KR100814367B1 - How to purge residual gas when fuel cell stops - Google Patents

Abstract

 The process of exchanging anode and cathode gases with each other inhibits oxidation of cathode carbon material and uses excess anode as a sacrificial electrode to maintain initial cell performance for a long time despite purging due to repeated operation / stop. A residual gas purging method of a stationary fuel cell is provided. The residual gas purging method of a stationary fuel cell according to the present invention is a fuel cell system in which hydrogen is stopped on the anode side and air is left on the cathode side. (A) The first side is purged with hydrogen gas. Allowing hydrogen to contact both the subject cathode and the anode; (b) purging the anode side with air such that hydrogen is in contact with the cathode and air is in contact with the anode; And (c) second purging the cathode side with air such that air is in contact with both the cathode and the anode.

Description

Method for purging the remaining gas of fuel cell during the shutdown process}

1 is a schematic view for explaining the power generation principle of a polymer electrolyte fuel cell (PEMFC) mainly used in the cationic conductive polymer composite membrane of the present invention.

2 is a view for explaining a problem in the case of purging both the polymer electrolyte fuel cell (PEMFC) anode and cathode using air.

3A to 3G are diagrams for describing a residual gas purging method of a stationary fuel cell according to an exemplary embodiment of the present invention.

4 is a diagram schematically illustrating a purging system for implementing a method for purging residual gas of a stationary fuel cell according to an exemplary embodiment of the present invention.

FIG. 5 is a graph showing a change in current density of a fuel cell according to voltage when the conventional purging method is used.

6 is a graph showing a change in current density of a fuel cell according to a voltage when the purging method according to the present invention is performed.

<Explanation of symbols for the main parts of the drawings>

310: anode

320: cathode

The present invention relates to a method for purging fuel gas fuel gas, and more particularly, by replacing the anode and the cathode with air and hydrogen during the purging process for removing hydrogen gas and condensate remaining in the state where the fuel cell is stopped. The present invention relates to a method for purging residual gas at the stop of a fuel cell that can prevent electrochemical oxidation of cathode carbon materials by post-purging.

A fuel cell is an electrochemical device that converts chemical energy of hydrogen (H ₂ ) and oxygen (O ₂ ) directly into electrical energy, and supplies hydrogen and oxygen to an anode and a cathode, respectively. It is a new generation technology that produces electricity continuously.

These fuel cells are capable of high-efficiency power generation that increases the overall efficiency to more than 80%, and have no spots of NO _x or CO ₂ emissions and very little noise. .

These fuel cells are classified into phosphoric acid type (PAFC), alkali type (AFC), polymer electrolyte type (PEMFC), molten carbonate type (MCFC), solid oxide type (SOFC), and direct methanol type (DMFC) depending on the type of internal electrolyte. Each of these fuel cells operates essentially on the same principle, but differs in fuel type, operating temperature, catalyst and electrolyte.

1 is a schematic view for explaining the power generation principle of a polymer electrolyte fuel cell (PEMFC) mainly used in the cationic conductive polymer composite membrane of the present invention.

As shown in FIG. 1, in the polymer electrolyte fuel cell (PEMFC), an anode 110, a cathode 130, and a cationic conductive polymer membrane 120 are stacked between a hydrogen supply unit 100 and an oxygen supply unit 140. It is structured.

As shown in FIG. 1, hydrogen (H ₂ ) is supplied through an anode (also called a fuel electrode; 110) from a hydrogen supply unit 100, and oxygen (O ₂ ) is a cathode (also called an air electrode) in an oxygen supply unit 140; 130 It is supplied through

Supplied from the hydrogen supplying section 100, the hydrogen is fed to the anode 110, in cathode 110, hydrogen is decomposed by a catalytic hydrogen ions (H ⁺⁾ and electron (e ^-) to generate a.

(Scheme: 2H ₂   → 4H ⁺  + 4e ^- )

At this time, the hydrogen ion (4H ⁺⁾ generated at the anode (110) is transmitted to the cathode 130 through the cation-conducting polymer film 120, the electrons (4e ^-) generated from the anode 110 outer conductors (150 It moves to the cathode 130 along.

In the cathode 130, oxygen (O ₂ ) supplied through the oxygen supply unit 140 meets electrons (4e ⁻ ) moved along the outer conductor line 150 and hydrogen ions (4H ⁺ ) moved along the polymer electrolyte. Water is produced by the following scheme.

(Scheme: O ₂  + 4H ⁺  + 4e ^-  → 2 H ₂ O )

At this time, current flows due to the flow of electrons (e ⁻ ), and heat is incidentally generated in an exothermic reaction of water (H ₂ O).

The generated current is used as a direct current or used as a power of a direct current motor, or may be converted into an alternating current through a power converter (DC / AC invertor). In addition, the additional heat generated in the fuel cell reaction may be used for heating.

A fuel cell having the above-described power generation principle requires water for delivery of hydrogen ions (6H ⁺ ) through the polymer electrolyte membrane 120, which uses water generated through external humidification or the reaction.

Such a fuel cell does not always operate, but stops when power generation is not required. Even in such a stop state, hydrogen gas, air, and water are present in the fuel cell.

However, in the case of hydrogen remaining inside the anode 110, the fuel cell maintains a high voltage even when the fuel cell is stopped. In addition, the water generated in the cathode 130 expands in volume when frozen at a temperature below freezing point. This may cause damage to the electrode.

A typical method used to remove hydrogen and water remaining inside the fuel cell when the fuel cell is stopped is to purge both the cathode and the anode with air. By supplying the anode, the hydrogen of the anode is discharged and the water inside the fuel cell is discharged to the outside of the cell by forced circulation.

In this case, there is a method of supplying low pressure air and high temperature air for efficient discharge of water, but these are common in that they supply air to the anode and the cathode.

However, the method of purging both the anode and the cathode using air as described above has a problem in that the catalyst of the cathode is lost due to oxidation of the carbon material used for the electrode of the cathode.

2 is a view for explaining a problem in the case of purging both the anode and the cathode using air.

Referring to FIG. 2, when air is simultaneously injected for purging both the anode 210 and the cathode 220, which are filled with hydrogen and air in the initial stop state, a portion (②) into which air is injected to the anode 210 side (②). An electric potential difference is generated between the portion ③ in which the excess hydrogen exists and the electrons generated at the anode 210 move to the air contact portion ②. At the same time, the hydrogen ions (H ⁺ ) generated complementarily in the hydrogen contact portion (③) is moved in the cathode direction.

However, on the opposite side, the reaction opposite to the above reaction, that is, the hydrogen ion (H ⁺ ) is moved complementarily from the cathode 220 side to the air contacting unit (②) of the anode 210, wherein the cathode 220 Since the hydrogen ions (H ⁺ ) moving from the anode 210 to the cathode 210 currently flow only to the cathode 220, the cathode electrode is accompanied with oxidation to generate the hydrogen ions.

That is, carbon (C) constituting the cathode electrode is generated in the cathode 220 to produce an oxidation reaction with the remaining water (H ₂ O) to generate hydrogen ions (H ⁺ ) and electrons (e-) as shown in the following reaction formula ^: At this time, the generated hydrogen ions are moved to the air contact portion (②) of the anode 210, the electrons are moved to the region (④) opposite the cathode 220.

_{C + 2H 2 O → CO 2} + 4H + + 4e -

As a result, the carbon material constituting the cathode 220 in the region ① is damaged by electrochemical oxidation to generate hydrogen ions, and loss of catalyst material such as platinum (Pt) evenly distributed on carbon. Will cause.

Therefore, in order to prevent oxidation of the carbon material of the cathode 220, a method of increasing the purging speed is effective in order to minimize the formation time of the air / hydrogen interface of the anode 210 (see US 6,858,336).

However, even by the method of increasing the purging speed as described above, as the operation / stop count is repeated, ultimately, the loss of the cathode 220 may not be prevented, resulting in a decrease in fuel cell performance.

Considering this problem, it is possible to think of a method of using an excessive amount of the catalyst on the cathode 220 side, but it is a general view that due to the increase in unit cost and other deterioration due to the characteristics of the fuel cell is practically impossible.

As described above, in the fuel cell, a factor that determines performance is mainly a loss and degradation of the cathode. Therefore, if the loss or degradation of the cathode is minimized, there is room for improving the performance of the fuel cell.

The technical problem to be achieved by the present invention is a method of purging residual gas when the fuel cell is stopped, which can improve the performance and life of the fuel cell as a whole by inducing the anode electrode to be lost instead of preventing the loss due to the oxidation of carbon of the cathode. In providing.

Technical problems to be achieved by the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art from the following description.

In a fuel cell system in which a residual gas purging method for stopping a fuel cell according to an embodiment of the present invention for solving the above technical problem is stopped with hydrogen remaining on an anode side and air remaining on a cathode side, (a ) First purging the cathode side with hydrogen gas so that hydrogen is in contact with both the cathode and the anode; (b) purging the anode side with air such that hydrogen is in contact with the cathode and air is in contact with the anode; And (c) second purging the cathode side with air such that air is in contact with both the cathode and the anode.

Specific details of other embodiments are included in the detailed description and the accompanying drawings.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, only the present embodiments to make the disclosure of the present invention complete, and common knowledge in the art to which the present invention pertains. It is provided to fully inform the person having the scope of the invention, which is defined only by the scope of the claims. Like reference numerals refer to like elements throughout.

In addition, the size of the components constituting the invention in the drawings are exaggerated for clarity of the specification, when any component is described as "exists inside, or is installed in connection with" other components, any of the above configuration An element may be installed in contact with the other component, or may be installed at a predetermined distance from the other component, and when installed at a distance, a third element for fixing or connecting the component to the other component The description of the means of may be omitted.

In the present invention, by purging the air to remove the water of the cathode and hydrogen of the anode and at the same time to prevent the loss due to the oxidation of the cathode carbon material to improve the long-term operation performance of the fuel cell by repeated operation / stop significantly .

In order to achieve this purpose, it is noted that the characteristics of the anode and the cathode of the fuel cell, that is, the cathode is generally more likely to be lost than the anode, and that the deterioration of the cathode is a major factor in determining the performance of the entire fuel cell are considered. The concept of sacrificing electrode was introduced. In other words, by moving the area where the electrode is damaged from the cathode to the anode to prevent the degradation of battery performance due to the loss of the cathode catalyst.

This concept of the sacrificial electrode is, in general, the oxidation reaction of the hydrogen generated at the anode ^{_{(H 2 → 2H + + 2e}} -) to the resulting hydrogen ions (H ⁺⁾ and electrons (e ^-) is the feed rate of oxygen reduction of the anode The reaction proceeds about 10 times faster than the reaction, and the hydrogen supplied to the anode has a purity of 99% or more, but the oxygen supplied to the cathode is conceived in the form of air.

That is, in the case of the anode, even if the amount of the catalyst present is small compared to the cathode, the function can be sufficiently exhibited, and the amount of the catalyst of the anode may be used only about 50 to 75% of the cathode, but the excess of the anode The presence of a catalyst does not significantly affect the performance of the battery, so commercially available electrodes are commercially available in such a way that the catalyst amounts of the anode and the cathode do not differ significantly.

Therefore, when the catalyst amounts of the cathode and the anode are the same or similar as in the case of commercialization, a considerable amount of catalyst is present in the anode, so that even if a part of the catalyst is lost, the battery performance is not significantly affected.

As a result, the fuel cell performance may be maintained for a very long time even if the loss of the anode instead of the cathode is caused in performing the purging by air when the fuel cell is stopped as described above.

3A to 3G are diagrams for describing a residual gas purging method of a stationary fuel cell according to an exemplary embodiment of the present invention.

In order to purge the residual gas of the stationary fuel cell according to an exemplary embodiment of the present invention, as shown in FIG. 3A, the fuel is stopped while the cathode 320 is in contact with air and the anode 310 is in contact with hydrogen gas. In the battery, as shown in FIG. 3B, the cathode 320 is first purged with hydrogen (H ₂ ) gas.

At this time, the hydrogen gas is used as it is, but the hydrogen gas used in the operation of the fuel cell, it is also possible to use a separate hydrogen source.

In the case of FIG. 3B, the flow of hydrogen ions causes hydrogen ions (H ⁺ ) to move toward the anode 310 at the hydrogen gas contact portion of the cathode 320 at the start of purging, and on the opposite side of the anode 310 to compensate Hydrogen ions (H ⁺ ) are moved to the cathode 320.

At this time, since hydrogen gas (H ₂ ) is in contact with all of the regions where hydrogen ions are generated, the generation of hydrogen ions (H ⁺ ) can be supplied by decomposing or oxidizing the hydrogen gas, so that the anode 310 is shown in FIG. 3B. ) Or the loss of the cathode 320 electrode carbon C does not occur.

When the purging is performed as shown in FIG. 3B, the anode 310 and the cathode 320 are in a state similar to that of FIG. 3C in which hydrogen gas is in contact with each other.

Next, as shown in FIG. 3d, the anode 310 is purged with air. In this case, the air used may also be used as air as fuel in the operating state of the fuel cell.

In FIG. 3D, when the air is just provided to the anode 310, a potential difference exists in the anode 310, and hydrogen ions H ⁺ are moved to the cathode at the hydrogen contacting part of the anode 310. On the other side, hydrogen ions (H ⁺ ) are moved from the cathode 320 to the anode 310.

In this case, since hydrogen gas (H ₂ ) is in contact with all the regions where hydrogen ions are generated, hydrogen ions (H ⁺ ) may be generated by decomposing or oxidizing the hydrogen gas. ) Or the loss of the cathode 320 electrode carbon C does not occur.

If purging is performed as illustrated in FIG. 3D, hydrogen gas is brought into contact with the cathode 320 and air is brought into contact with the anode 310.

That is, the fuel completely opposite to the operating state flows into each of the cathode 320 and the anode 310.

Next, as shown in FIG. 3F, the cathode 320 is secondly purged with air, and at this time, hydrogen ions (H ⁺ ) are moved to the anode side at the hydrogen contact portion of the cathode 320 when purging is started. In the air contact portion of the cathode 320 on the opposite side, hydrogen ions (H ⁺ ) are moved from the anode 310 to the cathode 320.

The hydrogen ions produced at the cathode 320 (H ⁺⁾ are subject to generate hydrogen ions (H ⁺⁾ by oxidation naejineun decomposition of a hydrogen gas (H ₂₎ in contact with the cathode 320, the cathode 320 Loss of carbon (C) can be prevented, but since all the air is in contact with the anode 310 side, hydrogen ions (H ⁺ ) generated by oxidizing the carbon (C) of the anode 310 by the following reaction formula is a cathode ( It is moved to the side 320, resulting in the loss of carbon on the anode 310 side.

_{C + 2H 2 O → CO 2} + 4H + + 4e -

At this time, the water (H ₂ O) that combines with the carbon is generated during operation and the water remaining inside the anode 310 is used.

If the cathode 320 is secondly purged with air as shown in FIG. 3F, eventually, as shown in FIG. 3G, air is in contact with both the cathode 320 and the anode 310 as shown in FIG. 3G, thereby purging. The process is complete.

As described above, even if the anode 310 catalyst is lost due to oxidation of carbon (C) on the anode 310 side as in FIG. 3F, since the excess amount of the catalyst is present in the anode 310, the overall fuel Although the operation of the battery does not significantly affect the performance of the purging method according to the present invention, although the performance of the fuel cell may not be permanent, the cathode 320 catalyst is lost due to oxidation of the carbon of the cathode 320 as in the prior art. It may result in a very improved life or operation performance than in the case.

4 is a diagram schematically illustrating a purging system for implementing a method for purging residual gas of a stationary fuel cell according to an exemplary embodiment of the present invention.

In this case, the valve 5 is opened while the valves 1 to 4 are closed for the primary purging of the cathode 320. At this time, the air of the cathode 320 is replaced with hydrogen so that both the cathode 320 and the anode 310 are filled with hydrogen as shown in FIG. 3C.

Thereafter, the valve 6 is opened to supply air to the anode 310, which is in a state as shown in FIG. 3E, and the gas of the cathode 320 and the anode 310 is changed as compared to when the fuel cell is initially stopped. .

Finally, when the valve 5 is closed to shut off the hydrogen supply to the cathode 320 and the valve 7 is opened to supply air to the cathode 320, the state as shown in FIG. 3G is obtained.

However, at this time, when supplying air to the cathode 320 to increase the flow rate to minimize the air / hydrogen interface formation time.

After the purging step as described above, after the anode 310 and the cathode 320 are finally purged with air, the oxidation of the cathode 320 carbon (C) does not occur, and the anode 310 carbon is oxidized instead.

Through the purging process, the anode 310 is used as a sacrificial electrode instead of the cathode 320, and the same principle is applied to reoperation.

That is, when hydrogen is directly supplied to the anode 310, the cathode 320 carbon material is also oxidized, so that the step applied at the stop is performed in the reverse direction.

Hereinafter, a specific experimental example will be described that there is almost no change in the performance of the fuel cell due to the change in the amount of anode catalyst when the residual gas purging method of the stopped fuel cell according to the embodiments of the present invention. Details not described herein are omitted because they can be sufficiently inferred by those skilled in the art.

FIG. 5 is a graph showing a change in current density of a fuel cell according to a voltage according to a conventional purging method, and FIG. 6 is a current density of a fuel cell according to a voltage according to a purging method according to the present invention. (current density) is a graph showing the change.

As shown in FIG. 5, it can be seen that the performance of the battery is deteriorated after the repeated 200 times of operation / stop by the conventional purging method. As the purging process is repeated, the cause of deterioration of the battery is known because the conventional purging method causes oxidation of the cathode carbon material and thus the loss of the catalyst. Therefore, when the conventional purging method is used, it is a natural result that performance decreases after 200 operations / stops.

On the other hand, as shown in Figure 6 in the purging method according to the present invention can be seen that the battery performance is almost unchanged even if repeated 200 times operation / stop. This result is because the cathode carbon material can be effectively protected by oxidizing the anode carbon material when performing the purging process according to the present invention.

Although the embodiments of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to the above embodiments but may be manufactured in various forms, and having ordinary skill in the art to which the present invention pertains. It will be understood by those skilled in the art that the present invention may be embodied in other specific forms without changing the technical spirit or essential features of the present invention. Therefore, it should be understood that the embodiments described above are exemplary in all respects and not restrictive.

According to the method of purging the residual gas when the fuel cell is stopped according to an embodiment of the present invention, the process of exchanging the gas of the anode and the cathode is repeated to suppress oxidation of the cathode carbon material and to use the excess anode as a sacrificial electrode instead. The initial battery performance can be maintained for a long time despite purging due to phosphorus operation / stop.

Claims (5)

In a fuel cell system in which hydrogen is stopped on the anode side and air is left on the cathode side, (a) first purging the cathode side with hydrogen gas so that hydrogen is in contact with both the cathode and the anode; (b) purging the anode side with air such that hydrogen is in contact with the cathode and air is in contact with the anode; And and (c) purging the cathode side with air to allow air to contact both the cathode and the anode. The method of claim 1, The purge rate in the step (a) is a residual gas purging method of the fuel cell, characterized in that for removing the residual gas on the cathode side within 0.01 seconds. The method of claim 1, The purge rate in the step (b) is the residual gas purging method of the fuel cell, characterized in that for removing the residual gas on the anode side within 0.01 seconds. The method of claim 1, The purge rate in the step (c) is the residual gas purging method of the fuel cell, characterized in that to remove the residual gas on the cathode side within 0.01 seconds. A method of purging residual gas at a stop of a fuel cell, wherein the fuel cell is restarted by reversing the purging step of claim 1 when the fuel cell stopped after the purging process is completed.

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