JPH11219715A - Operation control method for fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove the CO adsorbed to the surface of an electrode by oxidation and to avoid and recover the deterioration of battery performance due to impurities such as CO contained in fuel gas (H) by including a process for temporarily setting the electrode potential of a fuel electrode to a potential higher than the standard hydrogen electrode potential by a specific voltage or above. SOLUTION: The potential of a fuel electrode is temporarily set to a potential higher than the standard hydrogen electrode potential by +0.3 V or above with the operation of a fuel cell kept continued, and impurities such as CO accumulated on the electrode catalyst of the fuel electrode and deactivating the fuel electrode are removed by oxidation. The battery voltage drops as the discharge duration elapses, the feed of fuel is stopped when the battery voltage becomes lower than 0.6 V, and the feed of fuel is quickly resumed when the battery voltage becomes 0.1 V. The potential of the fuel electrode quickly rises when the feed of fuel is stopped, and the potential quickly drops to the original potential when the feed of fuel is resumed. Power can be generated while the potential of the fuel electrode is maintained at the average potential +0.1 V or below by repeating these actions.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel cell operation control method, and more particularly, to a fuel electrode catalyst (a noble metal catalyst such as Pt or Pt-Ru) used for a fuel electrode in a polymer electrolyte fuel cell or the like. The present invention relates to an operation control method for preventing poisoning and deactivation by carbon monoxide (CO) gas or the like contained in fuel gas.

[0002]

2. Description of the Related Art Conventionally, as a fuel cell of this type, for example, a polymer electrolyte fuel cell has a basic structure as shown in FIG. That is, this polymer electrolyte fuel cell
A fuel electrode (negative electrode) 14 is provided on one side of the hydrogen ion conductive polymer solid electrolyte membrane 12, and an air electrode (positive electrode) is provided on the other side.
16 are provided integrally, and a current collector (separator) 18 is provided on each electrode surface.

A flow path for a fuel gas (mainly hydrogen gas) is formed on the surface of the current collector 18 facing the fuel electrode 14, and an oxidizing gas (mainly air) is formed on the surface facing the air electrode 16. Are formed. As a material of the electrolyte membrane 12, a fluorine-based cation exchange membrane, for example, "Nafion" (trade name) of DuPont is generally used, and carbon black, carbon paper, carbon cloth, or the like is used as an electrode substrate. And a current collector (separator) such as graphite.

The basic principle of this battery is that, as shown in FIG. 6, when hydrogen is supplied to the fuel electrode and oxygen is supplied to the air electrode, an oxidation reaction is performed at the fuel electrode and a reduction reaction is performed at the air electrode as shown in the following reaction formula. Occurs, and hydrogen ions move through the electrolyte membrane to generate an electromotive force. Fuel ^{_{electrode: H 2 → 2H + + 2e}} - ^{_{cathode: 1 / 2O 2 + 2H +}} + 2e - → H 2 O

In this type of fuel cell, an electrode catalyst is used for both electrodes so that a sufficiently high reaction rate can be obtained. Since this reaction speed appears as a generated current, a high power output can be obtained. Generally, in a polymer electrolyte fuel cell, both a fuel electrode and an air electrode are a Pt catalyst or a multi-component catalyst containing Pt (for example, a Pt-Ru catalyst).
Is used. These catalysts are usually supported on carbon electrodes.

On the other hand, as a fuel gas to be used, a hydrogen-rich gas generated by reforming a low molecular weight hydrocarbon such as methane or an alcohol such as methanol may be used as a fuel. FIG. 7 shows a schematic configuration of the plant. Methanol as a fuel reacts with water on, for example, a Cu—Zn-based catalyst in a reforming section, and becomes a mixed gas of hydrogen and carbon dioxide as in the following equation. Is sent to the fuel electrode of the fuel cell. CH ₃ OH + H ₂ O → 3H ₂ + CO ₂

However, at this time, the above-mentioned reaction does not always proceed completely. For example, several% of CO is added by a route such as CH ₃ OH → 2H ₂ + CO CO ₂ + H ₂ → CO + H ₂ O. Be born. This by-product C
O acts as a catalyst poison for the Pt-based electrode catalyst, which is the electrode catalyst for the fuel electrode. CO is strongly adsorbed on the electrode catalyst and inhibits a hydrogen oxidation reaction which is an original reaction of the fuel electrode.

[0008] Therefore, in order to reduce the concentration of CO contained in the fuel gas, a CO removing section is usually provided after the reforming section. Here, the CO concentration is reduced by utilizing a shift reaction and a CO selective oxidation reaction as shown in the following equation. Shift _{reaction: CO + H 2 O → CO} 2 + H 2 CO selective oxidation reaction: 2CO + O ₂ → 2CO ₂

As described above, by providing the CO removing section at the subsequent stage of the reforming section, the C at the reformer outlet (fuel cell inlet) is provided.
O concentration can be reduced to several tens of ppm.
The operating temperature of the battery is relatively low (200 ° C. or less), and the battery is usually operated at a temperature of 100 ° C. or less.

[0010]

However, even with such a CO concentration, if a pure Pt catalyst is used for the fuel electrode, the fuel electrode is poisoned and the cell performance is reduced. It is known that if a generally known CO-resistant catalyst such as a Pt-Ru binary catalyst is used, at a battery temperature of about 80 ° C. or higher, if the CO concentration is several tens of ppm, the performance degradation hardly causes a problem. However, when the battery temperature is not sufficiently raised, such as at the time of startup, the performance is significantly reduced even at several tens of ppm.

Further, when there is a load fluctuation and the gas processing amount in the reformer changes over time, the heat balance may be temporarily lost and a large amount of CO may be sent to the fuel cell.
In this case, even the CO-resistant catalyst is poisoned and its performance is reduced. Therefore, as a countermeasure against CO in the fuel electrode, it is not always sufficient to use a CO-resistant catalyst such as Pt-Ru to reduce the CO concentration by the CO removing section and use a CO-resistant catalyst.

Therefore, as a further advanced technique for avoiding CO poisoning, about 2% of oxygen is mixed into fuel gas.
A method for oxidizing and removing CO molecules adsorbed on an electrode catalyst has been proposed [S. Gottesfeld and J. Pafford, J. Electro
chem. Soc. 135 , 2651 (1988); S. Gottesfeld, US Patent
4910099 (Mar. 20, 1990); DP Wilkinson, HHVoss, J. Du
dley, GJ Lamont and V. Basura, US Patent 5432021 (1
995), US patent 5482680 (1996)]. When this method is used, a lower temperature (up to room temperature) and a higher CO concentration (1
(00 ppm or more), it is said that performance degradation does not become a problem.

However, if the fuel and the flow rate of the blown air are incorrectly controlled, the danger of causing ignition or explosion in the battery cannot be denied. Including a power source for an automobile, in order to ensure the safety of the power generation device for general consumer is must be kept at least blowing O ₂ concentration of 1% or less, in the state of the art, lowering the O ₂ concentration effect Is not obtained.

An object of the present invention is to provide a fuel cell using a reformed gas obtained by steam reforming hydrocarbons such as methane and methanol or alcohols as a fuel gas. It is intended to oxidize and remove CO and the like adsorbed on the electrode surface of a fuel electrode on which an active catalyst such as a Ru-based catalyst is carried, thereby avoiding and recovering a decrease in battery performance due to impurities such as CO contained in a fuel gas (hydrogen). It is an object of the present invention to provide an operation control method which can be performed.

[0015]

According to a first aspect of the present invention, there is provided a method of controlling operation of a fuel cell, wherein a poisonous gas such as carbon monoxide is deposited on a fuel electrode carrying an electrode catalyst such as a platinum-based catalyst. In a fuel cell that supplies fuel gas containing oxidant gas to the cathode while supplying oxidizing gas to the cathode, and generates an electromotive force by oxidation of the anode and reduction reaction of the cathode, the electrode potential of the anode is temporarily changed during the power generation. In addition, the gist of the present invention is to include a step of setting the potential to be +0.3 V or more to the standard hydrogen electrode potential.

In the operation control method of the present invention having the above configuration, the potential of the fuel electrode is temporarily reduced with respect to the standard hydrogen electrode potential while the operation of the fuel cell is continued while impurities such as CO accumulated in the electrode catalyst of the fuel electrode are maintained. By setting the potential to be more noble than +0.3 V, CO and the like that have deactivated the fuel electrode are oxidized and removed. Therefore, the surface of the electrode catalyst on the surface of the fuel electrode that has been poisoned or deactivated by CO or the like is cleaned, and the hydrogen oxidation activity is restored.

According to the second aspect of the present invention, the supply of the oxidizing gas to the air electrode is continued as a process of temporarily setting the electrode potential of the fuel electrode to a potential nobler than +0.3 V with respect to the standard hydrogen electrode potential. The gist of the present invention is to reduce the supply amount or concentration of the fuel gas as it is or to stop the supply of the fuel gas.

Usually, a reaction capable of electrochemically oxidizing and removing CO and the like adsorbed on the fuel electrode catalyst is in a potential range of +0.3 V (standard hydrogen electrode standard) or higher. The potential of the fuel electrode when the fuel cell is operating is +0.1 V (standard hydrogen electrode standard) or lower, but the oxygen electrode of the counter electrode is sufficiently high as long as an oxidizing agent such as oxygen is supplied. Because it has a potential,
By temporarily reducing or stopping the supply or concentration of the fuel gas, the fuel electrode potential is set to 0.3 with respect to the standard hydrogen electrode.
It can be set to a noble potential of V or more. It is preferable to perform an operation such as restarting the supply of the fuel gas when the fuel electrode potential rises and reaches a certain potential of +0.3 V or more. The potential of the fuel electrode can theoretically rise to the same potential as the air electrode (+1.0 V, about the standard hydrogen electrode standard).
Is sufficiently obtained to electrochemically oxidize and remove.

Therefore, a potential for reducing or stopping the supply amount or concentration of the fuel gas to the fuel electrode is set in advance (for example, 0.1 V), and when the potential is exceeded, the supply of the fuel gas is reduced or stopped. In addition, the potential for restarting the supply of the fuel gas is also set in advance (for example,
0.6V), it is desirable to automate the operation control so that the supply of the fuel gas is restarted when the potential is reached.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail. (First Embodiment) First, FIG. 1 shows a structure of a small test polymer electrolyte fuel cell (unit cell) manufactured for this embodiment. In this embodiment, Pt-Ru is used as the fuel electrode catalyst.
(1: 1 atomic ratio) An alloy catalyst supported on carbon black, an air electrode catalyst supported on a pure Pt catalyst on carbon black, and Nafion (trade name of DuPont) used as an electrolyte membrane. The area of the electrode was 10 cm ² .

As an experimental method, first, pure hydrogen was supplied to the fuel electrode and air was supplied to the air electrode, the operating temperature of the battery was set to 60 ° C., and an electromotive force was generated at a constant current of 5 A. As a result, the voltage between terminals became stable at 0.7 V. In this state,
When 100 ppm of CO was added to the hydrogen supplied to the fuel electrode, the inter-terminal voltage decreased over time, and finally the inter-terminal voltage decreased to about 0.3 V. This is because CO was chemically adsorbed on the Pt-Ru catalyst of the fuel electrode and inhibited the oxidation reaction of hydrogen.

Here, the current was temporarily stopped, and first, the gas supplied to the air electrode was switched from air to pure hydrogen. Next, the gas flowing through the fuel electrode was switched from hydrogen containing CO to pure nitrogen while maintaining the potential of the fuel electrode at +0.05 V with respect to the air electrode switched to pure hydrogen. Thereafter, the potential of the fuel electrode is increased by +0.05 with respect to the air electrode supplying pure hydrogen (hereinafter referred to as “hydrogen reference electrode”).
Scanning was performed in a noble direction from V 1 (+ direction, a direction in which the oxidation reaction proceeds more easily), and the response of the current was examined.

FIG. 2 shows the relationship between the potential (V) of the fuel electrode with respect to the hydrogen reference electrode and the current density (mA / cm ² ). As shown in the data of FIG. 2, in the first scan shown by the solid line, +0.
A current in the oxidation direction began to flow at around 3 V, indicating that the CO adsorbed on the surface was electrochemically oxidized.
The oxidation of CO was completed at about +0.5 V with respect to the hydrogen reference electrode. After scanning the potential of the hydrogen reference electrode to +1.0 V, the potential was returned to +0.05 V again, and after holding for a while, the second potential scanning (shown by a broken line) was performed.

In the second scan, a current due to the oxidation of adsorbed hydrogen, which was not observed in the first scan, was observed in the potential region of +0.35 V or less, indicating that the activity for the hydrogen oxidation reaction was restored. In addition, the oxidation current caused by the oxidation of CO disappeared. From this result, it is found that the CO adsorbed on the electrode catalyst starts to oxidize at a potential higher than +0.3 V on the basis of the hydrogen electrode, and is completely oxidized and removed at +0.5 V or higher. It can be seen that the activity of hydrogen oxidation, which is the original function of the catalyst, is restored.

(Second embodiment) Next, using the test fuel cell described above, 75% hydrogen and CO100
Supplying methanol reformed gas containing 0.5 ppm, 0.5 A / c
Discharge at a constant current of m ² was performed under the conditions of an operating temperature of 60 ° C. and a battery internal pressure of 1.5 atm.

In this experiment, under a constant current discharge state, the battery voltage decreases with the passage of discharge time. When the battery voltage falls below 0.6 V, the supply of fuel is stopped, and the battery voltage is reduced to 0.6. When the voltage reached 1 V, the fuel supply was immediately restarted. After the fuel supply is resumed, the electrode activity is restored to the initial state, and the original voltage (0.6) is restored.
V), the supply of fuel was stopped when the voltage fell below 0.6 V again, and this operation was repeated.

FIG. 3 shows the potential of the fuel electrode (with reference to the hydrogen electrode) in the discharge state at the constant current. As a comparison, a change in potential in the case of continuous operation (prior art) is shown by a thin line or a broken line. Normally, when the fuel cell is operating, the potential of the fuel electrode is +0.1 V (standard hydrogen electrode standard) or less, and the CO adsorbed on the fuel electrode catalyst is
Are accumulated without being removed.

When the active point of the fuel electrode catalyst is occupied by CO, the hydrogen oxidation reaction, which is a normal fuel cell reaction, is hindered. When power is generated under a constant current or a constant load, the overvoltage increases. The potential of the fuel electrode increases with time. The CO adsorbed on the fuel electrode catalyst can be electrochemically oxidized and removed, but this reaction usually occurs in a potential range of +0.3 V or more (based on a standard hydrogen electrode).

As a result, when the electrode potential rises to a potential at which CO can be oxidized, a portion of CO is oxidized, thereby opening the catalytically active site, thereby reducing the overvoltage and lowering the electrode potential. When the voltage falls below 0.3 V, CO adsorption to the fuel electrode catalyst occurs again, and the electrode potential rises. CO adsorption and oxidation are repeated across 0.3 V, so that a thin line as shown in FIG. The power of raising and lowering the potential is repeated in a zigzag manner. Alternatively, as shown by a broken line in the figure, the potential changes at a constant potential near 0.3 V.

On the other hand, in the operation method of the present invention, as shown by the thick solid line, the potential of the fuel electrode rapidly rises when the fuel supply is stopped, and suddenly returns to the original potential when the fuel supply is restarted. To decline. By repeating this, it became clear that power generation was possible while maintaining the potential of the fuel electrode at an average potential of +0.1 V or less. As described above, according to the operation control of the method of the present invention, the catalyst surface of the fuel electrode is once completely cleaned by temporarily stopping the fuel, so that the hydrogen oxidation activity at the fuel electrode is completely recovered.

FIG. 4 shows a change in voltage of the fuel cell in the discharge state at a constant current. For comparison, the voltage change in the case of continuous operation (prior art) is indicated by a broken line. As can be seen from this data, the voltage change of the prior art shown by the broken line shows that the battery voltage decreases with the passage of time and shows a gradual decrease tendency even after decreasing to about 0.3 V.

On the other hand, in the case of the present invention, as shown by the solid line, the battery voltage suddenly drops when the supply of fuel is stopped, and rapidly recovers to the original voltage when the fuel supply is restarted. By repeating this, a state is obtained in which the cell potential is maintained at 0.6 V or more except for the time for stopping and restarting the fuel. According to the present invention, power generation can be performed while maintaining a high average voltage by repeating this process.

As another method for increasing the fuel electrode electrode potential so that CO is oxidized and removed and increasing the potential on the basis of the hydrogen electrode to +0.5 V or more, there is a method of increasing the power generation current.

The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention. For example, although not described in the above embodiment, a hydrogen electrode reference potential sensor is provided in the fuel electrode, and when the potential of the fuel electrode becomes higher than a certain level (for example, +0.1 V) by a detection signal from the potential sensor, The supply of fuel to the anode is stopped, and a certain level (for example, +0.6)
V) It is better to control so that the supply of fuel is restarted when it becomes more noble. Alternatively, a voltage sensor is provided in the fuel cell, and the same control is performed based on a detection signal from the voltage sensor (for example, the supply of fuel is stopped when the voltage becomes 0.6 V or less, and the fuel supply is stopped when the voltage becomes 0.1 V or less). Resume supply)
In this case, the operation of the fuel cell is automatically (automatically) controlled.

[0035]

According to the fuel cell operation control method of the present invention, the supply of fuel to the electrode is temporarily stopped during operation due to impurities such as CO adsorbed on the surface of the fuel electrode during operation. By doing so, the activity of the electrocatalyst supported on the fuel electrode is maintained by oxidation, so that the fuel cell can continue to generate high electromotive force, and as a result, The efficiency can be improved. In addition, since the fuel electrode is continuously recovered from the poisoning and deactivation of the electrode catalyst by CO, the fuel electrode can be used permanently and has a great economical advantage such as a maintenance-free state. It can be said that it is a driving method.

[Brief description of the drawings]

FIG. 1 is a diagram showing the structure of a test polymer electrolyte fuel cell used in the practice of the present invention.

FIG. 2 shows an experiment using the test fuel cell shown in FIG. 1 in which the potential of a fuel electrode with respect to hydrogen reference electrode (V) and the current density (m) were measured.
A / cm ² ).

FIG. 3 is a diagram showing, as another experiment, a potential change when hydrogen containing CO is supplied to a fuel electrode as a fuel, by comparing the method of the present invention with a conventional technique.

FIG. 4 is a graph showing a change in voltage of the fuel cell in the experiment of FIG.

FIG. 5 is a diagram showing a basic structure of a conventional polymer electrolyte fuel cell generally known.

FIG. 6 is a view for explaining a basic principle of the fuel cell shown in FIG. 5;

FIG. 7 is a diagram showing a schematic configuration of a plant when a reformed gas is used as fuel in the fuel cells shown in FIGS. 5 and 6.

[Explanation of symbols]

 Reference Signs List 10 polymer electrolyte fuel cell 12 polymer solid electrolyte membrane 14 fuel electrode 16 air electrode

Claims (2)

[Claims] A fuel gas containing a poisoning component is supplied to a fuel electrode on which an electrode catalyst is carried, and an oxidizing gas is supplied to an air electrode to generate an electromotive force by oxidation of the fuel electrode and a reduction reaction of the air electrode. In the resulting fuel cell, during its power generation,
A method for controlling operation of a fuel cell, comprising a step of temporarily setting the electrode potential of a fuel electrode to a potential nobler than +0.3 V with respect to a standard hydrogen electrode potential. 2. The step of temporarily setting the electrode potential of the fuel electrode to a potential nobler than +0.3 V with respect to the standard hydrogen electrode potential is performed by continuously supplying the oxidizing gas to the air electrode. 2. The method for controlling operation of a fuel cell according to claim 1, wherein the step is a step of decreasing the supply amount or concentration or a step of stopping the supply of the fuel gas.