Membrane electrode structure optimization method based on in-situ generated water volume change

Abstract

The invention discloses an in-situ generated water volume change-based membrane electrode structure optimization method, which comprises the following steps of: after activating a fuel cell, moderately purging a cathode side and an anode side by using air and hydrogen respectively, and applying a certain current while purging to ensure full wetting of a membrane; after purging is finished, the fuel cell is placed in a low-temperature box to be subjected to freezing and unfreezing circulation, gas and liquid transmission channels of a catalyst layer and a porous medium are fully opened by utilizing the tiny volume change of a small amount of residual in-situ generated water after purging in the fuel cell in the freezing and unfreezing circulation process, and the electrode structure is optimized to the maximum extent; the drainage capacity of the fuel cell is improved, the performance of a high-current-density area of the cell under the normal-temperature condition is improved, and the low-temperature starting capacity is improved. The method effectively solves the problem that liquid water is difficult to remove due to the phenomena that the pore size of the catalyst layer is too compact and the pore size distribution of the diffusion layer is unreasonable in the preparation process of the membrane electrode, and has very high practical value.

Description

Technical Field

[0001] The present invention relates to the technical field of fuel cells, and in particular to a membrane electrode structure optimization method based on volume changes of in-situ generated water. Background Art

[0002] Fuel cells are efficient and green energy conversion devices. Their cruising range is not affected by environmental factors. They are an ideal power source and have received extensive attention in the automotive field in recent years. However, their working principle makes efficient mass transfer in high-density areas and low-temperature conditions a technical problem. Drainage capacity is the main factor affecting mass transfer, which is mainly affected by the hydrophilicity, flow field structure, hydrophilicity and pore size distribution of the bipolar plate, and the catalyst layer and diffusion layer of the membrane electrode material. The appropriate pore size distribution can promote the rapid removal of water generated by the catalyst layer reaction, thereby improving the drainage capacity. During the preparation of the membrane electrode, the pore size of the catalyst layer cannot be accurately controlled due to the influence of the preparation process. In addition, although the pore size of the gas diffusion layer and the microporous layer can be designed accordingly, it cannot be accurately controlled during the production and processing. These will cause the pore size of the membrane electrode catalyst layer to be too dense and the pore size distribution of the diffusion layer to be unreasonable, which will hinder the removal of the water generated by the reaction, thereby affecting its drainage capacity. Therefore, developing a method that can simply and quickly improve the electrode structure and fully open up the reactant transmission channel and water transmission channel has a very high practical value for improving the mass transfer capacity of the new membrane electrode.

[0003] In the prior art, the method for optimizing the structure of a membrane electrode is to artificially create through holes of different pore sizes in the gas diffusion layer and the microporous layer during the preparation process to improve the drainage capacity, but it cannot improve the problem of uncontrollable pore size distribution caused by the production process during the membrane electrode manufacturing process. In addition, regarding a membrane electrode preparation method for optimizing the structure of the cathode catalyst layer, additives are added during the production process to adjust the pore size and porosity of the internal catalyst layer. Although it can effectively improve the mass transfer in the high-density area, it increases the complexity of the preparation process and cannot solve the drainage problem caused by the small pore size of the newly prepared membrane electrode. Summary of the invention

[0004] In view of the problems existing in the prior art, the present invention discloses a membrane electrode structure optimization method based on the volume change of in-situ generated water, which specifically includes the following steps:

[0005] Activating the fuel cell;

[0006] The cathode side and anode side of the fuel cell are purged with air and hydrogen respectively, and a certain current is applied during the purging to ensure the wetting of the membrane;

[0007] The fuel cell is placed in a cryogenic box, the storage temperature is controlled and subjected to freeze and thaw cycles;

[0008] Control the water content and freeze-thaw temperature in the membrane electrode after purging to indirectly control the volume expansion of the in-situ generated water and adjust the pore size distribution of the membrane electrode as needed;

[0009] By utilizing the volume change of a small amount of residual in-situ generated liquid water and membrane water in the fuel cell during the freeze-thaw cycle, the gas and liquid transmission channels of the catalyst layer and porous medium are fully opened, thereby maximizing the optimization of the electrode structure.

[0010] Furthermore, when the fuel cell is activated: the fuel cell is placed on a test platform for activation, the air humidity range is 50%-100%, the stoichiometric ratio range is 1.5-2.0, and the back pressure range is 50-100kPa; the hydrogen humidity range is 50%-100%, the stoichiometric ratio range is 1.5-2, the back pressure range is 50-100kPa, the activation voltage range is 0.4V-0.6V, and the activation time range is 4-8h.

[0011] Furthermore, dry air is introduced into the cathode side of the fuel cell with a gas flow rate of 8-10L / min and a back pressure of 40-80kPa, and dry hydrogen is introduced into the anode side with a gas flow rate of 2.5-5L / min and a back pressure of 50-80kPa. During the purge process, a current of 10-20A is applied to remove a large amount of liquid water in the porous medium and the flow channel, and the system is shut down after purging to a given voltage.

[0012] Furthermore, the purged fuel cell was placed in a cryogenic box, the temperature of the cryogenic box was set to -30°C, the freezing time was 12 hours, and after the freezing was completed, it was restored to room temperature to complete the experiment.

[0013] Due to the adoption of the above technical scheme, the present invention provides a membrane electrode structure optimization method based on the volume change of in-situ generated water. This method utilizes the tiny volume change of a small amount of in-situ generated water remaining after purging in the fuel cell during the freeze-thaw cycle, fully opens the gas and liquid transmission channels of the catalyst layer and the porous medium, optimizes the electrode structure to the maximum extent, plays a role in strengthening mass transfer, improves the drainage capacity of the fuel cell, improves the performance of the high-electric density area under normal temperature conditions of the battery, and improves the low-temperature starting ability. This method effectively solves the problem of difficulty in removing liquid water caused by the phenomenon of too dense pore size of the catalyst layer and unreasonable pore size distribution of the diffusion layer caused by the membrane electrode during the preparation process, and has very high practical value. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0015] Figure 1 This is a polarization curve diagram of the battery after constant voltage activation for 4 hours and freeze-thaw cycle once in the present invention;

[0016] Figure 2 It is a schematic diagram of the voltage change process of the new membrane electrode in the present invention being started at -5°C after constant voltage activation for 4 hours and being started at -5°C after one freeze-thaw cycle using the method described in this patent;

[0017] Figure 3 This is a graph showing the results of the diffusion layer mercury pressure test after constant pressure activation for 4 hours and one freeze-thaw cycle in the present invention;

[0018] Figure 4 This is a BET test result diagram of the catalyst layer after constant pressure activation for 4 hours and one freeze-thaw cycle in the present invention;

[0019] Figure 5 The voltage variation diagram of the proton exchange membrane fuel cell in Example 1 and Comparative Example 1 of the present invention during the -5°C low temperature start-up process;

[0020] Figure 6 The voltage variation diagram of the proton exchange membrane fuel cell in Example 1 and Comparative Example 3 of the present invention during the -5°C low temperature start-up process;

[0021] Figure 7 This is a voltage change diagram of the -7°C low-temperature startup process for the proton exchange membrane fuel cell in Example 2 and Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0022] In order to make the technical solutions and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention:

[0023] The present invention discloses a membrane electrode structure optimization method based on the volume change of in-situ generated water. The method indirectly controls the volume expansion coefficient of the in-situ generated water during the freezing process by controlling the water content and freeze-thaw temperature in the membrane electrode after purging, and realizes the on-demand regulation of the pore size distribution of the membrane electrode through different degrees of volume change, so as to achieve the optimization of the membrane electrode structure without any changes in mass production products, thereby enhancing the mass transfer capacity, promoting the removal of liquid water in the high-density area and the low-temperature start-up process, and improving the normal temperature performance and low-temperature start-up capability. The method mainly includes the following steps: first, activating the fuel cell; second, using air and hydrogen to appropriately purge the cathode side and the anode side of the fuel cell respectively, and applying a certain current to ensure the wetting of the membrane while purging; finally, placing the fuel cell in a low-temperature box, controlling the storage temperature, and performing a freeze and thaw cycle, and using the small volume change of the small amount of liquid water remaining in the fuel cell during the freeze-thaw cycle to fully open the gas and liquid transmission channels of the catalyst layer and the porous medium, and optimize the electrode structure to the maximum extent.

[0024] like Figure 1 The polarization curve of the battery after constant voltage activation for 4 hours and freeze-thaw cycle once is shown. Figure 1 It can be seen that after one freeze-thaw cycle using the method described in this application, the performance of the fuel cell in the high current density area is significantly improved.

[0025] Figure 2 It can be seen that when the newly prepared membrane electrode is activated at constant voltage for 4 hours and then started at low temperature, the voltage drops rapidly after about 25 seconds of reaction, indicating that a large amount of ice appears at this time, which seriously hinders gas transport and causes startup failure. When the method described in this patent is used to perform a freeze-thaw cycle once and then start at the same temperature, the battery voltage changes smoothly and starts successfully.

[0026] Figure 3 It can be seen that after one freeze-thaw cycle, the peak value of the pore size distribution of the diffusion layer increases by about 10 microns.

[0027] Figure 4 It can be seen that after one freeze-thaw cycle, the pores with a pore size of 3-6 nm in the catalytic layer disappeared, and the pores with a pore size of 5-50 nm increased.

[0028] In summary, after one freeze-thaw cycle, the pore size distribution of the gas diffusion layer and the catalyst layer was improved, and the transmission channels of gas and water were further opened up. The freeze-thaw cycle played a role in enhancing mass transfer, effectively promoted the removal of liquid water during the low-temperature start-up process, and improved the low-temperature start-up capability.

[0029] Example 1

[0030] This embodiment uses membrane electrode 1, and takes a full-area single cell with an effective area of ​​298 cm2 in actual engineering applications as the research object. A graphite bipolar plate is used, and a carbon paper coated with a microporous layer constitutes a gas diffusion layer. The thickness of the proton exchange membrane is 18 microns, and the platinum loadings of the anode and cathode are 0.4 mg / cm2 and 0.1 mg / cm2 respectively.

[0031] Battery activation process: The fuel cell is placed on the test platform, the air humidity is 100%, the stoichiometric ratio is 2.0, and the back pressure range is 100kPa; the hydrogen humidity is 100%, the stoichiometric ratio is 1.5, the back pressure is 100kPa, the activation voltage is 0.4V, and the activation time range is 4h;

[0032] Shutdown purge process: dry air is introduced into the cathode side of the fuel cell, with a gas flow rate of 10L / min and a back pressure of 40kpa, and dry hydrogen is introduced into the anode side, with a gas flow rate of 2.5L / min and a back pressure of 50kpa. During the purge process, a current of 10A is applied to remove a large amount of liquid water in the porous medium and the flow channel. The system is shut down after the voltage is purged to 0.8V.

[0033] Low-temperature starting experiment process: the starting temperature is -5℃, dry air and hydrogen are introduced, the air flow rate is 10L / min, the back pressure is 20kpa, the hydrogen flow rate is 2.5L / min, the back pressure is 40kpa, and the constant current starting method is adopted. The loading current is loaded to 105A at a loading rate of 3A / s. After loading is completed, the current is maintained unchanged.

[0034] Comparative Example 1:

[0035] 1. Battery activation: Place the fuel cell on the test platform, and introduce air with a humidity of 100%, a stoichiometric ratio of 2.0, and a back pressure range of 100kPa; introduce hydrogen with a humidity of 100%, a stoichiometric ratio of 1.5, a back pressure of 100kPa, an activation voltage of 0.4V, and an activation time range of 8h;

[0036] 2. Shutdown and purge: the purge conditions are the same as those in Example 1;

[0037] 3. Low temperature start-up experiment: The start-up conditions are the same as those in Example 1.

[0038] The results of the low temperature start-up experiment of Example 1 and Comparative Example 1 are compared as follows: Figure 5 As shown in the figure, it can be seen that extending the activation time cannot change the drainage capacity of the membrane electrode. When the voltage is loaded to about 60s, it is as low as 0.1V, and the low-temperature start-up performance of the fuel cell is not significantly improved.

[0039] Comparative Example 2:

[0040] 1. Battery activation: Place the fuel cell on the test platform, and introduce air with a humidity of 100%, a stoichiometric ratio of 2.0, and a back pressure range of 100kPa; introduce hydrogen with a humidity of 100%, a stoichiometric ratio of 1.5, a back pressure of 100kPa, an activation voltage of 0.4V, and an activation time range of 4h;

[0041] 2. Shutdown and purge: dry air is introduced into the cathode side of the fuel cell, with a gas flow rate of 10L / min and a back pressure of 40kpa. Dry hydrogen is introduced into the anode side, with a gas flow rate of 2.5L / min and a back pressure of 50kpa. During the purge process, a current of 10A is applied to remove a large amount of liquid water in the porous medium and the flow channel. The system is shut down after the voltage is purged to 0.8V.

[0042] 3. Freeze-thaw cycle: Place the purged fuel cell in a cryogenic box, set the temperature of the cryogenic box to -30°C, freeze for 12 hours, heat the cryogenic box to 30°C and stabilize for 2 hours to complete the experiment;

[0043] 4. Low temperature start experiment: the starting temperature is -5℃, dry air and hydrogen are introduced, the air flow rate is 10L / min, the back pressure is 20kpa, the hydrogen flow rate is 2.5L / min, the back pressure is 40kpa, and the constant current start mode is adopted. The loading current is loaded to 105A at a loading rate of 3A / s. After loading is completed, the current is maintained unchanged.

[0044] The results of the low temperature start-up test of Example 1 and Comparative Example 2 are as follows: Figure 2 As shown in the figure, it can be seen that the low-temperature startup capability of the fuel cell can be greatly improved after a freeze-thaw cycle, indicating that the volume change of the in-situ generated water after a freeze-thaw cycle has improved the pore size distribution of the membrane electrode, further opened up the mass transfer channel, and improved the drainage capacity.

[0045] Comparative Example 3:

[0046] 1. Battery activation: same as in Example 1;

[0047] 2. Shutdown and purge: dry air is introduced into the cathode side of the fuel cell, with a gas flow rate of 10L / min and a back pressure of 40kpa, and dry hydrogen is introduced into the anode side, with a gas flow rate of 2.5L / min and a back pressure of 50kpa. During the purge process, a current of 10A is applied to remove a large amount of liquid water in the porous medium and the flow channel. The system is shut down after the voltage is purged to 0.65V;

[0048] 3. Freeze-thaw cycle: Place the purged fuel cell in a cryogenic box, set the temperature of the cryogenic box to -30°C, freeze for 12 hours, heat the cryogenic box to 30°C and stabilize for 2 hours to complete the experiment;

[0049] 4. Low temperature start-up experiment: same as comparative example 2.

[0050] The results of the low temperature start-up test of Example 1 and Comparative Example 3 are as follows: Figure 6 As shown in the figure, when the purge end voltage is 0.65V, there is no residual liquid water in the electrode due to the dry purge, and no phase change of water occurs during the freeze-thaw process to cause the volume expansion of liquid water. The electrode structure does not change, and the drainage capacity does not improve, which is reflected in the lack of improvement in low-temperature starting performance.

[0051] Example 2: This example uses membrane electrode 2, which is a full-area single cell with an effective area of ​​298 cm2 in actual engineering applications as the research object. It uses a graphite bipolar plate, and a carbon paper coated with a microporous layer constitutes a gas diffusion layer. The thickness of the proton exchange membrane is 12 microns, and the platinum loadings of the anode and cathode are 0.25 mg / cm2 and 0.05 mg / cm2 respectively.

[0052] 1. Battery activation: same as in Example 1;

[0053] 2. Shutdown and purge: same as in Example 1;

[0054] 3. Low temperature start-up experiment: The start-up temperature is -7°C, and the rest is the same as in Example 1.

[0055] Comparative Example 4:

[0056] 1. Battery activation: same as comparative example 2;

[0057] 2. Shutdown and purge: same as comparative example 2;

[0058] 3. Freeze-thaw cycle: same as comparative example 2;

[0059] 4. Low temperature start-up experiment: the start-up temperature is -7°C, and the rest is the same as comparative example 2.

[0060] The results of the low temperature start-up test of Example 2 and Comparative Example 4 are compared as follows: Figure 7 As shown in the figure, after the membrane electrode 2 is subjected to one freeze-thaw cycle, the volume change of the in-situ generated water can also be used to optimize the pore size distribution of the membrane electrode, improve its drainage capacity, and thus improve the low-temperature start-up capability of the fuel cell.

[0061] Figure 5 It can be seen that there is no obvious difference in the voltage change during the low-temperature starting process after activation for different time periods, indicating that sufficient activation cannot change its low-temperature starting performance.

[0062] Figure 6 It can be seen that when the purge end voltage is 0.65V, the low-temperature starting performance of the battery has not been improved after one freeze-thaw cycle experiment, indicating that the purge is relatively dry and no phase change of water occurs during the freeze-thaw process to cause changes in the electrode structure.

[0063] Figure 7It can be seen that the membrane electrode 2 can also effectively improve the battery performance during the low-temperature startup process after one freeze-thaw cycle.

[0064] The method for optimizing the membrane electrode structure of a fuel cell proposed in the present invention is simple, practical and effective. The method uses air and hydrogen to appropriately purge the cathode side and the anode side of the activated battery respectively, and loads current during the purge process to achieve the removal of a large amount of liquid water in the porous medium while ensuring that a small amount of free water and membrane water remain in the proton exchange membrane, the catalyst layer and the gas diffusion layer. The volume expansion of a small amount of free water and membrane water during the freeze-thaw cycle improves the initial pore size distribution of the catalyst layer and the gas diffusion layer, further strengthens the establishment of water transmission channels, improves the drainage capacity of the membrane electrode, and thereby improves the high current density area performance and low-temperature startup capability of the fuel cell.

[0065] The method described in the patent of this invention can effectively solve the problem of the initial pore size being too dense caused by the membrane electrode preparation process. Reasonable control of the purge time can ensure that the freeze-thaw cycle does not cause damage to the membrane electrode while improving the pore size distribution, further opening up the mass transfer channel and greatly improving the performance of the high current density area of ​​the fuel cell.

[0066] The method described in the patent of this invention does not require any adjustment to the membrane electrode preparation process. The optimization of the membrane electrode pore structure can be achieved by accurately controlling the water content at the end of the purge and the freeze-thaw cycle temperature, thereby achieving the purpose of enhancing mass transfer and effectively improving the low-temperature startup capability at the membrane electrode structure level.

[0067] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A membrane electrode structure optimization method based on the volume change of in-situ generated water, characterized in that include: Activating the fuel cell; The cathode side and anode side of the fuel cell are purged with air and hydrogen respectively, and a certain current is applied during the purging to ensure the wetting of the membrane; The fuel cell is placed in a cryogenic box, the storage temperature is controlled and subjected to freeze and thaw cycles; Control the water content and freeze-thaw temperature in the membrane electrode after purging to indirectly control the volume expansion of the in-situ generated water and adjust the pore size distribution of the membrane electrode as needed; By utilizing the volume changes of a small amount of residual in-situ generated liquid water and membrane water in the fuel cell during the freeze-thaw cycle, the gas and liquid transmission channels of the catalytic layer and porous medium are fully opened, thereby maximizing the optimization of the electrode structure.

2. The membrane electrode structure optimization method based on the volume change of in-situ generated water according to claim 1, characterized in that: When activating the fuel cell: place the fuel cell on a test platform for activation, introduce air with a humidity range of 50%-100%, a stoichiometric ratio range of 1.5-2.0, and a back pressure range of 50-100kpa; introduce hydrogen with a humidity range of 50%-100%, a stoichiometric ratio range of 1.5-2, a back pressure range of 50-100kpa, an activation voltage range of 0.4V-0.6V, and an activation time range of 4-8h.

3. The membrane electrode structure optimization method based on the volume change of in-situ generated water according to claim 1, characterized in that: Dry air is introduced into the cathode side of the fuel cell with a gas flow rate of 8-10L / min and a back pressure of 40-80kpa. Dry hydrogen is introduced into the anode side with a gas flow rate of 2.5-5L / min and a back pressure of 50-80kpa. During the purge process, a current of 10-20A is applied to remove a large amount of liquid water in the porous medium and the flow channel. The system is shut down after purging to a given voltage.

4. The membrane electrode structure optimization method based on in-situ generated water volume change according to claim 1, characterized in that: The purged fuel cell was placed in a cryogenic box, the temperature of the cryogenic box was set to -30°C, the freezing time was 12 hours, and after freezing, it was restored to room temperature to complete the experiment.

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