Reversible solid oxide electrochemical cells with hybrid oxygen electrode

Abstract

Solid oxide electrochemical cells (SOECs) stand out as a highly promising clean energy technology that offers several benefits, showing significant potential to play a pivotal role in the transition towards a sustainable and low-carbon energy future. SOECs can efficiently convert the chemical energy stored in fuels to electricity in fuel cell mode, and produce various chemicals from abundant feedstocks (e.g., CO2, H2O, N2) and intermittent solar / wind-based renewable electricity. In-situ formed hybrid oxygen electrode materials have been developed from solid composite materials comprising a double perovskite phase and a single perovskite phase, which significantly improve the surface oxygen exchange coefficient and bulk oxygen-ion diffusion coefficient, enhancing the OER and ORR electrocatalytic activities. The SOECs equipped with these newly-developed oxygen electrode materials achieve exceptional performance for power generation using both hydrogen and propane as fuels. Additionally, the SOECs attain unprecedented performance in steam electrolysis mode. The SOECs also deliver remarkable stability during the accelerated stability testing, highlighting the great potential the solid composite materials as a high-performance oxygen electrode for next generation SOECs.

Description

REVERSIBLE SOLID OXIDE ELECTROCHEMICAL CELLS WITH HYBRID OXYGEN ELECTRODE STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0001] This invention was made with government support under Contract No. W911NF22P0021 awarded by Army Research Office, and Contract No. DE-AC07-05ID14517 awarded by Department of Energy. The government has certain rights in the invention. RELATED APPLICATIONS

[0002] This application claims the priority benefit of U.S. Provisional Patent Application Serial No. 63 / 598,322 entitled “REVERSIBLE SOLID OXIDE ELECTROCHEMICAL CELLS WITH HYBRID OXYGEN ELECTRODE,” filed November 13, 2023, and U.S. Provisional Patent Application Serial No. 63 / 580,913 entitled “REVERSIBLE SOLID OXIDE ELECTROCHEMICAL CELLS WITH HYBRID OXYGEN ELECTRODE,” filed September 6, 2023, the entire disclosures of which are incorporated herein by reference. BACKGROUND 1. Field of the Invention

[0003] The present disclosure generally relates to electrochemical cells and to methods for forming the same. More particularly, the present disclosure relates to reversible solid oxide electrochemical cells containing a hybrid oxygen electrode formed from solid composite materials comprising a double perovskite phase and a single perovskite phase. 2. Description of the Related Art

[0004] The growing concern over global greenhouse effects and climate change has led to a surge in demands for clean and sustainable energy across the globe. This has generated significant interest in highly efficient energy technologies that produce considerably less greenhouse gas emissions. Among such technologies, Solid Oxide Electrolyzer Cells (“SOECs”) have emerged as a promising option for next-generation electrochemical energy conversion and storage systems, due to their exceptional efficiency and minimal environmental impact. In fuel cell mode, SOECs convert fuels, such as hydrogen and hydrocarbons, into electricity with ahigher efficiency than combustors. When there is excess electricity available, SOECs function as electrolyzers to produce hydrogen, carbon monoxide, or syngas with different reactants fed to the fuel electrode.

[0005] SOEC technology has undergone drastic improvements over the past decade, but for its commercialization, further research and development are needed to increase performance (i.e., energy efficiency and durability) at reduced operating temperatures. However, the notoriously sluggish oxygen reduction reaction (“ORR”) and oxygen evolution reaction (“OER”) that occur at the oxygen electrode inevitably have increased the area-specific electrode polarization resistance (“ASRP”) with lower operating temperatures, which is ascribed to their thermally activated nature. Therefore, extensive efforts have been invested in developing oxygen electrode materials to alleviate the restricted electrochemical activity and decreased performance.

[0006] Recently, the application of double perovskite-based oxygen electrodes, for example, PrBa0.5Sr0.5Co1.5Fe0.5O6-^ (“PBSCF-a”) to fuel electrode-supported YSZ-based SOECs, have attracted a great deal of attention due to their enhanced ORR and OER activity and improved durability. An in-situ assembly process for fabricating oxygen electrodes has been demonstrated, which leads to the PBSCF-GDC composite oxygen electrode achieving a peak power density (PPD) of 1.37 W cm-2in fuel cell mode at 750 °C. PBSCF-a has been employed as the SOEC oxygen electrode, which achieved an electrolysis current density of 1.74 A cm-2at 2.0 V at 800 °C. An oxygen electrode with a PBSCF-infiltrated YSZ scaffold structure has been reported, attaining a low ASRPof 0.03 Ω cm2and stable electrolysis performance at 700°C for 600 hours. However, to fabricate effective nanostructured oxygen electrodes, several repeated infiltration / annealing cycles in the wet infiltration process are required, which limits its scale-up and commercial applications. Other novel oxygen electrode processing methods, such as freeze- casting, atomic layer deposition, and electrostatic spray deposition are also limited by high fabrication cost, complex procedures, and low scalability issues.

[0007] More efforts should be, therefore, devoted to developing highly active oxygen electrodes via efficient and facile fabrication methods. A nanostructured composite oxygen electrode has been developed, which is composed of PrBa0.8Ca0.2Co2O5+^ (PBCC) that is decorated with Gd0.2Ce0.8O1.9(GDC), via an in-situ assembly approach. The resulting SOECs achieve a PPD of 1.74 W cm-2in fuel cell mode at 750 °C and an electrolysis current density of1.77 A cm-2at 1.3 V. With employing a similar oxygen electrode processing method, an Er0.4Bi1.6O3-^ (ESB) functionalized La0.8Sr0.2MnO3-^ (LSM) oxygen electrode has been designed, which leads to improved performance in both fuel cell and electrolysis modes. Despite the outstanding progress that has been achieved with developing and employing novel oxygen electrode materials, the power density in fuel cell mode and energy efficiency in electrolysis mode can be further enhanced to boost the benefits of SOECs. SUMMARY

[0008] One or more embodiments generally relate to a hybrid oxygen electrode that comprises a solid composite material comprising a double perovskite phase and a single perovskite phase. The double perovskite phase comprises PrBaSrCoFeO (“PBSCF”) and the single perovskite phase comprises BaSrCoO (“BSC”).

[0009] One or more embodiments generally relate to a hybrid oxygen electrode that comprises a solid composite material. The solid composite material comprises: (a) a double perovskite phase and (b) a single perovskite phase. Generally, the double perovskite phase has a formula PraBabSrcCodFeeO6-δ1, wherein a is from 1 to 1.75, b is from 0.05 to 0.5, c is from 0.3 to 0.7, d is from 1.25 to 1.75, e is from 0.4 to 0.9, and δ1 is greater than zero, but less than 1. Additionally, the single perovskite phase having a formula BaxSryCozO3-δ2, wherein x is from 0.4 to 0.75, y is from 0.25 to 0.5, z is from 0.75 to 1.25, and δ2 is greater than zero, but less than 0.5.

[0010] One or more embodiments generally relate to a solid oxide electrochemical cell (“SOEC”). Generally, the reversible SOEC comprises an oxygen electrode comprising a solid composite material according to any embodiment described herein. The SOEC further comprises a fuel electrode that comprises any material conventional for such electrodes, including, but not limited to, NiO. Additionally, the SOEC comprises an electrolyte located between the oxygen and fuel electrodes. The electrolyte can be any conventional electrolyte material, including, but not limited to yttria-stabilized zirconia (“YSZ”). Typically, the oxygen and fuel electrodes have a thickness of at least 10 μm, from 10 to 100 μm, from 15 to 75 μm, or from 20 to 50 μm. In addition, the electrolyte material may have a thickness of at least 5 μm, from 5 to 30 μm, from 7 to 20 μm, or from 8 to 15 μm.

[0011] One or more embodiments generally relate to a method of producing hydrogen. Generally, the method comprises: providing the solid oxide electrochemical cell describedherein; reacting water at a first electrode to generate oxygen and protons; causing the protons to pass through the proton-conducting electrolyte to a second electrode; and supplying an electrical current to the first electrode and / or second electrode and reducing at least a portion of the protons to form hydrogen.

[0012] One or more embodiments generally relate to a method of generating electricity. Generally, the method comprises: providing the solid oxide electrochemical cell described herein; supplying hydrogen to the electrochemical cell and oxidizing the hydrogen at a first electrode to form protons and electrons; causing the protons to pass through the proton- conducting electrolyte to a second electrode; causing the electrons generated at the first electrode to induce an electrical current and passing the electrical current through an electrical load that is connected with the first electrode and the second electrode; and reacting the protons with oxygen at the second electrode to produce water. BRIEF DESCRIPTION OF THE FIGURES

[0013] Embodiments of the present invention are described herein with reference to the following drawing figures, wherein:

[0014] FIG. 1 depicts an exemplary solid oxide electrolyzer cell comprising an electrode formed from the solid composite materials described herein; and

[0015] FIG. 2 depicts an exemplary hybrid oxygen electrode, which is composed of a PBSCF phase and a minor BSC phase that is dispersed on the PBSCF phase. DETAILED DESCRIPTION

[0016] Hybrid oxygen electrodes have been developed to enhance SOEC performance for power generation and hydrogen production. Despite the improvements made by previous technology, further improvements were needed to achieve the U.S. Department of Energy (DOE) renewable hydrogen production cost goal of <$1 / kg, while increasing the hydrogen production rate per active area. Therefore, a new hybrid oxygen electrode has been developed that comprises a solid composite material comprising a double perovskite phase and a single perovskite phase dispersed on the double perovskite phase, wherein the double perovskite phase comprises PrBaSrCoFeO (“PBSCF”) and the single perovskite phase comprises BaSrCoO (“BSC”). These new and inventive hybrid oxygen electrodes may be produced via an in-situ formation process,which simplifies the oxygen electrode fabrication process and improves the electrocatalytic activity. As discussed below in greater detail, the PBSCF phase may provide an improved bulk oxygen-ion diffusion coefficient and the BSC phase may exhibit a much higher surface oxygen exchange coefficient. Therefore, this composite electrode provides synergistic effects, which improve the ORR and OER activity.

[0017] Although not wishing to be bound by theory, it is believed that the BSC phase synergizes with the PBSCF phase to enhance OER and ORR performance in the resulting electrode. Furthermore, the inventive electrodes described herein may simultaneously improve the surface oxygen exchange and bulk oxygen-ion diffusion coefficients.

[0018] The resulting SOECs formed with the inventive electrodes may exhibit exceptional performance for power generation using both hydrogen and propane as fuels. For example, at 750°C, an exceptional peak power density of at least about 2.4 W cm-2may be achieved. Additionally, the resulting SOECs may exhibit unprecedented performance in steam electrolysis mode. For instance, a current density of at least about 4.4 A cm-2may be achieved at 1.3 V and 750 °C, which represents the highest performance among all Yttria-stabilized zirconia (“YSZ”) electrolyte-based SOECs. Such SOECs may also exhibit remarkable stability during the accelerated stability testing, highlighting the great potential of the solid composite materials as a high-performance oxygen electrode for next generation SOECs.

[0019] Accordingly, the present disclosure is generally directed to a hybrid oxygen electrode that comprises solid composite material comprising a double perovskite phase and a single perovskite phase. The hybrid oxygen electrode may be fabricated via an in-situ formation process. The solid composite materials described herein may be employed as the oxygen electrode for oxygen-ion conducting SOECs, which may lead to improved performance in both fuel cell and electrolysis modes. The Solid Composite Materials

[0020] As discussed above, a solid composite material has been developed that may be used for forming hybrid oxygen electrodes in electrochemical cells. The solid composite materials described herein may comprise a double perovskite phase and a single perovskite phase. Due to its unique chemical formulation, the solid composite material of the presentdisclosure may be used to produce electrodes and electrochemical cells that exhibit superior performance.

[0021] According to various embodiments, a solid composite material may be produced that comprises a double perovskite phase and a single perovskite phase. The double perovskite phase comprises PrBaSrCoFeO (“PBSCF”) and the single perovskite phase comprises BaSrCoO (“BSC”).

[0022] As used herein, the term “perovskite” and “single perovskite phase” means any material with a crystal structure having the formula ABX3, wherein “A” and “B” refer to cations and “X” refers to an anion. Furthermore, as used herein a “double perovskite” refers to a complex perovskite material with a crystal structure of AA’BB’X6, wherein “A” and “A’” represent two or more cations, such as cations of alkaline earth metals and / or rare earth metals; “B” and “B’” represent two or more cations, such as those containing a transition metal; and “X” represents at least one anion. Generally, in various embodiments, a double phase perovskite may have a unit cell that is twice that of a single perovskite phase, with two cations ordered on the “B” site.

[0023] Generally, in one or more embodiments, the solid composite material, such as the double perovskite phase and the single perovskite phase, may be synthesized using a wet- chemistry method, wherein two or more cations and one or more anions are dissolved in water in stoichiometric amounts. Exemplary precursor reagents for the cations and / or anions may include, for example, Pr6O11, Ba(NO3)2, Sr(NO3)2, Co(NO3)•6H2O, Fe(NO3)3•9H2O, and / or Fe(NO3)3•6H2O.

[0024] During the wet-chemistry method, in one or more embodiments, Ba(NO3)2, Sr(NO3)2, Co(NO3)3•6H2O, and Fe(NO3)3•6H2O (or Fe(NO3)3•9H2O) may be at least partially dissolved in deionized water in stoichiometric amounts to form an initial reaction solution. Additionally, a specific amount of Pr6O11 may be dissolved in diluted nitric acid and subsequently added into the reaction solution. For example, a calculated amount of Pr6O11 may be dissolved in a about 20 weight percent diluted nitric acid solution at about 100°C to obtain the praseodymium nitrate solution. The praseodymium nitrate solution may be added to the reaction solution under stirring for 10 to 60 minutes, 15 to 45 minutes, or about 30 minutes at temperatures in the range of about 15 to about 100 °C, about 20 to about 80 °C, or about 20 to about 50 °C.

[0025] In one or more embodiments, the reaction solution comprises: (a) 0.05 to 0.5 moles, 0.1 to 0.4 moles, or 0.15 to 0.25 moles of Pr6O11; (b) 0.05 to 1.0 moles, 0.1 to 0.8, or 0.3 to 0.6 moles of Ba(NO3)2; (c) 0.05 to 1.0 moles, 0.1 to 0.8, or 0.3 to 0.6 moles of Sr(NO3)2; (d) 0.1 to 2.0 moles, 0.5 to 1.9 moles, or 1.0 to 1.7 moles of Co(NO3)3; and / or (e) 0.05 to 1.0 moles, 0.1 to 0.8, or 0.3 to 0.6 moles of Fe(NO3)3.

[0026] In one particular embodiment, the reaction solution comprises about 0.17 moles of Pr6O11, about 0.5 moles of Ba(NO3)2, about 0.5 moles of Sr(NO3)2, about 1.5 moles of Co(NO3)3, and about 0.5 moles of Fe(NO3)3.

[0027] After forming the initial reaction solution, one or more complexing and chelating agents, such as citric acid (“CA”) and / or ethylenediaminetetraacetic acid (“EDTA”), may be added to the reaction solution. The complexing and chelating agents may be added to the reaction solution under continuous stirring for 10 to 90 minutes, 15 to 75 minutes, or about 60 minutes at temperatures in the range of about 15 to about 100 °C, about 20 to about 80 °C, about 20 to about 50 °C, or about 23°C in order to obtain complete complexation.

[0028] In one or more embodiments, after adding the complexing and chelating agents, the resulting reaction solution may have a CA:total cation molar ratio of at least 1:1, 1.5:1, or 2:1 and / or less than 4:1, 3.5:1, or 3:1. Additionally, or in the alternative, after adding the complexing and chelating agents, the resulting reaction solution may have an EDTA:total cation molar ratio of at least 1:1, 1.5:1, or 2:1 and / or less than 4:1, 3.5:1, or 3:1. In certain embodiments, after adding the complexing and chelating agents, the resulting reaction solution may have a CA:EDTA:total cation molar ratio of about 2:2:1.

[0029] Subsequently, after adding the complexing and chelating agents, the pH of the resulting reaction solution may be adjusted to about 9 via the addition of a pH adjustment agent (e.g., an aqueous NH4OH solution).

[0030] After adjusting the pH of the reaction solution, the reaction solution may then be heated to thereby form a gel. Generally, in one or more embodiments, the reaction solution may be heated at a temperature in the range of about 200 to about 400 °C, about 225 to about 350 °C, or about 250 to about 300 °C. The reaction solution may be heated using conventional equipment known in the art, such as via a hot plate or oven, until a gel is formed.

[0031] Once the gel has formed, at least a portion of the resulting gel may be immediately placed in a dryer, such as a drying oven, to further dry. In one or more embodiments, the drying step can occur at a temperature in the range of about 50 to about 250 °C, about 75 to about 225 °C, about 100 to about 200 °C, or about 175°C over a time period of about 3 to about 36 hours, about 6 to about 30 hours, or about 24 hours.

[0032] After drying, in various embodiments, the resulting powder may then be calcined under air at a temperature in the range of about 400 to about 700 °C, about 500 to about 675 °C, or about 600 to about 650 °C over a time period of 1 to 24 hours, 2 to 18 hours, 3 to 8 hours, or about 5 hours.

[0033] Afterwards, in various embodiments, at least a portion of the calcined powder may be subjected to milling, such as ball-milling, in an alcohol (e.g., ethanol) for 1 to 48 hours, 6 to 36 hours, or about 24 hours. Subsequently, at least a portion of the milled powder may be subjected to drying in a dryer (e.g., a drying oven) at a temperature in the range of about 50 to about 250 °C, about 75 to about 225 °C, about 100 to about 200 °C, about 100°C, or about 175°C over a time period of about 3 to about 36 hours, about 6 to about 30 hours, or about 24 hours.

[0034] Finally, in various embodiments, the dried and calcined powder may be sintered at a temperature in the range of about 500 to about 900 °C, about 600 to about 800 °C, or about 750 to about 775 °C until the dried and calcined powder has crystallized into the solid composite material comprising a double perovskite phase and a single perovskite phase. In various embodiments, the resulting solid composite material may be in the form of a powder.

[0035] In one or more embodiments, the phase structures within the double perovskite phase and the single perovskite phase are such that each perovskite is a separately identifiable structure within the overall material with each phase having its own identifiable crystalline structure.

[0036] In one or more embodiments, solid composite material comprises a double perovskite phase having a formula PraBabSrcCodFeeO6-δ1and a single perovskite phase having a formula BaxSryCozO3-δ2, wherein: a is from 1 to 1.75, 1.1 to 1.6, 1.2 to 1.5, or 1.3 to 1.5; b is from 0.05 to 0.5, 0.05 to 0.4, 0.05 to 0.3, or 0.1 to 0.2; c is from 0.3 to 0.7, 0.35 to 0.65, or 0.4 to 0.6;d is from 1.25 to 1.75, 1.3 to 1.7, or 1.35 to 1.65; e is from 0.4 to 0.9, 0.5 to 0.85, or 0.6 to 0.8; δ1 is greater than zero, but less than 1; x is from 0.4 to 0.75, 0.45 to 0.7, or 0.5 to 0.7; y is from 0.25 to 0.5, 0.3 to 0.45, or 0.3 to 0.4; z is from 0.75 to 1.25, 0.8 to 1.2, or 0.9 to 1.1; and / or δ2 is greater than zero, but less than 0.5.

[0037] In one or more embodiments, solid composite material comprises a double perovskite phase having a formula PraBabSrcCodFeeO6-δ1 and a single perovskite phase having a formula BaxSryCozO3-δ2, wherein: a is from 1 to 1.75; b is from 0.05 to 0.5; c is from 0.3 to 0.7; d is from 1.25 to 1.75; e is from 0.4 to 0.9; δ1 is greater than zero, but less than 1; x is from 0.4 to 0.75; y is from 0.25 to 0.5; z is from 0.75 to 1.25; and δ2 is greater than zero, but less than 0.5.

[0038] In one or more embodiments, the solid composite material has a molar ratio of barium to strontium in the double perovskite phase of from 0.1:1 to 1:1, 0.2:1 to 0.8:1, or 0.2:1 to 0.5:1; a molar ratio of praseodymium to barium in the double perovskite phase of from 1.5:1 to 15:1, 2:1 to 14:1, 3:1 to 13:1, or 4:1 to 12:1; a molar ratio of praseodymium to strontium in the double perovskite phase of from 1.5:1 to 3.5:1, 2:1 to 3.5:1, or 2.5:1 to 3.5:1; a molar ratio of praseodymium to cobalt in the double perovskite phase of from 0.5:1 to 1.5:1, 0.7:1 to 1.4:1, or 0.8 to 1.3:1; a molar ratio of praseodymium to iron in the double perovskite phase of from 1.5:1 to 2.5:1, 1.6:1 to 2.4:1, or 1.7:1 to 2.3:1; a molar ratio of barium to strontium in the single perovskite phase of from 1.5:1 to 2.0:1 or 1.6:1 to 1.95:1; a molar ratio of barium to cobalt in the single perovskite phase of from 0.5:1 to 0.75:1 or 0.6:1 to 0.7:1; and / or a molar ratio of strontium to cobalt in the single perovskite phase of from 0.25:1 to 0.5:1 or 0.3:1 to 0.45:1.

[0039] In one or more embodiments, the solid composite material has a molar ratio of barium to strontium in the double perovskite phase of from 0.1:1 to 1:1; a molar ratio of praseodymium to barium in the double perovskite phase of from 1.5:1 to 15:1; a molar ratio of praseodymium to strontium in the double perovskite phase of from 1.5:1 to 3.5:1; a molar ratio of praseodymium to cobalt in the double perovskite phase of from 0.5:1 to 1.5:1; a molar ratio of praseodymium to iron in the double perovskite phase of from 1.5:1 to 2.5:1; a molar ratio of barium to strontium in the single perovskite phase of from 1.5:1 to 2.0:1; a molar ratio of barium to cobalt in the single perovskite phase of from 0.5:1 to 0.75:1; and / or a molar ratio of strontium to cobalt in the single perovskite phase of from 0.25:1 to 0.5:1.

[0040] In an exemplary embodiment, the double perovskite phase has a formula of Pr1.44Ba0.11Sr0.45Co1.32Fe0.68O6-δand the single perovskite phase has a formula of Ba0.62Sr0.38CoO3-δ-.

[0041] In another exemplary embodiment, the double perovskite phase has a formula of Pr1.39Ba0.14Sr0.53Co1.48Fe0.76O6-^- and the single perovskite phase has a formula of Ba0.66Sr0.34CoO3-^.

[0042] In one or more embodiments, the double perovskite phase has a tetragonal (P4 / mmm) crystal structure and / or the single perovskite phase has a hexagonal P63 / mmc crystal structure, as confirmed by transmission electron microscopy (“TEM”) and X-ray diffraction (“XRD”).

[0043] In one or more embodiments, the solid composite material comprises at least 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, or 89 weight percent of the double perovskite phase. Additionally, or in the alternative, the solid composite material may comprise less than 99, 95, or 90 weight percent of double perovskite phase. In certain embodiments, the solid composite material comprises about 89 weight percent of double perovskite phase. These weight percentages and those disclosed below may be confirmed by XRD.

[0044] In one or more embodiments, the solid composite material comprises at least 1, 5, 10, 15, 20, or 25 weight percent of the single perovskite phase. Additionally, or in the alternative, the solid composite material may comprise less than 60, 50, 45, 40, 35, 30, 25, 20, 25, 20, 15, or 11 weight percent of single perovskite phase. In certain embodiments, the solid composite material comprises about 10.5 weight percent of single perovskite phase.

[0045] In one or more embodiments, the solid composite material may have an average particle size of at least 1, 5, 10, 15, or 20 nm and / or less than 100, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, or 40 nm, as confirmed by TEM. The Hybrid Oxygen Electrodes and SOECs Formed Therefrom

[0046] The solid composite materials described herein may be used in a variety of applications. In one or more embodiments, the solid composite materials may be used to produce a hybrid oxygen electrode for power generation in an electrochemical cell, such as a fuel cell. Generally, a fuel cell is an electrochemical cell that reacts hydrogen fuel with oxygen or another oxidizing agent, to convert chemical energy to electricity. An exemplary fuel cell that may be produced from electrodes containing the solid composite materials described herein is a solid oxide electrolyzer cell (“SOEC”), such as a reversible SOEC.

[0047] In one or more embodiments, a hybrid oxygen electrode may be from the solid composite materials described herein. In such embodiments, the hybrid oxygen electrode may comprise, consist essentially of, or consist of the solid composite materials described herein. In various embodiments, the hybrid oxygen electrode may comprise at least 50, 75, 90, 95, or 99 weight percent of the solid composite materials.

[0048] Generally, in one or more embodiments, the hybrid oxygen electrode may be formed by combining the solid composite material powder with at least one dispersant (e.g., Sosperse 28000 by Lubrizol) and at least one binder (e.g., V-006 by Heraeus), and then applying the slurry mixture on the desired substrate. This mixture of the solid composite materials, the dispersant, and the binder may be applied using any method known in the art, such as, brush painting, spray coating, or tape casting. In certain embodiments, resulting slurry mixture may comprise 40 to 95, 50 to 90, or 55 to 85 weight percent of the solid composite material powder; 1 to 30, 2 to 25, or 3 to 20 weight percent of at least one dispersant; and 0.5 to 25, 1 to 20, or 2 to 15 weight percent of at least one binder. In order to prepare the mixture slurry for application, the slurry may be subjected to mixture and grinding to obtain a fine slurry.

[0049] In one or more embodiments, and as shown in FIG. 1, a reversible SOEC 10 may be formed that comprises a fuel electrode 12, an electrolyte layer 14, and a hybrid oxygen electrode 16.

[0050] The present disclosure is also concerned with a method of forming a reversible SOEC 10 with the solid composite materials described herein. In one or more embodiments, the method comprises: (a) forming a fuel electrode 12; (b) forming an electrolyte layer on at least a portion of the fuel electrode 12, thereby forming a stack comprising the fuel electrode 12 and the electrolyte layer 14; (c) sintering the stack to form a SOEC half-cell; (d) forming the hybrid oxygen electrode 16 on at least a portion of the electrolyte layer 14 of the half-cell to form a second stack; and (f) optionally sintering the second stack to form the SOEC 10. The sintering for the aforementioned steps may occur at a temperature of from about 500 to about 1,950 ℃, about 800 to about 1,650 ℃, or about 1,200 to about 1,450 ℃ for a time period of from about 1 hour to about 40 hours, about 2 hours to about 24 hours, or about 5 hours to about 15 hours.

[0051] In one or more embodiments, the SOEC 10 comprises a fuel electrode 12. It will be appreciated that, depending on the operating mode employed, the fuel electrode 12 can function as an anode or cathode in the SOEC 10. Particularly, when the SOEC 10 is operated in a hydrogen-producing mode, the fuel electrode 12 (functioning as a cathode) reduces protons to hydrogen, and when the cell is operated in an electricity-producing mode, the fuel electrode 12 (functioning as an anode) forms protons from hydrogen.

[0052] In various embodiments, the fuel electrode 12 comprises, consists essentially of, or consists of any material conventional for such electrodes, including, but not limited to, NiO and / or YSZ.

[0053] In one or more embodiments, the composite powder for forming the fuel electrode 12 may comprise 10 to 99, 25 to 90, or 50 to 80 weight percent of NiO; 1 to 70, 10 to 60, or 25 to 50 weight percent of YSZ; and / or 1 to 50, 5 to 40, or 10 to 25 weight percent of additives (e.g., corn starch, fish oil, PVB, polyethylene glycol, cyclohexanone, di-n-butyl phthalate, xylenes, and / or ethanol). The resulting slurry can be applied to a temporary substrate using conventional methods known in the art, such as spray coating, paint brushing, screen printing, or tape casting.

[0054] In various embodiments, the composite powder for forming the fuel electrode 12 may comprise about 40 weight percent of YSZ (e.g., TZ-8Y by Tosoh), about 60 weight percent of NiO (Sigma Aldrich), and 20 weight percent corn starch (Sigma Aldrich). Subsequently, in such embodiments, this mixture may then be mixed with menhaden fish oil, b-98 polyvinyl butyral, polyethylene glycol 400, cyclohexanone, di-n-butyl phthalate, xylenes, and / or ethanol,and then ball-milled using YSZ beads (e.g., diameter = 10 mm) for about 24 hours to obtain the fuel electrode slurry.

[0055] Once formed, the fuel electrode 12 may have a thickness in a range of about 10 µm to about 70 µm, about 20 µm to about 60 µm, or about 30 µm to about 50 µm.

[0056] In one or more embodiments, the SOEC 10 comprises a proton-conducting electrolyte layer 14. The proton-conducting electrolyte 14 may comprise a perovskite oxide electrolyte material. Examples of perovskite oxide electrolyte materials include, but are not limited to, yttrium-doped barium cerate, yttrium-doped barium zirconate, yttrium-doped barium cerate-zirconate, ytterbium-doped barium cerate, ytterbium-doped barium zirconate, ytterbium- doped cerate-zirconate, yttrium- and ytterbium-doped barium cerate, yttrium- and ytterbium- doped barium zirconate, yttrium- and ytterbium-doped barium cerate-zirconate, or combinations thereof. In certain embodiments, the proton-conducting electrolyte 14 may comprise YSZ.

[0057] In various embodiments, the composite powder for forming the electrolyte layer 14 may comprise YSZ. Subsequently, in such embodiments, this mixture may then be mixed with menhaden fish oil, b-98 polyvinyl butyral, polyethylene glycol 400, cyclohexanone, di-n- butyl phthalate, xylenes, and / or ethanol, and then ball-milled using YSZ beads (e.g., diameter = 10 mm) for about 24 hours to obtain the electrolyte slurry.

[0058] In one or more embodiments, the resulting electrolyte slurry can be applied onto at least a portion of a surface of the formed fuel electrolyte 12 using conventional methods known in the art, such as spray coating, paint brushing, screen printing, or tape casting. In certain embodiments, the resulting electrolyte slurry can be applied onto a mylar substrate, dried for a designated period of time (e.g., about 5 hours) and then cast on at least a portion of a surface of the formed fuel electrolyte 12. Subsequently, in certain embodiments, the resulting two layers may be dried for a designated amount of time (e.g., about 24 hours) and then sintered at about 1350°C for about 5 hours to fabricate SOEC half-cells.

[0059] Once formed, the proton-conducting electrolyte 14 may have a thickness in a range of about 0.01 µm to about 75 µm, about 1 µm to about 25 µm, about 5 µm to about 15 µm, or about 10 µm.

[0060] In one or more embodiments, the SOEC 10 comprises a hybrid oxygen electrode 16. It will be appreciated that, depending on the operating mode employed, the hybrid oxygen electrode 16 can function as an anode or cathode. Particularly, when the SOEC 10 is operated inhydrogen-producing mode, the hybrid oxygen electrode 16 (functioning as an anode) may form protons and oxygen from water using an electrical current, and when the SOEC 10 is operated in electricity-producing mode, the hybrid oxygen electrode 16 (functioning as a cathode) may react protons and electrons with oxygen to form water.

[0061] In various embodiments, the hybrid oxygen electrode 16 comprises, consists essentially of, or consists of the solid composite materials described herein. In one or more embodiments, the hybrid oxygen electrode 16 comprises at least 50, 75, 90, 95, or 99 weight percent of the solid composite materials described herein.

[0062] In one or more embodiments, the solid composite material powder may be applied to at least a portion of a surface of the electrolyte layer 14 to thereby form the hybrid oxygen electrode 16. The solid composite powder may be applied using conventional methods known in the art, such as spray coating, paint brushing, screen printing, or tape casting. In certain embodiments, the solid composite material powder may be mixed with various additives to form a slurry, which can then be applied (e.g., brush painted) onto the electrolyte layer 14. In various embodiments, the solid composite powder may be mixed with menhaden fish oil, b-98 polyvinyl butyral, polyethylene glycol 400, cyclohexanone, di-n-butyl phthalate, xylenes, and / or ethanol, and then ball-milled using YSZ beads (e.g., diameter = 10 mm) for about 24 hours to obtain the slurry. After the slurry has been applied to the electrolyte layer 14, the resulting hybrid oxygen electrode layer 16 can be dried at a temperature in the range of 140 to 220 °C for about 0.5 to 2 hours.

[0063] Once formed, the hybrid oxygen electrode 16 may have a thickness in a range of about 10 µm to about 70 µm, about 20 µm to about 60 µm, about 30 µm to about 50 µm, or about 40 µm.

[0064] FIG. 2 schematically depicts the hybrid oxygen electrode 16, which is composed of the PBSCF phase and a minor BSC phase that is dispersed on the PBSCF phase.

[0065] The performance characteristics and properties of the resulting SOECs may be tested by applying gold paste on both electrodes, along with a silver grid for current collection. Furthermore, the obtained SOECs may be directly sealed on an alumina tube with the MO∙SCI glass sealant (OL-GL1709P / -45) without pre-sintering the oxygen electrode. The testing station temperature was increased to 765°C with a heating rate of 4 °C / min, to melt the sealing glass while the hybrid oxygen electrode was obtained simultaneously. Then the SOEC single cell wastested at temperatures ranging from 550 to 750 °C. Ambient air (300 sccm) and humidified hydrogen (or propane) with a flow rate of 50 sccm, were fed to the oxygen electrode and fuel electrode, respectively. To evaluate the fuel cell performance, hydrogen was flowed through a bubbler at room temperature, while for electrolysis evaluation, a boiler was used instead, to obtain a hydrogen flow with 40% humidity.

[0066] Prior to the electrochemical characterization, a mixture of H2 and Ar gas flow (20 sccm) was fed to reduce the NiO to Ni at the fuel electrode. The electrochemical impedance spectroscopy (EIS) and I-V curve measurements were performed using Gamry Reference series potentiostat / galvanostat / ZRAs, including 1000, 3000 and 5000. EIS data was collected over a frequency range of 10 kHz – 0.1 Hz under open circuit conditions or applied voltage.

[0067] The hydrogen production rate and cost were calculated according to the following equation:

[0068] Where V is the voltage applied on, j is the corresponding current density (A cm-2), represents the unit electricity cost, F is the Faraday constant (96485 s A mol-1), and is the molecular weight of hydrogen (0.001 kg mol-1).

[0069] Electrical conductivity relaxation (“ECR”) testing was utilized to evaluate the surface exchange kinetics of the inventive hybrid oxygen electrode made from the solid composite materials described herein and prior art electrodes formed from PBSCF-a. The PBSCF-a powder was first axially pressed at 100 bar for 2 minutes using a die (diameter = 2 cm), then sintered at 900°C for 3 hours. The resulting dense pellets were cut by a high-precision diamond saw. The electrode from the solid composite material sample was also prepared in a similar manner. PBSCF dense bars were obtained and a BSC slurry was then coated on the surface. ECR measurements of these samples were conducted at temperatures ranging from 600 to 700 °C. In each measurement, the initial oxygen concentration was set as 2 volume percent (490 sccm Ar + 10 sccm O2) and a current of 40 mA was applied. After the voltage reached equilibrium, the oxygen concentration was changed to 20 volume percent (400 sccm Ar + 100sccm O2) and the voltage response was recorded each time. The resulting data was analyzed using the NETL ECR analysis tool.

[0070] The crystal structures of the inventive hybrid oxygen electrode and PBSCF-a electrode were determined by using X-ray diffraction (“XRD”) (Rigaku MiniFlex diffractometer) with a Cu K-alpha radiation source operated at 15 mA and 30 kV and a scanning rate of 1 degree per minute. The Rietveld refinement was further performed via the GSAS program to probe detailed crystal structural information of the solid composite material. The morphological and cross-sectional properties of the single cells were characterized by scanning electron microscopy (SEM, Hitachi / S-3400 N). Transmission electron microscopy (S / TEM, FEI Talos F200X, Thermo Fisher Scientific) operated at 200 kV was employed to obtain the morphological, and structural information about the oxygen electrode materials, with EDX detectors were utilized for chemical composition characterization.

[0071] It has been observed that SOECs equipped with the new hybrid oxygen electrode may achieve remarkable performance for both hydrogen production and power generation.

[0072] It has been observed that SOECs equipped with the new hybrid oxygen electrode may simultaneously attain high hydrogen production rates, while achieving lower hydrogen production cost than SOECs reported in literature. In one or more embodiments, SOECs equipped with the hybrid oxygen electrodes described herein may exhibit a hydrogen production cost of less than 1.2, 1.1, or 1.0 $ kg-1at a hydrogen production rate of 0.02 kg h-1cm-2. Additionally, or in the alternative, SOECs equipped with the hybrid oxygen electrodes described herein may exhibit a hydrogen production cost of less than 1.3, 1.2, or 1.1 $ kg-1at a hydrogen production rate of 0.04 kg h-1cm-2. Additionally, or in the alternative, SOECs equipped with the hybrid oxygen electrodes described herein may exhibit a hydrogen production cost of less than 1.3, 1.2, or 1.1 $ kg-1at a hydrogen production rate of 0.06 kg h-1cm-2. Additionally, or in the alternative, SOECs equipped with the hybrid oxygen electrodes described herein may exhibit a hydrogen production cost of less than 1.5, 1.4, 1.3, or 1.2 $ kg-1at a hydrogen production rate of 0.08 kg h-1cm-2. Additionally, or in the alternative, SOECs equipped with the hybrid oxygen electrodes described herein may exhibit a hydrogen production cost of less than 1.4, 1.3, or 1.2 $ kg-1at a hydrogen production rate of 0.12 kg h-1cm-2.

[0073] Furthermore, SOECs equipped with the new hybrid oxygen electrode may exhibit higher performance than all other known YSZ-based SOECs. In one or more embodiments,SOECs equipped with the hybrid oxygen electrodes described herein may exhibit a current density at 750°C and 1.3 V of at least 3, 3.5, 3.8, 3.9, 4, 4.1, 4.2, 4.3, or 4.4 A cm-2and / or less than 8, 7, 6, or 5 A cm-2.

[0074] Moreover, SOECs equipped with the new hybrid oxygen electrode may exhibit exceptional fuel cell performance using hydrogen and propane as fuel. These promising results have emphasized that the solid composite material described herein is a high-performing oxygen electrode for SOECs. In one or more embodiments, SOECs equipped with the hybrid oxygen electrodes described herein may exhibit a peak power density at 750°C using hydrogen and / or propane as the fuel of at least about 1.8, 1.9, 2, 2.1, 2.2, 2.3, or 2.4 W cm-2and / or less than about 5, 4, or 3 W cm-2.

[0075] Transmission electron microscopy (“TEM”), energy dispersive X-ray spectroscopy (“EDX”), and selected area electron diffraction (SAED) may be performed to better analyze the microstructure, crystal structure, and chemical compositions of the new hybrid oxygen electrode. The hybrid electrode powder may display a microstructure of nanosized particles with an average particle size of at least 1, 5, 10, 15, or 20 nm and / or less than 100, 90, 85, 80, 75, 70, 65, 60, or 55 nm, as confirmed by TEM. The SAED patterns further validate that this hybrid electrode is composed of the BSC phase and a PBSCF-b phase.

[0076] The diameter of each ring corresponds to the interplanar distance (d-spacing) of crystal planes present in the electrode. In certain embodiments, d-spacing values of 0.166 nm, 0.223 nm, and 0.282 nm correspond to (202), (002), and (110) planes of BSC, respectively. Additionally, in such embodiments, the d-spacing values of 0.202 nm, 0.404 nm, and 0.273 nm correspond to (200), (100), and (102) planes of the PBSCF phase.

[0077] Additionally, S / TEM-EDX images and spectra were collected to quantitively confirm the stoichiometries of both perovskite phases presented in the hybrid oxygen electrode. EDX mapping images confirmed that the hybrid oxygen electrode may comprise one Ba-rich phase and one Pr-rich phase, which correspond to BSC and PBSCF, respectively.

[0078] It was also observed that the inventive hybrid oxygen electrode may enhance the surface oxygen exchange coefficient and bulk oxygen-ion diffusion coefficient, thereby leading to improved electrochemical performance by the SOEC. Although not wishing to be bound by theory, it is believed that the two perovskite phases in the hybrid oxygen electrode can synergize to achieve improved electrochemical properties, such as faster surface oxygen exchange and bulkoxygen-ion diffusion kinetics. Therefore, ECR may be performed, and subsequent analysis using the NETL-developed software may be conducted to determine the surface oxygen exchange coefficient (kchem) and bulk oxygen-ion diffusion coefficient (Dchem).

[0079] Generally, it has been observed that the inventive hybrid oxygen electrode may significantly accelerate surface oxygen exchange kinetics. In one or more embodiments, the kchem of the inventive hybrid oxygen electrode may be at least 1.2 × 10-4, 1.3 × 10-4, 1.4 × 10-4, 1.5 × 10-4cm s-1, or 1.2 × 10-4cm s-1at 650°C, 675°C, or 700°C. Thus, the inventive hybrid oxygen electrode may exhibit a kchem that is significantly higher than conventional electrodes made from PBSCF-b. Additionally, in one or more embodiments, the corresponding activation energy of the inventive hybrid oxygen electrode may be significantly lower than the conventional electrodes made from PBSCF-b, indicating that the surface oxygen exchange process exhibits a lower energy barrier, which is essential for achieving high ORR and OER activity at reduced operating temperatures. For example, the inventive hybrid oxygen electrode may exhibit an activation energy of less than 120, 110, 100, 90, or 88 kJ mol-1.

[0080] Furthermore, the Dchem of the inventive hybrid oxygen electrode may also be higher than that of conventional electrodes formed from PBSCF-a. In one or more embodiments, the inventive hybrid oxygen electrode may exhibit a Dchemof at least 1.0 × 106cm2s-1at 550°C, at least 1.0 × 106cm2s-1at 600°C, at least 4.0 × 106cm2s-1at 650°C, at least 1.0 × 105cm2s-1at 675°C, and / or at least 1.0 × 105cm2s-1at 700°C. Moreover, the corresponding activation energy of the hybrid oxygen electrode may also be lower than that of conventional electrodes formed from PBSCF-a. For example, the inventive hybrid oxygen electrode may exhibit an activation energy of less than 190, 185, 180, 175, 170, 165, 160, or 155 kJ mol-1. These results imply that the hybrid oxygen electrodes can be employed to build SOECs operating at reduced operating temperatures.

[0081] The area-specific electrode polarization resistance (ASRp) and ohmic resistance (ASRohm) of SOECs with the hybrid oxygen electrode may also be superior to those of conventional electrodes. More particularly, the new hybrid oxygen electrode may lead to a much lower ASRp than a PBSCF-a electrode, confirming that the hybrid oxygen electrode can achieve improved electrocatalytic activity. In one or more embodiments, SOECs containing the inventive hybrid oxygen electrode may exhibit an ASRpof 0.1 to 0.3 Ω cm2at 750°C; 0.1 to 0.5 Ω cm2at 700°C; 0.1 to 1.0 Ω cm2or 0.2 to 0.9 Ω cm2at 650°C; 0.2 to 2 Ω cm2at 600°C; and / or 0.1 to 6 Ωcm2or 0.2 to 4 Ω cm2at 550°C. Furthermore, the Arrhenius plots of ASRpas a function of operating temperature show that the hybrid electrode may exhibit a lower activation energy than that of the PBSCF-a electrode, which is consistent with the kchem and Dchem results. For example, SOECs containing the inventive hybrid oxygen electrode may exhibit an activation energy of less than 48, 47, 46, 45, or 44 kJ mol-1.

[0082] On the other hand, employing the new hybrid oxygen electrode does not obviously affect the ASRohm, indicating the hybrid oxygen electrode does not lead to any deleterious chemical reactions between the electrolyte and electrode, indicating that the new hybrid electrode is chemically compatible with the electrolyte. In one or more embodiments, SOECs containing the inventive hybrid oxygen electrode may exhibit an ASRohmof 0.01 to 0.06, 0.01 to 0.1, 0.01 to 0.2, or 0.01 to 0.3 Ω cm2at 550°C, 600°C, 650°C, 700°C, or 750°C.

[0083] To gain an in-depth understanding of why the hybrid oxygen electrode leads to improved electrochemical performance, the distribution of relaxation time (“DRT”) analysis of EIS spectra may be performed. The DRT analysis is a convenient and potent method for distinguishing the resistance ascribed to different processes occurring at the electrodes. To transform EIS data into the time domain, the DRT software developed by Ciucci et al. was used, and a Gaussian distribution was employed. Measurements at EIS spectra at 700 °C and 0.8 V and corresponding DRT analysis results were taken. Each DRT analysis plot consists of five distinct peaks, which are denoted as P1-P5, starting from the high frequency (i.e., the highest frequency peak being P1). In general, the processes associated with P4 and P5 correspond to mass transfer processes, including the gas diffusion within the pores of both the oxygen electrode and the fuel electrode, and the transport of ionic / electronic defects within the porous solid electrodes. The process associated with P3 is likely to be the hydrogen oxidation reaction occurring at the hydrogen electrode (i.e., fuel electrode). The process represented by P2 is deemed as the key step because the integrated area of each peak represents the resistance of the process, and P2 shows the highest resistance. This process can be ascribed to the surface oxygen exchange process, including oxygen adsorption, dissociation, and surface transport, which is also the main difference between the inventive hybrid electrode and conventional electrodes made from PBSCF-a. The process represented by P1 is widely accepted to be the charge transfer process across the electrode / electrolyte interface.

[0084] As demonstrated by DRT and EIS, employing the inventive hybrid oxygen electrode primarily reduces the resistance corresponding to the surface oxygen exchange (P2), which further confirms that the hybrid oxygen electrode can significantly facilitate the surface oxygen exchange. This new electrode does not affect the electrolyte / electrode interfacial charge transfer, where the new electrode does not affect ASRohm. EIS data also indicates that the new hybrid electrode slightly reduces the resistance ascribed to the mass transport, which could be due to its relatively high porosity as the hybrid electrode may be sintered at a lower temperature than PBSCF-a.

[0085] In one or more embodiments, SOECs containing the inventive hybrid oxygen electrode may exhibit a DRT P2 peak of at least 0.1, 0.15, or 0.2 Ω cm2. Additionally, or in the alternative, the SOECs containing the inventive hybrid oxygen electrode may exhibit a DRT P1, P3 peak, P4 peak, and / or P5 peak of at least 0.03, 0.04, or 0.05 Ω cm2.

[0086] After analyzing the DRT peaks, the temperature dependencies of the area-specific resistances of each peak may be plotted for the hybrid oxygen electrode and conventional electrodes made from PBSCF-a. The resistance of the P2 process has the highest activation energy for both electrode materials, dominating especially at lower temperatures, and this is particularly evident for conventional electrodes made from PBSCF-a. For the inventive hybrid electrode, the P1 process is the major contribution in the higher temperature range. However, due to its much lower activation energy, its influence on the total resistance decreases as the temperature decreases. The temperature dependence of the resistance of the P3 process is strong for both electrode materials, with 142.05 kJ mol-1for the hybrid electrode and 103.73 kJ mol-1for PBSCF-a. Compared to other processes, the resistances associated with P4 and P5 are less significant.

[0087] To assess the SOEC performance in fuel cell mode, a set of SOECs equipped with the inventive hybrid oxygen electrode and an PBSCF-a electrode were tested. Testing SOECs were manufactured via co-tape casting, producing SOEC half cells that were highly reproducible, highly uniform, and defect-free, and had dense electrolytes with a thickness of about 8 µm, which was essential for fairly investigating different oxygen electrode materials. The inventive hybrid oxygen electrodes subjected to testing had an effective area of about 0.5 cm2.

[0088] Fuel cell performance may be measured using the aforementioned SOECs at an OCV of ~1.1 V at 750 °C, which is close to the theoretical value, indicating there is no gasleakage and the performance evaluation is valid. During testing, hydrogen was fed to the fuel electrode as fuel. In one or more embodiments, the SOEC with the inventive hybrid oxygen electrode may attain peak power densities of: 1.5 to 2.5 W cm-2or about 2.4 W cm-2at 750°C; 1.0 to 1.9 W cm-2or about 1.6 W cm-2at 700°C; 0.6 to 1.2 W cm-2or about 0.9 W cm-2at 650°C; 0.2 to 0.5 W cm-2or about 0.4 W cm-2at 600°C; and / or 0.1 to 0.3 W cm-2or about 0.2 W cm-2at 550°C. The peak power densities achieved with the inventive hybrid electrode may be two to four times as high as that achieved with a conventional PBSCF-a oxygen electrode. The demonstrated improvement in fuel cell performance can be investigated by conducting EIS. Notably, the ASRp was observed to be much lower using the inventive hybrid oxygen electrode compared to the PBSCF-a electrode.

[0089] Furthermore, the excellent fuel cell performance may also be demonstrated using C3H8 as the fuel. In one or more embodiments, the SOEC with the inventive hybrid oxygen electrode may attain peak power densities of 1.0 to 3.2 W cm-2, 1.3 to 2.9 W cm-2, or about 2.5 W cm-2at 750°C; 0.5 to 3.0 W cm-2, 1.0 to 1.9 W cm-2, or about 1.6 W cm-2at 700°C; 0.6 to 1.4 W cm-2or about 0.9 W cm-2at 650°C; 0.2 to 0.8 W cm-2or about 0.5 W cm-2at 600°C; and / or 0.1 to 0.5 W cm-2or about 0.2 W cm-2at 550°C.

[0090] The durability of SOECs is crucial for reliable power generation. Short-term stability testing of the SOECs in fuel cell mode may be conducted with both H2 and C3H8 fed to the fuel electrode. With humidified H2 fed to the fuel electrode as the fuel, it was observed that SOECs with the inventive electrode achieved durable operation with negligible degradation at a current density of 0.8 A cm-2and temperature of 650°C. Additionally, the SOEC was durable at 550°C and at a current density of 0.2 A cm-2, which validates the durability of the inventive hybrid oxygen electrode at an intermediate operating temperature. Moreover, the inventive electrode’s potential for utilizing alternative fuels (e.g., C3H8) for durable power generation at a current density of 0.4 A cm-2and temperature of 600 °C, with C3H8 being fed to the fuel electrode, may also be demonstrated. Overall, short-term stability testing conducted on SOECs with the inventive hybrid electrode may demonstrate its potential for durable power generation using multiple fuel streams.

[0091] SOECs containing the inventive oxygen hybrid electrodes may exhibit exceptional performance in steam electrolysis mode. In one or more embodiments, the SOECs containing the inventive oxygen hybrid electrodes may exhibit unprecedented performance insteam electrolysis mode. For instance, in various embodiments, the SOECs containing the inventive oxygen hybrid electrodes may exhibit a current density of at least about 1, 2, 3, 3.5, 4, or 4.4 A cm-2and / or less than 8, 7, or 6 A cm-2at 1.3 V and 750°C, while in steam electrolysis mode. Additionally, or in the alternative, the SOECs containing the inventive oxygen hybrid electrodes may exhibit a current density of at least about 0.5, 1, 1.5, 2, or 2.3 A cm-2and / or less than 8, 7, or 6 A cm-2at 1.3 V and 700°C, while in steam electrolysis mode. Additionally, or in the alternative, the SOECs containing the inventive oxygen hybrid electrodes may exhibit a current density of at least about 0.5, 1, or 1.4 A cm-2and / or less than 5, 4, or 3 A cm-2at 1.3 V and 650°C, while in steam electrolysis mode. Additionally, or in the alternative, the SOECs containing the inventive oxygen hybrid electrodes may exhibit a current density of at least about 0.1, 0.5, or 0.9 A cm-2and / or less than 4, 3, or 2 A cm-2at 1.3 V and 600°C, while in steam electrolysis mode.

[0092] Furthermore, to validate the stability of employing these SOECs for reliable H2 production, short-term stability may be initially tested at 650°C with a current density of 0.8 A cm-2. It was observed that SOECs containing the inventive oxygen hybrid electrodes may display a minor degradation at the early stages and stable operation for the remaining 40 hours. To accelerate the evaluation of SOEC stability, SOECs may be tested under an extremely high current density. For example, SOECs may be tested at 650°C and a current density of 3.0 A cm-2. During accelerated testing, it was observed that SOECs containing the inventive oxygen hybrid electrodes do not show degradation upon the accelerated stability testing. A second accelerated stability test was performed at 750°C and a current density of 5.0 A cm-2, which also indicated that the SOECs equipped with the new hybrid electrode are stable. Post-mortem characterizations were performed to evaluate the structural stability of the SOECs. SEM images confirmed that there was not any cracking or electrolyte-electrode delamination, implying the SOEC structure was robust under the harsh testing conditions.

[0093] In one or more embodiments, an electrical assembly is provided that comprises an SOEC described herein operably connected to a source of electricity and / or an electrical load, such as a light bulb. In various embodiments, the SOEC is operably connected to a source of water, such as steam.

[0094] Depending on the operating mode employed, the SOEC 10 can produce hydrogen or generate electricity.

[0095] The present disclosure is also concerned with a method of producing hydrogen. In one or more embodiments, the method comprises providing a SOEC described herein; reacting water at the fuel electrode to generate oxygen and protons; causing the protons to pass through the electrolyte to the oxygen electrode; and supplying an electrical current to the oxygen electrode and reducing at least a portion of the protons to form hydrogen.

[0096] To produce hydrogen, the SOEC 10 operates in a hydrogen-producing configuration. While in this configuration, water, preferably in the form of steam, is reacted at the fuel electrode 12 to generate oxygen, electrons, and protons via an oxygen evolution reaction, which occurs at a temperature of from about 300℃ to about 800℃. The generated protons may then pass through the proton-conducting electrolyte 14 to the oxygen electrode 16. Simultaneously, an electric current may be supplied to the oxygen electrode 16 from an external source, such as a solar panel. The oxygen electrode 16 may reduce at least a portion of the protons to form hydrogen via a hydrogen evolution reaction, which occurs at a temperature of from about 300℃ to about 800℃.

[0097] To generate electricity, the SOEC operates in an electricity-producing configuration. In this configuration, hydrogen is supplied to the SOEC, which is oxidized at the fuel electrode 12 to generate electrons and protons via a hydrogen oxidation reaction occurring at a temperature of from about 300℃ to about 800℃. In most embodiments, the supplied hydrogen is hydrogen previously formed while the SOEC operated in the hydrogen-producing configuration. Once generated, the protons pass through the proton-conducting electrolyte 14 to the oxygen electrode 16 and react with oxygen to produce water via an oxygen reduction reaction, which occurs at a temperature of from about 300℃ to about 800℃. Simultaneously, the generated electrons induce an electrical current and pass the electrical current through an electrical load to the oxygen electrode 16. In various embodiments, the electrons induce an open circuit voltage of the cell of from about 0.5 V to about 1.50 V, about 0.75 V to about 1.25 V, about 1.0 V to about 1.1 V, or about 1.05 V.

[0098] During operation, the SOEC operates at a temperature of from about 200℃ to about 1000℃, about 350℃ to about 850℃, or about 500℃ to about 700℃.

[0099] The present disclosure is also concerned with a method of generating electricity. In one or more embodiments, the method comprises: (a) providing an electrochemical cell; (b) supplying hydrogen to the electrochemical cell and oxidizing the hydrogen at the first electrodeto form protons and electrons; (c) causing the protons to pass through the electrolyte to the second electrode; (d) causing the electrons generated at the first electrode to induce an electrical current and passing the electrical current through an electrical load that is connected with the first electrode and the second electrode; and (e) reacting the protons with oxygen at the second electrode to produce water.

[0100] The present disclosure provides a new hybrid oxygen electrode, which is composed the solid composite materials described herein. This new hybrid electrode significantly improves the surface oxygen exchange and bulk oxygen-ion diffusion coefficients, enhancing both oxygen reduction reaction and oxygen evolution reaction kinetics. YSZ-based SOECs equipped with this novel hybrid electrode may attain unprecedented performance for both power generation in fuel cell mode and green hydrogen production in steam electrolysis mode. For example, at 750°C, a peak power density of about 2.4 W cm-2may be achieved using H2 as the fuel. With propane fed to the fuel electrode, a peak power density of about 2.6 W cm-2may be attained, representing a record in this field. In steam electrolysis mode, a current density of about 4.4 A cm-2was demonstrated at 1.3 V and 750°C, which outperforms all previously reported YSZ-based SOECs. This achievement led to a cell-level hydrogen production cost of about 0.997 $ kg-1at an H2production rate of about 0.0187 kg h-1cm-2, which approaches the DOE H2 production cost target (<$1 / kg). Additionally, accelerated stability testing performed in steam electrolysis mode under extremely high current densities (5.0 A cm-2) validated the stability of the SOECs equipped with the newly developed hybrid electrode. These promising results highlight that the solid composite materials described herein could serve as the next- generation oxygen electrode of high-performance SOECs. DEFINITIONS

[0101] It should be understood that the following is not intended to be an exclusive list of defined terms. Other definitions may be provided in the foregoing description, such as, for example, when accompanying the use of a defined term in context.

[0102] As used herein, the terms “a,” “an,” and “the” mean one or more.

[0103] As used herein, the term “about” refers to a range within ±10% of the stated value. For example, “about 10” would cover a range of 9 to 11.

[0104] As used herein, the term “and / or,” when used in a list of two or more items, means that any one of the listed items can be employed by itself or any combination of two or more of the listed items can be employed. For example, if a composition is described as containing components A, B, and / or C, the composition can contain A alone; B alone; C alone; A and B in combination; A and C in combination, B and C in combination; or A, B, and C in combination.

[0105] As used herein, the terms “comprising,” “comprises,” and “comprise” are open- ended transition terms used to transition from a subject recited before the term to one or more elements recited after the term, where the element or elements listed after the transition term are not necessarily the only elements that make up the subject.

[0106] As used herein, the terms “having,” “has,” and “have” have the same open-ended meaning as “comprising,” “comprises,” and “comprise” provided above.

[0107] As used herein, the terms “including,” “include,” and “included” have the same open-ended meaning as “comprising,” “comprises,” and “comprise” provided above. NUMERICAL RANGES

[0108] The present description uses numerical ranges to quantify certain parameters relating to the invention. It should be understood that when numerical ranges are provided, such ranges are to be construed as providing literal support for claim limitations that only recite the lower value of the range as well as claim limitations that only recite the upper value of the range. For example, a disclosed numerical range of 10 to 100 provides literal support for a claim reciting “greater than 10” (with no upper bounds) and a claim reciting “less than 100” (with no lower bounds). CLAIMS NOT LIMITED TO DISCLOSED EMBODIMENTS

[0109] The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention.

[0110] The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as it pertains to anyapparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.

Claims

What is claimed is:

1. A hybrid oxygen electrode for a reversible solid oxide electrochemical cell comprising a solid composite material, wherein the solid composite material comprises a double perovskite phase and a single perovskite phase.

2. The hybrid oxygen electrode of claim 1, wherein the double perovskite phase comprises PrBaSrCoFeO (PBSCF) and the single perovskite phase comprises BaSrCoO (BSC).

3. The hybrid oxygen electrode of claim 2, wherein the molar ratio of barium to strontium in the double perovskite phase is from 0.1:1 to 1:

1.

4. The hybrid oxygen electrode of claim 3, wherein the molar ratio of praseodymium to barium in the double perovskite phase is from 1.5:1 to 15:

1.

5. The hybrid oxygen electrode of claim 4, wherein the molar ratio of praseodymium to strontium in the double perovskite phase is from 1.5:1 to 3.5:

1.

6. The hybrid oxygen electrode of claim 5, wherein the molar ratio of praseodymium to cobalt in the double perovskite phase is from 0.5:1 to 1.5:

1.

7. The hybrid oxygen electrode of claim 6, wherein the molar ratio of praseodymium to iron in the double perovskite phase is from 1.5:1 to 2.5:

1.

8. The hybrid oxygen electrode of claim 7, wherein the molar ratio of barium to strontium in the single perovskite phase is from 1.5:1 to 2.0:

1.

9. The hybrid oxygen electrode of claim 8, wherein the molar ratio of barium to cobalt in the single perovskite phase is from 0.5:1 to 0.75:1.

10. The hybrid oxygen electrode of claim 9, wherein the molar ratio of strontium to cobalt in the single perovskite phase is from 0.25:1 to 0.5:

1.

11. The hybrid oxygen electrode of claim 2, wherein the double perovskite phase has a formula of PraBabSrcCodFeeO6-δ1 and the single perovskite phase has a formula of BaxSryCozO3- δ2, wherein – a is from 1 to 1.75, b is from 0.05 to 0.5, c is from 0.3 to 0.7, d is from 1.25 to 1.75, e is from 0.4 to 0.9, δ1 is greater than zero, but less than 1, x is from 0.4 to 0.75, y is from 0.25 to 0.5, z is from 0.75 to 1.25, and / or δ2 is greater than zero, but less than 0.

5.

12. The hybrid oxygen electrode of claim 11, wherein the double perovskite phase has a tetragonal (P4 / mmm) crystal structure and / or the single perovskite phase has a hexagonal P63 / mmc crystal structure.

13. The hybrid oxygen electrode of claim 11, wherein the solid composite material comprises at least 50 weight percent of the double perovskite phase, based on the total weight of the solid composite material.

14. The hybrid oxygen electrode of claim 11, wherein the solid composite material comprises at least 10 weight percent and less than 50 weight percent of the single perovskite phase, based on the total weight of the solid composite material.

15. The hybrid oxygen electrode of claim 11, wherein the hybrid oxygen electrode exhibits a Dchemof at least 1.0 × 105cm2s-1at 700°C.

16. The hybrid oxygen electrode of claim 11, wherein the hybrid oxygen electrode exhibits a Dchem of at least 4.0 × 106cm2s-1at 650°C.

17. The hybrid oxygen electrode of claim 11, wherein the hybrid oxygen electrode exhibits an ASRp of 0.1 to 0.3 Ω cm2at 750°C.

18. The hybrid oxygen electrode of claim 11, wherein the hybrid oxygen electrode exhibits an ASRp of 0.1 to 0.5 Ω cm2at 700°C.

19. The hybrid oxygen electrode of claim 11, wherein the hybrid oxygen electrode exhibits an ASRohm of 0.01 to 0.3 Ω cm2at or 750°C.

20. The hybrid oxygen electrode of claim 11, wherein the hybrid oxygen electrode exhibits kchem of the inventive hybrid oxygen electrode may be at least 1.2 × 10-4cm s-1at 700°C.

21. A reversible solid oxide electrochemical cell comprising: a fuel electrode comprising a first electrode material; a hybrid oxygen electrode comprising a second electrode material; and a proton-conducting electrolyte positioned between the fuel electrode and the hybrid oxygen electrode, wherein the second electrode material comprise a solid composite material, wherein the solid composite material comprises a double perovskite phase and a single perovskite phase.

22. The electrochemical cell of claim 21, wherein the double perovskite phase comprises PrBaSrCoFeO (PBSCF) and the single perovskite phase comprises BaSrCoO (BSC).

23. The electrochemical cell of claim 22, wherein the molar ratio of barium to strontium in the double perovskite phase is from 0.1:1 to 1:

1.

24. The electrochemical cell of claim 23, wherein the molar ratio of praseodymium to barium in the double perovskite phase is from 1.5:1 to 15:

1.

25. The electrochemical cell of claim 24, wherein the molar ratio of praseodymium to strontium in the double perovskite phase is from 1.5:1 to 3.5:

1.

26. The electrochemical cell of claim 25, wherein the molar ratio of praseodymium to cobalt in the double perovskite phase is from 0.5:1 to 1.5:

1.

27. The electrochemical cell of claim 26, wherein the molar ratio of praseodymium to iron in the double perovskite phase is from 1.5:1 to 2.5:

1.

28. The electrochemical cell of claim 27, wherein the molar ratio of barium to strontium in the single perovskite phase is from 1.5:1 to 2.0:

1.

29. The electrochemical cell of claim 28, wherein the molar ratio of barium to cobalt in the single perovskite phase is from 0.5:1 to 0.75:1.

30. The electrochemical cell of claim 29, wherein the molar ratio of strontium to cobalt in the single perovskite phase is from 0.25:1 to 0.5:

1.

31. The electrochemical cell of claim 21, wherein the double perovskite phase has a formula of PraBabSrcCodFeeO6-δ1 and the single perovskite phase has a formula of BaxSryCozO3-δ2, wherein – a is from 1 to 1.75, b is from 0.05 to 0.5, c is from 0.3 to 0.7, d is from 1.25 to 1.75, e is from 0.4 to 0.9, δ1 is greater than zero, but less than 1, x is from 0.4 to 0.75, y is from 0.25 to 0.5, z is from 0.75 to 1.25, and / or δ2 is greater than zero, but less than 0.

5.

32. The electrochemical cell of claim 31, wherein the double perovskite phase has a tetragonal (P4 / mmm) crystal structure and / or the single perovskite phase has a hexagonal P63 / mmc crystal structure.

33. The electrochemical cell of claim 31, wherein the solid composite material comprises at least 50 weight percent of the double perovskite phase, based on the total weight of the solid composite material.

34. The electrochemical cell of claim 31, wherein the solid composite material comprises at least 10 weight percent and less than 50 weight percent of the single perovskite phase, based on the total weight of the solid composite material.

35. The electrochemical cell of claim 31, wherein the hybrid oxygen electrode exhibits a Dchem of at least 1.0 × 105cm2s-1at 700°C.

36. The electrochemical cell of claim 31, wherein the hybrid oxygen electrode exhibits a Dchem of at least 4.0 × 106cm2s-1at 650°C.

37. The electrochemical cell of claim 31, wherein the hybrid oxygen electrode exhibits an ASRp of 0.1 to 0.3 Ω cm2at 750°C.

38. The electrochemical cell of claim 31, wherein the hybrid oxygen electrode exhibits an ASRpof 0.1 to 0.5 Ω cm2at 700°C.

39. The electrochemical cell of claim 31, wherein the hybrid oxygen electrode exhibits an ASRohmof 0.01 to 0.3 Ω cm2at or 750°C.

40. The electrochemical cell of claim 31, wherein the hybrid oxygen electrode exhibits kchemof the inventive hybrid oxygen electrode may be at least 1.2 × 10-4cm s-1at 700°C.

41. The electrochemical cell of claim 31, wherein the electrochemical cell exhibits a peak power density at 750°C using hydrogen and / or propane as a fuel of at least 1.8 W cm-2.

42. The electrochemical cell of claim 31, wherein the electrochemical cell exhibits a current density at 750°C and 1.3 V of at least 3 A cm-2.

43. An electrical assembly comprising the electrochemical cell of claim 21 operably connected to a source of electricity and / or an electrical load.

44. The electrical assembly of claim 43, wherein the electrical assembly is operably connected to a source of steam.

45. A method of producing hydrogen comprising:providing the electrochemical cell of claim 21; reacting water at the fuel electrode to generate oxygen and protons; causing the protons to pass through the proton-conducting electrolyte to the hybrid oxygen electrode; and supplying an electrical current to the fuel electrode and / or the hybrid oxygen electrode and reducing at least a portion of the protons to form hydrogen.

46. A method of generating electricity comprising: providing the electrochemical cell of claim 21; supplying hydrogen to the electrochemical cell and oxidizing the hydrogen at the fuel electrode to form protons and electrons; causing the protons to pass through the proton-conducting electrolyte to the hybrid oxygen electrode; causing the electrons generated at the fuel electrode to induce an electrical current and passing the electrical current through an electrical load that is connected with the fuel electrode and the hybrid oxygen electrode; and reacting the protons with oxygen at the hybrid oxygen electrode to produce water.

47. A method for forming the electrochemical cell of claim 21 comprising: (a) providing the fuel electrode; (b) forming the proton-conducting electrolyte on a surface of the fuel electrode; and (c) forming the hybrid oxygen electrode on a surface of the proton-conducting electrolyte.

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