JP7067496B2 - Fuel cell system - Google Patents

Description

The present invention relates to a fuel cell system that suppresses the influence of metal ions eluted in the generated water on the electrolyte membrane.

A fuel cell supplies chemical energy by supplying reaction gas of fuel gas (hydrogen gas) and oxidant gas (oxygen gas) to two electrically connected electrodes and electrochemically causing oxidation of the fuel. Is directly converted into electrical energy. This fuel cell usually has a single cell in which a membrane electrode assembly in which an electrolyte membrane is sandwiched between a pair of electrodes is sandwiched by a separator as a constituent unit, and includes a cell stack formed by stacking a plurality of the single cells. Among them, the polymer electrolyte fuel cell using the polymer electrolyte membrane as the electrolyte membrane has advantages such as easy miniaturization and operation at a low temperature, so that it is particularly portable and mobile. It is attracting attention as a power source for the body.

In the above fuel cell, a metal separator formed into a wavy shape is generally used as the separator. The metal separator is provided with a reaction gas flow path for flowing a fuel gas or an oxidant gas in the surface thereof. For example, Patent Document 1 discloses a stainless steel separator for use as a separator for a fuel cell.

In the polymer electrolyte fuel cell, the reaction of the following equation (1) proceeds at the fuel electrode (anode) to which hydrogen is supplied.
H ₂ → 2H ⁺ + 2e -... ⁽ 1)

The electrons generated by the above equation (1) pass through an external circuit, work with an external load, and then reach the oxidant electrode (cathode). On the other hand, the protons generated by the above equation (1) move from the fuel electrode side to the oxidant electrode side in the solid polymer electrolyte membrane by electro-osmosis in a state of being hydrated with water.

On the other hand, at the oxidizing agent electrode, the reaction of the following formula (2) proceeds.
2H ^＋ ＋ 1 / 2O ₂ ＋ 2e ^－ → H ₂ O ・ ・ ・ (2)

Therefore, the chemical reaction represented by the following equation (3) proceeds in the entire battery, an electromotive force is generated, and electrical work is performed on the external load.
H ₂ + 1 / 2O ₂ → H ₂ O ・ ・ ・ (3)

As described above, in the fuel cell, the generated water is generated at the oxidant electrode along with the power generation, and the generated water is back-diffused at the fuel electrode via the electrolyte membrane.

Further, in a fuel cell provided with a solid polymer electrolyte membrane represented by a perfluorocarbon sulfonic acid resin membrane, it is important to maintain a wet state of the electrolyte membrane and the catalyst layer in order to secure ionic conductivity. Therefore, in general, the reaction gas is supplied to the electrode in a pre-humidified state. At that time, the humidifying water may be liquefied without being absorbed by the electrolyte membrane and may stay in the reaction gas flow path of the separator.

As the polymer solid electrolyte membrane, a fluororesin-based polymer membrane having high hydrogen ion conductivity represented by a perfluorocarbon sulfonic acid resin membrane is generally used, and in the generated water that has passed through the electrolyte membrane, the electrolyte membrane is used. Fluorine compounds and fluorine ions may elute. When such generated water stays in the reaction gas flow path of a metal separator such as stainless steel, metal ions constituting the separator, such as iron ion, nickel ion, chromium ion, molybdenum ion, etc., are eluted into the generated water. It becomes a cause.

When such metal ions are incorporated into the polyelectrolyte film, the deterioration of the polyelectrolyte film is promoted and the performance of the fuel cell is deteriorated.

Therefore, conventionally, in a fuel cell, various attempts have been proposed in order to suppress the outflow of metal ions into the generated water.

For example, Patent Document 2 provides a fuel cell system provided with a means for detecting the concentration of ions in the generated water discharged from the fuel cell and lowering the temperature of the fuel cell when the ion concentration is higher than a predetermined value. Is disclosed.

Patent Document 3 describes a means for storing the generated water discharged from the fuel cell and prohibiting the discharge of the stored generated water when the ion concentration contained in the stored generated water is equal to or higher than a predetermined concentration. A fuel cell control device provided is disclosed.

Patent Document 4 discloses a fuel cell system provided with a means for acquiring the amount of ions in the fuel cell and detecting the deterioration of the electrolyte membrane based on the amount of ions.

Patent Document 5 discloses a fuel cell system including an ion adsorption device for storing generated water discharged from a fuel cell and adsorbing ions contained in the generated water.

Japanese Unexamined Patent Publication No. 2001-325969 Japanese Unexamined Patent Publication No. 2005-332768 Japanese Unexamined Patent Publication No. 2006-269156 Japanese Unexamined Patent Publication No. 2008-140661 Japanese Unexamined Patent Publication No. 2005-174608

Metal ions eluted from pipes and the like can be reduced before reaching the electrolyte membrane. However, when a metal separator such as stainless steel is used as the separator for a single cell, the separator is arranged in the vicinity of the electrolyte membrane, so that the eluted metal ions cannot be removed before reaching the electrolyte membrane. It was difficult.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a fuel cell system provided with means for suppressing the uptake of metal ions into an electrolyte membrane.

The present invention achieves the above object by the following means.

A fuel cell having a cell stack in which a plurality of single cells having a membrane electrode assembly sandwiched between stainless steel separators are stacked, and a fuel cell system having a control unit.
The control unit determines whether or not the metal ion concentration in the generated water discharged from the fuel cell is equal to or higher than the specified value, and if the metal ion concentration is equal to or higher than the specified value, detects the restraining pressure of the cell stack. A fuel cell system that reduces the restraining pressure of the cell stack to less than the specified pressure when the restraining pressure is equal to or higher than the specified value.

According to the present invention, when the metal ion concentration in the generated water is as high as a specified value or more, the thickness of the gas diffusion layer constituting the single cell is increased by reducing the restraining pressure of the cell stack constituting the fuel cell. , Thereby increasing the diffusion distance of the metal ions moving through the cell. As a result, the uptake of metal ions into the electrolyte membrane is suppressed, and deterioration of the membrane electrolyte membrane can be suppressed.

It is sectional drawing of the single cell which constitutes a fuel cell. It is sectional drawing of a fuel cell. It is a schematic diagram of the fuel cell system of this invention. It is a graph which shows the relationship between the iron ion concentration and the cross leak amount in a membrane electrode assembly. It is a graph which shows the relationship between the iron ion concentration in a membrane electrode assembly, and the endurance time. It is a graph which shows the relationship between the thickness of a gas diffusion layer and the iron ion uptake rate. It is a flowchart explaining the operation of the fuel cell system of this invention.

The fuel cell system of the present invention is
A fuel cell equipped with a cell stack formed by stacking a plurality of single cells in which a membrane electrode assembly is sandwiched by a stainless steel separator, and a control unit are provided.
The control unit determines whether or not the metal ion concentration in the generated water discharged from the fuel cell is equal to or higher than the specified value, and if the metal ion concentration is equal to or higher than the specified value, detects the restraining pressure of the cell stack. When the restraining pressure is equal to or higher than the specified value, the restraining pressure of the cell stack is reduced to less than the specified pressure.

FIG. 1 shows the basic structure of a single cell constituting a fuel cell. In the single cell 10, the catalyst layer 12 is formed on both sides of the proton conductive electrolyte membrane 11 made of the solid polymer material, and the gas diffusion layer 13 is further formed on both sides of the catalyst layer 12, and they are integrated. The membrane electrode assembly 14 is formed by sandwiching the membrane electrode assembly 14 with a stainless steel separator 15.

During the operation of such a fuel cell composed of a single cell, as described above, the water generated at the oxidant electrode passes through the electrolyte membrane and moves to the fuel electrode. At this time, if metal ions are present in the generated water, OH radicals are generated. The OH radicals generated during the operation of this fuel cell attack the electrolyte membrane 11 and decompose the electrolyte membrane 11. When the electrolyte membrane 11 is decomposed, the membrane becomes thin, and finally a hole is opened in the electrolyte membrane 11, and the power generation performance of the fuel cell is deteriorated.

Therefore, by suppressing the uptake of metal ions into the electrolyte membrane 11, deterioration of the electrolyte membrane 11 can be suppressed, and deterioration of the power generation performance of the fuel cell can be suppressed.

FIG. 2 shows the basic structure of the fuel cell. As shown in FIG. 2, the fuel cell 20 includes a cell stack 21 formed by stacking a plurality of single cells 10. A pair of terminal plates 16 for taking out electric power are arranged at both ends of the cell stack 21. A pair of end plates 17 are arranged on the outside of the terminal plate 16 and the cell stack 21 is fastened and fixed in the stacking direction. Further, a tension shaft 19 extending over the entire cell stack 21 is provided in the stacking direction of the cell stack 21, and the tension shaft 19 is fixed to the end plate 17 by bolts 18. The tension shaft 19 does not necessarily have to be provided, but by providing the tension shaft 19, it becomes easy to apply a restraining pressure to the cell stack 21 and adjust the restraining pressure.

As described above, in order to secure the adhesion between the electrolyte membrane and the catalyst layer, the cell stack usually applies pressure to the cell stack in the stacking direction. At this time, the gas diffusion layer is in a crushed state.

In the fuel cell, it is considered that the metal ions diffuse in the liquid water generated by the generated water or the humidifying gas and are taken into the electrolyte membrane. Therefore, in the present invention, the control unit determines whether or not the metal ion concentration in the generated water discharged from the fuel cell is equal to or higher than the specified value, and if the metal ion concentration is equal to or higher than the specified value, the restraining pressure of the cell stack is obtained. When this restraining pressure is equal to or higher than the specified value, the restraining pressure of the cell stack is lowered to less than the specified pressure. By reducing the restraining pressure of the cell stack, the thickness of the crushed gas diffusion layer becomes large, and the diffusion distance of metal ions until reaching the electrolyte membrane becomes long. As a result, the number of metal ions reaching the electrolyte membrane is reduced, the amount of metal ions taken up by the electrolyte membrane is reduced, and as a result, deterioration of the electrolyte membrane 11 can be suppressed.

Hereinafter, embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and can be variously modified and implemented within the scope of the gist of the present invention.

<Fuel cell system configuration>
FIG. 3 is an explanatory diagram showing a schematic configuration of a fuel cell system 100 as an embodiment of the present invention.

The fuel cell system 100 shown in FIG. 3 includes a fuel supply system for supplying fuel gas to the fuel electrode of the fuel cell 30, and an oxidant supply system for supplying oxidant gas to the oxidant electrode. The fuel cell 30 includes a cell stack having a configuration as shown in FIG.

In FIG. 3, the fuel supply system includes a fuel supply device 31, a pipe 32 connecting the fuel supply device 31 and a fuel inlet provided in the fuel cell 30, a fuel outlet provided in the fuel cell 30, and a cooler 33. The pipe 34 connecting the fuel separator 35, the fuel pipe 36 connecting the cooler 33 and the fuel-liquid separator 35, the pipe 38 connecting the gas-liquid separator 35 and the circulation pump (or compressor) 37, and the pump 37 and the pipe 32 are reversed. It is provided with a pipe 40 connected via a stop valve 39.

The fuel gas sent out from the fuel supply device 31 is supplied to the fuel electrode of the fuel cell 30 through the pipe 32 and used for the reaction at the fuel electrode. The fuel gas that has passed through the fuel electrode is discharged to the pipe 34 from the fuel outlet of the fuel cell 30. The fuel gas discharged to the pipe 34 is cooled by the cooler 33 as needed, and then reaches the gas-liquid separator 35 via the pipe 36. The fuel gas from which the liquid layer component has been removed by the gas-liquid separator 35 is sent to the pipe 40 by the circulation pump 37, and is again supplied to the fuel electrode of the fuel cell 30 through the pipe 32. In this way, a hydrogen circulation system (fuel gas circulation path) is configured in which residual hydrogen contained in the fuel gas discharged from the fuel cell 30 and not used for the reaction at the fuel electrode is supplied to the fuel cell 30 again. Has been done.

Further, in FIG. 3, the oxidant supply system includes an oxidant supply device 41, a pipe 42 connecting the oxidant supply device 41 and an oxidant inlet provided in the fuel cell 30, and an oxidation provided in the fuel cell 30. A pipe 44 connecting the agent outlet and the cooler 43, a pipe 46 connecting the cooler 43 and the gas-liquid separator 45, a pipe 48 connecting the gas-liquid separator 45 and the circulation pump (or compressor) 47, and a pump. A pipe 50 for connecting the 47 and the pipe 42 via a check valve 49 is provided.

The oxidant gas sent out from the oxidant supply device 41 is supplied to the fuel cell 30 oxidant electrode through the pipe 42 and used for the reaction at the oxidant electrode. The oxidant gas that has passed through the oxidant electrode is discharged to the pipe 44 from the oxidant outlet of the fuel cell 30. The oxidant gas discharged to the pipe 44 is cooled by the cooler 43 as needed, and then reaches the gas-liquid separator 45 via the pipe 46. The oxidant gas from which the liquid layer component has been removed by the gas-liquid separator 45 is sent to the pipe 50 by the circulation pump 47, and is again supplied to the oxidant electrode of the fuel cell 30 through the pipe 42. In this way, the oxygen circulation system (circulation route of the oxidant gas) in which the residual oxygen not used for the reaction at the oxidant electrode contained in the oxidant gas discharged from the fuel cell 30 is supplied to the fuel cell 30 again. ) Is configured.

By the way, in the fuel cell 30, water (generated water) is generated by the reaction at the oxidant electrode. The generated water reaches the fuel electrode from the oxidant electrode through the solid polyelectrolyte membrane. Here, a fluororesin-based polymer membrane is applied to the solid polymer electrolyte membrane. Therefore, fluorine compounds (for example, hydrofluoric acid) and fluorine ions may elute in the generated water that moves to the fuel electrode.

The generated water that has reached the fuel electrode passes through the passage of the separator 15 shown in FIG. 1, and is discharged to the outside (pipe 34) of the fuel cell 30 together with the fuel gas. At this time, the fluorine compounds and fluorine ions in the generated water react with the metals constituting the separator 15 (iron, nickel, chromium, molybdenum, etc. in the case of a stainless steel separator), which causes the ions of these metals to elute. Become.

Here, FIG. 4 shows the results of a durability test of a membrane electrode assembly in which iron ions are intentionally added as metal ions. In FIG. 4, the vertical axis shows the gas leakage amount (cross leak amount) between the two poles due to the thinness of the electrolyte membrane as a relative ratio when the reference value line is 1.0, and the horizontal axis shows. Shows the endurance time as a relative ratio when the target value is 1.0t. As is clear from this figure, it can be seen that as the amount of Fe (d) added to the membrane electrode assembly increases, the time for exceeding the standard cross-leakage amount becomes faster and the progress of membrane thinning becomes faster.

Further, FIG. 5 shows the relationship between the amount of Fe in the membrane electrode assembly and the durability time when the reference cross leak amount is exceeded. In FIG. 5, the vertical axis indicates the endurance time as a relative ratio when the target value is 1.0 t, and the horizontal axis indicates the amount of Fe as the amount of Fe in the membrane electrode assembly at the endurance time of 1.0 t. Is shown as a relative ratio when d is. There is a correlation between the amount of Fe in the membrane electrode assembly and the durability time, and the larger the amount of Fe, the shorter the durability time tends to be. In order to satisfy the target durability time, it is necessary that the amount of Fe in the membrane electrode assembly is equal to or less than the reference value. From the above, it can be seen that it is necessary to suppress the amount of Fe in the electrolyte membrane to a predetermined amount or less.

In view of the above problems, the fuel cell system shown in FIG. 3 includes a metal ion concentration detecting means for detecting the metal ion concentration in the generated water and an ECU 53 as a control unit. As the metal ion concentration detecting means, for example, the metal ion detection sensors 51 and 52 for detecting the concentration of metal ions in the generated water separated by the gas-liquid separators 35 and 45, which are shown in FIG. 3, can be used. The metal ion detection sensors 51 and 52 detect the concentration of metal ions, for example, iron ions, nickel ions, chromium ions, and molybdenum ions in the generated water by using, for example, an atomic absorption method. The output signal of the metal ion detection sensor is given to the ECU 53.

The ECU 53 is configured by using a CPU, a memory, an input / output interface, and the like, and the CPU executes a control program stored in the memory from a metal ion concentration detecting means such as the metal ion detecting sensors 51 and 52. Based on the sensor output of the cell stack, the constraint pressure of the cell stack is detected, and the constraint pressure of the cell stack is controlled. The restraint pressure of the cell stack is detected by a restraint pressure detecting means such as a pressure sensor provided in the fuel cell 30. Further, the restraint pressure of the cell stack can be controlled by adjusting the tension applied to the tension shaft 19 shown in FIG. 2, for example, by a restraint pressure control means such as the cell stack restraint pressure adjusting device 54.

FIG. 6 shows the relationship between the thickness of the gas diffusion layer in a single cell and the Fe uptake rate into the electrolyte membrane (the ratio of the Fe amount taken into the membrane electrode assembly to the eluted Fe ion amount). There is a correlation between the thickness of the gas diffusion layer and the Fe uptake rate, and the thicker the gas diffusion layer, the smaller the Fe uptake rate. It is considered that Fe ions diffuse in the generated water and are taken into the electrolyte membrane, and as the gas diffusion layer becomes thicker, the diffusion distance of Fe ions becomes longer and the number of Fe ions reaching the electrolyte membrane decreases. It is thought that this is because of the fact.

Therefore, in the present invention, the metal ion concentration detecting means (for example, the metal ion detection sensors 51 and 52) determines whether or not the metal ion concentration in the generated water is equal to or higher than the specified value, and the metal ion concentration is equal to or higher than the specified value. , The restraining pressure of the cell stack is detected by the restraining pressure detecting means (for example, a sensor provided in the fuel cell). When the restraint pressure of the cell stack is equal to or higher than the specified value, the restraint pressure control means (for example, the cell stack restraint pressure adjusting device 54) reduces the restraint pressure of the cell stack to less than the specified pressure. By setting the restraining pressure of the cell stack to less than the specified pressure in this way, the compressed gas diffusion layer in the single cell becomes thicker, and the amount of metal ions reaching the electrolyte membrane can be reduced.

The control in the fuel cell system is performed by the ECU 53, which is a control unit. The flow of the operation control process in the fuel cell system of the present invention is shown in the flowchart of FIG.

<Operation control processing>
The process shown in FIG. 7 is started, for example, by starting the fuel cell 30. When the process shown in FIG. 7 is started, the ECU 53 acquires the metal ion concentration in the discharged generated water by the metal ion detection sensors 51 and 52 (S01). Examples of the metal ion include one kind or a combination thereof among iron, nickel, chromium, molybdenum and the like.

Next, the ECU 53 determines whether or not the metal ion concentration is equal to or higher than a predetermined concentration specified value (S02). That is, the ECU 53 determines whether or not the metal ion concentration is equal to or higher than the prepared concentration specified value. At this time, if the metal ion concentration is equal to or higher than the specified concentration value (S02: Yes), the process proceeds to step S03, and if not (S02: No), the process is stopped.

In step S03, the ECU 53 acquires the cell restraint pressure in the cell stack by the restraint pressure sensor arranged in the fuel cell 30 (S03).

Next, the ECU 53 determines whether or not the cell restraint pressure is equal to or higher than the specified pressure value (S04). That is, the ECU 53 determines whether or not the cell restraint pressure is equal to or higher than the prepared pressure specified value. At this time, if the cell restraint pressure is equal to or higher than the specified pressure value (S04: Yes), the process proceeds to step S05, and if not (S04: No), the process is stopped.

In step S05, the ECU 53 starts the cell stack restraint pressure adjusting device 54, lowers the cell stack restraint pressure to less than the specified pressure value, and ends this process.

By the above treatment, the amount of metal ions reaching the electrolyte membrane can be suppressed, deterioration of the electrolyte membrane can be suppressed, and deterioration of the performance of the fuel cell can be suppressed.

10 Single cell 11 Electrolyte film 12 Catalyst layer 13 Gas diffusion layer 14 Membrane electrode assembly 15 Separator 16 Terminal plate 17 End plate 18 Bolt 19 Tension shaft 20 Fuel cell 21 Cell stack 30 Fuel cell 31 Fuel supply device 32 Piping 33 Cooler 34 Piping 35 Air-liquid separator 36 Piping 37 Circulation pump 38 Piping 39 Check valve 40 Piping 41 Oxidizing agent supply device 42 Piping 43 Cooler 44 Piping 45 Gas-liquid separator 46 Piping 47 Circulation pump 48 Piping 49 Check valve 50 Piping 51 Metal ion detection sensor 52 Metal ion detection sensor 53 ECU
54 Cell stack restraint pressure regulator

Claims (2)

A fuel cell having a cell stack in which a plurality of single cells having a membrane electrode assembly sandwiched between stainless steel separators are stacked, and a fuel cell system having a control unit.
The control unit determines whether or not the metal ion concentration in the generated water discharged from the fuel cell is equal to or higher than the specified value, and if the metal ion concentration is equal to or higher than the specified value, detects the restraining pressure of the cell stack. A fuel cell system that reduces the restraining pressure of the cell stack to less than the specified pressure when the restraining pressure is equal to or higher than the specified value. The fuel cell system according to claim 1, wherein the metal ion is an iron ion.

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