Method for manufacturing polymer electrolyte membrane fuel cell

Abstract

The present invention relates to a method for manufacturing a polymer electrolyte membrane fuel cell, the method comprising: preparing an intermediate sheet comprising a fibrous carbon material; obtaining a carbon sheet by performing at least one of heat treatment or acid treatment on the intermediate sheet; and manufacturing a unit cell including the electrolyte membrane. The electrodes are located on the first surface and the second surface of the electrolyte membrane, the gas diffusion layer is located on the electrodes, and the carbon sheet is sandwiched between the electrodes and the gas diffusion layer. The electrochemical performance of a polymer electrolyte membrane fuel cell is improved by removing impurities such as Fe particles and amorphous carbon from a carbon sheet via heat treatment or acid treatment.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a polymer electrolyte membrane fuel cell. More specifically, the present invention relates to a method for manufacturing a polymer electrolyte membrane fuel cell having improved electrochemical performance by removing impurities such as Fe particles and amorphous carbon from carbon sheets via heat treatment or acid treatment. Background Art

[0002] A fuel cell is a power generation system that generates electric power through an electrochemical reaction between hydrogen and oxygen. Depending on the type of electrolyte used, fuel cells can be classified into phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte membrane fuel cells, and alkaline fuel cells. The basic working principle of these fuel cells is the same, but the types of fuels used, operating temperatures, catalysts, electrolytes, etc. of these fuel cells are all different.

[0003] Among them, compared with other fuel cells, the polymer electrolyte membrane fuel cell (PEMFC) has significant high-output characteristics, a lower operating temperature, a shorter start-up time, and a fast response to load changes. In addition to these characteristics, the polymer electrolyte membrane fuel cell (PEMFC) also has the advantages of being able to generate various ranges of output and having a wide range of applications, that is, it can be used as a portable power source for portable electronic devices, for example, or a transportation power source for electric vehicles, for example, and a distributed power source for fixed power plants in houses and public buildings, for example.

[0004] The polymer electrolyte membrane fuel cell (PEMFC) is used in the form of a stack obtained by stacking and assembling dozens to hundreds of unit cells to meet the required output level. The unit cell includes bipolar plates (i.e., separators), gas diffusion layers (GDLs), electrodes (i.e., an anode and a cathode), and a polymer electrolyte membrane (i.e., a proton exchange membrane), and an assembly obtained by attaching two electrodes to the polymer electrolyte membrane is called a membrane electrode assembly (MEA). The composition and performance of such an MEA can be said to be the core of the polymer electrolyte membrane fuel cell.

[0005] As shown in the following reaction formula [1], in the electrochemical reaction of the fuel cell, hydrogen supplied to the anode as the oxidation electrode of the fuel cell is separated into protons and electrons through the hydrogen oxidation reaction (HOR), and then the protons migrate through the membrane to the cathode as the reduction electrode, and the electrons migrate to the cathode through an external circuit. The protons and electrons react with oxygen supplied from the outside through the oxygen reduction reaction (ORR) at the cathode, thereby generating electric power and heat energy, and simultaneously generating water as a reaction by-product. H2 → 2H + + 2e - ,E o = 0.000 V (versus SHE)[1]. 1 / 2O 2 + 2H + + 2e - → H2O, E o = 1.229 V (versus SHE)[2].

[0006] Here, E o represents the standard electrode potential, and SHE represents the standard hydrogen electrode.

[0007] Each electrode (i.e., the negative electrode and the positive electrode) includes a catalyst layer, and each GDL includes a microporous layer containing carbon particles and a backing layer containing carbon fibers. Since the structure of the microporous layer in contact with the catalyst layer has a particle porous property similar to the structure of the catalyst layer, it is difficult to increase the electrochemical active area of the fuel cell.

[0008] In addition, the backing layer in contact with the bipolar plate has a porous property (wherein the carbon fibers are irregularly arranged), so that the products obtained during the charge and discharge process can be easily discharged, and this structural property results in uneven contact between the bipolar plate and the GDL, so it may have an adverse effect on the performance and durability of the fuel cell.

[0009] The above information disclosed in this background art section is only used to enhance the understanding of the background of the present invention, so it may include information that does not constitute the prior art already known to those of ordinary skill in the art in the country. SUMMARY OF THE INVENTION

[0010] To solve the above problems, a method of inserting a carbon nanotube sheet between the electrode and the gas diffusion layer or inserting a carbon nanotube sheet between the gas diffusion layers is disclosed. Chemical vapor deposition, particularly the direct spinning method, is used to synthesize the carbon nanotube sheet over a large area, and since an Fe-containing catalyst (ferrocene, C 10 H 10 Fe) is used in this process, Fe particles may remain in the carbon nanotube sheet.

[0011] When Fe particles remain in the carbon nanotube sheet, during the process of manufacturing the membrane electrode assembly by hot pressing or during the charge and discharge process of the fuel cell, the Fe particles are ionized to form free radicals, and the ionomer in the electrode or the electrolyte membrane may undergo chemical degradation.

[0012] The present invention aims to solve the above problems related to the prior art, and an object of the present invention is to remove Fe particles from a sheet by subjecting the sheet including fibrous carbon materials such as carbon nanotubes to a specified post-treatment.

[0013] Another object of the present invention is to remove impurities such as amorphous carbon remaining in the sheet by subjecting the sheet including fibrous carbon materials such as carbon nanotubes to a specified post-treatment.

[0014] On the one hand, the present invention provides a method for manufacturing a polymer electrolyte membrane fuel cell, the method comprising: preparing an intermediate sheet including fibrous carbon materials; obtaining a carbon sheet by performing at least one of heat treatment or acid treatment on the intermediate sheet; and manufacturing a unit cell including an electrolyte membrane, electrodes on two surfaces of the electrolyte membrane, a gas diffusion layer on the electrodes, and a carbon sheet sandwiched between the electrodes and the gas diffusion layer.

[0015] In a preferred embodiment, the preparation of the intermediate sheet can be carried out by a direct spinning method.

[0016] In another preferred embodiment, the fibrous carbon materials may include one selected from the group consisting of carbon nanofibers, carbon nanotubes, vapor-grown carbon fibers, and combinations thereof. fibers, carbon nanotubes, vapor-grown carbon fibers, and combinations thereof.

[0017] In another preferred embodiment, the porosity of the carbon sheet can be greater than or equal to the porosity of the electrodes and less than or equal to the porosity of the gas diffusion layer.

[0018] In another preferred embodiment, the porosity of the carbon sheet can be 20% to 40%. In addition, the average diameter of the pores in the carbon sheet can be 5 nm to 50 nm.

[0019] In another preferred embodiment, as a result of the analysis of the carbon sheet using Raman spectroscopy, the peak intensity I of the G band G and the peak intensity I of the D band D of the ratio I G / I D can be 6.5 or more.

[0020] In another preferred embodiment, as a result of the thermogravimetric analysis of the carbon sheet, the amount of impurities remaining in the carbon sheet can be less than 14% by weight. In particular, the amount of impurities remaining in the carbon sheet can be less than 3% by weight.

[0021] In another preferred embodiment, the heat treatment can be carried out in a temperature range greater than 450 °C but less than 600 °C.

[0022] In another preferred embodiment, the acid treatment can be carried out using a strong acid.

[0023] In another preferred embodiment, the heat treatment or acid treatment can be carried out for 5 minutes to 30 minutes.

[0024] In another aspect, the present invention provides a method for manufacturing a polymer electrolyte membrane fuel cell, the method comprising: preparing an intermediate sheet including a fibrous carbon material; obtaining a carbon sheet by performing at least one of a heat treatment or an acid treatment on the intermediate sheet; and manufacturing a unit cell including an electrolyte membrane, electrodes on two surfaces of the electrolyte membrane, a gas diffusion layer on the electrodes, a bipolar plate on the gas diffusion layer, and a carbon sheet sandwiched between the gas diffusion layer and the bipolar plate.

[0025] In a preferred embodiment, the preparation of the intermediate sheet can be carried out using a direct spinning method.

[0026] In another preferred embodiment, the porosity of the carbon sheet can be greater than or equal to the porosity of the gas diffusion layer but less than or equal to the porosity of the bipolar plate.

[0027] In another preferred embodiment, as a result of the analysis of the carbon sheet using Raman spectroscopy, the peak intensity I G of the G band and the peak intensity I D of the D band G The ratio I D / I

[0028] In another preferred embodiment, as a result of the thermogravimetric analysis of the carbon sheet, the amount of impurities remaining in the carbon sheet can be less than 14% by weight. In particular, the amount of impurities remaining in the carbon sheet can be less than 3% by weight.

[0029] In another preferred embodiment, the acid treatment can be carried out using a strong acid.

[0030] In another preferred embodiment, the method can further include performing a waterrepellent treatment on the carbon sheet.

[0031] Other aspects and preferred embodiments of the present invention are discussed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments of the present invention shown in the accompanying drawings, which are given by way of illustration only and thus do not limit the present invention, wherein:

[0033] Figure 1 Showing a schematic cross-sectional view of a polymer electrolyte membrane fuel cell according to a first embodiment of the present invention;

[0034] Figure 2Shows a view schematically depicting the process of synthesizing carbon nanotube fibers using the direct spinning method;

[0035] Figure 3 Shows a schematic cross-sectional view of a polymer electrolyte membrane fuel cell according to a second embodiment of the present invention;

[0036] Figure 4 Shows the results of a scanning electron microscope (SEM) analysis of a carbon sheet that has not undergone the heat treatment and acid treatment according to the present invention;

[0037] Figure 5 Shows the results of a scanning electron microscope (SEM) analysis of a carbon sheet that has undergone the heat treatment and acid treatment according to the present invention;

[0038] Figure 6 Shows the results of Raman spectroscopic analysis of a carbon sheet that has undergone the heat treatment and acid treatment according to the present invention and a carbon sheet that has not undergone the heat treatment and acid treatment according to the present invention;

[0039] Figure 7 Shows the Raman spectroscopic results of a carbon sheet that has only undergone heat treatment;

[0040] Figure 8 Shows the thermogravimetric analysis results of carbon sheets according to Production Example 1 and Comparative Production Example 1;

[0041] Figure 9 Shows the results of evaluating the electrochemical performance of a fuel cell obtained by sandwiching carbon sheets according to Production Example and Comparative Production Example, which were manufactured under the condition of 100% relative humidity, between a gas diffusion layer and an electrode;

[0042] Figure 10 Shows the results of evaluating the electrochemical performance of a fuel cell obtained by sandwiching carbon sheets according to Production Example and Comparative Production Example, which were manufactured under the condition of 40% relative humidity, between a gas diffusion layer and an electrode;

[0043] Figure 11 Shows Figure 9 the impedance spectrum analysis results of a fuel cell;

[0044] Figure 12 Shows Figure 10 the impedance spectrum analysis results of a fuel cell;

[0045] Figure 13 Shows the results of evaluating the electrochemical performance of a fuel cell obtained by sandwiching carbon sheets according to Production Example and Comparative Production Example, which were manufactured under the condition of 100% relative humidity, between a gas diffusion layer and a bipolar plate;

[0046] Figure 14Shows the evaluation results of the electrochemical performance of a fuel cell obtained by sandwiching carbon paper clips according to production examples and comparative production examples, which are manufactured under conditions of a relative humidity of 40%, between a gas diffusion layer and a bipolar plate;

[0047] Figure 15 Shows Figure 13 the results of impedance spectrum analysis of the fuel cell;

[0048] Figure 16 Shows Figure 14 the results of impedance spectrum analysis of the fuel cell.

[0049] It should be understood that the drawings are not necessarily drawn to scale, but rather represent various preferred features illustrating the basic principles of the present invention in a somewhat simplified manner. Specific design features of the present invention disclosed herein, including, for example, specific dimensions, orientations, positions, and shapes, will be determined in part by the particular intended application and use environment.

[0050] In the figures, reference numerals refer to the same or equivalent parts of the present invention in several views of the drawings. Detailed Description

[0051] The above objects, other objects, advantages, and features of the present invention will become apparent from the following description of embodiments given with reference to the drawings. However, the present invention is not limited to the embodiments disclosed herein, but may be implemented in various different forms. These embodiments are provided to make the description of the present invention more thorough and to fully convey the scope of the present invention to those skilled in the art.

[0052] In the following description of embodiments, terms such as "including", "comprising", and "having" should be construed as indicating the presence of the features, numbers, steps, operations, elements, or components described in the description, or combinations thereof, and do not exclude the presence of one or more other features, numbers, steps, operations, elements, components, or combinations thereof, or the possibility of adding one or more other features, numbers, steps, operations, elements, components, or combinations thereof. Additionally, it can be understood that when a component such as a layer, film, region, or plate is referred to as being "on" another component, the component can be "directly" on the other component or other components can be interposed between the two components. Similarly, it can be understood that when a component such as a layer, film, region, or plate is referred to as being "under" another component, the component can be "directly" under the other component or other components can be interposed between the two components.

[0053] All numbers, numerical values, and / or expressions representing amounts of components, reaction conditions, polymer compositions, and mixtures used in the description are approximate values, which reflect the various uncertainties in measurements when these values are obtained from essentially different things. Thus, it is understood that they are all modified by the term "about" unless otherwise specified. Additionally, it is understood that if a numerical range is disclosed in the description, unless otherwise specified, the range includes all continuous values from the minimum value to the maximum value of the range. Further, if the range refers to integers, unless otherwise specified, the range includes all integers from the smallest integer to the largest integer.

[0054] Figure 1 is a schematic cross-sectional view of a polymer electrolyte membrane fuel cell according to a first embodiment of the present invention. Referring to Figure 1 , a method of manufacturing a polymer electrolyte membrane fuel cell is provided, the method including: preparing an intermediate sheet including a fibrous carbon material; obtaining carbon sheets 40 and 40' by performing at least one of heat treatment or acid treatment on the intermediate sheet; and manufacturing unit cells, where each unit cell includes an electrolyte membrane 10, electrodes 20 and 20' located on two surfaces of the electrolyte membrane 10, gas diffusion layers 30 and 30' located on the electrodes 20 and 20', and carbon sheets 40 and 40' (hereinafter commonly denoted by reference numeral 40) interposed between the electrodes 20 and 20' (hereinafter commonly denoted by reference numeral 20) and the gas diffusion layers 30 and 30'. Here, each unit cell may further include a bipolar plate (not shown) located on the gas diffusion layers 30 and 30' (hereinafter commonly denoted by reference numeral 30).

[0055] Hereinafter, "two surfaces" of any structure may be expressed as "one surface and the other surface" or "the first surface and the second surface".

[0056] First, the electrolyte membrane 10 may include a proton-conductive polymer electrolyte membrane used in a general unit cell of a fuel cell. The electrolyte membrane can be roughly classified into an acidic type and a basic type. The acidic type electrolyte membrane has durability and can resist free radicals generated during the energy conversion process, such as hydroperoxyl radicals (HOO). In particular, a perfluorosulfonic acid-based electrolyte membrane formed by TM (manufactured by DuPont) can be used. The main chain of this electrolyte membrane has a perfluoro group, and the side chain has a sulfonic acid group.

[0057] One of the electrodes 20 and 20' located on one surface of the electrolyte membrane 10 can be the positive electrode corresponding to the air electrode, while the other of the electrodes 20 and 20' located on the other surface of the electrolyte membrane 10 can be the negative electrode corresponding to the fuel electrode. The electrode 20 can include a catalyst, and the catalyst can include, for example, platinum (Pt) alone, or can include an alloy of platinum (Pt) with a metal selected from the group consisting of ruthenium (Ru), osmium (Os), chromium (Cr), nickel (Ni), manganese (Mg), cobalt (Co), and combinations thereof. In addition, the catalyst can also be loaded on a carbon material.

[0058] The gas diffusion layer 30 is an element that is generally formed on the surface of the electrode 20 forming the membrane electrode assembly, allows the reaction gas flowing through the bipolar plate to pass through, and distributes the reaction gas to the electrolyte membrane 10. The gas diffusion layer 30 includes a microporous layer containing carbon particles and a substrate layer containing carbon fibers. The microporous layer can be arranged to face the electrode 20, and the substrate layer can be arranged to face the bipolar plate.

[0059] Here, the microporous layer can include particulate porous carbon particles, such as carbon black, acetylene black carbon, or carbon powder of BlackPearls. In addition, a mixture of a polytetrafluoroethylene (PTFE)-based hydrophobic agent and carbon powder can also be used.

[0060] The substrate layer can include irregularly arranged carbon fibers and can have an irregular fibrous porous structure. In addition, the substrate layer can also include a polytetrafluoroethylene (PTFE)-based hydrophobic agent. The substrate layer can include, for example, carbon fiber cloth, carbon fiber felt, carbon fiber paper, etc.

[0061] The fibrous carbon material contained in the interlayer can be a carbon material having a fibrous porous structure, such as including one selected from the group consisting of carbon nanofibers, carbon nanotubes, vapor-grown carbon fibers, and combinations thereof.

[0062] Figure 2 is a view schematically depicting the process of synthesizing carbon nanotube fibers using the direct spinning method. Referring to Figure 2 , the interlayer can be prepared by the direct spinning method. The direct spinning method is a method for producing fibers using carbon nanotubes and is mainly used for synthesizing fibrous carbon materials because carbon nanotube fibers can be produced by the direct spinning method.

[0063] The direct spinning method can represent a process for synthesizing a carbon nanotube aerogel by injecting a precursor solution including a fibrous carbon material, a catalyst, a promoter, and a carrier gas into an electric furnace of a vertical chemical vapor deposition (CVD) apparatus at a constant rate, and then obtaining a fibrous carbon material processed from the carbon nanotube aerogel in a fibrous form by a winding roll. Here, the fibrous carbon material can be obtained in a sheet form.

[0064] In the process of obtaining fibrous carbon materials by the direct spinning method, fibrous carbon materials with various morphologies, thicknesses, and porosities can be synthesized by adjusting the raw materials of the fibrous carbon materials, the temperature of the electric furnace, the winding speed of the winding roller, etc.

[0065] The raw materials of the fibrous carbon materials may include at least one selected from the group consisting of C2 to C10 saturated and unsaturated hydrocarbons, alcohols, and ketones. The catalyst may include at least one selected from the group consisting of ferrocene, copper (Cu), chromium (Cr), molybdenum (Mo), tungsten (W), iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt), and preferably includes ferrocene. The promoter may be one of thiophene and carbon disulfide. The carrier gas is not limited to a specific gas, as long as it is a gas used for synthesizing fibrous carbon materials by CVD and the direct spinning method. For example, hydrogen (H 2 ) can be used.

[0066] The temperature of the electric furnace is not limited to a specific temperature. For example, it can be 1,250 °C to 1,450 °C.

[0067] In addition, in the process of obtaining fibrous carbon materials by the direct spinning method, when the winding speed of the winding roller is increased, the porosity of the fibrous carbon materials can be increased. The winding speed of the winding roller is not limited to a specific speed. For example, it can be 6 m / min to 10 m / min.

[0068] The thickness of the intermediate sheet synthesized by the direct spinning method can be 5 μm to 40 μm.

[0069] Although in the present invention, the intermediate sheet including the fibrous carbon material is prepared by the direct spinning method, any intermediate sheet that needs to remove impurities such as iron particles or amorphous carbon remaining therein can be applied without particular limitation.

[0070] After preparing the intermediate sheet including the fibrous carbon material by this process, at least one of heat treatment and acid treatment can be performed on the intermediate sheet. Preferably, both heat treatment and acid treatment are performed.

[0071] Heat treatment is a process of improving the crystallinity and porosity of the carbon sheet 40 and improving the conductivity of the carbon sheet 40 by removing iron particles remaining in the intermediate sheet and suppressing the generation of amorphous carbon.

[0072] In one embodiment, the heat treatment can be carried out in a temperature range higher than 450 °C but lower than 600 °C. When the heat treatment temperature is below 450 °C, the temperature is too low, so amorphous carbon may not be completely removed. When the heat treatment temperature is 600 °C or higher, the intermediate sheet including the fibrous carbon material starts to burn (or oxidize), and the porous structure is damaged, so the carbon sheet 40 may not be able to function as a gas diffuser.

[0073] In addition, the heat treatment can be carried out for 5 minutes to 30 minutes. When the heat treatment time is 5 minutes or shorter, the heat treatment time is too short, so amorphous carbon may not be completely removed. When the heat treatment time exceeds 30 minutes, the intermediate sheet starts to burn locally, so the crystallinity of the fibrous carbon material may deteriorate, and the conductivity of the fibrous carbon material may decrease.

[0074] Furthermore, the heating rate to reach the heat treatment temperature can be 5 °C / minute to 15 °C / minute, preferably 10 °C / minute. When the heating rate is less than 5 °C / minute, it takes a long time to reach the heat treatment temperature, so the processability may be reduced, and the heat absorption may increase, which may cause physical damage to the intermediate sheet. In addition, when the heating rate exceeds 15 °C / minute, rapid thermal deformation occurs, which may lead to deformation of the microstructure of the intermediate sheet.

[0075] The acid treatment is a process of preventing the oxidation of the Fe particles remaining in the intermediate sheet and having an adverse effect on the chemical durability of the electrolyte membrane 10 by removing them.

[0076] In one embodiment, a strong acid can be used for the acid treatment. A strong acid refers to an acid that is mostly ionized in an aqueous solution and has a pK a value of 0 or less. For example, it can include hydrochloric acid (HCl), nitric acid (HNO 3 ), sulfuric acid (H 2 SO 4 ), hydrobromic acid (HBr), hydroiodic acid (HI), chloric acid (HClO 3 ), perchloric acid (HClO 4 ), etc.

[0077] When the acid treatment is carried out with a weak acid having a pK a value that is a positive value less than the pK a value of water (i.e., 15.74), the Fe particles may not be completely removed. In addition, when an acid stronger than sulfuric acid is used for the acid treatment in an aqueous solution, the intermediate sheet will dissolve locally, and the crystallinity of the fibrous carbon material will deteriorate, so the chemical and electrical properties of the carbon sheet 40 may deteriorate.

[0078] In addition, the acid treatment can be carried out for 5 minutes to 30 minutes. When the acid treatment time is 5 minutes or shorter, the acid treatment time is too short, so the Fe particles may not be completely removed. When the acid treatment time exceeds 30 minutes, the intermediate sheet starts to dissolve locally, so the crystallinity of the fibrous carbon material may deteriorate and the chemical and electrical properties of the carbon sheet 40 may deteriorate.

[0079] In addition, the acid treatment can be carried out in a temperature range lower than the boiling point of the acid used. Preferably, the acid treatment can be carried out in a temperature range similar to but not exceeding the boiling point of the acid.

[0080] Since the acid treatment is carried out in a temperature range lower than the boiling point of the acid used, the acid treatment is carried out in the gas phase, so the acid can effectively penetrate into the intermediate sheet with fibrous porosity, thereby removing the Fe particles.

[0081] Therefore, the carbon sheet 40 can be obtained by performing at least one of heat treatment and acid treatment on the intermediate sheet including the fibrous carbon material (preferably both heat treatment and acid treatment are performed).

[0082] The carbon sheet 40 obtained by the above process can be sandwiched between the electrode 20 and the gas diffusion layer 30. Here, the carbon sheet 40 can be sandwiched between the electrode 20 and the gas diffusion layer 30 without any additional bonding treatment.

[0083] The unit cell of a conventional polymer electrolyte membrane fuel cell includes a gas diffusion layer formed on the electrode, and the microporous layer structure in the gas diffusion layer in contact with the electrode exhibits particulate porous characteristics similar to the electrode structure. Since the particulate porous structure formed by the aggregation of carbon particles is affected by the size of the carbon particles, and the size of the carbon particles is smaller than the size of the fibrous carbon material particles, the porosity of the particulate porous structure may be lower than the porosity of the fibrous porous structure formed by the aggregation of the fibrous carbon material particles. Therefore, there is a problem that it is difficult to increase the electrochemical active area of the fuel cell in the unit cell of the conventional polymer electrolyte membrane fuel cell.

[0084] The carbon sheet 40 includes a fibrous carbon material and has a fibrous porous structure different from the particulate porous structure. The carbon sheet 40 having a fibrous porous structure has a very low in-plane resistance (for example, 1 Ω / cm or lower), so it can compensate for the high in-plane resistance of the electrode 20 (for example, 100 Ω / cm or higher), thereby improving the electrical connectivity between the catalyst particles included in the electrode 20. Therefore, the electrochemical active area can be increased, and the increased electrochemical active area can be confirmed as a lower charge transfer resistance.

[0085] At the positive electrode of the fuel cell, water can be produced as a reaction by-product through the oxygen reduction reaction (ORR). The water can be discharged through the electrode 20, the gas diffusion layer 30, and the bipolar plate.

[0086] To more smoothly discharge the water generated at the electrode 20, the capillary pressure should decrease from the electrode 20 toward the bipolar plate. Since the capillary pressure is inversely proportional to the porosity or the pore size, the porosity of each component in the unit cell can increase in the direction from the electrode 20 to the bipolar plate.

[0087] The carbon sheet 40 is sandwiched between the electrode 20 and the gas diffusion layer 30. Therefore, the porosity of the carbon sheet 40 can be greater than or equal to the porosity of the electrode 20, but can be less than or equal to the porosity of the gas diffusion layer 30.

[0088] Specifically, the porosity of the carbon sheet 40 can be 20% to 40%. When the porosity of the carbon sheet 40 is less than 20%, the porosity of the carbon sheet 40 can be less than the porosity of the electrode 20. When the porosity of the carbon sheet 40 exceeds 40%, the porosity of the carbon sheet 40 can be greater than the porosity of the gas diffusion layer 30. When the porosity of the carbon sheet 40 is outside the above range, the water generated at the electrode 20 may not be discharged smoothly.

[0089] In addition, in one embodiment, the average diameter of the pores in the carbon sheet 40 can be 5 nm to 50 nm. Preferably, the average diameter of the pores in the carbon sheet 40 can be 35 nm to 50 nm. When the average diameter of the pores in the carbon sheet 40 exceeds 50 nm, the water may not be discharged smoothly.

[0090] In one embodiment, as a result of analyzing the carbon sheet 40 using Raman spectroscopy, the ratio I G of the peak intensity I D of the G band to the peak intensity I G of the D band D can be 6.5 or more.

[0091] The G band represents a peak that appears at around 1580 cm -1 . The G band is observed in carbon materials (such as graphite or carbon nanotubes) having a hexagonal lattice of SP 2 -bonded carbon as a basic configuration unit and may indicate the presence of fibrous carbon materials. The D band represents a peak that appears at around 1350 cm -1 . The D band is a peak caused by crystal defects and may indicate the presence of amorphous carbon with low crystallinity.

[0092] When the untreated carbon sheet is heat-treated according to the present invention, the amorphous carbon is removed, and the peak intensity IG The ratio I of the peak intensity I of the D band D to G / I D can be increased.

[0093] In addition, when the carbon sheet is subjected to the acid treatment according to the present invention, the crystallinity of the fibrous carbon material in the carbon sheet can still be maintained even after the acid treatment. Therefore, it can be observed that the amorphous carbon ratio in the carbon sheet subjected to the acid treatment according to the present invention is at a similar level to the amorphous carbon ratio in the carbon sheet not subjected to the acid treatment.

[0094] When the ratio I of the peak intensity I of the G band of the carbon sheet 40 G to the peak intensity I of the D band D is less than 6.5, the growth of the amorphous carbon present in the carbon sheet 40 may not be completely suppressed. The upper limit of the ratio I of the peak intensity I of the G band of the carbon sheet 40 G / I D to the peak intensity I of the D band G is not particularly limited, and can be, for example, 11 or less. D G D / I D The upper limit of the ratio I of the peak intensity I of the G band of the carbon sheet 40

[0095] In one embodiment, as a result of the thermogravimetric analysis of the carbon sheet 40, the amount of impurities remaining in the carbon sheet 40 can be less than 14% by weight. Preferably, the amount of impurities remaining in the carbon sheet 40 can be less than 3% by weight.

[0096] When the amount of impurities remaining in the carbon sheet 40 is 14% by weight or more, it can be understood that the Fe particles remaining in the carbon sheet 40 have not been completely removed by heat treatment or acid treatment, but are oxidized in the high temperature range to generate impurities.

[0097] Figure 3 is a schematic cross-sectional view of a polymer electrolyte membrane fuel cell according to the second embodiment of the present invention. Refer to Figure 3, a method for manufacturing a polymer electrolyte membrane fuel cell is provided, the method comprising: preparing an intermediate sheet including a fibrous carbon material; obtaining carbon sheets 40 and 40' by performing at least one of heat treatment or acid treatment on the intermediate sheet; and manufacturing unit cells, where each unit cell includes an electrolyte membrane 10, electrodes 20 and 20' on two surfaces of the electrolyte membrane 10, gas diffusion layers 30 and 30' on the electrodes 20 and 20' (hereinafter commonly denoted by reference numeral 20), bipolar plates 50 and 50' on the gas diffusion layers 30 and 30', and carbon sheets 40 and 40' (hereinafter commonly denoted by reference numeral 40) between the gas diffusion layers 30 and 30' (hereinafter commonly denoted by reference numeral 30) and the bipolar plates 50 and 50' (hereinafter commonly denoted by reference numeral 50).

[0098] The method for manufacturing a polymer electrolyte membrane fuel cell according to the second embodiment is substantially the same as the method for manufacturing a polymer electrolyte membrane fuel cell according to the first embodiment, except that the carbon sheet 40 is between the gas diffusion layer 30 and the bipolar plate 50, and thus the description of redundant elements will be omitted.

[0099] The structure of the bipolar plate 50 is the same as that of the bipolar plate used in a general unit cell of a fuel cell. The bipolar plate 50 may have flow channels for supplying reaction gases such as hydrogen and air to the outside of the gas diffusion layer 30 and discharging the water generated by the reaction.

[0100] The carbon sheet 40 obtained through the above process can be sandwiched between the gas diffusion layer 30 and the bipolar plate 50. Here, the carbon sheet 40 can be sandwiched between the gas diffusion layer 30 and the bipolar plate 50 without any additional bonding treatment.

[0101] The substrate layer of the gas diffusion layer 30 is formed into a porous structure in which carbon fibers are combined with a PTFE-based polymer resin and has a relatively high surface roughness (R z ) value (for example, 50 μm to 100 μm). Such a high surface roughness value of the gas diffusion layer 30 may cause uneven contact between the bipolar plate 50 and the gas diffusion layer 30, which may increase the ohmic resistance (R ohm ) of the unit cell and have an adverse effect on the electrochemical performance of the fuel cell.

[0102] Therefore, the surface roughness value of the carbon sheet 40 obtained by performing heat treatment and acid treatment on the intermediate sheet may be lower than the surface roughness value of the gas diffusion layer 30. For example, the surface roughness of the carbon sheet 40 can be 13 μm to 15 μm.

[0103] When a carbon sheet 40 having a relatively low surface roughness is sandwiched between the gas diffusion layer 30 and the bipolar plate 50 as in the second embodiment, non-uniform contact between the two can be improved, and the interfacial contact resistance can be reduced.

[0104] In order to more smoothly discharge the water generated at the electrode 20, the capillary pressure should be reduced in the direction from the electrode 20 to the bipolar plate 50. Since the capillary pressure is inversely proportional to the porosity or the pore size, the porosity of each element in the unit cell can be increased in the direction from the electrode 20 to the bipolar plate 50.

[0105] The carbon sheet 40 is sandwiched between the gas diffusion layer 30 and the bipolar plate 50, so the porosity of the carbon sheet 40 can be greater than or equal to the porosity of the gas diffusion layer 30, but can be less than or equal to the porosity of the bipolar plate 50.

[0106] Specifically, the porosity of the carbon plate 40 can be 20% to 40%. When the porosity of the carbon sheet 40 is less than 20%, the porosity of the carbon sheet 40 can be less than the porosity of the gas diffusion layer 30, and when the porosity of the carbon sheet 40 exceeds 40%, the porosity of the carbon sheet 40 can be greater than the porosity of the bipolar plate 50. When the porosity of the carbon sheet 40 exceeds the above range, the water generated at the electrode 20 may not be discharged smoothly.

[0107] In addition, the average diameter of the pores in the carbon sheet 40 can be 5 nm to 50 nm. Preferably, the average diameter of the pores in the carbon sheet 40 can be 35 nm to 50 nm. When the average diameter of the pores in the carbon sheet 40 exceeds 50 nm, the water may not be discharged smoothly.

[0108] In one embodiment, the method can further include performing a waterproof treatment on the carbon sheet 40. The material for the waterproof treatment can include a hydrophobic polymer, for example, at least one selected from the group consisting of polytetrafluoroethylene (PTFE), 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxolane-tetrafluoroethylene, fluorinated ethylene propylene (FEP), polyvinylidene fluoride (PVdF), and Fluorosarf (manufactured by Fluoro Technology).

[0109] Since an electrochemical reaction (oxidation or reduction) generates heat from the electrode 20, the interfacial temperature between the electrode 20 and the gas diffusion layer 30 may be higher than the interfacial temperature between the gas diffusion layer 30 and the bipolar plate 50. When the carbon sheet 40 is sandwiched between the gas diffusion layer 30 and the bipolar plate as in the second embodiment, the temperature between the two is lower than that in the first embodiment, so there is a high possibility that water exists in the liquid phase. The drainage performance can be improved by performing a waterproof treatment on the carbon sheet 40.

[0110] In the following, the present invention will be described in more detail by the following examples and comparative examples. The following examples and comparative examples are only used to exemplarily describe the present invention and are not intended to limit the scope and spirit of the present invention. Production Example 1. Relative humidity is 100%

[0111] A precursor solution was prepared by mixing ferrocene as a transition metal source for catalysis and thiophene as a promoter with ethanol as a raw material for the fibrous carbon material, and then sonicating for 2 hours.

[0112] The precursor solution and carrier gas (H 2 , 2,200 sccm) were injected into the electric furnace of a vertical CVD device at a constant rate, and carbon nanotube sheets were synthesized by the direct spinning method. Here, the winding speed of the roller for obtaining the carbon nanotube sheets was 6 m / min to 10 m / min, and the thickness of the synthesized carbon nanotube sheets was 15 μm.

[0113] To remove the residual Fe particles and amorphous carbon in the carbon nanotube sheets, the carbon nanotube sheets were heat-treated at a temperature of 550 °C for 30 minutes at a heating rate of 10 °C / min in an oxygen environment, and then the carbon nanotube sheets were immersed in HCl at a temperature of 80 °C for 30 minutes for acid treatment. The heat-treated and acid-treated carbon nanotube sheets were dried in a vacuum oven maintained at 120 °C for 12 hours to obtain carbon sheets. Here, the heat treatment and acid treatment were carried out under the condition of a relative humidity of 100%. Production Example 2. Relative humidity is 40%

[0114] Carbon sheets were manufactured by the same process as in Production Example 1, except that the heat treatment and acid treatment were carried out under the condition of a relative humidity of 40%. Comparative Production Example 1. Untreated (raw) carbon sheet

[0115] Carbon sheets were manufactured by the same process as in Production Example 1, except that the heat treatment and acid treatment were not carried out. Comparative Production Example 2. Relative humidity is 100%

[0116] Carbon sheets were manufactured by the same process as in Production Example 1, except that only the heat treatment among the heat treatment and acid treatment was carried out. Comparative Production Example 3. Relative humidity is 40%

[0117] Carbon sheets were manufactured by the same process as in Production Example 2, except that only the heat treatment among the heat treatment and acid treatment was carried out. Examples and Comparative Examples Example 1. First embodiment and relative humidity is 100%

[0118] A NAFION TM(Manufactured by DuPont) An electrolyte membrane, an electrode paste including a platinum-on-carbon catalyst, CNT paper as a gas diffusion layer, and a bipolar plate having flow channels formed therein. The prepared electrode paste was coated on both surfaces of the electrolyte membrane by spraying, and then the paste was dried to form electrodes. Carbon sheets prepared according to Production Example 1 were stacked on the corresponding electrodes, and then the gas diffusion layer and the bipolar plate having flow channels were sequentially stacked on the carbon sheets without performing a separate bonding process, thereby manufacturing a unit cell of a fuel cell. Example 2. First embodiment and relative humidity is 40%

[0119] By the same process as in Implementation Example 1, a unit cell of a fuel cell was manufactured, except that carbon sheets prepared according to Production Example 2 were stacked on the electrodes. Example 3. Second embodiment and relative humidity is 100%

[0120] An electrolyte membrane formed of NAFION TM (Manufactured by DuPont), an electrode paste including a platinum-on-carbon catalyst, CNT paper as a gas diffusion layer, and a bipolar plate having flow channels formed therein. The prepared electrode paste was coated on both surfaces of the electrolyte membrane by spraying, and then the paste was dried to form electrodes. The gas diffusion layer, carbon sheets prepared according to Production Example 1, and the bipolar plate having flow channels were sequentially stacked on the corresponding electrodes, thereby manufacturing a unit cell of a fuel cell. Example 4. Second embodiment and relative humidity is 40%

[0121] By the same process as in Implementation Example 3, a unit cell of a fuel cell was manufactured, except that carbon sheets according to Production Example 2 were stacked on the gas diffusion layer. Comparative Example 1. No carbon sheet is inserted

[0122] An electrolyte membrane formed of NAFION TM (Manufactured by DuPont), an electrode paste including a platinum-on-carbon catalyst, CNT paper as a gas diffusion layer, and a bipolar plate having flow channels formed therein. The prepared electrode paste was coated on both surfaces of the electrolyte membrane by spraying, and then the paste was dried to form electrodes. A unit cell of a fuel cell was manufactured by sequentially stacking the gas diffusion layer and the bipolar plate having flow channels on the electrodes. Comparative Example 2. First embodiment and untreated carbon sheet

[0123] A unit cell of a fuel cell was manufactured by the same process as in Example 1, except that carbon sheets according to Comparative Production Example 1 were stacked on the electrodes. Comparative Example 3. First embodiment, heat-treated carbon sheet and relative humidity is 100%

[0124] The unit cell of the fuel cell was fabricated by the same process as in Example 1, except that the carbon sheet according to Comparative Production Example 2 was stacked on the electrode. Comparative Example 4. First embodiment, heat-treated carbon sheet and relative humidity is 40%

[0125] The unit cell of the fuel cell was fabricated by the same process as in Example 1, except that the carbon sheet according to Comparative Production Example 3 was stacked on the electrode. Comparative Example 5. Second embodiment and untreated carbon sheet

[0126] The unit cell of the fuel cell was fabricated by the same process as in Example 3, except that the carbon sheet according to Comparative Production Example 1 was stacked on the gas diffusion layer. Comparative Example 6. Second embodiment, heat-treated carbon sheet and relative humidity is 100%

[0127] The unit cell of the fuel cell was fabricated by the same process as in Example 3, except that the carbon sheet according to Comparative Production Example 2 was stacked on the gas diffusion layer. Comparative Example 7. Second embodiment, heat-treated carbon sheet and relative humidity is 40%

[0128] The unit cell of the fuel cell was fabricated by the same process as in Example 3, except that the carbon sheet according to Comparative Production Example 3 was stacked on the gas diffusion layer. Test Example 1. Structural analysis of carbon sheet

[0129] The carbon sheets according to Comparative Production Example 1 and Production Example 1 were photographed with a scanning electron microscope (SEM), and the results are shown in Figure 4 and Figure 5 respectively.

[0130] Referring to Figure 4 , it can be seen that a large number of Fe particles are attached to the carbon nanotube fibers in the carbon sheet according to Comparative Production Example 1 without performing the heat treatment and acid treatment according to the present invention. Referring to Figure 5 , no Fe particles were observed in the carbon sheet according to Production Example 1 after performing the heat treatment and acid treatment according to the present invention.

[0131] In addition, as a result of image processing analysis, it was confirmed that the carbon nanotube fiber bundles in Figure 4 were reduced compared to Figure 5 . Therefore, it can be predicted that the porosity of the carbon sheet will be improved when the carbon sheet is subjected to heat treatment and acid treatment. Test Example 2. Composition analysis of carbon sheet

[0132] The carbon sheets prepared according to Comparative Production Example 1 and Production Example 1 were analyzed by Raman spectroscopy, and the results are shown in Figure 6 respectively. The carbon sheet according to Comparative Production Example 2 was analyzed by Raman spectroscopy, and the results are shown in Figure 5as shown

[0133] Referring to Figure 6 and Figure 7 , in all carbon flakes according to Comparative Production Example 1, Comparative Production Example 2, and Production Example 1, peaks were observed at around 1350 cm -1 for the D band and at around 1580 cm -1 for the G band. Here, the D band represents a peak caused by crystal defects, indicating the presence of amorphous carbon with low crystallinity, while the G band is observed in carbon materials (such as graphite or carbon nanotubes) with a hexagonal lattice of SP 2 bonded carbon as the basic configuration unit, indicating the presence of carbon nanotubes.

[0134] As Figure 6 and Figure 7 shown, the peak intensity I G of the G band of the carbon flakes according to Production Example 1 D and the peak intensity I G of the D band D gave a ratio I G / I D of 6.59, while the peak intensity I G of the G band and the peak intensity I D of the D band of the carbon flakes according to Comparative Production Example 1 and Comparative Production Example 2 gave ratios I

[0135] / I G of 9.04 and 10.36 respectively. D Since the ratio I G / I D of the peak intensity I

[0136] of the G band to the peak intensity I Figure 8 of the D band for the carbon flakes according to Comparative Production Example 1 and Production Example 1 was similar, it was confirmed that the growth of amorphous carbon was suppressed due to heat treatment and acid treatment of the carbon flakes.

[0137] In addition, thermogravimetric analysis was performed on the carbon flakes according to Comparative Production Example 1 and Production Example 1, and the results are as Figure 8 shown.

[0138] However, the weight of the carbon sheet according to Comparative Production Example 1 increased after about 750 °C and its slope increased, while the weight of the carbon sheet according to Production Example 1 hardly increased after about 750 °C and its slope was parallel. It is considered that the weight increase of the carbon sheet according to Comparative Production Example 1 is due to the oxidation of the residual Fe particles in the carbon sheet at about 750 °C or higher temperature, while the Fe particles in the carbon sheet according to Production Example 1 were removed by heat treatment and acid treatment.

[0139] In addition, the impurity content of the carbon sheet according to Production Example 1 was observed to be about 2.7 wt%, while the impurity content of the carbon sheet according to Comparative Production Example 1 was observed to be about 14 wt%. It can be confirmed that Fe particles and impurities caused by Fe particles were more successfully removed from the carbon sheet according to Production Example 1 where both acid treatment and heat treatment were carried out. Test Example 3. Performance evaluation of polymer electrolyte membrane fuel cell according to the first embodiment

[0140] In order to confirm the influence of the carbon sheet sandwiched between the gas diffusion layer and the electrode and the heat treatment and acid treatment of the carbon sheet under the condition of 100% relative humidity on the performance of the fuel cell including the carbon sheet, the electrochemical performances of the unit cell according to Example 1 (heat + acid), the unit cell according to Comparative Example 1 (CONV), the unit cell according to Comparative Example 2 (untreated), and the unit cell according to Comparative Example 3 (heat) were evaluated, and the results are as Figure 9 shown. The performance evaluation was carried out at a cell temperature of 60 °C.

[0141] Both the maximum current density and the power density of the unit cell according to Example 1 were superior to those of the unit cells according to Comparative Examples 1 to 3. In addition, both the maximum current density and the power density of the unit cell according to Comparative Example 3 where only the carbon nanotube sheet was heat-treated were superior to those of the unit cell according to Comparative Example 2 where no treatment was carried out and the unit cell according to Comparative Example 1 where no carbon sheet was used.

[0142] In Figure 9 it is expected that the electrochemical performance of the unit cell according to Comparative Example 2 including the carbon sheet is lower than that of the unit cell according to Comparative Example 1 not including the carbon sheet because the porosity of the carbon sheet is reduced due to the residual Fe particles and amorphous carbon in the carbon sheet, thus restricting the discharge of water generated during the charge and discharge of the fuel cell.

[0143] To confirm the effects of the carbon sheet sandwiched between the gas diffusion layer and the electrode, as well as heat treatment and acid treatment of the carbon sheet under a relative humidity of 40%, on the performance of a fuel cell including the carbon sheet, the electrochemical performances of the unit cells according to Example 2 (heat + acid), the unit cell according to Comparative Example 1 (CONV), the unit cell according to Comparative Example 2 (untreated), and the unit cell according to Comparative Example 4 (heat) were evaluated, and the results are as Figure 10 shown. The performance evaluation was carried out at a cell temperature of 80 °C.

[0144] Both the maximum current density and the power density of the unit cell according to Example 2 were superior to those of the unit cells according to Comparative Example 1, Comparative Example 2, and Comparative Example 4. In addition, both the maximum current density and the power density of the unit cell according to Comparative Example 4, in which only the carbon nanotube sheet was heat-treated, were superior to those of the unit cell according to Comparative Example 2, in which no treatment was carried out, and the unit cell according to Comparative Example 1, in which no carbon sheet was used.

[0145] To confirm the effects of the carbon sheet sandwiched between the gas diffusion layer and the electrode, as well as heat treatment and acid treatment of the carbon sheet under a relative humidity of 100%, on the electrochemical state of a fuel cell including the carbon sheet, the unit cells according to Example 1 (heat + acid), the unit cell according to Comparative Example 1 (CONV), the unit cell according to Comparative Example 2 (untreated), and the unit cell according to Comparative Example 3 (heat) were analyzed using impedance spectroscopy, and the results are as Figure 11 shown. The analysis using impedance spectroscopy was carried out at a cell temperature of 60 °C.

[0146] Referring to Figure 11 , it can be confirmed that, compared with the unit cells of Comparative Examples 1 to 3, both the ohmic resistance (R ohm ) and the charge transfer resistance (R ct ) of the unit cell according to Example 1, in which both heat treatment and acid treatment were performed on the carbon sheet, were improved.

[0147] To confirm the effects of the carbon sheet sandwiched between the gas diffusion layer and the electrode, as well as heat treatment and acid treatment of the carbon sheet under a relative humidity of 40%, on the electrochemical state of a fuel cell including the carbon sheet, the unit cells according to Example 2 (heat + acid), the unit cell according to Comparative Example 1 (CONV), the unit cell according to Comparative Example 2 (untreated), and the unit cell according to Comparative Example 4 (heat) were analyzed using impedance spectroscopy, and the results are as Figure 12 shown. The analysis using impedance spectroscopy was carried out at a cell temperature of 80 °C.

[0148] Referring to Figure 12, it can be confirmed that compared with the unit cells of Comparative Example 1, Comparative Example 2, and Comparative Example 4, the ohmic resistance (R ohm ) and charge transfer resistance (R ct ) of the unit cell according to Example 2, in which both heat treatment and acid treatment were performed on the carbon sheet, were improved. Test Example 4. Performance evaluation of polymer electrolyte membrane fuel cell according to the second embodiment

[0149] To confirm the effects of the carbon sheet sandwiched between the gas diffusion layer and the bipolar plate and the heat treatment and acid treatment of the carbon sheet under the condition of 100% relative humidity on the performance of the fuel cell including the carbon sheet, the electrochemical performances of the unit cell according to Example 3 (heat + acid), the unit cell according to Comparative Example 1 (CONV), the unit cell according to Comparative Example 5 (untreated), and the unit cell according to Comparative Example 6 (heat) were evaluated, and the results are as Figure 13 shown. The performance evaluation was carried out at a cell temperature of 60 °C.

[0150] Both the maximum current density and power density of the unit cell according to Example 3 were superior to those of the unit cells according to Comparative Example 1, Comparative Example 5, and Comparative Example 6. In addition, the maximum current density and power density of the unit cell according to Comparative Example 6, in which only heat treatment was performed on the carbon nanotube sheet, were superior to those of the unit cell according to Comparative Example 5 without treatment and the unit cell according to Comparative Example 1 without using the carbon sheet.

[0151] To confirm the effects of the carbon sheet sandwiched between the gas diffusion layer and the bipolar plate and the heat treatment and acid treatment of the carbon sheet under the condition of 40% relative humidity on the performance of the fuel cell including the carbon sheet, the electrochemical performances of the unit cell according to Example 4 (heat + acid), the unit cell according to Comparative Example 1 (CONV), the unit cell according to Comparative Example 5 (untreated), and the unit cell according to Comparative Example 7 (heat) were evaluated, and the results are as Figure 14 shown. The performance evaluation was carried out at a cell temperature of 80 °C.

[0152] Both the maximum current density and power density of the unit cell according to Example 4 were superior to those of the unit cells according to Comparative Example 1, Comparative Example 5, and Comparative Example 7. In addition, the maximum current density and power density of the unit cell according to Comparative Example 7, in which only heat treatment was performed on the carbon nanotube sheet, were superior to those of the unit cell according to Comparative Example 5 without treatment and the unit cell according to Comparative Example 1 without using the carbon sheet.

[0153] To confirm the effects of the carbon sheet sandwiched between the gas diffusion layer and the bipolar plate, and the heat treatment and acid treatment of the carbon sheet under the condition of 100% relative humidity on the electrochemical state of the fuel cell including the carbon sheet, the unit cells according to Example 3 (heat + acid), the unit cells according to Comparative Example 1 (CONV), the unit cells according to Comparative Example 5 (untreated), and the unit cells according to Comparative Example 6 (heat) were analyzed by impedance spectroscopy, and the results are as Figure 15 shown. The impedance spectroscopy analysis was carried out at a cell temperature of 60 °C.

[0154] Referring to Figure 15 , it can be confirmed that, compared with the unit cells according to Comparative Example 1, Comparative Example 5, and Comparative Example 6, the ohmic resistance (R ohm ) and the charge transfer resistance (R ct ) of the unit cell according to Example 3, in which both heat treatment and acid treatment were performed on the carbon sheet, were improved.

[0155] To confirm the effects of the carbon sheet sandwiched between the gas diffusion layer and the bipolar plate, and the heat treatment and acid treatment of the carbon sheet under the condition of 40% relative humidity on the electrochemical state of the fuel cell including the carbon sheet, the unit cells according to Example 4 (heat + acid), the unit cells according to Comparative Example 1 (CONV), the unit cells according to Comparative Example 5 (untreated), and the unit cells according to Comparative Example 7 (heat) were analyzed by impedance spectroscopy, and the results are as Figure 16 shown. The impedance spectroscopy analysis was carried out at a cell temperature of 80 °C.

[0156] Referring to Figure 16 , it can be confirmed that, compared with the unit cells according to Comparative Example 1, Comparative Example 5, and Comparative Example 7, the ohmic resistance (R ohm ) and the charge transfer resistance (R ct ) of the unit cell according to Example 4, in which both heat treatment and acid treatment were performed on the carbon sheet, were improved.

[0157] It can be clearly seen from the above description that according to the present invention, a carbon sheet can be obtained by performing at least one of heat treatment or acid treatment on an intermediate sheet including a fibrous carbon material, thereby being able to remove impurities such as Fe particles or amorphous carbon remaining in the carbon sheet, for example, and prevent chemical degradation of the ionomer in the electrode or the electrolyte membrane.

[0158] In addition, the carbon sheet from which impurities such as Fe particles or amorphous carbon have been removed can be sandwiched between the electrode and the gas diffusion layer, thereby being able to increase the electrical connectivity between the catalyst particles in the electrode and increase the electrochemically active area.

[0159] In addition, carbon sheets from which impurities such as Fe particles or amorphous carbon have been removed can be sandwiched between the gas diffusion layer and the bipolar plate, thereby enabling improvement of the non-uniform contact between the gas diffusion layer and the bipolar plate and reduction of the interfacial contact impedance.

[0160] The present invention has been described in detail with reference to the preferred embodiments. However, those skilled in the art will understand that these embodiments can be changed without departing from the principles and spirit of the present invention, and the scope of the present invention is defined in the appended claims and their equivalents.

Claims

1. A method for manufacturing a polymer electrolyte membrane fuel cell, comprising: preparing an intermediate sheet including a fibrous carbon material; Obtaining a carbon sheet by performing at least one of a heat treatment and an acid treatment on the intermediate sheet; as well as A unit cell including an electrolyte membrane is manufactured, wherein electrodes are located on a first surface and a second surface of the electrolyte membrane, a gas diffusion layer is located on the electrodes, and the carbon sheet is sandwiched between the electrodes and the gas diffusion layer.

2. The method according to claim 1, wherein: The preparation of the intermediate sheet is performed using a direct spinning method.

3. The method according to claim 1, wherein: The fibrous carbon material includes one selected from the group consisting of carbon nanofibers, carbon nanotubes, vapor-grown carbon fibers, and combinations thereof.

4. The method according to claim 1, wherein: The porosity of the carbon sheet is greater than or equal to the porosity of the electrode, and the porosity of the carbon sheet is less than or equal to the porosity of the gas diffusion layer.

5. The method according to claim 1, wherein: The porosity of the carbon sheet is 20% to 40%.

6. The method according to claim 1, wherein: The average diameter of the pores in the carbon sheet is 5 nm to 50 nm.

7. The method according to claim 1, wherein: The analysis results of the carbon sheet using Raman spectroscopy include the peak intensity of the G band I G The peak intensity of the D band is D The ratio I G / I D It is above 6.

5.

8. The method according to claim 1, wherein: Thermogravimetric analysis results of the carbon sheet included that the amount of impurities remaining in the carbon sheet was less than 14 wt %.

9. The method according to claim 1, wherein: Thermogravimetric analysis results of the carbon sheet included that the amount of impurities remaining in the carbon sheet was less than 3 weight %.

10. The method according to claim 1, wherein: The heat treatment is performed in a temperature range of greater than 450°C and less than 600°C.

11. The method according to claim 1, wherein: The acid treatment is performed using a strong acid.

12. The method according to claim 1, wherein: The heat treatment or the acid treatment is performed for 5 minutes to 30 minutes.

13. A method for manufacturing a polymer electrolyte membrane fuel cell, comprising: preparing an intermediate sheet including a fibrous carbon material; Obtaining a carbon sheet by performing at least one of a heat treatment and an acid treatment on the intermediate sheet; as well as A unit cell including an electrolyte membrane is manufactured, wherein electrodes are located on a first surface and a second surface of the electrolyte membrane, a gas diffusion layer is located on the electrodes, a bipolar plate is located on the gas diffusion layer, and wherein the carbon sheet is sandwiched between the gas diffusion layer and the bipolar plate.

14. The method according to claim 13, wherein: The preparation of the intermediate sheet is performed using a direct spinning method.

15. The method according to claim 13, wherein: The porosity of the carbon sheet is greater than or equal to the porosity of the gas diffusion layer, and the porosity of the carbon sheet is less than or equal to the porosity of the bipolar plate.

16. The method according to claim 13, wherein: The analysis results of the carbon sheet using Raman spectroscopy include the peak intensity of the G band I G The peak intensity of the D band is D The ratio I G / I D It is above 6.

5.

17. The method according to claim 13, wherein: Thermogravimetric analysis results of the carbon sheet included that the amount of impurities remaining in the carbon sheet was less than 14 wt %.

18. The method according to claim 13, wherein: Thermogravimetric analysis results of the carbon sheet included that the amount of impurities remaining in the carbon sheet was less than 3 weight %.

19. The method according to claim 13, wherein: The acid treatment is performed using a strong acid.

20. The method according to claim 13, further comprising performing a water-repellent treatment of the carbon sheet.

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