ, where r and s are dopant levels and δ is an oxygen deficiency (e.g., BaCe_0.4Zr_0.4Y_0.1Yb_0.1O_3−δ, BaCe_0.5Zr_0.3Y_0.1Yb_0.1O_3−δ, BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ); a yttrium- and ytterbium-doped barium-strontium-niobate (BSNYYb) such as Ba₃(Sr_1−gNb_2−hY_gYb_h)O_9−δ, where g and h are dopant levels, and δ is an oxygen deficiency; a doped barium-cerate (BaCeO₃) (e.g., yttrium-doped BaCeO₃ (BCY)); a doped barium-zirconate (BaZrO₃) (e.g., yttrium-doped BaCeO₃ (BZY)); a barium-yttrium-stannate (Ba₂(YSn)O_5.5); a barium-calcium-niobate (Ba₃(CaNb₂)O₉); or any combination thereof. In some embodiments, the proton-conducting ceramic electrolyte 104 comprises BCZYYb. In some embodiments, the proton-conducting ceramic electrolyte 104 comprises yttrium- and ytterbium-doped barium-cerate-zirconate of formula BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ.

The proton-conducting ceramic electrolyte 104 may have a thickness of from about 6 microns (μm) to about 18 μm. In some embodiments, the proton-conducting ceramic electrolyte 104 has a thickness of from about 6 microns (μm) to about 10 μm. In some embodiments, the proton-conducting ceramic electrolyte 104 has a thickness of less than about 10 microns.

The contact material 103 may be formed of and include a material having a high electrical conductivity and exhibiting compatibility with the material compositions of the oxygen electrode 102, the proton-conducting ceramic electrolyte 104, and the proton-conducting ceramic electrolyte 104 under the operating conditions (e.g., temperature, pressure, current density, etc.) of the PCEC 100. The contact material 103 may provide high conductivity between the oxygen electrode 102 and an interconnect. The contact material 103 may be formulated to demonstrate improved bonding with the oxygen electrode 102 and stability at the operating conditions of the PCEC 100 (e.g., high steam concentration).

In some embodiments, the contact material 103 has an electrical conductivity of from about 1000 S/cm to about 1600 S/cm at a temperature range of from about 350° C. to about 850° C. In some embodiments, the contact material 103 has an electrical conductivity of from about 1200 S/cm to about 1400 S/cm at a temperature range of from about 450° C. to about 750° C. In some embodiments, the contact material has an electrical conductivity of about 1300 S/cm at a temperature of about 600° C.

The contact material 103 may exhibit a similar thermal expansion coefficient value as the thermal expansion coefficient value of the oxygen electrode 102 of the PCEC 100. Therefore, there are sufficient electron transportation through the contact material 103, and a bonding between the contact material 103 and the oxygen electrode 102 of the PCEC 100.

In some embodiments, the contact material 103 exhibits a high electric conductivity, an enhanced stability, and an improved bonding with the oxygen electrode 102 of the PCEC 100 at a temperature range of from about 300° C. to about 700° C. and a steam concentration of at least about 50% (e.g., greater than or equal to about 50%) by volume in air.

In some embodiments, the contact material 103 includes a chemical formula of LaM_xNi_1−xO_3−δ where M and N are independently selected from a transition metal; x is a real number in a range of 0≤x≤1; and δ is an oxygen deficiency. The performance of the contact material 103 may be modified by varying the ratio of the transition metals M and N therein. In other words, the performance of the contact material 103 may be modified by varying the value of x. As a non-limiting example, the value of x may be adjusted such that the contact material 103 exhibits an increased electric conductivity, an enhanced stability, and an improved bonding with the oxygen electrode at the operating conditions of the PCEC 100 (e.g., a temperature of from about 400° C. to about 600° C., a steam concentration of ≥50% by volume in air). The contact material 103 may be formed by conventional techniques.

In some of such embodiments, the contact material includes a chemical formula of LaM_xN_1−xO_3−δ where M and N are transition metals independently selected from cobalt (Co), nickel (Ni), iron (Fe), copper (Cu), or zinc (Zn); x is a real number in a range of 0≤x≤1; and δ is an oxygen deficiency.

In some embodiments, the contact material includes LaNi_xCo_1−xO_3−δ, where x is a real number in a range of 0≤x≤1, and δ is an oxygen deficiency. As a non-limiting example, the contact material may include LaNi_0.5Co_0.5O_3−δ (LNC55) and LaNi_0.7Co_0.3O_3−δ (LNC73).

In some embodiments, the contact material includes a Co-doped LaNiO_3−δ. By doping cobalt (Co) into a lanthanum nickel alloy, the electrical conductivity may be improved.

The oxygen electrode 102, the hydrogen electrode 106, the proton-conducting ceramic electrolyte 104, and the contact material 103 may each individually exhibit any desired dimensions (e.g., length, width, thickness) and any desired shape (e.g., a cubic shape, a cuboidal shape, a tubular shape, a tubular spiral shape, a spherical shape, a semi-spherical shape, a cylindrical shape, a semi-cylindrical shape, a conical shape, a triangular prismatic shape, a truncated version of one or more of the foregoing, or an irregular shape). The dimensions and the shapes of the oxygen electrode 102, the hydrogen electrode 106, the proton-conducting ceramic electrolyte 104, and the contact material 103 may be selected relative to one another such that the proton-conducting ceramic electrolyte 104 substantially intervenes between opposing surfaces of the oxygen electrode 102 and the hydrogen electrode 106, and the contact material 103 is sufficiently bonded to the oxygen electrode 102.

The PCEC 100 (including the oxygen electrode 102, the hydrogen electrode 106, the proton-conducting ceramic electrolyte 104, and the contact material 103) may be formed using conventional processes (e.g., rolling process, milling processes, shaping processes, pressing processes, consolidation processes, etc.). The PCEC 100 may be mono-faced or bi-faced, and may have a prismatic, folded, wound, cylindrical, or jelly rolled configuration.

FIG. 3 is a simplified isomeric view of a protonic ceramic electrochemical cell (PCEC) stack, according to some embodiments of the disclosure.

The PCEC stack 300 includes a first PCEC 100₁, a second PCEC 100₂ adjacent to the first PCEC 100₁, and a third PCEC 100₃ adjacent to the second PCEC 100₂. The first PCEC 100₁ includes a first oxygen electrode 102₁, a first hydrogen electrode 106₁, a first proton-conducting ceramic electrolyte 104₁ between the first hydrogen electrode 106₁ and one side of the first oxygen electrode 102₁, and a first contact material 103₁ adjacent to another side of the first oxygen electrode 102₁ opposite to the one side of the first oxygen electrode 102₁. The second PCEC 100₂ includes a second oxygen electrode 102₂, a second hydrogen electrode 106₂, a second proton-conducting ceramic electrolyte 104₂ between the second hydrogen electrode 106₂ and one side of the second oxygen electrode 102₂, and a second contact material 103₂ adjacent to another side of the second oxygen electrode 102₂ opposite to the one side of the second oxygen electrode 102₂. The third PCEC 100₃ includes a third oxygen electrode 102₃, a third hydrogen electrode 106₃, a third proton-conducting ceramic electrolyte 104₃ between the third hydrogen electrode 106₃ and one side of the third oxygen electrode 102₃, and a third contact material 103₃ adjacent to another side of the third oxygen electrode 102₃ opposite to the one side of the third oxygen electrode 102₃. At least one of the first contact material 103₁ of the first PCEC 100₁, the second contact material 103₂ of the second PCEC 100₂, and the third contact material 103₃ of the third PCEC 100₃ includes a chemical formula of LaM_xN_1−xO_3−δ where M and N are independently selected from a transition metal; x is a real number in a range of 0≤x≤1; and δ is an oxygen deficiency.

The PCEC stack 300 further includes an interconnect 200 disposed between the first contact material 103₁ of the first PCEC 100₁ and the second hydrogen electrode 106₂ of the second PCEC 100₂, as well as an additional interconnect 200 disposed between the second contact material 103₂ of the second PCEC 100₂ and the third hydrogen electrode 106₃ of the third PCEC 100₃. The interconnect 200 may be a metal-based interconnect. In some embodiments, the interconnect 200 includes stainless steel. In some embodiments, the interconnect 200 includes stainless steel having a high chromium content (e.g., a chromium-based alloy). As a non-limiting example shown in FIG. 2, the interconnect may electrically connect several PCECs in series in the PCEC stacks (e.g., a planar PCEC stacks). Without being bound by any theory, it is believed that the material composition of the contact materials 103 improves ohmic contact resistance in the PCEC stack 300.

The PCEC stack in FIG. 3 is shown as including three (3) PCECs for ease of illustration. However, the disclosure is not limited, and the PCEC stack may include different numbers of the PCECs therein.

The ability to stack PCECs is important in the development and scale up of the PCECs for applications at industrial scale. Stacking PCECs often uses a contact material for the oxygen electrode of each PCEC to be durable and stable under the operating conditions of the PCECs (e.g., high steam concentration, intermediate to low temperatures), as well as cost-effective. Furthermore, there should be a high electrical conductivity between the contact material, the oxygen electrode, and the interconnect 200. The PCEC stack 300 includes a contact material 103 inserting between the oxygen electrode 102 and the interconnect 200. During the operation of the PCEC stack 300, water electrolysis occurs at the oxygen electrode 102, thus subjecting the contact material 103 and the oxygen electrode 102 to high steam concentrations (e.g., ≥50% by volume in air). Accordingly, the contact material 103 is formulated to maintain a high electrical conductivity, a high chemical stability, and a good bonding with the oxygen electrode 102 under the harsh operating conditions of the PCEC stack 300.

One conventional contact material used for the oxygen electrode of a conventional PCEC includes a precious metal (e.g., gold, silver, and platinum) due to the high electrical conductivity and chemical stability of the precious metal. However, precious metals are costly and prone to migration and evaporation at high operating temperatures, leading to the deposition of precious metal elements at or near the electrolyte-electrode surface. This can adversely affect transient behavior and degrade cell electrochemical performance of the PCEC.

The contact material 103 according to embodiments of the disclosure is substantially free of any precious metal (e.g., silver (Ag), gold (Au), platinum (Pt)), yet provides the PCEC with a comparable electrochemical performance and cell durability (e.g., stable, long-term durability) as the conventional PCEC that utilizes a precious metal-based contact material. Due to the substantially lower cost of the contact material 103 compared to the conventional precious metal-based contact material, the cost of the PCEC stack system operation of the disclosed PCECs can be significantly reduced compared to the cost of the PCEC stack system operation of the conventional PCECs.

Another conventional contact material comprising an alkaline earth metal (e.g., Sr, Ba, and Ca) has several limitations for the oxygen electrode of the PCEC. If the contact material and the oxygen electrode are subjected to a high steam concentration (e.g., ≥50%) during the operation of the PCEC, the alkaline earth metal is prone to phase segregation and contaminant poisoning under such severe conditions. The segregation of alkaline earth metals increases with a prolonged operation of the PCEC, leading to the formation of secondary phases with low stability and electrical conductivity. This degradation reduces electrical conductivity and jeopardizes material compatibility, ultimately resulting in cell degradation.

The contact material according to embodiments of the disclosure may include a chemical formula LaM_xN_1−xO_3−δ, where M and N are independently selected from a transition metal; x is a real number in a range of 0≤x≤1; and δ is an oxygen deficiency. In some embodiments, the contact material is substantially free of alkaline earth metal (e.g., Sr). Thus, the contact material is less susceptible to a phase segregation from the oxygen electrode and a contaminant poisoning (e.g., chromium poisoning) under the operating conditions of the PCEC, compared to the convention contact material comprising alkaline earth metal.

In some embodiments, the contact material includes a chemical formula LaM_xN_1−xO_3−δ, where M and N are transition metals independently selected from cobalt (Co), nickel (Ni), iron (Fe), copper (Cu), or zinc (Zn); x is a real number in a range of 0≤x≤1; and δ is an oxygen deficiency.

In some embodiments, the contact material comprises lanthanum nickel cobaltite (LNC) of chemical formula LaNi_xCo_1−xO_3−δ, where x is a real number in a range of 0≤x≤1; and δ is an oxygen deficiency. Non-limiting examples of the LNC materials suitable for use as a contact material for the oxygen electrode of the PCEC are LaNi_0.7Co_0.3O_3−δ (LNC73) and LaNi_0.5Co_0.5O_3−δ (LNC55).

Although the disclosure describes the use of the LNC material (e.g., LNC55, LNC73) as the contact material for the oxygen electrode of PCEC, the disclosure is not limited. Any contact material including a chemical formula of LaM_xN_1−xO_3−δ, where M and N are independently selected from a transition metal; x is a real number in a range of 0≤x≤1; and δ is an oxygen deficiency may be used. The description for the use of the LNC material (e.g., LNC55, LNC73) as the contact material for the oxygen electrode of PCEC is to illustrate and enable a non-limiting example of the contact material according to embodiments of the disclosure.

The contact material (e.g., LNC material) according to embodiments of the disclosure exhibits a superior electrical conductivity at a temperature range of from about 400° C. to about 1000° C., compared to conventional contact materials (e.g., lanthanum strontium manganite (LSM), Fe-doped lanthanum nickelate (LNF)). See EXAMPLE 3, FIG. 4. The LNC material (e.g., LNC55, LNC73) is a perovskite material exhibiting a high electrical conductivity, which may be attributed to the presence of electron hopping sites created by Ni and Co with mixed valence state. The LNC material (e.g., LNC55, LNC73) exhibits a good electrical conductivity under a high steam concentration (e.g., ≥50% by volume), which is the typical operation condition for the PCEC under the electrolysis mode. At the steam concentration of about 50%, the electrical conductivity of the LNC material is substantially higher than the threshold electrical conductivity of 100 S·cm⁻¹. For example, LNC55 and LNC73 show an electrical conductivity of about 1198 S·cm⁻¹ and 818 S·cm⁻¹, respectively, at a steam concentration of about 50% by volume and a temperature of about 600° C., which is the typical operating conditions of the PCEC. See EXAMPLE 3, FIG. 5. In addition, the LNC material maintains its high conductivity properties even after being exposed to a high steam concentration (e.g., about 50% by volume in the air) for about 500 hours. For example, the LNC material (e.g., LNC55, LNC73) maintains its electrical conductivity after a long-term operation (e.g., 500 hours) of the PCEC under a high steam concentration (e.g., a steam concentration of 50%) at a temperature of about 600° C. This indicates the long-term stability of the LNC material (LNC55, LNC73) as the contact material for the PCEC application. See EXAMPLE 3, FIG. 6.

The contact material of the disclosure (e.g., LNC material) has a similar (e.g., substantially the same) thermal expansion coefficient value as that of the oxygen electrode comprising a praseodymium cobalt nickelate (PNC) material (e.g., PNC73) (also referred herein as “PNC oxygen electrode” or “PNC73 oxygen electrode”) at an operating temperature range of the PCEC (e.g., from about 400° C. to about 600° C.). For example, the LNC55 material shows a thermal expansion coefficient (TEC) value of about 16.5×10⁻⁶ K⁻¹ at a temperature of about 600° C., which closely matches that of the PNC73 oxygen electrode (about 17.2×10⁻⁶ K⁻¹). See EXAMPLE 4, FIG. 8. This minimizes thermal mismatch and promotes a stable interface between the oxygen electrode and the contact material under high-steam conditions. Good bonding between the PNC oxygen electrode and the LNC contact material after long-term operation of PCECs in electrolysis mode indicates a high bond strength between the PNC oxygen electrode and the LNC contact material.

Compared to conventional conductive materials (e.g., silver (Ag), lanthanum cobalt oxide (LCO)), the LNC material (e.g., LNC55, LNC73) shows a much lower mismatch in the thermal expansion coefficient value with the PNC oxygen electrode. See EXAMPLE 4, FIG. 7. A higher mismatch in the thermal expansion coefficient values between the contact material and the oxygen electrode material results in a weaker bonding between the contact material and the oxygen electrode material, and therefore an increased risk of delamination of the contact material from the oxygen electrode material. Therefore, the LNC material (e.g., LNC55 and LNC73) according to embodiments of the disclosure is more stable as the contact material for the PNC oxygen electrode compared to conventional contact materials (e.g., Ag, LCO).

Stainless steel may be conventionally used as an interconnect in the stack of electrochemical cells (e.g., PCEC stack, SOEC stack) based on its electrical conductivity, thermal expansion behavior, mechanical properties (formability, ease of fabrication, creep resistance), and low cost.

During a high temperature operation of the electrochemical cells (e.g., PCECs, SOECs) within the stack, volatilization of chromium species (e.g., CrO₃) from the stainless steel interconnect may take place. These gaseous chromium species (e.g., CrO₃) have a tendency to undergo electrochemical reduction and form Cr₂O₃ at gas/oxygen electrode/electrolyte three-phase boundaries or to chemically react with the oxygen electrode, resulting in a chromium (Cr) poisoning of the oxygen electrode. The Cr-poisoning of oxygen electrode not only jeopardizes the performance of the PCEC stack but also shortens the lifetime of the PCEC stack. Therefore, it is desirable that the contact material, which is disposed between the oxygen electrodes and the interconnects of the PCEC stack, to have an improved Cr-poisoning resistance.

Furthermore, several of the conventional contact materials comprise an alkaline earth metal, such as lanthanum strontium cobaltite (LSC). When LSC is used as a contact material within the PCEC stack that utilizes a stainless steel interconnect, the gaseous chromium species generated from the interconnect at elevated temperatures may migrate to and react chemically with the contact material to form chromium-alkaline metal oxides. The chemical reaction between the chromium species and the LSC contact material leads to the formation of SrCrO₄ on the surface of the LSC contact material. The formation of a thick insulating layer of chromium-alkaline metal oxides (e.g., SrCrO₄) negatively impacts the surface conductivity and gas permeability of the PCEC stack, turning the interface between the interconnect material and the contact material into a barrier. Furthermore, the introduction of divalent alkaline earth metals creates more oxygen vacancies, which reduces the solubility of electron holes, leading to a decreased electrical conductivity at higher temperatures.

The contact material of the disclosure (e.g., LNC material) may have a high Cr resistance, and thereby minimize (if not prevent) the Cr poisoning of the oxygen electrode. The LNC material (e.g., LNC55, LNC73) exhibits a high Cr-resistance, which not only enables the stable operation of the PCEC at a high steam condition (e.g., ≥50%) and at intermediate temperatures (e.g., a temperature range of from about 400° C. to about 600° C.), but also blocks potential Cr contamination of the oxygen electrode. See EXAMPLE 5. Due to the absence of alkaline earth metal elements and the high reaction energy barrier with chromium oxide (Cr₂O₃), the LNC material according to embodiments of the disclosure functions effectively as a blocking layer to prevent chromium contamination of the oxygen electrode attached thereto. Furthermore, the contact material is substantially free of alkaline earth metal (e.g., Sr). Thus, the contact material is less susceptible to a phase segregation from the oxygen electrode and a contaminant poisoning (e.g., chromium poisoning) under the operating conditions of the PCEC, compared to the conventional contact material comprising alkaline earth metal.

In addition, the contact material of the disclosure (e.g., LNC material) when used as the contact material for the oxygen electrode of PCEC provides the PCEC with comparable electrochemical performance as when the precious metal (e.g., Ag) is used as the contact material for the oxygen electrode of PCEC. See EXAMPLES 6 through 8, FIGS. 8 and 9. However, the LNC material is significantly lower in cost than the conventional contact materials that include precious metals (e.g., Ag).

The following examples serve to explain embodiments of the disclosure in more detail. These examples are not to be construed as being exhaustive, exclusive, or otherwise limiting as to the scope of the disclosure.

EXAMPLES

Example 1. Preparation of Lanthanum Nickel Cobaltite (LNC) as a Contact

Material

Lanthanum nickel cobaltite (LNC) materials of chemical formula LaM_xN_1−xO_3−δ were synthesized and utilized as a contact material for the oxygen electron of the PCEC. Two LNC materials were studied: LaNi_0.5Co_0.5O_3−δ (LNC55) and LaNi_0.7Co_0.3O_3−δ (LNC73).

The LNC material was prepared using a combustion method starting with La(NO₃)₃·6H₂O (99.99%, Sigma-Aldrich), Ni(NO₃)₂·6H₂O (98%, Sigma-Aldrich), and Co(NO₃)₂·6H₂O (98%, Sigma-Aldrich) as the precursors. Stoichiometric amounts of precursor nitrate solution were added to distilled water along with the proper amounts of glycine and citric acid (e.g., a 2:1 molar ratio of glycine to citric acid). The resulting mixture was heated on a hot plate to form a gel, which was then calcined at a temperature of about 400° C. The calcinated powder was then sintered at a temperature of about 900° C., where the powder crystallized into LNC powder.

Example 2. Physical Characterization of the LNC Contact Material

The LNC materials (LNC55, LNC73) obtained from EXAMPLE 1 were characterized to determine the particle size, the crystal structure, the surface electronic structure, the morphology in different dimensions, the lattice structure, and the element distribution.

The particle size distribution of the LNC powders (LNC55, LNC73) was characterized using a particle size analyzer (Litesizer 500, Anton Paar) to measure the dynamic light scattering. The particle size analysis showed that an average particle size of LNC55 was smaller than that of LNC73.

The crystal structure of LNC contact materials (LNC55, LNC73) was examined by X-ray diffraction (XRD, 2008 Bruker D8) with Cu Kα radiation (λ=0.15406 nm) at room temperature. The LNC powders showed a pure phase of LaNiO₃ with a space symmetry of R-3c belonging to the rhombohedral families. The Co and Ni ions (B-site cations) were distributed at the center of the BO6 octahedra coordination. The trivalent La atoms were located at the A-site, surrounded by tiled octahedra. The Ni doping at B sites had a mild effect on changing the unit-cell parameters.

The surface electronic structure of LNC contact materials (LNC55, LNC73) was investigated using X-ray photoelectron spectroscopy (XPS). The XPS results showed that the shape of the Ni 2p and La 3d was complex, which may be due to the Ni 2P_3/2, La 3d_3/2, and La satellite (Sat.) peak overlap. The XPS results also showed the presence of multiple satellite and plasmon loss features associated with the different oxidation states of Ni and La. La showed stable valence state of +3. Ni and Co showed mixed valence state of +2/+3, which was beneficial to improve electron conductivity. The transition metal (TM) cations of Ni and Co in B-site performed a valence change and created TM³⁺/TM²⁺ couples, which acted as hopping sites for electrons (n type)/holes (p type).

The morphology in different dimensions, lattice structure and element distribution of the LNC materials (LNC55, LNC73) was evaluated. The scanning electron microscopy (SEM) and mapping images (not shown) were used to provide an overview of LNC morphology. The images of LNC73 and LNC55 each showed uniform fine particles of less than 500 nm. The elements of La, Ni, Co, and O were homogeneously distributed in each of LNC55 and LNC73 without precipitation.

The high-resolution transmission electron microscopy (TEM) image of LNC73 and LNC55 each showed good crystallization nature with an interplanar lattice distance of 0.384 nm, corresponding to the (101) crystal plane. The diffraction pattern corresponded well to the (101) and (110) crystal planes of LNC55, which was in good agreement with the XRD results. The uniform distribution of the cations and oxygen was confirmed by TEM-EDX mapping of single particles.

Example 3. Electrical Conductivity Study of the LNC Material

The electrical conductivities of the LNC materials (LNC55, LNC73) was studied at a temperature range of from about 400° C. to about 1000° C. and compared with the following conventional contact materials: lanthanum strontium cobalt ferrite of chemical formula La_xSr_1−xCo_1−yFe_yO_3−δ (LSCF), where x is a real number in a range of 0≤x≤1, y is a real number in a range of 0≤y≤1, and δ is an oxygen deficiency; Ba_xSr_1−xCo_1−yFe_yO_3−δ (BSCF), where x is a real number in a range of 0≤x≤1, y is a real number in a range of 0≤y≤1, and δ is an oxygen deficiency; lanthanum cobalt oxide of chemical formula LaCoO₃ (LCO), that has been used as a contact material in the oxide-ion conducting solid oxide cell (SOC) stacks; lanthanum strontium cobalt oxide of chemical formula La_1−xSr_xCoO_3−δ (LSC), where x is a real number in a range of 0≤x≤1, and δ is an oxygen deficiency; Sm-doped lanthanum strontium cobalt oxide of chemical formula La_1−xSr_xCoO_3−δ (LSmSC), where x is a real number in a range of 0≤x≤1, and δ is an oxygen deficiency; lanthanum nickelate of chemical formula LaNi_1−xFe_xO_3−δ (LNF), where x is a real number in a range of 0≤x≤1, and δ is an oxygen deficiency; lanthanum strontium ferrite La_xSr_1−xFeO_3−δ (LSF), where x is a real number in a range of 0≤x≤1, and δ is an oxygen deficiency; Fe-doped LaNi_xSr_1−xCuO_3−δ (LSCuF), where x is a real number in a range of 0≤x≤1, and δ is an oxygen deficiency; and lanthanum strontium manganate of chemical formula La_xSr_1−xMnO_3−δ (LSM), where x is a real number in a range of 0≤x≤1, and δ is an oxygen deficiency.

About 1 g of LNC powder (LNC55, LNC73) was pressed with a 4 mm×20 mm die at a pressure of about 0.4 MPa, and then fired at a temperature of about 1250° C. for about 5 hours to obtain a dense LNC bar. The resistant signal of the LNC material was collected at a temperature of from about 500° C. to about 700° C., and the related bulk conductivity (σ) was measured in the S cm⁻¹ unit by a four-probe method using a digital multimeter (Siglent SDM3065X). The electrical conductivity was calculated accordingly from the resistivity (φ as below:

σ = 1 ρ = 1 A × R

• • where:
  • R is the resistance of the sample (Q),
  • l is the length (cm), and
  • A is the cross-sectional area (cm², which is h×w: h is height and w is width).

FIG. 4 shows the comparative electrical conductivities of the studied materials at a temperature range of from about 400° C. to about 1000° C. The LNC55 material and the LNC73 material exhibited electrical conductivities of about 1279 S·cm⁻¹ and about 1147 S·cm⁻¹ at a temperature of about 600° C., respectively, which were much higher than the threshold electrical conductivity of about 100 S·cm⁻¹ for solid oxide cells (SOCs). At the operating temperature of the PCEC (e.g., from about 400° C. to about 600° C.) shown as the dotted box, the electrical conductivities of the LNC materials were higher than those of conventional contact materials (e.g., LSM and LNF). This demonstrated that the LNC materials (e.g., LNC55, LNC73) were well-suited as the contact materials for the oxygen electrode of the PCECs.

FIG. 5 shows the effect of steam concentration during the operation of the PCEC on the electrical conductivities of the LNC materials (LNC55, LNC73) at a temperature of about 600° C. A four-probe method was carried out to test the densified LNC bars with dimensions of 17.0 mm×3.4 mm×2.2 mm. To evaluate the electrical conductive behavior of the LNC material under different steam concentrations, the electrical conductivity of LNC55 and LNC73 were tested in air with steam concentrations ranging from 0 to 50% by volume. As the steam concentration increased, the electrical conductivities of both LNC55 and LNC73 decreased. An increase in the steam concentration may reduce the oxygen partial pressure, and therefore a decrease in the electron holes formation rate. At about 50% steam in air, the electrical conductivities of LNC55 and LNC73 were 1198 S·cm⁻¹ and 818 S·cm⁻¹, respectively, which were still sufficient for electrical conduction in the PCEC application.

FIG. 6 shows the long-term stability of the LNC contact material (LNC55, LNC73) under a high steam concentration (e.g., a steam concentration of 50%) at a temperature of about 600° C. The initial electrical conductivities of LNC55 and LNC73 at about 50% steam in air were about 1262 S·cm⁻¹ and about 898 S·cm⁻¹, respectively. The electrical conductivities of LNC55 and LNC73 decreased to about 1131 S·cm⁻¹ and about 866 S·cm⁻¹, respectively, over 5 hours, which may be due to hydration of the LNC materials. Following a treatment for 500 hours, the electrical conductivities of LNC55 and LNC73 slightly decreased to about 1069 S·cm⁻¹ and about 794 S·cm⁻¹, respectively. The high electrical conductivity of the LNC material when used as a contact material for the oxygen electrode after a long-term operation demonstrated a high stability of the LNC contact material and a high electrical conductivity for the PCEC application.

Example 4. Thermal Expansion Coefficient Study of the LNC Contact

Material

The thermal expansion behavior was examined by a dilatometer (DIL 402C, NETZSCH). The oxygen electrode comprising PrNi_0.7Co_0.3O_3−δ (PNC73) was used as the oxygen electrode of the PCEC. The thermal expansion behavior of the LNC material (LNC55 and LNC73) was studied and compared to that of the PNC73 oxygen electrode. The conventional contact materials that were included in the comparative study were: silver (Ag), LCO, LSC, LSM, LSF, LNF, and LSCF.

FIG. 7 shows the comparative thermal expansion coefficient values at a temperature of about 600° C. of the LNC materials (LNC55 and LNC73), the PNC73 oxygen electrode material, Ag, LCO, LSC, LSM, LSF, LNF, and LSCF.

As shown in FIG. 7, the thermal expansion coefficient values of LNC55 and LNC73 were closer to that of the PNC73 oxygen electrode material. Conventional contact materials, such as Ag and LCO, showed a higher degree of mismatch in the thermal expansion coefficient values with the PNC73 oxygen electrode material. A higher mismatch in the thermal expansion coefficient values between the contact material and the oxygen electrode material resulted in weaker bonding between the contact material and the oxygen electrode material, and therefore an increased risk of delamination of the contact material from the oxygen electrode material. Therefore, the LNC material (e.g., LNC55 and LNC73) was more stable as the contact material for the PNC73 oxygen electrode compared to conventional contact materials (e.g., Ag, LCO).

FIG. 8 shows the comparative thermal expansion coefficient values of the LNC73 contact material, the LNC55 contact material, and the oxygen electrode material PNC73 at various temperatures from room temperature to a temperature of about 1000° C. with a ramping rate of about 3° C./min under an argon (Ar) atmosphere. The determination of thermal expansion coefficient was conducted using the sample bars fabricated through the same process used for the measurement of the electrical conductivity described in EXAMPLE 3.

At a typical operating temperature range of the PCEC (e.g., from about 400° C. to about 600° C.) shown as the dotted box in FIG. 9, the LNC73 contact material and the LNC55 contact material had approximately the same thermal expansion coefficient values as the oxygen electrode material PNC73. The difference in the thermal expansion coefficient values between LNC55 and LNC73 may be attributed to the varying Ni content.

Example 5. Investigation of the Chromium Poisoning Resistance of the

LNC Contact Material

When stainless steel is used as an interconnect material, the volatility of Cr-containing species from the stainless steel interconnects may take place at elevated temperatures. Therefore, the Cr₂O₃ poisoning resistance of the contact material for the oxygen electrode is evaluated to ensure long-term stability of the PCEC cell.

The XRD spectroscopy, the SEM spectroscopy, and the Raman spectroscopy were used along with reaction energy calculations to evaluate the chemical stability of the contact material with both PNC oxygen electrode and the gaseous chromium species (Cr₂O₃) under water splitting operation temperature (e.g., a temperature of about 600° C.) and steam-containing conditions.

The XRD spectroscopy was taken for a mixture of the LNC contact material and the PNC oxygen electrode after treatment at a temperature of about 600° C. and at a 3% steam in air for 20 hours. No impurities were observed in the treated LNC and PNC mixture, indicating no chemical reaction between the LNC contact material and the PNC oxygen electrode. The XRD spectroscopy was also taken for a mixture of the LNC contact material and the gaseous chromium species (Cr₂O₃) after treatment at a temperature of about 600° C. and at a 3% steam in air for 20 hours. No impurities were observed in the treated LNC and chromium mixture, indicating no chemical reaction between the LNC contact material and the volatile chromium species released from the stainless steel interconnect material.

To compare the Cr-poisoning resistant performance of the LNC material with the Cr-poisoning resistant performance of conventional contact materials, the similar study was performed using LSCF of the chemical formula La_xSr_1−xCo_1−yFe_yO_3−δ, where x is a real number in a range of 0≤x≤1, y is a real number in a range of 0≤y≤1, and δ is an oxygen deficiency, as the contact material. LSCF is commonly used as a contact material for the oxygen electrode of the solid oxide fuel cell (SOFC) stacks. The XRD spectroscopy was taken for a mixture of the blended LNC/LSCF and the gaseous chromium species (Cr₂O₃) after treatment at a temperature of about 600° C. and at a 3% steam in air for 20 hours. The XRD patterns showed peaks between two theta of 20° and 35°, which belonged to SrCrO₄.

Thus, the LNC material showed a higher chemical stability as a contact material for the oxygen electrode of the PCEC stacks compared to the LSCF material commonly used as a contact material for the oxygen electrode of the SOFC stacks.

The SEM and line scan images were used to further verify the chemical stability of the LNC contact material when mixed with the PNC oxygen electrode and the gaseous chromium species (Cr₂O₃) after treatment. The SEM and line scan images of (a) the mixture of the LNC contact material and the PNC oxygen electrode, (b) the mixture of the LNC contact material and Cr₂O₃, and (c) the mixture of the LSCF contact material and Cr₂O₃ were taken.

The SEM and line scan images of (a) the mixture of the LNC contact material and the PNC oxygen electrode showed that the LNC contact material and the PNC oxygen electrode material had similar particle sizes of less than 500 nm. The elemental profiles showed distinct La and Pr peaks corresponding to the LNC and PNC regions, respectively. No La was detected in the PNC region, and no Pr was found in the LNC region. These results indicated the absence of elemental diffusion or chemical reaction between the two phases, which was consistent with the XRD results.

The SEM and line scan images of (b) the mixture of the LNC contact material and Cr₂O₃ showed that Cr₂O₃ had a larger particle size (from about 500 nm to about 2 μm) and appeared brighter than the LNC material. The LNC material maintained its chemical integrity when in contact with Cr₂O₃, without Cr migration or secondary phase formation. These results supported that the LNC material was suitable as the Cr-resistant contact material for the oxygen electrode of the PCEC.

The SEM and line scan images of (c) the mixture of LSCF and Cr₂O₃ showed the un-uniform particle size. The interface of LSCF and Cr₂O₃ showed a clear Cr peak, demonstrating chemical reaction between LSCF and Cr₂O₃ during the treatment.

Furthermore, the Raman spectroscopy of a mixture of the blended LNC/LSCF and the gaseous chromium species (Cr₂O₃) after the treatment showed the peaks at about 855.9 cm⁻¹, which belongs to SrCrO₄. In contrast, the Raman spectroscopy of a mixture of LNC55 and Cr₂O₃ after the treatment showed pure phase. Thus, the Raman results confirmed the XRD and SEM results that the LNC material (e.g., LNC55, LNC73) was resistant to the chromium poisoning.

Additionally, the reaction energy of Cr₂O₃ with the three potential contact materials (LNC55, LNC73, and LSCF) was calculated by density functional theory (DFT) method based on 2×2×1 orthorhombic supercell following the chemical equations for chromium poisoning of LNC and LSCF. According to the XRD and Raman results, SrCrO₃ was the main product for the reaction between LSCF and Cr₂O₃. The reaction products between Cr₂O₃ and LNC55/LNC73 were assumed as LaCrO₃, Cr₂NiO₄, and Cr₂CoO₄. The DFT results show that LSCF has the lowest reaction energy with Cr₂O₃ compared to LNC55 and LNC73.

Thus, the experimental investigation showed that the LNC material was suitable as a contact material for the oxygen electrode of the PCEC with a high Cr-resistance, which not only enabled the stable operation of the PCEC at a high steam condition (e.g., ≥50%) and at intermediate temperatures (e.g., a temperature range of from about 400° C. to about 600° C.) but also blocked potential Cr contamination of the oxygen electrode.

Example 6. Fabrication of the PCEC Comprising the LNC Contact Material for the Oxygen Electrode

The NiO—BaCe_0.4Zr_0.4Y_0.1Yb_0.1O_3−δ (“NiO—BCZYYb”) hydrogen electrode supporting layer and the BCZYYb electrolyte layer of the PCEC was prepared by a tape casting method. The green tape with three layers of hydrogen electrode and one layer of electrolyte were laminated, and the laminated green tape was cut to a diameter of about 9/8 inch. After pre-sintering at a temperature of about 920° C. to remove the organic solvents and binder, the half-cells were sintered at a temperature of about 1430° C. for 5 hours in a furnace with a temperature ramping rate of about 3° C./minute to obtain the half-cells with a 1-inch diameter. To investigate the suitability of the LNC contact material for a large-scale PCEC stack, planar cells were fabricated with a 5 cm×5 cm area following the above half-cell fabrication process.

The oxygen electrode used in the study comprised the PNC powder (e.g., PrNi_0.7Co_0.3O_3−δ (PNC73)). The PNC powder was synthesized by a combustion method. Stoichiometric amounts of Pr(NO₃)₃·6H₂O (99.99%, Sigma-Aldrich), Ni(NO₃)₂·6H₂O, and Co(NO₃)₂·6H₂O were dissolved in deionized water with a concentration of about 0.02 mol/L to produce a precursor nitrate solution. Then, appropriate amounts of glycine and citric acid (2:1 mole ratio) were added to the precursor nitrate solution. The resulting solution was then heated up to a temperature of about 400° C. on a hot plate, followed by combustion to form powder ash. The resulting powders were then calcined at a temperature of about 1000° C. for 5 hours in air to obtain a crystallized perovskite phase. The oxygen electrode slurry was prepared by mixing the PNC (e.g., PNC73) powder with about 5% by weight of the BZCYYb powder.

After the aforementioned half-cell fabrication process, the oxygen electrode PNC73 slurry was integrated into the half-cells by brush-painting, followed by firing at a temperature of about 1000° C. for 5 hours to obtain the final full cells (e.g., button cells) with an effective oxygen electrode area of about 1.26 cm².

The LNC powder was prepared into an ink by mixing about 1 gram (g) of the LNC powder with about 1.5 g of organic binders (5% hydroxyethyl cellulose in terpineol) using a planetary centrifugal mixer (Thinky mixer ARE-310).

After the full PCEC cells (e.g., button PCEC cells) were prepared, the LNC contact material (e.g., LNC55) with a thickness of about 55 μm was integrated into the PCEC on the oxygen electrode as shown in FIGS. 1 and 2. The LNC contact material was integrated into the PCECs with different sizes and shapes, including discs with 10 mm diameter, discs with 1 inch diameter, disc with 5 cm×5 cm square. Then, the current collecting leads (silver wires) were pasted on the oxygen electrode through the LNC contact material. Silver was applied on the hydrogen electrode as the current collecting layer.

Example 7. Microscopy Characterization of the PCECs Comprising the LNC Contact Material for the Oxygen Electrode

Energy-dispersive X-ray spectroscopy (EDX) mapping was used to investigate the elemental distribution. The EDX mapping image showed a homogeneous distribution of La, Ni, Co, and O in the LNC55 contact material, as well as a homogeneous distribution of Pr, Ni, Co, and O in PNC73 oxygen electrode.

Scanning electron microscopy (SEM, FEI Quanta 650) was performed to determine the morphology of the PCEC button cells. No cracks or delamination between the PNC73 oxygen electrode and LNC55 contact material were observed in the cross-sectional SEM image. This demonstrated improved bonding between the PNC73 oxygen electrode and LNC55 contact material. Furthermore, the LNC55 contact material exhibited a greater quantity of larger pores than the PNC73 oxygen electrode after removing the organic binders from the LNC55 contact material by firing at a temperature of about 1000° C. The relatively larger pore size of the LNC55 contact material may ensure that sufficient feedstock (e.g., steam) can reach the oxygen electrode through the current collecting layer.

The element valence state was characterized by X-ray photoelectron spectroscopy (XPS, Nexsa G2) using a monochromatic Al Kα source (hv=1486.6 eV). A FEI Tecnai F30 super-twin field-emission-gun transmission electron microscope operated at 300 kV was used to acquire the TEM images, electron diffraction patterns and Energy Dispersive X-ray Spectroscopy (EDS) spectrum. The line scan across the thickness of the PCEC from the PNC73 oxygen electrode to the LNC55 contact material showed no element diffusion between the PNC73 oxygen electrode to the LNC55 contact material. This demonstrated that there was no reaction between the PNC73 oxygen electrode and the LNC55 contact material during firing the LNC55 contact material paste.

The integration of LNC contact material for the PNC73 oxygen electrode of the 1-inch PCEC button cells was evaluated, in comparison with the integration of Ag contact material for the PNC73 oxygen electrode of the 1-inch PCEC button cells. Silver (Ag) is a common contact material for the PCEC stacks due to its high stability and inertness toward chemical reaction with other components of the PCEC during the high-steam contact and elevated temperature operating conditions of the PCEC. To firmly stick the Ag contact to the PCEC with the same area, the Ag thickness of about 45 m was used, which was twice the thickness used for the LNC contact material (about 20 m). The thinner contact layer, the less gas transportation limitation and the stronger mechanical strength on the oxygen electrode were beneficial to the long-term stability of the PCEC. After the operation in electrolysis mode, peel off of the thick Ag contact material from the PCEC was observed. In contrast, no cracks and delamination of the contact material from the PCEC were observed when the LNC material (e.g., LNC55, LNC73) was used as the contact material. This demonstrated the enhanced compatibility between the LNC contact material and the PNC73 oxygen electrode, compared to the Ag contact material.

Example 8. Electrochemical Performance of the PCECs Comprising the LNC Contact Material for the Oxygen Electrode

The button cells were sealed with glass sealant (SCHOTT GM31107) on a custom built testing fixture, and the electrochemical performances of PCEC were collected by an electrochemical workstation (Solartron 1400). Under the fuel cell mode testing, the PNC oxygen electrode was exposed to dry O₂ at a rate of about 40 mL/min and the fuel electrode (hydrogen electrode) was flowing with dry H₂ at a rate of about 20 mL/min. Under the electrolysis mode testing, the feedstock was pure H₂ for the hydrogen electrode and humidified O₂ (50% steam) for the steam electrode (oxygen electrode). Electrochemical impedance spectra (EIS) were carried out with a frequency ranging from about 100 kHz to about 0.1 Hz with an AC amplitude of about 20 mV at about O CV. The reversible operation of the PCEC was conducted at a temperature of about 600° C. During the electrolysis mode, the voltage was controlled to 1.2, 1.3, and 1.4 V. While in the fuel cell mode, the voltage was controlled to 0.8, 0.7, and 0.5 V. During reversible operation, the voltage was switched between these two modes.

The electrochemical performance of the PCEC using three different contact materials (LNC55, LNC73, and silver (Ag)) were evaluated in electrolysis mode at a temperature of about 600° C. and a steam concentration of about 50%.

FIG. 9 shows the current densities of the PCEC at different temperatures when different voltages were applied to the PCECs, when the PNC73 was used as the oxygen electrode of the PCEC and three different materials (LNC55, LNC73 or Ag) were used as the contact material. As shown in FIG. 9, at a voltage of about 1.3 V, the current densities of PCECs with the LNC55 contact material, the LNC73 contact material, or silver as contact material were −1.27 A·cm⁻², −1.18 A·cm⁻², or −1.05 A·cm⁻², respectively. This demonstrated that the PCECs using the LNC73 or LNC556 as the contact material provided comparable electrochemical performance to the PCEC using Ag as the contact material.

FIG. 10 compared the polarization resistances of PCECs in the electrolysis mode at voltages of 1.1 V, 1.2 V, and 1.3 V, when the PNC73 oxygen electrode was used with different contact materials (LNC55, LNC73, or Ag). As shown in FIG. 10, the polarization resistances demonstrated that LNC material (LNC73, LNC55) did not suppress the electrochemical performance of PCECs during operation at a temperature of about 600° C. and a high steam concentration of 50%.

Furthermore, the reversible operation of the PCEC between the electrolysis mode and the fuel cell mode at different current densities was evaluated for its capability of converting hydrogen generated by electrolysis into electricity. The reversible fuel cell/electrolysis cell mode cycling was investigated with the dynamic voltages of from about 0.8 V to about 0.5 V for generating electricity and the dynamic voltages of from about 1.3 V to about 1.4 V for producing hydrogen at a temperature of about 600° C. with 50% steam. The LNC material exhibited comparable electrochemical performance to silver.

The embodiments of the disclosure described above and illustrated in the accompanying drawings do not limit the scope of the disclosure, which is encompassed by the scope of the appended claims and their legal equivalents. Any equivalent embodiments are within the scope of this disclosure. Indeed, various modifications of the disclosure, in addition to those shown and described herein, such as alternate useful combinations of the elements described, will become apparent to those skilled in the art from the description. Such modifications and embodiments also fall within the scope of the appended claims and equivalents.