ES2994204A1 - Iron oxides as electrode materials for fuel cells and high-temperature electrolyzers - Google Patents

Abstract

Iron oxides as materials for fuel cell electrodes and high-temperature electrolysers. The development of high-performance materials at relatively low temperatures (700°C) for SOFC (Solid Oxide Fuel Cell) electrodes and SOEC (Solid Oxide Electrolysis Cell) electrolysers 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. The present invention proposes new oxides based on Fe doped with Ni and Cu that present high electrocatalytic activity in air and at 700°C, which places them as potential oxygen electrodes, both for SOFC fuel cells (cathodes) and for SOEC electrolysers (anodes), as well as their method of obtaining. (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

Iron oxides as electrode materials for fuel cells and high-temperature electrolyzers

Technical sector

The present invention relates to ceramic materials with high electrocatalytic activity, their method of obtaining and their use as oxygen electrodes for application in SOFC type fuel cells and SOEC type electrolysers.

Background of the invention

Currently, the "standard" electroactive material in oxygen electrodes in SOFC (Solid Oxide Fuel Cell) and SOEC (Solid Oxide Electrolysis Cell) is a Mn oxide. The performance of this material has been improved by Co oxides doped with a low concentration of Fe. But Co is a strategic element and, in addition, this type of Co materials have high coefficients of thermal expansion, which is a problem from the point of view of the mechanical resistance of the device.

Therefore, the development of high-performance materials at relatively low temperatures (700°C) for electrodes in SOFC (Solid Oxide Fuel Cell) type fuel cells and SOEC (Solid Oxide Electrolysis Cell) type electrolysers 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.

For this reason, there are different proposals for this type of materials, such as the one shown in document CN102208662A where cobalt-free materials for oxygen electrodes for SOFC and SOEC with ABO<3> perovskite type structure are described. Table 1 lists materials close to the invention reported in the scientific literature, both Co-based materials and Co-free materials (cobalt-free).

The present invention proposes new oxides based on Fe doped with Ni and Cu that present high electrocatalytic activity in air and at 700°C, which places them as potential oxygen electrodes, both for SOFC type fuel cells (cathodes) and for SOEC electrolyzers (anodes), as well as their method of obtaining.

Explanation of the invention

The present invention focuses on the development of ceramic materials with high electrocatalytic activity that makes them competitive for use as oxygen electrodes in SOFC-type fuel cells and SOEC-type electrolysers. The materials are oxides of transition elements, mainly Fe, to avoid the use of strategic elements. The invention also relates to a method of synthesis of the materials as an alternative to the conventional ceramic method, which is fast and scalable for the preparation of large quantities.

The ceramic materials developed are oxides with a structure derived from the perovskite type, with ABO<3> stoichiometry, containing Ba, Ca and Gd in the A position and containing Fe doped with Ni and 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 means a reduction in the production costs of the materials.

In particular, the chemical composition of the materials object of the invention is Gao.sBao.sCao.4Fe2xNixO6-B and Ga0.sBa0.sCa0.4Fe2-xCuxO6-5 where 0 < x < 0.20.

Materials are prepared by two possible methods:

1. Ceramic method, by reaction of the oxides Gd<2>O<3>, Fe<2>O<3> and NiO or CuO and the carbonates of Ca and Ba (BaCO<3> and CaCO<3>). In this case the reagents are subjected to a first treatment at 1000°C followed by two others at 1250°C with intermediate grinding.

2. Combustion method, based on redox processes where a mixture of a fuel (reducer) and an oxidizer (oxidizer) is applied the activation energy necessary to initiate 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.

Combustion synthesis is carried out from an aqueous solution of nitrates of the metal cations in the stoichiometric quantities of the phase to be prepared and an organic fuel. Combustion occurs by evaporating the solvent and then igniting the mixture.

In particular, for these phases, we start from an aqueous solution prepared with the minimum quantity of water of the corresponding nitrates: Gd(NO<3>)<3>-<6>(H<2>O), Ba(NO<3>)<2>, Ca(NO<3>)<2>-<4>(H<2>O), Fe(NO<3>)<3>'<9>(H<2>O) and Ni(NO<3>)<3>'<6>(H<2>O) or Cu(NO<3>)<2>'<3>(H<2>O); glycine is used as organic fuel and, to help and promote the combustion reaction and reach the appropriate preparation temperature of the desired phase, a variable quantity of NH<4>NO<3> is added. Once the solvent has evaporated and the solid obtained has been subsequently ignited, the solid produced is collected and pressed into a pellet which is then introduced into a muffle furnace for heat treatment at 1050°C (for the Cu phase) or 1250°C (for the Ni phase). The material obtained is finally ground in a ball mill to reduce and homogenise the particle size.

The combustion method has several advantages over the ceramic method, mainly the reduction in temperature and preparation time of the materials, in addition to allowing obtaining particles with a size below 10 pm.

The Fe oxides obtained in the present invention show an electrocatalytic activity similar to that of Co oxides, with the additional advantage of presenting a lower thermal expansion and similar to that of the materials used as usual electrolytes in these devices, making them more compatible with these.

The invention is applicable to the development of electrodes for high-temperature fuel cell technologies (SOFC type), which are energy-generating devices, as well as for high-temperature electrolysers (SOEC type), which are energy storage devices. On the other hand, fuel cells can use “green” hydrogen as fuel, which can be obtained, in turn, by electrolysis of water with renewable energies in high-temperature electrolysers (SOEC type). The technological development of electrolysers is essential for the hydrogen economy. The implementation of these technologies is limited, in part, by the development of materials with adequate performance, such as the materials of the invention.

Brief description of the drawings

To complement the description being made and in order to help better understand the characteristics of the invention, a set of figures is included as an integral part of said description, which contain experimental results and where, for illustrative and non-limiting purposes, the following has been represented:

Figure 1.- SEM image of an oxide of the system Gao<.8>Bao<.8>Cao<.4>Fe<2>xNixO<6 -5>

Figure 2.- X-ray diffraction diagram of an oxide of the system Gao<.8>Bao<.8>Cao<.4>Fe<2>xNixO<6>-5.

Figure 3.- X-ray diffraction diagram of a mixture of CGO and an oxide of the Ga0.8Ba0.8Ca0.4Fe2xN ixO6-5 system.

Figure 4. Variation of the polarization resistance with temperature of an oxide of the system Ga<0>.<8>Ba<0>.<8>Ca<0>.<4>Fe<2>xNixO<6 -5>and an oxide of the system Gao.<8>Bao.<8>Cao.<4>Fe<2>xNixO<6 -5>prepared by the combustion method.

Preferred embodiment of the invention

The present invention is illustrated by the following examples, which are not intended to be limiting of its scope. The materials described in these examples have been obtained by both the ceramic method and the combustion method as detailed above.

Example 1.

This example concerns the structural characterization of the materials obtained.

Figure 1 shows a SEM image of an oxide of the Gao.<8>Bao.<8>Cao.<4>Fe<2>xNixO<6 -5> system where a particle size below <1 0> microns can be seen.

X-ray diffraction allows monitoring the formation reaction of materials, i.e. the degree of purity after their preparation. Figure 2 shows an X-ray diffraction diagram resulting from an oxide of the Gao.<8>Bao.<8>Cao.<4>Fe<2>xNixO<6 -5> system obtained by the combustion method and by the ceramic method (both are identical), which indicates that the oxide is monophasic with a crystalline structure derived from perovskite. Using the EDX technique (in TEM mode), it is confirmed that the materials obtained have compositions similar to the respective nominal ones.

Example 2.

This example concerns the chemical stability of the materials obtained.

One of the essential characteristics that the materials obtained must present 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.

Stability tests are carried out on homogeneous mixtures of 50% of the materials of the invention with 50% of CGO (CeO<2>doped with Gd<2>O<3>) subjected to 900°C for two weeks. The test results (verified by X-ray diffraction) indicate that the materials of the invention do not react with CGO at 900°C (higher than the working temperature of the devices, which is usually between 700 and 800°C).

Figure 3 shows (as an example) an X-ray diffraction diagram of mixtures of one of the compounds of the system Ga<0>.<8>Ba<0>.<8>Ca<0>.<4>Fe<2>xNixO<6-5>with<c>G<o>after treatment at 900°C.

Example 3.

This example concerns the mechanical stability of materials.

The mechanical strength of ceramic electrode materials at high working temperatures is assessed by the so-called 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.

In the case of the materials of the invention, the TECs 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 diagrams collected in the temperature range in which the TEC is to be determined. The typical TECs obtained for the materials are in the range between 15-10"6 K-1 and 1310-6 K-1. These values, well below the values of the Co oxide used in commercial devices, are in the order of the values of the usual solid electrolytes, which is of great importance to achieve optimal mechanical compatibility of the cells and, thus, high durability of these.

Example 4.

This example shows the electrocatalytic activity of the materials.

The electrocatalytic activity of electrode materials for SOFC and SOEC devices is evaluated by complex impedance measurements in air in symmetric cells constructed by a "composite" of the inventive material together with the electrolyte (typically 70:30% by weight) as electrodes and CGO as 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 mode cell) or to ion oxidation (in the SOEC mode cell); of the electrical resistance of the electrodes to the passage of electric current and to the conduction of oxide ions; and of the resistance to charge transfer (oxide ions) at the electro/electrolyte interfaces.

The materials developed in this invention present specific area resistances below 0.1 Q cm2 at 700°C, both for the samples prepared by the ceramic method and by the combustion method. 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 have high contents of critical elements (e.g. Pro.9Yo.1BaCo1.4Nio.2O6-5, SrCoo.8Feo.1Nbo.1O3-s, Nd1.5Pro.5Nio.95Moo.o5O4+5, Nd1.5Pro.5NiO4+s) and/or higher thermal expansion, or are prepared by less scalable synthesis methods (SrNb<0>.<2>Fe<0>.<8>O<3>-<5>and SrNb<0>.<1>W<0>.<1>Fe<0>.<8>O<3>-<5>for example).

Figure 5 shows the variation of the polarization resistance versus the inverse of the temperature of two of the materials object of the invention as an example.

Example 5.

In this example, the materials described in the present invention are compared with materials close to the invention reported in the scientific literature.

This comparison is shown in Table 1. The ASR and TEC values of the materials are indicated, as well as the synthesis method used for their preparation. The improvement of the materials of the invention with respect to those reported in terms of Co content, electrocatalytic and mechanical properties is also indicated.

Table 1. Comparison of previously known materials and the material of the invention

Claims (9)

1. Fe oxides with a perovskite-type structure and ABO<3> stoichiometry that contain Ba, Ca and Gd in the A position and contain Fe doped with Ni and Cu in the B position. 2. Fe oxides, according to claim 1, of composition Gao.<8>Bao.<8>Cao.<4>Fe<2>xNixO<6 - 5>where 0 < x <<0>.<2 0>. 3. Fe oxides, according to claim 1, of composition Gao.<8>Bao.<8>Cao.<4>Fe<2>-xCuxO<6-5>where 0 < x < 0.20. 4. Method for obtaining the claimed Fe oxides, which comprises reacting the oxides by reacting the oxides Gd<2>O<3>, Fe<2>O<3> and NiO or CuO and the carbonates BaCO<3> and CaCO<3> by means of a first treatment at 1000°C followed by two others at 1250°C with intermediate grinding. 5. Method for obtaining the claimed Fe oxides comprising preparing an aqueous solution of nitrates of the metal cations in the stoichiometric quantities of the phase to be prepared and an organic fuel, evaporating the solvent and subsequently igniting the mixture to produce combustion, where the aqueous solution is prepared with the minimum quantity of water of the corresponding nitrates: Gd(NO<3>)<3>-<6>(H<2>O), Ba(NO<3>)<2>, Ca(NO<3>)<2>-<4>(H<2>O), Fe(NO<3>)<3>'<9>(H<2>O) and Ni(No<3>)<3>'<6>(H<2>O) or Cu(NO<3>)<2>'<3>(H<2>O); Glycine is used as organic fuel and, to help and promote the combustion reaction and reach the appropriate preparation temperature of the desired phase, a variable quantity of NH<4>NO<3> 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 electrolysers. 9. Oxygen electrode containing the claimed Fe oxides.