Patent Description:
A fuel cell typically includes a unit cell including an electrolyte membrane, and an anode and a cathode each containing an electrode catalyst and attached to either side of the electrolyte membrane. An interconnector, which may be referred to as "separator" or "collector", having a gas channel is disposed on either side of the unit cell. The fuel cell typically has a stacking structure (fuel cell stack) of a plurality of such unit cells and a plurality of interconnectors.

The term "interconnector" means a component interconnecting between the unit cells in a narrow sense. In the present invention, however, the term includes not only "the interconnector in the narrow sense" but also a component that supplies a reaction gas to a unit cell located at an end of the fuel cell stack and supplies/receives electrons to/from the unit cell.

Fuel cells are classified into various types such as a solid oxide type, a solid polymer type, an alkaline type, and a phosphoric acid type, etc. according to the type of electrolyte. In each type, poisoning of an electrode by a poisonous substance deteriorates cell performance. To address such a difficulty, various approaches have been provided.

For example, Patent Literature <NUM> discloses a fuel cell system including.

Patent Literature <NUM> describes that using the absorbent made of the same material (perovskite oxide) as a cathode material makes it possible to trap the poisonous element before the poisonous element reacts with the cathode.

Patent Literature <NUM> discloses a sulfur-resistant anode collector layer containing Ni, Co, and samarium-doped ceria (Ce<NUM>Sm<NUM>O<NUM>).

Patent Literature <NUM> describes that using such an anode collector layer converts H<NUM>S contained in a fuel gas into SOx so that absorption of H<NUM>S is minimized.

Patent Literature <NUM> discloses an interconnector for a solid oxide fuel cell including perovskite oxide and silica, wherein the composition of the perovskite oxide is expressed by La<NUM>-xSrxTi<NUM>-yFeyO<NUM>-δ, where <NUM> ≤ x ≤ <NUM> and <NUM> < y ≤ <NUM> are satisfied and δ is determined to satisfy a charge neutrality condition, and the content of silica is <NUM> to <NUM> mass%.

Patent Literature <NUM> describes that using the interconnector including no Cr prevents deterioration in cell performance due to Cr poisoning.

To suppress poisoning of a cathode or an anode of a solid oxide fuel cell by Cr or S, Patent Literatures <NUM> to <NUM> disclose the respective methods of.

However, there has been no case where an oxide having a corundum structure or a titania-based oxide is used to prevent the electrode from being poisoned by a poisonous substance.

<CIT> discloses coatings for metal interconnects to reduce SOFC degradation.

An object of the invention is to provide a novel fuel cell system capable of capturing Cr and/or S each being a poisonous substance for a fuel cell electrode.

To achieve the object, a fuel cell system according to the present invention has a configuration in accordance with claim <NUM>.

Cr and S may each act as a poisonous substance for a fuel cell electrode. The oxide having the corundum structure and the titania-based oxide each has the function of capturing Cr and S. Accordingly, in a case where such an oxide is provided in an appropriate portion on the fuel gas supply passage and/or the oxidizer gas supply passage, it is possible to prevent the fuel cell electrode from being poisoned by Cr or S.

One embodiment of the present invention will now be described in detail.

A fuel cell system according to the present invention includes.

The present invention can be applied to all types of fuel cells. Examples of the fuel cell to which the present invention is applied include a solid oxide fuel cell, a solid polymer fuel cell, an alkaline fuel cell, and a phosphoric acid fuel cell.

Of these, the solid oxide fuel cell (SOFC) may include a peripheral component made of a material containing Cr (for example, Fe-Cr stainless steel) and is operated at high temperature. The present invention is therefore preferably applied to the solid oxide fuel cell.

The term "fuel gas supply passage" refers to a passage to supply a fuel gas from a fuel gas source to the anode of the fuel cell. Examples of structural components of the fuel gas supply passage include.

The term "oxidizer gas supply passage" refers to a passage to supply an oxidizer gas from an oxidizer gas source to the cathode of the fuel cell. Examples of structural components of the oxidizer gas supply passage include.

The term "capturing component" refers to a component for capturing the poisonous substance. The capturing component is provided in an appropriate portion on the fuel gas supply passage and/or the oxidizer gas supply passage.

In the present invention, details of the capturing component such as a structure and a setting method are not limited as long as the capturing component can capture the poisonous substance before the electrode is poisoned. Examples of the capturing component include.

In the case of the solid oxide fuel cell, the capturing component is preferably a poisoning prevention layer formed on the inner surface of the gas channel of the interconnector of the fuel cell.

The capturing agent needs to be heated to an appropriate adsorption temperature to adsorb the poisonous substance. On the other hand, the solid oxide fuel cell is operated at high temperature. Accordingly, when the poisoning prevention layer is formed on the inner surface of the gas channel of the interconnector of the solid oxide fuel cell, it is unnecessary to provide a heat source to maintain the poisoning prevention layer at the appropriate temperature.

Although various materials have been used for the interconnector of the solid oxide fuel cell, use of inexpensive Fe-Cr alloy (so-called ferritic stainless steel) is now specifically investigated. However, since the interconnector is disposed close to the electrode, if the Fe-Cr alloy is used for the material of the interconnector, the electrode is poisoned by the interconnector. In contrast, when a poisoning prevention layer is formed in the inner surface of the gas channel of the interconnector, Cr poisoning of the electrode can be suppressed even if the interconnector is made of a material to be a poisoning source such as Fe-Cr alloy.

In the present invention, the term "poisonous substance" refers to Cr and/or S.

In a case where a material containing Cr (for example, Fe-Cr stainless steel) is used for one of the fuel gas supply passage and the oxidizer gas supply passage, when the gas supply passage is exposed to high temperature, a vapor containing Cr is generated from the material containing Cr. The vapor containing Cr poisons, for example, the cathode and the anode (in particular, the cathode) of the solid oxide fuel cell, causing deterioration in fuel cell performance.

The fuel gas may contain a sulfur constituent derived from the raw material. When air is used as the oxidizer gas, the air may contain a sulfur constituent. When such a sulfur constituent is supplied to the anode and/or the cathode of the fuel cell, the sulfur constituent poisons the anode and/or the cathode, causing deterioration in fuel cell performance.

In the present invention, the capturing component is used to capture Cr and/or S before the electrode is poisoned by Cr and/or S that has entered the gas supply passage of the fuel cell.

In the present invention, the capturing agent contains the oxide having the corundum structure or the titania-based oxide. The present invention is different in this point from the existing fuel cell systems. The capturing agent may contain one or both of such oxides.

Examples of the oxide having the corundum structure include alumina, α-Ga<NUM>O<NUM> (gallium oxide), α-Ti<NUM>O<NUM> (titanium oxide), α-In<NUM>O<NUM> (indium oxide), α-Fe<NUM>O<NUM> (iron oxide), α-Cr<NUM>O<NUM> (chromium oxide) , α-V<NUM>O<NUM> (vanadium oxide) , and α-Rh<NUM>O<NUM> (rhodium oxide).

Alumina is preferable as the oxide having the corundum structure. This is because alumina is easily available and thermally stable and easily dissolves Cr as seen in ruby.

The oxide having the corundum structure (in particular, alumina) serves as the capturing agent of Cr and S. The reason for this is as follows.

That is, the oxide having the corundum structure easily forms a solid solution such as Fe<NUM>O<NUM>-Cr<NUM>O<NUM>, Ga<NUM>O<NUM>-Cr<NUM>O<NUM>, Al<NUM>O<NUM>-Cr<NUM>O<NUM>, V<NUM>O<NUM>-Cr<NUM>O<NUM>, and Al<NUM>O<NUM>-Cr<NUM>O<NUM>, and thus potentially has a capturing effect of Cr as typically seen in alumina.

The term "titania-based oxide" refers to an oxide mainly containing titanium oxide (TiOx).

Examples of the titania-based oxide include.

The titania-based oxide (in particular, titania-based solid solution) serves as the capturing agent of Cr and S. The reason for this is as follows.

That is, basicity of titania contributes to the capturing effect of Cr and S. The solid solution maintains the basicity and thus also serves as the capturing agent.

The present invention can be applied to various fuel cell systems. <FIG> shows a schematic diagram of a fuel cell system having a solid oxide fuel cell of an external reforming type. In <FIG>, a fuel cell system <NUM> includes a vaporizer <NUM>, a reformer <NUM>, a heat exchanger <NUM>, and a fuel cell (solid oxide fuel cell) <NUM>.

The vaporizer <NUM> vaporizes hydrocarbon (for example, methane) and water as raw materials for synthesizing a reformed gas and supplies such vapor to the reformer <NUM>. Two respective inlets of the vaporizer <NUM> are connected to an undepicted hydrocarbon source and an undepicted water source. An outlet of the vaporizer <NUM> is connected to an inlet of the reformer <NUM> via a gas tube 18a.

The reformer <NUM> generates a mixed gas (synthesis gas) of CO and H<NUM> through steam reforming of hydrocarbon. Since the steam reforming reaction is an endothermic reaction, the reformer <NUM> includes a reforming part 14a for steam reforming and a combustion part 14b to supply heat necessary for the steam reforming reaction to the reforming part 14a.

The inlet of the reforming part 14a is connected to the outlet of the vaporizer <NUM> via the gas tube 18a. An outlet of the reforming part 14a is connected to an inlet of an anode of the fuel cell <NUM> via a gas tube 18b.

The combustion part 14b reacts an unreacted fuel (H<NUM>, CO) contained in anode off-gas with residual oxygen contained in cathode off-gas, and supplies the combustion heat to the reforming part 14a. Two inlets of the combustion part 14b are connected to an outlet of the anode and an outlet of the cathode of the fuel cell <NUM> via gas tubes 18c and 18d, respectively. An outlet of the combustion part 14b is connected to an exhaust-gas-side inlet of the heat exchanger <NUM> via a gas tube 18e.

The fuel cell <NUM> generates electric power using the fuel such as hydrocarbon, CO, and H<NUM>, and oxygen. The fuel cell <NUM> of <FIG> is a solid oxide fuel cell. The solid oxide fuel cell is described in detail later.

The heat exchanger <NUM> heats air to be supplied to the cathode of the fuel cell <NUM>. Since the solid oxide fuel cell is operated at approximately <NUM>, if air at room temperature is directly supplied to the cathode, temperature of the fuel cell <NUM> is lowered. In the solid oxide fuel cell, therefore, the heat exchanger <NUM> is typically used to heat the air.

The atmosphere-side inlet of the heat exchanger <NUM> is connected to the atmosphere via a gas tube 18f. The atmosphere-side outlet of the heat exchanger <NUM> is connected to the inlet of the cathode of the fuel cell <NUM> via a gas tube <NUM>.

In the fuel cell system <NUM> of <FIG>, the fuel gas supply passage includes the vaporizer <NUM>, the gas tube 18a, the reforming part 14a, the gas tube 18b, and an anode gas channel of the fuel cell <NUM>.

The oxidizer gas supply passage includes the gas tube 18f, the heat exchanger <NUM>, the gas tube <NUM>, and a cathode gas channel of the fuel cell <NUM>.

In the present invention, the capturing component is provided in an appropriate portion on the fuel gas supply passage and/or the oxidizer gas supply passage.

For example, the capturing component may be provided in the middle of the gas tube 18b or <NUM>. This advantageously allows the capturing component to be easily replaced when the capturing component is reduced in ability of capturing the poisonous substance.

However, the capturing agent needs to be maintained at a certain temperature or higher to capture the poisonous substance. Specifically, the temperature of the capturing agent is preferably <NUM> or higher, and more preferably <NUM> or higher. A heat source is therefore required to maintain the capturing component at an appropriate temperature if the capturing component is provided in the middle of the gas tube 18b or <NUM>. Further, if a Cr emission source and/or a S emission source additionally exist between the setting point of the capturing component and the fuel cell <NUM>, the poisoning prevention effect is insufficient.

In contrast, when the capturing component is provided on a surface of the interconnector of the fuel cell <NUM>, the capturing component is maintained at a temperature equal to the temperature of the fuel cell <NUM>. It is therefore unnecessary to provide the heat source to maintain the capturing component at an appropriate temperature. In addition, the interconnector is located closest to the anode or the cathode. The capturing component is therefore preferably formed on the inner surface of the gas channel of the interconnector of the fuel cell <NUM>.

The present invention can be applied to fuel cell systems having various fuel cell stacks. <FIG> shows a schematic diagram of a solid oxide fuel cell stack having flat interconnectors. In <FIG>, a fuel cell stack <NUM> includes unit cells 30a and 30b and interconnectors 40a to 40c.

<FIG> merely exemplarily illustrates the two unit cells 30a and 30b, and the fuel cell stack may include any optional number of unit cells for each purpose.

The unit cells 30a and 30b each include an electrolyte membrane <NUM>, and a cathode <NUM> and an anode <NUM> bonded to two respective surfaces of the electrolyte membrane <NUM>. In the solid oxide fuel cell, an undepicted intermediate layer is typically provided between the electrolyte membrane <NUM> and the cathode <NUM> to prevent them from reacting with each other. In the present invention, various materials may be used for each of the electrolyte membrane <NUM>, the cathode <NUM>, the anode <NUM>, and the intermediate layer.

In the solid oxide fuel cell, materials of the electrolyte membrane <NUM> typically include yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia (ScSZ), and lanthanum gallate (LaGaO<NUM>).

Materials of the cathode <NUM> typically include lanthanum manganite (LaAeMnO<NUM>, where Ae is Sr or Ce), lanthanum cobaltite (LaSrCoO<NUM>), lanthanum ferrite (LaSrFeO<NUM>), lanthanum strontium cobalt ferrite (LaSrCoFeO<NUM>), lanthanum nickel oxide (LaNiO<NUM>), and samarium cobaltite (SmSrCoO<NUM>).

Materials of the anode <NUM> typically include composite materials of nickel oxide and a solid electrolyte such as NiO-YSZ and NiO-ScSZ.

Materials of the intermediate layer typically include gadolinium-doped ceria (GDC).

The interconnectors 40a to 40c are flat and provided at an upper end of the unit cell 30a, a lower end of the unit cell 30b, and between the unit cells 30a and 30b, respectively.

The interconnector 40a is provided at the anode <NUM>-side end of the fuel cell stack <NUM>. The interconnector 40a has a fuel gas channel <NUM> in its surface adjacent to the unit cell 30a.

The interconnector 40b is provided at the cathode <NUM>-side end of the fuel cell stack <NUM>. The interconnector 40b has an oxidizer gas channel <NUM> in its surface adjacent to the unit cell 30b.

The interconnector 40c is provided between the unit cells 30a and 30b. The interconnector 40c has an oxidizer gas channel <NUM> in its surface adjacent to the unit cell 30a. The interconnector 40c further has a fuel gas channel <NUM> in its surface adjacent to the unit cell 30b.

As described above, the capturing agent is preferably in a form of a thin film (poisoning prevention layer) including the capturing agent formed on an inner surface of the gas channel (the fuel gas channel <NUM> or the oxidizer gas channel <NUM>) of each of the interconnectors 40a to 40c.

The term "inner surface of the gas channel" refers to a surface to be in direct contact with the reaction gas (fuel gas or oxidizer gas) among the surfaces of the interconnectors 40a to 40c, i.e., does not include a surface to be in direct contact with the cathode <NUM> or the anode <NUM>.

That is, no poisoning prevention layer is formed on a surface in direct contact with the cathode <NUM> or the anode <NUM> among the surfaces of the interconnectors 40a to 40c so that each of the unit cells 30a and 30b is electrically connected to some of the interconnectors 40a to 40c.

The cathode <NUM> or the anode <NUM> is poisoned by Cr via a gas phase. Hence, when Fe-Cr alloy is used for the interconnectors 40a to 40c, substantially no Cr poisoning occurs through solid-phase diffusion even if some of the interconnectors 40a to 40c is in direct contact with the cathode <NUM> or the anode <NUM>.

Cr and S may each act as a poisonous substance for the fuel cell electrode. The oxide having the corundum structure and the titania-based oxide each has the function of capturing Cr and S. Accordingly, in a case where such an oxide is provided in an appropriate portion on the fuel gas supply passage and/or the oxidizer gas supply passage, it is possible to prevent the fuel cell electrode from being poisoned by Cr or S.

Titania powder (rutile type, from Kojundo Chemical Laboratory Co. ), ethanol, a dispersant (MALIALIM (registered tradename), from NOF CORPORATION), and a binder (S-LEC (registered tradename), from SEKISUI CHEMICAL CO. ) were mixed by a planetary ball mill (from Fritsch Japan Co. , Ltd) to prepare a titania slurry.

A surface of an alumina honeycomb was dip-coated with the titania slurry to form a titania thin film on the surface of the alumina honeycomb. <FIG> shows a photograph of the alumina honeycomb (aeration setter, from MINO CERAMIC CO. ) used in the experiment. <FIG> shows a photograph of the alumina honeycomb coated with titania (Example <NUM>).

The alumina honeycomb used in Example <NUM> was directly used for a test.

An LSCF (LaSrCoFeO<NUM>, lanthanum strontium cobalt ferrite) sintered body used for the cathode of the SOFC was used as a specimen (poisoning object). The LSCF sintered body was prepared through sintering for <NUM> hours at <NUM> in the atmosphere.

SUS <NUM> was used as a chromium source, and sulfide-dispersed lead-free copper alloy (CAC411) was used as a sulfur source.

<FIG> shows a schematic diagram of the poisoning test. In Examples <NUM> and <NUM>, the capturing component (titania-coated alumina honeycomb or alumina honeycomb) was placed on the poisoning sources (chromium source and sulfur source), and the LSCF sintered body was further placed on the capturing component. Such a sample was accommodated in an alumina crucible and subjected to heat treatment. The heat treatment was performed for <NUM> hours at <NUM> in the atmosphere.

In Comparative Example <NUM>, pillars having the same height as the alumina honeycomb were set up on the poisoning sources, and the LSCF sintered body was placed on the pillars. Such a sample was accommodated in an alumina crucible and subjected to heat treatment under the same condition as in Example <NUM>.

In Comparative Example <NUM> (reference), pillars having the same height as the alumina honeycomb were set up without the capturing component, and the LSCF sintered body was placed on the pillars. Such a sample was accommodated in an alumina crucible and subjected to heat treatment under the same condition as in Example <NUM>.

After the poisoning test, X-ray diffraction (XRD) measurement was performed on the poisoning-source-side surface of each LSCF sintered body. <FIG> shows each XRD pattern of the poisoning-source-side surface of the LSCF sintered body after the poisoning test. In Example <NUM>, impurity peaks other than the peaks derived from the LSCF were smallest in number and low in intensity.

After the poisoning test, X-ray fluorescence (XRF) measurement was performed on the poisoning-source-side surface of each LSCF sintered body. Table <NUM> shows the XRF measurement result of the poisoning-source-side surface of the LSCF sintered body after the poisoning test. Table <NUM> reveals the following.

Titania powder (rutile type, from Kojundo Chemical Laboratory Co. ), alumina powder (from Kojundo Chemical Laboratory Co. ), ethanol, a dispersant (MALIALIM (registered tradename), from NOF CORPORATION), and a binder (S-LEC (registered tradename), from SEKISUI CHEMICAL CO. ) were mixed by a planetary ball mill (from Fritsch Japan Co. , Ltd) to prepare an alumina/titania mixed slurry.

A surface of an interconnector material (Fe-Cr alloy) was dip-coated with the alumina/titania mixed slurry to form an alumina/titania thin film.

<FIG> shows a photograph of the interconnector material coated with the alumina/titania mixed slurry. A uniform alumina/titania thin film was successfully formed by dip coating on the surface of the interconnector material. A uniform alumina/titania thin film was successfully formed by the dip coating not only on the alumina honeycomb but also on a sheet material.

Claim 1:
A fuel cell system (<NUM>), comprising:
a solid oxide fuel cell (<NUM>);
a fuel gas supply passage supplying a fuel gas to an anode (<NUM>) of the solid oxide fuel cell;
an oxidizer gas supply passage supplying an oxidizer gas to a cathode (<NUM>) of the solid oxide fuel cell; and
a capturing component provided in a portion on the fuel gas supply passage and/or the oxidizer gas supply passage and containing a capturing agent to capture a poisonous substance,
the capturing component being a thin film including the capturing agent formed on an inner surface of a gas channel (<NUM>, <NUM>) of an interconnector (40a, 40b, 40c) of the solid oxide fuel cell (<NUM>) without being formed on a surface of the interconnector that is in direct contact with the cathode (<NUM>) or the anode (<NUM>),
the interconnector (40a, 40b, 40c) being made of ferritic stainless steel,
the poisonous substance being Cr and/or S, and
the capturing agent including an oxide having a corundum structure and/or a titania-based oxide.