JP2003282111A - Fuel cell operating method and fuel cell control device - Google Patents

Abstract

(57) [Summary] [Problem] To control the wetting state of an electrode without lowering the gas utilization rate, to maintain optimal moisture over the entire surface of the electrode,
An operation method of a fuel cell capable of maintaining high power generation efficiency for a long time is provided. A wetting state of an electrode is controlled by periodically changing a utilization rate of a fuel gas and / or an oxidizing gas. Specifically, by changing the supply amount of the fuel gas and / or the oxidizing gas or the current density in the fuel cell, the utilization rate is periodically changed to control the wet state of the electrode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of operating a fuel cell and a fuel cell controller for controlling the output power of the fuel cell.

[0002]

2. Description of the Related Art A fuel cell generates electric power by reacting a fuel gas supplied to a fuel electrode with an oxidant gas supplied to an oxidant electrode. Especially in the polymer electrolyte fuel cell,
A gas diffusion layer having pores that serve as a supply path for a reaction gas, a polymer electrolyte membrane that serves as a hydrogen ion conduction path, and a catalyst layer that has pores provided between these and also serves as an electron conductor. Electrolyte membrane electrode assembly (M
EA) is used. The gas diffusion layer and the catalyst layer form an electrode. The discharge performance of the battery depends on the size of the so-called three-phase interface formed by the pores, the polymer electrolyte and the catalyst.

By the way, since the polymer electrolyte membrane does not exhibit sufficient hydrogen ion conductivity unless it contains water, the reaction gas (fuel gas and oxidant gas) is humidified and supplied. In addition, water is generated with the power generation of the fuel cell due to the reaction of hydrogen and oxygen. Therefore, excess water in the supply gas and the generated water block the pores of the electrode,
There is a problem that the diffusion of fuel gas and oxidant gas is hindered and the power generation efficiency of the fuel cell is reduced. Therefore,
Efficient discharge of such water from the fuel cell was important for maintaining and improving its performance.

Conventionally, in order to efficiently discharge water from the fuel cell, a method of treating the electrodes with water repellent has been generally used. However, in the upstream portion of the flow of the supply gas, the amount of water contained in the supply gas is small, and the electrode tends to dry with the passage of time. On the other hand, in the downstream part, the supply gas reacts with the power generation and is consumed, resulting in a relative excess of water. In addition to this, water is also generated, and optimum water content is retained over the entire surface of the electrode. It is difficult.

Further, the water repellency of the electrode decreases with the passage of time, and the progress of the wetting of the electrode increases the region of excessive water content, whereby the voltage that is sufficiently exerted at the initial stage also with the passage of time. There is a problem of decrease. In order to solve this problem, it is possible to reduce the gas utilization rate and reduce the wet state on the electrode by increasing the amount of supply gas above the amount required for power generation. The amount of power generation for gas, that is, the power generation efficiency decreases. Therefore, in a fuel cell used in a cogeneration system or the like, which requires higher efficiency, such a method is not practical.

[0006]

In view of the above problems, the object of the present invention is to control the wet state of the electrode without lowering the gas utilization rate, to keep the optimum moisture over the entire surface of the electrode, and An object of the present invention is to provide a method of operating a fuel cell capable of maintaining power generation efficiency for a long period of time.

[0007]

In order to solve the above problems, the present invention provides an electrolyte membrane, a first electrode and a second electrode sandwiching the electrolyte membrane, and a fuel gas to the first electrode. A method of operating a fuel cell, comprising: a first separator plate having a gas flow path for supplying the gas; and a second separator plate having a gas flow path for supplying the oxidant gas to the second electrode, the method comprising: There is provided a method of operating a fuel cell including a step of controlling a wet state of the first electrode and the second electrode by periodically changing a utilization rate of at least one of a fuel gas and an oxidant gas.

In the operating method, in the step, the utilization rate is periodically changed by changing the supply amount of at least one of the fuel gas and the oxidant gas or the current density in the fuel cell. Preferably, it is a step of controlling the wet state of the first electrode and the second electrode. Further, it is preferable that the current density in the fuel cell is changed by periodically changing the current value taken out of the fuel cell.

The present invention also provides a fuel cell control device used for a fuel cell operated by the above-described operating method. That is, the present invention includes a power storage unit that stores the output power of the fuel cell and a control unit that flattens the output power, and the power is stably supplied even if the current density of the fuel cell changes periodically. There is provided a fuel cell control device for supplying. Further, the present invention comprises a control unit for complementarily changing the current density of each block of the fuel cell electrically divided into a plurality of blocks, wherein the current density of each of the blocks is periodically changed. Even so, a fuel cell control device that stably supplies electric power is also provided.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The first aspect of the present invention includes an electrolyte membrane, first and second electrodes sandwiching the electrolyte membrane, and a gas flow path for supplying and discharging a fuel gas to the first electrode. A separator plate and a second separator plate having a gas flow path for supplying and discharging an oxidant gas to and from the second electrode, the method comprising:
Alternatively, the present invention relates to a method for operating a fuel cell, including a step of controlling the wet state of the first electrode and / or the second electrode by periodically changing the utilization rate of the oxidant gas.

As described above, the polymer electrolyte fuel cell is composed of an electrolyte membrane and electrodes arranged on both sides of the electrolyte membrane. This electrode is composed of a gas diffusion layer that supplies a gas such as a fuel gas and an oxidant gas, and a catalyst layer that actually causes a chemical reaction. In such a fuel cell, it is necessary to retain an appropriate amount of water in the electrodes and optimize the amount of humidification of the gas. There is a tendency for the electrodes to dry over time, and for the electrodes on the downstream side to have excessive water content due to power generation.

The optimum amount of humidification for the electrode depends on the amount of supply gas and the current density, and the region having the optimum humidification state corresponds to a certain gas supply amount and current density, and is located on any part of the electrode. Exists in. Then, this optimum humidification region can be moved on the electrode by changing the gas supply amount and the current density. That is, when the gas supply amount is increased, the optimum humidification region can be moved to the downstream side of the gas flow in the electrode, and similarly, when the current density is increased, the optimum humidification region can be moved to the upstream side.

The power generation efficiency of the fuel cell is defined as (electrical energy taken out from the fuel cell) / (energy supplied to the fuel cell), and reduces the gas utilization rate (= the amount of gas contributing to power generation with respect to the gas supply amount). Then, the power generation efficiency also decreases. The amount of gas contributing to power generation can be uniquely determined from the number of electrons corresponding to the current based on the current density once the current density is determined. In addition, the water repellency of the electrode decreases with the passage of time, and the wet state on the electrode progresses, so that the region with excessive water content increases, and as a result, the sufficient voltage at which the fuel cell initially exerts also increases with the passage of time. descend.

Therefore, in such a case, the present inventors can create an optimum wet state over the entire surface of the electrode by periodically changing the gas utilization rate and periodically moving the optimum humidification region. It was found that the fuel cell voltage could be increased, and the present invention was completed. The fluctuation of the gas utilization rate is not preferable for the system from the viewpoint of efficiency and controllability, but in the present invention,
The optimum humidification region can be expanded by periodically changing the gas utilization rate, and the average gas utilization rate obtained by averaging the gas utilization rates over a certain period of time can be kept constant.

To change the gas utilization rate, either the gas supply amount or the current density may be changed. The gas supply amount can be controlled by a mass flow controller or the like. It is common that the effect of expanding the optimum humidification area is obtained by changing either of them, but changing the gas supply amount is often difficult in terms of system design. For example, when reforming city gas to produce hydrogen gas, such as in a cogeneration system, it is difficult to change the gas supply amount in a short cycle because the reformer is inferior in load fluctuation response. .

The "current density" referred to in the present invention can be defined as the current value per unit area flowing per unit cell constituting the fuel cell, and specifically, the total current flowing through the fuel cell. It is obtained by dividing the value by the area of the electrode of the single cell. Therefore, the current density can be uniquely determined by measuring the current flowing through the fuel cell.

On the other hand, changing the current density can be easily achieved by controlling the current value taken out of the fuel cell. For example, a power management circuit can be connected to the fuel cell to change the current density. Specifically, the current value taken out of the fuel cell can be controlled by changing the gate current of the power transistor forming the power management circuit or shifting the timing of the trigger signal of the thyristor.

The optimum cycle of the change of the current density varies depending on the material of the electrode, the size of the electrode, the shape of the separator plate, the configuration of the gas flow path, etc., but the preferable cycle is experimentally appropriately determined by those skilled in the art. It is also possible to do so. For example, in a fuel cell in which the wet state changes rapidly with load changes, it is desirable to change the current density in a short cycle. Further, in a fuel cell in which the wet state changes slowly with respect to load changes, it is desirable to change the current density in a long cycle.

For example, when the pores of the electrode are small, it is desirable that the period for changing the current density is short. Further, even when the gas flow passage has a small cross-sectional area, it is desirable that the period for changing the current density is short. On the other hand, when the water repellency of the electrode is high, a longer cycle for changing the current density tends to be better. Further, even when the size of the electrode is large, it is preferable that the cycle for changing the current density is long, and even if the shape of the separator plate is rectangular and the gas flow path is long, the cycle for changing the current density is longer. desirable.

A specific optimum cycle can be found in the range of 1 millisecond to 1 hour, but more preferably, 0.
1 second to 10 minutes. If the cycle is too short, the movement of the wetting of the electrodes cannot follow and the effect of varying the optimum humidification area cannot be obtained. On the other hand, if the cycle is too long, drying and wetting progress and the voltage drops.

Here, the fuel cell in which the wet state of the first electrode and / or the second electrode is controlled by periodically changing the utilization rate of the fuel gas and / or the oxidant gas as described above. From, the output power that changes periodically is obtained. At this time, it is desirable to use a fuel cell control device to flatten the output power and output it to the outside. Therefore, the present invention includes a power storage unit that stores the output power of the fuel cell, and a control unit that flattens the output power, and the power is stably supplied even if the current density of the fuel cell changes periodically. There is provided a fuel cell control device for supplying. This device
The power storage unit is characterized in that it has a power storage unit for flattening the output power, and the power storage unit can be composed of, for example, a capacitor or a storage battery.

However, when this device is used, flattening is possible when the period in which the current density changes is short, but it is not possible to deal with it appropriately when the period is long. When the cycle is long, the fuel cell is electrically divided into two or more blocks, and the current density of each block is changed complementarily, so that the current density of each block changes. Electric power can be taken out stably. Of course, a plurality of fuel cells may be used. Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.

[0023]

EXAMPLES << Example 1 >> First, an MEA was produced by the following method. 10 g of conductive carbon powder supporting platinum particles with an average particle size of about 30 Å (50 wt% platinum, TEC10E50E manufactured by Tanaka Kikinzoku Co., Ltd.), 10 g of water and 9% of hydrogen ion conductive polymer electrolyte 55 g of an ethanol solution (Flemion manufactured by Asahi Glass Co., Ltd.) was mixed to prepare a catalyst paste A. The catalyst paste A was applied on a polypropylene film with a bar coater and dried to obtain a catalyst layer A for an oxidizer electrode. The coating amount of the catalyst paste A was adjusted so that the platinum content was 0.3 mg per cm ² .

Next, platinum-ruthenium was added to the conductive carbon powder.
Supported with aluminum alloy (30% by weight platinum, 23% by weight
Is ruthenium, TEC61E54 from Tanaka Kikinzoku Co., Ltd. 10g
In addition, 10 g of water and 9 of hydrogen ion conductive polymer electrolyte
% Ethanol solution (Flemion manufactured by Asahi Glass Co., Ltd.) 50
g was mixed to prepare a catalyst paste B. This catalyst page
Strike B is applied on polypropylene film with a bar coder.
Then, the catalyst layer B for the fuel electrode was obtained by drying. catalyst
The amount of paste B applied is such that the platinum content is 1 cm. ²Hit
It was adjusted to be 0.3 mg.

The polypropylene film having the catalyst layer A and the polypropylene film having the catalyst layer B were each cut into 6 cm × 6 cm squares, and in the polypropylene films after cutting, the catalyst layer A and the catalyst layer B had high hydrogen ion conductivity. In contact with the molecular electrolyte membrane (Gore-Select made by Japan Gore-Tex, film thickness 30 μm),
The electrolyte membrane was sandwiched. Then, after performing hot pressing under the conditions of 130 ° C. and 10 minutes, the polypropylene film was removed to obtain an electrolyte membrane having a catalyst layer A and a catalyst layer B.

On the other hand, the gas diffusion layer of the electrode was prepared as follows. Carbon nonwoven fabric composed of conductive carbon particles having a thickness of 360 μm (TGP-H-12 manufactured by Toray Industries, Inc.)
0) was impregnated with a fluororesin-containing aqueous dispersion (Neoflon ND1 manufactured by Daikin Industries, Ltd.), dried, and heated at 400 ° C. for 30 minutes to impart water repellency. Further, an ink obtained by mixing a conductive carbon powder and a dispersion liquid in which polyethylene terephthalate (PTFE) fine powder is dispersed is applied to one surface of this carbon nonwoven fabric by a screen printing method, and then repelled. An aqueous layer was formed. At this time, a part of the water repellent layer was embedded in the carbon nonwoven fabric.

Next, the electrolyte membrane having the catalyst layer A and the catalyst layer B and the two carbon nonwoven fabrics were sandwiched so that the water repellent layers of the carbon nonwoven fabric were in contact with the catalyst layers A and B, respectively, and the whole. Were joined by hot pressing to obtain an electrode / electrolyte membrane assembly (MEA). M obtained here
A schematic vertical sectional view of the EA is shown in FIG. As shown in FIG. 1, the MEA 14 included an electrolyte membrane 11, a catalyst layer 12 and a gas diffusion layer 13. However, the water repellent layer was omitted.

Then, a groove was formed in a graphite plate having a thickness of 2 mm by cutting to form a gas flow path to obtain a separator plate.
The MEA was sandwiched between two obtained separator plates to assemble a single cell 23 (measuring cell) as a fuel cell. The gas channel is formed only in the part of the separator plate that contacts the electrode,
The dimensions of the groove forming the gas flow path are 1 mm width and 0.7 m depth.
The width of the rib between the grooves was 1 mm. Also,
The gas flow path was formed by serpentinely meandering on the surface of the separator plate. A schematic vertical sectional view of the fuel cell obtained here is shown in FIG. As shown in FIG. 2, the unit cell 23 includes a separator plate 21 having a gas passage 22 and an M
It contained EA14.

The temperature of the fuel cell was set to 75 ° C., and hydrogen gas humidified to have a dew point of 70 ° C. was supplied to the fuel electrode at a current density of 300 mA / cm ² with a utilization rate of 80%.
Was supplied. On the other hand, the oxidizer electrode has 7
Air humidified so that it has a dew point of 0 ° C., current density 3
An amount of 40% utilization was supplied at 00 mA / cm ² . Then, as shown in FIG. 3, the current density is 263 m.
A discharge test was carried out by the operating method according to the present invention by periodically changing to A / cm ² and 338 mA / cm ² . FIG. 3 is a graph showing the cycle of the current density in the operating method of Example 1 according to the present invention. As shown in FIG. 3, the cycle was 5 minutes.

As a result, the fuel utilization rate at a current density of 263 mA / cm ² corresponds to 70%, and the current density is 338 m.
The fuel utilization rate at A / cm ² was equivalent to 90%. Further, it was assumed that the wet state of the upstream side portion (a) and the downstream side portion (b) of the electrode at this time was in the state of FIG. FIG. 4 is a graph conceptually showing the wet state (water content) of the electrodes of the fuel cell in Example 1. Further, when the voltage of the fuel cell was measured, it fluctuated with the change of the current density, but as shown in FIG. 5, the average voltage hardly decreased even after a long time operation. FIG. 5 is a graph showing changes in the voltage of the fuel cell in Example 1 with respect to time. Further, FIG. 6 shows changes with time of the average value of the voltage of the fuel cell in Example 1 per hour.

Similar results were obtained even when the cycle was changed in the range of 1 minute to 10 minutes. When the cycle was changed from 5 seconds to 1 minute, the average voltage slightly decreased with time, but long-term operation was possible. On the other hand, even when the cycle was changed in the range of 10 minutes to 1 hour, long-term operation was possible, although the average voltage slightly decreased with time.

Example 2 First, in the same manner as in Example 1, a single cell for characteristic measurement was assembled as a fuel cell. The fuel cell was operated by connecting the fuel cell control device having the configuration shown in FIG. FIG. 7 is a diagram showing a configuration of a system in which a fuel cell control device 31 is connected to the fuel cell 30 of the present invention. A power smoothing circuit having an electrolytic capacitor as a main component was used for the power storage unit 32 in the fuel cell control device 31. Power storage capacity is 5W, load response time is 1m
It was set to m seconds. A discharge test was conducted in the same manner as in Example 1 using this control device. The cycle was 1 second.

FIG. 8 shows a graph plotting the change with time of the voltage of the fuel cell in Example 2. The voltage of the fuel cell was almost flat as shown in FIG. Although the voltage dropped slightly over time, long-term operation was possible. Moreover, in FIG. 6, the time-dependent change of the average value of the output voltage of the fuel cell in Example 2 per hour is shown.
Similar results were obtained even when the cycle was changed in the range of 1 millisecond to 1 minute.

Example 3 Similar to Example 1, a single cell for characteristic measurement was assembled as a fuel cell. Two sets of this fuel cell were prepared, and the fuel cell control device having the configuration shown in FIG. 9 was connected and operated. FIG. 9 is a diagram showing the configuration of another system in which the fuel cell control device is connected to the fuel cells 33 and 34 of the present invention. As this fuel cell control device,
A control unit that complementarily changes the current density of each block of the fuel cell that is electrically divided into a plurality of blocks (fuel cells A and B) is provided, and the current density of each block is periodically changed. A fuel cell control device that stably supplies electric power even when changed is used.

The current densities of the fuel cells A and B were changed as shown in FIGS. 10 (a) and 10 (b), respectively. FIG. 10 is a graph showing the cycle of changing the current density of each of the two fuel cells in Example 3, and the cycle was set to 5 minutes. FIG. 11 shows a graph plotting changes with time in the voltage obtained from the fuel cell control system in the third embodiment. This voltage was almost flat as shown in FIG. The voltage did not decrease even after running for a long time. Further, FIG. 6 shows a change with time of the average value of the output voltage of the fuel cell control device in Example 3 per hour (c). Similar results were obtained even when the cycle was changed in the range of 5 seconds to 10 minutes.

Comparative Example As in Example 1, a single cell for characteristic measurement was assembled as a fuel cell. This single cell was subjected to a discharge test with the current density fixed at 300 mA / cm ² . The voltage of the fuel cell dropped over time. Figure 6
Fig. 3D shows the change over time in the average value of the fuel cell voltage per hour in the comparative example.

[0037]

As described above, according to the present invention, the gas utilization factor is substantially changed and the electrode wet condition is made uniform over the entire surface by operating the fuel cell by periodically changing the current density. Thus, the output power of the fuel cell can be increased and high power generation efficiency can be maintained for a long time.

[Brief description of drawings]

FIG. 1 is a schematic vertical sectional view showing the structure of an MEA obtained in Example 1.

FIG. 2 is a schematic vertical cross-sectional view showing the structure of the fuel cell obtained in Example 1.

FIG. 3 is a graph showing a cycle of current density in the operating method of Example 1 according to the present invention.

FIG. 4 is a graph conceptually showing the wet state of the electrodes of the fuel cell in Example 1.

FIG. 5 is a graph showing changes with time in voltage of the fuel cell in Example 1.

FIG. 6 is a graph showing changes over time in the average value of the voltage of the fuel cell in Example 1 per hour.

FIG. 7 is a diagram showing a configuration of a system in which a fuel cell control device is connected to the fuel cell of the present invention.

FIG. 8 is a graph in which a change with time in the voltage of the fuel cell in Example 2 is plotted.

FIG. 9 is a diagram showing the configuration of another system in which a fuel cell control device is connected to the fuel cell of the present invention.

FIG. 10 is a graph showing a cycle in which the current density of each of the two fuel cells in Example 3 is changed.

FIG. 11 is a graph in which changes with time in voltage obtained from the fuel cell control device in Example 3 are plotted.

[Explanation of symbols]

11 Electrolyte membrane 12 Catalyst layer 13 Gas diffusion layer 14 MEA 21 Separator plate 22 gas flow path 23 single cells

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shinya Kosa             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. (72) Inventor Eiichi Yasumoto             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. (72) Inventor Yoshihiro Hori             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. (72) Inventor Yasushi Sugawara             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. (72) Inventor Shinichi Arisaka             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. F-term (reference) 5H026 AA06                 5H027 AA06 MM01 MM26

Claims (5)

[Claims] 1. An electrolyte membrane, a first electrode and a second electrode sandwiching the electrolyte membrane, a first separator plate having a gas flow path for supplying a fuel gas to the first electrode, and the first electrode and the second electrode. A method of operating a fuel cell, comprising: a second separator plate having a gas flow path for supplying an oxidant gas to the second electrode, wherein a utilization rate of at least one of the fuel gas and the oxidant gas is periodically measured. A method of operating a fuel cell, comprising the step of controlling the wet state of the first electrode and the second electrode by changing the wet state. 2. The step of cyclically changing the utilization rate by changing the supply amount of at least one of the fuel gas and the oxidant gas or the current density in the fuel cell in the step, The method for operating a fuel cell according to claim 1, which is a step of controlling a wet state of the electrode and the second electrode. 3. The method of operating a fuel cell according to claim 2, wherein the current density is changed by periodically changing a current value taken out of the fuel cell. 4. A power storage unit for storing the output power of the fuel cell, and a control unit for flattening the output power, the power being stably supplied even if the current density of the fuel cell changes periodically. Fuel cell controller to supply. 5. A control unit for complementarily changing the current density of each block of the fuel cell electrically divided into a plurality of blocks, wherein the current density of each block is periodically changed. Fuel cell control device that supplies power stably.