KR20220117593A - Self-Supported Gas diffusion layer comprising microporous structure, and Fuel cell comprising the same - Google Patents

Abstract

The present invention provides a self-supported gas diffusion layer having a microporous structure and a fuel cell including the same, wherein the self-supported gas diffusion layer is robust without a separate support unlike a pre-existing technology by combining polyvinylidene fluoride (PVDF) and acetylene black-multi-walled carbon nanotube composite, facilitates movement of gas and water during an operation of a fuel cell by realizing the gas diffusion layer with a microporous structure having a network structure that has larger number of larger pores compared to the pre-existing technology, and furthermore, effectively controls hydrophobicity of the pores by controlling the PVDF content within a specific range to minimize irreversible losses such as polarization behavior limitation and overpotential so as to optimize performance of the fuel cell.

Description

Self-Supported Gas diffusion layer comprising microporous structure, and Fuel cell comprising the same

The present invention relates to a self-supporting gas diffusion layer having a micropore structure and a fuel cell including the same.

A fuel cell is a device that produces electrical energy by electrochemically reacting fuel and oxygen. Depending on the type of electrolyte used, a polymer electrolyte membrane (PEM), phosphoric acid type, molten carbonate type, or solid It can be divided into solid oxide and alkaline aqueous solution types, and depending on the electrolyte used, the operating temperature of the fuel cell and the materials of components, etc. vary. Among them, the Polymer Electrolyte Membrane Fuel Cell (PEMFC) has a lower operating temperature and higher efficiency than other types of fuel cells, has high current and output densities, has a shorter starting time, and is more sensitive to load changes. It has a quick response characteristic.

Polymer electrolyte membrane fuel cell is a membrane electrode assembly (MEA) with catalytic electrode layers attached to both sides of the membrane where an electrochemical reaction occurs centering on an electrolyte membrane through which hydrogen ions move, airtightness of reactive gases and cooling water, and proper clamping pressure. Gaskets and fasteners for holding the battery, and a bipolar plate that electrically connects the cells and moves reactive gases and cooling water.

Meanwhile, in a fuel cell, hydrogen as a fuel and oxygen (air) as an oxidizing agent are respectively supplied to the anode and cathode of the membrane electrode assembly through the flow path of the separator, and hydrogen is Also called 'pole'), oxygen (air) is supplied to the cathode ('air electrode' or 'oxygen electrode', also called 'reduction electrode'). Hydrogen supplied to the anode is decomposed into hydrogen ions (proton, H ⁺ ) and electrons (electron, e ^- ) by the catalyst of the electrode layers on both sides of the electrolyte membrane, and only hydrogen ions of these selectively pass through the electrolyte membrane, which is a cation exchange membrane. is transferred to the cathode, and at the same time electrons are transferred to the cathode through the conductor gas diffusion layer and the separator. In the cathode, hydrogen ions supplied through the electrolyte membrane and electrons transferred through the separator meet with oxygen in the air supplied to the cathode by an air supply device to generate water. Due to the movement of hydrogen ions occurring at this time, the flow of electrons through the external conductor occurs, and a current is generated by the flow of these electrons.

On the other hand, the electrode of the polymer electrolyte fuel cell is composed of a catalyst layer (Catalyst Layer; CL) and a gas diffusion layer that supports the catalyst layer (Gas Diffusion Layer; GDL), and the gas diffusion layer is a microporous layer on a support made of a porous carbon membrane; MPL) is generally used as a support, and a porous carbon sheet, carbon paper, or carbon cloth is widely used as a support. It plays a role of diffusing to the catalyst layer, a current collector that moves the current generated in the catalyst layer to the separator, and a channel through which the generated water flows out of the catalyst layer.

On the other hand, in the gas diffusion layer manufacturing method according to the prior art, it is common to implement hydrophobicity by coating polytetrafluoroethylene (PTFE) particles of several micrometers (㎛) on a support made of carbon, and polytetrafluoroethylene is As a material having water repellency, it corresponds to a fluorinated polymer and is mainly obtained by free-radical polymerization. PTFE is chemically stable in most acid and alkali environments, has a high density, and is stable even at high temperatures. In addition, it is known that the intrinsic water repellency prevents flooding by moisture and prevents the macropores from filling up with the electrolyte in the catalyst layer to provide a passage for the reaction gas to move.

However, the method for manufacturing a gas diffusion layer using PTFE has a complicated manufacturing process, and if the content of PTFE in the catalyst layer is small, the proportion of electrolyte in the catalyst layer increases and the resistance to mass transfer of the electrolyte decreases, but many large pores are wet with the electrolyte. Therefore, there is a problem in that the resistance to mass transfer of the reactive gas increases. If the PTFE content is large, the water repellency of the macropores increases, making it easier to move the reactive gas, but the electrolyte penetration becomes difficult, which increases the resistance to electrolyte diffusion, and the catalyst/electrolyte There is a problem in that the contact area and the gas-liquid contact area are reduced.

In relation to this, published Korean Patent No. 10-0963274 discloses a composition including a microporous layer made by coating a slurry made by mixing PVDF and carbon filler fine powder on a support such as carbon cloth. , a gas diffusion layer with improved wear resistance and thermal conductivity, and a simplified manufacturing method thereof are disclosed.

Republic of Korea Patent Registration No. 10-963274 (2010.06.11.)

The present invention combines polyvinylidene fluoride (PVDF) and acetylene black-multi-walled carbon nanotube composite, unlike the prior art, it is robust without a separate support, and has a larger and more pore network structure than the prior art. By implementing a gas diffusion layer having a micropore structure, it facilitates the movement of gas and water between fuel cell driving, and furthermore, by controlling the PVDF content within a specific range, the hydrophobicity of pores is effectively controlled to limit current behavior and irreversible such as overpotential An object of the present invention is to provide a self-supported gas diffusion layer having a micropore structure, which can optimize the performance of a fuel cell by minimizing loss, and a fuel cell including the same.

In addition, the technical problems to be solved by the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned are clearly to those of ordinary skill in the art to which the present invention belongs from the description below. can be understood

In the present specification, as a gas diffusion layer for moving the reactive gas of the fuel cell to the catalyst layer and discharging the generated water to the outside, polyvinylidene fluoride (Polyvinylidene fluoride); and acetylene black-multi-walled carbon nanotube composites; It provides a micropore structure comprising a, self-supported gas diffusion layer.

In addition, in the present specification, the polyvinylidene fluoride is included in an amount of 15 to 25% by weight based on the total weight of the gas diffusion layer, and provides a self-supported gas diffusion layer.

In addition, in the present specification, the acetylene black-multi-walled carbon nanotube composite is formed by mixing 90 to 100 parts by weight of acetylene black to 1 to 10 parts by weight of multi-walled carbon nanotubes, a self-supported gas diffusion layer provides

In addition, in the present specification, the gas diffusion layer provides a self-supported gas diffusion layer in the form of a single sheet having a thickness in the range of 40 to 80 μm.

In addition, in the present specification, the gas diffusion layer provides a self-supported gas diffusion layer having a network structure.

In addition, in the present specification, a contact angle ( θ _c ) of the gas diffusion layer with respect to water is 130 to 140°, and a self-supported gas diffusion layer is provided.

In the present specification, an electrode for a fuel cell, a membrane electrode assembly for a fuel cell, or a polymer electrolyte fuel cell including the gas diffusion layer is provided.

In the present specification, as a method for manufacturing a self-supported gas diffusion layer constituting an electrode of a fuel cell, a) polyvinylidene fluoride is mixed with N-methyl-2-pyrrolidone (NMP), N,N - Forming an emulsion by dispersing in at least one solvent selected from among dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and acetone; b) mixing acetylene black and multi-walled carbon nanotubes to prepare an acetylene black-multi-walled carbon nanotube composite; c) preparing a slurry by mixing and homogenizing the emulsion obtained in step a and the complex obtained in step b; and d) coating and drying the slurry obtained in step c on a metal foil, followed by rolling and removing the metal foil; It provides a method of manufacturing a self-supported (self-supported) gas diffusion layer comprising a.

In addition, in the present specification, polyvinylidene fluoride in step a is emulsified to be 3 to 7% by weight in N-methyl-2-pyrrolidone (NMP) solvent, self-supported. A method for manufacturing a gas diffusion layer is provided.

In addition, in the present specification, in step b, acetylene black and multi-walled carbon nanotubes are mixed in a ratio of 90 to 100 parts by weight to 1 to 10 parts by weight, a method for preparing a self-supported gas diffusion layer is provided.

In addition, in the present specification, the slurry prepared in step c has a viscosity in the range of 1000 to 5000 mPa·s, and provides a method for manufacturing a self-supported gas diffusion layer.

In addition, in the present specification, step d is performed by coating the slurry on a metal foil, drying it within a temperature range of 70 to 90° C., and then removing the metal foil in water or a base solution after rolling. To provide a method for manufacturing a self-supported gas diffusion layer.

The gas diffusion layer and the fuel cell including the same according to the present invention are robust without a separate carbon support, and by implementing a gas diffusion layer having a micropore structure having a network structure larger and more pores than the prior art, gas between fuel cell driving Since it facilitates the movement of water and water, the energy efficiency of the fuel cell can be increased, and long life can be expected.

In addition, the gas diffusion layer and the fuel cell including the same according to the present invention, by controlling the PVDF content within a specific range, effectively control the hydrophobicity of the pores to limit the current behavior and minimize irreversible losses such as overpotentials, thereby improving the performance of the fuel cell can be optimized.

1 schematically illustrates a process for manufacturing a self-supported gas diffusion layer having a micropore structure according to an embodiment of the present invention.
2 is a schematic diagram of the force-distance curve obtained in each tapping cycle using (a) the peak force tapping operation principle using the tip trajectory, and (b) the peak force as a control parameter.
3 shows surface SEM images of (a) SGL29BC and (b) 20PVDF-SSMPL according to Examples and Comparative Examples of the present invention.
4 shows surface SEM images of (a) 10PVDF-SSMPL and (b) 30PVDF-SSMPL according to a comparative example of the present invention.
5 shows a pore size distribution (PSD) curve of the gas diffusion layer according to Examples and Comparative Examples of the present invention.
6 shows pore size distribution (PSD) curves of gas diffusion layers according to Examples and Comparative Examples of the present invention.
7 is a photograph of measuring a water contact angle of a gas diffusion layer according to an embodiment and a comparative example of the present invention.
8 shows the capillary pressure according to the moisture saturation of the gas diffusion layer according to Examples and Comparative Examples of the present invention.
9 shows the polarization curves of the PEMFC to which the gas diffusion layer according to Examples and Comparative Examples of the present invention is applied.
10 shows a Nyquist plot of a PEMFC to which a gas diffusion layer according to Examples and Comparative Examples of the present invention is applied.
11 shows a Nyquist plot and graphic analysis of PEMFC to which a gas diffusion layer is applied according to Examples and Comparative Examples of the present invention.
12 shows (a) polarization curves of PEMFCs using diluted oxygen in different GDLs, and (b) current density limitations by varying oxygen concentrations and cathode pressures in different GDLs according to Examples and Comparative Examples of the present invention. it has been shown
13 shows the total oxygen resistance (R _tot ) of the cathode to which the gas diffusion layer is applied according to the Examples and Comparative Examples of the present invention.
14 shows (a, d) 3D topological images, (b, e) 2D topological images, and (c, f) adhesion mapping images of gas diffusion layers according to Examples and Comparative Examples of the present invention.
15 schematically shows a water transport mechanism of a gas diffusion layer according to Examples and Comparative Examples of the present invention.

Hereinafter, with reference to the accompanying drawings, embodiments of the present invention will be described in detail so that those of ordinary skill in the art can easily implement them. However, the present invention may be embodied in several different forms and is not limited to the embodiments disclosed below.

Hereinafter, a self-supported gas diffusion layer having a microporous structure and a fuel cell including the same will be described in more detail.

Gas diffusion layer, electrode, membrane electrode assembly, polymer electrolyte fuel cell

A gas diffusion layer according to an embodiment of the present invention is a gas diffusion layer that moves a reactive gas of a fuel cell to a catalyst layer and discharges generated water to the outside, and includes polyvinylidene fluoride; and acetylene black-multi-walled carbon nanotube composites; It may have a micropore structure comprising a.

The fuel cell according to the present invention may be, for example, a polymer electrolyte membrane fuel cell (PEMFC). In general, PEMFC maintains airtightness and proper clamping pressure of a Membrane Electrode Assembly (MEA) with catalyst electrode layers attached to both sides of the membrane, in which an electrochemical reaction occurs around an electrolyte membrane through which hydrogen ions move, reactive gases and coolant. Gaskets and fasteners for this, and a bipolar plate that electrically connects the cells and moves reactive gases and cooling water may be included.

In general, the catalyst electrode layer is composed of a catalyst layer (CL) and a gas diffusion layer (Gas Diffusion Layer; GDL) supporting the catalyst layer, and the gas diffusion layer is coated with a microporous layer (MPL) on a support made of a porous carbon film. In general, as a support, a porous carbon sheet, carbon paper, or carbon cloth is widely used as a support.

On the other hand, the gas diffusion layer of the present invention, unlike the gas diffusion layer according to the prior art, is an integrated support and microporous layer, does not require a separate carbon support, etc. To provide a gas diffusion layer having a micropore structure in the form of a single sheet having a network structure with

More specifically, in the case of a fuel cell electrode according to the prior art, polytetrafluoroethylene (PTFE) resin is coated on a carbon support to form, or polyvinylidene fluoride (PVDF) resin is mixed with carbon aggregates. On the other hand, the gas diffusion layer of the present invention is prepared in the form of coating and forming on a separate carbon support, polyvinylidene fluoride (Polyvinylidene fluoride); and acetylene black-multi-walled carbon nanotube composites; It has a micropore structure, including, but is robust without a separate support, and has a network structure with larger and more pores than the prior art.

When the gas diffusion layer is manufactured in the form of a single sheet as described above, but has a network structure, it facilitates the movement of gas and water between driving of the fuel cell, so that the energy efficiency of the fuel cell can be increased and lifespan can be expected.

First, in the present invention, vinylidene fluoride (PVDF) is a hydrophobic fluororesin, and has excellent chemical resistance and good mechanical, thermal, and electrical properties, while having excellent mechanical strength, rigidity and creep resistance. Meanwhile, in the present invention, vinylidene fluoride not only serves as a binder between carbon aggregates when forming a gas diffusion layer having a microporous structure, but also permeates between carbon aggregates to impart hydrophobicity to the micropores. do it together

According to an embodiment of the present invention, polyvinylidene fluoride may be included in an amount of 15 to 25% by weight, specifically 20% by weight, based on the total weight of the gas diffusion layer. When polyvinylidene fluoride is included in the above weight % range in the gas diffusion layer, oxygen transfer to the adjacent catalyst layer (Catalyst Layer, CL) is smooth, and water can be effectively delivered and discharged from the catalyst layer.

In the present invention, the acetylene black-multi-walled carbon nanotube composite is a carbon aggregate, prepared in the form of a composite by mixing acetylene black and multi-walled carbon nanotubes, specifically acetylene black 90 to 100 weight The unit may be formed by mixing 1 to 10 parts by weight of multi-walled carbon nanotubes, specifically, it may be formed by mixing 95 parts by weight of acetylene black to 5 parts by weight of multi-walled carbon nanotubes.

When acetylene black and multi-walled carbon nanotubes are included in the ratio by weight in the above range when forming the complex, a more robust network structure can be formed, and the multi-walled carbon nanotubes included in the predetermined range are in the network structure. It plays the same role as the steel frame that serves as a skeleton, and forms relatively large pores in the state of being bonded to the carbon aggregate (acetylene black) that acts as concrete, while supporting the structure.

On the other hand, the gas diffusion layer according to an embodiment of the present invention may be in the form of a single sheet having a thickness in the range of 40 to 80 μm, and unlike the prior art, a single sheet having a solid structure without using a separate carbon substrate Since it can be manufactured in the form of a sheet of layers, it is superior in terms of process efficiency and economy compared to the prior art of forming a gas diffusion layer in a multi-layer structure by repeatedly coating the slurry on a carbon substrate.

On the other hand, the contact angle ( θ _c ) of the gas diffusion layer with respect to water according to the present invention may be 130 to 140 °, for example, 135.7 °. On the other hand, the contact angle ( θ _c ) of the gas diffusion layer containing PTFE according to the prior art with respect to water is about 150°, whereas when PVDF is included according to the present invention, PVDF has a PTFE density ( k = 2.20 gcm ^-3 ) This is because it has a lower density ( f = 1.77 gcm ^-3 ) compared to that, while having a high solubility in NMP, so that it is more uniformly distributed among the particles of carbon agglomerates. On the other hand, in the micropore structure of the gas diffusion layer of the present invention, the contact angle (hydrophobic property) as described above may be controlled by adjusting the PVDF content.

As described above, the gas diffusion layer according to the present invention can be applied to an electrode for a fuel cell, a membrane electrode assembly, or a polymer electrolyte fuel cell. Since the eggplant implements a gas diffusion layer with a micropore structure, it is possible to facilitate the movement of gas and water between fuel cell driving. Furthermore, since the hydrophobicity of the pores is effectively controlled by controlling the PVDF content within a specific range, the performance of the fuel cell can be optimized by limiting the current behavior and minimizing irreversible losses such as overpotentials.

For example, when the gas diffusion layer is applied to the cathode of a polymer electrolyte fuel cell of a hydrogen electric vehicle, a sudden flooding that may occur during operation can be alleviated.

Gas diffusion layer manufacturing method

The method for preparing a self-supported gas diffusion layer according to an embodiment of the present invention comprises: a) polyvinylidene fluoride mixed with N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and acetone (Acetone) dispersing in one or more solvents to form an emulsion; b) mixing acetylene black and multi-walled carbon nanotubes to prepare an acetylene black-multi-walled carbon nanotube composite; c) preparing a slurry by mixing and homogenizing the emulsion obtained in step a and the complex obtained in step b; and d) coating and drying the slurry obtained in step c on a metal foil, followed by rolling and removing the metal foil; may include (see FIG. 1).

In step a, polyvinylidene fluoride is one selected from N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and acetone (Acetone). It may be carried out by emulsifying in the above solvent, for example, it may be carried out by emulsifying so as to be 3 to 7% by weight in N-methyl-2-pyrrolidone (NMP) solvent. Meanwhile, in the present invention, the solvent may serve to adjust the slurry viscosity.

On the other hand, in step b, acetylene black, for example, acetylene black powder and multi-walled carbon nanotubes, for example, multi-walled carbon nanotube paste in a ratio of 90 to 100 parts by weight to 1 to 10 parts by weight, detail Preferably, it may be carried out by mixing in a ratio of 95 to 5 parts by weight.

On the other hand, the viscosity of the slurry prepared in step c may be in the range of 1000 to 5000 mPa·s, smooth coating can be performed within the range, and the occurrence of microcracks on the surface of the prepared gas diffusion layer can be effectively prevented. have.

On the other hand, in step d, the slurry is coated on a metal foil by a method such as tape-casting using a doctor blade or the like, and then, within a temperature range of 70 to 90° C., for example, convection. It may be carried out by drying in an oven (convection oven). In addition, after the gas diffusion layer is prepared, in order to remove the metal foil, the metal foil may be peeled off by immersion in water or a base solution after rolling.

Example

Since the present invention may have various changes and may have various forms, specific embodiments will be illustrated and described in detail below. However, this is not intended to limit the present invention to the specific disclosed form, it should be understood to include all modifications, equivalents and substitutes included in the spirit and scope of the present invention.

Example

An emulsion was prepared by preparing PVDF powder (Solvay), and emulsifying it in a pre-prepared NMP (Alfa aesar) solvent to about 5% by weight. Next, acetylene black (AB) carbon powder (Alfa aesar) and multi-walled carbon nanotube (MWCNT) paste (Cnano Technology LTD) were mixed in a 95: 5 parts by weight ratio to prepare an acetylene black-multi-walled carbon nanotube composite. .

Next, the AB-MWCNT complex was dispersed in the PVDF emulsion, and slurries were prepared with various PVDF contents using a planetary centrifugal mixer (ARM-310, Thinky Inc.) (ex. 10 wt%, 20 wt% and 30 wt%) weight%).

Next, the slurry was coated on a metal foil by a tape-casting method using a doctor blade in various thicknesses, and completely in a convection oven at 80° C. to evaporate the solvent on the slurry after coating. After drying, the metal foil was peeled off by immersing in water to observe the properties of the prepared gas diffusion layer having a micropore structure, and the gas diffusion layer was prepared (PVDF-SSMPLs).

comparative example

As a ready-made product, a gas diffusion layer (product name: SGL29BC) having a microporous structure containing 22 wt% of PTFE was prepared.

[Test method]

Physical and structural characterization

A mercury penetration porosimeter (Auto poreV, Micromeritics Inc.) was used to analyze the porous structure of PVDF-SSMPLs prepared according to the above example. The pore size distribution curves were expressed in two formats : v _avg to d V /dlog p p for a broad pore distribution and v _avg to d V / d _{p p} for capillary pore distribution.

Meanwhile, the surface contact angle measurement was performed by a sessile drop method using a contact angle analyzer (Phoenix 300 Touch S.E.O) according to ASTM D5946 standard.

Surface morphologies were observed with field emission scanning electron microscopy (FE-SEM, Inspect F-50).

Meanwhile, AFM measurements were performed using the Dimension Icon (Bruker Inc.) system, and all images were taken in the peak force QNM (Quantitative Nanoscale Mechanical) Tapping mode, where the material sensitive properties were determined by topology and adhesion through FIG. 2 and the like. It can be evaluated simultaneously with the mapping information.

Electrochemical analysis and limiting current measurement in a single PEMFC

Prepare a single PEMFC employing a 25 cm ² geometric active area and a commercial catalyst coated membrane (CNL Energy Inc.) loaded with 0.4 mg Pt in An/Ca, but with SGL29BC and a 4-channel type serpentine graphite block (ILDO F&C Inc.) was placed on the anode side, and the porous metal foam (foam) of PVDF-SSMPL and square grooved graphite block according to the self-made embodiment was placed on the cathode side.

All single cell tests were performed in a fully humidified RH _An/Ca = 100%, 65 °C and ambient environment using a fully automated test station (CNL Energy Inc.). Polarization curves were measured in constant current mode (and under constant stoichiometry) with a dwell time of 5 min to obtain a constant potential response.

EIS analysis was performed galvanostatically at 0.2 Acm ^-2 at a constant rate of 114 cm ³ min ^-1 over a frequency range of 50 kHz to 50 mHz by applying a 5% AC-amplitude signal. For the limiting current measurement, oxygen diluted with nitrogen (ex. oxygen concentration 1 - 5%) was introduced into the cathode, and a clear mass transfer limiting operation was confirmed at various cathode back pressures (ex. 101.3 - 303.9 kPa). Polarization measurements were performed using the potential step method, and the limiting current was 0.5 V vs. It was assumed to be RHE.

[Experimental results and evaluation]

First, FIG. 3 shows a surface image of SSMPL prepared according to Examples and Comparative Examples. 3 (a) is a gas diffusion layer according to a comparative example (SGL29BC), (b) is a gas diffusion layer (20PVDF-SSMPL) containing 20% by weight of PVDF in Example.

Referring to the results of FIG. 3 , an important difference between the gas diffusion layer according to the example and the comparative example is the presence and absence of microcracks formed on the surface of the MPL. It was expected that it was caused by heat treatment or the like. On the other hand, in the case of the gas diffusion layer according to the Example, it was confirmed that a large number of porous microstructures were observed compared to the Comparative Example, and a spatial network having relatively large voids compared to the Comparative Example.

In addition, referring to FIG. 4 , (a) a gas diffusion layer (10PVDF-SSMPL) containing 10% by weight of PVDF, (b) a gas diffusion layer containing 30% by weight of PVDF (30PVDF-SSMPL) also facilitates the transport of oxygen and water It was confirmed to have a network structure with more micropores.

Next, FIG. 5 shows the pore size distribution (PSD) curve (d V /dlog v _p ) of the gas diffusion layer according to Examples and Comparative Examples of the present invention, and in the case of Comparative Example, 0.09 (MPL-related) and 60 μm Mesopores of 0.2 to 30 μm are observed with a typical dual-PSD positioned (CB-related), and in the case of Examples, a clear single-PSD centered at 0.2 μm is observed, and in both Examples and Comparative Examples, , the mesopores found at > 200 μm appeared to derive mostly from pores near the surface on both sides of the GDL.

Referring to this, it was found that the 20PVDF-SSMPL according to the Example had a relatively larger pore size (2 times or more) and a wider micropore PSD compared to the SGL29BC of the Comparative Example, and in the case of the Example, it was inversely proportional to the pore size. It was found that the lower capillary pressure facilitates gas and water transport.

Referring to FIG. 6 (a), as the PVDF content increased in SSMPL, it was confirmed that the specific micropore volume (d V / d _{p p} ) decreased and the micropore distribution became bimodal, less than 0.7 μm. The PSD transition of and the shift of the peak as the PVDF concentration of PVDF-SSMPL increased was determined to be due to i) the formation of small voids (Voids) by the binder penetrating between the AB and MWCNT particles, and ii) the increase in the capillary diameter. .

Referring to Fig. 6(b), in the case of 20PVDF-SSMPL with MWCNTs, the reduction in micropore volume at 0.3 to 3 μm supports that MWCNTs make SSMPL more dense.

7 shows the contact angles of SSMPL according to Examples and Comparative Examples ((a) SGL29BC according to Comparative Example, (b) 10PVDF-SSMPL, (c) 20PVDF-SSMPL, (d) 30PVDF-SSMPL, respectively), SGL29BC The contact angle ( θ _c = 150°) of the Example 20PVDF-SSMPL ( θ _c = 137°) shows a higher contact angle, which indicates that PVDF ( f = 1.77 gcm ^-3 ) is compared to PTFE ( f = 2.20 g ). cm ^-3 ) compared to low density and high solubility in NMP, which means that it is uniformly injected into the carbon particles, thereby exhibiting lower MPL hydrophobicity.

On the other hand, referring to (b) to (d) of FIG. 7 , it can be seen that the contact angle on the surface of SSMPL increases as the PVDF content increases, and through this, the wettability of PVDF-SSMPL is determined by the PVDF concentration. It was confirmed that it can be effectively controlled.

On the other hand, referring to FIG. 8 and the like, the capillary pressure ( p _c ) of SGL29BC with a contact angle ( θ _c ) of 150° increases more rapidly compared to 20PVDF-SSMPL with a contact angle ( θ _c ) of 135.7° as the water saturation increases. , it was confirmed that 20PVDF-SSMPL had a faster water removal rate than SGL29BC.

In order to investigate the polarization characteristics, PEMFCs each having 20PVDF-SSMPL and SGL29BC according to Examples and Comparative Examples, respectively, were fabricated, and performance was measured using a polarization technique, which is shown in FIG. 9 .

Figure 9 (a) shows the polarization behavior of PEMFCs with different carbon amounts of 20PVDF-SSMPL, and as shown in Figure 9 (a), SSMPL at 0.93 mg _C cm ^-2 shows the lowest performance in the entire current density region. , it was confirmed that the slightly thicker 1.45 mg _C cm ^-2 significantly improved the PEMFC performance without severe mass transfer limitation. As the carbon content of SSMPL further increased, the performance reached a maximum at 2.81 mg _C cm ^-2 . However, the excess carbon at 4.22 mg _C cm ^-2 was not favorable for improving PEMFC performance, resulting in higher overpotentials with current behavior limitation.

Referring to the corresponding results, it is suggested that for very thin MPLs, the cathode is easily submerged, whereas for very thick MPLs, it interferes with water drainage and counter oxygen flow from the 3D porous metal foam at medium current and high current densities.

Meanwhile, polarization curves obtained using PEMFCs with different PVDF contents in MPL (2.81 mg _C cm ⁻² ) are shown in FIG. 9(b). Referring to FIG. 9(b), PVDF increased in the order of 10 < 30 < 20 wt%, indicating that 20PVDF-SSMPL minimized irreversible loss during fuel cell operation.

As mentioned above, the contact angle is related to the PVDF content of SSMPL, and an optimal hydrophobic binder promotes PEMFC performance by controlling pore properties including pore size and hydrophobicity for reactant and product flows. could check

Fig. 9(c) shows the polarization curves of SGL29BC and 20PVDF-SSMPL. These performances were nearly identical up to 1.0 A cm ^-2 , and at >1.0 A cm ^-2 , where the polarization behavior is considered to be significantly complicated due to mass transfer effects, the performance difference becomes larger with increasing current density. This means that the self-fabricated 20PVDF-SSMPL can facilitate water removal at high current densities when compared to SGL29BC, which is quantitatively consistent with the results for water saturation in both MPLs, as shown in Fig. 8 above. .

On the other hand, as mentioned, the performance of PEMFC is determined by kinetic, ohmic and irreversible losses including concentration overpotential. The ELS technique is a useful characterization method for differentiating the electrochemical processes occurring in PEMFCs because electrode processes, including dielectric and capacitive behavior, charge transfer, and mass transfer, exhibit intrinsic impedance responses with different relaxation times. . However, each process can be interfered with, resulting in a combined impedance operation on a timescale of inverse frequency.

In order to clearly distinguish the individual polarization losses, each Nyquist plot experimentally measured at 0.2 A cm ^-2 was fit to the concentrated equivalent circuit model used in the previous study, i) the overall polarization behavior. The anodic contribution is also negligibly small, and ii) the exchange current density available for ORR Pt surface availability was assumed to be constant even at low current densities.

These experimental EIS conditions show that in the high current density region, the oxygen diffusion constraint associated with water accumulation in the CL is kinetically governed by the Tafel slope for ORR across the CL, where it is affected by oxygen flux and ionomer hydration. ) and effective Pt surface area can be significantly altered.

Fig. 11 (a) is a method of isolating the overall impedance behavior of 20PVDF-SSMPL into four individual resistances: high-frequency resistance R _HF , proton (H ⁺ ) resistance at CL R _H+ , charge transfer resistance R _CT , and mass transfer resistance R _MT . indicates Detailed procedures for graphical analysis are described in Yi et al. initiated, etc.

Fig. 11 (b) shows the four resistances of SSMPL with different carbon amounts, respectively, using the Nyquist plot (see Fig. 10) suitable for the theoretical model. As expected from the polarization curve of FIG. 9(a), the SSMPL of 0.93 mg _C cm ^-2 had the highest value of each resistance among SSMPLs, especially among the four polarization resistances. This means that the oxygen diffusion resistance through the non-reactive region (mainly MPL) is very high, resulting in a lower rate of charge transfer at the interface between the catalyst and the electrolyte due to severe water saturation in the cathode. However, unlike SSMPL at 0.93 mg _C cm ^-2 , three SSMPLs at ≥ 1.45 mg _C cm ^-2 provide polarization resistance in the general order of R _ct > R _mt > R _HF > R _H+ , which is a hydrated membrane or that the Pt surface reaction in contact with the ionomer is the slowest step at a given current density of 0.2 A cm ^-2 . The values of R _ct and R _mt decrease with increasing carbon content in SSMPL and rise at 4.22 mg _C cm ^-2 , indicating that SSMPL at 2.81 mg _C cm ^-2 has a relatively small oxygen gradient at the cathode.

Similarly, the experimentally obtained Nyquist plots of PEMFCs with different PVDF contents (see Fig. 10(b)) were analyzed using a graphical method and shown in Fig. 11(c). All values were almost the same, and PEMFCs of different PVDF contents (2.81 mgc cm ^-2 ) did not show significant proton conductivity. However, it appears that the water distribution of each cathode is mainly due to the different pore properties such as wettability and pore shape. In particular, the decreasing trend of R _H+ is related to the higher water content of CL, which favors the movement of H ₃ O ⁺ ions as proton carriers. However, at the higher R _mt of the PEMFC using 10PVDF-SSMPL, more water was assumed to be saturated in the MPL, and oxygen transfer to the CL was relatively poor. In contrast, due to the relatively high limiting pressure of 30PVDF-SSMPL, we inferred that excessive hydrophobic binders in SSMPL inhibited water drainage from CL to MPL. Therefore, it was confirmed that 20PVDF-SSMPL could improve CL utilization by achieving proper moisture balancing between CL and MPL at the cathode. When 20PVDF-SSMPL was compared with SGL29BC, the overall polarization resistance was similar (Fig. 10(c)), but each resistance was slightly different. Except for R _ct , the values of R _HF , R _H+ and R _mt of 20PVDF-SSMPL were slightly higher than those of SGL29BC, while the water content of 20PVDF-SSMPL was somewhat higher, but the interfacial oxygen reaction rate was relatively fast, possibly This was thought to be due to the low water saturation of CL.

Although experimental and theoretical EIS analyzes have provided a clear understanding of individual electrochemical processes at low current densities, which are mainly governed by kinetics and ohmic resistance, oxygen diffusion resistances in multiscale pores of cathodes with different GDLs in the concentration-dominant region. will be useful for estimating In order to clearly distinguish the oxygen resistance in the diffusion region, the following procedure was performed step by step. First, limiting currents were measured at various oxygen partial pressures under different cathode backpressures using PEMFCs with SGL29BC and 20PVDF-SSMPL (see Fig. 12(a)). Next, the total oxygen resistance R _tot of the cathode was calculated in the plane limiting the current and oxygen partial pressure at a constant total pressure of the cathode using Faraday's law and ideal gas law (see Fig. 12(b)).

where F, R, and T are Faraday's constant (= 96,485 C mol ^-1 ), gas constant, and temperature (in Kelvin). Then, the values at various total cathode pressures can be approximated linearly with the slope of the molecular diffusion resistance and the y-intercept of the non-molecular diffusion resistance based on the simplified Stefan-Maxwell equation as have.

where R _MD,P , R _MD,0 , and R _ext represent pressure-dependent molecular oxygen diffusion resistance, pressure-independent molecular oxygen diffusion resistance, and additional oxygen diffusion resistance, respectively. Finally, R _MD and R _non-MD were determined using the linear relationship between R _tot and P . Figure 13 shows the R _tot values of PEMFCs using SGL29BC and 20PVDF-SSMPL at various P values, indicating that the total oxygen diffusion resistance is linearly dependent on the total pressure of the cathode. SGL29BC has a higher R _tot than 20PVDF-SSMPL at 101.3 kPa, and the difference becomes larger as P increases.

This is related to the relatively low performance in the high current density region of the PEMFC adopting the SGL29BC. According to the above four-step procedure, the R _MD and R _non-MD of PEMFCs employing SGL29BC and 20PVDF-SSMPL were estimated. 13, the R _MD size of PEMFC with SGL29BC was relatively high (about 1.14 times), while R _non-MD was about 2.36 times higher than that of 20PVDF-SSMPL. In general, molecular diffusion at > d _p = 50 nm mainly occurs in GDL (including MPL) and GFF, whereas Knudsen diffusion at 10 < d _p < 50 nm and thin film diffusion at < d _p = 10 nm is It is mainly observed in CL.

Therefore, in the small difference between SGL29BC and 20PVDF-SSMPL, R _MD does not show significant water accumulation in both GDLs even at high current densities, whereas in the CL of PEMFCs using SGL29BC, severe water as a result of significantly larger R _non-MD . saturation occurs. Also, the Nussen diffusion between non-molecular diffusion processes is considered to be remarkably small. This can be explained by several reasons. i) Nussen diffusion coefficient is the only function of pore diameter at constant temperature supplied with air, ii) catalyst-coated membranes prepared under the same conditions were applied to PEMFCs with two different GDLs, iii) both PEMFCs have limited current is considered to be driven without significant water overflow due to the clearly observed relatively moderate current density, so there is no appreciable change in pores controlled by Nussen diffusion, iv) thin film diffusion is Complex multi-step processes such as oxygen adsorption/dissolution/desorption via ionomers, which are reported to be three times slower, and other diffusion through various barrier layers on the Pt catalyst surface may be involved. Therefore, 20PVDF-SSMPL reduced the water saturation of CL (or faster water removal through MPL), resulting in higher oxygen concentration of the thin ionomer-covered Pt catalyst surface in CL corresponding to a smaller R _ct in EIS analysis. was judged to occur.

Consequently, the surface morphology, internal structure and wettability of SGL29BC and 20PVDF-SSMPL were determined by the carbon material and hydrophobic binder through SEM analysis and contact angle measurement, and these MPL properties were analyzed in terms of the water management of the cathode and the polarization resistance and oxygen diffusion process. has been shown to be correlated with However, more sophisticated microstructures such as surface topology and carbon aggregates bound with hydrophobic binders provide more insight into electrochemical processes including reactant and product flows at the cathode. To obtain these fields of view, SGL29BC and 20PVDF-SSMPL were characterized using AFM. 14 (a) and (d) show the 3D surface profiles of two different GDLs. 14(a)-(d), a longer and tortuous surface roughness (up to 1.87 times greater height difference between peak and valley) was evident in SGL29BC, and 20PVDF-SSMPL It showed a relatively smooth surface and a porous structure.

These results confirm that the surface roughness was controlled by the evaporation rate of the solvent and additives, the binder properties, and the presence (or absence) of the macroporous carbon matrix allowing the carbon slurry to partially penetrate into the macropores. Interestingly, from the SEM and 2D topological images shown in Figs. 3 and 14, it is judged that MWCNTs interconnected with carbon aggregates in 20PVDF-SSMPL act as steel frames in reinforced concrete-like structures, forming relatively large pores around small carbon aggregates. .

In addition, we successfully observed the distribution of hydrophobic binders between carbon aggregates, which could not be seen using SEM from adhesion mapping images. As can be seen in FIGS. 14(c) and 14(f), the spots contain softer materials such as hydrophobic polymeric binders. In Fig. 14(c), SGL29BC shows large (see white circles), medium (see green circles), and small aggregates of hydrophobic binders scattered without distinct boundaries, i) PTFE particles agglomerated in MPL loaded on CB during heat treatment. melting and agglomeration, ii) PTFE particles dispersed in the MPL were melted by heat, but did not spread widely, resulting in relatively large binder agglomerates in the MPL like snow mountain. Conversely, since PVDF dissolved in NMP moved freely around carbon particles and aggregates, numerous polymer-impregnated carbon aggregates of different sizes were found in 20PVDF-SSMPL, although somewhat found as relatively large fractions (see green circles). .

The markedly different microstructure and binder distribution in SGL29BC and 20PVDF-SSMPL are significantly related to gaseous oxygen diffusion and liquid water flow to and from the CL. The small pores inside the carbon agglomerates of MPL are available for gaseous oxygen diffusion, whereas the larger pores between the carbon agglomerates of MPL are mainly used for capillary-driven water flow.

Moreover, as affected by pore wettability, the contact angle of the composite surface composed of hydrophilic carbon and hydrophobic polymer affects water transport through MPL and pore shape. Thus, SGL29BC with larger carbon agglomerates heterogeneously bound with PTFE provides small micropores for water transport, while 20PVDF-SSMPL containing relatively larger micropores between carbon agglomerates uniformly impregnated with PVDF. facilitates drainage of water through individual micropores. In addition, several points without PTFE of SGL29BC (see blue circles) were clearly observed in Fig. 14(c), indicating that local water pooling may occur inside the MPL and at the interface between CL and MPL. , thus reducing the oxygen diffusion path towards CL.

Based on the experimental and theoretical results obtained using various GDLs in this study, we can propose a water transport mechanism through hydrophobic MPL: PTFE-bound MPL and PVDF-bound MPL. Figure 15 shows two MPLs bonded with optimal PTFE or PVDF content.

PTFE-bonded MPL consists of larger carbon agglomerates partially bonded with coarse fragments of scattered PTFE, whereas smaller carbon agglomerates are found in PVDF-bonded MPL that are uniformly impregnated with a thin, smooth PVDF film.

PTFE-bonded MPL exhibits smaller pores between the carbon aggregates, which initiates water flow at the hydrophobic interface of CL/MPL and creates a high limiting pressure to transport liquid water along the pores inside, whereas PVDF-bonded MPL Larger and more pores between carbon aggregates can be used for capillary-driven water flow in the CL/MPL interface, lowering the limiting pressure within the pores.

In addition, the structural features of PTFE-bonded MPL are due to the fact that water intensively attacks the unbonded carbon particles with the hydrophobic polymer in the carbon agglomerates, resulting in binder redistribution and further structural collapse inside the MPL. considered to be susceptible to erosion by However, PVDF-bonded MPL is not suitable for extended fuel cell operation because of i) smaller, ii) uniform impregnation of the polymer film into the carbon aggregates, and iii) binder-bonded dispersed MWCNTs such as three-dimensional landslide guard nets. It is considered to be more structurally robust.

In the foregoing, specific embodiments of the present invention have been described and illustrated, but it is common knowledge in the art that the present invention is not limited to the described embodiments, and that various modifications and variations can be made without departing from the spirit and scope of the present invention. It is self-evident to those who have Accordingly, such modifications or variations should not be individually understood from the technical spirit or point of view of the present invention, and modified embodiments should be said to belong to the claims of the present invention.

Claims (14)

As a gas diffusion layer for moving the reactive gas of the fuel cell to the catalyst layer and discharging the generated water to the outside,
Polyvinylidene fluoride; and acetylene black-multi-walled carbon nanotube composites; A micropore structure comprising a, self-supported gas diffusion layer. The method of claim 1,
The polyvinylidene fluoride is contained in an amount of 15 to 25% by weight based on the total weight of the gas diffusion layer, a self-supported gas diffusion layer. The method of claim 1,
Wherein the acetylene black-multi-walled carbon nanotube composite is formed by mixing 90 to 100 parts by weight of acetylene black to 1 to 10 parts by weight of multi-walled carbon nanotubes. The method of claim 1,
The gas diffusion layer is a single sheet (Sheet) form having a thickness in the range of 40 to 80 ㎛, self-supported (self-supported) gas diffusion layer. The method of claim 1,
The gas diffusion layer has a network structure, a self-supported gas diffusion layer. The method of claim 1,
A contact angle of the gas diffusion layer with respect to water ( θ _c ) is 130 to 140°, a self-supported gas diffusion layer. An electrode for a fuel cell comprising the gas diffusion layer of claim 1 . A membrane electrode assembly for a fuel cell comprising the gas diffusion layer of claim 1. A polymer electrolyte fuel cell comprising the gas diffusion layer of claim 1. A method for manufacturing a self-supported gas diffusion layer constituting an electrode of a fuel cell, comprising:
a) at least one solvent selected from among N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and acetone (Acetone) for polyvinylidene fluoride dispersing in to form an emulsion;
b) mixing acetylene black and multi-walled carbon nanotubes to prepare an acetylene black-multi-walled carbon nanotube composite;
c) preparing a slurry by mixing and homogenizing the emulsion obtained in step a and the complex obtained in step b; and
d) coating and drying the slurry obtained in step c on a metal foil, followed by rolling and removing the metal foil; A method of manufacturing a self-supported gas diffusion layer comprising a. 11. The method of claim 10,
In step a, the polyvinylidene fluoride is emulsified in an N-methyl-2-pyrrolidone (NMP) solvent in an amount of 3 to 7% by weight, a method for producing a self-supported gas diffusion layer . 11. The method of claim 10,
In step b, acetylene black and multi-walled carbon nanotubes are mixed in a ratio of 90 to 100 parts by weight to 1 to 10 parts by weight, a method for producing a self-supported gas diffusion layer. 11. The method of claim 10,
The slurry prepared in step c has a viscosity in the range of 1000 to 5000 mPa·s, a method for producing a self-supported gas diffusion layer. 11. The method of claim 10,
In step d, the slurry is coated on a metal foil, dried within a temperature range of 70 to 90° C., and after rolling, the metal foil is removed in water or a base solution. Self-supporting type A method of manufacturing a (self-supported) gas diffusion layer.

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