WO2009040649A1

WO2009040649A1 - Fuel cell system and fuel cell system control method

Info

Publication number: WO2009040649A1
Application number: PCT/IB2008/002513
Authority: WO
Inventors: Kyoichi Tange; Yoshitsugu Kojima; Takayuki Ichikawa; Chie Oomatsu; Satoshi Hino
Original assignee: Toyota Jidosha Kabushiki Kaisha; Hiroshima University
Priority date: 2007-09-28
Filing date: 2008-09-26
Publication date: 2009-04-02
Also published as: JP2009087726A

Abstract

A fuel cell system (10) includes a fuel cell stack (1), and a gas passage portion (2) through which fuel gas is supplied to the fuel cell stack. The gas passage portion is provided with ammonia adsorption units (23, 33). The ammonia adsorption units (23, 33) include first adsorbents (25, 35) and second adsorbents (24, 34) that differ in ammonia adsorption capacity.

Description

FUEL CELL SYSTEM AND FUEL CELL SYSTEM CONTROL METHOD

FIELD OF THE INVENTION

[0001] The invention relates to a fuel cell system where fuel gas from which ammonia has been removed is supplied to a fuel cell stack, and a method for controlling the fuel cell system.

BACKGROUND OF THE INVENTION [0002] A fuel cell is a device in which an electrochemical reaction is caused in a membrane electrode assembly (hereinafter, referred to as "MEA" where appropriate) that includes an electrolyte layer (hereinafter, referred to as "electrolyte membrane") and electrodes (anode and cathode) that are provided on respective sides of the electrolyte membrane, and which supplies the electric energy generated by this electrochemical reaction to an external component. Among various types of fuel cells, a polymer electrolyte fuel cell (hereinafter, referred to as "PEFC" where appropriate) that is used in a household cogeneration system, an automobile, etc. is able to operate at especially low temperatures. The PEFC exhibits high energy conversion efficiency, starts up quickly, and is compact and light-weight. Therefore, the PEFC is drawing attention as a power source for an electric automobile and as an electrical source for a mobile device. [0003] Each cell of a PEFC includes an MEA and paired collectors provided on respective sides of the MEA, and the MEA includes a proton conductive polymer that exhibits proton conductivity when being maintained in a hydrated condition. When the PEFC operates, gas that contains hydrogen (hereinafter, referred to as "hydrogen") is supplied to an anode while gas that contains oxygen (hereinafter, referred to as "air") is supplied to a cathode. The hydrogen supplied to the anode is separated into a proton and an electron under the action of a catalyst included in a catalyst layer of the anode (hereinafter, referred to as "anode catalyst layer" where appropriate), and the proton produced from the hydrogen passes through the anode catalyst layer and the electrolyte membrane and reaches a catalyst layer of the cathode (hereinafter, referred to as "cathode catalyst layer" where appropriate). Meanwhile, the electron passes through an external circuit and reaches the cathode catalyst layer. Electric energy is obtained through the above-described process. In addition, water is produced through a reaction between the proton and the electron that reach the cathode catalyst layer and the oxygen supplied to the cathode catalyst layer.

[0004] As described above, hydrogen is supplied to the anode when the PEFC operates. However, if ammonia is contained in the hydrogen, the function of the catalyst deteriorates, which may cause deterioration of the performance of the PEFC. Therefore, the PEFC needs to be supplied with gas from which ammonia has been removed.

[0005] Technologies related to a fuel cell that is supplied with gas from which ammonia has been removed are described in the following publications. Japanese Patent Application Publication No. 06-84537 (JP-A-06-84537) describes a technology related to a fuel cell power generator that is provided with an ammonia removal device which removes ammonia from reformed gas that is supplied to a fuel cell power generation unit. In the ammonia removal device according to JP-A-06-84537, ammonia is adsorbed to an adsorbent, for example, zeolite or activated carbon. Japanese Patent Application Publication No. 2003-45470 (JP-A-2003-45470) describes a technology related to a digestive gas fuel cell power generation system in which an ammonia adsorption tower is filled with vanadium oxide series ammonia adsorbent. Japanese Patent Application Publication No. 06-68894 (JP-A-06-68894) describes a technology related to a fuel cell power generator that is provided with an ammonia adsorption unit which is arranged downstream of a fuel reforming device and which adsorbs ammonia contained in the reformed gas supplied from the fuel reforming device to remove the ammonia from the reformed gas.

[0006] It is considered to be possible to remove ammonia from the reformed gas according to the technologies described above. However, when ammonia is adsorbed to an adsorbent, fine pores of the adsorbent are plugged with the ammonia, etc. If the fine pores are plugged, diffusivity of the gas deteriorates. Accordingly, with the technologies described above, the diffiisivity of the hydrogen deteriorates as the amount of ammonia adsorbed to the adsorbent increases. Therefore, with the technologies described above, it is difficult to maintain sufficient diffusivity of the hydrogen that is supplied to a fuel cell over a long period of time, which may cause a problem that the performance of the fuel cell is likely- to deteriorate over time.

SUMMARY OF THE INVENTION

[0007] The invention provides a fuel cell system and a fuel cell system control method with which ammonia adsorption capacity is maintained at a sufficient level over a long period of time.

[0008] A first aspect of the invention relates to a fuel cell system. The fuel cell system includes a fuel cell stack, and a gas passage portion through which fuel gas is supplied to the fuel cell stack. The gas passage portion is provided with an ammonia adsorption unit. The ammonia adsorption unit includes a first adsorbent and a second adsorbent that differ in ammonia adsorption capacity.

[0009] The term "fuel cell stack" signifies a structure formed by stacking a plurality of fuel cells, or a structure formed by stacking fuel cell modules each of which includes a plurality of fuel cells. Furthermore, the phrase "the ammonia adsorption unit includes a first adsorbent and a second adsorbent that differ in ammonia adsorption capacity" means that a plurality of adsorbents which differ in ammonia adsorption capacity are provided in the ammonia adsorption unit. In other words, the ammom'a adsorption unit according to the first aspect of the invention may include only the first adsorbent and the second adsorbent that differ in ammonia adsorption capacity, or may include three or more types of adsorbents that differ in ammonia adsorption capacity. [0010] With the fuel cell system according to the first aspect of the invention, ammonia is removed by the ammonia adsorption unit which includes the first adsorbent and the second adsorbent. Therefore, the ammonia adsorption capacity is maintained at a sufficient level over a long period of time while sufficient diffusivity of the hydrogen is ensured. Therefore, according to the first aspect of the invention, it is possible to provide the fuel cell system which exhibits sufficient performance over a long period of time by maintaining sufficient hydrogen diffusivity over a long period of time.

[0011] The first adsorbent may be higher in ammonia adsorption capacity than the second adsorbent, and the second adsorbent may be arranged upstream of the first adsorbent in the direction of flow of the fuel gas.

[0012] The second adsorbent which is lower in ammonia adsorption capacity than the first adsorbent is arranged upstream of the first adsorbent in the direction of flow of the fuel gas. Thus, it is possible to maintain the hydrogen diffusivity over a long period of time. [0013] A plurality of the ammonia adsorption units may be provided, and at least two of the plurality of the ammonia adsorption units may be arranged in parallel with each other.

[0014] The phrase "arranged in parallel with each other" means that the two ammonia adsorption units are arranged in such a manner that a line that connects the inlet of one ammonia adsorption unit to the inlet of the other ammonia adsorption unit is connected to an upstream side portion of the gas passage portion, and a line that connects the outlet of the one ammonia adsorption unit to the outlet of the other ammonia adsorption unit is connected to a downstream side portion of the gas passage portion.

The same applies if three or more ammonia adsorption units are arranged in parallel with each other.

[0015] Because the plurality of the ammonia adsorption units arranged in parallel with each other are provided, it is possible to maintain both the ammonia adsorption capacity and the hydrogen diffusivity at sufficient levels over a long period of time.

[0016] In the fuel cell system in which at least two of the plurality of the ammonia adsorption units are arranged in parallel with each other, while the fuel gas is supplied to one of the plurality of the ammonia adsorption units that are arranged in parallel with each other, a removal process for removing at least part of ammonia adsorbed to the second adsorbent may be performed on another ammonia adsorption unit among the plurality of the ammonia adsorption units, to which the fuel gas is not being supplied. [0017] "Another ammonia adsorption unit to which the fuel gas is not being supplied" means the ammonia adsorption unit which is among the plurality of the ammonia adsorption units arranged in parallel with each other and to which the fuel gas is not being supplied. [0018] The fuel cell system is operated in such a manner that the fuel gas is supplied to only a part of the plurality of the ammonia adsorption units and the removal process is performed on the ammonia adsorption unit, to which the fuel gas is not being supplied, in order to recover the ammonia adsorption capacity. Thus, it is possible to provide the fuel cell system in which the ammonia adsorption capacity is maintained at a sufficient level over a long period of time and the hydrogen dififusivity is easily maintained at a sufficient level.

[0019] The aforementioned fuel cell system may include heating means for heating the ammonia adsorption unit in order to remove ammonia adsorbed to the ammonia adsorption unit. [0020] The heating means may use the heat generated in the fuel cell stack to heat the ammonia adsorption unit.

[0021] The ammonia adsorbed to the second adsorbent may be removed from the second adsorbent at a temperature equal to or lower than a temperature of heat that is generated in the fuel cell stack. [0022] A second aspect of the invention is related to a method for controlling a fuel cell system that includes a fuel cell stack, a gas passage portion through which fuel gas is supplied to the fuel cell stack, and a plurality of ammonia adsorption units which are provided in the gas passage portion, which are arranged in parallel with each other, and each of which includes a first adsorbent and a second adsorbent that differ in ammonia adsorption capacity. The method includes removing, while the fuel gas is supplied to one of the plurality of the ammonia adsorption units, at least part of ammonia adsorbed to the second adsorbent of another ammonia adsorption unit among the plurality of the ammonia adsorption units, to which the fuel gas is not being supplied. BRIEF DESCRIPTION OFTHE DRAWINGS

[0023] The foregoing and further features and advantages of the invention will become apparent from the following description of an example embodiment with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein,

FIG 1 is a view schematically showing a fuel cell system according to an embodiment of the invention in a simplified form.

DETAILED DESCRIPTION OF THE EMBODIMENT [0024] FIG 1 is a view schematically showing a fuel cell system according to an embodiment of the invention in a simplified form. As shown in FIG 1, a fuel cell system 10 according to the embodiment of the invention includes a fuel cell stack 1 that has a plurality of fuel cells (not shown), and a pipe 2 that serves as a gas passage portion through which fuel gas is supplied to the fuel cell stack 1. The pipe 2 is provided with ammonia adsorption units 23 and 33 that are arranged in parallel with each other. The ammonia adsorption units 23 and 33 include mesoporoυs bodies 24 and 34 that serve as second adsorbents having relatively low ammonia adsorption capacity, and microporous bodies 25 and 35 that serve as first adsorbents having ammonia adsorption capacity that is higher than that of the mesoporous bodies 24 and 34, respectively. The ammonia adsorption unit 23 is provided with a valve 26 located on its inlet side and a valve 27 located on its outlet side. Similarly, the ammonia adsorption unit 33 is provided with a valve 36 located on its inlet side and a valve 37 located on its outlet side. The inlets of the ammonia adsorption units 23 and 33 are connected to each other via a pipe, and the outlets of the ammonia adsorption units 23 and 33 are connected to each other via a pipe. Further, return pipes 28 and 38 through which the ammonia that has been adsorbed to the ammonia adsorption units 23 and 33 is introduced into an ammonia processing unit (not shown) are connected to the ammonia adsorption units 23 and 33, respectively. The mesoporous bodies 24 and 34 included in the ammonia adsorption units 23 and 33 are provided with heating means 29 and 39, respectively. [0025] When the fuel cell system 10 operates, fuel gas (hereinafter, referred to as "hydrogen") from which most of ammonia has been removed by the ammonia adsorption units 23 and 33 is supplied to the fuel cell stack 1, and used to generate electricity. In the early stage of an operation of the fuel cell system 10, most of the ammonia contained in the gas supplied to the ammonia adsorption units 23 and 33 is adsorbed to the mesoporous bodies 24 and 34, and the gas that contains only a small amount of ammonia is delivered to the microporous bodies 25 and 35. Then, the ammonia is adsorbed to the microporous bodies 25 and 35, and the hydrogen that has passed through the microporous bodies 25 and 35 (hydrogen that contains ammonia of, for example, 0.3 ppm or lower) is delivered to the fuel cell stack 1. .If an automobile is stopped after the fuel cell system 10 operates for a prescribed period of time, the temperature of the fuel cell system 10 is decreased due to the stop of the automobile, which may disrupt the uniformity of chemical reaction. If high concentration of ammonia is supplied to the ammonia adsorption units 23 and 33 due to, for example, such disruption of the uniformity of chemical reaction, a large amount of ammonia may be left even after the gas passes through the mesoporous bodies 24 and 34. The ammonia which the mesoporous bodies 24 and 34 fail to adsorb is adsorbed to the microporous bodies 25 and 35 that have high ammonia adsorption capacity. Thus, it is possible to supply the fuel cell stack 1 with hydrogen that contains only a small amount of ammonia. [0026] In the embodiment of the invention, if the ammonia adsorption units 23 and 33 are not provided with the microporous bodies 25 and 35 that serve as the first adsorbents, the fine pores of the mesoporous bodies 24 and 34 are plugged with ammonia in a short period of time. As a result, the ammonia adsorption capacity deteriorates over time and therefore the hydrogen diffusivity deteriorates over time. Furthermore, in the embodiment of the invention, if the ammonia adsorption units 23 and 33 are provided with only the microporous bodies 25 and 35 that serve as the first adsorbents, the fine pores of the microporous bodies 25 and 35 are immediately plugged with ammonia. As a result, it is difficult to maintain sufficient hydrogen diffusivity, and, in addition, the ammonia adsorption capacity of the ammonia adsorption units 23 and 33 deteriorates. Therefore, the fuel cell system 1 according to the embodiment of the invention needs to be structured in such a manner that the ammonia adsorption units 23 and 33 include the first adsorbents (microporous bodies 25 and 35) and the second adsorbents (mesoporous bodies 24 and 34) that differ in ammonia adsorption capacity. [0027] Any types of material may be used as the mesoporous bodies 24 and 34 provided in the fuel cell system 10 as long as the material has fine pores with which hydrogen is dispersed and is able to adsorb ammonia contained in the hydrogen. Concrete examples of material that may be used as the mesoporous bodies 24 and 34 include silica porous bodies typified by FSM (Folded Sheet Mesoporous Material), porous carbon materials typified by a carbon nanotube and a carbon nanohorn, and active carbon.

[0028] Meanwhile, any types of material may be used as the microporous bodies 25 and 35 provided in the fuel cell system 10 as long as the material has fine pores with which hydrogen is dispersed, is able to adsorb ammonia contained in the hydrogen, and has the ammonia adsorption capacity higher than that of the mesoporous bodies 24 and 34. Concrete examples of material that may be used as the microporous bodies 25 and 35 include the structure obtained by providing substance that strongly interacts with ammonia (for example, MgCl₂ or an ammine complex) in the fine pores of the material that forms the above-described mesoporous bodies 24 and 34. When MgCl₂ is adsorbed into the fine pores, preferably, approximately 0.3 ml to approximately 0.5 ml of MgCl₂ is adsorbed into 1 ml of fine pores that form the mesoporous bodies 24 and 34. With this configuration, it is possible to provide the microporous bodies 25 and 35 having favorable ammonia adsorption capacity without excessively suppressing the hydrogen diffusivity. [0029] When MgCl₂ is provided in the fine pores of the microporous bodies 25 and 35 provided in the fuel cell system 10, the microporous bodies 25 and 35 in which MgCl₂ is provided in the fine pores are formed by, for example, the following method. According to the method, the mesoporous bodies 24 and 34 are immersed in a MgCl₂ aqueous solution for a prescribed period of time, taken out of the aqueous solution, and dried. Then MgCl₂ is provided in the fine pores of the mesoporous bodies 24 and 34. When the microporouies bodies 25 and 35 are formed by this method, the amount of MgCI₂ provided in the fine pores of the microporous bodies 25 and 35 may be controlled by adjusting the concentration of MgCl₂ contained in the aqueous solution. [0030] It is considered that when ammonia is adsorbed to the microporous bodies 25 and 35 in which MgCl₂ (hereinafter, referred to as "ammonia adsorbent", where appropriate, regardless of whether reaction with ammonia has occurred) is provided in the fine pores, the ammonia adsorbent provided in the fine pores expands. However, because the ammonia adsorbent provided in the microporous bodies 25 and 35 is arranged in the fine pores surrounded by walls, expansion is restricted by these walls and gaps through which the hydrogen flows are not excessively reduced. Therefore, providing these microporous bodies 25 and 35 makes it possible to provide the fuel cell system 10 in which the ammonia adsorption capacity and the hydrogen diffusivity are maintained at sufficient levels over a long period of time. [0031] It is considered that using the ammonia adsorption units 23 and 33 that include the mesoporous bodies 24 and 34 and the microporous bodies 25 and 35 makes it possible to maintain the ammonia adsorption capacity at a sufficient level over a long period of time. However, even when the thus structured ammonia adsorption units 23 and 33 are used, there is a limit on the amount of ammonia that is adsorbed. Therefore, if the fuel cell system 10 is operated over a long period of time, many fine pores of the ammonia adsorption units 23 and 33 to which a large amount of ammonia has been adsorbed are plugged with the ammonia, which may deteriorate the hydrogen diffusivity. Therefore, in the fuel cell system 10, the heating means 29 and 39 are provided in the mesoporous bodies 24 and 34, respectively. [0032] In the fuel cell system 10, a removal process is performed. In the removal process, heat generated in the fuel cell stack 1 (waste heat) is transferred through the heating means 29 and 39 to the mesoporous bodies 24 and 34, whereby the ammonia that has been adsorbed into the fine pores of the mesoporous bodies 24 and 34 is removed by the heat, respectively. The ammonia may be removed from the fine pores of the mesoporous bodies 24 and 34 at a temperature that is lower than that of the heat generated in the fuel cell stack 1. Then, the ammonia that has been removed through the removal process is delivered through the return pipes 28 and 38 to the ammonia processing unit. In order to improve the efficiency of the removal process for removing the ammonia that has been adsorbed into the fine pores, preferably, the gas that contains ammonia is not supplied to the ammonia adsorption units 23 and 33 that undergo the removal process. Therefore, in the fuel cell system 10, the ammonia adsorption units 23 and 33 are arranged in parallel with each other. The gas is not supplied to the ammonia adsorption unit 23 while the ammonia adsorption unit 23 undergoes the removal process, whereas the gas that contains ammonia is supplied to the ammonia adsorption unit 33 that does not undergo the removal process. Similarly, the gas is not supplied to the ammonia adsorption unit 33 while the ammonia adsorption unit 33 undergoes the removal process, whereas the gas that contains ammonia is supplied to the ammonia adsorption unit 23 that does not undergo the removal process. With this configuration, the efficiency of the removal process is improved. Furthermore, alternately performing the removal process on the ammonia adsorption unit 23 and the removal process on the ammonia adsorption unit 33, the ammonia adsorption units 23 and 33 being arranged in parallel with each other, makes it easier to maintain the ammonia adsorption capacity and the hydrogen diffusivity at sufficient levels over a long period of time. [0033J In the fuel cell system 10, the removal process may be performed not only on the mesoporous bodies 24 and 34 but also on the microporous bodies 25 and 35. Furthermore, the temperature to which the mesoporous bodies 24 and 34 are heated by the heating means 29 and 39 during the removal process is not limited to a specific value. However, in order to remove approximately 70% of the ammonia that has been adsorbed to the mesoporous bodies 24 and 34, preferably, the mesoporous bodies 24 and 34 are heated to approximately 8O⁰C. It is considered to be possible to remove approximately 30% of the ammonia that has been adsorbed to the mesoporous bodies 24 and 34 if the mesoporous bodies 24 and 34 are heated to approximately 60⁰C.

[0034] In the embodiment of the invention, any types of means that are able to heat the mesoporous bodies 24 and 34 may be used as the heating means 29 and 39 that are used in the removal process. Concrete examples of the heating means 29 and 39 that may be used in the embodiment of the invention include heating means provided with for example, various types of heaters or heat exchangers. [0035] In the fuel cell system 10 according to the embodiment of the invention, the manner for arranging the ammonia adsorption units 23 and 33 is not particularly limited as long as the ammonia contained in the hydrogen that flows through the pipe 2 is adsorbed to the ammonia adsorption units 23 and 33. For example, the ammonia adsorption units 23 and 33 may be provided to a main tank that stores high-pressure hydrogen.

[0036] In the fuel cell system 10 according to the embodiment of the invention, the mesoporous bodies 24 and 34 are used repeatedly by performing the removal process. However, with the configuration in which the heat (waste heat) produced in a polymer electrolyte fuel cell (PEFC) is mainly used, removing the ammonia that has been adsorbed to the microporous bodies 25 and 35 may be difficult. Therefore, if the hydrogen diffusivity or the ammonia adsorption capacity of the microporous bodies 25 and 35 deteriorate, preferably, the ammonia adsorption units 23 and 33, more specifically, the microporous bodies 25 and 35, are removed from the fuel cell system 10 and replaced when the main tank is replaced. [0037] In the fuel cell system 10 according to the embodiment of the invention, the size of the mesoporous bodies 24 and 34 (second adsorbents) and the size of the microporous bodies 25 and 35 (first adsorbents) that are provided in the ammonia adsorption units 23 and 33, respectively, are not particularly limited, and may be changed on an as-required basis depending on the design of the main tank, the ammonia adsorption capacity and service life required of the ammonia adsorption units 23 and 33. It is considered that the maximum amount of ammonia that is discharged from the main tank is substantially constant independently of the operating condition in which a fuel cell automobile equipped with the fuel cell system 10 according to the embodiment of the invention is driven. Therefore, ammonia poisoning of the fuel cell stack 1 is suppressed by, for example, repeatedly performing the removal process before the ammonia adsorption capacity and the hydrogen diffusivity excessively deteriorate. As a result, it is possible to provide a fuel cell automobile with high reliability.

[0038] Hereafter, the embodiment of the invention will be described in more detail with reference to the results of evaluations and FIG 1.

1. Evaluation of Ammonia Adsorption Capacity

[0039] The ammonia adsorption units 23 and 33 (hereinafter, referred to as "adsorption units according to the embodiment of the invention" where appropriate) that include the mesoporous bodies 24 and 34 and the micoporous bodies 25 and 35, respectively, were produced. The mesoporous bodies 24 and 34 were made from FSM with fine pores each having a diameter of 1.5 nm (nanometers) to 2 nm. The microporous bodies 25 and 35 were produced by immersing the mesoporous bodies 24 and 34 in an aqueous solution having a MgCl₂ concentration of 0.5 mol/L to 1 mol/Lfor 10 minutes to 60 minutes and then drying them. Then, the hydrogen gas that contains 100 ppm of ammonia was supplied to the ammonia adsorption units 23 and 33. Then, the ammonia adsorption capacity of the ammonia adsorption units according to the embodiment of the invention was evaluated by measuring the concentration of ammonia contained in the hydrogen gas passed through the ammonia adsorption units 23 and 33.

[0040] Meanwhile, ammonia adsorption units that include only the mesoporous bodies 24 and 34 (hereinafter, referred to as "adsorption units according to a first comparative example"), ammonia adsorption units that include only coconut shell active carbon with fine pores each having a diameter of 1 nm or smaller (hereinafter, referred to as "adsorption units according to a second comparative example"), and ammonia adsorption units that include only microporous bodies 25 and 35 (hereinafter, referred to as "adsorption units according to a third comparative example") were produced. Then, hydrogen gas that contains 100 ppm of ammonia was supplied to each of the ammonia adsorption units. Then, the ammonia adsorption capacity of the ammonia adsorption units according to each of the first to third comparative examples was evaluated by measuring the concentration of ammonia contained in the hydrogen gas passed through the ammonia adsorption units. Table 1 shows the result of evaluation performed on the ammonia adsorption capacity of each of the ammonia adsorption units according to the embodiment of the invention, the ammonia adsorption units according to the first comparative example, the ammonia adsorption units according to the second comparative example, and the ammonia adsorption units according to the third comparative example. [0041] Table 1

[0042] Table 1 shows that the concentration of ammonia contained in the hydrogen gas that has passed through the ammonia adsorption units according to the first comparative example was 4 ppm to 10 ppm, but the concentration of ammonia contained in the hydrogen gas that has passed through each of the ammonia adsorption units according to the embodiment of the invention, the ammonia adsorption units according to the second comparative example, and the ammonia adsorption units according to the third comparative example was lower than 1 ppm. Therefore, it was confirmed that the ammonia adsorption units according to the embodiment of the invention, the ammonia adsorption units according to the second comparative example, and the ammonia adsorption units according to the third comparative example have favorable ammonia adsorption capacity.

2. Evaluation of Hydrogen Diffusivity

[0043] Hydrogen gas was supplied to each of the ammonia adsorption units according to the embodiment of the invention, the ammonia adsorption units according to the first comparative example, the ammonia adsorption units according to the second comparative example, and the ammonia adsorption units according to the third comparative example. Then, the hydrogen diffusivity of each of the ammonia adsorption units was evaluated by measuring the diffusion flux of hydrogen after three hours and after ten hours. Table 2 shows the results.

[0044] Table 2

[0045] "Hydrogen diffusion flux (g/sec ^• 1 kg)" in table 2 is an index that indicates how many grams of hydrogen were diffused in one second when one kilogram of hydrogen was supplied to each of the ammonia adsorption units according to the embodiment of the invention, the ammonia adsorption units according to the first comparative example, the ammonia adsorption units according to the second comparative example and the ammonia adsorption units according to the third comparative example. A larger value indicates higher hydrogen diffusivity. Note that, "-" in table 2 means that it was not possible to diffuse the hydrogen.

[0046] Table 2 shows that the ammonia adsorption units according to the embodiment of the invention exhibited a hydrogen diffusion flux of 0.4 (g/sec * 1 kg) after three hours and a hydrogen diffusion flux of 0.3 (g/sec ^• 1 kg) after ten hours. That is, the hydrogen diffusivity of the ammonia adsorption units according to the embodiment of the invention was still favorable even after 10 hours. In contrast, the ammonia adsorption unit according to the first comparative example exhibited a hydrogen diffusion flux of 0.2 to 0.4 (g/sec ^• 1 kg) after 3 hours and a hydrogen diffusion flux of 0.1 (g/sec ^• 1 kg) after 10 hours. That is, the hydrogen diffusivity of the ammonia adsorption units according to the first comparative example considerably deteriorated. Furthermore, the ammonia adsorption units according to the second comparative example exhibited a hydrogen diffusion flux of only 0.04 (g/sec* 1 kg) after three hours and it was not possible to diffuse the hydrogen after ten hours. Furthermore, the ammonia adsorption units according to the third comparative example exhibited a hydrogen diffusion flux of 0.3 (g/sec ^• 1 kg) after three hours, but it was not possible to diffuse the hydrogen after ten hours. That is, the hydrogen diffusivity of the ammonia adsorption units according to the third comparative example significantly deteriorated. [0047] As described above, with the ammonia adsorption units (ammonia adsorption units 23 and 33) that include the first adsorbents (microporous bodies 25 and 35) and the second adsorbents (mesoporous bodies 24 and 34), the ammonia adsorption capacity and the hydrogen diffusivity were maintained at sufficient levels over a long period of time. However, with the ammonia adsorption units that include only the first adsorbents or only the second adsorbents, especially the hydrogen diffusivity was not maintained at a sufficient level over a long period of time. In other words, in the fuel cell system 10 that includes the ammonia adsorption units 23 and 33, it is possible to maintain the ammonia adsorption capacity and the hydrogen diffusivity at sufficient levels over a long period of time. Therefore, according to the embodiment of the invention, it is possible to provide the fuel cell system which exhibits sufficient performance over a long period of time.

[0048] While the invention has been described with reference to an example embodiment thereof, it is to be understood that the invention is not limited to the described embodiment or construction. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the disclosed invention are shown in various example combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the appended claims.

Claims

CLAIMS:

1. A fuel cell system, comprising: a fuel cell stack; and a gas passage portion through which fuel gas is supplied to the fuel cell stack, wherein the gas passage portion is provided with an ammonia adsorption unit, and the ammonia - adsorption unit includes a first adsorbent and a second adsorbent that differ in ammonia adsorption capacity.

2. The fuel cell system according to claim 1, wherein: the first adsorbent is higher in ammonia adsorption capacity than the second adsorbent; and the second adsorbent is arranged upstream of the first adsorbent in a direction of flow of the fuel gas.

3. The fuel cell system according to claim 1 or 2, wherein: a plurality of the ammonia adsorption units are provided; and at least two of the plurality of the ammonia adsorption units are arranged in parallel with each other.

4. The fuel cell system according to claim 3, wherein while the fuel gas is supplied to one of the plurality of the ammonia adsorption units that are arranged in parallel with each other, a removal process for removing at least part of ammonia adsorbed to the second adsorbent is performed on another ammonia adsorption unit among the plurality of the ammonia adsorption units, to which the fuel gas is not being supplied.

5. The fuel cell system according to any one of claims 1 to 4, further comprising: heating means for heating the ammonia adsorption unit in order to remove ammonia adsorbed to the ammonia adsorption unit.

6. The fuel cell system according to claim 5, wherein the heating means uses the heat generated in the fuel cell stack to heat the ammonia adsorption unit.

7. The fuel cell system according to any one of claims 1 to 6, wherein ammonia adsorbed to the second adsorbent is removed from the second adsorbent at a temperature equal to or lower than a temperature of heat that is generated in the fuel cell stack.

8. A fuel cell system, comprising: a fuel cell stack; a gas passage portion through which fuel gas is supplied to the fuel cell stack; and an ammonia adsorption unit that is provided in the gas passage portion, and that includes a first adsorbent and a second adsorbent which differ in ammonia adsorption capacity.

9. A method for controlling a fuel cell system that includes a fuel cell stack, a gas passage portion through which fuel gas is supplied to the fuel cell stack, and a plurality of ammonia adsorption units which are provided in the gas passage portion, which are arranged in parallel with each other, and each of which includes a first adsorbent and a second adsorbent that differ in ammonia adsorption capacity, the method comprising: removing, while the fuel gas is supplied to one of the plurality of the ammonia adsorption units, at least part of ammonia adsorbed to the second adsorbent of another ammonia adsorption unit among the plurality of the ammonia adsorption units, to which the fuel gas is not being supplied.