JP7650404B2 - Electrochemical Cell - Google Patents

Description

The present invention relates to an electrochemical cell.

An anode-supported fuel cell is known as an example of an electrochemical cell (see, for example, Patent Document 1). An anode-supported fuel cell includes a support layer, a hydrogen electrode disposed on the support layer, an oxygen electrode, and an electrolyte disposed between the hydrogen electrode and the oxygen electrode.

The support layer can be composed of YSZ (yttria-stabilized zirconia) and Ni (nickel), and the hydrogen electrode can be composed of a ceria-based oxide with rare earth elements added and Ni. The Ni contained in the hydrogen electrode and the support layer exists in the form of NiO in an oxidizing atmosphere.

JP 2017-123231 A

When the support layer and the hydrogen electrode are exposed to a reducing atmosphere, the NiO contained in the support layer and the hydrogen electrode is reduced to Ni, causing a volume change in each of the support layer and the hydrogen electrode. In addition, the ceria (CeO ₂ ) in the hydrogen electrode expands greatly in a reducing atmosphere, whereas the zirconia (ZrO ₂ ) in the support layer expands relatively little in a reducing atmosphere, so that a difference in the amount of expansion occurs between the hydrogen electrode and the support layer in a reducing atmosphere. When start-up and stop are repeated under this condition, the stress accumulated in the hydrogen electrode causes cracks.

The object of the present invention is to provide an electrochemical cell capable of suppressing cracks in the hydrogen electrode.

The electrochemical cell according to the present invention comprises a support layer, an intermediate layer disposed on the support layer, a hydrogen electrode disposed on the intermediate layer, an oxygen electrode, and an electrolyte disposed between the hydrogen electrode and the oxygen electrode. The support layer is composed of yttria-stabilized zirconia and nickel. The hydrogen electrode is composed of a ceria-based oxide having a rare earth element added thereto, and nickel. The intermediate layer is composed of a solid solution of yttria-stabilized zirconia and a ceria-based oxide having a rare earth element added thereto, and nickel.

According to the present invention, an electrochemical cell can be provided that can suppress cracks in the hydrogen electrode.

FIG. 1 is a cross-sectional view of a fuel cell.

(Fuel cell 10)
The fuel cell 10 is a so-called anode-supported type fuel cell, and includes a support layer 11, an intermediate layer 12, a hydrogen electrode 13, an electrolyte 14, a reaction prevention layer 15, and an oxygen electrode 16.

[Support layer 11]
The support layer 11 supports the intermediate layer 12. The shape of the support layer 11 is not particularly limited, and it can be formed into, for example, a plate shape, a hollow plate shape, a cylinder shape, or the like.

The support layer 11 is made of a porous material having electronic conductivity. Specifically, the support layer 11 contains YSZ (yttria-stabilized zirconia) and Ni (nickel). As the YSZ, ZrO ₂ (zirconia) stabilized with 3 mol % to 10 mol % of Y ₂ O ₃ (yttria) can be used. Ni becomes metallic Ni in a reducing atmosphere where a fuel gas is supplied, and becomes NiO in an oxidizing atmosphere where no fuel gas is supplied.

The YSZ content in the support layer 11 can be 25 mol% or more and 55 mol% or less. The Ni content in the support layer 11 can be 45 mol% or more and 75 mol% or less in terms of NiO.

The respective contents of YSZ and Ni in the support layer 11 are obtained by line analysis using atomic concentration profiles, i.e., element mapping using an EPMA (Electron Probe Micro Analyzer). Specifically, concentration distribution data for each element is obtained by performing line analysis in the thickness direction using an EPMA on a cross section along the thickness direction. Note that EPMA is a concept that includes EDS (Energy Dispersive x-ray Spectroscopy).

The porosity of the support layer 11 can be, for example, 10% or more and 50% or less. In this specification, the porosity is the ratio of the area of the gas phase to the total area of the solid phase and the gas phase in cross-sectional observation using a SEM (scanning electron microscope). The thickness of the support layer 11 can be, for example, 50 μm or more and 1 mm or less.

[Intermediate layer 12]
The intermediate layer 12 is disposed on the support layer 11. The intermediate layer 12 is disposed between the support layer 11 and the hydrogen electrode 13.

The intermediate layer 12 is composed of a porous material having electronic and ionic conductivity. Specifically, the intermediate layer 12 contains a solid solution of YSZ and a ceria-based oxide to which a rare earth element has been added, and Ni. The solid solution refers to a state in which YSZ and a ceria-based oxide to which a rare earth element has been added are dissolved together to form a uniform solid phase.

When the support layer 11 and the hydrogen electrode 13 are exposed to a reducing atmosphere, the NiO contained in the support layer 11 and the hydrogen electrode 13 is reduced to Ni, causing a change in volume in each of the support layer 11 and the hydrogen electrode 13. In addition, while the _CeO2 in the hydrogen electrode 13 expands greatly in a reducing atmosphere, the _ZrO2 in the support layer 11 expands relatively little in a reducing atmosphere, causing a difference in the amount of expansion between the hydrogen electrode 13 and the support layer 11 in a reducing atmosphere. If the fuel cell 10 is repeatedly started and stopped under this circumstance, cracks are likely to occur due to the stress accumulated in the hydrogen electrode 13.

Therefore, the intermediate layer 12 according to this embodiment contains both YSZ, which is the constituent material of the support layer 11, and a ceria-based oxide with added rare earth elements, which is the constituent material of the hydrogen electrode 13 described below. This intermediate layer 12 functions as a buffer layer that buffers the difference in the amount of expansion between the hydrogen electrode 13 and the support layer 11, thereby making it possible to suppress the accumulation of stress in the hydrogen electrode 13 compared to when the support layer 11 and the hydrogen electrode 13 are directly connected. This makes it possible to suppress the occurrence of cracks in the hydrogen electrode 13.

The intermediate layer 12 is a region between the support layer 11 and the hydrogen electrode 13, in which a solid solution of YSZ and a ceria-based oxide to which a rare earth element is added exists. Examples of ceria-based oxides to which a rare earth element is added include, but are not limited to, gadolinium-doped ceria (GDC), samarium-doped ceria (SDC), and yttrium-doped ceria (YDC). CeO ₂ (ceria) doped with 5 mol % to 20 mol % of Gd (gadolinium) oxide can be used as GDC. Ni becomes metallic Ni in a reducing atmosphere to which a fuel gas is supplied, and becomes NiO in an oxidizing atmosphere to which no fuel gas is supplied.

The YSZ content in the intermediate layer 12 can be 15 mol% or more and 35 mol% or less. The ceria-based oxide to which a rare earth element has been added in the intermediate layer 12 can be 15 mol% or more and 35 mol% or less. The Ni content in the intermediate layer 12 can be 35 mol% or more and 65 mol% or less in terms of NiO. The respective contents of YSZ, ceria-based oxide to which a rare earth element has been added, and Ni in the support layer 11 are obtained by line analysis using the above-mentioned atomic concentration profile.

The thickness of the intermediate layer 12 can be, for example, 1 μm or more and 50 μm or less. The thickness of the intermediate layer 12 is preferably 20 μm or less. This can prevent the ion conduction path in the intermediate layer 12 from being broken, and therefore can prevent the initial performance of the fuel cell 10 from being reduced. The thickness of the intermediate layer 12 is preferably 3 μm or more. This can further prevent cracks from occurring in the hydrogen electrode 13. Therefore, by setting the thickness of the intermediate layer 12 to 3 μm or more and 20 μm or less, it is possible to ensure the initial performance of the fuel cell 10 and further prevent cracks from occurring at the same time.

The thickness of the intermediate layer 12 is obtained by observing a cross section of the intermediate layer 12 along the thickness direction at 1000x magnification using an FE-SEM, and calculating the arithmetic average of the thicknesses of the intermediate layer 12 at five points randomly selected from the observed image. The thickness direction is the direction perpendicular to the hydrogen electrode side surface of the electrolyte 14.

The porosity of the intermediate layer 12 can be, for example, greater than or equal to 10% and less than or equal to 50%.

[Hydrogen electrode 13]
The hydrogen electrode 13 is disposed on the intermediate layer 12. The hydrogen electrode 13 is disposed between the intermediate layer 12 and the electrolyte 14. A fuel gas is supplied to the hydrogen electrode 13 via the support layer 11 and the intermediate layer 12. In the hydrogen electrode 13, an electrode reaction represented by the following formula (1) occurs.

_H2 + ^O2- → _H2O +2e ^-... (1)

The hydrogen electrode 13 is made of a porous material having electronic conductivity and ionic conductivity. Specifically, the hydrogen electrode 13 includes a ceria-based oxide to which a rare earth element is added and Ni. Examples of the ceria-based oxide to which a rare earth element is added include, but are not limited to, GDC, SDC, and YDC. CeO ₂ (ceria) doped with 5 mol % to 20 mol % of an oxide of Gd (gadolinium) can be used as GDC. The ceria-based oxide to which a rare earth element is added contained in the hydrogen electrode 13 may be the same type as or different from the ceria-based oxide to which a rare earth element is added contained in the intermediate layer 12. Ni becomes metallic Ni in a reducing atmosphere to which a fuel gas is supplied, and becomes NiO in an oxidizing atmosphere to which no fuel gas is supplied. In this way, the hydrogen electrode 13 contracts and expands in response to the reduction and oxidation of Ni. However, as described above, the volume change of the hydrogen electrode 13 is absorbed by the intermediate layer 12, thereby preventing cracks from occurring in the hydrogen electrode 13.

The content of the ceria-based oxide to which a rare earth element has been added in the hydrogen electrode 13 can be 35 mol% or more and 65 mol% or less. The content of Ni in the hydrogen electrode 13 can be 35 mol% or more and 65 mol% or less in terms of NiO. The respective contents of the ceria-based oxide to which a rare earth element has been added and Ni in the hydrogen electrode 13 are obtained by line analysis using the atomic concentration profile described above.

The porosity of the hydrogen electrode 13 can be, for example, 10% or more and 50% or less. The thickness of the hydrogen electrode 13 can be, for example, 5 μm or more and 0.1 mm or less.

[Electrolyte 14]
The electrolyte 14 is disposed on the hydrogen electrode 13. The electrolyte 14 is disposed between the hydrogen electrode 13 and the oxygen electrode 16. In this embodiment, since the fuel cell 10 is provided with a reaction prevention layer 15, the electrolyte 14 is sandwiched between the hydrogen electrode 13 and the reaction prevention layer 15.

The electrolyte 14 is made of a dense material having ion conductivity but no electronic conductivity. The electrolyte 14 can be made of, for example, ZrO ₂ stabilized with Y ₂ O ₃ , CeO ₂ , Sc ₂ O ₃ , Yb ₂ O _{3 ,} or CeO ₂ doped with Y ₂ O ₃ , Sm ₂ O ₃ , Gd ₂ O _{3 ,} or the like. Alternatively, the electrolyte 14 can be made of lanthanum gallate, or a lanthanum gallate-type perovskite structure oxide in which a portion of the lanthanum or gallium of the lanthanum gallate is replaced with strontium, calcium, barium, magnesium, aluminum, indium, cobalt, iron, nickel, copper, or the like. The electrolyte 14 may be made of one type of electrolyte material, or may be made of two or more types of electrolyte materials.

The porosity of the electrolyte 14 can be, for example, 0% or more and 7% or less. The thickness of the electrolyte 14 can be, for example, 3 μm or more and 50 μm or less. The electrolyte 14 may have a single layer structure or a multilayer structure.

[Reaction prevention layer 15]
The reaction prevention layer 15 is disposed on the electrolyte 14. The reaction prevention layer 15 is disposed between the electrolyte 14 and the oxygen electrode 16. The reaction prevention layer 15 is provided to prevent a reaction between the constituent material of the electrolyte 14 and the constituent material of the oxygen electrode 16, resulting in the formation of a reaction layer with high electrical resistance.

The reaction prevention layer 15 is made of an ion-conductive material. The reaction prevention layer 15 can be made of ceria doped with oxides of rare earth elements such as Gd, Sm, and Y.

The porosity of the reaction prevention layer 15 can be, for example, 0.1% or more and 50% or less. The thickness of the reaction prevention layer 15 can be, for example, 1 μm or more and 50 μm or less.

[Oxygen electrode 16]
The oxygen electrode 16 is disposed on the reaction prevention layer 15. A gas containing oxygen (e.g., air) is supplied to the oxygen electrode 16. In the oxygen electrode 16, an electrode reaction represented by the following formula (2) occurs.

(1/2)・O ₂ +2e ⁻ →O ^{2 −} …(2)

The oxygen electrode 16 is made of a porous material having electronic conductivity, and can be made of, for example, LSCF=(La,Sr)(Co,Fe) _O3 (lanthanum strontium cobalt ferrite), LSF=(La,Sr) _FeO3 (lanthanum strontium ferrite), LNF=La(Ni,Fe) _O3 (lanthanum nickel ferrite), LSC=(La,Sr) _CoO3 (lanthanum strontium cobaltite), or the like.

The porosity of the oxygen electrode 16 can be, for example, 10% or more and 50% or less. The thickness of the oxygen electrode 16 can be, for example, 10 μm or more and 100 μm or less.

(Modification of the embodiment)
Although the embodiments of the present invention have been described above, the present invention is not limited to these, and various modifications are possible without departing from the spirit of the present invention.

[Modification 1]
In the above embodiment, a so-called anode-supported fuel cell has been described as an example of a fuel cell, but the electrochemical cell is not limited to this. The present invention is applicable to an electrochemical cell in which a hydrogen electrode and an oxygen electrode are disposed on either side of an electrolyte layer.

An electrochemical cell is a general term for an element in which a pair of electrodes are arranged so that an electromotive force is generated from an overall oxidation-reduction reaction in order to convert electrical energy into chemical energy, and an element for converting chemical energy into electrical energy.

Examples of electrochemical cells include anode-supported fuel cells, horizontally striped fuel cells, vertically striped fuel cells, flat plate fuel cells, cylindrical fuel cells, and electrolysis cells that generate hydrogen by utilizing the electrolysis reaction of water. In the above embodiment, O ²⁻ (oxygen ion) is used as the carrier, but OH ⁻ (hydroxide ion) or protons may also be used as the carrier.

[Modification 2]
In the above embodiment, the fuel cell 10 is provided with the reaction prevention layer 15 , but the reaction prevention layer 15 may not be provided.

Below, we will explain examples of the electrolytic cell according to the present invention, but the present invention is not limited to the examples described below.

(Preparation of Examples 1 to 7)
The electrolytic cells according to Examples 1 to 7 were prepared as follows.

First, a support layer molded body was produced by sheet molding a support layer slurry made by mixing YSZ powder, NiO powder, binder, pore-forming agent, plasticizer, dispersion medium and solvent.

Next, an intermediate layer paste prepared by mixing YSZ powder, GDC powder, NiO powder, binder, pore former, plasticizer, dispersion medium, and solvent was printed on the support layer body to form an intermediate layer body. At this time, the thickness of the intermediate layer was adjusted for each of Examples 1 to 7 by changing the printing thickness of the intermediate layer paste, as shown in Table 1.

Next, a hydrogen electrode paste prepared by mixing GDC powder, NiO powder, binder, pore-forming agent, plasticizer, dispersion medium and solvent was printed onto the intermediate layer compact to form a hydrogen electrode compact.

Next, an electrolyte paste prepared by mixing YSZ powder, binder, plasticizer, dispersion medium and solvent was printed onto the hydrogen electrode compact to form an electrolyte compact.

Next, a reaction prevention layer paste prepared by mixing GDC powder, a binder, and a solvent was printed onto the electrolyte molded body to form a reaction prevention layer molded body.

A laminate of a support layer molded body, an intermediate layer molded body, a hydrogen electrode molded body, an electrolyte molded body and a reaction prevention layer molded body was fired (1300°C, 5 hours) to produce a fired body consisting of a support layer, an intermediate layer, a hydrogen electrode, an electrolyte and a reaction prevention layer.

An oxygen electrode paste prepared by mixing LSCF powder, binder and solvent was printed onto the reaction prevention layer to form an oxygen electrode body.

Next, the oxygen electrode molded body was sintered (1050°C, 2 hours) to obtain the electrolytic cells of Examples 1 to 7.

(Preparation of Comparative Example 1)
An electrolytic cell according to Comparative Example 1 was produced by the same steps as those of Examples 1 to 7, except that no intermediate layer was formed.

(Thermal cycle test)
A first thermal cycle test was carried out on the electrolytic cells according to Examples 1 to 7 and Comparative Example 1. Specifically, while supplying Ar gas and hydrogen gas (4% relative to Ar gas) to the hydrogen electrode, one cycle was repeated 10 times, in which the temperature was increased from room temperature to 750° C. in 2 hours and then decreased from 750° C. to room temperature in 4 hours.

The cross section of the hydrogen electrode was then observed using a FE-SEM (field emission scanning electron microscope) to check for the presence or absence of cracks.

Next, a second thermal cycle test was conducted on the electrolytic cells according to Examples 1 to 7 and Comparative Example 1. Specifically, while supplying Ar gas and hydrogen gas (4% relative to Ar gas) to the hydrogen electrode, one cycle was repeated 10 times, in which the temperature was increased from room temperature to 850°C in 2 hours and then decreased from 850°C to room temperature in 4 hours.

Then, the cross section of the hydrogen electrode was observed using FE-SEM to check for the presence or absence of cracks.

In Table 1, samples that developed cracks in the first thermal cycle test were rated as "X", samples that developed cracks for the first time in the second thermal cycle test were rated as "O", and samples that did not develop cracks in the second thermal cycle test were rated as "◎".

(Initial performance evaluation test)
An initial performance evaluation test was carried out on the electrolytic cells according to Examples 1 to 7 and Comparative Example 1. Specifically, the electrolytic voltage was measured when a mixed gas of water vapor and hydrogen gas (mixing ratio 50:50) was supplied to the hydrogen electrode and air was supplied to the oxygen electrode while sweeping a current value of 0.5 A/ ^cm2 .

Then, the increase rate of the electrolysis voltage of Examples 1 to 7 was calculated relative to the electrolysis voltage of Comparative Example 1. In Table 1, the increase rate of the electrolysis voltage of less than 1% was evaluated as "good", and the increase rate of the electrolysis voltage of 1% or more was evaluated as "good".

As shown in Table 1, in Examples 1 to 7 in which an intermediate layer was provided between the support layer and the hydrogen electrode, the occurrence of cracks in the hydrogen electrode was suppressed. This result was obtained because the intermediate layer contained both YSZ, which is the constituent material of the support layer, and ceria-based oxide with added rare earth elements, which is the constituent material of the hydrogen electrode, and thus the intermediate layer was able to buffer the difference in the amount of expansion between the hydrogen electrode and the support layer during the thermal cycle.

In addition, in Examples 1 to 6, in which the thickness of the intermediate layer was set to 20 μm or less, it was possible to prevent the initial performance from decreasing compared to Example 7. This result was obtained because it was possible to prevent the ion conduction path in the intermediate layer from being broken.

Furthermore, in Examples 2 to 7, in which the thickness of the intermediate layer was 3 μm or more, the occurrence of cracks in the hydrogen electrode was further suppressed compared to Example 1.

10 Fuel cell 11 Support layer 12 Intermediate layer 13 Hydrogen electrode 14 Electrolyte 15 Reaction prevention layer 16 Oxygen electrode

Claims (1)

A supporting layer having electronic conductivity;
an intermediate layer disposed on the support layer;
a hydrogen electrode disposed on the intermediate layer;
An oxygen electrode;
an electrolyte disposed between the hydrogen electrode and the oxygen electrode;
Equipped with
The support layer comprises yttria-stabilized zirconia and nickel;
The hydrogen electrode contains a ceria-based oxide to which a rare earth element is added and nickel,
the intermediate layer contains nickel and a solid solution of yttria-stabilized zirconia and a ceria-based oxide to which a rare earth element is added;
The thickness of the intermediate layer is 3 μm or more and 20 μm or less.
Electrochemical cell.

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