Fuel cells with multidirectional fuel flow

In a stack of non-circulating-type fuel cells 20 where a supply of a fuel gas is not recirculated, a control circuit 50 sets a changeover valve assembly 41 in a disconnected state from both high-pressure hydrogen tanks 30 and outside of a cell stack body 21, while setting residual changeover valve assemblies 40, 42, and 43 in a connecting state to connect inside of the cell stack body 21 with the high-pressure hydrogen tanks 30. The supply of the fuel gas is accordingly fed into the stack of fuel cells 20 via the changeover valve assemblies 40, 42, and 43 and goes through electrochemical reactions. An impurity-containing gas after the electrochemical reactions is accumulated in the vicinity of a connection port 22. The control circuit 50 then sets the changeover valve assembly 41 in a connecting state to connect the inside of the fuel cells 20 with the outside and discharge the impurity-containing gas to the outside of the fuel cells 20. The control circuit 50 sequentially changes over the changeover valve assemblies 40 to 43 to the connecting state and thereby prevents the impurity-containing gas from being accumulated in the vicinity of any specific connection port. This arrangement desirably restrains potential deterioration of power generation performances of fuel cells and enhances the durability of the fuel cells.

FIELD OF THE INVENTION

The present invention relates to a fuel cell including only one unit cell, as well as to a stack of fuel cells including multiple unit cells laid one upon another.

DESCRIPTION OF THE RELATED ART

There are two typical methods to feed a supply of a fuel gas (hydrogen-rich gas) to fuel cells. The first method is a circulating-type supply that the fuel gas fed to fuel electrodes or anodes to go through electrochemical reactions is recirculated to the anodes. The second method is a non-circulating-type supply that the fuel gas fed to the anodes is not recirculated to the anodes.

The non-circulating-type fuel cells do not require any piping structures and pumps for recirculation of the fuel gas to the anodes. This leads to size reduction of a fuel cell system.

In the prior art non-circulating-type fuel cells, sufficient power generation is performed in unit cells in an area from an inlet to the approximate center of the fuel gas flow path, while unit cells in an area close to the outlet of the fuel gas flow path have a relatively little contribution to power generation. The unit cells in the area from the inlet to the approximate center of the fuel gas flow path accordingly supplement the insufficient power generation by the unit cells in the area close to the outlet of the fuel gas flow path. This leads to thermal deterioration of the anodes (fuel electrodes) and cathodes (air electrodes) of the unit cells in the area from the inlet to the approximate center of the fuel gas flow path, as well as deterioration of the catalyst and carrier in the cathodes of the unit cells in the area close to the outlet of the fuel gas flow path.

The increasing power generation raises the reaction heat in the unit cells and thermally deteriorates or decomposes the material of the anode electrodes. This is the thermal deterioration of the cathodes and the anodes. This problem arises in both the unit cell structure and the stack structure of multiple unit cells.

An increase in potential of the cathodes in the unit cells in the area close to the outlet of the fuel gas flow path having a little contribution to power generation causes electrochemical damages of the cathodes. This is the deterioration of the catalyst and the carrier in the cathodes. Convection of nitrogen and water, which are transmitted from the cathodes, in the anodes forms a quasi internal cell. The electrons, which are originally to be flowed from the anodes to the cathodes, inversely run from the cathodes to the anodes in the stack structure of multiple unit cells. The electrons also run on the plane of the cathode and on the plane of the anode both in the unit cell structure and in the stack structure of multiple unit cells. Generation of electrons on the cathodes is required for such phenomena. The electrons are produced by reaction of carbon with water or by ionization of platinum. This results in consumption of carbon as the carrier and platinum as the catalyst on the cathodes.

DISCLOSURE OF THE INVENTION

The object of the invention is thus to restrain potential deterioration of power generation performances of non-circulating-type fuel cells where a supply of a fuel gas is not recirculated, as well as to enhance the durability of the fuel cells.

In order to attain at least part of the above and the other related objects, a first aspect of the present invention provides a stack of fuel cells, which are a fuel gas non-circulating-type where a supply of a fuel gas is not recirculated. The stack of fuel cells of the first aspect of the invention includes: a cell stack body that includes multiple unit cells laid one upon another and has a fuel gas flow path formed therein to make a flow of the fuel gas; multiple connection mechanisms that are provided with the cell stack body and function to connect the fuel gas flow path formed in the cell stack body with outside of the cell stack body and disconnect the fuel gas flow path in the cell stack body from the outside of the cell stack body; and a connection mechanism control module that selectively changes over the multiple connection mechanisms to a connecting state, when a preset condition is satisfied.

In the first aspect of the invention, under the preset condition, the multiple connection mechanisms provided with the cell stack body are selectively changed over to the connecting state to connect the fuel gas flow path formed in the cell stack body with the outside of the cell stack body. This selectively changes the flow direction of the fuel gas through the fuel gas flow path and accelerates diffusion of the fuel gas inside the cell stack body. This arrangement desirably restrains potential deterioration of power generation performances of the fuel cells and enhances the durability of the fuel cells.

In the first aspect of the invention, the selective changeover of the multiple connection mechanisms to the connecting state may be attained by setting at least one connection mechanism located downstream the fuel gas flow path in a connecting position. This arrangement activates the flow of the fuel gas in the fuel gas flow path and effectively discharges the fuel gas with a high content of impurities, which is accumulated downstream the fuel gas flow path, to the outside of the cell stack body.

A second aspect of the invention provides a stack of fuel cells, which are a fuel gas non-circulating-type where a supply of a fuel gas is not recirculated. The second stack of fuel cells of the second aspect of the invention includes: a cell stack body that includes multiple unit cells laid one upon another and has a fuel gas flow path formed therein to make a flow of the fuel gas; multiple connection mechanisms that are provided with the cell stack body and function to connect the fuel gas flow path formed in the cell stack body with either outside of the cell stack body or a fuel gas supply source and disconnect the fuel gas flow path in the cell stack body from both the outside of the cell stack body and the fuel gas supply source; and a connection mechanism control module that, in a normal operating condition, sets at least one connection mechanism among the multiple connection mechanisms in a disconnected state from both the outside of the cell stack body and the fuel gas supply source, while setting residual connection mechanisms in a connecting state with the fuel gas supply source. When a preset condition is satisfied, the connection mechanism control module changes over the at least one connection mechanism, which is set in the disconnected state from both the outside of the cell stack body and the fuel gas supply source, to connect the fuel gas flow path to the outside of the cell stack body.

In the second aspect of the invention, under the preset condition, among the multiple connection mechanisms provided with the cell stack body, the at least one connection mechanism set in the disconnected state from both the outside of the cell stack body and the fuel gas supply source is changed over to connect the fuel gas flow path with the outside of the cell stack body. Such changeover effectively activates the flow of the fuel gas in the cell stack body. A sufficient quantity of the fuel gas is thus fed to the cell stack body in the vicinity of the at least one connection mechanism, which is set in the disconnected state from both the outside of the cell stack body and the fuel gas supply source. This arrangement desirably restrains potential deterioration of power generation performances of the fuel cells and enhances the durability of the fuel cells.

In the second aspect of the invention, it is preferable that the connection mechanism control module sequentially selects different connection mechanisms as the at least one connection mechanism, which is set in the disconnected state from both the outside of the cell stack body and the fuel gas supply source, to be successively connected with the outside of the cell stack body. The embodiment of this structure sequentially selects the different connection mechanisms as the at least one connection mechanism, which is set in the disconnected state from both the outside of the cell stack body and the fuel gas supply source, to be connected with the outside of the cell stack body. This selectively changes the flow direction of the fuel gas through the fuel gas flow path and accelerates diffusion of the fuel gas inside the cell stack body. This arrangement desirably restrains potential deterioration of power generation performances of the fuel cells and enhances the durability of the fuel cells.

The first stack of fuel cells or the second stack of fuel cells may further include a voltage measurement unit that measures an output voltage. The preset condition is satisfied when the measured output voltage is less than a preset level. This structure estimates an increase in variation of the power generation among the respective unit cells. Such estimation is effectively usable to restrain potential deterioration of power generation performances of the fuel cells and enhance the durability of the fuel cells.

The first stack of fuel cells or the second stack of fuel cells also may further include multiple connection ports that connect an air flow path formed in the cell stack body with the outside of the cell stack body; multiple reference electrodes arranged respectively close to the multiple connection mechanisms and the multiple connection ports; and a determination module that determines an increase rate in potential measured by at least one of the reference electrodes. The preset condition is satisfied when the determination module determines that the increase rate in potential in a neighborhood of the at least one connection mechanism, which is set in the disconnected state from both the outside of the cell stack body and the fuel gas supply source, is not less than a preset level. This structure estimates an increase in potential of the cathode. Such estimation is effectively usable to restrain potential deterioration of power generation performances of the fuel cells and enhance the durability of the fuel cells.

In either of the first or second aspect of the invention, the preset condition is satisfied when a predetermined time period has elapsed since setting of the at least one connection mechanism in the disconnected state from both the outside of the cell stack body and the fuel gas supply source. This structure regularly varies the flow of the fuel gas in the cell stack body and sufficiently diffuses the fuel gas inside the cell stack body.

A third aspect of the invention provides a fuel cell, which is a fuel gas non-circulating-type where a supply of a fuel gas is not recirculated. The fuel cell of the third aspect of the invention has a membrane electrode assembly located between an anode separator and a cathode separator. This fuel cell further includes: a fuel gas passage defined by the anode separator and the membrane electrode assembly; multiple connection mechanisms that are formed in the anode separator and function to connect the fuel gas passage with outside of the fuel cell and disconnect the fuel gas passage from the outside of the fuel cell; and a connection mechanism control module that selectively changes over the multiple connection mechanisms to a connecting state, when a preset condition is satisfied.

The third aspect of the invention exerts the same functions and effects as those of the stack of fuel cells of the first aspect of the invention discussed above. The diverse arrangements adopted for the stack of fuel cells of the first aspect of the invention are also applicable to the fuel cell of the third aspect of the invention.

A fourth aspect of the invention provides a fuel cell, which is a fuel gas non-circulating-type where a supply of a fuel gas is not recirculated. The fuel cell of the fourth aspect of the invention has a membrane electrode assembly located between an anode separator and a cathode separator. This fuel cell further includes: a fuel gas passage defined by the anode separator and the membrane electrode assembly; multiple connection mechanisms that are formed in the anode separator and function to connect the fuel gas passage with either outside of the fuel cell or a fuel gas supply source and disconnect the fuel gas passage from both the outside of the fuel cell and the fuel gas supply source; and a connection mechanism control module that, in a normal operating condition, sets at least one connection mechanism among the multiple connection mechanisms in a disconnected state from both the outside of the fuel cell and the fuel gas supply source, while setting residual connection mechanisms in a connecting state with the fuel gas supply source. When a preset condition is satisfied, the connection mechanism control module changes over the at least one connection mechanism, which is set in the disconnected state from both the outside of the fuel cell and the fuel gas supply source, to connect the fuel gas passage to the outside of the fuel cell.

The fourth aspect of the invention exerts the same functions and effects as those of the stack of fuel cells of second aspect of the invention discussed above. The diverse arrangements adopted for the stack of fuel cells of the second aspect of the invention are also applicable to the fuel cell of the fourth aspect of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The fuel cells of the invention are described with some preferable embodiments with reference to the drawings.

A. First Embodiment

A fuel cell system including fuel cells is described as a first embodiment of the invention with reference toFIGS. 1 and 2.FIG. 1schematically illustrates the configuration of a fuel cell system10including a fuel cell stack20in the first embodiment.FIG. 2schematically illustrates the internal structure of the fuel cell stack20in the first embodiment.

The fuel cell system10of the first embodiment includes the fuel cell stack20of multiple unit cells laid one upon another, high-pressure hydrogen tanks30that store hydrogen gas to be supplied as a fuel gas to the fuel cello stack20, and a control circuit50that controls changeover valve assemblies40,41,42, and43provided for the fuel cell stack20.

The fuel cell stack20has a cell stack body21of multiple unit fuel cells210as minimum units laid one upon another, and connection ports22,23,24, and25connecting the inside of the cell stack body21with the outside. The unit fuel cell210typically includes a membrane-electrode assembly, which has a pair of electrodes arranged on both faces of an electrolyte membrane and is interposed between an anode separator and a cathode separator. As shown inFIG. 2, fuel gas flow paths211,212, and213for the flow of fuel gas are formed inside the cell stack body21. The ends of the fuel gas flow paths211and212are connected with the connection ports22,23,24, and25.

The fuel cell stack20is constructed as non-circulating-type fuel cells, where the fuel gas flowing through the fuel gas flow paths211,212, and213and going through electrochemical reactions is not resupplied to the fuel gas flow paths211,212, and213(the fuel cell stack20) but is discharged outside the fuel cell stack20. A load55is linked to the fuel cell stack20as shown inFIG. 1. A voltage sensor51is located on a feed line551connecting the fuel cell stack20with the load55to measure the output voltage of the fuel cell stack20.

The fuel gas flow paths211,212, and213are connected with intra-cell fuel gas passages214formed in the respective unit fuel cells210. The fuel gas flows through the fuel gas flow paths211,212, and213and is led into the respective unit fuel cells210via the fuel gas passages214.

The changeover valve assemblies40,41,42, and43are respectively connected with the connection ports23,22,24, and25of the fuel cell stack20. In the structure of this embodiment, each of the changeover valve assemblies40,41,42, and43has a three-way valve V3and a two-way valve V2for electromagnetic changeover between a connecting state and a disconnecting state. The changeover valve assemblies40,41,42, and43function to selectively connect the fuel gas flow paths211and212formed in the cell stack body21with either the high-pressure hydrogen tanks30as fuel gas supply sources or the outside of the cell stack body21or disconnect the fuel gas flow paths211and212from both the high-pressure hydrogen tanks30and the outside of the cell stack body21. In the normal operating conditions, one of the changeover valve assemblies40,41,42, and43is disconnected from both the high-pressure hydrogen tanks30and the outside of the cell stack body21, so that the fuel cell stack20works as the non-circulating-type fuel cells. The changeovers of the respective changeover valve assemblies40,41,42, and43are determined by the driving conditions of the fuel cell system10.

Although not specifically illustrated, the control circuit50includes a CPU that executes diverse series of operations, a ROM that stores changeover programs for the changeover valve assemblies40,41,42, and43and diversity of other processing programs, and a RAM that temporarily stores the results of the operations by the CPU and various data. The control circuit50controls the operating conditions of the fuel cell system10. Actuators (electromagnetic actuators) of the respective changeover valve assemblies40,41,42, and43and the voltage sensor51are connected to the control circuit50.

The description now regards the flow state of the fuel gas in the cell stack body of the fuel cell stack20with reference toFIG. 2. Here it is assumed that the connection port22is closed to make the fuel cell stack20work as the non-circulating-type fuel cells. In the closed position of the connection port22, the changeover valve assembly41is disconnected from both the high-pressure hydrogen tanks30and the outside of the cell stack body21.

The flow of fuel gas supplied from one of the high-pressure hydrogen tanks30is fed to the fuel gas flow paths211and212via the connection ports23,24, and25. The fuel gas supplied to the fuel gas flow paths211and212flows through the intra-cell fuel gas passages214into the fuel gas flow path213. The fuel gas is thus homogeneously fed to the respective unit fuel cells210.

The fuel gas that has supplied to the fuel gas flow paths211and212but has not been flowed to the intra-cell fuel gas passages214or has flowed into the fuel gas flow path213eventually reaches the vicinity of the closed connection port22. The fuel gas accumulated in the vicinity of the connection port22tends to have a relatively low hydrogen concentration and a large quantity of impurities including nitrogen.

The flow states of the fuel gas and the impurity-containing gas in the fuel cell stack20of the first embodiment and the operating states of the changeover valve assemblies40,41,42, and43are described below with reference toFIGS. 3 to 14.FIG. 3shows a first flow state of the fuel gas in the fuel cell stack20in normal operating conditions.FIG. 4shows a first flow state of the fuel gas and the impurity-containing gas in the fuel cell stack20in impurity-containing gas discharge conditions.FIG. 5shows a second flow state of the fuel gas in the fuel cell stack20in the normal operating conditions.FIG. 6shows a second flow state of the fuel gas and the impurity-containing gas in the fuel cell stack20in the impurity-containing gas discharge conditions.FIG. 7shows a third flow state of the fuel gas in the fuel cell stack20in the normal operating conditions.FIG. 8shows a third flow state of the fuel gas and the impurity-containing gas in the fuel cell stack20in the impurity-containing gas discharge conditions.FIG. 9shows a fourth flow state of the fuel gas in the fuel cell stack20in the normal operating conditions.FIG. 10shows a fourth flow state of the fuel gas and the impurity-containing gas in the fuel cell stack20in the impurity-containing gas discharge conditions.FIG. 11shows a first operating state of a changeover valve assembly to feed a supply of the fuel gas in the normal operating conditions and in the impurity-containing gas discharge conditions.FIG. 12shows a second operating state of the changeover valve assembly to discharge the impurity-containing gas in the normal operating conditions.FIG. 13shows a third operating state of the changeover valve assembly to discharge the impurity-containing gas in the impurity-containing gas discharge conditions.FIG. 14shows a variation in contribution to power generation among unit cells in a prior art fuel cell stack.

At the initial stage of normal operation, the control circuit50sets the changeover valve assembly41in the second operating state (seeFIG. 12) to disconnect the changeover valve assembly41from both the high-pressure hydrogen tanks30and the outside of the cell stack body21, while setting the other changeover valve assemblies40,42, and43in the first operating state (seeFIG. 11) to connect the fuel gas flow paths211and212formed in the cell stack body21with the high-pressure hydrogen tanks30. The flow of the fuel gas in the fuel cell stack20is accordingly set in the first flow state shown inFIG. 3. The fuel gas is supplied through the changeover valve assemblies40,42, and43, goes through the electrochemical reactions, and is accumulated in the vicinity of the connection port22(the changeover valve assembly41). The fuel gas accumulated in the vicinity of the connection port22has a low hydrogen concentration and a relatively large quantity of impurities including nitrogen and water.

When the output voltage of the fuel cell stack20measured by the voltage sensor51is lowered below a preset level, the control circuit50detects the necessity of discharge of the impurity-containing gas and sets the changeover valve assembly41in the third operating state (seeFIG. 13) to connect the fuel gas flow path211in the cell stack body21with the outside of the cell stack body21. The output voltage of the fuel cell stack20is accordingly lowered with an increase in quantity of the impurities, such as nitrogen and water, which are transmitted through the electrolyte membrane from the cathode side to the anode side. The impurity-containing gas accumulated in the vicinity of the connection port22is then discharged to the outside of the cell stack body21, for example, to the atmosphere, as shown inFIG. 4.

After a preset time period has elapsed since the changeover of the changeover valve assembly41to the third operating state, the control circuit50sets the changeover valve assembly43in the second operating state (seeFIG. 12) to disconnect the changeover valve assembly43from both the high-pressure hydrogen tanks30and the outside of the cell stack body21, while setting the other changeover valve assemblies40,41, and42in the first operating state (seeFIG. 11) to connect the fuel gas flow paths211and212formed in the cell stack body21with the high-pressure hydrogen tanks30. This sets the driving state of the fuel cell stack20to the normal driving conditions. The flow of the fuel gas in the fuel cell stack20is accordingly set in the second flow state shown inFIG. 5. The fuel gas is supplied through the changeover valve assemblies40,41, and42, goes through the electrochemical reactions, and is accumulated in the vicinity of the connection port25(the changeover valve assembly43). The fuel gas accumulated in the vicinity of the connection port25has a low hydrogen concentration and a relatively large quantity of impurities including nitrogen and water.

When the output voltage of the fuel cell stack20measured by the voltage sensor51is lowered below the preset level, the control circuit50detects the necessity of discharge of the impurity-containing gas and sets the changeover valve assembly43in the third operating state (seeFIG. 13) to connect the fuel gas flow path211in the cell stack body21with the outside of the cell stack body21. The impurity-containing gas accumulated in the vicinity of the connection port25is then discharged to the outside of the cell stack body21, for example, to the atmosphere, as shown inFIG. 6.

After a preset time period has elapsed since the changeover of the changeover valve assembly43to the third operating state, the control circuit50sets the changeover valve assembly42in the second operating state (seeFIG. 12) to disconnect the changeover valve assembly42from both the high-pressure hydrogen tanks30and the outside of the cell stack body21, while setting the other changeover valve assemblies40,41, and43in the first operating state (seeFIG. 11) to connect the fuel gas flow paths211and212formed in the cell stack body21with the high-pressure hydrogen tanks30. This sets the driving state of the fuel cell stack20to the normal driving conditions. The flow of the fuel gas in the fuel cell stack20is accordingly set in the third flow state shown inFIG. 7. The fuel gas is supplied through the changeover valve assemblies40,41, and43, goes through the electrochemical reactions, and is accumulated in the vicinity of the connection port24(the changeover valve assembly42). The fuel gas accumulated in the vicinity of the connection port24has a low hydrogen concentration and a relatively large quantity of impurities including nitrogen and water.

When the output voltage of the fuel cell stack20measured by the voltage sensor51is lowered below the preset level, the control circuit50detects the necessity of discharge of the impurity-containing gas and sets the changeover valve assembly42in the third operating state (seeFIG. 13) to connect the fuel gas flow path212in the cell stack body21with the outside of the cell stack body21. The impurity-containing gas accumulated in the vicinity of the connection port24is then discharged to the outside of the cell stack body21, for example, to the atmosphere, as shown inFIG. 8.

After a preset time period has elapsed since the changeover of the changeover valve assembly42to the third operating state, the control circuit50sets the changeover valve assembly40in the second operating state (seeFIG. 12) to disconnect the changeover valve assembly40from both the high-pressure hydrogen tanks30and the outside of the cell stack body21, while setting the other changeover valve assemblies41,42, and43in the first operating state (seeFIG. 11) to connect the fuel gas flow paths211and212formed in the cell stack body21with the high-pressure hydrogen tanks30. This sets the driving state of the fuel cell stack20to the normal driving conditions. The flow of the fuel gas in the fuel cell stack20is accordingly set in the fourth flow state shown inFIG. 9. The fuel gas is supplied through the changeover valve assemblies41,42, and43, goes through the electrochemical reactions, and is accumulated in the vicinity of the connection port23(the changeover valve assembly40). The fuel gas accumulated in the vicinity of the connection port23has a low hydrogen concentration and a relatively large quantity of impurities including nitrogen and water.

When the output voltage of the fuel cell stack20measured by the voltage sensor51is lowered below the preset level, the control circuit50detects the necessity of discharge of the impurity-containing gas and sets the changeover valve assembly40in the third operating state (seeFIG. 13) to connect the fuel gas flow path212in the cell stack body21with the outside of the cell stack body21. The impurity-containing gas accumulated in the vicinity of the connection port22is then discharged to the outside of the cell stack body21, for example, to the atmosphere, as shown inFIG. 10.

The control circuit50sequentially repeats the four patterns discussed above. Instead of the measured output voltage of the fuel cell stack20, an empirically determined reference time may be used for detection of the necessity of discharge of the impurity-containing gas. The necessity of discharge may otherwise be determined according to an increase rate or a decrease rate of voltage measured by reference electrodes located in the respective unit fuel cells210.

As described above, the fuel cell stack20of the first embodiment sequentially shifts the changeover valve assembly to discharge the impurity-containing gas. Such sequential shift successively changes the accumulation area of the impurity-containing gas (that is, the position of the closed connection port). This arrangement desirably prevents the impurity-containing gas from being accumulated in a fixed area of the fuel cell stack20, thus restraining potential deterioration of the power generation performances of the fuel cell stack20and enhancing the durability of the fuel cell stack20.

The successive change of the accumulation area of the impurity-containing gas fully diffuses the fuel gas over the whole area of the fuel cell stack20and enables all the unit fuel cells210included in the fuel cell stack20to evenly generate electric power. This arrangement desirably eliminates the drawback of the prior art structure of a fuel cell stack70shown inFIG. 14. In the prior art structure of FIG.14, upstream unit fuel cells71a(that is, unit cells in an area from the inlet to the approximate center) among all the unit fuel cells71aof the fuel cell stack70mainly contribute to power generation, while downstream unit fuel cells71b(that is, unit cells in an area close to the outlet) make only a little contribution to power generation. The arrangement of the embodiment effectively prevents potential damage of the anodes and the cathodes in the upstream unit fuel cells (in the area from the inlet to the center) due to a high reaction heat by the excessive reaction (power generation), as well as potential damage of the cathodes in the downstream unit fuel cells (in the area close to the outlet) due to electrochemical reactions with an increase in potential.

In the structure of the first embodiment, each of the connection ports22to25is one hole having both the function of feeding the fuel gas and discharging the impurity-containing gas. This desirably simplifies the structure of the cell stack body21and the fuel gas flow paths211,212, and213, while reducing the total size of the fuel cell stack20.

B. Second Embodiment

A fuel cell60in a second embodiment of the invention is described below with reference toFIGS. 15 and 16.FIG. 15schematically illustrates the internal structure of the fuel cell60in the second embodiment.FIG. 16shows a variation in power generation area as a drawback of a prior art fuel cell (unit cell).

The fuel cell60of the second embodiment has only one unit cell. The potential damage of the electrode catalyst due to a difference in contribution to power generation is not the unique problem arising in a stack of fuel cells having a difference between upstream unit fuel cells and downstream unit fuel cells, but also arises in an intra-cell fuel gas passage formed in the unit cell.

The fuel cell60of the second embodiment includes a separator61having two inlet/outlet ports611and612and an intra-cell fuel gas passage613that interconnects the two inlet/outlet ports611and612to make a flow of the fuel gas supplied from a high-pressure hydrogen tank. Reference electrodes R for measuring the potential are located in the intra-cell fuel gas passage613close to the respective inlet/outlet ports611and612in the separator61. The fuel cell60also has changeover valve assemblies62and63, which selectively connect the intra-cell fuel gas passage613with either the outside of the fuel cell60or the high-pressure hydrogen tank or disconnect the intra-cell fuel gas passage613from both the outside of the fuel cell60and the high-pressure hydrogen tank.

At the initial stage of normal operation, the changeover valve assembly62is connected with the high-pressure hydrogen tank, whereas the changeover valve assembly63is disconnected from both the outside of the fuel cell60and the high-pressure hydrogen tank. Continuous operation in this state causes the impurity-containing gas to be accumulated in an area of the intra-cell fuel gas passage613in the vicinity of the inlet/outlet port612. Such accumulation raises the potential measured by the reference electrodes R located close to the inlet/outlet port612.

When an increase rate of the potential measured by the reference electrodes R (that is, a potential increase rate per unit time) exceeds a preset level, the changeover valve assembly63is connected with the outside of the fuel cell60to discharge the impurity-containing gas accumulated in the vicinity of the inlet/outlet port612to the outside of the fuel cell60.

The changeover valve assembly63is then connected with the high-pressure hydrogen tank, while the changeover valve assembly62is disconnected from both the outside of the fuel cell stack60and the high-pressure hydrogen tank. Continuous operation in this state causes the impurity-containing gas to be accumulated in an area of the intra-cell fuel gas passage613in the vicinity of the inlet/outlet port611. Such accumulation raises the potential measured by the reference electrodes R located close to the inlet/outlet port611.

When the increase rate of the potential measured by the reference electrodes R (that is, the potential increase rate per unit time) exceeds the preset level, the changeover valve assembly62is connected with the outside of the fuel cell60to discharge the impurity-containing gas accumulated in the vicinity of the inlet/outlet port611to the outside of the fuel cell60.

These two patterns discussed above are sequentially repeated. The necessity of discharge of the impurity-containing gas may be detected according to the measured output voltage of the fuel cell60or according to an empirically determined reference time, instead of the increase rate of the potential measured by the reference electrodes R.

As described above, the fuel cell60of the second embodiment sequentially shifts the changeover valve assembly to discharge the impurity-containing gas. Such sequential shift successively changes the accumulation area of the impurity-containing gas on the separator61(electrode). This arrangement desirably prevents the impurity-containing gas from being accumulated in a fixed area of the fuel cell60, thus restraining potential deterioration of the power generation performance of the fuel cell60and enhancing the durability of the fuel cell60.

The successive change of the accumulation area of the impurity-containing gas fully diffuses the fuel gas over the whole area of the fuel cell60(the intra-cell fuel gas passage613) and enables the whole area of the fuel cell60to evenly generate electric power. This arrangement desirably eliminates the drawback of the prior art structure of a fuel cell shown inFIG. 16. In the prior art structure ofFIG. 16, only an upstream area713a(that is, an area from the inlet to the approximate center) of an intra-cell fuel gas passage713mainly contributes to power generation, while a downstream area713b(that is, an area close to the outlet) makes only a little contribution to power generation. The arrangement of the second embodiment effectively prevents potential damage of the anode and the cathode in the area from the inlet to the center due to a high reaction heat by the excessive reaction (power generation), as well as potential damage of the cathode in the area close to the outlet due to electrochemical reactions with an increase in potential.

In the structure of the first embodiment, the connection ports22to25are formed in parallel with the vertical sides on the transverse sections of the fuel cell stack20. The connection ports may have a three-dimensional arrangement, for example, may be located diagonally on the transverse section.

The configuration of the fuel gas flow paths211and212in the fuel cell stack20of the first embodiment is only illustrative and not restrictive at all. The fuel gas flow paths may have any adequate arrangement, for example, a diagonal arrangement.

The fuel cell stack20(the cell stack body21) of the first embodiment has the four connection ports22to25. The number of the connection ports is not restrictive to 4, but may be any suitable value of not less than 2. Each of the connection ports22to25has both the function of supplying the fuel gas and the function of discharging the impurity-containing gas. Connection ports having only one of these functions may be arranged in pairs or separately. In this modified structure, each connection port is provided with a changeover valve assembly, which includes both a changeover valve for connecting the fuel gas flow path with the outside of the fuel cell20and a changeover valve for connecting the fuel gas flow path with the high-pressure hydrogen tank30.

In the structure of the first embodiment, the connection ports22to25are formed on the opposing side faces of the fuel cell stack20(the cell stack body21) to face each other in the horizontal direction. The connection ports may otherwise be formed on the front face and the rear face of the fuel cell stack20to face each other in the horizontal direction or formed on the top face and the bottom face of the fuel cell stack20to face each other in the vertical direction. The connection ports may be arranged both in the horizontal direction and in the vertical direction. The fuel gas flow paths211,212, and213formed in the cell stack body21are only illustrative and not restrictive at all. The fuel gas flow paths may be extended either in the horizontal direction or in the vertical direction. Plural fuel gas flow paths may be arranged to cross over each other.

The configuration of the intra-cell fuel gas passage613in the separator61in the structure of the second embodiment is only illustrative and not restrictive at all. The fuel gas passage may have any of diverse structures, for example, a facing structure.

In the structure of the second embodiment, the connection ports611and612are formed on one side face of the fuel cell60. The connection ports may be formed on opposing side faces to face each other in the horizontal direction. The connection ports may otherwise be formed on at least one of the bottom face and the top face to face each other in the horizontal direction or in the vertical direction. The connection ports may be formed in both a vertically extending face and a horizontally extending face. The configuration of the intra-cell fuel gas passage613is only illustrative and not restrictive at all. The intra-cell fuel gas passage may have a single flow path or plural flow paths and may be extended either in the horizontal direction or in the vertical direction.

The embodiments and their modified examples discussed above are to be considered in all aspects as illustrative and not restrictive. There may be many modifications, changes, and alterations without departing from the scope or spirit of the main characteristics of the present invention. All changes within the meaning and range of equivalency of the claims are intended to be embraced therein. The scope and spirit of the present invention are indicated by the appended claims, rather than by the foregoing description.