KR20110078903A - Direct methanol fuel cell having micro-perforated carbon papers as diffusion layers - Google Patents

Abstract

PURPOSE: A direct methanol fuel cell including a carbon paper diffusion layer having micropores is provided to maximize the output and efficiency of a battery by rapidly discharging by-products and smoothing the supply of fuel and oxidizing agents by forming micropores on a carbon paper constituting a diffusion layer. CONSTITUTION: A direct methanol fuel cell including a carbon paper diffusion layer having micropores comprises: an electrolyte film in which a catalyst layer and a diffusion layer are successively arranged on one side of the electrolyte film; a fuel electrode to which a methanol solution is supplied as fuel; and an air electrode supplied with oxidation gas in which a catalyst layer and a diffusion layer are successively arranged on the other side of the electrolyte film.

Description

Direct methanol fuel cell having micro-perforated carbon papers as diffusion layers

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct methanol fuel cell, and more particularly, to a direct methanol fuel cell using carbon paper in which micropores are formed as diffusion layers of an anode and a cathode.

Recently, as the depletion of existing energy resources such as oil and coal is predicted, interest in energy that can replace them is increasing. As one of such alternative energy-use power generation technologies, fuel cells are particularly attracting attention due to their advantages such as higher efficiency, quieter, and no emission of pollutants such as nitrogen oxides or sulfur oxides.

A fuel cell is a power generation system for converting chemical reaction energy of fuel and oxidant into electrical energy. Hydrogen or hydrocarbon fuel such as methanol, methane, butane, etc. is mainly used as fuel, and air is typically used as oxidant.

Among them, the direct methanol fuel cell (DMFC) using methanol as fuel has a lower electrode density than the fuel cell using hydrogen directly, so the power density is lower, but the energy density of methanol as fuel is lower. It is high and easy to store, which is very advantageous for low power and long time use. Taking advantage of these advantages, the development of direct methanol fuel cell as a power source for mobile devices has been actively conducted to replace existing secondary batteries. However, CO ₂ and water from the anode and cathode of the direct methanol fuel cell, respectively, inhibit the diffusion of fuel and air into the catalyst layer, which reduces the output. This phenomenon is remarkable in high power density operation.

The electrochemical reactions occurring in the anode and cathode catalyst layers of a direct methanol fuel cell are as follows.

_{Anode: CH 3 OH + H 2 O} → CO 2 + 6H + + 6e -

^{_{Cathode: 3/2 O 2 + 6H + +}} 6e - → 3H 2 O

Total reaction: CH ₃ OH + H ₂ O → CO ₂ + 3H ₂ O

When the liquid phase methanol aqueous solution is supplied from the anode, CO ₂ in the gas phase is generated as a by-product, and conversely, in the cathode, liquid water is produced (when bubbling) to the air in the gas phase. Therefore, in the two diffusion layers that deliver fuel and air to the catalyst layer and discharge the by-product CO ₂ and water into the flow path, there is an interface between the liquid phase and the gas phase, and this interface provides the fuel and air supply. Will be inhibited. The inhibition of reactant (fuel and air) supply due to the interface is more marked when operating at high current density conditions because the amount of by-products produced per unit catalyst layer area is proportional to the current density.

Therefore, when the discharge of the by-products not smooth, an air electrode catalyst layer becomes a cathode flooding (cathode flooding) that prevent smooth supply of the air standing covered with water, the air electrode by-product generation, the fuel electrode catalyst layer is the fuel electrode by-product CO ₂ bubbles to the fuel Fuel starvation (anode starving) to cut off the supply of a problem occurs that the output is lowered.

1 is a cross-sectional view of a typical direct methanol fuel cell, which will be described in detail below.

As shown in FIG. 1, a direct methanol fuel cell generally includes a unit cell composed of one electrolyte membrane, two catalyst layers, and two diffusion layers. The stack is formed by stacking, and a separator (bipolar plate) for inserting a flow path and connecting electrodes is inserted between unit cells. The final product is therefore a stack of unit cells.

The unit cell includes a membrane electrode unit, in which a membrane electrode unit is stacked on one side in the order of a seal member on a frame, a fuel flow path plate and a current collector plate, and the stacked arrangement of the structure described above is a membrane electrode unit. Is formed on the opposite side of the. The membrane electrode unit is provided with a fuel electrode supplied with fuel, an air electrode supplied with an oxidizing gas, and an electrolyte membrane interposed between the electrodes. The anode includes a catalyst layer in contact with an electrolyte membrane and a diffusion layer having carbon paper stacked on the catalyst layer, and the cathode includes a catalyst layer in contact with the electrolyte membrane and a diffusion layer having carbon paper stacked on the catalyst layer.

As described above, in the direct methanol fuel cell, CO ₂ bubbles and water generated as by-products in the anode and the cathode, respectively, form an interface inside each diffusion layer, thereby inhibiting the supply of reactant methanol and air. . Therefore, by improving the discharge of the by-products can be expected to improve the output of the fuel cell, this requires more detailed consideration of the discharge mechanism of the by-products.

In the direct methanol fuel cell, the process of discharging the by-products consists of two stages: movement through the inside of the diffusion layer and desorption from the surface of the diffusion layer, and the by-products after desorption move along the flow path.

First, since the desorption from the surface of the diffusion layer corresponds to an inlet through which methanol or air diffuses into the catalyst layer, the CO ₂ bubbles and water droplets respectively attached to the surface of the anode and the cathode diffusion layer are not properly desorbed. A decrease in output will occur due to a shortage of reactants. Attachment of the bubbles and droplets is by surface tension, and their desorption is by drag force acting on the bubbles or droplets by the fluid flowing along the flow path. Therefore, the desorption can be controlled by reducing the surface tension by controlling the wettability of the surface of the diffusion layer, that is, hydrophilicity and hydrophobicity, or increasing drag through the flow path design.

On the other hand, the movement of the by-products through the diffusion layer is formed in a gaseous phase / liquid interface inside an amorphous and complex microcapillary structure in which carbon fibers forming a diffusion layer of carbon species form a layered structure. The by-products are discharged by advancing in the outward direction of the diffusion layer (ie flow path direction). Therefore, the surface tension acting between the carbon fiber inside the capillary and the interface acts as a force to prevent the discharge of by-products, and the by-product pressure on the interface increases as the by-products continuously generated in the catalyst layer in contact with the diffusion layer accumulate. This pressure acts as a force to discharge the byproduct.

Therefore, the surface tension control can facilitate the desorption of the by-product from the surface of the diffusion layer, so that the carbon fiber surface inside the diffusion layer can be hydrophilic or hydrophobic to facilitate the discharge of the by-product. However, the method is cheaper and easier than the above method by easily reducing the minimum force, that is, critical pressure, required by the physical method to advance the interface formed in the diffusion layer, as in the method of the present invention. It can be achieved.

FIG. 2 is an enlarged surface photograph and a cross-sectional photograph of carbon paper forming a diffusion layer of a direct methanol fuel cell, and shows a structure in which irregularly arranged carbon fibers are stacked. The lattice structures produced by the carbon fibers that make up each layer are connected between layers, forming irregular and complex interconnected capillaries. Therefore, in the direct methanol fuel cell in which the carbon paper is used as the diffusion layer, the movement of the by-product is the shortest linear distance from the point of generation of the by-product on one side of the diffusion layer in contact with the catalyst layer to the discharge point on the opposite side of the diffusion layer exposed to the flow path side. It does not move to, but moves along a capillary tube with high tortuosity ( τ = L / C , where L is the length of the curve connecting two points, and C is the linear distance between the two points).

For example, when an interface of by-products is located in one layer (or several layers) of a layered structure, and the pressure on the by-product side increases due to continuous by-product generation, a force acts to force the interface to move to the next layer. Due to the structure, the lattice of the next layer acts as an obstacle to the movement of the interface. Therefore, the movement of the interface to the next layer can be performed by dividing the interface into two by the intersecting lattice, or by-products exiting only in one of the divided directions. It must be higher than usual, and in order for such a high pressure to be formed, a larger amount of by-products must be accumulated in the catalyst bed region, and thus, by-products are not discharged smoothly.

The present invention is to solve the problems of the prior art as described above, by forming a plurality of fine perforations in the carbon paper constituting the diffusion layer of the anode and the cathode to facilitate the discharge of the by-product, thereby maximizing the output of the battery A direct methanol fuel cell can be provided.

The present invention to achieve the above object,

A catalyst layer and a diffusion layer are sequentially arranged on one surface of the electrolyte membrane and the electrolyte membrane, a fuel electrode supplied with an aqueous methanol solution as a fuel, and a catalyst layer and a diffusion layer are sequentially arranged on the surface of the electrolyte membrane in the other direction, and an air electrode supplied with an oxidizing gas. A direct methanol fuel cell having an electrode unit comprising: a direct methanol fuel cell, wherein fine perforations are formed in the carbon paper constituting the diffusion layer.

Preferably, PTFE (polytetrafluoroethylene) is impregnated in the carbon paper constituting the diffusion layer, or a micro-porous layer (MPL) made of carbon or graphite powder is attached to one side of the carbon paper.

Fine perforation in the diffusion layer according to the present invention, after overlapping a number of stainless steel plate of 50 ~ 500㎛ thickness, forming a wedge by using a wire cutting method, between the respective wedge-shaped stainless plate between Inserting, aligning, and fixing the plate-shaped member to maintain a constant spacing is formed using a fabric mill to form a plurality of wedges at regular intervals, the wedge shape in the wedge-shaped stainless plate is a chemical etching method Can be used.

In addition, the present invention comprises the steps of preparing a plurality of sheets of stainless steel plate of a constant thickness; Forming a wedge shape at regular intervals on the overlapped stainless plate using a wire cutting method; Inserting and aligning a plate-shaped member having a constant thickness between the overlapping stainless plates to produce a fabric mill having a plurality of wedge shapes; And arranging the graphite sheet together with the carbon paper in the punching device, and forming a plurality of microperforations on the carbon paper at a time by a roll press method.

The wedge shape formed on the overlapping stainless plate may be formed using a wire cutting method or a chemical etching method.

According to the present invention, by forming fine perforations in the carbon paper constituting the diffusion layer in a direct methanol fuel cell, by-products are quickly discharged, thereby smoothly supplying fuel and oxidant, thereby maximizing output and efficiency of the cell. It works.

The present invention sequentially arranges the catalyst layer and the diffusion layer on one surface of the electrolyte membrane and the electrolyte membrane, arranges the anode in which the aqueous methanol solution is supplied as a fuel, and arranges the catalyst layer and the diffusion layer on the surface of the electrolyte membrane in the other direction, and supplies air. In a direct methanol fuel cell having an electrode unit including a cathode, the direct methanol fuel cell is characterized in that fine perforations are formed in the carbon paper constituting the two diffusion layers.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings. The accompanying drawings show exemplary forms of the present invention, which are provided to explain the present invention in more detail, and the technical scope of the present invention is not limited thereto.

The present invention is designed to reduce the minimum critical pressure required for the discharge of these by-products in the existing methanol fuel cell, the degree of bending in the vertical direction of the diffusion layer carbon paper stacked in an irregular shape to form a layered lattice structure τ = 1 By forming a large number of fine pores, the by-products move along the shortest distance from the formation point in contact with the catalyst layer to the discharge point in contact with the flow path, and also by removing the carbon fiber lattice structure that prevents the advance of the by-product interface along the perforated holes. The aim is to reduce the pressure needed to advance the by-products and ultimately increase the output and efficiency of the direct methanol fuel cell.

First, a process and a puncturing apparatus for forming a microperforation on the carbon paper constituting the diffusion layer will be described. Preferably, PTFE (polytetrafluoroethylene) is impregnated in the carbon paper constituting the diffusion layer, or a micro-porous layer (MPL) made of carbon or graphite powder is attached to one side of the carbon paper.

Regarding the size of the fine perforations to be performed in the diffusion layer, if the size is too large, the catalyst of the catalyst layer may be washed away, or the contact area between the catalyst layer and the diffusion layer may decrease, resulting in a drop in electrical conductivity. On the contrary, when the size of the perforation is about or smaller than the size of the carbon fiber lattice constituting the diffusion layer, the effect of the perforation is hardly shown. Therefore, the size of the microperforation is preferably several tens of micrometers or more and several hundred micrometers or less.

In addition, with respect to the number of fine perforations, the number should be large enough. This is because, if the number of perforations is small, the average distance from the by-product generation point of the catalyst layer to the perforation point increases, so that the effect of the critical pressure drop due to the perforation will not be so great. Of course, if the number of perforations is too large, it should be noted that the diffusion layer structure may collapse. Therefore, with respect to the size and number of fine perforations to be performed on the diffusion layer, it is necessary to sufficiently generate the above-described perforations over the entire area, and for this purpose, in the present invention, a plurality of perforations can be performed at a low cost. The method was devised.

In order to finely perforate the diffusion layer, a fabric mill was prepared as shown in FIG. 3. To this end, first, several sheets of 100 μm-thick stainless plates are overlapped, and then each wire is cut at a time by overlapping each stainless plate to a square plate of a desired size by using a wire cutting method, and then one side of the overlapping square plate The wedge shape at regular intervals along the surface is formed by a wire cutting method or a chemical etching method. Then, each plate is separated, and a plate having an appropriate thickness without a wedge process is inserted so that a predetermined gap is formed between the plate and the plate, and properly aligned, a perforation device as shown in FIG. 3 can be manufactured. As shown in Figure 3, in the punching device of the present invention, for proper alignment of each plate, two circular holes are formed in the same position in each plate, and alignment is made by inserting a circular rod through the inside. To lose.

4A to 4F illustrate a process of forming a microperforation on carbon paper using the punching mill of FIG. 3, and first stacking a carbon paper and a graphite sheet having an appropriate thickness on the punching device (FIG. 4A). , 4b, 4c), the circular roller was passed over the punching device while pressing the carbon paper and graphite sheet (Fig. 4d, 4e), finally to form a fine perforation on the carbon paper (Fig. 4f). The reason why the graphite sheet is used is that the graphite sheet deeply presses the carbon paper when the roller passes, so that a hole completely formed in the thickness direction in the carbon paper can be formed. There is also an advantage in that the size of the hole to be drilled can be adjusted as desired by appropriately adjusting the thickness of the graphite sheet and the height of the roller.

FIG. 5 is a photograph of a carbon paper diffused layer having fine perforations formed by using the above-described puncturing apparatus and the puncturing method, so that one hole is formed per area of about 0.3 mm ² , and the LCD monitor was photographed using a backlight. . As can be seen in FIG. 5, it can be seen that thousands of holes having a size of about 100 μm are completely formed in the thickness direction of the diffusion layer in the diffusion layer.

6 illustrates a direct methanol fuel cell using carbon paper punched by the above method as a diffusion layer between the anode and the cathode. In the conventional direct methanol fuel cell illustrated in FIG. 1, only the diffusion layers of the anode and the cathode are replaced with the diffusion layers in which the perforations are performed, and it can be seen that the present invention can be easily applied to the existing fuel cells.

Hereinafter, the present invention will be described in detail with reference to the following examples. However, the following examples are not limited to the present invention only, and the analysis of the product in the embodiments of the present invention is performed by the following method. did.

The electrolyte used in the experiment was Nafion 115, and PtRu / C (Johnson Matthey, HISPEC 12100, Pt loading 1.8 ~ 2mg / ㎠), Pt / C (HISPEC 13100, Pt loading 1.5 ~ 1.6 Mg / cm 2) was used as the active area (catalyst coated membrane) of 3.1 × 3.2 cm 2. By attaching the diffusion layer of carbon paper on both sides of the CCM, a MEA (membrane electrode assembly) is manufactured. In this experiment, ordinary carbon paper without perforation was used as the diffusion layer, and carbon paper with perforation was used. The case where it used as a diffusion layer was made into the comparison object. In the case of using the two different CCM for the above two cases can not be excluded the effect of the electrolyte and catalyst coating, the experiment was performed by changing only the diffusion layer for one CCM and compared the results. Toray 060 was used for the anode diffusion layer, and 25BC (SGL Carbon) containing 5 wt% polytetrafluoroethylene (PTFE) was attached to the cathode diffusion layer with a micro porous layer (MPL) attached to one side.

Next, a single cell was manufactured using the cathode and the anode diffusion layer on which the CCM and the perforation were performed, and the cell was made of a visible cell to observe the behavior of bubbles and droplets. Place the CCM, the gasket and the diffusion layer between two current collector plates with serpentine flow paths with a cross-section of 1 × 1 mm2, cover them with acrylic plates with a thickness of 3 cm, and combine them with a torque of 50-70 kg _f / cm using 6M screws. It was. The 5W film heater (Watlow Kapton heater) was attached to the current collector plate extended to the outside of the unit cell to allow the experiment in the elevated temperature state. Methanol aqueous solution of 1 mol concentration was used as a fuel, and was placed in a sealed container pressurized with nitrogen and then quantitatively supplied at a flow rate of 4.2 cc / min using a liquid MFC. Air was used as the oxidizing agent, and the MFC for gas was supplied quantitatively at a flow rate of 474 cc / min. Meanwhile, bubble and droplet behaviors that can be observed from the outside of the visualization unit were observed using a high resolution CCD camera.

As described above, only one CCM was used in comparing a single cell using a diffusion layer without perforation and a single diffusion cell using a perforation diffusion layer, and to exclude the effect of performance change with the wetting time of the fuel cell. During the same time, wetting was performed and then I (current) -V (voltage) measurements were performed. More specifically, OCV (open circuit voltage) measurement is performed simultaneously with the supply of fuel and air, and then operated for 30 minutes (at room temperature) to 1 hour (at elevated temperature) under I = 0.5A and then CC ( I - V measurements were performed in constant current mode.

7 and 8 show the output results of the fuel cell without diffusion layer drilling and the fuel cell with diffusion layer drilling measured through the above method. First, FIG. 7 is a result of measuring the temperature of a fuel cell by applying a constant voltage to the film heater attached to the current collector plate, and measuring the result. In the low current density region of 50 mA / cm ² or less, there is no significant difference. In the density region, it can be seen that the fuel cell subjected to the perforation shows a higher power density. As described above, since the production of by-products increases in proportion to the increase in current density, it shows that the rapid by-products are discharged through the perforation in the high current density region. Meanwhile, in the results of FIG. 7, the maximum output of the fuel cell that performed the puncture was increased by 21.3% compared to the fuel cell that did not perform the puncture.

Next, FIG. 8 shows the results of operating at room temperature. As shown in FIG. 7, it can be seen that the output of the fuel cell that performs the puncturing is higher as the result of the high current density region, and the maximum output is not performed. 3.8% increase over the fuel cell.

As described above, it can be seen that the direct methanol fuel cell having a perforation in the diffusion layer provides a higher output regardless of the operating temperature through the temperature raising and the room temperature experiments of the fuel cell having the perforation in the diffusion layer and the fuel cell not performing the diffusion layer. This is evidence that the present invention, which performs fine perforation in the diffusion layer to improve the output and efficiency of the fuel cell by smoothly discharging the by-products through the diffusion layer, is working effectively. As a result, by applying the present invention, it is possible to reduce mass transport polarization, which is a major factor in directly lowering the output of the methanol fuel cell, thereby improving the output of the fuel cell.

Although the present invention has been described with reference to one embodiment shown in the accompanying drawings, this is merely exemplary, and it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible. Could be. Accordingly, the true scope of protection of the present invention should be determined only by the appended claims.

1 is a cross-sectional view of a typical direct methanol fuel cell.

2 is a surface photograph and a cross-sectional photograph of carbon paper directly used as a diffusion layer of a methanol fuel cell. As shown in FIG. 2, a structure in which irregularly arranged carbon fibers are stacked is shown.

3 is a perspective view showing a perforation apparatus for forming a fine perforation on carbon paper.

4A to 4F are step views illustrating a process of forming micropores on carbon paper using the fabricator of FIG. 3.

FIG. 5 is a photograph of carbon paper that is punched using the punching device of FIG. 3 and the punching method of FIG. 4.

FIG. 6 is a cross-sectional view of a direct methanol fuel cell using carbon paper obtained by drilling using the drilling device of FIG. 3 and the drilling method of FIG. 4 as a diffusion layer of an anode and a cathode.

FIG. 7 illustrates a result of operating a single cell fabricated using the carbon paper obtained by using the drilling device of FIG. 3 and the drilling method of FIG. 4 as a diffusion layer of an anode and a cathode.

FIG. 8 illustrates a result of operating a single cell fabricated using carbon paper obtained by using the drilling device of FIG. 3 and the drilling method of FIG. 4 as a diffusion layer of a fuel electrode and an air electrode at room temperature.

Claims (8)

A catalyst layer and a diffusion layer are sequentially arranged on one surface of the electrolyte membrane and the electrolyte membrane, a fuel electrode supplied with an aqueous methanol solution as a fuel, and a catalyst layer and a diffusion layer are sequentially arranged on the surface of the electrolyte membrane in the other direction, and an air electrode supplied with an oxidizing gas. In a direct methanol fuel cell having an electrode unit comprising: Direct methanol fuel cell, characterized in that fine perforation is formed on the carbon paper constituting the diffusion layer. The method according to claim 1, A direct methanol fuel cell, wherein the carbon paper constituting the diffusion layer is impregnated with PTFE (polytetrafluoroethylene) and fine pores are formed. The method according to claim 1, A direct methanol fuel cell, characterized in that a micro-porous layer (MPL) made of carbon or graphite powder is attached to one surface of the carbon paper constituting the diffusion layer, and micropores are formed. The method according to claim 1, In performing the fine perforation on the diffusion layer, a plurality of 50 ~ 500㎛ thick stainless plate is overlapped, and then a wedge shape is formed by a wire cutting method, and the wedge shape is formed between each of the stainless plate formed. Inserting a plate member to maintain a predetermined interval, and by forming and aligning by fixing a plurality of wedge shape by using a milling machine to be formed at a predetermined interval to perform a perforation in the diffusion layer, and includes the perforated diffusion layer Direct methanol fuel cell, characterized in that. The method of claim 4, wherein In order to provide the wedge-shaped silver in the wedge-shaped stainless plate, by forming a plurality of wedges by using a chemical etching method, after fabricating the fabricator in the same manner as in claim 4, and comprises a diffusion layer for performing the perforation Direct methanol fuel cell, characterized in that. Preparing a plurality of sheets of stainless steel plate of constant thickness; Forming a wedge shape at regular intervals on the overlapped stainless plate using a wire cutting method; Inserting and aligning a plate-shaped member having a constant thickness between the overlapping stainless plates to produce a fabric mill having a plurality of wedge shapes; And The method of manufacturing a carbon paper having a fine perforation comprising the step of superimposing the graphite sheet with carbon paper in the perforating device, and forming a plurality of microperforations on the carbon paper at a time by a roll press method. The method according to claim 6, The wedge shape formed on the overlapping stainless plate is a carbon paper manufacturing method having a fine perforation, characterized in that using a wire cutting method. The method according to claim 6, The wedge shape formed on the overlapping stainless plate is a carbon paper manufacturing method having a fine perforation, characterized in that using a chemical etching method.

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