JP4547853B2 - Operation method and characteristic recovery method of polymer electrolyte fuel cell - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel cell useful as a consumer cogeneration system or a power generator for a moving body, and more particularly to a polymer electrolyte fuel cell using a polymer electrolyte and a method for recovering its characteristics.
[0002]
[Prior art]
In a fuel cell, a fuel such as hydrogen and an oxidant gas such as air are electrochemically reacted by a gas diffusion electrode to simultaneously supply electricity and heat. There are several types of fuel cells depending on the type of electrolyte used. A polymer electrolyte fuel cell using a polymer as an electrolyte has the above-mentioned polymer on both surfaces of the back and front of a polymer electrolyte membrane in which —CF ₂ — is a main chain skeleton and sulfonic acid is introduced at the end of a side chain. An electrode paste in which a carbon powder carrying a platinum-based metal catalyst is mixed with an electrolyte dispersion solution is applied and dried to form an air electrode and a fuel electrode. Outside the air electrode and the fuel electrode, a conductive porous body such as carbon paper is usually used as an electrode base material, and is arranged as a diffusion layer of air and fuel gas. At this time, an electrode paste may be applied to the carbon paper described above, and an electrolyte membrane may be bonded thereto.
[0003]
Outside this, a conductive separator plate for mechanically fixing the joined body of the electrode and the electrolyte membrane and electrically connecting adjacent joined bodies to each other in series is disposed. In the separator plate, a reaction gas is supplied to the electrode, and a gas flow path for carrying water and surplus gas generated by the reaction between hydrogen and oxygen is formed. A sealing member such as a gasket or a sealing agent is disposed around the gas flow path and the electrode to prevent the reaction gas from being directly mixed or leaked to the outside.
[0004]
When this is used as a power generation device, in order to increase the output voltage, it is usual to stack a plurality of single cells made of a polymer electrolyte layer, a gas diffusion electrode layer, a separator plate, a gas flow path, and the like. Each gas flow path supplies fuel gas such as hydrogen and air from the outside to the gas diffusion electrode through the manifold. The current generated in the electrode reaction layer is collected by the electrode base material and taken out through the separator plate. The separator plate is often made of a carbon material that is electrically conductive and has both gas tightness and corrosion resistance. However, a separator using a metal material such as stainless steel is also being studied from the viewpoint that it is easy to reduce the thickness of the separator in addition to molding processability and low cost.
[0005]
[Problems to be solved by the invention]
Since the polymer electrolyte described above has hydrogen ion conductivity in a state of containing water, it is generally performed to humidify the fuel gas supplied to the fuel cell. Further, since water is generated by the battery reaction at the air electrode, water is always present inside the battery. As a result, when the battery is operated for a long period of time, ionic impurities, inorganic impurities, and organic impurities contained in the carbon material, sealing material, resin material, and metal material, which are constituent materials of the battery, are eluted. In addition, air supplied from the outside of the battery contains air pollutants, such as trace amounts of nitrogen oxides and sulfur oxides, and trace amounts of metal oxides contained in the hydrogen purifier are also included in the fuel gas. Sometimes. These impurities accumulate in the electrolyte membrane, the air electrode, the catalytic reaction layer in the fuel electrode, and the like, leading to a decrease in the conductivity of the polymer electrolyte and a decrease in the catalytic reaction activity. As a result, the battery performance gradually deteriorates during long-term battery operation. Further, when a metal is used for the separator plate, the metal ions eluted from the separator plate further damage the electrolyte membrane and the catalytic reaction layer.
[0006]
[Means for Solving the Problems]
In order to solve the above problems, a method for operating a polymer electrolyte fuel cell according to the present invention includes a polymer electrolyte membrane, a fuel electrode and an air electrode arranged with the polymer electrolyte membrane sandwiched therebetween, and an oxidant on the air electrode. gas supply discharge, the battery main body formed by laminating a single cell constituted by a pair of separator plates to have a gas flow path for supplying and discharging a fuel gas to the fuel electrode, the oxidizer gas and the fuel into the cell main body It means for supplying and discharging the gas, and means for controlling the extraction of power generated by the battery main body, a method of operating a polymer electrolyte fuel cell and a battery performance recovery means, the polymer electrolyte The fuel cell is operated for a predetermined period of time sufficient to allow impurity ions present in the fuel cell to be mixed with the product water of the electrode reaction and discharged to the outside in an operation mode at a current 1.5 times or more that in normal operation. Or before In a polymer electrolyte fuel cell, impurity ions present in the fuel cell are mixed with the water generated in the electrode reaction and discharged to the outside in an operation mode at a current where the output voltage per unit cell is 0.2 V or less. It is characterized by driving for a predetermined time sufficient.
[0007]
A method for recovering characteristics of a polymer electrolyte fuel cell according to the present invention includes a polymer electrolyte membrane, a fuel electrode and an air electrode arranged with the polymer electrolyte membrane interposed therebetween, and an oxidant gas supplied to and discharged from the air electrode, A battery main body part in which unit cells constituted by a pair of separator plates having a gas flow path for supplying and discharging fuel gas to and from the fuel electrode, and means for supplying and discharging the oxidant gas and fuel gas to the battery main body part And a method for recovering characteristics of a polymer electrolyte fuel cell comprising: means for controlling extraction of electric power generated in the battery main body portion; and means for recovering battery characteristics, wherein the polymer electrolyte fuel cell is By operating in an operation mode with a current 1.5 times or more that of the operation, the impurity ions in the battery are mixed with the water generated in the electrode reaction and discharged to the outside, or the polymer electrolyte fuel cell A cell When the output voltage of or is driven at a driving mode of a current to be 0.2V or less, wherein the impurity ions in the battery is discharged to the outside mixed in produced water of electrode reaction.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The ionic conductivity of the electrolyte used in the polymer electrolyte fuel cell as described above is expressed by the hydrogen ion of the sulfone group at the tip of the side chain pendant to the polymer main chain. However, when metal ions such as iron and sodium are present as impurities, they are replaced with hydrogen ions, and the ionic conductivity of the electrolyte membrane is lowered. Moreover, since the metal ion which infiltrated into the electrolyte has a hydration state different from that of hydrogen ions, the water content of the electrolyte is lowered, thereby reducing the ionic conductivity of the electrolyte membrane. Such a decrease in ionic conductivity and a decrease in water content not only increase the direct current resistance component of the battery, but also reduce the reaction area of the catalytic reaction layer in the electrode, further reducing the battery performance. In addition, the above-described metal ions adhere to the catalyst surface or form an oxide to seal the catalyst, thereby causing a decrease in battery performance. Further, as an anionic impurity, sulfur oxide poisons the catalyst to lower the battery performance, and nitrogen oxide ions and carboxylate ions corrode and alter the constituent members as acidic substances.
[0010]
Such contaminating ions are present at a high concentration at a specific site in the battery in a normal operation state. For example, the above-described metal ions are distributed at a high concentration at the interface between the electrolyte membrane and the electrode layer and at the polymer electrolyte portion kneaded inside the electrode. Accumulation of acidic substances derived from anionic impurities progresses to the electrode base material and the separator plate surface that become the gas diffusion layer. Since these metal ions, cationic anionic impurities, and anionic impurities are not discharged out of the battery in a normal operation state, the battery performance gradually decreases.
[0011]
The ionic impurities present in the battery include those that move easily and those that do not move easily depending on the ionic species. However, in any case, it moves at a constant rate along with the current flowing in the battery. Taking advantage of this point, when the load is taken with the current density 1.5 times or more than during normal operation (for example, rated operation), the distribution of accumulated impurity ions changes and is expelled from the electrolyte, causing electrode reactions. It can be discharged out of the battery mixed with the generated water.
[0012]
In addition, when the gas supply is switched between the fuel electrode and the air electrode and the current direction is reversed, the impurity ions can be moved and discharged in the reverse direction.
[0013]
Further, the movement and discharge of impurity ions can be promoted by pressurizing the reaction gas or using oxygen as the oxidizing gas.
[0014]
Further, since the impurity ions in the electrolyte are discharged to the outside in the form of replacement with hydrogen ions, they can be discharged to the outside by washing the electrolyte and the electrode with an acidic solution.
[0015]
【Example】
Examples suitable for the present invention will be specifically described below.
[0016]
Example 1
A catalyst for the reaction electrode was prepared by supporting 25% by weight of platinum particles having an average particle diameter of about 30 mm on acetylene black carbon powder. A dispersion solution in which perfluorocarbon sulfonic acid powder was dispersed in ethyl alcohol was mixed with a solution in which the catalyst powder was dispersed in isopropanol to obtain an electrode paste.
[0017]
On the other hand, carbon paper having a thickness of 300 microns was dipped in an aqueous dispersion of polytetrafluoroethylene (PTFE) and dried to obtain a water-repellent porous electrode substrate. An electrode was prepared by applying and drying the electrode paste on one side of the porous electrode substrate. Next, the polymer electrolyte membrane is sandwiched between a pair of the electrodes with the surface on which the electrode paste is applied facing inside, and this is hot-pressed at a temperature of 110 ° C. for 30 seconds, whereby an electrolyte-electrode assembly (MEA) ) Was produced. Here, as the polymer electrolyte membrane, a perfluorocarbon sulfonic acid membrane having a thickness of 50 μm (Nafion manufactured by DuPont) was used.
[0018]
In addition to the above-mentioned carbon paper, the porous electrode base material is a carbon cloth woven with carbon fibers as a flexible material, and a carbon felt formed by mixing carbon fibers and carbon powder and adding an organic binder. Can also be used.
[0019]
Next, as a separator plate, a carbon plate made by cold press-molding a carbon powder material is used, and a resin-impregnated carbon plate that has been impregnated and cured with phenol resin to improve gas sealability is formed. did. Manifold holes for supplying and discharging cooling water for gas supply and discharge and for controlling the temperature of the battery were provided in the periphery of the gas flow path. In addition to the carbon separator described above, a metal separator in which a gas channel and a manifold hole were formed on a metal plate made of SUS304 was also prepared.
[0020]
Around the MEA with an electrode area of 25 cm ² , a gasket made of silicon rubber serving as a gas seal material is placed, and the battery is pressed and fastened from both ends with a pressure of 20 kgf / cm ² through a current collector plate made of SUS304. did.
[0021]
A practical battery is generally used by laminating a plurality of unit cells with a separator engraved with a cooling water flow path interposed therebetween. However, it is considered that the above-mentioned contaminating ions rarely move between different cells, and the evaluation in this example was performed with a single cell. A gas supply system for supplying humidified gas to the air side and hydrogen side to the unit cell thus created, an electric output system for setting and adjusting the load current to be extracted from the battery, and further adjusting the battery temperature to discharge A heat regulation system that effectively utilizes heat was attached to form a polymer electrolyte fuel cell of this example.
[0022]
For the fuel cell created by the above method, the following operating conditions are driven in the normal mode. evaluated. First, the current density taken out was set to 0.6 A / cm ² . Next, a gas utilization rate, which is an index indicating how much gas actually reacts with the supplied fuel gas and oxidant gas, is 70% on the fuel electrode side and 30% on the air side. %. Further, the cooling water was adjusted so that the battery temperature was 75 ° C. Then, using the pure hydrogen and air as the feed gas, supply pressure 0.2 kgf / cm ² air side, the hydrogen side is 0.05 kgf / cm ^2, the outlet of the gas was air release.
[0023]
As a result of driving the battery system under these conditions, the performance of both separators using carbon as separator plates and those using SUS304 has deteriorated after continuous operation for 500 hours. Therefore, the current was increased to 0.8 A / cm ² and operation was performed for 20 hours under these conditions. Thereafter, the current was returned again to 0.6 A / cm ² and the battery was operated, but the performance was not improved so much. Therefore, again, the current was increased to 1.0 A / cm ² , and the battery performance was recovered by removing and discharging the contaminated ions by operating for 20 hours under these conditions. Furthermore, removal and discharge of contaminating ions were similarly attempted at 1.5 A / cm ² and 2.0 A / cm ² . The results of these battery continuous tests are shown in FIG.
[0024]
In FIG. 1, when the current density is increased to 1.0 A / cm ² , the battery voltage is restored from 570 mV to 590 mV in the battery using the carbon separator, and 530 mV is 580 mV in the battery using the SUS304 separator. Performance has been recovered. In addition, the battery voltage was improved in the same manner for those having current densities of 1.5 A / cm ² and 2.0 A / cm ² .
[0025]
Through the above processing, the water discharged from the battery was analyzed when the output current was increased. Iron ions were detected in the battery using the SUS304 separator, and the phenol component was detected in the battery using the carbon separator. did. This result proves that the contaminated ions accumulated in the battery by long-term driving can be removed and discharged by the treatment of the present invention, and thereby the battery performance can be recovered.
[0026]
As described above, it was confirmed that the process for recovering the performance of the battery deteriorated by the continuous operation was performed by changing the current density, and the original purpose could be realized. Therefore, as a process for recovering the performance of the battery, the current load was increased, the output voltage of the single cell was kept at a voltage of 0.2 V or less for a certain period of time, and then returned to the normal operation state. As a result, it has been found that the output voltage can be recovered in the same manner as described above even if such a processing method is used.
[0027]
Next, a battery performance recovery process was attempted by reversing the direction of the current to be taken out from the battery whose voltage had been reduced by continuous operation for 500 hours. That is, during normal operation (output current = 0.6 A / cm ² ), air is sent to the fuel electrode side where hydrogen is flowing, and hydrogen is supplied to the air electrode side where air is supplied. The direction was reversed and the operation was performed at 0.6 A / cm ² for 20 hours. Thereafter, the normal operation mode was restored. When such a treatment was performed, the carbon separator battery was found to recover from 570 mV to 585 mV, and the SUS304 separator battery was found to recover from 530 mV to 565 mV.
[0028]
In this way, when the process of changing the magnitude and direction of the load current and the reverse process of the supply gas are performed, the pollutants accumulated in the battery can be mixed into the exhaust gas and the exhaust water and eliminated. The battery performance was recovered.
[0029]
In addition, the performance of the battery could be recovered by changing the direction in which the supply gas was introduced, that is, by supplying gas from the outlet of the reaction gas (air, hydrogen) in normal operation. Furthermore, it has also been found that this battery performance recovery effect can be promoted by introducing pure oxygen instead of air or by performing treatment using a pressurized reaction gas.
[0030]
(Example 2)
Next, by using the same fuel cell as that prepared in Example 1 and forcibly washing the contaminating ions present in the battery with degraded performance, the concentration is reduced, and the characteristics of the battery are improved. An attempt was made to recover.
[0031]
First, the above-described fuel cell was continuously operated for 500 hours in the normal operation mode described in Example 1, and the operation was stopped when the voltage of the battery decreased from the initial stage. Next, the battery was boiled in pure water for 1 hour, so that the boiled pure water was circulated inside the battery through the reaction gas supply gas passage. As a result of operating again in the normal operation mode after this operation, the battery voltage recovered from 570 mV to 580 mV in the battery using the carbon separator, and the output voltage from 530 mV to 555 mV in the battery using the SUS304 separator. Recovered.
[0032]
(Example 3)
In the treatment described in Example 2, boiling water was used for washing the battery. In this example, washing was performed using dilute sulfuric acid having pH 2 and pH 1. After the same fuel cell as that prepared in Example 1 was operated in the same normal operation mode as in Example 1, the operation was stopped, and a reaction gas supply port (air side / hydrogen side) was supplied to this battery. Then, dilute sulfuric acid was introduced through the tube and discharged from the outlet. After washing with dilute sulfuric acid for 2 hours, pure water was introduced and washed thoroughly, and washing was performed until the pH of the washing water coming out from the outlet became 5 or more.
[0033]
As a result of operating again in the normal operation mode after this operation, the output voltage recovered from 580 mV to 588 mV in the battery using the carbon separator, and the output voltage recovered from 555 mV to 572 mV in the battery of the SUS304 separator.
[0034]
In the above treatment, weakly acidic dilute sulfuric acid was used as the cleaning solution. However, when a cleaning solution made of weak alkali, that is, about pH 9, was used, although a slight cleaning effect was confirmed, no significant recovery was observed. The above results are shown in FIG.
[0035]
Thus, it has been found that the performance of the battery can be recovered by washing the inside of the battery with the cleaning liquid. At that time, it was also confirmed that the recovery effect was enhanced by washing at a higher temperature. Further, it was also found that the recovery of the battery voltage can be further promoted when the recovery treatment by the high current density operation performed in Example 1 and the cleaning with the weakly acidic cleaning water are used in combination. It was also confirmed that even if dilute acetic acid or ammonium sulfate was used as the weakly acidic cleaning solution, the same effect was obtained.
[0036]
Summarizing the effects of the present invention shown in Examples 1, 2, and 3 above from the difference in separator material that is a constituent element of the battery body, a battery using a metal separator is eluted from the separator during long-time operation. Although the battery performance deteriorates due to the metal ions, the characteristics of the battery could be recovered by removing the metal ions accumulated in the battery by high current operation or washing with weakly acidic washing water.
[0037]
On the other hand, batteries with carbon separators as constituent elements do not generate metal ions and various cations as much as metal separators, but according to ashing analysis, it was found that trace amounts of iron and calcium were contained in the separators. . Therefore, even if the battery is operated for a long time, the performance is not lowered as much as the battery using the metal separator, but there is a slight performance drop due to the contained metal ions. In addition, the organic substance eluted from the resin to be mixed in order to improve the gas tightness of the carbon separator, and trace amounts of sulfur compounds and nitrogen oxides contained in the air reduced the performance by about 30 mV after 500 hours of continuous testing. It is considered a thing. The recovery treatment of the present invention was also effective for a battery using a carbon separator.
[0038]
To summarize the implementation requirements of the present invention, the polymer electrolyte fuel cell first has means for adjusting the load current and output voltage in the electrical output system, or the gas supply system introduces cleaning liquid into the gas flow path. The battery can be cleaned directly, and the operation method is to adjust the load current and output voltage after a certain period of time, after a certain period of operation, or after the battery performance has deteriorated, or to clean the battery interior. In addition to the above battery, gas supply system, heat adjustment system, and electric output system, a system that includes a fuel reforming system, a control system, a charger system, etc., for example, an electric vehicle equipped with a fuel cell And cogeneration systems and portable power systems.
[0039]
【The invention's effect】
According to the present invention, it was possible to effectively recover performance degradation due to long-term operation of the polymer electrolyte fuel cell, and as a result, high durability could be realized.
[Brief description of the drawings]
FIG. 1 is a graph showing recovery characteristics of battery performance according to a first embodiment of the present invention. FIG. 2 is a graph showing recovery characteristics of battery performance according to another embodiment of the present invention.

Claims (8)

To chromatic polymer electrolyte membrane, a fuel electrode and an air electrode arranged to sandwich the polymer electrolyte membrane, an oxidant gas is supplied discharged into the air electrode, a gas flow path for supplying and discharging a fuel gas to the fuel electrode A battery main body portion in which unit cells constituted by a pair of separator plates are stacked, means for supplying and discharging the oxidant gas and fuel gas to and from the battery main body portion, and taking out of electric power generated in the battery main body portion is controlled. And a method for operating a polymer electrolyte fuel cell comprising a means for recovering battery characteristics and a means for recovering battery characteristics,
In the polymer electrolyte fuel cell, the impurity ions present in the fuel cell are sufficient to be discharged outside by being mixed with the water generated in the electrode reaction in an operation mode at a current 1.5 times or more that in normal operation. A method for operating a polymer electrolyte fuel cell, characterized by operating for a predetermined time . To chromatic polymer electrolyte membrane, a fuel electrode and an air electrode arranged to sandwich the polymer electrolyte membrane, an oxidant gas is supplied discharged into the air electrode, a gas flow path for supplying and discharging a fuel gas to the fuel electrode A battery main body portion in which unit cells constituted by a pair of separator plates are stacked, means for supplying and discharging the oxidant gas and fuel gas to and from the battery main body portion, and taking out of electric power generated in the battery main body portion is controlled. And a method for operating a polymer electrolyte fuel cell comprising a means for recovering battery characteristics and a means for recovering battery characteristics,
In the polymer electrolyte fuel cell, impurity ions existing in the fuel cell are mixed with the generated water of the electrode reaction and discharged to the outside in an operation mode at a current where the output voltage per unit cell is 0.2 V or less. A method for operating a polymer electrolyte fuel cell, characterized by operating for a predetermined time . The method for operating a polymer electrolyte fuel cell according to claim 1 or 2, wherein in the operation mode, an introduction direction of the oxidant gas or the fuel gas is changed. 3. The method for operating a polymer electrolyte fuel cell according to claim 1, wherein, in the operation mode, the oxidant gas or the fuel gas is supplied under pressure from a normal operation time. A pair having a polymer electrolyte membrane, a fuel electrode and an air electrode arranged with the polymer electrolyte membrane interposed therebetween, and a gas flow path for supplying and discharging oxidant gas to the air electrode and supplying and discharging fuel gas to the fuel electrode A battery main body portion in which unit cells composed of the separator plate are stacked, means for supplying and discharging the oxidant gas and fuel gas to and from the battery main body portion, and taking out of electric power generated in the battery main body portion is controlled. And a method for recovering the characteristics of a polymer electrolyte fuel cell comprising a battery characteristics recovery means,
By operating the polymer electrolyte fuel cell in an operation mode at a current 1.5 times or more that in normal operation, impurity ions in the cell are mixed with the water generated in the electrode reaction and discharged to the outside. A method for recovering characteristics of a polymer electrolyte fuel cell. A pair having a polymer electrolyte membrane, a fuel electrode and an air electrode arranged with the polymer electrolyte membrane interposed therebetween, and a gas flow path for supplying and discharging oxidant gas to the air electrode and supplying and discharging fuel gas to the fuel electrode A battery main body portion in which unit cells composed of the separator plate are stacked, means for supplying and discharging the oxidant gas and fuel gas to and from the battery main body portion, and taking out of electric power generated in the battery main body portion is controlled. And a method for recovering the characteristics of a polymer electrolyte fuel cell comprising a battery characteristics recovery means,
By operating the polymer electrolyte fuel cell in an operation mode with an electric current at which the output voltage per unit cell is 0.2 V or less, impurity ions in the cell are mixed with the water generated in the electrode reaction and discharged to the outside. A method for recovering the characteristics of a polymer electrolyte fuel cell. 7. The method for recovering characteristics of a polymer electrolyte fuel cell according to claim 5, wherein the direction of introduction of the oxidant gas or the fuel gas is changed in the operation mode. The method for recovering characteristics of a polymer electrolyte fuel cell according to claim 5 or 6, wherein, in the operation mode, the oxidant gas or the fuel gas is supplied under pressure from the normal operation.

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