CN114497582B - Preparation method of membrane electrode catalytic layer - Google Patents

Abstract

The invention discloses a preparation method of a membrane electrode catalytic layer. The membrane electrode comprises an anode catalytic layer, a proton exchange membrane and a cathode catalytic layer; the membrane electrode is stretched by the catalytic layer substrate, so that cracks appear in the catalytic layer. And transferring the catalytic layer with the cracks onto the proton membrane, and improving the water vapor transmission efficiency of the catalytic layer by utilizing the cracks. The method can reduce mass transfer polarization of the battery and optimize the performance of the battery.

Description

Preparation method of membrane electrode catalytic layer

Technical Field

The invention belongs to the technical field of fuel cells, and particularly relates to a preparation method of a membrane electrode catalytic layer.

Background

The membrane electrode consists of a diffusion layer, a catalytic layer and a proton exchange membrane, is an electrochemical generation place of a proton exchange membrane fuel cell, and is a core component for chemical energy and electric energy conversion. The performance of a fuel cell suffers from three aspects, namely electrochemical polarization, ohmic polarization, mass transfer polarization. The catalytic layer has low porosity and few through holes, and is a main component for mass transfer polarization of the membrane electrode. Therefore, the improvement of the mass transfer efficiency of the catalytic layer is of great significance for improving the battery performance. Currently, there are several kinds of methods for improving the mass transfer efficiency of the catalytic layer. Firstly, the slurry for preparing the catalytic layer is regulated by a solvent and an additive, and a large number of secondary holes are formed in the preparation and forming process of the catalytic layer, so that a convenient channel for oxygen is formed. The method needs to greatly study the compatibility and the suitability of various materials, has low flexibility in material use and is not beneficial to quickly manufacturing the membrane electrode applicable to different requirements. Second, platinum is supported on an ordered nanoarray as a catalytic layer. The catalytic layer has a large number of straight-through pore channels, and the pore channels have high through hole rate and low tortuosity, so that the catalytic layer is the most ideal oxygen mass transfer path. However, the cost of this preparation process is high, and there are few mass producers in addition to the NSTF series proposed by 3M company. The NSTF family is also stalled due to a number of problems. Third, the catalytic layer is prepared from fibrous material. By using fibrous platinum carbon catalyst or ion resin, the length thereof can be utilized to lap into a loose structure, forming large-sized secondary pores. However, the formation of the fibers requires the addition of polymers, and the effect of these polymers on catalyst activity and battery durability needs to be further considered.

The prior patent with the application number 201910136626.8 provides a method for continuously preparing a membrane electrode ordered catalytic layer in a large scale, wherein the catalytic layer is prepared by modifying polytetrafluoroethylene fibers by nano catalysts, then mixing, melting, extruding and calendaring the polytetrafluoroethylene fibers, perfluorosulfonic acid resin and carbon fibers to prepare a catalyst membrane embryo, and finally heating and longitudinally stretching the catalyst membrane embryo. The carbon fiber and the catalyst are orderly arranged in a fibrous shape, channels are provided for electron and proton transmission, and micro-channels which are beneficial to gas and water transmission are reserved after the catalyst is attached to the proton exchange membrane, so that the catalytic layer is provided with micro-channels, the hydrophobic transmission of gas and the removal of water are facilitated, and the catalytic activity and durability are improved. But the addition of polytetrafluoroethylene greatly improves the hydrophobicity of the catalytic layer, which is unfavorable for the humidity condition of 30-40% commonly used in the industry at present. In addition, although the bulk mass transfer efficiency after stretching is enhanced, polytetrafluoroethylene is attached to the surface of the catalyst, but local mass transfer resistance is increased, and the low-platinum design of the future membrane electrode is not facilitated. For the most commonly used platinum carbon catalysts supported on carbon black, it is difficult to form a continuous, uniform catalytic layer even with the addition of polytetrafluoroethylene.

In order to improve the performance of the membrane electrode and even the fuel cell, it is necessary to propose a preparation method of a catalytic layer of the membrane electrode, so as to promote the technical development in the field of fuel cells.

Disclosure of Invention

The invention aims at overcoming the defects in the prior art and provides a preparation method of a membrane electrode catalytic layer. The invention utilizes a simplified preparation method to introduce a large-scale oxygen transmission channel into the catalytic layer.

The membrane electrode comprises an anode catalytic layer, a proton exchange membrane and a cathode catalytic layer; the membrane electrode is stretched by the catalytic layer substrate, so that cracks appear in the catalytic layer. And transferring the catalytic layer with the cracks onto the proton membrane, and improving the water vapor transmission efficiency of the catalytic layer by utilizing the cracks. The method can reduce mass transfer polarization of the battery and optimize the performance of the battery.

The invention aims at realizing the following technical scheme:

The invention provides a preparation method of a membrane electrode catalytic layer, which comprises the following steps:

s1, selecting a material easy to deform to prepare a stretchable substrate;

s2, adding the platinum carbon catalyst and the ion resin solution into a solvent and stirring to obtain a catalyst layer slurry;

And S3, coating the catalyst layer slurry prepared in the step S2 on the stretchable substrate prepared in the step S1, and stretching the substrate after the slurry is dried to generate the required deformation quantity, so as to obtain the membrane electrode catalyst layer coated on the stretchable substrate.

As an embodiment of the present invention, the membrane electrode catalytic layer has gas transmission channels formed by stretching distributed therein. Compared with the original secondary pore canal of the catalytic layer, the pore canal has larger size and more orderly direction, so that the oxygen transmission efficiency is greatly improved. The invention stretches the substrate after preparing the catalytic layer on the deformable substrate. Because the strength and state of the catalytic layer are similar to dry soil, the catalytic layer may crack due to stretching without strength. And finally, thermally compounding the catalytic layer with the cracks on the proton exchange membrane. Under the condition that the preparation process of the material and the catalytic layer is determined, the generated crack size and density are basically the same, and the oxygen mass transfer efficiency is improved.

As one embodiment of the present invention, the length of the gas transmission channel is 0.8mm-1.2mm and the width is 0nm-200nm. The direction of cracks generated by stretching the catalytic layer is vertical to the stretching direction, the length distribution is about 1mm, the maximum width reaches 100-200nm, the catalytic layer is far larger than the secondary channels in the unstretched catalytic layer (the gaps accumulated by the spherical catalyst are the secondary channels in the conventional catalytic layer without stretching, and the pore diameters are generally distributed at 30-60 nm), and the large-size cracks are favorable for water and gas flowing. The size and the density of the crack of the catalytic layer can also be controlled by controlling the deformation of the substrate so as to adapt to different battery working condition characteristics.

As one embodiment of the present invention, the Pt loading of the catalytic layer is 0.25-0.3mg cm ^-2.

As an embodiment of the present invention, the deformable material described in step S1 comprises one of PTFE, PI, ETFE, PE, PET.

As an embodiment of the present invention, the mass ratio of the ionic resin to the carbon in the platinum carbon catalyst in step S2 is 0.6 to 1:1.

As an embodiment of the present invention, the ionic resin solution in step S2 is a 20wt% Nafion solution.

As an embodiment of the present invention, in step S2, the solvent includes a binary or ternary solvent composed of isopropyl alcohol, n-propyl alcohol, t-butyl alcohol, water, DMF.

As an embodiment of the present invention, the deformation amount in step S3 is 5% -15%.

As one embodiment of the present invention, the stretching in step S3 has a stretching rate of 1 to 2 μm/S.

As an embodiment of the present invention, the stretching in step S3 is stretching using a universal tester.

As an embodiment of the present invention, the substrate is stretched in step S3 and then subjected to secondary stretching. The secondary stretching is to recover the width thereof by secondary stretching in the width direction. The substrate is stretched along the length direction under the action of the tensile force, and after stretching, the substrate can shrink to a certain extent along the width direction, and then the width of the substrate is restored by secondary stretching.

The invention also provides a membrane electrode, which comprises a proton exchange membrane, and anode catalytic layers and the cathode catalytic layers on two sides of the proton exchange membrane.

The invention also provides a preparation method of the membrane electrode, which comprises the following steps:

a1, adding a platinum carbon catalyst and an ionic resin solution into a solvent, stirring to obtain a catalytic layer slurry, coating the catalytic layer slurry on a substrate, and drying to obtain an anode catalytic layer;

a2, thermally compounding the anode catalytic layer prepared in the step A1 and the membrane electrode catalytic layer in the claim 1 on two sides of the proton exchange membrane, and stripping the substrate to obtain the membrane electrode.

In step A1, the anode catalyst layer has a Pt loading of 0.05 to 0.1mg cm ^-2 as an embodiment of the present invention.

The invention utilizes mechanical stretching to manufacture cracks on the catalytic layer of the fuel cell, and utilizes the cracks to improve the water vapor transmission efficiency. The invention focuses on the construction mode of the catalytic layer to improve the oxygen mass transfer performance, but does not take measures for changing the catalyst, but improves the distribution mode of the ion resin to improve the electrochemical reaction efficiency in the catalytic layer, which is all attributed to the construction method of the catalytic layer.

Compared with the prior art, the invention has the following beneficial effects:

1. The invention utilizes a simple physical method to manufacture the high-efficiency gas transmission channel in the catalytic layer, improves the gas transmission efficiency, can obviously reduce the mass transfer polarization of the battery and improves the battery performance;

2. the invention only uses conventional platinum-carbon catalyst, ion resin and other materials, does not need pore-forming agent and other chemical auxiliary agents, and avoids the preparation of ordered arrays or carbon fiber and other complex materials.

3. Based on the mechanical characteristics of materials (a substrate and a catalytic layer), the invention utilizes a universal experiment machine to accurately stretch a sample, and can effectively control the size of a crack, thereby adjusting the performance of a battery.

Drawings

Other features, objects and advantages of the present invention will become more apparent upon reading of the detailed description of non-limiting embodiments, given with reference to the accompanying drawings in which:

fig. 1 is a graph showing the results of battery performance tests of examples 1 to 3 and comparative example 4;

FIG. 2 shows the mass transfer resistance test results of examples 1-3 and comparative example 4.

Detailed Description

The invention will now be described in detail with reference to the drawings and specific examples. The following examples will assist those skilled in the art in further understanding the present invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications could be made by those skilled in the art without departing from the inventive concept. These are all within the scope of the present invention.

According to the invention, the catalytic layer of the fuel cell is cracked by mechanical stretching, and the water vapor transmission efficiency is improved by utilizing the cracks. The appearance of large-size cracks provides a smooth channel for water vapor transmission, can effectively reduce mass transfer polarization of the catalytic layer, and improves the battery performance. In the present invention, the battery performance test temperature is 80℃and the humidity is 40% or 100%. The test backpressure was 150KPaabs. The flow channel selected by the battery is a 5-channel serpentine flow field with 5cm by 5cm, and the metering ratio of the test gas is H2:air=2:2. The mass transfer resistance test temperature of the battery is 80 ℃ and the humidity is 67%. The test backpressure was 150KPaabs. The selected flow channel of the cell is a 5-channel straight flow field with the flow channel of 2cm x 1cm, the test hydrogen gas quantity is 800cc min ^-1, and the oxygen-nitrogen mixed gas quantity is 1500cc min ^-1. The oxygen partial pressure in the oxygen-nitrogen mixture was 4%.

To optimally embody the performance characteristics of the present invention, the present invention is implemented in the following manner.

Example 1

The embodiment provides a preparation method of a membrane electrode, which comprises the following specific steps:

s1, adding a platinum carbon catalyst and an ion resin solution (Nafion, 20% by weight) into a solvent, and stirring for 24 hours to obtain a catalyst layer slurry; wherein, the mass ratio (I/C) of the ionic resin to the carbon contained in the platinum carbon catalyst is controlled to be 0.7:1. The solvent was a mixed solvent of isopropyl alcohol and water (1:1 by vol.) with a platinum carbon catalyst concentration in the solvent of 10 mg.ml ^-1.

S2, coating the cathode catalytic layer slurry in the step S1 on a PTFE substrate by using a spraying machine, and drying, wherein the platinum loading of the catalytic layer on the substrate is controlled to be 0.25mg cm ^-2. And (3) placing the substrate carrying the catalytic layer on a universal testing machine, setting a tensile force to stretch the substrate by 3% along the length direction, generating certain shrinkage on the stretched substrate along the width direction, and secondarily stretching the substrate along the width direction to recover the width of the substrate, so as to obtain the stretched cathode catalytic layer coated on the stretchable substrate.

And S3, coating the catalytic layer slurry in the step S1 on a PTFE substrate by using a spraying machine, and drying, wherein the platinum loading of the substrate is 0.07mg cm ^-2. And (5) drying and then carrying out no stretching to obtain the anode catalytic layer coated on the substrate.

S4, thermally compounding the cathode catalytic layer and the anode catalytic layer which are described in S2 and S3 on two sides of the proton exchange membrane, and stripping the substrate to obtain the membrane electrode.

Example 2

Example 2 was identical to the preparation of example 1, except that the amount of stretch deformation in step S2 was 10%.

Example 3

Example 3 was identical to the preparation of example 1, except that the amount of stretch deformation in step S2 was 20%.

Comparative example 1

Comparative example 1 was the same as the preparation method of example 1 except that stretching was not performed in step S2.

Fig. 1 shows the battery test performance of each example and comparative example. As shown in fig. 1, when the substrate was stretched by 3% (example 1), the battery performance was substantially consistent with that of the unstretched comparative example, indicating that the stretching degree of 3% was small, and insufficient to have a substantial effect on the battery performance. When the degree of stretching reached 10% (example 2), the performance of the battery in the high current region was significantly improved, indicating that example 2 has superior mass transfer efficiency. The mass transfer advantage of example 2 is related to the cracks formed by stretching. The crack can be used as a large-scale water-gas transmission channel, so that water generated by the reaction is discharged in time, and the flooding of the battery is effectively prevented. When the elongation reached 20% (example 3), it was found that the catalytic layer was sufficiently broken, and the proton and electron transport channels inside it were not effectively connected, so example 3 exhibited poor battery performance. At the same time, the overstretching of the area significantly reduces the platinum loading per unit area, which is also a significant cause of poor performance in example 3.

The mass transfer resistances of the examples and comparative examples are given in fig. 2. The test results show that example 1 stretches to a lesser extent and fails to form effective fissures, thus exhibiting a higher mass transfer resistance similar to the comparative example. And in the embodiments 2 and 3, more cracks are formed after the materials are fully stretched, so that efficient water and gas transmission can be realized, and the mass transfer resistance of the materials is obviously reduced. It can also be demonstrated that the too low cell performance of example 3 is not due to mass transfer polarization, but rather to electrochemical polarization and ohmic polarization.

The foregoing describes specific embodiments of the present invention. It is to be understood that the invention is not limited to the particular embodiments described above, and that various changes and modifications may be made by one skilled in the art within the scope of the claims without affecting the spirit of the invention.

Claims (4)

1. A method for preparing a membrane electrode catalytic layer, the method comprising the steps of:

s1, selecting a material easy to deform to prepare a stretchable substrate;

s2, adding the platinum carbon catalyst and the ion resin solution into a solvent and stirring to obtain a catalyst layer slurry;

S3, coating the catalyst layer slurry prepared in the step S2 on the stretchable substrate prepared in the step S1, and stretching the substrate under the action of a tensile force after the slurry is dried, wherein the substrate is stretched along the length direction to generate a required deformation amount, so that the membrane electrode catalyst layer coated on the stretchable substrate is obtained;

the mass ratio of the ionic resin to the carbon in the platinum-carbon catalyst in the step S2 is 0.6-1:1;

In the step S2, the solvent comprises a binary or ternary solvent consisting of isopropanol, n-propanol, tertiary butanol, water and DMF;

the deformation amount in the step S3 is 5% -15%;

the stretching speed of the stretching in the step S3 is 1-2 mu m/S;

In the step S3, the substrate is stretched and then subjected to secondary stretching, wherein the secondary stretching is performed in the width direction to recover the width of the substrate;

The catalytic layer has no strength, and the catalytic layer is stretched to generate cracks;

gas transmission channels formed by stretching are distributed in the membrane electrode catalytic layer;

the length of the gas transmission channel is 0.8mm-1.2mm, and the width is 0nm-200nm.

2. The method of claim 1, wherein the deformable material in step S1 comprises one of PTFE, PI, ETFE, PE, PET.

3. A membrane electrode comprising a proton exchange membrane and anode catalytic layers on both sides of the proton exchange membrane and the membrane electrode catalytic layer of claim 1.

4. A method of producing the membrane electrode according to claim 3, comprising the steps of:

a1, adding a platinum carbon catalyst and an ionic resin solution into a solvent, stirring to obtain a catalytic layer slurry, coating the catalytic layer slurry on a substrate, and drying to obtain an anode catalytic layer;

a2, thermally compounding the anode catalytic layer prepared in the step A1 and the membrane electrode catalytic layer in the claim 1 on two sides of the proton exchange membrane, and stripping the substrate to obtain the membrane electrode.