KR20080045458A - Method of activating stack for fuel cell - Google Patents

Abstract

A method for activating a stack for a direct oxidation fuel cell is provided to activate the membrane-electrode assembly within a short time, and to measure the activation degree. A method for activating a stack for a direct oxidation fuel cell includes a step of changing and applying an electric current value while supplying 1-3 M of a hydrocarbon fuel and an oxidant, measuring voltage, and stopping the driving of the stack when the output does not increase anymore. The stack comprises at least one membrane-electrode assembly which has an anode electrode and a cathode electrode placed to face each other, and a polymer electrolyte membrane placed between the anode and cathode electrodes.

Description

Activation method of stack for direct oxidation fuel cell {METHOD OF ACTIVATING STACK FOR FUEL CELL}

1 is a view schematically showing the structure of a fuel cell system of the present invention.

[Industrial use]

The present invention relates to a method for activating a stack for a direct oxidation fuel cell, and more particularly, to a method for activating a stack for a direct oxidation fuel cell capable of activating a membrane-electrode assembly in a short time.

[Prior art]

A fuel cell is a power generation system that directly converts the chemical reaction energy of hydrogen and oxygen contained in hydrocarbon-based materials such as methanol, ethanol and natural gas into electrical energy. This fuel cell is a clean energy source that can replace fossil energy, and has the advantage of generating a wide range of outputs by stacking unit cells, and having an energy density of 4-10 times that of a small lithium battery. It is attracting attention as a compact and mobile portable power source.

Representative examples of the fuel cell include a polymer electrolyte fuel cell (PEMFC) and a direct oxidation fuel cell (Direct Oxidation Fuel Cell). When methanol is used as a fuel in the direct oxidation fuel cell, it is called a direct methanol fuel cell (DMFC).

The polymer electrolyte fuel cell has an advantage of having a high energy density and a high output, but requires attention to handling hydrogen gas and reforms fuel for reforming methane, methanol, natural gas, etc. to produce hydrogen as fuel gas. There is a problem that requires additional equipment such as a device.

On the other hand, the direct oxidation fuel cell has a lower energy density than the polymer electrolyte fuel cell, but it is easy to handle fuel and has a low operating temperature, so that it can be operated at room temperature, in particular, it does not require a fuel reformer.

In such fuel cell systems, the stack that substantially generates electricity comprises several unit cells consisting of a membrane-electrode assembly (MEA) and a separator (also called a bipolar plate). It has a structure laminated to several tens. The membrane-electrode assembly is called an anode electrode (also called "fuel electrode" or "oxidation electrode") and a cathode electrode (also called "air electrode" or "reduction electrode") with a polymer electrolyte membrane containing a hydrogen ion conductive polymer therebetween. ) Is located.

Before using the stack, an activation process is performed using methanol and air to improve the output. However, there is a problem that the time required for the activation process is required for too long, about 3 days or more, and there is a problem that it is impossible to measure whether the battery activation has reached a satisfactory level.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a method for activating a stack for a direct oxidation fuel cell that can be activated in a short time and can measure the degree of activation.

In order to achieve the above object, the present invention provides a direct oxidation type comprising at least one membrane-electrode assembly including an anode electrode and a cathode electrode positioned opposite each other, and a polymer electrolyte membrane positioned between the anode electrode and the cathode electrode. A method for activating a stack for a fuel cell, which is applied while varying a current value while supplying hydrocarbon fuel and an oxidant at a concentration of 1 to 5 M, and measuring a voltage for 30 minutes to 3 hours to drive the stack at a time when the output no longer increases. It provides a method for activating a stack for a direct oxidation fuel cell comprising the step of stopping.

Hereinafter, the present invention will be described in more detail.

The present invention relates to a method for activating a stack for a direct oxidation fuel cell, and more particularly, to a method for activating a stack for a passive oxidation fuel cell (called a passive type or air breathing type) for supplying an oxidant in a diffusion manner. .

The fuel cell stack has a structure in which one or more membrane-electrode assemblies and separators are stacked and substantially generates electricity. Such fuel cell stacks must undergo activation before use to maximize cell output. Therefore, the conditions for the cell activation process are very important factors in maximizing the cell performance, but the active type cell which supplies the conventional oxidant using a pump is approximately 3 days, which is about 2 hours per day. Only conditions were present, and there are no established criteria for the activation process, particularly in passive types such as the present invention. In addition, even if an active type activation process is applied, it takes about three days as described above, so it takes a long time, and there is a problem that it is not possible to measure whether the activation is as much as desired.

The present invention establishes an activation process in a passive type direct oxidation fuel cell, and provides an activation method which enables activation in a short time and can measure the degree of activation.

In the activation process of the present invention, while supplying hydrocarbon fuel and oxidant at a concentration of 1 to 5M, the current is applied while varying the current value, and the voltage is measured for 30 minutes to 3 hours to drive the stack when the output is no longer increased. It includes the process of stopping. At this time, the change of the current value was increased from 0A to 0.1A / 10sec until the battery voltage became 0.2V.

The voltage measurement time is to measure the difference in voltage while changing the current, and since the output can be known from this, it is the time until the output becomes maximum and no longer increases. If the output does not increase in this way, it can be determined that the activation process is completed. In addition, when the voltage value is changed during the measurement time, once, the time required for one measurement is generally about 20 minutes. Since 5 to 8 times are generally performed, a total of about 100 to 160 minutes Is required.

At this time, the hydrocarbon fuel supplied during the measurement process may use the same concentration of fuel within the concentration of 1 to 5M, may be used while gradually increasing the concentration, or may be used while increasing in steps such as 1M, 3M and 5M. . When increasing in steps of 1M, 3M and 5M, it is preferable to perform about 3 to 5 times at 1M, about 2 to 3 times at 3M, and may be optionally further performed at 1 to 2 times at 5M.

As the hydrocarbon fuel, methanol, ethanol, propanol, butanol, natural gas, or a combination thereof may be used, and methanol is preferable. Oxygen or air may be used as the oxidant, and air is preferable.

In the membrane-electrode assembly constituting the stack of the present invention, the cathode electrode and the anode electrode include a catalyst layer and an electrode substrate.

The catalyst layer is platinum, ruthenium, osmium, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy, platinum-M alloy (M is M Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu , A transition metal selected from the group consisting of Zn, Sn, Mo, W, Rh, Ru, and combinations thereof), and a catalyst selected from the group consisting of combinations thereof. More preferably, Pt, Pt / Ru, Pt / W, Pt / Ni, Pt / Sn, Pt / Mo, Pt / Pd, Pt / Fe, Pt / Cr, Pt / Co, Pt / Ru / W, Pt / One or more selected from the group consisting of Ru / Mo, Pt / Ru / V, Pt / Fe / Co, Pt / Ru / Rh / Ni, and Pt / Ru / Sn / W can be used.

In addition, such a metal catalyst may be used as the metal catalyst (black) itself, or may be supported on a carrier. As the carrier, carbonaceous materials such as graphite, denka black, ketjen black, acetylene black, carbon nanotubes, carbon nanofibers, carbon nanowires, carbon nanoballs or activated carbon may be used, or alumina, silica, zirconia, Inorganic fine particles such as titania may be used, but carbon-based materials are generally used. In the case of using the noble metal supported on the carrier as a catalyst, a commercially available commercially available product may be used, or the noble metal supported on the carrier may be prepared and used. Since the process of supporting the precious metal on the carrier is well known in the art, detailed descriptions thereof will be readily understood by those skilled in the art even if the detailed description is omitted.

The catalyst layer may further include a binder resin for improving the adhesion of the catalyst layer and the transfer of hydrogen ions.

It is preferable to use a polymer resin having hydrogen ion conductivity as the binder resin, more preferably a cation exchange group selected from the group consisting of sulfonic acid group, carboxylic acid group, phosphoric acid group, phosphonic acid group and derivatives thereof in the side chain. Any polymer resin which has can be used. Preferably, a fluorine polymer, a benzimidazole polymer, a polyimide polymer, a polyetherimide polymer, a polyphenylene sulfide polymer, a polysulfone polymer, a polyether sulfone polymer, a polyether ketone polymer, a polyether One or more hydrogen ion conductive polymers selected from ether ketone polymers or polyphenylquinoxaline polymers, more preferably poly (perfluorosulfonic acid), poly (perfluorocarboxylic acid), Copolymers of tetrafluoroethylene with fluorovinyl ethers containing sulfonic acid groups, defluorinated sulfided polyether ketones, aryl ketones, poly (2,2'-m-phenylene) -5,5'-bibenzimidazole One containing one or more hydrogen ion conductive polymers selected from (poly (2,2 '-(m-phenylene) -5,5'-bibenzimidazole) or poly (2,5-benzimidazole) may be used. .

The hydrogen ion conductive polymer may replace H with Na, K, Li, Cs or tetrabutylammonium in an ion exchange group at the side chain end. In case of replacing H with Na in the side chain terminal ion exchanger, NaOH is substituted for the preparation of the catalyst composition, and tetrabutylammonium hydroxide is substituted for tetrabutylammonium, and K, Li or Cs may be substituted with appropriate compounds. It can be substituted using. Since this substitution method is well known in the art, detailed description thereof will be omitted.

The binder resin may be used in the form of a single substance or a mixture, and may also be optionally used with a nonconductive polymer for the purpose of further improving adhesion to the polymer electrolyte membrane. It is preferable to adjust the usage-amount so that it may be suitable for a purpose of use.

Examples of the nonconductive polymer include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoro alkylvinyl ether copolymer (PFA), and ethylene / tetrafluoro Copolymer of ethylene / tetrafluoroethylene (ETFE), ethylenechlorotrifluoro-ethylene copolymer (ECTFE), polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), dode At least one selected from the group consisting of silbenzenesulfonic acid and sorbitol is more preferred.

The electrode substrate plays a role of supporting the electrode and diffuses the fuel and the oxidant to the catalyst layer, thereby serving to easily access the fuel and the oxidant to the catalyst layer. The electrode substrate is a conductive substrate, and representative examples thereof include carbon paper, carbon cloth, carbon felt, or metal cloth (porous film or polymer fiber composed of metal cloth in a fibrous state). The metal film is formed on the surface of the cloth formed with)) may be used, but is not limited thereto.

In addition, it is preferable to use a water-repellent treatment with a fluorine-based resin as the electrode base material because it can prevent the reactant diffusion efficiency from being lowered by water generated when the fuel cell is driven. Examples of the fluorine-based resin include polytetrafluoroethylene, polyvinylidene fluoride, polyhexafluoropropylene, polyperfluoroalkyl vinyl ether, polyperfluorosulfonyl fluoride alkoxy vinyl ether, and fluorinated ethylene propylene ( Fluorinated ethylene propylene), polychlorotrifluoroethylene or copolymers thereof can be used.

In addition, a microporous layer may be further included to enhance the reactant diffusion effect in the electrode substrate. These microporous layers are generally conductive powders having a small particle diameter, such as carbon powder, carbon black, acetylene black, activated carbon, carbon fiber, fullerene, carbon nanotubes, carbon nanowires, and carbon nanohorns. -horn or carbon nano ring.

The microporous layer is prepared by coating a composition including a conductive powder, a binder resin, and a solvent on the gas diffusion layer. The binder resin may be polytetrafluoroethylene, polyvinylidene fluoride, polyhexafluoropropylene, polyperfluoroalkyl vinyl ether, polyperfluorosulfonyl fluoride, alkoxy vinyl ether, polyvinyl alcohol, cellulose acetate Or copolymers thereof and the like can be preferably used. As the solvent, alcohols such as ethanol, isopropyl alcohol, n-propyl alcohol, butyl alcohol, water, dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone, tetrahydrofuran, etc. may be preferably used. The coating process may be screen printing, spray coating, or coating using a doctor blade according to the viscosity of the composition, but is not limited thereto.

Generally used as a polymer electrolyte membrane in a fuel cell, anything made of a polymer resin having hydrogen ion conductivity can be used. Representative examples thereof include a polymer resin having a cation exchange group selected from the group consisting of sulfonic acid groups, carboxylic acid groups, phosphoric acid groups, phosphonic acid groups and derivatives thereof in the side chain.

Representative examples of the polymer resin include a fluorine polymer, a benzimidazole polymer, a polyimide polymer, a polyetherimide polymer, a polyphenylene sulfide polymer, a polysulfone polymer, a polyether sulfone polymer and a polyether ketone It may comprise one or more selected from polymers, polyether-etherketone-based polymers or polyphenylquinoxaline-based polymers, more preferably poly (perfluorosulfonic acid) (generally marketed as Nafion), poly (Perfluorocarboxylic acid), copolymers of tetrafluoroethylene and fluorovinyl ether containing sulfonic acid groups, defluorinated sulfide polyether ketones, aryl ketones, poly (2,2'-m-phenylene)- 5,5'-bibenzimidazole (poly (2,2 '-(m-phenylene) -5,5'-bibenzimidazole) or poly (2,5-benzimidazole) may be mentioned. have.

A schematic structure of the fuel cell system of the present invention is shown in FIG. 1, which will be described in more detail with reference to the following.

The fuel cell system 100 of the present invention includes at least one electricity generator 10 for generating electrical energy through an electrochemical reaction between a fuel and an oxidant, and a fuel supply unit for supplying fuel to the electricity generator 10. 30 and an oxidant supply unit 50 for supplying an oxidant to the electricity generating unit 10.

The electricity generating unit 10 includes one or more membrane-electrode assemblies, and the separator 15 is disposed on a surface in contact with the fuel supply unit 30.

The oxidant supply unit 50 is positioned on the surface of the electricity generator 10 that is not in contact with the fuel supply unit 30. The oxidant supply unit 50 serves as a separator, and also serves to supply an oxidant to the electricity generating unit 10 through a plurality of vent holes 50a.

The fuel supply unit 30 may be disposed in close contact with the electricity generator 10, and may supply fuel to the electricity generator 10 in a diffusion manner without depending on a driving force such as a pump.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention and the present invention is not limited to the following examples.

(Example 1)

Anode using 88 wt% Pt-Ru black (Johnson Matthey) and Pt black (Johnson Matthey) catalyst and 12 wt% Nafion / H ₂ O / 2-propanol (Solution Technology Inc.) at 5 wt% concentration as binder Catalyst compositions for electrodes and catalyst compositions for cathode electrodes were prepared, respectively. Applying the anode catalyst catalyst composition to a carbon paper electrode substrate having a carbon content of 0.2mg / cm ² to produce an anode electrode, the cathode electrode catalyst composition to a carbon paper electrode substrate having a carbon content of 1.3mg / cm ² It was applied to prepare a cathode electrode. At this time, the catalyst loading in the anode electrode and cathode electrode was 8mg / cm ² .

A membrane-electrode assembly was prepared using the prepared anode and cathode electrodes and a commercial Nafion 115 (perfluorosulfonic acid) polymer electrolyte membrane.

One or more membrane-electrode assemblies were stacked to prepare a stack.

1M methanol and air were injected into the stack, and the voltage was measured until the battery voltage became 0.2V while increasing the current from 0A to 0.1A / 10sec. This process was repeated once to five times. The measurement time took about 20 minutes in one run.

(Example 2)

Inject 3M methanol and air into the stack, increase the current from 0A to 0.1A / 10sec, and measure twice the voltage until the battery voltage reaches 0.2V, then inject 1M methanol and air the next day. Then, while increasing the current from 0A to 0.1A / 10sec, the voltage was measured twice until the battery voltage became 0.2V, and the results are shown as ⑥ and ⑦ in FIG. 2, respectively. The time required for one measurement was about 20 minutes.

In addition, the results of repeating 1 to 5 times in the process of Example 1 are shown as ① to ⑤ in FIG. As shown by ① to ⑤ in Fig. 2, the output of about 1W at 4V is obtained at one time, and the output gradually improves to 1.5W at about five times. In addition, when repeated four times and five times, the output is almost saturated and no longer increases, and the result is almost the same. As a result, it can be seen that the activation process of the stack is completed. That is, it took about 3 days to activate the conventional stack, but in Example 1, it can be seen that it was completed in a very short time, about 1 hour.

As shown in ⑥ and ⑦ of FIG. 2, after the activation process, the results were almost the same on the second day (3.2 W at 3.99 V and 3.2 W at 4.1 V), indicating that the stack was activated. Therefore, even in the case of using a high concentration of 3M methanol can be seen that the activation of the stack can be obtained on the second day if the activation process for about one day.

All simple modifications or changes of the present invention can be easily carried out by those skilled in the art, and all such modifications or changes can be seen to be included in the scope of the present invention.

The activation method of the stack for a direct oxidation fuel cell of the present invention can be carried out within a short time, and it is possible to measure whether the activation has reached a desired level.

Claims (10)

A method of activating a stack for a direct oxidation fuel cell comprising at least one membrane-electrode assembly comprising an anode electrode and a cathode electrode positioned opposite to each other, and a polymer electrolyte membrane positioned between the anode electrode and the cathode electrode, the method comprising: Supplying hydrocarbon fuel and oxidant at a concentration of 1 to 3M, changing and applying the current value, measuring the voltage, and stopping the operation of the stack when the output no longer increases. A method of activating a stack for a direct oxidation fuel cell comprising a process. The method of claim 1, And said hydrocarbon fuel in said measuring step is supplied with a hydrocarbon fuel having a constant concentration. The method of claim 1, The method of activating a stack for a direct oxidation fuel cell, wherein the hydrocarbon fuel is supplied while gradually increasing from 1M to 3M concentration in the measurement process. The method of claim 1, The method of activating a stack for a direct oxidation fuel cell in which the hydrocarbon fuel in the measurement process is supplied in increments of 1M and 3M concentration. The method according to claim 3 or 4, The method of activating a stack for a direct oxidation fuel cell that additionally uses a hydrocarbon fuel of 5M concentration as the hydrocarbon fuel. The method of claim 1, The voltage measurement is performed for 100 to 160 minutes. The method of claim 1, Wherein said hydrocarbon fuel is selected from the group consisting of methanol, ethanol, propanol, natural gas, and combinations thereof. The method of claim 7, wherein And the hydrocarbon fuel is methanol. The method of claim 1, And said stack is for a passive type fuel cell. The method of claim 1, The activation method is a method for activating a stack for a direct oxidation fuel cell that is carried out for 30 minutes to 3 hours.

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