KR102220867B1 - Solid oxide fuel cell having durable electrolyte under negative current conditions - Google Patents

Abstract

The present invention provides a solid oxide fuel cell stacked in the order of a cathode layer, an electrolyte layer, and a cathode layer, wherein the electrolyte layer comprises a first electrolyte layer in contact with the anode layer and a first electrolyte layer in contact with the cathode layer. Consisting of a bi-layer structure including a second electrolyte layer, the first electrolyte layer in contact with the anode layer is a praseodymium cerium oxide (PCO) solid oxide doped with praseodymium (Pr) and A solid oxide fuel cell comprising YSZ (Yttria-stabilized zirconia), wherein the second electrolyte layer in contact with the cathode layer is made of YSZ,
In the solid oxide fuel cell to which a bi-layered electrolyte layer according to the present invention is applied, only the first electrolyte layer, which is the electrolyte layer on the cathode/electrolyte interface, has electron conductivity (P-type), so that the cathode is in a reverse current situation. /Electrolyte peeling can be effectively prevented, and leakage current can also be prevented because only one layer of the electrolyte has electronic conductivity.

Description

Solid oxide fuel cell preventing deterioration due to reverse current {SOLID OXIDE FUEL CELL HAVING DURABLE ELECTROLYTE UNDER NEGATIVE CURRENT CONDITIONS}

The present invention relates to a solid oxide fuel cell having a novel stacked structure, and more particularly, comprising an electrolyte layer having a structure capable of preventing deterioration of an SOFC cell stack in a negative current condition. It is about solid oxide fuel cells.

A solid oxide fuel cell (SOFC) is a battery that converts chemical energy into electrical energy through a chemical reaction between hydrogen and oxygen. The solid oxide fuel cell stacks several cells that generate an electromotive force of up to about 1V per unit cell to produce a desired output. During such a stack operation, the supply of hydrogen fuel may be interrupted due to a system malfunction or natural disaster such as an earthquake. When the fuel supply is interrupted, the load is lowered and electricity production is stopped. If the supply of hydrogen is stopped at high temperatures, the nickel anode can be re-oxidized. When nickel is re-oxidized, it causes physical damage to the microstructure due to volume expansion of the anode, resulting in a decrease in the Triple Phase Boundary (TPB), resulting in a permanent decrease in performance. Therefore, in order to secure the durability of the battery when fuel supply is stopped, a technology capable of protecting the anode is required.

Looking at the potential at which Ni-NiO is in equilibrium (FIG. 1), it can be seen that an open circuit voltage of at least 0.7V or more must be maintained at an operating temperature of 700 to 800°C. However, when the fuel supply is interrupted, since the open circuit voltage of the cell can be reduced to 0.7V or less, reoxidation of nickel may occur, which may eventually lead to stack failure. The most effective way to keep the cell voltage high even when the hydrogen supply is interrupted is to reverse the current to maintain above the equilibrium potential. When current is supplied to the solid oxide battery, oxygen molecules that have penetrated toward the nickel anode are reduced (ionized) when the supply of hydrogen is stopped and passed through the oxygen ion conductive electrolyte, and the oxygen molecules can be discharged back to the oxygen molecule through oxidation at the opposite cathode (Fig. 2 and 3). A zirconia (YSZ) electrolyte doped with 8 mol% yttria, which is the most widely used material as an electrolyte for a solid oxide fuel cell, is very vulnerable under such reverse current operating conditions. This is because during reverse current operation, an abnormally high oxygen chemical potential is formed inside the solid electrolyte, which may cause separation between the electrode and the electrolyte (FIG. 4). That is, in the case of an electrolyte cell made of pure YSZ only, reverse current supply may prevent nickel reoxidation, but may cause a peeling phenomenon between the cathode/electrolyte.

Korean Patent Application Publication No. 10-2015-0128715 (published date: 2015. 11. 18) International Publication Patent WO 2010-078356 (Publication date: 2010. 07. 08)

The technical problem to be solved by the present invention is that it is possible to effectively prevent the separation of the cathode/electrolyte in a reverse current situation, which is a problem of the prior art, and has an electronic conductivity in only one layer inside the electrolyte, so that a solid that can also prevent leakage current. It is to provide oxide fuel cells.

When a cell using an existing pure Yttria-stabilized zirconia (YSZ) electrolyte is driven as an SOEC, not only a decrease in voltage per hour, but also a decrease in open circuit voltage and power density after operation, as well as a decrease in the open circuit voltage and power density, may occur at the interface between the cathode and the electrolyte.

The cause of the separation between the cathode and the electrolyte interface is due to the high oxygen partial pressure at the cathode/electrolyte interface due to reverse current. To prevent the separation, it is important to reduce the oxygen partial pressure in the electrolyte region close to the cathode/electrolyte interface. Do.

When YSZ containing a material such as PCO that exhibits electron conductivity is used as an electrolyte, mixed conductivity by PCO dopant occurs (electron hole conduction + ion conduction), so even when reverse current is operated, the oxygen partial pressure is stably maintained between the cathode and the electrolyte. It can prevent peeling.

However, when PCO is added to all regions of the YSZ electrolyte, it is important to provide mixed conductivity locally because the entire region of the electrolyte becomes a mixed conductor, and leakage current may be generated to degrade performance.

Accordingly, in the present invention, electron conductivity is provided only in the electrolyte region on the cathode/electrolyte interface where the oxygen partial pressure is formed high, and the other layer is a pure YSZ layer to prevent leakage current, thereby achieving both basic performance and stability against reverse current. As a solid oxide fuel cell, a pure YSZ layer is placed on the anode to reduce the oxygen partial pressure formed in the electrolyte region adjacent to the cathode during reverse current operation, and a YSZ layer added with a material having P-type mixed conductivity is placed on the cathode. A solid oxide fuel cell having a bi-layer structure is provided (FIG. 5).

That is, in order to achieve the above-described technical problem, the present invention provides the electron conductivity of the electrolyte region close to the air electrode (anode) where peeling occurs due to high oxygen partial pressure, the cathode layer, the electrolyte layer, and the cathode In the solid oxide fuel cell stacked in the order of (anode) layers, the electrolyte layer has a bi-layer structure of a first electrolyte layer in contact with an anode layer and a second electrolyte layer in contact with a cathode layer, , The first electrolyte layer in contact with the anode layer includes praseodymium cerium oxide (PCO) solid oxide and YSZ (Yttria-stabilized zirconia) doped with praseodymium (Pr), and a second electrolyte layer in contact with the anode layer The electrolyte layer is made of YSZ, and provides a solid oxide fuel cell in which deterioration due to reverse current is prevented.

In this case, the cathode layer may include an anode support (AS) and an anode functional layer (AFL) formed on the anode support layer.

The negative electrode support layer and the negative electrode functional layer may be made of a nickel (Ni)-based material, and further, the negative electrode support layer and the negative electrode functional layer may include nickel and YSZ.

In addition, the anode layer may include a cathode functional layer (CFL) formed on the electrolyte layer and an anode collector layer (Current Collector, CC) formed on the anode functional layer, for example, the The anode functional layer may include Sr-doped LaMnO3 (LSM) and YSZ, and the anode current collector layer may include LSM.

In another aspect, the present invention provides a solid oxide fuel cell stack including a plurality of unit cells made of the solid oxide fuel cell.

In the solid oxide fuel cell to which a bi-layered electrolyte layer according to the present invention is applied, only the first electrolyte layer, which is the electrolyte layer on the cathode/electrolyte interface, has electron conductivity (P-type), so that the cathode is in a reverse current situation. /Electrolyte peeling can be effectively prevented, and leakage current can also be prevented because only one layer of the electrolyte has electronic conductivity.

1 is a graph showing the potential at which Ni-NiO equilibrates with temperature.
2 is a graph showing a voltage increase according to a voltage decrease and reverse current supply in a situation in which fuel supply is stopped.
3 is a schematic diagram showing an oxygen molecule reduction reaction and an oxygen ion oxidation reaction occurring in each of the anode and the cathode that occur when a reverse current is supplied to the solid oxide cell.
4 is a schematic cross-sectional view of a solid oxide fuel cell including an electrolyte consisting of only conventional Yttria-stabilized zirconia (YSZ).
5 is a schematic cross-sectional view of a solid oxide fuel cell including an electrolyte having a bi-layer structure according to the present invention.
6 is a flowchart illustrating each step of a process of manufacturing a solid oxide fuel cell cell according to an exemplary embodiment of the present application.
FIG. 7(a) is a graph showing the voltage change over time when a constant voltage (CV) is applied with the supply of hydrogen fuel to a solid oxide fuel cell including a conventional YSZ electrolyte layer, and FIG. 7(b) is a corresponding constant voltage test. It is a scanning electron microscope (SEM) photograph showing the microstructure of the cell cross-section after.
FIG. 8(a) is a graph showing a voltage change over time when a constant voltage (CV) is applied to a solid oxide fuel cell cell including a bi-layer electrolyte prepared in the present embodiment, and FIG. 8( b) is a graph showing the IV measurement results before and after the corresponding constant voltage test.

In describing the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, a detailed description thereof will be omitted.

Since the embodiments according to the concept of the present invention can apply various changes and have various forms, specific embodiments are illustrated in the drawings and will be described in detail in the present specification or application. However, this is not intended to limit the embodiments according to the concept of the present invention to a specific form of disclosure, and it should be understood that all changes, equivalents, and substitutes included in the spirit and scope of the present invention are included.

The terms used in the present specification are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In the present specification, terms such as "comprise" or "have" are intended to designate the presence of a set feature, number, step, action, component, part, or combination thereof, but one or more other features or numbers It is to be understood that the possibility of addition or presence of, steps, actions, components, parts, or combinations thereof is not preliminarily excluded.

Hereinafter, the present invention will be described in more detail through examples.

<Example> Preparation of a unit cell having a bi-layer structure electrolyte according to the present invention

As shown in FIG. 6, the following steps (1) to (8) are sequentially performed to prepare a solid electrolyte fuel cell unit cell having an electrolyte of a bi-layer structure having a cross-sectional schematic diagram shown in FIG. I did.

(1) Anode support layer (AS) was prepared by die-pressing anode powder consisting of NiO and YSZ and then heat treatment at 900°C for 1 hour.

(2) To prepare the negative electrode functional layer (AFL), NiO and YSZ were mixed at a ratio of 60 wt%: 40 wt%, respectively, and an additional pulverization process was performed using a planetary ball mill to refine the particles.

(3) After pulverization, a slurry was prepared using a butanol solvent, and the AS layer prepared above was drop-coated and heat treated at 900°C for 1 hour.

(4) Drop coating of YSZ slurry on the heat-treated AS/AFL substrate was performed at 1000° C. for 1 hour.

(5) PCO: YSZ was added to the butanol solvent in a ratio of 8 mol%: 92 mol%, respectively, and a dispersant in an amount of 1% of the total powder was added so that the two types of powder were evenly dispersed.

(6) The prepared slurry was well mixed using sonication, and then drop-coated on the pure YSZ layer and sintered at 1460°C for 6 hours.

(7) The anode functional layer mixed with LSM+YSZ was screen printed on the sintered cell and heat-treated at 1170℃ for 1 hour.

(8) An anode made of LSM was screen printed on the anode functional layer and heat-treated at 1160℃ for 1 hour.

<Experimental Example> Reverse current durability test

In order to check the durability under reverse current operation, an initial performance check was performed, and a reverse current was supplied for a total of 100 hours after the hydrogen supply was cut off, and 0.8V was maintained so that the cell open circuit voltage became more than the nickel-nickel oxide equilibrium potential.

In the case of the pure YSZ electrolyte cell, when hydrogen was resupplied after the reverse current test, the 1.0 V open circuit voltage was not recovered and showed very unstable open circuit voltage (Fig. 7a).According to the SEM analysis result after the test, separation between the cathode and the electrolyte interface was observed. Was observed (Fig. 7b).

On the other hand, the Bi-layer cell showed an I-V curve similar to the performance of 100 hours ago even after the 0.8 V maintenance reverse current test for about 100 hours (FIG. 8A) (FIG. 8B).

In the solid oxide fuel cell to which the electrolyte layer having a bi-layer structure according to the present invention described in detail above is applied, only the first electrolyte layer, which is an electrolyte layer at the cathode/electrolyte interface, has electron conductivity (P-type), and thus reverse current In this situation, it is possible to effectively prevent the cathode/electrolyte from peeling, and since only one layer of the electrolyte has electron conductivity, leakage current can also be prevented.

In the above, embodiments of the present invention have been described with reference to the accompanying drawings, but those of ordinary skill in the art to which the present invention pertains can be implemented in other specific forms without changing the technical spirit or essential features. You can understand that there is. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not limiting.

Claims (8)

In the solid oxide fuel cell stacked in order of an anode layer, an electrolyte layer, and a cathode layer,
The electrolyte layer has a bi-layer structure including a first electrolyte layer in contact with the anode layer and a second electrolyte layer in contact with the cathode layer,
The first electrolyte layer in contact with the anode layer includes praseodymium cerium oxide (PCO) solid oxide and YSZ (Yttria-stabilized zirconia) doped with praseodymium (Pr),
The solid oxide fuel cell, characterized in that the second electrolyte layer in contact with the negative electrode layer is made of YSZ. The method of claim 1,
The first electrolyte layer, characterized in that containing 8 mol% of PCO, a solid oxide fuel cell in which deterioration due to reverse current is prevented. The method of claim 1,
The cathode layer,
Anode Support (AS); And
A solid oxide fuel cell comprising an anode functional layer (AFL) formed on the anode support layer, and preventing deterioration due to reverse current. The method of claim 3,
The anode support layer and the anode functional layer are made of a nickel (Ni)-based material, wherein deterioration due to reverse current is prevented. The method of claim 3,
The negative electrode support layer includes nickel and YSZ,
The negative electrode functional layer comprises nickel and YSZ, wherein the solid oxide fuel cell from which deterioration due to reverse current is prevented. The method of claim 1,
The anode layer,
A cathode functional layer (CFL) formed on the electrolyte layer; And
A solid oxide fuel cell comprising an anode current collector (CC) formed on the anode functional layer. The method of claim 6,
The anode functional layer includes LSM (Sr-doped LaMnO ₃ ) and YSZ,
The positive electrode current collecting layer comprises an LSM, wherein the solid oxide fuel cell in which deterioration due to reverse current is prevented. A solid oxide fuel cell stack comprising a plurality of solid oxide fuel cell unit cells according to any one of claims 1 to 7.

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