Method and reactor system for decomposing water and / or carbon dioxide

Abstract

Methods and systems for decomposing one or more of water and carbon dioxide are disclosed. Exemplary methods can operate under substantially isothermal conditions. The methods can include the use of a material that includes two or more spinel phases in a solid solution. The solid solution can include oxygen, aluminum, and one or more transition metals.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application No. 63 / 270,030, filed October 20, 2021, entitled METHOD AND REACTOR SYSTEM FOR SPLITTING WATER AND / OR CARBON DIOXIDE, the contents of which are incorporated herein by reference to the extent not inconsistent with this disclosure.

[0002] The present disclosure generally relates to methods and systems for decomposing one or more of water and carbon dioxide. [Background technology]

[0003] The use of hydrogen as a renewable fuel is hampered by the inability to produce it cleanly and economically. Conventional solar thermochemical approaches consider perovskites in a two-stage redox cycle or temperature swing configuration with benchmark ceria, where reduction occurs at much higher temperatures than oxidation. Isothermal redox cycles are considered feasible and avoid the material stability challenges associated with solid-solid thermal regeneration techniques and large temperature swings; yet have long been considered inefficient due to the thermodynamic disadvantages of operating exothermic oxidation reactions at higher temperatures. Therefore, improved methods and systems suitable for splitting water (and / or carbon dioxide) in a relatively efficient manner under relatively isothermal conditions are desirable.

[0004] Any discussion, including discussion of problems and solutions, set forth in this section is included in this disclosure solely for the purpose of providing a context for the present disclosure, and such discussion should not be construed as an admission that any or all of the information constitutes prior art if not otherwise known and was publicly known at the time the invention was made. Summary of the Invention

[0005] Various embodiments of the present disclosure relate to methods suitable for decomposing water and / or carbon dioxide. The manner in which various embodiments of the present disclosure address the shortcomings of conventional methods and systems is discussed in further detail below, but generally, embodiments of the present disclosure provide improved methods and reactor systems for decomposing water and / or carbon dioxide under isothermal or near isothermal conditions in a relatively efficient manner.

[0006] According to an exemplary embodiment of the present disclosure, a method for decomposing one or more of water and carbon dioxide is provided. The exemplary method includes providing a first material in a first reactor of a reactor system, the first material including two or more spinel phases in solid solution, the solid solution including oxygen, aluminum, and one or more transition metals, and providing one or more of H2O and CO2 to the first reactor. The temperature in the reactor can be greater than 800°C. The partial pressure of oxygen in the reactor can be greater than 10 -7 According to examples of these embodiments, the method can further include providing a second material in a second reactor of the reactor system, the second material having the same chemical formula as the first material, and providing N2 and / or another inert gas to the second reactor. The temperature in the second reactor can be greater than 800° C. The partial pressure of oxygen in the second reactor can be greater than 10 -7 According to various aspects of these embodiments, the temperature in the first reactor and the temperature in the second reactor are between 800° C. and 1500° C. Additionally or alternatively, one or more of the partial pressure of oxygen in the first reactor and the partial pressure of oxygen in the second reactor are between 10 -7 10 from bar -1 According to various embodiments, the first material and the second material each have a thickness of (M ζ Al 1-ζ ) 3-δO4, where ζ is greater than 1 / 3, M is one or more transition metals, and ζ can be less than 1. Exemplary first and second materials described herein can exhibit large variations in oxygen content within the range of oxygen partial pressures expected in large-scale systems. When operated near isothermally, e.g., at 1400° C., the (e.g., iron aluminate-based) materials described herein can exhibit a large variation in oxygen content at 500 μmol g of material. -1 A capacity for hydrogen production greater than 100% has been demonstrated, which remains feasible even under high conversion conditions (i.e., molar H2O / H2<500:1) and exceeds the hydrogen yield of the three oxygen vacancy-mediated candidates after a temperature swing of 400°C (or less). Isothermal water and / or carbon dioxide decomposition using (e.g., iron) aluminate-based materials opens the door to simpler, more robust, and more efficient production of renewable hydrogen. Additional examples of methods according to the present disclosure are described below.

[0007] According to an additional embodiment of the present disclosure, a reactor system is provided. An exemplary reactor system includes a first reactor, a first material in the first reactor, the first material comprising two or more spinel phases in solid solution, the solid solution comprising oxygen, aluminum, and one or more transition metals; one or more of a H2O source and a CO2 source fluidly connected to the first reactor, and a pressure regulator configured to control a temperature in the first reactor to greater than 800° C. and a partial pressure of oxygen in the first reactor to greater than 10 -7 and a controller configured to control the pressure to greater than bar. The exemplary system, and in particular the controller, can be further configured to perform the methods described herein.

[0008] These and other embodiments will become readily apparent to those skilled in the art from the following detailed description of specific embodiments in conjunction with the accompanying drawing figures; the invention is not necessarily limited to any specific embodiment(s) disclosed.

[0009] A more complete understanding of the exemplary embodiments of the present disclosure can be had by reference to the detailed description and claims when considered in conjunction with the following illustrative figures. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 illustrates reduction and oxidation cycles using the materials described herein. [Diagram 2] 1 illustrates an exemplary method and system according to an embodiment of the present disclosure. [Diagram 3] 1 illustrates an exemplary method and system according to an embodiment of the present disclosure. [Figure 4] Figure 1 shows the observed (Yobs) and calculated (Ycalc) diffraction patterns of (A) Fe33Al67, (B) Fe47Al53, and (C) Co13Fe20Al67 after synthesis and experimental campaigns. Characteristic diffraction peaks assigned to FeAl2O4 and Fe3O4 are indicated by vertical solid and dashed lines, while diffraction peaks assigned to corundum, hematite, and other spinel phases are referred to using triangle (▲), asterisk (*), and circle (●) symbols, respectively. [Diagram 5] FIG. 1 illustrates the stability of the (A) Fe—Al—O and (B) Co—Fe—Al—O systems as a function of temperature and oxygen partial pressure at selected cation compositions. The circular symbols (●) represent the reaction of Equation 6 (i.e.,

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[0011] It will be understood that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale, for example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of the illustrated embodiments of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Although certain specific embodiments and examples are disclosed below, it will be understood by those skilled in the art that the invention extends beyond the specifically disclosed embodiments and / or the use of the invention and obvious modifications and equivalents thereof, and therefore it is not intended that the scope of the disclosed invention should be limited by the specific disclosed embodiments described below.

[0013] In this disclosure, "gas" may include materials that are gases at standard temperature and pressure, evaporated solids and / or evaporated liquids, depending on the context, and may consist of a single gas or a mixture of gases. An inert gas may be a gas that does not participate in chemical reactions to any appreciable extent. Exemplary inert gases include nitrogen.

[0014] In this disclosure, continuously or consecutively or continuously can refer to no break in the time sequence, no intervening material steps, no change in process conditions, or immediately following as a step, depending on the context.

[0015] In this disclosure, any two numerical values ​​of a variable can constitute a workable range for the variable, and any range stated can include or exclude its endpoints. Furthermore, any value of a variable stated (whether stated with "about" or not) can refer to an exact value or an approximation, and in some embodiments can include equivalents, and can refer to an average, median, representative value, majority, and the like. Furthermore, in this disclosure, the terms "comprise", "comprise", "have", and "have" and variations thereof can independently refer to "typically or broadly include", "comprise", "consist essentially of", or "consist of", and variations thereof, in some embodiments. According to aspects of the present disclosure, any defined meaning of a term does not necessarily exclude the ordinary and customary meaning of the term.

[0016] Turning now to the figures, Figure 1 illustrates a reduction and oxidation cycle 100 using materials 102, 104 described herein. The materials 102, 104 can be or can include two or more spinel phases in a solid solution, the solid solution including oxygen, aluminum, and one or more transition metals. An exemplary material can be a metal oxide having the formula: (M ζ Al 1-ζ ) 3-δ O4, where ζ is greater than 1 / 3 and M is one or more transition metals. For example, ζ can be greater than 1 / 3 and less than 1. M can be selected from, for example, one or more of Fe, Co, Ti, Mn, Mg, Zn, Ni, and Cr. As described in more detail below, the material can include cationic vacancies (δ) that enable the removal of oxygen and the decomposition of one or more of water and carbon dioxide.

[0017] During the reduction stage 106 of the reduction-oxidation cycle 100, the material 104 is reduced and oxygen 110 is generated. During the oxidation stage 108 of the reduction-oxidation cycle 100, water or carbon dioxide - such as from a source 112 - can be added to the material 102 and hydrogen 114 can be generated.

[0018] The temperature during the reduction and oxidation cycle 100 can be substantially isothermal. In this context, substantially isothermal can mean that the temperature during the reduction stage 106 and the temperature during the oxidation stage 108 of the reduction and oxidation cycle 100 are within ±10 or ±25 or ±50° C. of each other during operation. According to an embodiment of the present disclosure, the temperature during the reduction stage 106 and / or the oxidation stage 108 is between 800° C. and 1500° C.

[0019] The partial pressure of oxygen during the reduction and oxidation cycle 100 is 10 -7 For example, the partial pressure of oxygen during the reduction step 106 can be greater than 10 -7 Can be greater than 10 bar -7 10 from bar -1 Additionally or alternatively, the partial pressure of oxygen during the oxidation step 108 can be between 10 -7 Can be greater than 10 bar -7 10 from bar -1 It can be done between bars.

[0020] 2 and 3, a reactor system 200 is illustrated that includes a first reactor 202 and a second reactor 204. The reactor system 200 may also include other components, such as a heat exchanger 203, a compressor 206, a gas separator 208, a condenser 210, valves 212-226, a controller 228, one or more of a H2O source and a CO2 source 230, 232 fluidly connected to at least one or more of the first reactor 202 and the second reactor 204, a nitrogen source 238, and lines 240-258.

[0021] The first reactor 202 and the second reactor 204 can each be or include a fluidized bed reactor with a fluidized material that includes the materials described herein. The material in the first reactor can be referred to as the first material, and the material in the second reactor can be referred to as the second material. The first and second materials can be represented by the same chemical formula. Furthermore, the first and second can be used to refer to different reactors. The first and second reactors can be synonymous.

[0022] Heat exchanger 203 may be any suitable heat exchanger. According to embodiments of the present disclosure, heat exchanger 203 is configured to use heat from gas exhausted from reactor 202 and / or reactor 204 (e.g., from lines 242, 246, 254, 258) to heat gas from one or more sources 230, 232, and 238. Gas exhausted from reactor 202 and / or reactor 204 may be at or near the operating temperature of the respective reactor. The sources may be at, for example, ambient temperature.

[0023] The compressor 206 may be or include any suitable compressor.

[0024] Gas separator 208 may be any suitable separator capable of separating H2 and / or CO from a mixture of H2, CO, and / or CO2, for example. By way of example, gas separator 208 may be or include a pressure swing adsorption or membrane separation unit.

[0025] The condenser 210 can be or include any suitable heat exchanger or the like that reduces the gas temperature to a temperature at or below which water will condense. By way of example, the condenser 210 can be or include a twin tower desiccant dryer.

[0026] Valves 212-226 may be or include any suitable valves, such as pneumatic valves.

[0027] The controller 228 can include electronic circuitry and software that selectively operates the valves (e.g., valves 212-226), manifolds, heaters, pumps (e.g., compressor 206), and other components included in the system 200. Such circuitry and components can operate to introduce reactants (e.g., from sources 230 and / or 232) or other gases from their respective sources. The controller 228 can control the timing of gas pulse sequences, the temperature in the reactor(s), the pressure in the reactor(s), the partial pressure of the gases, and various other operations to provide proper operation of the system. The controller can include control software that electrically or pneumatically controls valves to control the flow of gases into and / or out of the reactors. The controller can include modules, such as software or hardware components, e.g., FPGAs or ASICs, that perform certain tasks. The modules can be advantageously configured to reside in addressable storage media of the control system and can be configured to execute one or more processes.

[0028] In Figure 2, the first reactor 202 operates in the reduction stage or mode described above, and the second reactor 204 operates in the oxidation stage or mode. In the illustrated embodiment, nitrogen from a nitrogen source 238 is provided to the first reactor 202 via line 240 and valve 220, and material 234 is reduced in the first reactor 202 to generate oxygen. The oxygen can be mixed with the nitrogen already provided. The oxygen can be sent via line 242 and valve 212 to a compressor 206, where it can be compressed and stored if desired.

[0029] One or more of the water and carbon dioxide from sources 230, 232 can be provided to a second reactor operating in an oxidation mode to produce H2, CO, H2O, and / or CO2 via line 244 and valve 226. The first reactor 202 and the second reactor 204 can operate simultaneously - for example, over overlapping periods. The H2, CO, H2O, and / or CO2 can be sent to a condenser 210 via line 246 and valve 216 to remove heat as described herein. Product gases (e.g., H2, CO) can be separated using a gas separator 208 and stored if desired. As shown, CO2 from the gas separator 208 can be recycled to the second reactor 204 via line 248 and valve 226. Additionally or alternatively, the H2O from the condenser 210 can be recycled back to the second reactor 204 via line 250 and valve 226.

[0030] 3, the operation of the first reactor 202 and the second reactor 204 can be switched such that the first reactor 202 operates in an oxidation mode and the second reactor operates in a reduction mode. This allows the reactor system 200 to operate continuously while extracting product gases. The switch can be controlled by the controller 228 and can occur when the change in the extent of reaction (Δδ) between the reduction and oxidation steps is substantially equal (see FIG. 1). In this context, substantially equal can mean that the extent of reaction after the reduction stage 106 and the extent of reaction after the oxidation stage 108 of the reduction and oxidation cycle 100 are within ±1 or ±2 or ±5% of each other.

[0031] 3, nitrogen can be supplied to the second reactor 204 via line 252 and valve 224, where material 236 is reduced to generate oxygen. The oxygen can be mixed with the nitrogen already provided. The oxygen can be sent to compressor 206 via line 254 and valve 218, where it can be compressed using compressor 206 and stored if desired.

[0032] One or more of water and carbon dioxide from sources 230, 232 are provided to the first reactor 202 via line 256 and valve 222, and the first reactor operates in an oxidation mode to produce H2, CO, and / or CO2. Product gases (e.g., H2, CO) are transmitted via line 258 and valve 214 and can be separated using condenser 210 and gas separator 208 and stored if desired. As shown, CO2 from gas separator 208 can be recycled to the first reactor 202 via lines 248, 256 and valve 222. Additionally or alternatively, H2O from condenser 210 can be recycled back to the first reactor 202 via lines 250, 256 and valve 222.

[0033] During operation, the controller 228 can independently control the temperature in the first reactor 202 and the second reactor 204 to a temperature described herein, such as greater than 800° C. or between 800° C. and 1500° C. Additionally, the controller can control the temperature of the first reactor 202 and the second reactor 204 to a substantially isothermal temperature (about ±10 or ±25 or ±50° C.). Additionally, the controller 228 can control the partial pressure of oxygen in the first reactor 202 and the second reactor 204 to a temperature greater than 10 or between 10 or 25 or 50° C. -7 Higher than bar or 10 -7 and 10 -1 In some cases, the partial pressure of oxygen in the first reactor is controlled to be greater than the partial pressure of oxygen in the second reactor. In some cases, the partial pressure of oxygen in the second reactor is controlled to be greater than the partial pressure of oxygen in the first reactor.

[0034] According to further embodiments of the present disclosure, for example, a method of decomposing one or more of water and carbon dioxide using reactor system 200 may include providing a first material in a first reactor of the reactor system; and providing one or more of H2O and CO2 to the reactor, wherein a temperature in the first reactor is greater than 800° C. and a partial pressure of oxygen in the first reactor is greater than 10 -7The first material can be a material described herein, for example, a material comprising two or more spinel phases in a solid solution, the solid solution comprising oxygen, aluminum, and one or more transition metals. The method further comprises providing a second material into a second reactor of the reactor system, the second material comprising the same chemical formula as the first material; and providing N2 to the second reactor, the temperature in the second reactor being greater than 800° C. and the partial pressure of oxygen in the second reactor being greater than 10 -7 bar. The first and second materials, partial pressures, and temperatures can be as described above. Exemplary methods can further include switching operation of the first and second reactors from an oxidizing to a reducing mode during operation to enable continuous or substantially continuous operation of the reactor system including two or more reactors. In such cases, the method can include a two-stage reduction-oxidation process. The methods and systems described herein can be used to generate 500 μmol g per cycle or more. -1 A capacity for hydrogen production greater than 100% could be demonstrated, which remains feasible even under high conversion conditions (i.e., molar H2O / H2<500:1) and exceeds the hydrogen yield of the three oxygen vacancy-mediated candidates after a temperature swing of 400°C (or less). Isothermal water splitting using (e.g., ferro)aluminate-based materials opens the door for simpler, more robust, and more efficient production of renewable hydrogen. For example, when splitting water, the exemplary method can produce hydrogen in an atmosphere containing existing hydrogen, and therefore the partial pressure of oxygen in the second reactor is greater than that described by a 7:1 or 200:1 H2O:H2 ratio. When splitting CO2, the exemplary method can produce carbon monoxide in an atmosphere containing existing carbon monoxide, and therefore the partial pressure of oxygen in the second reactor is greater than that described by a 2:1 or 64:1 CO2:CO ratio.

[0035] Specific examples are presented below. The exemplary embodiments of the present disclosure do not limit the scope of the present invention, as these embodiments are merely examples of embodiments of the present invention. Any equivalent embodiments are intended to be within the scope of the present invention. Indeed, various modifications of the present disclosure, in addition to those shown and described herein, will become apparent to those skilled in the art from the description, such as alternative useful combinations of the elements described. Such modifications and embodiments are also intended to be encompassed within the scope of the appended claims.

[0036] The embodiments of the present disclosure can be used to convert intermittent solar radiation into storable and transportable chemical fuels that can make sustainable feedstocks and controllable power sources accessible regardless of geographic location. Heat can be obtained via light-concentrating optics and / or renewable power sources (e.g., photovoltaic devices). When coupled with established catalytic processes such as Fischer-Tropsch synthesis, the products H2 and CO can be converted into a variety of liquid hydrocarbons (e.g., diesel) and organic oxygenates (e.g., methanol) that are free of nitrogen- and sulfur-containing impurities. For industries that rely on chemical fuels produced through traditional means (i.e., coal gasification, methane reforming, etc.), solar-driven gas-to-liquid technology offers a viable alternative that can reduce dependency on dwindling fossil energy resources and thus mitigate associated greenhouse gas emissions.

[0037] By initiating the oxidation at a higher temperature, as described above, the reaction kinetics can be improved and the mechanical stresses induced by thermal cycling and the challenges associated with realizing solid-phase heat recovery can be avoided if the redox regime were operated substantially isothermally. Although the requirements for delicate heating of the process gas are increased, the integration of a very effective gas-phase heat recovery can be mitigated by removing the dominant source of irreversibility. A substantially isothermal redox cycle offers some practical advantages, but it reduces the cycle capacity of the oxide for the production of H2 (and / or CO), which in this case can be affected by the difference in oxygen chemical potential between the inert sweep gas and the hot oxidant. As a result, unlike redox schemes that follow temperature swings, the choice of materials is not strictly limited to those that exhibit large enthalpy and entropy changes; rather, materials that preferentially exhibit a fairly wide range of reactions within the feasible range of oxygen chemical potentials are more desirable.

[0038] Ultimately, the results demonstrate that (e.g., iron) aluminate-based materials exhibit excellent performance and remain viable under (less favorable) conditions expected in large-scale systems, where delivery of excess oxidant and achieving wide temperature swings are avoided, improving efficiency.

[0039] Materials synthesis and chemical characterization The following procedure was used to prepare a three-compound blend of cobalt-iron aluminate (Co x Fe 1-x+y Al 2-yO4) composition space: (1) x=0.40 and y=0, (2) x=0 and y=0, and (3) x=0 and y=0.40. For simplicity and to avoid implying the presence of specific phases, these formulas are hereafter referred to as Co13Fe20Al67, Fe33Al67, and Fe47Al53, respectively. The method outlined in this strategy was specifically tailored to ensure that the preparation of Fe33Al67 results in high purity hercynite, which was previously achieved by subjecting a homogeneous precursor mixture of the correct cation ratio (i.e., 33 mol% Fe) to prolonged heat treatment at high temperature and low oxygen partial pressure. To homogeneously distribute the metal cations, previous work has considered - for example - mechanical mixing of Fe2O3 and Al2O3 powders, dissolving Fe and Al ammonium alum in solution, or physically combining metallic Fe with dehydrated Al(OH)3. Here, however, a modified Pechini sol-gel method, commonly used in the synthesis of perovskites, was utilized to ensure that the cations were uniformly dispersed prior to calcination. First, representative samples of Co(NO3)2·6H2O, Fe(NO3)3·9H2O, and Al(NO3)3·9H2O (Sigma-Aldrich, ACS reagents, ≥98%) were dehydrated using thermogravimetric analysis (NETZSCH, STA 449 F1 Jupiter) to determine the nominal weight percent of metal cations. Stoichiometric amounts of these metal nitrates, formulated with the desired cation ratios for each formula, were then dissolved in 20 mL of deionized (DI) water along with dehydrated citric acid monohydrate (C6H8O7·H2O, Fisher Scientific, ACS certified). The molar ratio of C6H8O7·H2O to the total metal cations was set at 3:2. The aqueous solution contained within the glass beaker was continuously stirred at 300 RPM for 2 h under ambient conditions. Ethylene glycol (C2H6O2, Fisher Scientific, certified) was then introduced in a ratio of 2 moles C2H6O2 per mole C6H8O7·H2O to further promote homogeneity. After 10 min, the solution was slowly heated to 90 °C and the temperature was maintained until complete gelation was achieved; the magnetic stirrer speed was gradually increased from 300 to 900 RPM during heating.After cooling, the readily accessible material was transferred to two alumina combustion boats and air dried in a Lindberg Hevi-Duty tube furnace for 3 hours at 300° C. Any remaining material that adhered to the surface of the glass beaker was air dried overnight at 100° C. in a drying oven (Fisher Scientific, Isotemp 737F). The resulting material was separated by formula and drying procedure and then ground to a fine powder in a pestle and mortar.

[0040] The heat treatment consisted of three steps. First, each batch was reinserted into a Lindberg Hevi-Duty tube furnace and calcined individually at 850 °C in air for 24 h to (1) pyrolyze any remaining organic matter and (2) form solid solutions consisting mainly of binary metal oxides. Then, each solid solution was subsequently reduced to ensure that all components were of the spinel phase. In cases where this was not the case, the authors observed the persistence of impurities (e.g., metallic Fe or corundum) in the final product, which in those cases was synthetic hercynite. Here, reduction consisted of 6 h at 900 °C for approximately 10 min. -14 Finally, each batch was heated to 1380° C. and approx. -10 these conditions were chosen with the intention of promoting the formation of hercynite. A portion of the resulting iron aluminate spinel was cold pressed uniaxially at 2 tons for 120 seconds to form a dense cylindrical pellet with the following dimensions: diameter 6 mm, height 1-2 mm, and mass 50-65 mg.

[0041] The last two reduction steps were carried out in a stagnant flow reactor, a device well suited to ensure that the sample experienced a homogenous gas composition, where the pO2 of the system was precisely controlled by delivering a 15% CO / Ar mixture (Airgas, certified spec) at various flow rates along with CO2 (Airgas, grade 5.0).

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[0042] The equilibrium constants for the formation of CO and CO (i.e.,

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[0043] The iron aluminate-based materials were characterized by several techniques at different stages of the calcination process and after the experimental campaign. Powder X-ray diffraction (PXRD) was performed on a Bruker D8 Advance diffractometer equipped with a LYNXEYE XE-T detector and monochromatic Cu-Kα radiation; generator voltage and test tube current were 40 kV and 40 mA, respectively. PXRD patterns were recorded between 15° and 100° (2θ) at a scan rate of 2° min. -1, step size of 0.007°, and time per step of approximately 40 s were recorded at room temperature. Each measurement was processed with DIFFRAC.EVA software (Bruker AXS), which was used to (1) perform phase identification using reference patterns derived from the Crystallography Open Database (COD), and (2) correct for background effects observed at low diffraction angles (i.e., 2θ<20°) introduced by the PMMA specimen holder. Fe33Al67 and Fe47Al53 PXRD patterns were further analyzed with FullProf Suite software to ensure precise quantification of phase proportions. Peak profiles were modeled using COD standards and pseudo-Voigt functions, and refined parameters included scale factors, atomic positions, occupancies, isotropic atomic displacement parameters, and lattice parameters. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were performed with a silicon drift detector (Oxford Instruments, X-Max N The analysis was performed using an FEI Nova NanoSEM 450 equipped with a 3000 NA 1 ... -1 It was further diluted until it was less than

[0044] thermal analysis The equilibrium behavior of pelletized iron aluminate-based samples was evaluated using a STA 449 F1 Jupiter thermal analyzer equipped with a vertically oriented sample carrier that allows for thermogravimetric (TG) measurements. An S-type thermocouple embedded within the TG sample carrier provided temperature measurements at the location of the sample (T S ). An exposed thermocouple junction directly supports a flat alumina crucible, which was chosen to reduce external mass transfer restrictions to the pellets; a 6 mm sapphire disk was embedded between each pellet and the crucible to prevent any interactions. Upstream of the sample chamber, O2 / Ar mixtures containing 10% O2 (Airgas, certified spec) or 0.2% O2 (Airgas, certified spec) were diluted with additional Ar (Airgas, grade 5.0) via two electronic mass flow controllers (Bronkhorst, El-FLOW Select) and a manual rotameter (Vogtlin Instruments, Q-Flow 140). Before starting the experimental campaign, all flow controllers were calibrated (Mesa Labs, FlexCal Series) to ensure precise delivery of inlet gas (standardized to 25°C and 760 Torr) and thus control pO2 in a well-mixed sample chamber and hermetically sealed system. Independent of the flow configuration, the total volumetric flow rate (

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[0045] Before each experiment, the samples were heated at either 700 °C or 1100 °C and approximately 10 -2The pellets were fully oxidized at a pO of 10 bar; preliminary tests confirmed that complete oxidation was achieved under these conditions, as indicated by the lack of mass gain and stabilization of the oxygen signal gradient. The experiment was then performed at 10 -2 bar and 10 -5 The experiment was carried out at a constant pO between 100 and 200 bar, so that any relative change in mass of the sample was measured at different furnace reference temperatures (T ref ) directly by heating or cooling; T ref The heating and cooling rate is 15°C / min. -1 Less than or equal to T s is T ref The experimental procedure consisted of randomly selected T ref The duration of each isotherm was maintained for either 1 or 2 hours. For each experiment, samples were ref The sequence was run twice, with each sequence being uniquely randomized. In addition, a 700°C isotherm was run at the beginning and end of the entire experiment, as well as at two T ref A series of lower temperature segments were realized across the array. These lower temperature segments provided a time frame of reference for the higher temperature mass relaxation studies, where higher oxygen evolution was predicted for the pO2 range considered. If the sample mass relative changes did not equilibrate within the allotted duration, separate experiments with longer reactions were performed to ensure that thermodynamic equilibrium was achieved at all temperatures. Here, the randomized temperature sequence was modified so that samples were simply evaluated without equilibrium having yet been established. In these cases, T ref The sequences were organized in ascending order. Regardless of the method, each experiment was immediately repeated in the absence of any reactive material to compensate for undesired buoyancy effects observed in the TG measurements.

[0046] calculation Thermodynamic analysis of the Fe-Al-O and Co-Fe-Al-O systems was performed using the software package FactSage (version 8.0) to calculate phase equilibria based on the Gibbs free energy minimization principle. The calculations considered a database of both solutions (i.e., slags, corundum, spinel, and monoxides) and pure compounds (i.e., gases and solids), and each system was evaluated for various cation compositions (ζ), temperatures, and oxygen partial pressures at constant pressure (760 Torr).

[0047] The defect model is proposed to complement thermogravimetry by providing insight into material behavior outside the range of experimentally tested conditions. Importantly, the description of the effect of T and pO2 on equilibrium is based on only a few parameters, namely the standard molar enthalpy (

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[0048] The comparison was evaluated in terms of the degree of non-stoichiometry (δ), which in the latter case corresponds to the spinel (M st ) and oxygen (M O ) and the relative change in mass (m) measured between the equilibrium and stoichiometric states of the spinel (denoted by the subscripts eq and st, respectively).

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[0049] Note that in this study, equilibrium was considered to be established only when (1) the furnace was set to isothermal conditions and (2) the mass change with time was less than one tenth of a microgram per minute. These criteria are mathematically expressed in Equation 4.

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[0050] The close agreement between theory and experiment, in addition to providing support for the proposed mechanism, allows for the determination of δ at any condition if the defect model assumptions remain valid. As a result, the standard partial molar enthalpy of vacancy formation or consumption (

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[0051] To obtain these properties, we manipulate equation 5 into the linear form of the van't Hoff equation, -ln(

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[0052] water splitting The effectiveness of the oxide to facilitate thermochemical water splitting is measured by the change in the extent of reaction achieved between the reduction and oxidation steps (i.e., Δδ). To predict Δδ, which is proportional to the amount of oxygen and hydrogen cycled out of the oxide, one must first establish the operating conditions obtainable at each step, in particular pO2, as T is independently controlled. At ambient pressure (i.e., 1 bar), the inlet pO2 of the reduction reaction is defined as the oxygen content in the inert sweep gas, which is typically below 10 ppm. Conversely, the inlet pO2 of the oxidation reaction is determined according to the temperature-dependent equilibrium of the thermal splitting of water (i.e.,

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[0053] Here, we introduce a reaction coordinate (ε) to obtain a unique solution for pO2, where the initial number of moles of each chemical species (n i ) and independent reaction conditions (i.e., T and P) are specified;

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[0054] Mechanistic insight and fault models The X-ray diffraction patterns of the iron aluminate powders after synthesis and the experimental campaign are shown in Figure 4. Superimposed on the plot are the characteristic diffraction peaks of FeAl2O4 and Fe3O4 indicated by the vertical solid and dashed lines, respectively; the former diffraction peaks are located at higher 2θ values ​​due to the larger lattice parameter of Fe3O4 (i.e. a=b=c=8.15 Å and 8.40 Å for FeAl2O4 and Fe3O4, respectively). Qualitative phase analysis was performed in a CO / CO2 atmosphere (pO2≈10 -10 After calcination at 1380 °C in 10 bar, the Fe33Al67 sample consisted mainly of the FeAl2O4 phase, while the Fe47Al53 sample - which exhibits peaks between the characteristic peaks of each spinel - appeared to contain a higher proportion of Fe3O4. -2 After an experimental campaign exposed to oxygen partial pressures as high as 1000 bar, peaks assigned to corundum and hematite were also observed for both Fe33Al67 and Fe47Al53. Analysis of Co13Fe20Al67 samples revealed similar behavior: non-spinel phases (e.g., Al2O3) were only observed after the samples were subjected to more oxidizing conditions. In the case of cobalt-containing formulations, the spinel phase may consist of CoAl2O4, Co3O4, FeAl2O4, and Fe3O4; the lattice parameters of CoAl2O4 and Co3O4 are close to those of FeAl2O4 (i.e., a=b=c=8.10 Å and 8.08 Å for CoAl2O4 and Co3O4, respectively).

[0055] To quantify the phase composition, especially for the Fe33Al67 and Fe47Al53 samples, a multiphase Rietveld refinement of the PXRD data was performed. As shown as the black line in Figure 4, the calculations yielded a final goodness of fit (χ 2) show a close agreement with the diffraction pattern observed by . Both iron aluminates were of the spinel phase after calcination and were confirmed to consist of spinel, corundum, and hematite phases after the experimental campaign; the phase composition and structure refinement parameters are presented in Table 1. For Fe33Al67, the lattice parameter is close to 8.15 Å (i.e., the value for FeAl2O4), thus indicating that the as-synthesized samples are nearly phase pure, as hoped. Refinement also showed that the lattice parameter of the as-synthesized Fe47Al53 sample is larger - a result of the larger amount of Fe3O4 (i.e., 26.8 wt%). After the experimental campaign, the spinel lattice parameter of both samples increased to about 8.35 Å as the ratio of Fe3O4 to FeAl2O4 increased. Thus, although defect effects may contribute to the change in peak position, it is clear that the spinel peak shifts to lower 2θ values ​​as the samples oxidize, with a concomitant increase in the ratio of Fe3O4 to other spinels.

[0056] Table 2 presents the elemental compositions of the as-synthesized iron aluminate-based materials determined by ICP-OES. These results are all close to their respective targets and also agree with the elemental compositions calculated from the corresponding phase compositions reported in Table 1. According to the Rietveld refinement, the Fe33Al67 sample had a 0.34 mol Fe Mol c -1 and 0.66 mol Al Mol c -1 while the Fe47Al53 sample consisted of 0.48mol Fe Mol c -1 and 0.52 mol Al Mol c -1 It consists of:

[0057] [Table 1]

[0058] [Table 2]

[0059] To provide further insight into the PXRD and Rietveld refinement results, phase diagrams for the Fe-Al-O and Co-Fe-Al-O systems were constructed as shown in FIG. 4. Under the conditions tested, the following phases exist in the Fe-Al-O system: solid solution (ss) of spinel (i.e., FeAl2O4 and Fe3O4), solid solution of corundum, solid solution of hematite, and solid solution of intermediate compounds (1:1 Fe2O3:Al2O3). The spinel region indicates the spinel phase, the equilibrium region indicates the equilibrium of spinel and corundum phases, and the unshaded region represents the fully oxidized state of iron aluminate, as revealed by the presence of hematite. The solid line between the equilibrium and unshaded regions - hereafter referred to as the solid solution phase boundary - remains constant over a wide range of iron cation compositions (e.g., from 0.125 to 0.5). In contrast, the phase boundary between the spinel and equilibrium regions shifts toward lower temperatures and higher oxygen partial pressures as ζ increases; the regions created by the shift in the phase boundary (e.g., from ζ=0.35 to ζ=0.45, as shown by the dashed and dot-dash lines, respectively) are shaded, and the spinel and equilibrium combinations are shown superimposed. In general, the Co-Fe-Al-O system shows similar behavior, but only three phases are present: solid solutions of spinel (i.e., CoAl2O4, Co3O4, FeAl2O4, and Fe3O4), solid solutions of corundum, and solid solutions of hematite. Here, both phase boundaries - including those between the equilibrium and colorless regions - vary as a function of cobalt cation composition and shift toward lower temperatures and higher oxygen partial pressures as ζ increases. Thus, the introduction of cobalt in the iron aluminate lowers the temperature required for the transition from the oxidized to the reduced state. The results of these calculations indicate that the final step of the experimental campaign is -2 This is supported by the observations in FIG. 4, such as the appearance of corundum and / or hematite in the post-cycle samples, since rapid cooling to room temperature under oxygen partial pressures as high as 1 bar was involved.

[0060] 4 and 5 show that, in contrast to previous assertions, the redox behavior of the iron aluminate-based materials cannot be attributed solely to hercynite or mixed cobalt-iron aluminate compounds. It would appear that otherwise phase-pure hercynite is metastable under ambient conditions, but does not rapidly revert to its original initial constituents (i.e., Fe2O3 and Al2O3) when exposed to a sufficiently oxidizing environment at high temperatures. Instead, under conditions associated with the isothermal dissociation of H2O (see symbols in FIG. 5), the iron aluminate-based materials exist as solid solutions composed primarily of magnetite and hercynite spinel. Although magnetite cannot individually produce appreciable amounts of hydrogen when combined with hercynite, the altered equilibrium of the solid solution makes water splitting particularly possible. This phenomenon - a consequence of the change in Gibbs free energy due to mixing - is similar to the effect of inert zirconia on the thermodynamic properties of ceria; i.e., ceria-zirconia solid solutions exhibit a significantly greater degree of reduction than does undoped ceria.

[0061] Therefore, according to the defect structure of iron oxide established with high oxygen activity, we assign cation-not-oxygen-vacancies as the primary point defects responsible for the water splitting ability of iron aluminate-based materials. The general chemical reaction for the removal (i.e., reduction) of oxygen from, for example, an iron aluminate spinel solid solution can then be written as shown in Equation 7.

number

[0062] Note that δ, the deviation from stoichiometry, is a measure of the concentration of crystal lattice defects, and the subscripts i and f refer to the initial and final states, respectively. To drive this reaction in the forward direction, Δδ-δ i and δ fThis suggests that -, defined as the difference between δ and δ, must be positive, thus suggesting that unlike materials that accommodate oxygen vacancies, the degree of reduction increases with decreasing δ. In other words, the removal of lattice oxygen requires that a cation vacancy be consumed, which is then displaced from the neighboring Fe vacancies such that charge neutrality is maintained, as described using the Kroger-Vink notation in Equation 8. 3+ Cation to F 2+ This is compensated for by conversion to a cation.

number

[0063] According to the law of mass action, if the activity coefficients are assumed to be 1, the lattice species introduced above can be related as follows:

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[0064] Although Equation 9 allows for a qualitative interpretation of the bulk nonstoichiometry of iron aluminate spinel solid solutions, to more rigorously represent the underlying physics, it was necessary to take into account the temperature-dependent site preferences of the lattice species. In the spinel structure, cations (and vacancies) can be coordinated to either four or six oxygen anions, depending on whether tetrahedral or octahedral sites are occupied. The distribution of cations among the sites can be determined by the general formula A 1-λ B λ (A λ / 2 B 1-λ / 2 )2O4, where the symbols A and B are divalent (e.g., Fe 2+ ) and trivalent (e.g., Fe 3+ and Al 3+) cations, brackets indicate the octahedral sublattice, and λ refers to the degree of inversion. At room temperature, magnetite adopts a predominantly inverted (λ=1) distribution, while hercynite adopts a predominantly normal (λ=0) distribution; however, as the temperature increases, the tetrahedral and octahedral cations exchange their lattice sites (0<λ<1) as a result of -entropic effects. Here the disorder in the spinel structure was described by introducing the following reaction, also written in Kroger-Vink notation:

number

[0065] To reduce the number of model equations, only exchange between lattice species with different charges (or oxidation states) is considered, i.e., octahedral Fe 2+ Tetrahedral Fe with 3+ , octahedral Al 3+ Tetrahedral Fe with 2+ , and tetrahedral Fe with a doubly ionized octahedral vacancy 3+ For each reaction presented in Equation 10, the law of mass action can be similarly applied to relate the concentrations of the corresponding lattice species to unique equilibrium constants; these relationships are shown in Equation 11.

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[0066] Furthermore, the aforementioned chemical reactions must obey the bulk conservation equations: Tetrahedral Partial Balance:

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[0067] Although evidence of defect associations has been observed in iron oxides, particularly wüstite, the formation of such clusters in the Fe-Al-O system remains unresolved, and point defects have therefore been assumed to form ideal solutions on their respective sublattices. Taken together, Equation 9 and Equations 11 through 17 define the model used to describe the redox behavior of iron aluminates under conditions relevant to two-stage thermochemical fuel production. For a given T, pO2, and ζ, the concentrations of all lattice species can be calculated using the standard molar enthalpy (

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[0068] It is important to note that while the mechanistic insights presented herein pertain to cobalt-containing formulations, the method may not be able to quantify δ without additional information since both Co3O4 and CoAl2O4 are present when the cobalt-iron aluminate is fully oxidized (see FIG. 5). As a result, the failure models presented were developed only for the iron aluminates tested herein (i.e., Fe33Al67 and Fe47Al53).

[0069] Thermodynamic characterization FIG. 6 shows the thermogravimetric response of pelletized Co13Fe20Al67, Fe33Al67, and Fe47Al53 samples when subjected to temperature changes at various oxygen partial pressures. For each sample, the relative percent change in mass from the fully oxidized state (Δm / m i The extent of reduction, expressed as T ref pO2 increases with increasing oxygen partial pressure and decreasing pO2. Furthermore, formulations with a higher amount of iron (i.e., the primary redox active element) showed a greater degree of reduction when compared under conditions in which the spinel solid solution was present (see FIG. 5). As can be seen in FIG. 6(A) for temperatures below 1400°C, the iron aluminates Fe33Al67 and Fe47Al53 otherwise remain in a fully oxidized state (i.e., solid solution of corundum and hematite), whereas the cobalt-containing counterpart Co13Fe20Al67 was visibly reduced even at temperatures as low as −1000°C. This inherent aspect of the Fe-Al-O system led to slower kinetics when approaching or crossing the solid solution phase boundary, especially for Fe33Al67 at low oxygen partial pressures (FIG. 6(B)). As a result, separate experiments were performed to ensure that equilibrium was obtained at all conditions.

[0070] Experimental results such as those displayed in FIG. 6 are evaluated according to the criteria defined in Equation 4 to characterize the equilibrium as a function of isothermal temperature and oxygen partial pressure; the sum of the equilibrium thermogravimetric measurements is presented in FIG. 7. Here the effect of the cation composition on the redox behavior of the samples, and in particular the location of the solid solution phase boundary, is even more evident. For example, the equilibrium state of Co13Fe20Al67 depends on pO2 throughout the conditions tested, and such a dependence is only observed for Fe33Al67 and Fe47Al53 at temperatures above 1100°C (i.e., after the spinel solid solution phase is established). This phase transition - depicted in FIG. 5 and described by the chemical reaction presented in Equation 18 - occurs over a period of about 2×10 as seen for both Fe33Al67 and Fe47Al53 (FIGS. 7(B) and 7(C) respectively). -2 From 5×10 -2 This is indicated by the significant change in mass at 1300°C over a narrow range of pO2 between bar and 1300°C.

number

[0071] The standard molar enthalpies and entropies used to generate the reported model predictions (solid lines) are listed in Table 3. As expected, the defect reactions responsible for the removal of oxygen (Eq. 8), as well as the reaction of tetrahedral Fe 2+ Parts and octahedral Al 3+ The exchange between the sites (Eq. 10b) is endothermic. Moreover, the standard molar properties of the latter are -Al with respect to octahedral coordination. 3+ The reaction tends to the left - due to the known preference for cation vacancies (i.e., K3<<1 at practical temperatures). These observations, as well as the close agreement between experimental data and thermodynamic (phase diagram and defect model) calculations, support the validity of the proposed defect mechanism and argue that the water splitting ability of iron aluminate-based materials is due to (1) the presence of cation vacancies and (2) the interaction of two or more spinels in solid solution. Note that the model is only applicable when a spinel solid solution phase is predicted.

[0072] To improve the clarity of the figures, uncertainties in the relative percent change in oxygen partial pressure and mass are independent of temperature and error bars are shown only for the results at 1400°C; where not shown, the errors are within the size of the symbols.

[0073] [Table 3]

[0074] In FIG. 8, equilibrium thermogravimetries and corresponding defect model predictions for Fe33Al67 and Fe47Al53 are plotted in terms of δ and compared with relevant data extracted from the literature at 1400°C. Independent of the iron aluminate composition, δ (i.e., the degree of oxidation) increases with increasing pO2, consistent with materials such as magnetite (ζ=1) that are known to accommodate cation vacancies. For a given pO2, δ also increases as the proportion of redox active ions (ζ) decreases, thus confirming that the presence of aluminum limits the degree of reduction that can be achieved. Notably, only compositions containing high amounts of aluminum (ζ<0.5) exhibit the behavior required to facilitate water splitting at 1400°C, i.e., the equilibrium pO2 for the thermal decomposition of water (in this case, 3.84×10 -4 bar) indicates a positive slope.

[0075] Physically, the formation of cationic vacancies is achieved by outward diffusion of cations - a phenomenon responsible for the well-known growth of layering scale on iron and iron oxides under highly oxidizing conditions. Under reducing conditions, when cationic vacancies are consumed, the direction of cationic diffusion shifts inward and interstitials become the dominant point defects, in the limit where, for example, magnetite approaches stoichiometry (i.e., δ=0). Although the growth of multiphase scale can be avoided by the introduction of aluminum (due to the formation of a stable solid solution phase), the mechanism of the transition from vacancy-mediated (δ>0) to interstitial-mediated (δ<0) in magnetite may also occur in iron aluminates, thus explaining the discrepancy between measurements and thermodynamic predictions at low δ.

[0076] The definition of standard partial molar enthalpies and entropies of reduction (or oxidation) of metal oxides allows the determination of thermodynamic equilibria at known T and pO2 without requiring an understanding of defect chemistry. An illustration of the procedure for obtaining such properties is presented in FIG. 9(A), which can be used to calculate the entropy of a metal oxide, e.g.

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[0077] FIG. 10 shows the redox behavior of some candidate materials as a function of oxygen partial pressure at 1400° C. To ensure a fair comparison, the results are expressed as the relative percent change in mass (or oxygen content) between the fully oxidized and equilibrium states, since the interpretation of δ depends on the type of defects the material accommodates. The vertical dashed-dotted line represents the maximum achievable pO2 for water splitting at 1400° C. and 1 bar (see Equation 6), and therefore when operating isothermally, the reduction step must start at a lower pO2 to produce hydrogen. In this case, quantification of the material's cycling capacity for hydrogen production involves determining the vertical distance between the oxygen content at reduced and oxidized conditions. As a result, for a given T and reduced pO2, the highest slope (i.e., Δm per unit change in pO2) is obtained. eq / m i The materials with the highest yields will have the highest change in δ (change in δ) for the iron aluminate-based materials. The data suggest that the observed slope for the iron aluminate-based materials increases with increasing iron content, but the trend is obscured by the discrepancy between the measurements at low δ and the thermodynamic predictions. A linear approximation (dashed line) was therefore included to provide additional insight at the experimentally evaluated temperatures. Despite the discrepancy, it is clear that the iron aluminate-based materials exhibit a slope that exceeds that of ceria and the perovskites considered, and are therefore the more desirable candidates for isothermal redox cycling at 1400 °C.

[0078] The thermochemical water splitting performance of candidate redox materials under isothermal conditions is examined in Figure 11. In general, in isothermal operation, the thermodynamic capacity of oxides for hydrogen production (Figure 11(A)) increases with increasing temperature, as the oxidation pO2 - determined according to the equilibrium of the thermal decomposition of water at 1 bar - increases simultaneously. Notably, the highest capacity is observed for iron aluminate-based materials, a result of their retention of partial molar properties, which allows a uniquely large variation in the extent of reaction within the feasible range of pO2 (see Figure 10). For example, at 1400 °C, Fe33Al67 has a hydrogen production of 450 μmol g -1hydrogen production, whereas LSMA6464 - a perovskite predicted to do so efficiently under isothermal conditions - produces 370 μmol g -1 ; a linear approximation suggests that the capacity of Fe47Al53 appears to exceed that of Fe33Al67. The higher capacity for hydrogen production suggests that the material is more tolerant to conditions expected in practice, and the amount of water vapor delivered must be constrained to reduce sensitive heating penalties and improve efficiency. As can be seen from Figure 11(B), the material exhibiting the highest capacity (i.e., the iron aluminate-based material) remains effective under "high conversion" conditions, reaching 200 μmol g when exposed to water vapor to hydrogen as low as 500:1. -1 An increase in hydrogen yield while maintaining high reactant conversion (i.e., H2O / H2<500:1) is possible if the reduction step is initiated at an even lower pO2; note in this comparison that all yields converge at a steam to hydrogen ratio of 200:1 since the oxidation pO2 at this condition is lower than that defined for the reduction (i.e., 10 ppm residual oxygen).

[0079] To provide further context regarding the significance of iron aluminate performance under isothermal conditions, for example, the results are also presented as a function of temperature swing, as shown in FIG. 12. For water splitting at high conversion (i.e., H2O / H2=500:1), Fe33Al67 is capable of exceeding the hydrogen yield of oxygen vacancy-mediated alternatives after a temperature swing of -400°C (or lower) when operated isothermally at -1400°C. Alternatives tested include ceria, widely recognized as a benchmark material, and CTM55, a recently developed perovskite marketed as having "outstanding properties" for two-stage thermochemical fuel production. Especially when lower conversions are observed (e.g., H2O / H2=1500:1), the potential for higher yields with Fe33Al67 is much greater than that of ceria, since ceria operates near its full oxidation state at lower temperatures. The superior isothermal capability over materials well suited for temperature swing operation suggests that solar-to-hydrogen conversion with iron aluminate-based materials could be more efficient than conventional approaches, since comparable yields can be achieved without incurring excessive energy losses from heating and cooling between redox regimes.

[0080] Low-cost iron aluminate-based materials were characterized to confirm the extent of their performance under such conditions. Thermogravimetry was used to quantify the extent of equilibrium of the reaction as a function of cation composition, temperature, and oxygen partial pressure. The measurements were supplemented with a defect model to provide insight into the behavior outside the scope of the experimental campaign, as well as to clarify previous misunderstandings regarding the mechanism by which these materials operate. Guided by X-ray diffraction data and phase equilibrium calculations, the model was based on the observation that under water splitting conditions, iron aluminate exists as a solid solution consisting mainly of hercynite and magnetite spinel, the latter of which mediates oxygen exchange via cation-but not oxygen-vacancies. Open-system thermodynamic analysis further showed that, unlike pure magnetite, iron aluminate-based spinel solid solutions are capable of isothermally splitting water as a result of the change in Gibbs free energy due to mixing. Notably, these materials possess an exceptional ability to isothermally generate hydrogen, which therefore remains feasible even under high conversion conditions. For example, in water splitting with a steam-hydrogen ratio as low as 500:1, iron aluminate Fe33Al67 still exists at 200 μmol g -1 More than 1000 keV of hydrogen can be produced at 1400 °C, exceeding the capabilities predicted for ceria and two attractive perovskite candidates after a temperature swing of 400 °C (or less). The demonstration of improved performance without the need for excess oxidant and / or wide temperature swings represents a leap towards sustainable hydrogen production at a scale using only renewable energy sources.

[0081] The exemplary embodiments of the present disclosure described above do not limit the scope of the present invention, as these embodiments are merely examples of embodiments of the present invention. Any equivalent embodiments are intended to be within the scope of the present invention. Indeed, various modifications of the present disclosure in addition to the embodiments shown and described herein, such as alternative useful combinations of the elements described, may become apparent to those skilled in the art from the description. Such modifications and embodiments are also intended to be encompassed within the scope of the appended claims.

Claims

1. 1. A method for decomposing one or more of water and carbon dioxide, the method comprising: providing a first material in a first reactor of a reactor system, the first material comprising two or more spinel phases in a solid solution, the solid solution comprising oxygen, aluminum, and one or more transition metals; and H ２ O and CO ２ providing one or more of: Including, the temperature in the first reactor is greater than 800°C; The partial pressure of oxygen in the first reactor is 10 －７ Bigger than a bar A method for decomposing one or more of water and carbon dioxide.

2. providing a second material in a second reactor of the reactor system, the second material having the same chemical formula as the first material; and The second reactor is charged with N ２ to provide further comprising the temperature in the second reactor is greater than 800°C; The partial pressure of oxygen in the second reactor is 10 －７ Bigger than a bar The method of claim 1.

3. 3. The method of claim 1 or claim 2, wherein one or more of the temperature in the first reactor and the temperature in the second reactor is between 800°C and 1500°C.

4. One or more of the partial pressure of oxygen in the first reactor and the partial pressure of oxygen in the second reactor is 10 －７ 10 minutes from the bar －１ The method of claim 1 , wherein the pressure is between 100 and 150 bar.

5. The first material and the second material are each (M ζ Al １－ζ ) ３－δ O ４ wherein ζ is greater than 1 / 3 and M is one or more transition metals.

6. 6. The method of claim 5, wherein ζ is greater than 1 / 3 and less than 1.

7. 6. The method of claim 5, wherein M is selected from one or more of Fe, Co, Ti, Mn, Mg, Zn, Ni, and Cr.

8. 6. The method of claim 5, wherein one or more of the first material and the second material comprises cation vacancies (δ) that allow for the removal of oxygen and the decomposition of one or more of water and carbon dioxide.

9. 10. The method of claim 1 comprising a two-stage reduction-oxidation process.

10. 10. The method of claim 9, wherein the two-stage reduction-oxidation process is substantially isothermal.

11. The partial pressure of oxygen in the second reactor relative to water is 7:1 H ２ O:H ２ 10. The method of claim 1, wherein hydrogen can be generated in an existing hydrogen-containing atmosphere such that the ratio is greater than that described by the ratio.

12. CO ２ With respect to the partial pressure of oxygen in the second reactor, ２ 10. The method of claim 1, wherein carbon monoxide can be generated in an existing carbon monoxide-containing atmosphere such that the CO:CO ratio is greater than that described by the CO:CO ratio.

13. 10. The method of claim 1, wherein the partial pressure of oxygen in the first reactor is greater than the partial pressure of oxygen in the second reactor.

14. 10. The method of claim 1, wherein the partial pressure of oxygen in the second reactor is greater than the partial pressure of oxygen in the first reactor.

15. 10. The method of claim 1, capable of producing greater than 500 μmol of hydrogen and / or carbon monoxide per gram of material per cycle.

16. a first reactor; and a first material in the first reactor, the first material comprising two or more spinel phases in a solid solution, the solid solution comprising oxygen, aluminum, and one or more transition metals; fluidly connected to the first reactor; ２ O source and CO ２ one or more of the sources; controlling the temperature in the first reactor to greater than 800°C; and The partial pressure of oxygen in the first reactor is 10 －７ Control it more than bar The configured controller and a reactor system comprising:

17. a second reactor containing a second material having the same chemical formula as the first material; The controller controlling the temperature in the second reactor to greater than 800°C; and The partial pressure of oxygen in the second reactor is 10 ７ further configured to control the bar to a value greater than 17. The reactor system of claim 16.

18. 18. The reactor system of claim 16, wherein the first reactor and the second reactor operate substantially isothermally.

19. 17. The reactor system of claim 16, further comprising a nitrogen source coupled to said first reactor and said second reactor.

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