KR20080047078A - Method of activating stack for direct oxidation fuel cell - Google Patents

Abstract

A method for activating the stack of a direct oxidation fuel cell is provided to maximize the performance of a battery within a short time. A method for activating the stack of a direct oxidation fuel cell which comprises at least one membrane electrode assembly provided with an anode and a cathode facing each other and a polymer electrolyte membrane located between the anode and the cathode, comprises the step of injecting the humidification hydrogen to an anode. Preferably the relative humidity of the humidification hydrogen is 20-100 % and the amount of the humidification hydrogen injected to the anode is 1.0-3.0 lambda.

Description

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

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

2 is a graph showing the performance of a fuel cell manufactured according to Example 1 and Comparative Example 1 of the present invention.

3 is a graph showing the cell performance according to the active time of the fuel cell prepared according to Example 1 and Comparative Example 1 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 maximizing cell performance 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, which is 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. Has a structure in which

The principle of generating electricity in a fuel cell is that fuel is supplied to an anode electrode, which is a fuel electrode, adsorbed to a catalyst of the anode electrode, the fuel is ionized by an oxidation reaction, and electrons are generated, and the generated electrons are oxidized according to an external circuit. Reaching the cathode, which is the pole, hydrogen ions pass through the polymer electrolyte membrane and are delivered to the cathode. An oxidant is supplied to the cathode, and the oxidant, hydrogen ions, and electrons react on the catalyst of the cathode to generate electricity while producing water.

It is an object of the present invention to provide a method for activating a stack for a direct oxidation fuel cell that can maximize cell performance in a short time.

In order to achieve the above object, the present invention provides a direct oxidation fuel 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 battery, the method for activating a stack for a direct oxidation fuel cell comprising a step of injecting humidified hydrogen into an anode electrode of the stack.

The humidified hydrogen preferably has a relative humidity of 20 to 100%.

The amount of humidified hydrogen injected into the anode electrode is preferably 1.0 to 3.0λ.

It is preferable that the fuel cell is a passive fuel cell.

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

In the direct oxidation fuel cell system, a fuel is injected into an anode electrode and an oxidant is injected into a cathode electrode to induce an electrochemical reaction to generate electricity. In this case, the fuel and oxidant may be classified into an active type and a passive type (called a passive type or an air breathing type). The active type is a method of supplying a fuel and an oxidant by using a pump. The passive type is a method of supplying fuel by a diffusion method or a pump and the oxidant is supplied by a convection phenomenon.

In the case of passive fuel cells, there are few known methods for maximizing the performance of membrane-electrode assemblies in a short time as compared to active fuel cells. One of the known methods, the active method using methanol solution or water, It takes a long time of at least 20 hours to 72 hours to reach the maximum performance, and there is also a difficulty in maximizing battery performance in this manner.

Accordingly, the present invention provides an active method that can maximize the performance of a passive fuel cell as well as an active fuel cell in a short time.

The activation method of the present invention comprises a step of injecting humidified hydrogen into the anode electrode of the membrane-electrode assembly constituting the stack.

Here, the humidified hydrogen means that a certain volume of hydrogen gas contains water, and the content of moisture contained in the humidified hydrogen may be expressed by relative humidity%. In the present invention, the humidified hydrogen preferably has a relative humidity of 20 to 100%, more preferably 60 to 90%. When the relative humidity of the humidified hydrogen is in the above range, there is an advantage that the polymer electrolyte membrane and the electrode can be properly hydrated, which is preferable.

The humidified hydrogen increases the hydrogen ion conductivity by hydrating the polymer electrolyte membrane and the electrode, thereby improving the activity of the fuel cell system in a short time and maximizing battery performance.

The injection amount of the humidified hydrogen is preferably 1.0 to 3.0 lambda (Stoichiometry), more preferably 1.2 to 1.5 lambda. In the present invention, λ means a theoretical injection amount that must be supplied to the anode electrode in order to obtain the highest output.

When the injection amount of humidified hydrogen is in the said range, since it can fully hydrate a polymer electrolyte membrane and an electrode, it is preferable. If the injected amount of humidified hydrogen exceeds 3.0λ, the relative humidity of the gas decreases as the flow rate increases, so that it is difficult to supply hydrogen with sufficient humidity, which may cause a problem that the polymer electrolyte membrane and the electrode cannot be sufficiently hydrated. I can't. In addition, when less than 1.0λ, the amount of hydrogen is small, which is difficult to cause sufficient current discharge, which is not preferable.

As such, when humidified hydrogen is injected into the anode electrode of the direct oxidation fuel cell system and activated, the fuel cell system can reach maximum performance within 2 to 18 hours. Thus, the battery reaches the maximum performance compared to the conventional 20 to 72 hours. Time can be greatly reduced, and battery performance can also be maximized.

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

As the catalyst, any one that can be used as a catalyst may participate in the reaction of the fuel cell, and a platinum-based catalyst may be used as a representative example. The platinum-based catalyst is platinum, ruthenium, osmium, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy, or platinum-M alloy (M is Ga, Ti, V, Cr, Mn, Fe, Co, Ni At least one catalyst selected from the group consisting of Cu, Zn, Sn, Mo, W, Rh, and Ru). Specific examples include Pt, Pt / Ru, Pt / W, Pt / Ni, Pt / Sn, Pt / Mo, Pt / Pd, Pt / Fe, Pt / Cr, Pt / Co, Pt / Ru / W, Pt / Ru One or more selected from the group consisting of / Mo, Pt / Ru / V, Pt / Fe / Co, Pt / Ru / Rh / Ni, and Pt / Ru / Sn / W can be used.

In addition, the platinum-based catalyst may be used in a black form not supported on the carrier or not. As the carrier, carbon-based materials such as graphite, denka black, ketjen black, acetylene black, carbon nanotube, carbon nanofiber, carbon nanowire, carbon nanoball, 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 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 due to water generated when the fuel cell is driven. As the fluorine-based resin, polyvinylidene fluoride, polytetrafluoroethylene, fluorinated ethylene propylene, polychlorotrifluoroethylene, fluoroethylene polymer, or the like may 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 comprising a conductive powder, a binder resin and a solvent on the electrode substrate. As the binder resin, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl alcohol, cellulose acetate, and the like may be preferably used. The solvent may be ethanol, isopropyl alcohol, n-propyl alcohol, butyl alcohol, or the like. Alcohol, water, dimethylacetamide, dimethylsulfoxide, N-methylpyrrolidone and the like can 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.

In the membrane-electrode assembly of the present invention, the polymer electrolyte membrane includes a polymer having excellent hydrogen ion conductivity, and representative examples thereof include a sulfonic acid group, a carboxylic acid group, a phosphoric acid group, a phosphonic acid group and derivatives thereof in the side chain. The polymer resin which has a cation exchange group selected can be mentioned.

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 include 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.

In the hydrogen ion conductive group of the hydrogen ion conductive polymer, H may be substituted with Na, K, Li, Cs or tetrabutylammonium. 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 stack for the direct oxidation fuel cell of the present invention is more suitable for a passive type in which fuel is supplied in a diffusion manner or using a pump, and an oxidant is supplied in a convection phenomenon.

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 generating unit 10, and may supply fuel to the stack 10 under no load 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)

88 wt% Pt-Ru black (Johnson Matthey) catalyst and 12 wt% Nafion aqueous dispersion were added to 88 wt% Pt black (Johnson Matthey) catalyst to prepare an anode electrode catalyst composition and a cathode electrode catalyst composition, respectively. .

The anode composition and the cathode composition were applied to a carbon paper electrode substrate to prepare an anode electrode and a cathode electrode. At this time, the catalyst loading of the anode electrode and the cathode electrode was 4 mg / cm ^2, respectively.

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

The prepared membrane-electrode assembly is inserted between polytetrafluoroethylene-coated glass fiber gaskets, and then inserted into two separators having a gas flow channel and a cooling channel having a predetermined shape. The unit cell, ie stack, was prepared by pressing between copper end plates.

Humidified hydrogen with a relative humidity of 95% was supplied to an anode electrode of the stack at 1.2λ for 2 hours to activate, and then the output of the battery was measured.

(Example 2)

The same procedure as in Example 1 was carried out except that humidified hydrogen having a relative humidity of 80% was supplied to the anode electrode of the stack at 1.0λ for 2 hours.

(Example 3)

The same procedure as in Example 1 was carried out except that humidified hydrogen having a relative humidity of 75% was supplied to the anode electrode of the stack at 1.3λ for 2 hours.

(Example 4)

The same procedure as in Example 1 was conducted except that humidified hydrogen having a relative humidity of 63% was supplied to the anode electrode of the stack at 1.5λ for 2 hours.

(Example 5)

The same procedure as in Example 1 was carried out except that humidified hydrogen having a relative humidity of 47% was supplied to the anode electrode of the stack at 1.74 lambda for 2 hours.

(Example 6)

The same procedure as in Example 1 was carried out except that humidified hydrogen having a relative humidity of 33% was supplied to the anode electrode of the stack at 2.0 lambda for 2 hours.

(Example 7)

The same procedure as in Example 1 was conducted except that humidified hydrogen having a relative humidity of 55% was supplied to the anode electrode of the stack at 2.3 lambda for 2 hours.

(Example 8)

The same procedure as in Example 1 was carried out except that humidified hydrogen having a relative humidity of 60% was supplied to the anode electrode of the stack at 2.7 lambda for 2 hours.

(Example 9)

The same procedure as in Example 1 was conducted except that humidified hydrogen having a relative humidity of 87% was supplied to the anode electrode of the stack at 2.87 lambda for 2 hours.

(Comparative Example 1)

The same process as in Example 1 was conducted except that humidified hydrogen was not injected into the anode electrode of the stack.

Performance of the batteries prepared according to Examples 1 to 9 and Comparative Example 1 were measured, among which the battery performance measurement results of Example 1 and Comparative Example 1 and the battery performance measurement results according to the active time are shown in FIGS. 2 and 3. Represented in the graph respectively.

Referring to FIG. 2, Example 1, in which hydrogen was supplied to the anode electrode of the stack, showed better battery characteristics than Comparative Example 1. FIG.

Referring to FIG. 3, Example 1, in which the humidified hydrogen was supplied and the battery was activated, took 2 hours to activate the battery, and the output density was high as 10 W / cm ² , and after 2 hours after battery activation ( At 4 hours from the start of activation, the power density was very high at 25W / cm ² . In addition, it took 12 hours for the cell to reach maximum performance, and the output density at the maximum performance was very high, above 30 W / cm ² .

However, in Comparative Example 1 in which the cell was activated without supplying humidified hydrogen, the time taken for activation was equal to 2 hours, but the output density was 5 W / cm ² , which was lower than that in Example 1, and 2 hours after the cell activation. The output density (at 4 hours from the start of activation) was very low, 10 W / cm ² . Also, It took a long time of 20 hours for the battery to reach maximum performance, and the output density at the maximum performance was 20W / cm ^{2, which} is very low compared to Example 1.

In other words, when the battery is activated by supplying humidified hydrogen to the anode electrode, it can be confirmed that not only shortening the activation time but also reducing the time for the battery to reach maximum performance, and the battery performance is greatly improved.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited thereto, and various modifications and changes can be made within the scope of the claims and the detailed description of the invention and the accompanying drawings. Naturally, it belongs to the scope of the invention.

In the fuel cell stack activation method of the present invention, by supplying humidified hydrogen at the time of battery activation, the battery performance can be maximized in a short time.

Claims (6)

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, A method of activating a stack for a direct oxidation fuel cell comprising the step of injecting humidified hydrogen into the anode electrode. The method of claim 1, The relative humidity of the humidified hydrogen is 20 to 100% of the activation method of the stack for a direct oxidation fuel cell. The method of claim 2, The relative humidity of the humidified hydrogen is 60 to 90% of the method for activating a stack for a direct oxidation fuel cell. The method of claim 1, The amount of humidified hydrogen injected into the anode electrode is 1.0 to 3.0λ activation method for a direct oxidation fuel cell stack. The method of claim 4, wherein The amount of the humidified hydrogen is 1.2 to 1.5λ activation method of a stack for a direct oxidation fuel cell. The method of claim 1, And the fuel cell is a passive fuel cell.

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