Fuel cell system and control method of the same

[0001] The disclosure of Japanese Patent Application No. 2002-116950 filed on Apr. 19, 2002, including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to a fuel cell system and a control method thereof.

[0004] 2. Description of the Related Art

[0005] A fuel cell stack is formed by stacking a plurality of unit cells. Each of the unit cells is configured such that a membrane electrode assembly (hereinafter, referred to as MEA) is sandwiched by two separators. The MEA is formed from an electrolyte membrane in which platinum as a catalytic electrode is applied to both surfaces thereof, and a pair of gas diffusion electrodes which sandwich the electrolyte membrane. The catalytic electrode and the gas diffusion electrode on one surface of the electrolyte membrane form an anode, and the catalytic electrode and the gas diffusion electrode on the other surface form a cathode. A fuel gas passage through which hydrogen gas as fuel gas is distributed to a unit cell is formed in the separator that faces the anode, and an oxidizing gas passage through which air as oxidizing gas is distributed to the unit cell is formed in a separator that faces the cathode. Also, based on the fact that the electrolyte membrane normally exhibits good proton conductivity in a humid state, fuel gas and oxidizing gas are supplied after being humidified such that the electrolyte membrane is maintained to be humid.

[0006] In a fuel cell stack of this type, when a circuit which connects an output terminal of a fuel cell and a load is opened, a phenomenon in which the hydrogen gas on the anode side passes through the electrolyte membrane and reaches the cathode side as it is without being protonated, that is, so-called cross leak is caused. As a result, there is a possibility that sufficient durability cannot be obtained due to the cross leak of the hydrogen gas. For example, when the amount of the cross-leaked hydrogen gas is large, hydrogen directly reacts with oxygen on the cathode side, which generates a large amount of heat, and the electrolyte membrane is damaged due to the heat. As a result, there is a possibility that sufficient durability cannot be obtained. Therefore, in order to prevent such cross leak of the hydrogen gas, a method is conceivable, in which the hydrogen gas that remains in each of the unit cells is consumed by short-circuiting a positive electrode and a negative electrode of the output terminal after the supply of the fuel gas is stopped.

[0007] However, normally, distribution of the fuel gas to each of the unit cells is configured such that the fuel gas is supplied from a fuel gas supply source to a fuel gas supply manifold of the fuel cell stack through gas piping, and the fuel gas is distributed from the manifold to each of the unit cells. Also, since interruption of the supply of the fuel gas is configured to be performed by a valve or the like which is provided in the gas piping, a considerable amount of the fuel gas remains in the gas piping and the manifold after the supply of the gas is interrupted. When the positive electrode and the negative electrode of the output terminal are short-circuited in this state, a large amount of heat is generated. Accordingly, there still exists a problem concerning durability. Also, there is another problem that it becomes difficult to obtain good fuel economy, which is supposed to be obtained, since a large amount of fuel gas is unnecessarily consumed.

[0008] A configuration in which a positive electrode and a negative electrode of an output terminal are short-circuited when operation of a fuel cell is stopped is disclosed in Japanese Patent Laid-Open Publication No. 9-139221. However, an object thereof is to eliminate residual voltage so as to prevent electric shock which is caused during a checkup after the operation. Accordingly, prevention of the cross leak of hydrogen gas, and the fuel economy are not considered.

SUMMARY OF THE INVENTION

[0009] The invention is made in consideration of the above-desctibed problem. It is an object of the invention to provide a fuel cell system in which cross leak can be prevented when an output terminal of a stack is disconnected from a load, and a control method thereof. Also, it is another object of the invention to provide a fuel cell system in which deterioration of fuel economy can be prevented when cross leak is prevented, and a control method thereof.

[0010] According to an exemplary embodiment of the invention, a fuel cell system in which fuel gas and oxidizing gas are supplied to a fuel cell stack that is formed by stacking a plurality of unit cells, and power generation is performed by electrochemical reaction between the fuel gas and the oxidizing gas is provided. This system includes a fuel gas supply manifold which is extended in the unit cell stacking direction and which distributes fuel gas to a fuel gas passage of each of the unit cells, a switching device which is provided inside the fuel gas supply manifold and which switches between permission and prohibition of distribution of the fuel gas to the fuel gas passage of each of the unit cells, a load disconnect device which disconnects the output terminal of the stack from the load, a short-circuit device which short-circuits the positive electrode and the negative electrode of the output terminal of the stack, and a control device which short-circuits the positive electrode and the negative electrode of the output terminal by the short-circuit device after distribution of the fuel gas to the fuel gas passage of each of the unit cells is prohibited by the switching device when the output terminal is disconnected from the load by the load disconnect device.

[0011] In a fuel cell system with such a configuration, when the output terminal of the fuel cell stack is disconnected from the load, the hydrogen in each of the unit cells is consumed by short-circuiting the positive electrode and the negative electrode of the output terminal of the stack after distribution of the fuel gas to the fuel gas passage is prohibited in the fuel gas supply manifold. Accordingly, cross leak of hydrogen can be prevented. Also, distribution of the fuel gas is stopped in the fuel gas supply manifold, unlike a conventional method in which the supply of the fuel gas is stopped in gas piping which is provided outside the stack. Accordingly, the amount of the hydrogen which remains after distribution of the fuel gas is stopped is smaller, the amount of the hydrogen which is consumed when the positive electrode and the negative electrode of the output terminal are short-circuited is smaller, and the amount of heat generation is smaller, as compared with the conventional method. Accordingly, deterioration of durability due to heat generated at the time of short-circuit can be suppressed, and unnecessary consumption of large amount of hydrogen can be avoided. Therefore, according to this fuel cell system, cross leak can be prevented when the output terminal of the stack is disconnected from the load, and deterioration of fuel economy can be avoided.

[0012] Also, according to another exemplary embodiment of the invention, a control method of a fuel cell system in which fuel gas and oxidizing gas are supplied to a fuel cell stack that is formed by stacking a plurality of unit cells, and power generation is performed by electrochemical reaction between the fuel gas and the oxidizing gas is provided. This control method includes the following steps of:

[0013] prohibiting distribution of the fuel gas to the fuel gas passage of each of the unit cells in the fuel gas supply manifold which is extended in the unit cell stacking direction when the output terminal of the stack is disconnected from the load, and

[0014] short-circuiting the positive electrode and the negative electrode of the output terminal after distribution of the fuel gas is prohibited.

[0015] According to the method, when the output terminal of the fuel cell stack is disconnected from the load, the hydrogen in each of the unit cells is consumed by short-circuiting the positive electrode and the negative electrode of the output terminal of the stack after distribution of the fuel gas to the fuel gas passage of each of the unit cells is prohibited in the fuel gas supply manifold. Accordingly, cross leak of the hydrogen can be prevented. Also, distribution of the fuel gas is stopped in the fuel gas supply manifold, unlike the conventional method in which the supply of the fuel gas is stopped in gas piping which is provided outside the stack. Accordingly, the amount of the hydrogen which remains after distribution of the fuel gas is stopped is smaller, the amount of the hydrogen which is consumed when the positive electrode and the negative electrode of the output terminal are short-circuited is smaller, and an amount of heat generation is smaller, as compared with the conventional method. Accordingly, deterioration of durability due to heat generated at the time of short-circuit can be suppressed, and unnecessary consumption of large amount of hydrogen can be avoided. Therefore, according to the control method of the fuel cell, cross can be prevented when the output terminal of the stack is disconnected from the load, and deterioration of fuel economy can be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The above-mentioned and other objects, features, advantages, technical and industrial significances of this invention will be better understood by reading the following detailed description of the exemplary embodiments of the invention, when considered in connection with the accompanying drawings, in which:

[0017]FIG. 1 is a block diagram schematically showing a fuel cell system according to an exemplary embodiment of the invention;

[0018]FIG. 2 is a perspective view of an FC stack of the fuel cell system according to the embodiment;

[0019]FIG. 3 is an exploded perspective view of a unit cell shown in FIG. 2;

[0020]FIG. 4 is a flowchart showing a control process of the fuel cell system according to an exemplary embodiment of the invention;

[0021]FIG. 5 is a block diagram showing a vicinity of a blower, as a first modified example of the embodiment; and

[0022]FIG. 6 is an explanatory diagram showing switching means, as a second modified example of the embodiment:

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0023] In order to further clarify the invention, hereafter, an exemplary embodiment of the invention will be described with reference to accompanying drawings. FIG. 1 is a block diagram schematically showing a fuel cell system <b>10</b> according to the embodiment, FIG. 2 is a perspective view of an FC stack <b>20</b>, and FIG. 3 is an exploded perspective view of a unit cell <b>210</b>.

[0024] The fuel cell system <b>10</b> according to the embodiment mainly includes a fuel cell stack (hereinafter, referred to as an FC stack) <b>20</b> of a solid polyelectrolyte type, a first interrupter <b>30</b> which is provided between output terminals <b>25</b>, <b>26</b> of the FC stack <b>20</b> and a load <b>60</b>, a second interrupter <b>40</b> which is provided between the output terminals <b>25</b>, <b>26</b> of the FC stack <b>20</b>, and a control device <b>50</b> which controls a supply of fuel gas and oxidizing gas to the FC stack <b>20</b> and controls connection and disconnection by the first interrupter <b>30</b> and the second interrupter <b>40</b>.

[0025] The FC stack <b>20</b>, which will be described later in detail, is formed by stacking a plurality of unit cells <b>210</b>. In the FC cell <b>20</b>, the fuel gas is supplied from a fuel gas supply source <b>15</b> to a fuel gas supply manifold M<b>1</b>, and the fuel gas which passes through each of the unit cells <b>210</b> is released to the outside from a fuel gas release manifold M<b>3</b>. Also, the oxidizing gas which is humidified by a humidifier <b>13</b> is supplied from a blower <b>11</b> that is an oxidizing gas supply source to an oxidizing gas supply manifold M<b>2</b>, and the oxidizing gas which passes through each of the unit cells <b>210</b> is released to the outside from an oxidizing gas supply manifold M<b>4</b>. In this case, the fuel gas is gas which is rich in hydrogen, and the oxidizing gas is air (including oxygen). Since an electrochemical reaction occurs between the oxygen in the oxygen gas and the hydrogen in the fuel gas which are supplied to the FC stack <b>20</b> when the oxygen and the hydrogen pass through each of the unit cells <b>210</b>, the FC stack <b>20</b> generates electric power, and a potential difference is generated between the output terminals <b>25</b>, <b>26</b>. Examples of the fuel gas supply source <b>15</b> are a reformer which generates hydrogen gas from hydrocarbon type fuel (methanol, methane, propane, gasoline, or the like), a high-pressure hydrogen gas cylinder, a hydrogen absorbing alloy, or the like.

[0026] Also, inside the FC stack <b>20</b>, a cylindrical valve <b>21</b> is extended in the direction of stacking the unit cell <b>210</b>. The cylindrical valve <b>21</b>, which will be described later in detail, functions as the fuel gas supply manifold M<b>1</b> which distributes the fuel gas to each of the unit cells <b>210</b>, and functions as the switching means for permitting or prohibiting distribution of the fuel gas to each of the unit cells <b>210</b>. When the cylindrical valve <b>21</b> is rotated around the axis by the motor <b>22</b> and is positioned at a permission position for permitting distribution of the fuel gas to each of the unit cells <b>210</b>, the cylindrical valve <b>21</b> permits distribution of the fuel gas to each of the unit cells <b>210</b>. Meanwhile, when the cylindrical valve <b>21</b> is positioned at a prohibition position for prohibiting distribution of the fuel gas, the cylindrical valve <b>21</b> prohibits distribution of the fuel gas to each of the unit cells <b>210</b>.

[0027] The first interrupter <b>30</b> connects the output terminals <b>25</b>, <b>26</b> of the FC stack <b>20</b> with the load <b>60</b>, or disconnects the output terminals <b>25</b>, <b>26</b> from the load <b>60</b> depending on excitation or non-excitation of a solenoid (not shown). An example of the load <b>60</b> is a traction motor with an inverter, for example, in the case of an electric vehicle.

[0028] In the second interrupter <b>40</b>, a short circuiting switch <b>41</b> short-circuits or does not short-circuit the output terminals <b>25</b>, <b>26</b> of the FC stack <b>20</b> through a resistor <b>42</b> depending on excitation or non-excitation of a solenoid (not shown).

[0029] The control device <b>50</b> is formed as a microprocessor in which a CPU <b>51</b> is a central portion. In addition to the CPU <b>51</b>, the control device <b>50</b> includes ROM <b>52</b> which stores various programs, RAM <b>53</b> which temporarily stores data, an input port (not shown), an output port (not shown), and a communication port (not shown). A load request signal is input into the control device <b>50</b> from another control unit (not shown) through the input port. Also, a driving signal to the blower <b>11</b>, the fuel gas supply source <b>15</b>, a motor <b>22</b> or the like, and a switching control signal to the first interrupter <b>30</b>, the second interrupter <b>40</b>, or the like are output from the control device <b>50</b> through the output port.

[0030] Next, the FC stack <b>20</b> will be described. The FC stack <b>20</b> is formed by stacking a plurality of unit cells <b>210</b>, and then by sequentially arranging current collecting plates <b>211</b>, <b>212</b>, insulating plates <b>213</b>, <b>214</b>, and end plates <b>215</b>, <b>216</b> on both ends of the stack, as shown in FIG. 2. Each of the current collecting plates <b>211</b>, <b>212</b> is formed of a gas impermeable conductive member such as a dense carbon, or a steel plate, each of the insulating plates <b>213</b>, <b>214</b> is formed of an insulative member such as rubber, or resin, and each of the end plates <b>215</b>, <b>216</b> is formed of a metal such as steel with rigidity. The output terminals <b>25</b>, <b>26</b> are provided on the current collecting plates <b>211</b>, <b>212</b> respectively. One of the output terminals <b>25</b>, <b>26</b> is the positive electrode, and the other is the negative electrode. Also, the end plates <b>215</b>, <b>216</b> apply pressure to the FC stack <b>20</b> in the unit cell stacking direction by a pressure device (not shown) so as to support the FC stack <b>20</b>.

[0031] The unit cell <b>210</b> is formed by stacking a separator <b>230</b>, a first seal member <b>240</b>, an MEA <b>250</b>, a second seal member <b>260</b>, and a separator <b>230</b> sequentially in this order, as shown in FIG. 3.

[0032] The MEA <b>250</b> is a membrane electrode assembly in which an electrolyte membrane <b>251</b> is sandwiched by an anode <b>252</b> and a cathode <b>253</b>. In this case, the electrolyte membrane <b>251</b> is an ion-exchange membrane (for example, a Nafion membrane manufactured by Dupont) of a proton conductive type which is formed of a solid polymeric material such as fluorocarbon resin, and exhibits good proton conductivity in the humid state. On both surfaces of the electrolyte membrane <b>251</b>, catalytic electrodes are formed by applying platinum or an alloy of platinum and another metal on both surfaces of the electrolyte membrane <b>251</b>. Outside the electrolyte membrane, a pair of gas diffusion electrodes which are formed of carbon cloth that is woven from thread formed of carbon fiber is provided. The catalytic electrode and the gas diffusion electrode on one surface form the anode <b>252</b>, and the catalytic electrode and the gas diffusion electrode on the other surface form the cathode <b>253</b>.

[0033] The separator <b>230</b> is formed from a gas impermeable conductive member such as a formed carbon which is formed by compressing a carbon such that the carbon becomes gas impermeable. In this separator <b>230</b>, a first concave portion <b>231</b> is provided on a surface which faces the anode <b>252</b> of the MEA <b>250</b>, and a fuel gas passage <b>233</b> through which the fuel gas passes is formed in the first concave portion <b>231</b>. Also, a second concave portion <b>232</b> is provided on a surface which faces the cathode <b>253</b> of the MEA <b>250</b>, and an oxidizing gas passage (not shown) through which oxidizing gas passes is formed in the second concave portion <b>232</b>. First to fourth through holes <b>235</b> to <b>238</b> are provided on four corners of the separator <b>230</b> respectively. The first through hole <b>235</b> and the third through hole <b>237</b> which are arranged on one diagonal line are provided in an area that is inside the first concave portion <b>231</b> and outside the second concave portion <b>232</b>. The second through hole <b>236</b> and the fourth through hole <b>238</b> which are arranged on the other diagonal line are provided in an area that is outside the first concave portion <b>231</b> and inside the second concave portion <b>232</b>.

[0034] The first seal member <b>240</b> is provided between the anode <b>252</b> of the MEA <b>250</b> and the separator <b>230</b>, and has a large hole <b>241</b> which coincides with a contour of the first concave portion <b>231</b>, and two small holes <b>246</b>, <b>248</b> which coincide with the second through holes <b>236</b> and the fourth through hole <b>238</b> respectively. The second seal member <b>260</b> is provided between the cathode <b>253</b> of the MEA <b>250</b> and the separator <b>230</b>, and has a large hole <b>261</b> which coincides with an contour of the second concave portion <b>232</b> and two small holes <b>265</b>, <b>267</b> which coincide with the first through hole <b>235</b> and the third through hole <b>237</b> respectively.

[0035] Inside the FC stack <b>20</b>, the fuel gas supply manifold M<b>1</b>, the fuel gas release manifold M<b>3</b>, the oxidizing gas supply manifold M<b>2</b> and the oxidizing gas release manifold M<b>4</b> are formed.

[0036] The cylindrical valve <b>21</b>, which can be rotated around the axis, is inserted into a cylindrical cavity which is formed by stacking the first through hole <b>235</b> of the separator <b>230</b>, the large hole <b>241</b> of the first seal member <b>240</b>, and the small hole <b>265</b> of the second seal member <b>260</b> in the unit cell stacking direction. The cylindrical valve <b>21</b> functions as the fuel gas supply manifold M<b>1</b>. In the cylindrical valve <b>21</b>, a plurality of slit windows <b>21</b><i>a </i>is provided along the axial direction, as shown in FIG. 3. When the cylindrical valve <b>21</b> is positioned such that each slit window <b>21</b><i>a </i>communicates with the first concave portion <b>231</b> of each of the unit cells <b>210</b>, distribution of fuel gas to each of the unit cells <b>210</b> is permitted (this position is referred to as a permission position). Meanwhile, when the cylindrical valve <b>21</b> is positioned such that the slit window <b>21</b><i>a </i>cannot communicate with the first concave portion <b>231</b> of each of the unit cells <b>210</b>, distribution of fuel gas to each of the unit cells <b>210</b> is prohibited (this position is referred to as a prohibition position). The cylindrical valve <b>21</b> is positioned at the permission position or the prohibition position by the motor <b>22</b> (refer to FIG. 1) which is attached to the end plate <b>216</b> of the FC stack <b>20</b>. Then, the fuel gas which is supplied to the cylindrical valve <b>21</b> that is positioned at the permission position passes through the fuel gas passage <b>233</b> provided in the first concave portion <b>231</b> of each of the unit cells <b>210</b>, reaches the fuel gas release manifold M<b>3</b> which is formed by stacking the third through hole <b>237</b> of the separator <b>230</b>, the large hole <b>241</b> of the first seal member <b>240</b>, and the small hole <b>267</b> of the second seal member <b>260</b>, and is released to the outside from the fuel gas release manifold M<b>3</b>.

[0037] The oxidizing gas supply manifold M<b>2</b> is formed by stacking the second through hole <b>286</b> of the separator <b>230</b>, the large hole <b>261</b> of the second seal member <b>260</b>, and the small hole <b>246</b> of the first seal member <b>240</b> in the unit cell stacking direction. The oxidizing gas which is supplied to the oxidizing gas supply manifold M<b>2</b> passes through an oxidizing gas passage (not shown) provided in the second concave portion <b>232</b>, reaches the oxidizing gas release manifold M<b>4</b> which is formed by stacking the fourth through hole <b>238</b> of the separator <b>230</b>, the large hole <b>261</b> of the second seal member <b>60</b>, and the small hole <b>248</b> of the first seal member <b>240</b> in the unit cell stacking direction direction, and is released to the outside from the oxidizing gas release manifold M<b>4</b>.

[0038] Next, the operation of the fuel cell system <b>10</b> according to the embodiment will be described. FIG. 4 is a flowchart showing a cell control process. The CPU <b>51</b> of the control device <b>50</b> reads a cell control program from the ROM <b>52</b> at each predetermined time so as to perform the cell control process. When the cell control process is started, the CPU <b>51</b> initially determines whether the load request which is input from another control device is zero (step S<b>100</b>). When the load request is not zero, the CPU <b>51</b> performs regular control corresponding to a load request (step S<b>150</b>), afterwhich the cell control process ends. In the regular control corresponding to a load request, the output terminals <b>25</b>, <b>26</b> of the FC stack <b>20</b> are connected with the load <b>60</b> by the first interrupter <b>30</b>, and the short-circuiting switch <b>41</b> is opened by the second interrupter <b>40</b> such that the output terminals <b>25</b>, <b>26</b> of the FC stack <b>20</b> are not short-circuited, and the cylindrical valve <b>21</b> is positioned at the permission position by the motor <b>22</b>. When the fuel gas and the oxidizing gas are supplied to the FC stack <b>20</b> by the amount that corresponds to the load request, electric power which corresponds to the supply amounts is supplied to the load <b>60</b>. Since the humidified oxidizing gas is supplied to the FC stack <b>20</b>, the electrolyte membrane <b>251</b> is continuously humid, and good proton conductivity can be maintained.

[0039] Meanwhile, when the load request is zero in step S<b>100</b>, in order to realize OCV (Open Circuit Voltage) state, the output terminals <b>25</b>, <b>26</b> of the FC stack <b>20</b> are disconnected from the load <b>60</b> by opening the first interrupter <b>30</b> which has been closed (step S<b>110</b>). Next, the cylindrical valve <b>21</b> is moved from the permission position and is positioned at the prohibition position by driving the motor <b>22</b> such that the inlet of the first concave portion <b>231</b>, that is, the portion in the vicinity of the inlet of the fuel gas passage <b>233</b> is closed (step S<b>120</b>), and the driving of the blower <b>11</b> is stopped (step S<b>130</b>). Thus, the amount of the hydrogen gas which remains in the FC stack <b>20</b> at the OCV time becomes equivalent to the capacity of the first concave portion <b>231</b> (approximately the capacity of the fuel gas passage <b>233</b>). Also, the supply of the oxidizing gas is stopped. Then, the short-circuiting switch <b>41</b> is closed by the second interrupter <b>40</b> such that the output terminals <b>25</b>, <b>26</b> of the FC stack <b>20</b> are short-circuited through a resistor <b>42</b> (step S<b>140</b>). At this time, the hydrogen gas whose amount is equivalent to the capacity of the first concave portion <b>231</b> is consumed, which generates corresponding heat. Due to this heat generation, each of the unit cells <b>210</b> becomes relatively dry, which is preferable for preventing cross leak of the hydrogen. The fact that the load request is zero signifies that, for example, in the case of an electric vehicle, the vehicle is in the idling state or the like. When the secondary battery is charged, the fact that the load request is zero signifies that the battery is fully charged.

[0040] In the fuel cell system <b>10</b> according to the embodiment, which has been described in detail, when the output terminals <b>25</b>, <b>26</b> of the FC stack <b>20</b> are disconnected from the load <b>60</b>, the hydrogen in each of the unit cells <b>210</b> is consumed by short-circuiting the output terminals <b>25</b>, <b>26</b> of the FC stack <b>20</b> after distribution of the fuel gas to the fuel gas passage <b>233</b> of each of the unit cells <b>210</b> is prohibited in the fuel gas supply manifold M<b>1</b>. Accordingly, cross leak of hydrogen can be prevented. Also, distribution of the fuel gas is stopped in the fuel gas supply manifold M<b>1</b>, unlike the conventional method in which the supply of the fuel gas is stopped in the gas piping which is provided outside the stack. Accordingly, the amount of the hydrogen which remains after distribution of the fuel gas is stopped is smaller, the amount of the hydrogen which is consumed when the output terminal <b>25</b>, <b>26</b> are short-circuited is smaller, and the amount of heat generation is smaller, as compared with the conventional method. Thus, deterioration of durability due to heat generated at the time of short-circuit can be suppressed, and unnecessary consumption of large amount of hydrogen can be avoided. Therefore, cross leak can be prevented when the output terminals <b>25</b>, <b>26</b> of the FC stack <b>20</b> are disconnected from the load <b>60</b>, and deterioration of fuel economy can be avoided.

[0041] Also, each of the unit cells <b>210</b> becomes relatively dry due to heat which is generated when the output terminals <b>25</b>, <b>26</b> of the FC stack <b>20</b> are short-circuited, which is advantageous for the prevention of cross leak.

[0042] Further, since the cylindrical valve <b>21</b> performs opening and closing in the vicinity of the inlet of the fuel gas passage <b>233</b> of each of the unit cells <b>210</b>, the amount of the hydrogen which is consumed at the time of short-circuit can be further reduced, and heat which is generated at the time of short-circuit is further reduced, which is advantageous for the prevention of deterioration of fuel economy.

[0043] It is apparent that the invention is not limited to the above-mentioned embodiment, and that the invention may be realized in various other embodiments within the scope of the invention.

[0044] For example, in the above-mentioned embodiment, the oxidizing gas which is humidified by the humidifier <b>13</b> is supplied from the blower <b>11</b> to the FC stack <b>20</b>. However, as shown in FIG. 5, a three-way valve <b>12</b> may be provided between the blower <b>11</b> and the humidifier <b>13</b> such that unhumidified oxidizing gas is supplied from the three-way valve <b>12</b> to the FC stack <b>20</b> through a bypass route <b>14</b> which bypasses the humidifier <b>13</b>. At the time of regular control corresponding to a load request, the control device <b>50</b> may control the three-way valve <b>12</b> such that the oxidizing gas is supplied from the blower <b>11</b> to the FC stack <b>20</b> through the humidifier <b>13</b>. Meanwhile, at the OCV time, the control device <b>50</b> may control the three-way valve <b>12</b> such that the oxidizing gas is supplied from the blower <b>11</b> to the FC stack <b>20</b> through the bypass route <b>14</b>. In this manner, at the OCV time, dry oxidizing gas is supplied to the FC stack <b>20</b>. Thus, since the water which is generated at the cathode by electrochemical reaction is efficiently released by the dry oxidizing gas, each of the unit cells <b>210</b> becomes dry, and cross leak becomes difficult to cause.

[0045] Also, in the above-mentioned embodiment, the cylindrical valve <b>21</b> is employed as switching means. However, as shown in FIG. 6, a switching mechanism <b>70</b> which has a lid portion <b>71</b> that can close an inlet of the fuel gas passage at the tip thereof, and has a rotational shaft <b>72</b> at the base end may be employed. At the time of regular control corresponding to a load request, the lid portion <b>71</b> may open the inlet of the fuel gas passage of each of the unit cells. Meanwhile, at the OCV time, the inlet of the fuel gas passage of each of the unit cells may be closed by the lid portion <b>71</b> by rotating a rotational shaft <b>72</b>. In this case, the lid portion <b>71</b> needs to be formed of resin, rubber or the like. Alternatively, as disclosed in Japanese Patent Laid-Open Publication No. 9-312168, a slide slit may be employed. At the time of regular control corresponding to a load request, the slit may open the inlet of the fuel gas passage of each of the unit cells. Meanwhile, at the OCV time, the inlet of the fuel gas passage of each of the unit cells may be closed by sliding the slit.

[0046] Further, in place of the motor <b>22</b> which is employed in the above-mentioned embodiment, an actuator which is driven by air pressure or hydraulic pressure may be employed. Furthermore, a ground switch may be provided between the short-circuiting switch <b>41</b> of the second interrupter <b>40</b> and the output terminal <b>26</b> in the above-mentioned embodiment such that the ground switch is opened and closed in accordance with opening and closing of the short-circuiting switch <b>41</b>.

[0047] When the load request is zero, the control device of the fuel cell system disconnects both of the output terminals of the FC stack from the load (step S<b>110</b>). Next, the control device performs closing in the vicinity of the inlet of the fuel gas passage (step S<b>120</b>), and stops the supply of the oxidizing gas (step S<b>130</b>). Thus, the amount of the hydrogen gas which remains in the FC stack at the OCV time becomes approximately equivalent to the capacity of the fuel gas passage. Then, both of the output terminals of the FC stack are short-circuited (step S<b>140</b>). Since the remaining hydrogen gas is consumed at this time, cross leak of the hydrogen is not caused thereafter. Also, although heat is generated at the time of short-circuit, the amount of remaining hydrogen gas is smaller, as compared with the conventional method. Accordingly, since the amount of generated heat is small, a negative effect can be avoided.

[0048] While the invention has been described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the exemplary embodiments or constructions. To the contrary, the invention is intended to cover various elements of the exemplary embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.