JP2008004319A - Fuel cell system, and its operation stop method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable to effectively restrain excessive current from flowing at short circuiting of cells, without adding new components. <P>SOLUTION: In the fuel cell system provided with cells generating power by electrochemical reaction of fuel gas and oxidizing gas, a cell stack made by laminating the cells, a control means for controlling a supply flow of the fuel gas and the oxidizing gas, a short-circuiting means short-circuiting a cathode and an anode of an output terminal of the cell stack, both electrodes are made short-circuited after an operation with a supply ratio of oxygen to hydrogen lower than normal, at the time of power generation stoppage. It is preferable to operate with oxygen supply volume against sweep current made lower than stoichiometry. Further, it is provided with a voltage measurement means for measuring voltage between both electrodes of the cell stack and it is preferable if the both electrodes are made short-circuited when the voltage between the electrodes gets below a given value. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a fuel cell system and an operation stop method thereof. More specifically, the present invention relates to improvement of processing contents when the fuel cell system is stopped.

In general, a fuel cell (for example, a polymer electrolyte fuel cell) is configured by stacking a plurality of cells each having an electrolyte sandwiched between separators. With regard to the fuel cell having such a configuration or a fuel cell system including the fuel cell, it is desired to avoid a situation in which an abnormal potential is generated at the cathode during the standing state after the operation is stopped until the next operation is started. It is rare. In order to avoid such a situation, a processing technique is disclosed in which each cell is short-circuited when the fuel cell system is stopped (see, for example, Patent Document 1).
JP 2003-317770 A

  However, when each cell is short-circuited as described above during the shutdown process, there is a problem in that an excessive current can flow during the short-circuit. As a means for avoiding such a problem, for example, a resistor having a large value is provided in a short circuit of each cell. However, this is not necessarily a preferable means in that a new part needs to be added.

  Accordingly, an object of the present invention is to provide a fuel cell system and an operation stop method thereof that can effectively suppress an excessive current from flowing when a cell is short-circuited without adding a new component.

  In order to solve this problem, the present inventor has made various studies. For example, if the cell voltage is short-circuited directly from a high cell voltage (OCV: Open Circuit Voltage) with the same gas composition as during normal power generation, hydrogen on the anode side and oxygen on the cathode side will react to a sudden decrease at the time of the short-circuit. As a result, an excessive current flows. The inventor who has further studied this point has come up with an idea that leads to the solution of such a problem, that is, an idea of forming a state in which an excessive current does not flow.

  The present invention is based on such an idea, and controls a cell for generating electricity by electrochemically reacting a fuel gas and an oxidizing gas, a cell stack formed by stacking the cells, and a supply flow rate of the fuel gas and the oxidizing gas. In a fuel cell system comprising a control means and a short-circuit means for short-circuiting the positive electrode and the negative electrode of the output terminal of the cell stack, when the power generation is stopped, the two electrodes are operated after operating at a lower oxygen to hydrogen supply ratio than usual. It is characterized by short-circuiting.

  Furthermore, the present invention provides a cell for generating electricity by electrochemically reacting a fuel gas and an oxidizing gas, a cell stack formed by stacking the cells, a control means for controlling a supply flow rate of the fuel gas and the oxidizing gas, In a fuel cell system shutdown method comprising a short-circuit means for short-circuiting the positive and negative electrodes of the cell stack output terminal, when the shutdown command is received, the oxygen-to-hydrogen supply ratio is set lower than usual. Then, the voltage between both electrodes of the cell stack is lowered, and then both electrodes of the cell stack are short-circuited using the short-circuit means.

  As described above, the processing technique of short-circuiting each cell at the time of shutdown in the fuel cell system is used as described above. However, if a cell is simply short-circuited in this way, an excessive current may flow. is there. In this regard, in the present invention, after receiving an operation stop command, the operation is performed with the supply ratio of oxygen to hydrogen lower than usual, that is, the operation is performed with a lower so-called air stoichiometric ratio, and thereby the positive electrode and the negative electrode are operated. Reduce the potential difference. Then, after the cell voltage is lowered, both poles of the cell stack are short-circuited. That is, in the present invention, the cell voltage is reduced prior to the cell short circuit, and then both poles of the cell stack are short-circuited. Is possible.

  Moreover, since the present invention realizes effective suppression of excessive current by constructing a new stop processing procedure at the time of operation stop, a new value such as providing a resistor with a large value in the short circuit is provided. There is also an advantage that it is not necessary to add parts. Therefore, it can be easily applied to the current state or existing facilities.

  In this case, it is preferable to operate with the oxygen supply amount with respect to the sweep current being lower than the stoichiometric ratio. To reduce the potential difference between the positive and negative electrodes more effectively by supplying oxygen or air that is lower than the stoichiometric ratio (the air stoichiometric ratio is lower than 1) to the current being swept. Can do.

  In addition, the fuel cell system as described above may include voltage measuring means for measuring a voltage between both electrodes of the cell stack, and short-circuit these electrodes when the voltage between both electrodes becomes a predetermined value or less. preferable. Similarly, in the operation method of the fuel cell system as described above, it is preferable to short-circuit both electrodes when the voltage between both electrodes of the cell stack becomes a predetermined value or less. By short-circuiting both poles after detecting or confirming that the voltage has fallen below a predetermined value after the process of reducing the cell voltage, it is possible to more reliably prevent an excessive current from flowing.

  According to the present invention, it is possible to effectively suppress an excessive current from flowing when a cell is short-circuited without adding new parts.

  Hereinafter, the configuration of the present invention will be described in detail based on an example of an embodiment shown in the drawings.

  1 to 5 show an embodiment of a fuel cell system according to the present invention. The fuel cell system 100 includes a cell 2 that generates electricity by electrochemically reacting a fuel gas and an oxidizing gas, a cell stack 3 formed by stacking the cells 2, and a control unit that controls the supply flow rates of the fuel gas and the oxidizing gas. The short circuit means 4 for short-circuiting the positive electrode and the negative electrode of the output terminal of the cell stack 3 is configured as a system. When the fuel cell system 100 of the present embodiment receives an operation stop command, the fuel cell system 100 is operated with a supply ratio of oxygen to hydrogen in the fuel gas and the oxidizing gas supplied to the cell stack 3 lower than normal, The voltage between the two poles of the cell stack 3 is lowered, and then the two poles of the cell stack 3 are short-circuited using the short-circuit means 4.

  In the following, first, the overall configuration of the fuel cell system 100 and the configuration of the cell 2 constituting the fuel cell 1 will be described, and then an excessive current can be effectively suppressed from flowing when the cell is short-circuited. The configuration will be described.

  FIG. 1 shows a schematic configuration of a fuel cell system 100 in the present embodiment. As shown in the figure, a fuel cell system 100 includes a fuel cell 1, an oxidizing gas supply / discharge system (hereinafter also referred to as an oxidizing gas piping system) 300 for supplying air (oxygen) as an oxidizing gas to the fuel cell 1, a fuel A fuel gas supply / discharge system (hereinafter also referred to as a fuel gas piping system) 400 that supplies hydrogen as a gas to the fuel cell 1, a refrigerant piping system 500 that supplies the refrigerant to the fuel cell 1 and cools the fuel cell 1, A power system 600 that charges and discharges the power of the system and a control unit 700 that serves as a control unit that controls the entire system are provided.

  The fuel cell 1 is formed of, for example, a solid polymer electrolyte type and has a stack structure in which a large number of cells (single cells) 2 are stacked (see FIG. 3). Each cell 2 has an air electrode on one surface of an electrolyte made of an ion exchange membrane, a fuel electrode on the other surface, and a pair of separators 20 (see FIG. 5) so as to sandwich the air electrode and the fuel electrode from both sides. 2 are indicated by reference numerals 20a and 20b, respectively). Fuel gas is supplied to the fuel gas flow path of one separator 20, and oxidizing gas is supplied to the oxidizing gas flow path of the other separator 20, and the fuel cell 1 generates electric power by this gas supply.

  The oxidizing gas piping system 300 has a supply path 111 through which the oxidizing gas supplied to the fuel cell 1 flows, and a discharge path 112 through which the oxidizing off gas discharged from the fuel cell 1 flows. The supply path 111 is provided with a compressor 114 that takes in the oxidizing gas via the filter 113, and a humidifier 115 that humidifies the oxidizing gas fed by the compressor 114. The oxidizing off-gas flowing through the discharge path 112 is subjected to moisture exchange by the humidifier 115 through the back pressure regulating valve 116, and is finally exhausted into the atmosphere outside the system as exhaust gas. The compressor 114 takes in the oxidizing gas in the atmosphere by driving the motor 114a.

  The fuel gas piping system 400 includes a hydrogen supply source 121, a supply path 122 through which hydrogen gas supplied from the hydrogen supply source 121 to the fuel cell 1 flows, and a supply path for supplying hydrogen offgas (fuel offgas) discharged from the fuel cell 1. A circulation path 123 for returning to the confluence point A of 122, a pump 124 that pumps the hydrogen off gas in the circulation path 123 to the supply path 122, and a discharge path 125 that is branched and connected to the circulation path 123. .

  The hydrogen supply source 121 is composed of, for example, a high-pressure tank or a hydrogen storage alloy, and is configured to be able to store, for example, 35 MPa or 70 MPa of hydrogen gas. When the main valve 126 of the hydrogen supply source 121 is opened, hydrogen gas flows out to the supply path 122. The hydrogen gas is finally depressurized to, for example, about 200 kPa by the pressure regulating valve 127 and other pressure reducing valves and supplied to the fuel cell 1.

  A shutoff valve 128 is provided on the upstream side of the confluence point A of the supply path 122. The hydrogen gas circulation system is configured by sequentially communicating a flow path downstream of the confluence point A of the supply path 122, a fuel gas flow path formed in the separator of the fuel cell 1, and the circulation path 123. Yes. The hydrogen pump 124 circulates and supplies the hydrogen gas in the circulation system to the fuel cell 1 by driving the motor 124a.

  The discharge passage 125 is provided with a purge valve 133 that is a shut-off valve. When the purge valve 133 is appropriately opened when the fuel cell system 100 is in operation, impurities in the hydrogen off gas are discharged together with the hydrogen off gas to a hydrogen diluter (not shown). By opening the purge valve 133, the concentration of impurities in the hydrogen off-gas in the circulation path 123 decreases, and the hydrogen concentration in the hydrogen off-gas supplied in circulation increases.

  The refrigerant piping system 500 includes a refrigerant circulation channel 141 communicating with a cooling channel in the fuel cell 1, a cooling pump 142 provided in the refrigerant circulation channel 141, and a radiator that cools the refrigerant discharged from the fuel cell 1. 143, a bypass flow path 144 that bypasses the radiator 143, and a three-way valve (switching valve) 145 that sets the flow of cooling water to the radiator 143 and the bypass flow path 144. The cooling pump 142 circulates and supplies the refrigerant in the refrigerant circulation channel 141 to the fuel cell 1 by driving the motor 142a.

  The power system 600 includes a high-voltage DC / DC converter 161, a battery 162, a traction inverter 163, a traction motor 164, and various auxiliary inverters 165, 166, and 167. The high-voltage DC / DC converter 161 is a direct-current voltage converter that adjusts the direct-current voltage input from the battery 162 and outputs it to the traction inverter 163 side, and the direct-current input from the fuel cell 1 or the traction motor 164. And a function of adjusting the voltage and outputting it to the battery 162. The charge / discharge of the battery 162 is realized by these functions of the high-voltage DC / DC converter 161. Further, the output voltage of the fuel cell 1 is controlled by the high voltage DC / DC converter 161.

  The battery 162 is configured such that battery cells are stacked and a constant high voltage is used as a terminal voltage, and surplus power can be charged or power can be supplementarily supplied under the control of a battery computer (not shown). The traction inverter 163 converts a direct current into a three-phase alternating current and supplies it to the traction motor 164. The traction motor 164 is, for example, a three-phase AC motor, and constitutes a main power source of, for example, a vehicle on which the fuel cell system 100 is mounted.

  Auxiliary machine inverters 165, 166, and 167 are motor control devices that control driving of corresponding motors 114a, 124a, and 142a, respectively. Auxiliary machine inverters 165, 166, and 167 convert a direct current into a three-phase alternating current and supply it to motors 114a, 124a, and 142a, respectively. Auxiliary machine inverters 165, 166, and 167 are, for example, pulse width modulation type PWM inverters that convert a DC voltage output from fuel cell 1 or battery 162 into a three-phase AC voltage in accordance with a control command from control unit 700. The rotational torque generated by each motor 114a, 124a, 142a is controlled.

  The control unit 700 is configured as a microcomputer including a CPU, a ROM, and a RAM inside. The CPU executes a desired calculation according to the control program, and performs various processes and controls such as a thawing control of the pump 124 described later. The ROM stores control programs and control data processed by the CPU. The RAM is mainly used as various work areas for control processing. The control unit 700 inputs detection signals such as various pressure sensors, temperature sensors, and outside air temperature sensors used in the gas system (300, 400) and the refrigerant piping system 500, and outputs control signals to each component.

  Then, the schematic structure of the cell 2 of the fuel cell 1 in this embodiment is shown in FIG. The cells 2 configured as shown in the figure are sequentially stacked to form a cell stack (see FIG. 3). Further, the cell stack formed in this way is, for example, a load in the stacking direction in a state where both ends are sandwiched between a pair of end plates 8 and a tension plate 9 is arranged so as to connect the end plates 8 to each other. Is applied and concluded.

  The fuel cell 1 composed of such cells 2 and the like can be used as, for example, an in-vehicle power generation system of a fuel cell vehicle (FCHV), but is not limited thereto. It can also be used as a power generation system mounted on a mobile object (for example, a ship or an airplane), a self-propelled device such as a robot, or a stationary power generation system.

  The cell 2 includes an electrolyte, specifically, a membrane-electrode assembly (hereinafter referred to as MEA; Membrane Electrode Assembly) 30 and a pair of separators 20 that sandwich the MEA 30 (in FIG. 2, reference numerals 20a and 20b respectively). Etc.). The MEA 30 and the separators 20a and 20b are formed in a substantially rectangular plate shape. Further, the MEA 30 is formed so that its outer shape is smaller than the outer shape of each separator 20a, 20b.

  The MEA 30 includes a polymer electrolyte membrane (hereinafter also simply referred to as an electrolyte membrane) 31 made of a polymer material ion exchange membrane, and a pair of electrodes (an anode side diffusion electrode and a cathode side diffusion electrode) sandwiching the electrolyte membrane 31 from both sides. 32a and 32b (see FIG. 2). The electrolyte membrane 31 is formed larger than the electrodes 32a and 32b. The electrodes 32a and 32b are joined to the electrolyte membrane 31 by, for example, a hot press method while leaving the peripheral edge portion 33.

  The electrodes 32a and 32b constituting the MEA 30 are made of, for example, a porous carbon material (diffusion layer) carrying a catalyst such as platinum attached to the surface thereof. One electrode (anode) 32a is supplied with hydrogen gas as a fuel gas (reactive gas), and the other electrode (cathode) 32b is supplied with an oxidizing gas (reactive gas) such as air or an oxidant. An electrochemical reaction is generated in the MEA 30 by the gas, and the electromotive force of the cell 2 is obtained.

  The separator 20 (20a, 20b) is made of a gas impermeable conductive material. Examples of the conductive material include carbon and a hard resin having conductivity, and metals such as aluminum and stainless steel. The base material of the separator 20 (20a, 20b) of the present embodiment is a so-called metal separator formed of a plate-like metal. It is preferable that a film having excellent corrosion resistance (for example, a film formed by gold plating) is formed on the surface of the base material on the electrodes 32a and 32b side.

  Further, a groove-like flow path constituted by a plurality of concave portions is formed on both surfaces of the separators 20a and 20b. These flow paths can be formed by press molding in the case of the separators 20a and 20b of the present embodiment in which the base material is formed of, for example, a plate-like metal. The groove-shaped flow path thus formed constitutes an oxidizing gas flow path 34, a hydrogen gas flow path 35, or a cooling water flow path 36. More specifically, a plurality of gas passages 35 for hydrogen gas are formed on the inner surface on the electrode 32a side of the separator 20a, and a plurality of cooling water passages 36 are formed on the back surface (outer surface). (See FIG. 2). Similarly, a plurality of gas channels 34 for oxidizing gas are formed on the inner surface of the separator 20b on the electrode 32b side, and a plurality of cooling water channels 36 are formed on the back surface (outer surface) (FIG. 2). reference). Further, in the present embodiment, regarding two adjacent cells 2 and 2, when the outer surface of the separator 20 a of one cell 2 and the outer surface of the separator 20 b of the cell 2 adjacent to this are combined, The water flow path 36 is integrated to form a flow path having a rectangular or honeycomb cross section (see FIG. 2).

  Furthermore, as described above, the separators 20a and 20b have a relationship in which at least the uneven shape for forming a fluid flow path is reversed between the front surface and the back surface. More specifically, in the separator 20a, the back surface of the convex shape (convex rib) forming the hydrogen gas gas flow path 35 is a concave shape (concave groove) forming the cooling water flow path 36, and the gas flow path The back surface of the concave shape (concave groove) forming 35 is a convex shape (convex rib) forming the cooling water channel 36. Furthermore, in the separator 20b, the back surface of the convex shape (convex rib) that forms the gas flow path 34 of the oxidizing gas has a concave shape (concave groove) that forms the cooling water flow path 36, and the concave that forms the gas flow path 34. The back surface of the shape (concave groove) is a convex shape (convex rib) forming the cooling water flow path 36.

  Further, in the vicinity of the longitudinal ends of the separators 20a and 20b (in the case of this embodiment, in the vicinity of one end shown on the left side in FIG. 2), the manifold 15a on the inlet side of the oxidizing gas, hydrogen gas An outlet side manifold 16b and a cooling water outlet side manifold 17b are formed. For example, in the case of this embodiment, these manifolds 15a, 16b, and 17b are formed by substantially rectangular or trapezoidal through holes provided in the separators 20a and 20b (see FIG. 2). Further, an oxidant gas outlet side manifold 15b, a hydrogen gas inlet side manifold 16a, and a cooling water inlet side manifold 17a are formed at opposite ends of the separators 20a and 20b. In the case of this embodiment, these manifolds 15b, 16a, and 17a are also formed by substantially rectangular or trapezoidal through holes (see FIG. 2).

  Among the manifolds as described above, the inlet side manifold 16a and the outlet side manifold 16b for the hydrogen gas in the separator 20a are connected to the inlet side communication passage 61 and the outlet side communication passage 62 formed in the separator 20a in a groove shape. Each communicates with a gas flow path 35 of hydrogen gas. Similarly, the inlet side manifold 15a and the outlet side manifold 15b for the oxidizing gas in the separator 20b are oxidized via the inlet side communication passage 63 and the outlet side communication passage 64 formed in the separator 20b in a groove shape. The gas communicates with the gas flow path 34 (see FIG. 2). Further, the inlet side manifold 17a and the outlet side manifold 17b of the cooling water in each separator 20a, 20b are connected to each separator 20a, 20b through an inlet side communication passage 65 and an outlet side communication passage 66 formed in a groove shape. Each communicates with the cooling water passage 36. With the configuration of the separators 20a and 20b as described above, the cell 2 is supplied with oxidizing gas, hydrogen gas, and cooling water. As a specific example, when the cells 2 are stacked, for example, hydrogen gas passes from the inlet side manifold 16a of the separator 20a through the communication passage 61 and flows into the gas flow path 35, and is supplied to the power generation of the MEA 30. After that, the fluid passes through the communication passage 62 and flows out to the outlet side manifold 16b.

  The first seal member 13a and the second seal member 13b are both formed of a plurality of members (for example, four small rectangular frames and a large frame for forming a fluid flow path) (FIG. 2). Among these, the first seal member 13a is provided between the MEA 30 and the separator 20a. More specifically, a part of the first seal member 13a is a peripheral portion 33 of the electrolyte membrane 31 and a gas flow path 35 of the separator 20a. It is provided so that it may interpose between the surrounding parts. The second seal member 13b is provided between the MEA 30 and the separator 20b. More specifically, a part of the second seal member 13b is a peripheral portion 33 of the electrolyte membrane 31 and the gas channel 34 of the separator 20b. It is provided so as to be interposed between the surrounding portions.

  Furthermore, a plurality of members (for example, four small rectangular frames and a large frame for forming a fluid flow path) are formed between the separators 20b and 20a of the adjacent cells 2 and 2. A third seal member 13c is provided (see FIG. 2). The third seal member 13c is provided so as to be interposed between a portion around the cooling water passage 36 in the separator 20b and a portion around the cooling water passage 36 in the separator 20a, and seals between them. It is.

  In addition, as the first to third seal members 13a to 13c, an elastic body (gasket) that seals a fluid by physical contact with an adjacent member, or an adhesive that is bonded by chemical bonding with an adjacent member. An agent or the like can be used. For example, in this embodiment, a member that is physically sealed by elasticity is employed as each of the seal members 13a to 13c, but instead, a member that is sealed by a chemical bond such as the adhesive described above may be employed.

  The frame-shaped member 40 is a member made of, for example, resin (hereinafter also referred to as a resin frame) that is sandwiched between the separators 20a and 20b together with the MEA 30. For example, in this embodiment, a thin frame-shaped resin frame 40 is interposed between the separators 20a and 20b, and the resin frame 40 sandwiches at least a part of the MEA 30, for example, a portion along the peripheral edge 33 from the front side and the back side. I have to. The resin frame 40 provided in this way functions as a spacer between the separators 20 (20a, 20b) that supports the fastening force, functions as an insulating member, and as a reinforcing member that reinforces the rigidity of the separator 20 (20a, 20b). Demonstrate the function.

  The configuration of the fuel cell 1 will be briefly described as follows (see FIG. 3 and the like). The fuel cell 1 in the present embodiment has a cell stack in which a plurality of single cells 2 are stacked, and a current collector plate with an output terminal is sequentially placed outside the single cells 2 and 2 located at both ends of the cell stack, The insulating plate and the end plate 8 are each arranged (see FIG. 3). Such a cell stack is restrained in a stacked state by a tension plate 9. The tension plate 9 is provided so as to bridge between both the end plates 8, 8. For example, a pair of the tension plates 9 are arranged so as to face both sides of the cell stack. In addition, an elastic module is further provided for applying a compressive force to the cell stack by an elastic force. The elastic module is a member for continuing to apply a load while absorbing a change even when the cell laminate is thermally expanded or contracted, or when both are repeated. In this case, it is composed of a plurality of elastic bodies (not shown) arranged in parallel to each other, a pair of pressure plates 12 that sandwich the plurality of elastic bodies from the stacking direction of the cells 2 (see FIG. 3).

  Subsequently, in the fuel cell system 100 of the present embodiment, a configuration that can effectively suppress an excessive current from flowing when a cell is short-circuited will be described (see FIG. 4 and the like).

  In the fuel cell system 100 of the present embodiment, while the operation of the fuel cell 1 is stopped, both electrodes of the cell stack 3 are short-circuited to have the same potential, thereby avoiding the occurrence of an abnormal potential during the neglected state. It is said. Further, when an operation stop command is received, the operation is performed with the supply ratio of oxygen to hydrogen lower than usual, the voltage between both electrodes of the cell stack 3 is lowered, and then the cell stack is used by using the short circuit means 4. 3 poles are short-circuited. Specifically, in the present embodiment, the oxygen supply amount with respect to the sweep current is made lower than the stoichiometric ratio when the oxygen to hydrogen supply ratio is made lower than usual. By making the oxygen supply amount lower than the stoichiometric ratio with respect to the swept current (that is, the air stoichiometric ratio is lower than 1), the potential difference between the positive electrode and the negative electrode can be reduced more effectively. .

  The configuration for realizing the processing at the time of shutdown as described above is not particularly limited. For example, in this embodiment, the processing at the time of shutdown is performed by using the oxidizing gas bypass valve 118 and the short-circuit means 4. It is supposed to be implemented (see FIGS. 1 and 4).

  The oxidizing gas bypass valve 118 can reduce the supply amount of the oxidizing gas to the fuel cell 1 as necessary, and can appropriately reduce the supply ratio of oxygen to hydrogen as necessary. For example, in the case of the present embodiment, a bypass 117 that connects the oxidizing gas supply path 111 and the oxidizing off gas discharge path 112 is provided, and a three-way valve provided at a branch point of the supply path 111 and the bypass 117 is a bypass valve 118. (See FIG. 1). The bypass valve 118 opens and closes the valve in response to a control signal from the control unit 700, and appropriately changes the gas flow rate to the fuel cell 1 side and the gas flow rate to be bypassed.

  The short-circuit means 4 is provided as a means for short-circuiting both poles of the cell stack 3 when an operation stop command for the fuel cell 1 is issued. Such a short-circuit means 4 can be constituted by a member for simply short-circuiting both electrodes, or can be constituted by using another member such as a cell monitor for sensing a cell voltage. Here, the short-circuit means 4 (hereinafter also referred to as cell monitor 4) configured using a cell monitor as in the latter is illustrated (see FIG. 4).

  The cell monitor 4 shown in FIG. 4 is provided to detect the power generation status of each stacked cell 2. The terminal 41 provided corresponding to each cell 2, and these terminals 41 are connected to each cell 2. A cell monitor cable 42 for connecting to a part (for example, the separator 20) is provided. Each terminal 41 is in a state in which a limiting resistor 43 and a relay 44 are connected in series to each other, and are connected via the limiting resistor 43 and the relay 44. In the case of the present embodiment, odd-numbered terminals 41 are connected to each other, and a plurality of terminals 41 are connected to each other, that is, every other terminal 41 is connected to every other terminal. (See FIG. 4). Reference numeral 45 is a capacitor, and reference numeral 46 is another relay. Reference symbol PB denotes a substrate of the cell monitor 4.

  Next, the content of the process when the operation of the fuel cell 1 is stopped will be described below using a flowchart (see FIG. 5).

  First, when there is an operation stop command for the fuel cell 1 (Yes in Step 1), the operation is performed for a short time with the air stoichiometric ratio set to 1 or less immediately before stopping the power generation (Step 2). For example, in the case of this embodiment, a control means for controlling the oxidizing gas supply flow rate constituted by the bypass valve 118 and the compressor 114, and a control for controlling the fuel gas supply flow rate constituted by the pressure regulating valve 127 and the pump 124, etc. Each of the means is controlled by the control unit 700 so that the supply ratio of oxygen to hydrogen is lower than that during normal operation. The operation stop command is issued, for example, when the user performs an operation to turn off the ignition switch.

  Thus, when the supply ratio of oxygen to hydrogen is lower than that during normal operation, the air stoichiometric ratio is 1 or less, that is, the amount of oxidizing gas relative to the fuel gas is relatively insufficient. In such a case, the chemical reaction in the fuel cell 1 is not performed as much as during normal operation. As a result, the voltage across the cell stack 3 decreases, the cell voltage decreases, and the potential difference between the positive electrode and the negative electrode decreases. Become. Preferably, the potential difference between the positive electrode and the negative electrode can be reduced more effectively by making the oxygen supply amount lower than the stoichiometric ratio corresponding to the sweep current (the air stoichiometric ratio is lower than 1). .

  Subsequently, the voltage between both electrodes of the cell stack 3 is measured using a voltage measuring means. In the present embodiment, it is determined whether or not the voltage between both electrodes has become a predetermined value or less using the terminal 41 of the cell monitor 4 described above (step 3).

  When the voltage between the two electrodes of the cell stack 3 becomes a predetermined value or less (Yes in Step 3), the positive and negative electrodes of the output terminal of the cell stack 3 are short-circuited by the short-circuit means (Step 4). In this embodiment, each relay 44 in the cell monitor 4 described above is closed to short-circuit predetermined cells 2 (see FIG. 4).

  As described so far, according to the fuel cell system 1 of the present embodiment in which both the poles of the cell stack 3 are short-circuited after performing the process of reducing the cell voltage, an excessive current flows at the time of the short-circuit. It can be effectively suppressed. In addition, since the line used for the cell monitor 4 is generally thin, there is a risk of fusing if it is short-circuited in a high voltage state. Therefore, the line such as the cell monitor cable 42 used at the time of the short circuit does not melt.

  Further, in the present embodiment, it is possible to effectively suppress an excessive current by constructing a new stop processing procedure or method at the time of operation stop, and the cell monitor 4 (terminal) used for sensing a normal cell voltage is realized. In this case, the cell stack 3 can be short-circuited without adding hardware or a large change in logic or the like. It is possible. Therefore, it can be applied to the current state or existing facilities, and it is not necessary to add other parts (hardware) such as various resistors.

  In addition, according to this embodiment, the following effects can be obtained. That is, generally, if the supply of the oxidizing gas to the cathode is sufficient (the air stoichiometric ratio exceeds 1), water is generated from oxygen, hydrogen ions and electrons, but the supply of the oxidizing gas to the cathode is not possible. When the amount is insufficient (air stoichiometric ratio is 1 or less), a reaction occurs in which hydrogen ions and electrons are recombined to generate hydrogen (pumping hydrogen) according to the amount of oxidizing gas that is insufficient. Therefore, when the operation in which the supply ratio of oxygen to hydrogen is lower than usual when the operation is stopped as in the present embodiment, both the cathode and the anode can be substantially filled with hydrogen. In such a state, since the potential difference becomes small and the two electrodes have substantially the same potential, it is possible to effectively suppress an excessive current from flowing at the time of a short circuit in combination with such a situation. Contrary to this, it is difficult to avoid the deterioration phenomenon that the carbon material is oxidized when the treatment is performed by supplying an oxidant gas to the anode side, while it is filled with hydrogen as in this embodiment. In such a state, such a deterioration phenomenon can be suppressed. For example, there is an advantage that the abnormal potential is small at the time of restart.

  The above-described embodiment is an example of a preferred embodiment of the present invention, but is not limited thereto, and various modifications can be made without departing from the scope of the present invention. For example, in the present embodiment, the cell monitor 4 used as the short-circuit means is exemplified by a configuration in which a plurality of terminals 41 are connected every other one, but this is only an example, and the cell monitor 4 having another configuration is used. Of course it is possible. As another example, a short-circuit unit that short-circuits the positive electrode and the negative electrode of the cell stack 3 may be used, or a short-circuit unit that short-circuits one cell at a time. If the short-circuit means that short-circuits one cell at a time like the latter, the current that flows at the time of the short-circuit can be divided and borne by each circuit, and the potential of each cell 2 under the short-circuit condition is averaged over the entire stack You can also

It is a figure which shows the structure of the fuel cell system in this embodiment. It is a disassembled perspective view which decomposes | disassembles and shows the cell of a cell laminated body. It is a perspective view which shows the structure of the cell stack of the fuel cell in this embodiment. It is a figure which shows schematically the circuit of the cell monitor which comprises a short circuit means. It is a flowchart which shows the content of the process at the time of the driving | operation stop of a fuel cell.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Fuel cell, 2 ... Cell, 3 ... Cell stack, 4 ... Cell monitor (short circuit means, voltage measurement means), 100 ... Fuel cell system, 700 ... Control part (control means)

Claims (6)

A cell for generating electricity by electrochemically reacting a fuel gas and an oxidizing gas; a cell stack formed by stacking the cells; a control means for controlling a supply flow rate of the fuel gas and the oxidizing gas; and an output terminal of the cell stack. In a fuel cell system comprising a short-circuit means for short-circuiting the positive electrode and the negative electrode,
A fuel cell system characterized in that when power generation is stopped, both electrodes are short-circuited after operation with a lower supply ratio of oxygen to hydrogen than usual.   2. The fuel cell system according to claim 1, wherein the fuel cell system is operated with an oxygen supply amount with respect to a sweep current being lower than a stoichiometric ratio.   3. The fuel cell according to claim 1, further comprising: a voltage measuring unit configured to measure a voltage between both electrodes of the cell stack, wherein the two electrodes are short-circuited when a voltage between the two electrodes becomes a predetermined value or less. system. A cell for generating electricity by electrochemically reacting a fuel gas and an oxidizing gas; a cell stack formed by stacking the cells; a control means for controlling a supply flow rate of the fuel gas and the oxidizing gas; and an output terminal of the cell stack. A short-circuit means for short-circuiting the positive electrode and the negative electrode;
When an operation stop command is received, operate with the oxygen to hydrogen supply ratio lower than normal,
Reducing the voltage across the cell stack,
Then, the method of stopping the operation of the fuel cell system, wherein both electrodes of the cell stack are short-circuited using the short-circuit means.   The method for stopping the operation of the fuel cell system according to claim 4, wherein the operation is performed with the oxygen supply amount with respect to the sweep current being lower than the stoichiometric ratio. 6. The method for stopping operation of a fuel cell system according to claim 4, wherein when the voltage between both electrodes of the cell stack becomes a predetermined value or less, these electrodes are short-circuited.

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