Protonic ceramic electrochemical cells having composite positive electrode

Abstract

Protonic ceramic electrochemical cells (PCECs) can be employed for power generation and sustainable hydrogen production. Achieving high energy efficiency and long-term durability at low operating temperatures is a long-standing challenge in PCEC research. A simple and scalable approach for fabricating ultrathin, chemically homogeneous, and robust protonconducting electrolytes and demonstrate an in-situ formed composite positive electrode, Bao.62Sr0.38Co03-8-Pr1.44Bao.11Sro.45Co1.32Feo.6s06-6, which significantly reduces ohmic resistance, positive electrode-electrolyte contact resistance, and electrode polarization resistance. The PCECs attain high power densities in fuel cell mode and exceptional current densities in steam electrolysis mode.

Description

PROTONIC CERAMIC ELECTROCHEMICAL CELLS HAVING COMPOSITE POSITIVE ELECTRODE RELATED APPLICATIONS

[0001] This application claims the priority benefit of U.S. Provisional Patent Application Serial No. 63 / 598,327 entitled “PROTONIC CERAMIC ELECTROCHEMICAL CELLS HAVING COMPOSITE POSITIVE ELECTRODE,” filed November 13, 2023, and U.S. Provisional Patent Application Serial No. 63 / 580,917 entitled “PROTONIC CERAMIC ELECTROCHEMICAL CELLS HAVING COMPOSITE POSITIVE ELECTRODE,” filed September 6, 2023, the entire disclosures of which are incorporated herein by reference. BACKGROUND 1. Field of the Invention

[0002] The present disclosure generally relates to positive electrodes that can be used in protonic ceramic electrochemical cells (“PCECs”). More particularly, the present disclosure generally relates to positive electrodes that comprise a solid composite material comprising a double perovskite phase and a single perovskite phase. 2. Description of the Related Art

[0003] Ceramic electrochemical cells based on both oxygen-ion conductors and proton- conducting oxides have been demonstrated for efficient power generation and green hydrogen production. Ceramic electrochemical cells have key merits, including fuel flexibility in fuel cell mode and higher energy efficiency in steam electrolysis mode compared to low-temperature polymer electrolyte membrane electrochemical cells. However, the scaleup and commercialization of ceramic electrochemical cells have lagged behind those of polymer electrolyte membrane electrochemical cells, which is mainly ascribed to the high operating temperature of ceramic electrochemical cells (>500°C). Therefore, creating low-temperature ceramic electrochemical cells has been one of the primary challenges and opportunities in this research field.

[0004] The operating temperature of solid oxide electrochemical cells based on oxygen- ion conductors (“O-SOECs”) has been successfully reduced to ~500°C. However, due to the relatively high activation energy of oxygen-ion conduction, reducing the operating temperatureof O-SOECs to <450°C is challenging. Due to proton conduction having a lower activation energy than oxygen-ion conduction, protonic ceramic electrochemical cells (“PCECs”) have been recurringly researched and proposed to operate at <450°C.

[0005] Only a handful of studies have reported that PCECs can operate at <450°C. Haile et al. demonstrated that depositing a dense interfacial layer between the electrolyte and positive electrode reduced the ohmic resistance and increased the PPD to 0.36 W cm-2on H2-air at 450°C. To reduce the electrode polarization resistance, Ding et al. fabricated a highly porous PrBa0.5Sr0.5Co1.5Fe0.5O6-δ (“PBSCF-1”) positive electrode using a complicated template method, which improved the mass transport kinetics, leading to a PPD of 0.41 W cm-2on H2-O2 at 400°C. A recent study by the same team reported that employing an acid-etched electrolyte and a positive electrode fabricated via this template method further increased the PPD to 0.65 W cm-2on H2-O2 at 450°C. However, most of these approaches are either complicated or unscalable. Therefore, more straightforward and scalable methods are desired to fabricate PCECs with low- resistance electrolytes and active positive electrodes to achieve good PCEC performance at <450°C.

[0006] It has been observed that there is a discrepancy between intrinsic electrolyte conductivity and PCEC ohmic conductivity, implying that PCECs contribute additional ohmic resistance, which mainly includes high electrolyte-grain boundary resistance, and positive electrode-electrolyte contact resistance. Therefore, PCECs should be better fabricated to optimize their microstructure, aiming at minimizing the grain boundary resistance while enhancing the bonding between the electrode and electrolyte, particularly the weakly bonded positive electrode-electrolyte interface created in a separated high-temperature step. Furthermore, it has been widely recognized that the oxygen reduction reaction (“ORR”) and oxygen evolution reaction (“OER”), which occur at the positive electrode, tend to be more sluggish with decreasing operating temperatures. These results suggest that both the electrolyte and positive electrode should be optimized to enhance PCEC performance at <450°C. SUMMARY

[0007] One or more embodiments generally relate to a positive electrode that can be used in protonic ceramic electrochemical cells (PCECs) that comprises a solid composite material comprising a double perovskite phase and a single perovskite phase. The double perovskitephase comprises PrBaSrCoFeO (PBSCF) and the single perovskite phase comprises BaSrCoO (BSC).

[0008] One or more embodiments generally relate to a positive electrode that comprises a solid composite material. The 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.

[0009] One or more embodiments generally relate to a protonic ceramic electrochemical cell (PCEC) comprising a positive electrode formed from a solid composite material according to any embodiment described herein. The PCEC further comprises a negative electrode that comprises any material conventional for such electrodes, including, but not limited to, BCZYYb7111 and NiO. The PCEC also comprises an electrolyte located between the oxygen and fuel electrodes. The electrolyte can be any conventional electrolyte material, including, but not limited to BCZYYb4411. In certain embodiments, the positive and negative 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 certain embodiments, the electrolyte material has a thickness of at least 1 μm, from 1 to 30 μm, from 2 to 20 μm, or from 5 to 15 μm.

[0010] One or more embodiments generally relate to a method of producing hydrogen. Generally, the method comprises: providing the protonic ceramic electrochemical cell described herein; reacting water at the negative electrode to generate oxygen and protons; causing the protons to pass through the proton-conducting electrolyte to the positive electrode; and supplyingan electrical current to the negative electrode and reducing at least a portion of the protons to form hydrogen.

[0011] One or more embodiments generally relate to a method of generating electricity. Generally, the method comprises: providing the protonic ceramic 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 second electrode to induce an electrical current and passing the electrical current through an electrical load that is connected with the second electrode and the first electrode; and reacting the protons with oxygen at the second electrode to produce water. BRIEF DESCRIPTION OF THE FIGURES

[0012] Embodiments of the present invention are described herein with reference to the following drawing figures, wherein:

[0013] FIG. 1 depicts an exemplary protonic ceramic electrochemical cell electrochemical cell comprising an electrode formed from the solid composite materials described herein; and

[0014] FIG. 2 depicts schematic illustrations of PCEC electrolytes fabricated using: (a) an ultrasonic spray coater and (b) a traditional air brush. DETAILED DESCRIPTION

[0015] A solid composite material has been developed that may be used for various applications, including forming 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 present disclosure may be used to produce electrodes and electrochemical cells that exhibit superior performance. For instance, it has been discovered that positive electrodes may be formed from the solid composite materials that can be used in protonic ceramic electrochemical cells (“PCECs”) and which exhibit superior performance. More particularly, it has been discovered that high-performance, low-temperature (<450°C) PCECs (“LT-PCECs”) may be produced using the solid composite materials described herein.

[0016] As described below in greater detail, a protonic ceramic electrochemical cell may be provided that comprises a positive electrode comprising a solid composite material according to any embodiment described herein. The PCEC further comprises a negative electrode that comprises any material conventional for such electrodes, including, but not limited to, BCZYYb7111 and NiO. Additionally, the PCEC comprises an electrolyte located between the positive and negative electrodes. The electrolyte can be any conventional electrolyte material, including, but not limited to BCZYYb4411. In certain embodiments, the positive and negative 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. Additionally, or in the alternative, the electrolyte material has a thickness of at least 1 μm, from 1 to 30 μm, from 2 to 20 μm, or from 5 to 15 μm. The Solid Composite Materials

[0017] 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”).

[0018] 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.

[0019] 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.

[0020] 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 Pr6O11may 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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 certainembodiments, 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.

[0025] 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).

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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, 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] In an exemplary embodiment, the double perovskite phase has a formula of Pr1.44Ba0.11Sr0.45Co1.32Fe0.68O6-δ and the single perovskite phase has a formula of Ba0.62Sr0.38CoO3-δ-.

[0037] 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-^.

[0038] 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”).

[0039] The crystal structure of the as-synthesized powders was measured using XRD (Rigaku MiniFlex diffractometer with a Cu Kα radiation source operated at 30 kV and 15 mA). The scan speed was 1° / min. The morphology, microstructure, and chemical composition of the solid composite materials were examined using transmission electron microscopy (TEM, FEI Tecnai Osiris 200 Kv S / TEM system and Talos F200X S / TEM system operated at 200 kV, equipped with an FEG (field emission gun) cathode and four in-column Super-X energy dispersive X-ray spectrometer (EDS) detectors with a total collection angle of 0.9 sr). The microstructure of the PCECs was examined by using scanning electron microscopy (Hitachi / S- 3400 N).

[0040] In one or more embodiments, the solid composite material comprises at least 25, 30, 35, 40, 45, 50, 55, 60, 65, or 70 weight percent of the double perovskite phase. Additionally, or in the alternative, the solid composite material may comprise less than 99, 95, 90, 85, 80, or 75 weight percent of double perovskite phase. In certain embodiments, the solid composite material comprises about 70 weight percent of double perovskite phase. These weight percentages and those disclosed below may be confirmed by XRD.

[0041] 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, or 35 weight percent of single perovskite phase. In certain embodiments, the solid composite material comprises about 30 weight percent of single perovskite phase.

[0042] 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 Protonic Ceramic Electrochemical Cells

[0043] 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 an 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. Exemplary fuel cells that may beproduced from electrodes containing the solid composite materials described herein are protonic ceramic electrochemical cells (“PCECs”).

[0044] As shown in FIG. 1, and as described in greater detail below, a protonic ceramic electrochemical cell 10 comprising an electrode formed from the solid composite materials described herein may comprise: a negative electrode 12; an electrolyte 14 in contact with the negative electrode 12; and a positive electrode 16 in contact with the electrolyte 14.

[0045] Various approaches are described herein for overcoming the aforementioned challenges and fabricating high-performance low-temperature (<450°C) PCECs (“LT-PCECs”).

[0046] The present disclosure is also concerned with a method of forming a protonic ceramic electrochemical cells (“PCEC”) with the solid composite materials described herein. In one or more embodiments, the method comprises: (a) forming a single-layer or double-layer negative electrode 12 on a temporary substrate; (b) forming an electrolyte 14 on the negative electrode 12 to thereby form a stack; (c) optionally sintering the stack to form a portion of the electrochemical cell 10; and (d) forming a positive electrode 16 on the electrolyte 14, thereby forming a second stack comprising the negative electrode 12, the electrolyte 14, and the positive electrode 16; and (f) optionally sintering the second stack to form the electrochemical cell 10.

[0047] In one or more embodiments, the electrochemical cell 10 comprises a negative electrode 12. It will be appreciated that, although referred to as a “negative electrode,” depending on the operating mode employed, the negative electrode 12 can function as an anode or cathode in the cell 10. Particularly, when the cell 10 is operated in a hydrogen-producing mode, the negative electrode 12 (functioning as a cathode) reduces protons to hydrogen, and when the cell is operated in an electricity-producing mode, the first electrode (functioning as an anode) forms protons from hydrogen.

[0048] In one or more embodiments, the negative electrode 12 comprises any conventional and commercially-available electrode material currently available. In certain embodiments, the negative electrode 12 comprises and may be formed from BaCe0.7Zr0.1Y0.1Yb0.1O3-δ(“BCZYYb7111”). In certain embodiments, the negative electrode 12 comprises at least 50, 75, 90, or 99 weight percent of BCZYYb7111.

[0049] In one or more embodiments, negative electrode powder may be composed of BaCe0.7Zr0.1Y0.1Yb0.1O3-δ(“BCZYYb7111”), NiO, and corn starch with a weight ratio of 2:3:1. It has been widely recognized that an additional Ba reservoir can facilitate electrolytedensification and mitigate elemental segregation, which consequently improves proton conductivity. Thus, an anode substrate with minimized Ba nonstoichiometry (i.e., low Ba deficiency) can promote electrolyte densification and proton conduction. In various embodiments, BCZYYb7111 may be synthesized via a fast-heating solid-state sintering (“SSS”) method. In such embodiments, a stoichiometric amount of BaCO3, CeO2, ZrO2, Y2O3, Yb2O3, and about one weight percent of NiO (the amount of NiO may be based on the total weight of crystallized BCZYYb7111) may be first ball-milled for about 48 hours. After drying at 120°C for 24 hours, the homogeneous mixture may be calcined at 1,350°C with a fast-heating rate of 10°C / min for 5 hours in the air to crystallize BCZYYb7111, which reduces Ba vaporization during the anode powder crystallization process and thereby maintains the Ba stoichiometry. The obtained chuck powder may then be crushed into a fine powder and then ball-milled again and dried at 120°C overnight. The as-synthesized BCZYYb7111 fine powder may then be ball- milled with NiO and corn starch for 48 hours. After drying at 120°C, a negative electrode powder with a low Ba deficiency may be synthesized. During the following cosintering process, the anode substrate with a low Ba deficiency may function as the Ba reservoir to assist in electrolyte densification and retain chemical homogeneity.

[0050] In one or more embodiments, the negative electrode functional layer powder may be composed of at least 20, 30, or 40 weight percent and / or less than 80, 70, 60, or 50 weight percent of BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb7111) and at least 20, 30, 40, 50, or 60 weight percent and / or less than 95, 80, 70, or 65 weight percent of NiO. The addition of corn starch to the powder may be optional.

[0051] In one or more embodiments, the negative electrode powder may be mixed with various additives (e.g., menhaden fish oil, b-98 polyvinyl butyral, polyethylene glycol 400, cyclohexanone, di-n-butyl phthalate, and / or xylenes) and an alcohol (e.g., ethanol) and then ball- milled for a duration (e.g., 24 hours) to obtain a negative electrode slurry.

[0052] In one or more embodiments, the negative electrode slurry may be coated on desired substrate (e.g., Mylar) via tape-casting to form a negative electrode function layer. After drying for a designated time period (e.g., in air for 5 hours), the negative electrode slurry may then be cast on the negative electrode function layer. The resulting two layers may be dried (e.g., in air for 48 hours) and then presintered at about 800°C for about 30 minutes to thermally degrade all the organic materials.

[0053] Once formed, the negative electrode 12 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.

[0054] In one or more embodiments, the electrochemical cell 10 comprises a proton- conducting electrolyte layer 14.

[0055] In one or more embodiments, the proton-conducting electrolyte layer 14 may be formed from any known or conventional electrolyte material currently available. In certain embodiments, the proton-conducting electrolyte layer 14 may be formed from BaCe0.4Zr0.4Y0.1Yb0.1O3-δ (“BCZYYb4411”).

[0056] In one or more embodiments, the electrolyte powder used to form the electrolyte layer 14 may be prepared using a wet-chemistry method to obtain nanosized electrolyte powder. BaCe0.4Zr0.4Y0.1Yb0.1O3-δ (BCZYYb4411) may be synthesized via a wet-chemistry method using Y2O3, Yb2O3, Ba(NO3)2, Ce(NO3)3•6H2O, and zirconyl nitrate solution (35 wt. % in diluted nitric acid) as the precursors. The calculated amounts of Y2O3and Yb2O3may be first dissolved in 20 weight percent diluted nitric acid solution at 100°C to obtain yttrium and ytterbium nitrate solutions. Subsequently, stoichiometric amounts of Ba(NO3)2, Ce(NO3)3•6H2O, and zirconyl nitrate may be subsequently added into the yttrium and ytterbium nitrate solution, and then stirred for 30 minutes. The complexing and chelating agents, citric acid (CA) and ethylenediaminetetraacetic acid (EDTA), may then be added to the above solution with a CA:EDTA:total cation molar ratio of 2:2:1, which may be continuously stirred for 1 hour at room temperature for complete complexation. The pH may then be adjusted to ~9 by adding an aqueous NH4OH solution. The obtained solution may then be heated at 250°C until a gel is formed. The gel is then dried at 175°C for 24 hours in a drying oven, followed by calcining the powder at 1,250°C for 5 hour to obtain the BCZYYb4411 electrolyte powder.

[0057] In various embodiments, the BCZYYb4411 electrolyte powder may be mixed with 1 wt.% NiO (NiO functions as the sintering aid, the amount of which is based on the mass of BCZYYb4411) and binders (e.g., fish oil, b-98 polyvinyl butyral, di-n-butyl phthalate, and ethanol). The electrolyte powder may be mixed with a specific amount of fish oil, b-98 polyvinyl butyral, di-n-butyl phthalate, and ethanol to prepare well-dispersed ink. To further improve its dispersion, the ink should be ball milled for four days. The particle size distribution of the electrolyte powder in the ink may be evaluated using a Malvern Zetasizer Nano S. Toensure the reliability of these results, three measurements may be conducted. The obtained data may be analyzed utilizing the Mie theory. In certain embodiments, approximately 10 vol.% of the powder exhibits a particle size of ~150 nm, while the majority of the powder exhibits a particle size of ~500 nm. In various embodiments, the average particle size may be 360 to 375 nm. An electrolyte powder with a small particle size may be essential for achieving an ultrathin and uniform electrolyte layer.

[0058] In one or more embodiments, an ultrasonic coating system may be used to fabricate the ultrathin (quasi-two dimensional), chemically homogeneous, and robust proton- conducting electrolyte layer 14. In such embodiments, at least a portion of the electrolyte ink may be loaded into an ultrasonic syringe. The flow rate of the electrolyte ink fed to the ultrasonic nozzle may be controlled using a syringe pump. An ultrasonic nozzle installed on the x-y-z robotic arm may be used for coating the ultrathin electrolyte layer. The coating time, spray spot size, coating speed, ultrasonic power, and substrate temperature may be optimized to achieve the desired electrolyte thickness and density. In certain embodiments, the obtained electrolyte layer 14 applied on the negative electrode layer 12 may be co-sintered at about 1,450°C for about 5 hours to fabricate the PCEC half-cells.

[0059] During the ultrasonic spray coating process, to keep the ink well dispersed, the ink may be loaded in an ultrasonic reservoir. Moreover, the ultrasonic nozzle creates highly dispersed droplets, which allows the fabrication of uniform and ultrathin proton-conducting electrolyte. The negative substrate is also important for high-quality electrolyte. In the present disclosure, anode substrates with a two-layers structure may be utilized. The electrolyte ink was coated on the negative electrode functional layer. This negative electrode functional layer may not contain any pore former, which exhibits low porosity, creating a surface with high flatness. This smooth negative functional layer is one of the main factors for fabricating thin, uniform, and dense electrolyte. The porous negative electrode layer can increase the active surface area and ensure fast mass transport.

[0060] In various embodiments, LT-PCECs with ultrathin, bamboo-structured (single- grain thick), and chemically homogeneous electrolytes may be fabricated via a scalable and cost- effective ultrasonic spray-coating process, which may minimize the resistance of proton conduction across the grain boundary, as shown in FIG. 2. More particularly, as shown in FIG. 2, schematic illustrations are provided of PCEC electrolytes fabricated using: (a) an ultrasonicspray coater and (b) a traditional air brush. The ultrasonic spray coating may yield an ultrathin, bamboo-structured (single-grain thick), and chemically homogeneous electrolyte with a larger grain size and less grain boundary. The traditional air brush method (compressed air spray) may lead to a thin, multiple-grains thick electrolyte with a smaller grain size and more grain boundaries. Consequently, the grain boundary has a much lower proton conductivity than the grain interior. The arrows in FIG. 2 show that proton conduction encounters high resistance at the grain boundaries; thus, the electrolyte prepared via the ultrasonic spray coating has a higher proton conductivity.

[0061] Consequently, the use of ultrasonic spray coating reduced the area-specific ohmic resistance (“ASRo”) attributed to the electrolyte.

[0062] Poor sinterability and Ba evaporation are two main challenges associated with proton-conducting electrolyte fabrications. It is also well known that the grain boundary is less conductive than the grain bulk. Therefore, fabricating PCECs with ultrathin, large-grain, and chemically homogeneous electrolytes may be one of the most essential steps for realizing high performance. Herein, we employed an ultrasonic spray coating system to coat electrolyte on the negative electrode, which is scalable and reproducible. Additionally, high-quality PCECs with relatively large areas have been fabricated using the ultrasonic spray coating system, which yields dense and defect-free PCEC electrolytes with a uniform thickness, further validating the feasibility of employing this process for large-scale PCEC manufacturing. Furthermore, the ultrasonic spray coating improved the PCEC electrolyte microstructure and conductivity compared to the traditional compressed air spray coater.

[0063] The conductivity of the electrolytes was measured by using the EIS and a comprehensive analysis of the EIS spectra was performed to determine the grain bulk and grain boundary conductivity. The electrolyte powder was subjected to uniaxial dry pressing to form a 20 mm diameter pellet at a pressure of 150 bar, which was then sintered for 5 hours at 1450°C. The resulting pellet was polished with sandpaper to attain a flat surface with a thickness of ~1 mm. Platinum (Pt) paste was applied on both sides of the pellet (with an effective area of 0.5 cm2) as the current collector via brush painting, which was then calcined at 350°C for 30 minutes. The ohmic resistance of this BCZYYb4411 electrolyte pellets was measured by performing EIS under a humidified argon atmosphere (3% steam) from 500 to 250 °C. Beforeeach measurement, the pellet was held at each temperature for approximately one hour to achieve a steady state.

[0064] The EIS spectra were collected using a Gamry 1010 interface. The obtained impedance spectra were then analyzed using Zview software to separate and analyze the bulk and grain boundary resistances. The “brick layer model” was used to calculate the bulk and grain boundary conductivities.

[0065] Due to the ultrasonic spray coating method, the electrolyte layer may be dense, with a uniform thickness. Furthermore, it has been observed that these electrolytes contain a bamboo structure, leading to a low ASRO. PCECs manufactured via this simple coating and cosintering process are reproducible. In certain embodiments, the electrolyte surface has an average grain size that is larger than the electrolyte thickness. Therefore, in such embodiments, the PCEC obtained via this process has a quasi-2D electrolyte, which exhibits a low grain boundary area and reduced ASRO.

[0066] As previously noted, the ultrathin electrolyte layer 14 may exhibit a bamboo structure where the grain boundaries appear perpendicular to the negative electrode 12. Furthermore, the thin electrolyte layer 14 may be chemically homogeneous and also show a homogeneous elemental distribution, implying that the chemical composition is consistent throughout the electrolyte. Element segregation at the grain boundary can lead to a positive potential, which decreases proton conduction. Consequently, the PCECs fabricated in accordance with the present disclosure may achieve an ASROthat is lower or comparable to that of PCECs fabricated via costly and complicated prior art techniques.

[0067] In one or more embodiments, the proton-conducting electrolyte layer 14 has an average grain size of at least 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, or 7 µm. Additionally, or in the alterative, the proton-conducting electrolyte layer 18 has an average grain size less than 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, or 3 µm. This smaller grain size may be derived from the ultrasonic spray coating methodology. It has also been observed that PCEC’s fabricated using ultrasonic spray coating, rather than traditional air brushing, may exhibit reduced ASRO, indicating that the electrolyte with a bigger grain size and fewer grain boundaries has a higher proton conductivity.

[0068] Once formed, the proton-conducting electryolyte 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.

[0069] In one or more embodiments, the electrochemical cell 10 comprises a positive electrode 16. It will be appreciated that, although it is referred to as a “positive electrode,” depending on the operating mode employed, the positive electrode 16 can function as an anode or cathode. Particularly, when the cell 10 is operated in hydrogen-producing mode, the positive electrode 16 (functioning as an anode) forms protons and oxygen from water using an electrical current, and when the cell 10 is operated in electricity-producing mode, the positive electrode 16 (functioning as a cathode) reacts protons and electrons with oxygen to form water.

[0070] In various embodiments, the positive electrode 16 comprises, consists essentially of, or consists of the solid composite materials described herein. In one or more embodiments, the positive electrode 16 comprises at least 50, 75, 90, 95, or 99 weight percent of the solid composite materials.

[0071] An objective of the present disclosure was to reduce the ASRP of the PCEC to less than 0.4 Ω cm2at 450°C, which would further improve the PCEC performance at less than 450°C. Thus, an in-situ formed composite positive electrode was developed that was composed of the solid composite materials described herein. More particularly, a positive electrode was developed with double perovskite phase and single perovskite phase nanoparticles to reduce the ASRP.

[0072] In one or more embodiments, the in-situ formed positive electrode powder was synthesized via the above-referenced wet-chemistry method described above for producing the solid composite materials described herein. For example, in certain embodiments, stoichiometric amounts of Pr6O11, Ba(NO3)2, Sr(NO3)2, Co(NO3)2•6H2O, and Fe(NO3)3•9H2O may be utilized as the precursors. Briefly, a calculated amount of Pr6O11 may be first dissolved in 20 weight percent diluted nitric acid solution at 100°C to obtain a praseodymium nitrate solution. Subsequently, in such embodiments, stoichiometric amounts of Ba(NO3)2, Sr(NO3)2, Co(NO3)2•6H2O, and Fe(NO3)3•9H2O may be subsequently added into the praseodymium nitrate solution, and then stirred for 30 minutes. The complexing and chelating agents, citric acid (CA) and ethylenediaminetetraacetic acid (EDTA), may then be added to the above solution with a CA:EDTA:total cation molar ratio of 2:2:1, and then continuously stirred for 1 hour at roomtemperature for complete complexation. The pH may be adjusted to ~9 by adding an aqueous NH4OH solution. The solution may then be heated at 250°C until a gel is formed. The gel may be immediately placed in a drying oven at 175°C for 24 hours. The obtained black powder may then be calcined under air at 600°C for 5 hours, followed by ball-milling in ethanol for 1 day and then drying at 100°C. Upon sintering at 765°C, the calcined powder will be crystallized to form the solid composite materials powder.

[0073] In one or more embodiments, a designated amount (e.g., 5 grams) of the solid composite materials powder may be dispersed in a dispersant (e.g., one gram of 20 weight percent solsperse 28000 (Lubrizol) dissolved in α-terpinol), and a binder (e.g., 0.5 grams of 5 weight percent V-006 (Heraeus) dissolved in α-terpinol) and then mixed and ground to thereby form a positive electrode slurry.

[0074] The resulting slurry may be applied using conventional application methods, such as ultrasonic spray coating, to the electrolyte layer 14 in order to form the positive electrode layer 16.

[0075] In one or more embodiments, at least a portion of the positive electrode slurry may be brush painted on the electrolyte layer 14, and the PCEC single-cell may then be directly sealed on an alumina tube without presintering the positive electrode.

[0076] Once formed, the positive 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 20 µm to about 50 µm, or about 30 µm.

[0077] It has been observed that the use of positive electrodes formed from the solid composite materials described herein may lead to improved electrocatalytic activity.

[0078] In one or more embodiments, the positive electrodes formed from the solid composite materials may exhibit an area-specific electrode polarization resistance (“ASRP”) at 400°C of at least 0.1, 0.5, 1.0, or 1.2 Ω cm2and / or less than 9, 8, 7, 6, 5, 4, 3, 2, or 1.5 Ω cm2. Additionally, or in the alternative, the positive electrodes formed from the solid composite materials may exhibit an ASRPat 450°C of at least 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, or 0.7 Ω cm2and / or less than 4, 3, 2, or 1 Ω cm2. Additionally, or in the alternative, the positive electrodes formed from the solid composite materials may exhibit an ASRP at 500°C of at least 0.05, 0.1, or 0.15 Ω cm2and / or less than 1, 0.9, 0.8, 0.7, or 0.6 Ω cm2. Additionally, or in the alternative, the positive electrodes formed from the solid composite materials may exhibit an ASRPat 550°C ofat least 0.01, 0.05, or 0.1 Ω cm2and / or less than 1, 0.9, 0.8, 0.7, 0.6, 0.5, or 0.4 Ω cm2. Additionally, or in the alternative, the positive electrodes formed from the solid composite materials may exhibit an ASRP at 600°C of at least 0.01 or 0.05 Ω cm2and / or less than 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, or 0.3 Ω cm2.

[0079] Furthermore, in one or more embodiments, the positive electrodes formed from the solid composite materials may exhibit an activation energy of at least 10, 25, 50, 75, 80, or 85 kJ / mol and / or less than 150, 130, 110, 105, or 100 kg / mol. This indicates that the positive electrodes described herein can function as a promising positive electrode at lower operating temperatures.

[0080] In one or more embodiments, the positive electrodes formed from the solid composite materials may exhibit oxygen temperature-programmed desorption (O2-TPD) with: (a) one peak at a temperature of at least 200°C, 215°C, or 230°C and / or less than 320°C, 310°C, 300°C, 290°C, 280°C, 270°C, or 260°C and (b) a second peak at a temperature of at least 200°C, 225°C, 250°C, 275°C, or 290°C and / or less than 380°C, 370°C, 360°C, 350°C, 340°C, 330°C, 320°C, 310°C, or 300°C. These indicate the oxygen mobility and diffusion the electrodes. For this test, two main peaks were observed for the positive electrode materials, possibly due to the oxygen molecules absorbed on the surfaces exhibiting different binding energies. The corresponding peak temperatures of the positive electrode materials of the present disclosure were observed to be lower than those of the prior art, indicating that the surface oxygen on the inventive electrodes is more easily released from the surface, which leads to enhanced oxygen surface oxygen exchange kinetics.

[0081] Both surface oxygen exchange kinetics and bulk oxygen-ion diffusion kinetics are essential for high OER and ORR activities, which are determined by electrochemical conductivity relaxation (“ECR”) experiments. In one or more embodiments, the positive electrodes formed from the solid composite materials may exhibit bulk oxygen-ion diffusion coefficients (“Dchem”) of at least 10-7or 10-6cm2s-1and / or less than 10-4cm2s-1at 500°C or 550°C. It has been observed that the inventive electrodes have higher bulk oxygen-ion diffusion coefficients (Dchem) than prior art electrodes, suggesting that the inventive electrodes exhibit enhanced oxygen-ion diffusion in the bulk.

[0082] Two samples were prepared for the ECR testing. The samples were first axially pressed at 100 bar for 2 minutes using a 2 cm (diameter) die, followed by sintering at 1,250°Cfor 3 hours to densify the pellets. A bar was prepared using a high-precision diamond saw. ECR measurement was conducted at 700°C, 650°C, 600°C, 550°C, and 500°C. At each temperature, the oxygen concentration was initially set as 2 vol.% (10 sccm O2 + 490 sccm Ar). ECR was measured via a four-probe method with a current of 40 mA applied and voltage measured. Once the voltage reached equilibrium, the oxygen concentration was immediately changed from 2 volume percent to 20 volume percent. The voltage response was recorded. The relative conductivity change data was calculated using the NETL ECR analysis tool, which determines the surface exchange coefficient (kchem) and bulk diffusion coefficient (Dchem).

[0083] Likewise, in one or more embodiments, the positive electrodes formed from the solid composite materials may exhibit a surface oxygen exchange coefficient (kchem) of at least 10-5or 10-4cm s-1and / or less than 10-3cm s-1at 500°C or 550°C. It has been observed that the inventive electrodes exhibit higher surface oxygen exchange coefficients (kchem) relative to prior art electrodes formed from just double phase perovskites. For example, at 550°C, the kchem of inventive electrodes may be more than twice than that of prior art electrodes formed from just a double perovskite phase, implying that the single perovskite phase is beneficial for accelerating the surface oxygen exchange kinetics.

[0084] This is also consistent with the O2-TPD profiles. In one or more embodiments, the positive electrodes formed from the solid composite materials may exhibit an activation energy associated with Dchem of at least 50, 75, or 100 kJ / mol and / or less than 175, 150, or 125 kJ / mol. Likewise, in various embodiments, the positive electrodes formed from the solid composite materials may exhibit an activation energy associated with kchemof at least 20, 40, or 60 kJ / mol and / or less than 100, 90, 80, or 70 kJ / mol. Both of these activation energies were observed to be lower than those of the prior art electrodes formed from just a double perovskite phase, further highlighting promising performance at lowered operating temperatures. Although not wishing to be bound by theory, there appears to be a synergy between the double perovskite phase and the single perovskite phase in the solid composite materials described herein, which results in improved electrocatalytic activity, especially at less than 450°C.

[0085] As demonstrated by electrochemical impedance spectroscopy (EIS) of PCECs collected at 450°C under OCV conditions, the inventive positive electrodes formed from the solid composite materials may reduce the ASRp by a factor of five relative to prior art electrodes.

[0086] The distribution of relaxation time (“DRT”) analysis may be provided by graph plots (as defined by the graph peaks) divided into three main regions, i.e., a low-frequency region (<100 Hz), an intermediate-frequency region (100-10,000 Hz), and a high-frequency region (>10,000 Hz), which correspond to oxygen mass transport, oxygen exchange on the positive electrode, and the charge transfer process, respectively. In one or more embodiments, the positive electrodes formed from the solid composite materials may exhibit a DRT in a low- frequency region at 450°C of at least 0.01 and / or less than 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, or 0.2 Ω / cm2. Additionally, or in the alternative, the positive electrodes formed from the solid composite materials may exhibit a DRT in an intermediate-frequency region at 450°C of at least 0.01 and / or less than 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, or 0.2 Ω / cm2. Additionally, or in the alternative, the positive electrodes formed from the solid composite materials may exhibit a DRT in a high-frequency region at 450°C of at least 0.01 and / or less than 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, or 0.1 Ω / cm2. Generally, the inventive electrodes may exhibit lower DRT peaks at low and medium frequencies relative to prior art electrodes formed from just a double perovskite phase, indicating that the inventive electrodes exhibit enhanced surface oxygen exchange kinetics, which may be attributed to the porous structure of the inventive electrode.

[0087] In one or more embodiments, the positive electrodes formed from the solid composite materials may have a BET-measured surface area of at least 5, 6, 7, 8, or 9 m2 / g and / or less than 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, or 10 m2 / g. Nitrogen Brunauer–Emmett– Teller (BET) surface area analysis and O2-temperature-programmed desorption (O2-TPD) may be conducted using a ChemBET Pulsar chemisorption analyzer.

[0088] Spin-polarized DFT calculations were performed to study the bulk oxygen vacancy formation and oxygen diffusion in the double perovskite phase, the single perovskite phase, and a prior art double perovskite phase. Based on DFT calculations, it was observed that a higher concentration of Sr and a proper mixing of Fe help stabilize Ov and lower the oxygen vacancy formation energies.

[0089] The inventive solid composite materials display a superior ORR performance owing to the higher oxygen diffusivity of the combined double perovskite phase and single perovskite phase. It was observed that the energy barriers (Ea,bulk) exhibit a decreasing order of the inventive single perovskite phase (BSC) (2.12 eV) > the prior art double perovskite phase (PBSCF-1) (0.88 eV) > the inventive double perovskite phase (PBSCF-2) (0.51 eV). The Ea,bulkin the inventive double perovskite phase is 0.37 eV lower than in the prior art double perovskite phase. The energy differences will have a meaningful impact on atomic oxygen diffusion and correlate very well with the measured diffusivities.

[0090] The climbing image-nudged elastic band (CI-NEB) and the dimer methods were employed to model oxygen diffusion in bulk lattices. The two vacancy sites with the lowest formation energies (Ov1 and Ov2) were marked. The structure with the most stable oxygen vacancy (“Ov1”) is considered the initial state (“IS”) for lattice oxygen diffusions, whereas the structure with the most stable nearest oxygen vacancy is determined as the final state (“FS”). The diffusion pathways in the double perovskite phases cross the “critical triangle” formed by one Co atom at the B site and two Pr atoms at the A sites, as described by Mastrikov. The diffusion energy barriers (Ea,bulk) were calculated with Equation:

[0091] Ea,bulk = Ets – Einitial,

[0092] where Ets and Einitial are the total energies of the transition and initial states, respectively.

[0093] The observed surface exchange rates may also be observed using DFT analysis. It was observed that the oxygen surface exchange coefficient (kchem) varies linearly against the bulk oxygen vacancy formation energy (ΔEOv). Furthermore, a lower ΔEOvwould indicate a higher kchem. As such, the high kchem observed in the single perovskite phase (BSC) can also be successfully explained based on the trends in the calculated oxygen vacancy formation energies.

[0094] All DFT calculations presented were performed using the Vienna Ab Initio Simulation Package (VASP). The projector augmented wave (PAW) pseudopotential, and the GGA-PBE functional were used to account for the core electrons and the electron exchange- correlation effects. The plane-wave basis set was expanded with a cutoff energy of 400 eV and Gaussian smearing (σ = 0.02 eV). The reciprocal first Brillouin zone of bulk models was sampled using a 4×4×4 k-point mesh generated from the Monkhorst-Pack scheme. The structures and energies of all transition states were obtained from the climbing image-nudged elastic band (CI-NEB) method and the dimer method.

[0095] The oxygen vacancy sites were tested at all symmetry-distinct locations in the lattice. All structures of the perovskite phases were visualized using the VESTA software. The bulk oxygen vacancy formation energy (ΔEOv) was defined by Equation (1):

[0096] ΔEOv= EOv+ 0.5*EO2- Ebulk

[0097] where EOv,EO2, and Ebulkare energies of bulk structure with an oxygen vacancy, gas phase oxygen molecule, and vacancy-free structure, respectively.

[0098] Combining the trends in oxygen diffusivity and the surface exchange coefficients, a synergistic effect may be supported by the inventive solid composite materials described herein, in which the single perovskite phase (BSC) top layer facilitates surface oxygen exchange. Then, the double perovskite phase (PBSCF) enhances the bulk diffusions with low Ea,bulk, both owing to the promoted oxygen vacancy formations.

[0099] It was observed that positive electrodes formed from the solid composite materials described herein simultaneously reduced the ohmic resistance and the corresponding activation energy. For instance, in one or more embodiments, PCECs containing a positive electrode formed from the solid composite materials described herein may exhibit an ASROat 400°C of at least 0.01 Ω cm2and less than 0.5, 0.4, or 0.3 Ω cm2. Additionally, or in the alternative, PCECs containing a positive electrode formed from the solid composite materials described herein may exhibit an ASROat 450°C of at least 0.01 Ω cm2and less than 0.4, 0.3, or 0.25 Ω cm2. Additionally, or in the alternative, PCECs containing a positive electrode formed from the solid composite materials described herein may exhibit an ASRO at 500°C of at least 0.01 Ω cm2and less than 0.4, 0.3, 0.25, or 0.2 Ω cm2.

[0100] Additionally, in one or more embodiments, PCECs containing a positive electrode formed from the solid composite materials described herein may exhibit an activation energy of the ASROof at least 1, 5, 10, or 20 kJ / mol and less than 50, 40, 33, 32, or 31 kJ / mol.

[0101] Dilatometry results also indicate that the positive electrode formed from the inventive solid composite material has a lower coefficient of thermal expansion (CTE, 18.0×10-6K-1) than that of the prior art positive electrode (23.4 ×10-6K-1), which can improve the positive electrode-electrolyte compatibility and interfacial bonding. Additionally, the in-situ formed positive electrodes described herein achieve a much lower CTE than other Co-based positive electrodes reported in the literature.

[0102] The dilatometry measurements were carried out using the Netzsch DIL 402 Expedis instrument to determine the linear thermal expansion profiles of the positive electrode samples. Cylindrical samples were used. Briefly, 0.6 g of each sample powder was first dry pressed by using a carbonized stainless-steel die set under a pressure of 50 bar for 1 minute, producing a green cylinder. The cylinders were then sintered in air to get the desired samplecomposition (the inventive cylinder was calcined at 765°C for 5 minutes; the prior art cylinders were was calcined at 950°C for 5 hours). The linear thermal expansion profiles of the samples were measured using a heating and cooling rate of 10°C / min. The dilatometer was calibrated and corrected for each measurement.

[0103] Peeling strength testing was also conducted to quantify the positive electrode- electrolyte bonding strength and determine the coherence of the positive electrode-electrolyte interface. It was observed that a more uniform and stronger peeling strength was obtained when using the inventive positive electrode described herein, indicating that the solid composite materials led to a positive electrode-electrolyte interface with improved chemical bonding and coherence, which reduced the positive electrode-electrolyte contact resistance and consequently decreases the ASRO.

[0104] In one or more embodiments, the inventive positive electrodes formed from the solid composite materials described herein may exhibit an average peeling strength of at least 4, 5, 6, 7, 8, or 9 N and / or less than 20, 15, or 14 N over a time period of 2, 3, 4, 5, or 6 minutes.

[0105] It was observed that the inventive positive electrode may exhibit an average peeling strength of 6.8 N, 42% higher than that of the prior art positive electrode (4.8 N). Therefore, the inventive positive electrode simultaneously improved the electrochemical performance and reduced the ohmic resistance at lower operating temperatures.

[0106] The peeling strength of the positive electrodes was measured according to the ASTM D903 180-degree peeling strength testing standard. Briefly, the specimen was placed in the peeling strength test machine by fixing the negative electrode using heavy-duty double-sided tape. One end of a piece of rectangular tape was stuck on the positive electrode, while the other end was fixed to the peeling strength testing machine. By pulling the tape back in a “U” shape (180°) at the same speed, the applied force was recorded as a function of time.

[0107] HAADF and EDS images of the inventive positive electrode-electrolyte interface showed that both BSC and PBSCF were observed, which is thought to enhance the chemical bonding of the positive electrode with the electrolyte, thus, reducing ASRO. Forming the Protonic Ceramic Electrochemical Cells and Performance Properties

[0108] In various embodiments, the high-performance PCECs capable of operating at less than 450°C may be fabricated via via tape-casting followed by an ultrasonic spray-coatingprocess and then employing the in-situ formed solid composite materials as the positive electrode.

[0109] In one or more embodiments, the inventive PCECs may be manufactured with both tape-casting and spray-coating methods.

[0110] As noted above, the inventive PCECs may comprise an in-situ formed positive electrode composed of the solid composite materials described herein. This positive electrode may reduce the coefficient of thermal expansion (“CTE”), improve the positive electrode- electrolyte bonding, and reduce the contact resistance, further reducing the ASRO. Additionally, this positive electrode may simultaneously facilitate the surface oxygen exchange and bulk oxygen-ion diffusion, particularly at low operating temperatures, which can significantly decrease the ASRP, thereby allowing an extraordinarily low ASRP(e.g., about 0.38 Ω cm2) to be achieved at 450°C (versus the 0.76 Ω cm2ASRP of the prior art positive electrode).

[0111] Furthermore, as discussed below, the LT-PCECs equipped with low-resistance electrolytes and the positive electrodes described herein may exhibit remarkable performance at <450°C, as demonstrated by the peak power density. For example, at 400°C, a fuel cell PPD of about 0.34 W cm-2may be achieved using H2, which doubles the PPD of the prior art PCECs. Additionally, for example, in the steam electrolysis mode, a durable current density of about - 1.28 A cm-2may be obtained at 1.4 V and 450°C.

[0112] The PCECs described herein may achieve effective PCEC fuel cell operation at 275°C, with a practical PPD of ~0.1 W cm-2achieved on H2-O2. Good stability was also demonstrated at 400°C in terms of both power generation (degradation rate <0.07 mV / h) and hydrogen production (degradation rate <0.15 mV / h). Furthermore, the LT-PCECs of the present disclosure can produce hydrogen with a comparable or higher rate and electrolysis energy efficiency at 400°C compared to the previously reported PCECs that operate at >500°C.

[0113] It was observed that the open-circuit voltages (“OCVs”) for the inventive LT- PCECs under fuel cell operation may be higher than 1.05 V, which indicates that the inventive PCECs with an ultrathin and bamboo-structured electrolyte do not exhibit obvious electronic leakage or gas leakage. Furthermore, it was observed that the inventive LT-PCECs may achieve a much higher fuel cell performance than PCECs formed with prior art positive electrodes. For example, the inventive LT-PCECs can exhibit a peak power density (with air, ammonia, hydrogen, and / or oxygen fed to the positive electrode and / or the negative electrode) of at least0.01, 0.05, or 0.1 W / cm2and / or less than 1.8, 1, 0.5, 0.4, or 0.3 W / cm2at 275°C, 300°C, or 350°C. Additionally, or in the alternative, the inventive LT-PCECs can exhibit a peak power density (with air, ammonia, hydrogen, and / or oxygen fed to the positive electrode and / or the negative electrode) of at least 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or 0.5 W / cm2and / or less than 1.8, 1.6, or 1.4 W / cm2at 350°C, 400°C, 450°C, or 500°C. Additionally, or in the alternative, the inventive LT-PCECs can exhibit a peak power density (with air, ammonia, hydrogen, and / or oxygen fed to the positive electrode and / or the negative electrode) of at least 1.0, 1.1, or 1.2 W / cm2and / or less than 2.0, 1.9, or 1.8 W / cm2at 550°C or 600°C. For example, with air fed to the positive electrode, a peak power density (“PPD”) of 1.64 W / cm2may be attained at 600°C for the inventive LT-PCEC, which is greater than that of previously reported PCEC fuel cells in the literature.

[0114] In certain embodiments, to compare the inventive LT-PCECs with the most recently developed PCECs, oxygen was fed to the positive electrode, and a PPD of about 2.0 W cm-2was achieved at 600°C, which was higher than the PCECs with acid-etched electrolytes and 3D positive electrodes. The inventive LT-PCECs may also exhibit remarkable performance at 450°C and less. For example, in a certain embodiment, a PPD of ~0.77 W cm-2was reached at 450°C. Additionally, at 450°C in a certain embodiment, the inventive positive electrode facilitated an ~2.7-fold increase in PPD compared to the prior art positive electrode (0.77 W / cm2versus 0.29 W / cm2). It was also demonstrated that when the inventive LT-PCEC operated at 275°C, it may function as a practical fuel cell PPD of about 0.1 W cm-2on H2-O2.

[0115] The LT-PCECs operating at <450°C may also provide good fuel flexibility. In one or more embodiments, with ammonia fed to the anode, without using any additional ammonia cracking catalysts, the inventive LT-PCECs achieved PPDs of about 0.65 W / cm2at 500°C, about 0.28 W / cm2at 450°C, and about 0.16 W / cm2at temperatures as low as 400°C.

[0116] Furthermore, as shown above in such embodiments, the direct-ammonia PCECs may also exhibit outstanding performance at 500-600°C, achieving a PPD of about 1.38 W / cm2at 600°C, which well exceeds the results reported in the literature. After conducting the performance evaluation under ammonia, there was no evidence of grain boundary fractures, indicating that the ultrathin electrolyte fabricated in the inventive LT-PCECs was robust under ammonia. The PCEC performance on ammonia was comparable to that on H2, indicating that thenegative electrode was active for ammonia cracking, which suggested that ammonia can serve as a H2 carrier for power generation in PCECs.

[0117] To determine the PCEC fuel-cell performance using ammonia as the fuel, the ammonia flow rate may be set as 40 sccm at 600°C, 35 sccm at 550°C, 30 sccm at 500°C, 25 sccm at 450°C, 20 sccm at 400°C, and 10 sccm at 400°C, and varied with the operating temperature to maximize the fuel utilization rate.

[0118] It was observed that the inventive LT-PCECs can directly utilize methane for power generation. To demonstrate this, an internal steam methane reforming (SMR) catalyst, Sm0.2Ce0.7Ni0.1Ru0.05O2-δ, was coated on a PCEC negative electrode to further achieve high performance using methane as the fuel at low operating temperatures. Its catalytic activity was first evaluated in a packed bed reactor, which indicated that the synergy between Ni and Ru enhances the SMR activity. For example, Sm0.2Ce0.7Ni0.1Ru0.05O2 achieved the highest CH4 conversion at 500°C, which was ~35% higher than that of Sm0.2Ce0.7Ni0.15O2-δ and ~26% higher than that of Sm0.2Ce0.7Ru0.15O2-δ. At 450°C, the CH4conversion achieved over Sm0.2Ce0.7Ni0.1Ru0.05O2 reached ~27%, suggesting that the PCEC equipped with this catalyst can deliver viable power density. To test the PCEC fuel-cell performance on methane, the SDC-Ni- Ru ink was brush-painted on the negative electrode as a catalytic reforming layer. A mixture of methane with steam at a methane-to-steam ratio of 1:1 was fed to the negative electrode. Herein, the CH4 flow rate, which was set as 16 sccm at 500°C, 12 sccm at 450°C, and 8 sccm at 400°C, was also varied with the operating temperatures.

[0119] The CH4conversion (XCH4) may be calculated as follows:

[0120] )of inlet gas or outlet gas, respectively.

[0122] In certain embodiments, at 500°C, a PPD of 0.55 W / cm2was observed on methane versus, e.g., 0.37 W / cm2at 500°C for the prior art solid oxide fuel cells, which approached the PPD performance of PCECs on pure H2.

[0123] In such embodiments, the PCEC fuel cell performance on methane shows that a viable PPD was still achieved at 400°C. Thus, in such embodiments, the LT-PCEC fuel-cell performance using both ammonia and methane validates the fuel flexibility of PCECs operating at <450°C.

[0124] Additionally, the hydrogen production of the inventive LT-PCECs may be evaluated in electrolysis mode at <450°C. To evaluate the PCEC electrolysis performance for hydrogen production, a mixture of steam and nitrogen was supplied to the positive electrode with a steam concentration of 40%. The steam concentration was controlled by bubbling nitrogen (100 sccm) through a customized temperature-controlled bubbler. Pure hydrogen gas (15 sccm) was fed to the negative electrode as the sweep gas.

[0125] To evaluate the Faradaic efficiency (FE), the negative electrode outlet gas was mixed with a nitrogen gas stream, which was then injected into an online gas chromatograph (INFICON 3000 Micro GC) to quantify the hydrogen production rate. This method allowed us to evaluate the PCEC electrolysis performance under realistic conditions and accurately determine the FE. The FE (%) was calculated using the following equations:

[0126] different current densities, is the measured hydrogen production rate (mol / s), n is 2, F is the Faradaic constant1), and I is the applied current (A).

[0128] To simulate the PCEC electrolysis performance under industrially relevant conditions, an H2stream may be fed to the negative electrode as the sweep gas. Conventionally, an Ar or diluted H2 stream may be used as the sweep gas for testing lab-scale PCECs, which allows one to accurately measure the outlet gas concentration and quantify the FE using the GC. However, in a PCEC stack or system, dry and pure H2is produced in the negative electrode. No sweep gas is needed for hydrogen production at the stack or system level. Therefore, in the present disclosure, pure H2 was used as the sweep gas to evaluate the lab-scale PCEC electrolysis performance, which can better represent the PCEC performance at the commercial scale. The outlet gas was then mixed with nitrogen to dilute the H2 stream, enabling quantification of the hydrogen production rate using GC.

[0129] In one or more embodiments, the inventive LT-PCECs in electrolysis mode exhibit a current density of at least -1.6. -1.5, or -1.4 A cm-2and / or less than -0.5, -0.8, -1.0, or - 1.1 A cm-2at 1.4 V and 450°C. Additionally, or in the alternative, the inventive LT-PCECs in electrolysis mode may exhibit a current density of at least -1.0, -0.9, or -0.8 A cm-2and / or less than -0.5, -0.6, or -0.7 A cm-2at 1.4 V and 400°C. Additionally, or in the alternative, theinventive LT-PCECs in electrolysis mode may exhibit a current density of at least -0.01, -0.02, or -0.03 A cm-2and / or less than -0.5, -0.6, or -0.7 A cm-2at 1.4 V and 350°C.

[0130] For example, it was observed that the inventive PCECs can attain a current density of about -1.28 A cm-2at 1.4 V and 450°C versus, e.g., 1.04 A cm-2at 450°C for the prior art PCECs. Furthermore, in one or more embodiments, the inventive LT-PCECs in electrolysis mode may exhibit a Faradaic efficiency (“FE”) of at least 70, 75, 80, 85, 90, or 95 percent at 400°C and a current density of 0.4, 0.6, or 0.8 A cm-2.

[0131] Moreover, at 400°C and a current density of 0.6 A cm-2, the cell-level electrical energy-to-chemical energy conversion efficiency of the inventive LT-PCECs may reach at least 77 percent, validating the feasibility of producing hydrogen with the inventive PCECs at <450°C.

[0132] It is important to recognize that a relatively low FE (%) of PCECs may be one of the main challenges. The low FE (%) is typically ascribed to the p-type electronic leakage across the electrolyte. The p-type electronic leakage is due to the oxidation reaction ( ). Thus, it is suggested an electrolyte, which suppresses the oxidation and favors hydration, could be developed to enhance the FE (%).

[0133] PCEC stability testing was also evaluated for power generation and hydrogen production at <450°C. In certain embodiments, at a current density of 300 mA cm-2and 400°C, the inventive LT-PCEC may continuously operate in fuel cell mode for at least 100 hours, achieving a degradation rate of less than 0.07 mV / h. After this time, there can be no observable change in the microstructure and no electrolyte cracking. The positive electrode-electrolyte interfaces may show no delamination, suggesting that the interfacial bonding is good and the thermal expansions are compatible.

[0134] The inventive LT-PCECs also exhibited superior long-term stability in electrolysis mode at 400°C. In one or more embodiments, the inventive PCECs, when operated at a current density of -0.6 A cm-2for at least 250 hours, may achieve a low degradation in the applied voltage (less than 0.15 mV / h) and FE. Although not wishing to be bound by theory, it is believed that the durability of the inventive LT-PCECs allows for H2 production with PCECs at 400°C for the first time, which dramatically reduces the required operating temperature and exceeds previous PCEC performance. The stable terminal voltage and FE of the inventivePCECs suggest no noticeable degradation. In certain embodiments, postmortem microstructural analysis of the inventive PCECs shows no evidence of degradation after the aforementioned long-term durability testing parameters. Specifically, no cracking may be observed on the electrolyte, essential for high Faradaic efficiencies. Moreover, in certain embodiments, no evidence of positive electrode decomposition and / or electrolyte-positive electrode delamination may be present after the aforementioned long-term durability test parameters, thereby highlighting the robustness of the inventive PCECs for H2production.

[0135] By fabricating ultrathin, single-grain thick, chemically homogeneous, and low- resistance electrolytes and developing an in-situ formed composite positive electrode, high- performance PCECs have been fabricated that are capable of both power generation and hydrogen production at 450°C and lower temperatures. The PCEC electrolyte fabricated via the simple and scalable process may exhibit a resistance comparable to or lower than that of PCECs prepared via costly and complicated processes. Furthermore, the in-situ formed positive electrodes formed the solid composite materials described herein may exhibit a significantly improved electrocatalytic activity and reduced positive electrode-electrolyte contact resistance. At 450°C, the inventive PCECs exhibited exceptional performance in both fuel cell and steam electrolysis modes. The inventive PCECs may exhibit long-term durable fuel cell performance and remarkable durability in electrolysis mode at 400°C. These reproducible results further highlight the promise of using the inventive PCECs for efficient power generation and hydrogen production at a large scale. Electrical Assembly and Other Uses

[0136] Depending on the operating mode employed, the protonic ceramic electrochemical cell 10 can produce hydrogen or generate electricity.

[0137] To produce hydrogen, the protonic ceramic electrochemical cell 10 operates in a hydrogen-producing configuration. While in this configuration, water, preferably in the form of steam, is reacted at the negative electrode 12 to generate oxygen, electrons, and protons via an oxygen evolution reaction, which occurs at a temperature of from about 300℃ to about 700℃. The generated protons may then pass through the proton-conducting electrolyte 14 to the positive electrode 16. Simultaneously, an electric current may be supplied to the positive electrode 16 from an external source, such as a solar panel. The positive electrode 16 may reduce at least aportion of the protons to form hydrogen via a hydrogen evolution reaction, which occurs at a temperature of from about 300℃ to about 700℃.

[0138] To generate electricity, the protonic ceramic electrochemical cell 10 operates in an electricity-producing configuration. In this configuration, hydrogen is supplied to the cell 10, which is oxidized at the negative electrode 12 to generate electrons and protons via a hydrogen oxidation reaction occurring at a temperature of from about 300℃ to about 700℃. In most embodiments, the supplied hydrogen is hydrogen previously formed while the cell 10 operated in the hydrogen-producing configuration. Once generated, the protons pass through the proton- conducting electrolyte 14 to the positive 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 700℃. Simultaneously, the generated electrons induce an electrical current and pass the electrical current through an electrical load to the positive electrode 16. The electrical load is connected with the negative electrode 12 and positive 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.

[0139] The present disclosure is also concerned with a method of producing hydrogen. In one or more embodiments, the method comprises providing a protonic ceramic electrochemical cell; reacting water at the negative electrode to generate oxygen and protons; causing the protons to pass through the electrolyte to the positive electrode; and supplying an electrical current to the negative electrode and reducing at least a portion of the protons to form hydrogen.

[0140] During operation, the protonic ceramic electrochemical cell 10 operates at a temperature of from about 200℃ to about 1000℃, about 350℃ to about 850℃, about 400℃ to about 700℃, less than 450℃, or less than 400℃.

[0141] The present disclosure is also concerned with a method of generating electricity. In one or more embodiments, the method comprises: (a) providing a protonic ceramic electrochemical cell; (b) supplying hydrogen to the electrochemical cell and oxidizing the hydrogen at the negative electrode 12 to form protons and electrons; (c) causing the protons to pass through the electrolyte layer 14 to the positive electrode 16; (d) causing the electrons generated at the negative electrode 12 to induce an electrical current and passing the electrical current through an electrical load that is connected with the negative electrode 12 and thepositive electrode 16; and (e) reacting the protons with oxygen at the positive electrode 16 to produce water. DEFINITIONS

[0142] 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.

[0143] As used herein, the terms “a,” “an,” and “the” mean one or more.

[0144] 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.

[0145] 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.

[0146] 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.

[0147] As used herein, the terms “having,” “has,” and “have” have the same open-ended meaning as “comprising,” “comprises,” and “comprise” provided above.

[0148] As used herein, the terms “including,” “include,” and “included” have the same open-ended meaning as “comprising,” “comprises,” and “comprise” provided above. NUMERICAL RANGES

[0149] 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 claimreciting “greater than 10” (with no upper bounds) and a claim reciting “less than 100” (with no lower bounds). CLAIMS NOT LIMITED TO DISCLOSED EMBODIMENTS

[0150] 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.

[0151] 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 any apparatus 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 positive electrode for a protonic ceramic electrochemical cell comprising a solid composite material, wherein the solid composite material comprises a double perovskite phase and a single perovskite phase.

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

3. The positive 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 positive 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 positive 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 positive 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 positive 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 positive 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 positive 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 positive 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 positive 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 positive electrode of claim 2, 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 positive electrode of claim 2, 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 positive electrode of claim 2, 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 positive electrode of claim 2, wherein the solid composite material has an average particle size of at least 1 nm and / or less than 100 nm, as confirmed by TEM.

16. The positive electrode of claim 2, wherein the positive electrode exhibits an ASRP at 400°C of 0.1 to 4 Ω cm2.

17. The positive electrode of claim 2, wherein the positive electrode exhibits an ASRP at 450°C of 0.1 to 4 Ω cm2.

18. The positive electrode of claim 2, wherein the positive electrode exhibits an activation energy of 10 to 150 kJ / mol.

19. The positive electrode of claim 2, wherein the positive electrode has a BET- measured surface area of 5 to 20 m2 / g.

20. The positive electrode of claim 2, wherein the positive electrode exhibits an ASRO at 400°C of 0.01 to 0.5 Ω cm2.

21. A protonic ceramic electrochemical cell comprising: a negative electrode comprising a first electrode material; a positive electrode comprising a second electrode material; and a proton-conducting electrolyte positioned between the negative electrode and the positive 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 any one 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 positive electrode exhibits an ASRP at 400°C of 0.1 to 4 Ω cm2.

36. The electrochemical cell of claim 31, wherein the positive electrode exhibits an ASRP at 450°C of 0.1 to 4 Ω cm2.

37. The electrochemical cell of claim 31, wherein the positive electrode exhibits an activation energy of 10 to 150 kJ / mol.

38. The electrochemical cell of claim 31, wherein the positive electrode has a BET- measured surface area of 5 to 20 m2 / g.

39. The electrochemical cell of claim 31, wherein the positive electrode exhibits an ASROat 400°C of 0.01 to 0.5 Ω cm2.

40. The electrochemical cell of claim 31, wherein the positive electrode exhibits an average peeling strength of 4 to 20 N as measured according to ASTM D903.

41. The electrochemical cell of claim 31, wherein the electrochemical cell exhibits a peak power density of 0.01 to 1 W / cm2at 350°C.

42. The electrochemical cell of claim 31, wherein the electrochemical cell exhibits a peak power density of 0.1 to 1.4 W / cm2at 400°C.

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 negative electrode to generate oxygen and protons; causing the protons to pass through the proton-conducting electrolyte to the positive electrode; and supplying an electrical current to the negative 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 negative electrode to form protons and electrons; causing the protons to pass through the proton-conducting electrolyte to the positive electrode; causing the electrons generated at the negative electrode to induce an electrical current and passing the electrical current through an electrical load that is connected with the negative electrode and the positive electrode; and reacting the protons with oxygen at the positive electrode to produce water.

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

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