JP7088061B2 - Fuel cell system - Google Patents

Description

The present invention relates to a fuel cell system that suppresses deterioration of the anode catalyst when fuel gas is scarce.

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 polymer electrolyte fuel cell, the reaction of the following equation (1) proceeds at the anode (fuel electrode) 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 cathode (oxidizing agent electrode). On the other hand, the protons generated by the above equation (1) move from the anode side to the cathode side in the solid polymer electrolyte membrane by electroosmosis in a state of being hydrated with water.

On the other hand, at the cathode, the reaction of the following equation (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)

In a fuel cell having such a configuration, when the supplied fuel gas (hydrogen) is insufficient during the operation of the fuel cell, the reaction of generating protons proceeds due to the corrosion of the carbon carrier which is the catalyst carrier of the anode. There is. Such corrosion of the carbon carrier also causes elution of the catalytic metal carried on the carbon carrier, and as a result, the performance deterioration of the fuel cell is deteriorated.

In order to suppress such a problem, it has been proposed to generate an electrolysis reaction of produced water at the anode in order to supplement the proton (H ⁺ ) required for the battery reaction when the fuel gas is depleted.

Specifically, for example, in Patent Documents 1 to 3, a water electrolysis catalyst is mixed with the anode, and water is further moved to the anode, and when fuel is scarce, the water electrolysis catalyst promotes the electrolysis reaction of the produced water. , It has been proposed to suppress the deterioration of the carbon carrier.

Japanese Unexamined Patent Publication No. 2007-265921 JP-A-2007-265929 Japanese Unexamined Patent Publication No. 2007-305388

As described above, a water electrocatalyst has been added to the catalyst layer in order to suppress deterioration of the catalyst layer when fuel is scarce, but water is used below the freezing point and at low temperatures (for example, 0 ° C to 15 ° C). Since the reaction activity of the electrolytic catalyst is low, there is a problem that it is difficult to suppress the deterioration of the anode catalyst layer.

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 capable of suppressing deterioration of a carbon carrier in an anode catalyst layer due to fuel deficiency even at low temperatures.

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

<1> A fuel cell system including a fuel cell having an anode catalyst layer including a water electrolysis catalyst and a carbon carrier, and a fuel cell system including a control unit.
The control unit is a fuel cell system that raises the temperature of the fuel cell to a temperature higher than the threshold when the fuel cell is in a fuel-deficient state and the temperature of the fuel cell is lower than the threshold.

According to the present invention, when the fuel cell is in a fuel-deficient state and the temperature of the fuel cell is low, and it is determined that the activity of the water electrolysis catalyst is low, the temperature of the fuel cell is raised. Further, by improving the activity of the water electrolysis catalyst, deterioration of the anode can be prevented.

It is sectional drawing of the fuel cell. It is a schematic diagram of the fuel cell system of this invention. It is a figure which shows the power generation voltage of the fuel cell which is lacking fuel. It is a figure which shows the relationship between the temperature of an anode catalyst including a water electrolysis catalyst, and the anode potential. It is a figure which shows the relationship between a cathode stochastic ratio and a cell voltage. 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 with an anode catalyst layer containing a water electrocatalyst and a carbon carrier, and a control unit.
When the fuel cell is in a fuel-deficient state and the temperature of the fuel cell is lower than the threshold value, the control unit raises the temperature of the fuel cell to a temperature higher than the threshold value.

In a fuel cell, a problem may occur in which the inlet and outlet of the anode are blocked in various operating conditions. As an example, the outlet portion of the anode may freeze at the time of starting below the freezing point, resulting in a state in which the anode is blocked. In such a case, fuel hydrogen is not supplied to the electrode surface, resulting in a fuel deficiency state. In this fuel shortage state, the following equation (4)
C + 2H ₂ O → CO ₂ + 4H ⁺ + 4e -... ⁽ 4)
The reaction represented by (1) proceeds, and the carbon contained in the carbon carrier supporting the catalyst metal and the diffusion layer at the anode deteriorates.

Therefore, as a measure against fuel shortage, a water splitting catalyst has been placed at the anode. By arranging the water electrolysis catalyst on the anode in this way, even if the anode falls into a fuel-deficient state, the following equation (5)
2H ₂ O → O ₂ + 4H ⁺ + 4e -... ⁽ 5)
It is possible to cause the reaction represented by, suppress the reaction represented by the above formula (4), and suppress the deterioration of the carbon carrier and the like which are the constituent materials of the anode.

However, this water electrolysis catalyst has low activity at low temperature, and therefore, when fuel deficiency occurs at low temperature, the reaction represented by the above formula (5) cannot proceed, and is represented by the above formula (4). The reaction will proceed and the anode will deteriorate.

Therefore, in the present invention, when the fuel cell is in a fuel-deficient state and the temperature of the fuel cell is lower than the threshold value, the temperature of the fuel cell is raised to a temperature higher than the threshold value.

When the fuel cell is in a fuel-deficient state, it is necessary to proceed with the reaction represented by the above (5) by using a water electrolysis catalyst, and for that purpose, it is necessary that the water electrolysis catalyst exhibits activity. .. When the temperature of the fuel cell is lower than the temperature at which the water electrolysis catalyst can exhibit activity, the temperature of the fuel cell is raised to exert the activity of the water electrolysis catalyst, and the reaction represented by the above (5) is exhibited. Can be advanced.

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 configuration>
FIG. 1 shows the basic structure of a fuel cell constituting a fuel cell stack included in the fuel cell system of the present invention. In the fuel cell 10, catalyst layers 12 are formed on both sides of a proton conductive electrolyte membrane 11 made of a solid polymer material, and gas diffusion layers 13 are further formed on both sides of the catalyst layer 12 to integrate them. The membrane electrode assembly 14 is sandwiched between the separators 15.

The separator 15 is made of a conductive material such as stainless steel. A fuel gas flow path for the fuel gas to flow or an oxidant gas flow path for the oxidant gas to flow is formed on the surface of the separator 15 on the gas diffusion layer 13 side. The thickness of the separator 15 is, for example, about 0.3 mm.

The gas diffusion layer 13 is a layer for diffusing a fuel gas or an oxidant gas, and is made of a conductive material such as carbon paper. The layer thickness of the gas diffusion layer 13 is about 250 μm.

The catalyst layer 12 contains a catalyst metal such as Pt (platinum) or a Pt alloy for promoting the protonation of hydrogen or promoting the reaction between protons and oxygen. Further, one catalyst layer 12 (anode catalyst layer) contains a water electrolysis catalyst for promoting electrolysis of water. Examples of the water electrolytic catalyst include metals such as Pt, Ru (ruthenium), Ir (iridium), Au (gold), Ni (nickel) and Ag, alloys such as Pt-Ru and Pt _- Ir, _RuO2 , IrO2 and the like. Metal oxides and the like can be used. The catalyst layer 12 has a structure in which the water electrocatalyst is added to the catalyst metal supported on a carbon carrier such as carbon particles and kneaded.

The electrolyte membrane 11 is made of, for example, a solid polymer electrolyte such as Nafion (registered trademark), which is a perfluorosulphonic acid type polymer having proton conductivity. The film thickness of the electrolyte membrane 11 is, for example, about 10 μm to 50 μm.

<Fuel cell system configuration>
FIG. 2 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. 2 includes a fuel cell 20 and a control unit 40.

The fuel cell 20 includes a fuel cell stack 31, a heater 32, a temperature sensor 33, and a voltage sensor 34. The fuel cell stack 31 has a structure in which a plurality of fuel cell 10s shown in FIG. 1 are laminated and fastened. The heater 32 covers the fuel cell stack 31 and controls the temperature of the fuel cell stack 31 according to the instructions of the control unit 40.

The voltage sensor 34 detects the generated voltage of each fuel cell 10, and gives the detection result to the control unit 40. The temperature sensor 33 detects the temperature of the cooling water discharged from the fuel cell, and gives the detection result to the control unit 40. The control unit 40 determines the temperature of the fuel cell 10 based on the temperature measurement result of the cooling water.

The control unit 40 is composed of a CPU (central processing unit), a ROM (read-only memory), a RAM (random access memory), and the like. The control unit 40 controls each unit of the fuel cell system including the heater 32 based on the detection results of the temperature sensor 33 and the voltage sensor 34.

<Operation control processing>
Subsequently, the control of the fuel cell system 100 by the control unit 40 will be described. First, the control unit 40 controls a fuel gas supply means and an oxidant gas supply means (not shown) so that the fuel gas and the oxidant gas are supplied to the fuel cell 20. As a result, power is generated in the fuel cell 20. Next, the control unit 40 determines whether or not the fuel cell 20 is deficient in fuel. Here, in the fuel cell 10 in which the fuel is deficient, the generated voltage becomes negative. Details will be described with reference to FIG.

FIG. 3 is a diagram showing a power generation voltage of a fuel cell 10 that is deficient in fuel. The vertical axis of FIG. 3 shows the generated voltage of the fuel cell 10, and the horizontal axis of FIG. 3 shows the elapsed time. As shown in FIG. 3, when the fuel is insufficient, the generated voltage drops sharply. After that, the decrease in the generated voltage becomes smaller. This is because the reaction of the above formula (5) occurs via the water electrolysis catalyst, and the protons are continuously supplied to the electrolyte membrane 3. From the above, the control unit 40 can determine that the fuel cell 10 is deficient in fuel when the generated voltage of any of the fuel cell 10 is 0 V or less.

In a specific aspect of the present invention, when it is determined that the fuel is deficient, the control unit 40 determines whether or not the temperature of the fuel cell 10 is lower than the threshold value. This temperature is determined by the control unit 40 based on the detection result of the temperature sensor 33 that detects the temperature of the cooling water discharged from the fuel cell 10.

Here, the threshold value of the temperature of the fuel cell 10 will be described. When the anode potential fluctuation test (0V to upper limit potential) was performed in a nitrogen purge at 30 ° C, which simulates fuel deficiency at low temperature, using an actual vehicle cell, the durability requirement that the output decrease was 5% or less in the potential fluctuation test 1200 cycles was satisfied. The maximum anode potential to be satisfied was 1.65V. If the frequency of the higher potential load increases, the oxidative deterioration of carbon in the electrodes is promoted, the output is significantly reduced, and there is a concern that the vehicle may run in a problem.

Next, the relationship between the temperature of the anode catalyst including the water electrolysis catalyst and the anode potential will be described. FIG. 4 shows the relationship between the temperature and the anode potential in the current hold test of the anode catalyst to which Ir ₃ Ru _{7 Ox} was added as a water electrolysis catalyst at a metal grain of 0.01 mg / cm ² . Generally, the higher the temperature of a water electrocatalyst, the higher the catalytic activity, and the lower the anode potential. FIG. 4 shows that when Ir ₃ Ru _{7 Ox} is used as a water electrolysis catalyst at a metal basis weight of 0.01 mg / cm ² , the cell temperature should be 20 ° C. or lower in order to keep the anode upper limit potential of 1.65 V or less. Indicates that is required. The temperature at which the anode potential is 1.65 V or less is set as the threshold value (T1). Then, this threshold value can be experimentally obtained in advance depending on the catalyst type and the amount of catalyst contained in the anode catalyst layer. For example, when IrO _x was used as the water electrolysis catalyst, this threshold was 53 ° C.

In this way, the cell temperature at which the anode potential is 1.65 V or less is calculated from the relationship between the cell temperature and the anode potential, and is set as the threshold value (T1). At a cell temperature of T1 or less, the water electrolysis catalytic activity is not sufficient, the anode potential rises to 1.65 V or more, the oxidation of the carbon carrier is promoted, and there is a concern that the battery performance (output) is significantly reduced. Therefore, when it is determined that the temperature of the fuel cell 10 is lower than the threshold value, the control unit 40 raises the temperature of the fuel cell 10 to increase the activity of the water electrolysis catalyst.

To raise the temperature of the fuel cell 10, for example, the heater 32 is operated according to an instruction from the control unit 40 to heat the fuel cell stack 31. Further, the temperature of the fuel cell 10 can also be raised by lowering the stochastic ratio of the cathode.

A method of raising the temperature of the fuel cell by lowering the cathode stoichiometric ratio will be described. As shown in FIG. 5, when the cathode ratio is lowered, that is, when the air is deficient, the concentration overvoltage increases and the cell voltage decreases as the IV performance deteriorates. In a fuel cell, all the overvoltage component that is not used for power generation is dissipated as heat, so the cell temperature rises due to the heat dissipation of the increased overvoltage component. In the example shown in FIG. 5, when the cathode stoichiometric ratio is lowered from 2.5 to 0.8 at the time of 1.0 A / cm ² current sweep, the calorific value per cell is calculated for 1 second.
When the stok ratio is 2.5: 1.0A / cm ² x 250cm ² x 0.3V = 75J
When the stochastic ratio is 0.8: 1.0A / cm ² x 250cm ² x 0.5V = 125J
Therefore, the heat generation effect when the stoichiometric ratio is lowered is
125/75 = 1.67 (times)
Therefore, it becomes possible to raise the temperature of the cell about 1.7 times faster.

FIG. 6 is a flowchart showing an example of control of the fuel cell system 100 by the control unit 40 during power generation of the fuel cell 20. The control unit 40 executes the flowchart of FIG. 6 at a predetermined cycle. As shown in FIG. 6, first, the control unit 60 determines whether or not any of the fuel cell 10 is in the fuel shortage state (step S1). In this case, the control unit 40 determines whether or not the generated voltage of any of the fuel cell 10 is 0 V or less based on the detection result of the voltage sensor 34.

When it is determined in step S1 that any of the fuel cell 10 is in a fuel deficient state, the control unit 40 determines that the temperature of the cooling water discharged from the fuel cell 10 is based on a threshold value calculated in advance. It is determined whether it is low (step S2). In this case, the control unit 40 determines based on the detection result of the temperature sensor 33 that detects the temperature of the cooling water. When it is determined in step S2 that the temperature of the cooling water is lower than the threshold value, the control unit 40 determines that the temperature of the fuel cell is lower than the threshold value, and raises the temperature of the fuel cell (step S3).

Next, the control unit 40 determines whether the temperature of the cooling water discharged from the fuel cell 10 is lower than the threshold value calculated in advance (step S4). When it is determined in step S4 that the temperature of the cooling water is lower than the threshold value, the control unit 40 repeats from the operation of step S3.

If it is not determined in step S1 that any of the fuel cell 10 is in a fuel-deficient state, if it is not determined in step S2 that the temperature of the cooling water is lower than the threshold value, and in step S4, the cooling water is cooled. When it is determined that the temperature is higher than the threshold value, the control unit 40 ends the operation. As a result, the fuel cell 20 continues to generate electricity. If it is not determined in step S4 that the temperature of the cooling water is higher than the threshold value, the control unit 40 repeats from the operation of step S3.

As described above, by the control according to the flowchart of FIG. 6, even when the fuel is deficient and the temperature is low in any of the fuel cell 10, the activity of the water splitting catalyst is maintained and the water is electrolyzed. The reaction can be continued. As a result, deterioration of the fuel electrode 2 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 20 Fuel cell 31 Cell stack 32 Heater 33 Temperature sensor 34 Voltage sensor 40 Control unit

Claims (1)

A fuel cell with an anode catalyst layer containing a water electrolysis catalyst and a carbon carrier, and a fuel cell system with a control unit.
When the fuel cell is in a fuel-deficient state and the temperature of the fuel cell is 0 ° C. or higher and lower than the threshold value at which the anode potential is 1.65 V or lower, the control unit determines the fuel. A fuel cell system that raises the temperature of a battery cell above the threshold.

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