Iron oxides as materials for electrodes for fuel cells and high-temperature electrolysers
Fe-based oxides with Ni or Cu doping address the mechanical strength and cost issues of cobalt-based electrodes in SOFCs and SOECs, providing high electrocatalytic activity and reduced thermal expansion for efficient energy conversion.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- UNIV COMPLUTENSE DE MADRID
- Filing Date
- 2025-11-05
- Publication Date
- 2026-05-15
AI Technical Summary
The use of cobalt-based oxides in oxygen electrodes for SOFCs and SOECs is limited by high thermal expansion coefficients, posing mechanical strength issues, and cobalt is a strategic element, increasing production costs.
Development of Fe-based oxides doped with Ni or Cu, with a perovskite-derived structure, synthesized using ceramic or combustion methods, offering high electrocatalytic activity and reduced thermal expansion, suitable for use in SOFCs and SOECs.
The Fe-based oxides exhibit electrocatalytic performance comparable to Co oxides but with lower thermal expansion, reducing production costs and enhancing mechanical compatibility with electrolytes, enabling efficient energy generation and storage.
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Abstract
Description
[0001]
[0002] Iron oxides as materials for fuel cell electrodes and high-temperature electrolyzers
[0003] TECHNICAL SECTOR
[0004] The present invention relates to ceramic materials with high electrocatalytic activity, their method of obtaining them, and their use as oxygen electrodes for application in SOFC-type fuel cells and SOEC-type electrolyzers.
[0005] BACKGROUND OF THE INVENTION
[0006] Currently, the standard electroactive material for oxygen electrodes in SOFCs (Solid Oxide Fuel Cells) and SOECs (Solid Oxide Electrolysis Cells) is a manganese oxide. The performance of this material has been improved by cobalt oxides doped with a low concentration of iron. However, cobalt is a strategic element, and these types of cobalt materials exhibit high coefficients of thermal expansion, which poses a problem from the perspective of the device's mechanical strength.
[0007] Therefore, the development of high-performance materials at relatively low temperatures (700°C) for SOFC (Solid Oxide Fuel Cell) type electrodes and SOEC (Solid Oxide Electrolysis Cell) type electrolyzers based on non-critical elements is one of the most important and urgent challenges facing the technological advancement of these devices for the use and production of "green" hydrogen.
[0008] Therefore, there are different proposals for this type of material, such as the one shown in document CN102208662A, which describes cobalt-free materials for oxygen electrodes for SOFCs and SOECs with a perovskite-type structure ABO3. Table 1 lists materials close to the invention reported in the scientific literature, including both Co-based and Co-free (cobalt-free) materials.
[0009] The present invention proposes new Fe-based oxides doped with Ni or Cu that exhibit high electrocatalytic activity in air at 700°C, which positions them as potential oxygen electrodes, both for SOFC fuel cells (cathodes) and for SOEC electrolyzers (anodes), as well as their method of obtaining.
[0010] EXPLANATION OF THE INVENTION
[0011] The present invention focuses on the development of ceramic materials with high electrocatalytic activity, making them competitive for use as oxygen electrodes in SOFC fuel cells and SOEC electrolyzers. The materials are transition element oxides, primarily iron, to avoid the use of strategic elements. The invention also relates to a rapid and scalable method for synthesizing these materials, an alternative to the conventional ceramic method, for preparing large quantities.
[0012] The ceramic materials developed are oxides with a perovskite-derived structure, with ABO3 stoichiometry, containing Ba, Ca, and Gd in the A position and Fe doped with Ni or Cu in the B position. The fact that they are based on Fe instead of Co (as occurs in commercial oxides) is advantageous since, unlike Co, which is a critical element, Fe is the third most abundant element in nature, which implies a reduction in the production costs of the materials.
[0013] In particular, the chemical composition of the materials object of the invention is Gdo.8Bao8Cao.4Fe2-xN¡x06.5y Gdo.8Bao8Cao.4Fe2-xCu x 06-5 where 0 < x < 0.20.
[0014] The materials are prepared using two possible methods:
[0015] 1. Ceramic method, by reaction of the oxides Gd2Os, Fe2U3 and NiO or CuO and the carbonates of Ca and Ba (BaCO3 and CaCO3). In this case the reagents are subjected to a first treatment at 1000 °C followed by two others at 1250 °C with intermediate grinding.
[0016] 2. Combustion method, which is based on redox processes where a mixture of a fuel (reducer) and an oxidizer (oxidant) is given the activation energy necessary to start the reaction at a point, so that the energy emitted by the reaction at this point serves to propagate it to the rest of the mixture.
[0017] Combustion synthesis is carried out using a concentrated solution of nitrates of the metal cations in stoichiometric amounts of the phase to be prepared and an organic fuel. Combustion occurs by evaporating the solvent and subsequently igniting the mixture.
[0018] In particular, for these phases, the starting point is an aqueous solution prepared with the minimum amount of water of the corresponding nitrates: Gd(NO3)3-6(H2O), Ba(NO3)2, Ca(NO3)2-4(H2O), Fe(NO3)3-9(H2O) and
[0019] Ni(NO3)3'6(H2O) or Cu(NO3)23(H2O) are used; glycine is used as the organic fuel, and a variable amount of NH4NO3 is added to aid and promote the combustion reaction and reach the appropriate temperature for preparing the desired phase. Once the solvent has evaporated and the resulting solid has been ignited, the solid is collected and pressed into a pellet, which is then placed in a muffle furnace for heat treatment at 1050 °C (for the Cu phase) or 1250 °C (for the Ni phase). The resulting material is finally ground in a ball mill to reduce and homogenize the particle size.
[0020] The combustion method has several advantages over the ceramic method, mainly the reduction of temperature and the preparation time of the materials, in addition to allowing obtaining particles of size below 10 pm.
[0021] The Fe oxides obtained in the present invention show an electrocatalytic activity similar to that of Co oxides, with the additional advantage of having a lower thermal expansion and similar to that of the materials used as common electrolytes in these devices, making them more compatible with them.
[0022] The invention is applicable to the development of electrodes for high-temperature fuel cell (SOFC) technologies, which are energy-generating devices, as well as for high-temperature electrolyzers (SOEC) technologies, which are energy storage devices. Furthermore, fuel cells can use "green" hydrogen as fuel, which can be obtained through water electrolysis using renewable energy sources in high-temperature electrolyzers (SOEC). The technological development of electrolyzers is essential for the hydrogen economy. The implementation of these technologies is limited, in part, by the development of materials with suitable properties, such as the materials of the invention.
[0023] BRIEF DESCRIPTION OF THE DRAWINGS
[0024] To complement the description being made and in order to help a better understanding of the characteristics of the invention, a set of figures is included as an integral part of said description, which show experimental results and in which, for illustrative and non-limiting purposes, the following has been represented:
[0025] Figure 1.- SEM image of an oxide of the Gdo.8Bao8Cao.4Fe2-xNi system x 06-6
[0026] Figure 2.- X-ray diffraction pattern of an oxide of the Gdo.8Bao8Cao.4Fe2-xN¡xO6-6 system
[0027] Figure 3.- X-ray diffraction pattern of a CGO mixture and an oxide of the Gdo.8Bao8Cao.4Fe2-xNi system x 06-6
[0028] Figure 4. - Variation of the polarization resistance with temperature of an oxide of the Gdo.8Bao8Cao.4Fe2-xN system x 06.6 and an oxide of the Gdo.8Bao8Cao.4Fe2- XCU system X O6-6 prepared by the combustion method.
[0029] PREFERRED EMBODIMENT OF THE INVENTION
[0030] The present invention is illustrated by the following examples, which are not intended to limit its scope. The materials described in these examples have been obtained using both the ceramic method and the combustion method as detailed above.
[0031] Example 1.
[0032] This example refers to the structural characterization of the obtained materials. Figure 1 shows an SEM image of an oxide from the Gdao.8Bao8Cao.4Fe2- XN system. X O6-6, where a particle size below 10 microns is observed.
[0033] X-ray diffraction allows monitoring of the material formation reaction, i.e., the degree of purity after preparation. Figure 2 shows an X-ray diffraction pattern resulting from an oxide of the Gdo.8Bao8Cao.4Fe2-xN¡xO6-5 system obtained by the combustion method and by the ceramic method (both are identical), indicating that the oxide is single-phase with a perovskite-derived crystalline structure. EDX (in TEM mode) confirms that the obtained materials have compositions similar to their respective nominal compositions.
[0034] Example 2.
[0035] This example refers to the chemical stability of the materials obtained.
[0036] One of the essential characteristics that the materials obtained must have for use as oxygen electrodes in SOFC type fuel cells or SOEC type electrolyzers is their zero reactivity with the materials as electrolytes at the working temperatures of the devices.
[0037] Stability tests were performed on homogeneous mixtures of 50% of the materials of the invention with 50% CGO (CeO2 doped with Gd2U3) subjected to 900 °C for two weeks. The test results (verified by X-ray diffraction) indicated that the materials of the invention did not react with CGO at 900 °C (a temperature higher than the operating temperature of the devices, which is usually between 700 and 800 °C).
[0038] Figure 3 shows (as an example) an X-ray diffraction pattern of mixtures of one of the compounds of the Gdo.8Bao8Cao.4Fe2-xN system x 06-5 with CGO after treatment at 900 °C.
[0039] Example 3.
[0040] This example relates to the mechanical stability of materials. The mechanical strength of ceramic electrode materials at high operating temperatures is evaluated using the coefficient of thermal expansion (TEC). The TEC values of the electrodes must be similar to those of the electrolytes to ensure mechanical compatibility of the device interfaces.
[0041] In the case of the materials of the invention, the TEC values are determined from the variation of the lattice parameters of the crystalline unit cell with temperature. The lattice parameters are obtained from X-ray diffraction patterns collected within the temperature range for which the TEC value is to be determined. Typical TEC values obtained for the materials are in the range between 15 and 10°C. 6 K' 1 and 13-10 -6 K' 1These values, well below the values of Co oxide used in commercial devices, are on the order of the values of ordinary solid electrolytes, which is of great importance to achieve optimal mechanical compatibility of the cells and thus high durability of these.
[0042] Example 4.
[0043] This example shows the electrocatalytic activity of the materials.
[0044] The electrocatalytic activity of electrode materials for SOFC and SOEC devices is evaluated by measuring complex air impedance in symmetrical cells constructed from a composite of the invention material and the electrolyte (typically 70:30 wt%) as electrodes, and CGO as the electrolyte. The polarization resistance (specific area resistance) of the cell is the sum of the resistance associated with the catalytic activity of the material to oxygen reduction (in the SOFC cell mode) or to ion oxidation (in the SOEC cell mode); the electrical resistance of the electrodes to the passage of electric current and the conduction of oxide ions; and the resistance to charge transfer (oxide ions) at the electrode / electrolyte interfaces.
[0045] The materials developed in this invention exhibit specific area strengths below 0.1 Q cm 2at 700 °C, for both the samples prepared by the ceramic method and by combustion. These are very low values compared to other materials found in the literature (Table 1). In some cases, the ASR values reported for other materials are close to those found for the materials of this invention. However, those materials have high contents of critical elements (for example, Pro-9I-1BaCo-14NiO-2O-6, SrCoO-8FeO-1N2O-1O-3, NdO-2Pro-1NiO-2O-3, NdO-2Pro-1NiO-2O-3) and / or greater thermal expansion, or are prepared by less scalable synthesis methods (SrN2O-2FeO-2O-3 and SrN2O-1WO-1FeO-8O-3). 3-5 , For example).
[0046] Figure 5 shows the variation of the polarization resistance versus the inverse of the temperature of two of the materials of the invention as an example.
[0047] Example 5.
[0048] In this example, the materials described in the present invention are compared with materials close to the invention reported in the scientific literature.
[0049] Table 1 shows this comparison. It indicates the ASR and TEC values of the materials, as well as the synthesis method used for their preparation. It also indicates the improvement of the materials of the invention over those previously reported in terms of Co content, electrocatalytic properties, and mechanical properties.
[0050] Table 1. Comparison of previously known materials and the material of the invention
Claims
CLAIMS 1. Fe oxides with a perovskite-derived structure and ABO3 stoichiometry containing Ba, Ca and Gd in the A position and containing Fe doped with Ni or Cu in the B position.
2. Fe oxides, according to claim 1, of composition Gdo.8Bao8Cao.4Fe2-xNi x 06-5 where 0 < x < 0.
20.
3. Fe oxides, according to claim 1, of composition and grade 8BaO8CaO4Fe2- x Cu x 06-5 where 0 < x < 0.
20.
4. Method for obtaining the claimed Fe oxides comprising reacting the oxides by reaction of the oxides Gd2U3, Fe2U3 and NiO or CuO and the carbonates BaCOs and CaCOs by means of a first treatment at 1000 °C followed by two others at 1250 °C with intermediate grinding.
5. A method for obtaining the claimed Fe oxides comprising preparing an aqueous solution of nitrates of the metal cations in the stoichiometric amounts of the phase to be prepared and an organic fuel, evaporating the solvent and subsequent ignition of the mixture to produce combustion, wherein the aqueous solution is prepared with the minimum amount of water from the corresponding nitrates: Gd(NO3)3'6(H2O), Ba(NO3)2, Ca(NO3)2'4(H2O), Fe(NO3)3'9(H2O) and Ni(NO3)3-6(H2O) or Cu(NO3)2'3(H2O); glycine is used as the organic fuel and, to aid and promote the combustion reaction and reach the appropriate temperature for preparing the desired phase, a variable amount of NH4NO3 is added.
6. Use of the claimed Fe oxides as an oxygen electrode.
7. Use, according to claim 6, as a cathode in SOFC (Solid Oxide Fuel Cell) type fuel cells.
8. Use, according to claim 6, as an anode in SOEC (Solid Oxide Electrolysis Cell) type electrolyzers 9. Oxygen electrode containing the claimed Fe oxides.