An ordered hierarchical porous carbon electrode and a preparation method thereof

Abstract

This invention provides an ordered ladder-porous carbon electrode and its preparation method. The electrode preparation method includes: (1) synthesis of a carbonization precursor solution; (2) spin-coating a single layer of polystyrene microspheres onto a silica substrate; (3) filling the gaps between the microspheres with the precursor solution using sputtering or immersion methods; (4) carbonizing the template and precursor combination in a muffle furnace at high temperature and removing the template; (5) repeating steps (2)-(4) layer by layer using microspheres of different sizes to create pores, thereby obtaining an ordered ladder-porous microporous layer (MPL); (6) transferring the prepared ordered ladder-porous microporous layer from the silica substrate to a gas diffusion layer (GDL) and hot-pressing it with other components into a single cell. This invention forms ordered ladder-porous macropores and randomly distributed micropores inside the electrode, and gas-liquid separation channels are formed inside the electrode under capillary action, effectively improving the material transport efficiency inside the electrode. Meanwhile, the present invention can also regulate the balance of liquid water inside the electrode through ordered stepped macropores, which can retain liquid water under low pressure conditions to meet the hydration function of the proton exchange membrane and effectively remove excess liquid water under high pressure conditions to prevent water flooding inside the battery, thus effectively improving the electrochemical performance of the battery.

Description

Technical Field

[0001] This invention relates to the field of proton exchange membrane fuel cell electrodes, and more particularly to an ordered ladder porous carbon electrode and its preparation method. Background Technology

[0002] In recent years, advancements in hydrogen production using proton exchange membrane fuel cells (PEMs) have made them one of the most promising clean energy technologies, potentially alleviating energy shortages and environmental pollution crises. However, the transport and distribution of liquid water significantly impacts fuel cell performance. A fuel cell mainly comprises a proton exchange membrane (PEM), a catalyst layer (CL), a microporous layer (MPL), a gas diffusion layer (GDL), and bipolar plates (BP), among which the MPL plays a crucial role in water management. MPLs are typically made of carbon black powder, hydrophobic PTFE, and binder materials, with a thickness usually tens of micrometers. The function of the MPL is similar to that of the GDL, the biggest difference being its smaller pore size, primarily used to improve the transport of reactant gases and liquid water. Studies have shown that conventional MPLs are ineffective in achieving higher power outputs within fuel cells under high humidity conditions, while layered MPL designs can prevent flooding and maintain a balance in hydration. [1] Therefore, it is essential to improve the pore structure of the microporous layer to optimize the distribution of liquid water in the fuel cell. Summary of the Invention

[0003] Purpose of the invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide an ordered, stepped porous carbon electrode and its preparation method. This invention forms ordered stepped macropores and randomly distributed micropores within the electrode, creating gas-liquid separation channels under capillary action, effectively improving the mass transport efficiency within the electrode. Simultaneously, the ordered stepped macropores can regulate the balance of liquid water within the electrode, retaining liquid water under low-pressure conditions to meet the hydration requirements of the proton exchange membrane, and effectively removing excess liquid water under high-pressure conditions to prevent flooding within the battery, thus significantly improving the electrochemical performance of the battery.

[0005] Technical solution

[0006] The present invention provides a method for preparing an ordered ladder-type macroporous electrode, characterized by comprising the following steps:

[0007] (1) Dissolve the carbon precursor in the appropriate organic solvent to prepare a carbon precursor solution;

[0008] (2) Nanoscale microspheres and micron-scale microspheres are uniformly dispersed in an ethanol solution to prepare a first-layer polystyrene microsphere template on a silica substrate;

[0009] (3) The dried microsphere template is immersed in the precursor solution, and the precursor solution is used to fill the pores between the microspheres to obtain a combination of polystyrene microspheres and precursor.

[0010] (4) After drying the obtained conjugate, it is soaked in tetrahydrofuran to remove the template by dissolving the polystyrene microspheres, thereby forming microsphere-like pores;

[0011] (5) After cleaning and drying the obtained sample, it is placed in a tube furnace for high-temperature carbonization to obtain a porous carbon layer.

[0012] (6) Using the carbon layer prepared above as a substrate, use micron-sized microspheres of different sizes layer by layer, repeat steps (2)-(5) to obtain a ladder-like porous structure.

[0013] (7) Transfer the sample from the silica substrate to the gas diffusion layer and hot-press it with other components into a single cell.

[0014] Furthermore, the carbon precursor solution preparation scheme in step (1) is a general preparation method; preferably, the carbon precursor is one of carbon-containing compounds, carbon-containing polymers or sugars, which can be pyrolyzed into porous carbon at high temperature; preferably, the organic solvent is a methanol solution.

[0015] Further, the method of uniformly dispersing the microspheres in step (2) is ultrasonic dispersion; preferably, the diameter of the nanoscale microspheres is 100nm to 500nm; preferably, the diameter of the micron-scale microspheres is 10um to 100um.

[0016] Further, the preparation method of the single-layer polystyrene microsphere template in step (2) is spin coating; preferably, the mass concentration of the polystyrene microsphere dispersion used in the spin coating method is 0.1% to 10%; preferably, the spin coating speed used in the spin coating method is 500 rpm to 3000 rpm; preferably, the spin coating time used in the spin coating method is 30s to 60s.

[0017] Furthermore, the template described in step (3) is soaked in the precursor solution for 3 hours.

[0018] Further, the template removal in step (4) includes a drying and soaking process; preferably, the drying temperature is 50°C; preferably, the soaking time is 24 hours.

[0019] Further, the carbonization in step (5) includes centrifugal cleaning, drying, and high-temperature carbonization; preferably, the drying temperature is 90°C; preferably, the high-temperature carbonization temperature is 750°C. Preferably, the high-temperature carbonization time is 2 hours. Preferably, the protective gas for the high-temperature carbonization is any one of nitrogen, helium, argon, and neon.

[0020] Furthermore, in step (6), the substrate is changed to the porous carbon layer prepared previously during the repeated process (2)-(5).

[0021] Further, the sample transfer in step (7) involves chemically etching to remove the silicon dioxide substrate and transferring it to the gas diffusion layer; preferably, the gas diffusion layer is Toray carbon paper.

[0022] Furthermore, the single-cell assembly method described in step (7) is a conventional fuel cell hot-press assembly method.

[0023] Beneficial effects

[0024] The key point of this invention is to design larger vias, especially vias exhibiting a stepped structure, within a small-aperture MPL layer. The via sequence is a crucial factor, with a positive gradient being optimal. Specifically:

[0025] The invention forms ordered, stepped macropores and randomly distributed micropores inside the electrode. Under capillary action, the liquid water exits through the stepped macropores, while the reactant gases enter through the ordered micropores. This gas-liquid separation reduces the obstruction of liquid water to reactant gas transport, effectively improving the mass transport efficiency inside the electrode. Simultaneously, the invention can also balance the liquid water demand inside the electrode through the ordered stepped macropores. Under low-pressure (low-humidity) conditions, the micropores near the catalyst layer in the pore size-increasing structure from the MPL|CL interface to the MPL|GDL interface can retain liquid water to meet the hydration requirements of the proton exchange membrane, thereby improving the proton transport efficiency inside the electrode. Under high-pressure (high-humidity) conditions, after liquid water breaks through the first layer of micropores in the aforementioned increasing structure, excess liquid water can be quickly discharged, preventing overflow inside the electrode and effectively improving the electrochemical performance of the battery. Attached Figure Description

[0026] The disclosed settings are described below with reference to the accompanying drawings and examples, wherein:

[0027] Figure 1 This is a schematic diagram of the fabrication process of an ordered ladder porous electrode;

[0028] Figure 2 This is a schematic diagram of the main components and structure of a proton exchange membrane fuel cell;

[0029] Figure 3 This is a schematic diagram of the 3D structure of an ordered ladder-type porous electrode; wherein: buffer zone 1, ordered ladder-type microporous layer 2, gas diffusion layer 3, and gas channel 4.

[0030] Figure 4 This is a schematic diagram of the cross-section of an ordered stepped porous electrode with different gradients;

[0031] Figure 5 The comparison of liquid water saturation in the GDL of the three embodiment models under different driving pressure conditions is shown.

[0032] Figure 6 The comparison of reactant gas saturation in the MPL of the three embodiment models under different driving pressure conditions is shown. Detailed Implementation

[0033] The technical solution of this invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0034] Example 1

[0035] Ordered ladder porous electrodes prepared by template method, such as Figure 3 As shown, the fabrication process of the ordered ladder porous electrode is as follows: Figure 1 Perform the steps shown:

[0036] Step 1: Dissolve 10.87g of zinc nitrate crystals and 9g of 2-methylimidazole in 60ml of methanol solution to prepare a carbon precursor solution;

[0037] Step 2: Ultrasonically disperse polystyrene microspheres with a diameter of 500 nanometers and polystyrene microspheres with a diameter of 10 micrometers into an ethanol solution to prepare a suspension with a mass fraction of 5%.

[0038] Step 3: A 2.5% suspension is dropped onto a silica substrate with a spin coating process of 600 rpm * 30 s + 2000 rpm * 30 s to prepare the first layer of microsphere template. The uniform rotation of the substrate is achieved using a spin coater. Defects in the first layer of microsphere template are reduced by dropping the solution multiple times.

[0039] Step 4: After drying and fixing the prepared first-layer template at 60 degrees Celsius, immerse it in the precursor solution for 3 hours to ensure that the precursor solution fully fills the pores between the microspheres.

[0040] Step 5: Remove the template from the precursor solution and dry it at 50°C for 8 hours. Then, immerse the sample in tetrahydrofuran to remove the template.

[0041] Step 6: Centrifuge and clean the sample after removing the template, and dry it at 90℃ for 30 min. Then, place the dried sample in a tube furnace and carbonize it at 750℃ for 2 h.

[0042] Step 7: Use micron-sized microspheres with diameters of 12um, 16um, and 20um in sequence, and repeat steps (2)-(5) twice to prepare an ordered three-level positive gradient porous microporous layer.

[0043] Step 8: Transfer the sample from the silicon dioxide substrate to the gas diffusion layer, and hot-press it with other components to form an electrode, such as... Figure 3 As shown.

[0044] Example 2

[0045] The difference between this embodiment and Embodiment 1 is that the micron-sized microspheres used in step 7 are changed to 20µm, 16µm, and 12µm, thereby preparing an ordered three-level negative gradient porous microporous layer, such as... Figure 3 As shown.

[0046] Comparative Example 1

[0047] The through-hole design in this comparative example has been shown by some studies to be an improvement. [2] Therefore, the comparative example used in this invention can demonstrate the beneficial effects of this invention, and its structural preparation steps are as follows:

[0048] Step 1: Dissolve 10.87g of zinc nitrate crystals and 9g of 2-methylimidazole in 60ml of methanol solution to prepare a carbon precursor solution;

[0049] Step 2: Disperse polystyrene microspheres with a diameter of 500 nanometers of the same mass into an ethanol solution by ultrasonication to prepare a suspension with a mass fraction of 5%.

[0050] Step 3: A 2.5% suspension is dropped onto a silica substrate with a spin coating process of 600 rpm * 30 s + 2000 rpm * 30 s to prepare the first layer of microsphere template. The uniform rotation of the substrate is achieved using a spin coater. Defects in the first layer of microsphere template are reduced by dropping the liquid multiple times.

[0051] Step 4: After drying and fixing the prepared first-layer template at 60 degrees Celsius, immerse it in the precursor solution for 3 hours to ensure that the precursor solution fully fills the pores between the microspheres.

[0052] Step 5: Remove the template from the precursor solution and dry it at 50°C for 8 hours. Then, immerse the sample in tetrahydrofuran to remove the template.

[0053] Step 6: Centrifuge and clean the sample after removing the template, and dry it at 90℃ for 30 min. Then, place the dried sample in a tube furnace and carbonize it at 750℃ for 2 h.

[0054] Step 7: Use laser transfer technology to uniformly perforate the carbonized microporous layer, with the pore size set to 9 micrometers, to ensure consistent porosity;

[0055] Step 8: Transfer the sample from the silicon dioxide substrate to the gas diffusion layer, and hot-press it with other components to form an electrode, such as... Figure 3 As shown.

[0056] Testing: For the ordered ladder porous electrodes provided in Example 1, Example 2, and Comparative Example 1, the following were established based on the Lattice-Boltzmann method: Figure 2 and 3 The model was used to investigate the effect of an ordered, stepped porous structure on the distribution and transport of liquid water inside the electrode. The test results are as follows: Figure 4 As shown.

[0057] analyze:

[0058] Figure 5 The data shows that when the driving pressure is 10 kPa, the saturation of liquid water in the GDL of Example 1 is 0, indicating that the positive gradient macroporous structure can prevent liquid water loss under low humidity conditions. The negative gradient macroporous structure in Example 2 has a certain water retention effect compared to the through-hole structure in Comparative Example 1, but it is worse than that of Example 1. When the driving pressure increases, the saturation of liquid water in the GDLs of Examples 1, 2, and Comparative Example 1 is almost the same, indicating that under high humidity conditions, the positive gradient macroporous model, the negative gradient macroporous model, and the through-hole model can maintain the same efficient drainage capacity. The improvement of electrode drainage capacity by the through-hole model has been demonstrated in the research. [2] .

[0059] Figure 6 Data shows that the gas content in the MPL of Example 1 model was 270.5%, 82.6%, and 14.5% higher than that of Comparative Example 1 model at 10 kPa, 15 kPa, and 20 kPa, respectively. This result indicates that the positive gradient macroporous structure can provide more channels for reactant gas transport. Under low pressure, i.e., low humidity conditions, the increased reactant gas content in the positive gradient macroporous structure improves the reaction efficiency of the catalyst layer, thereby increasing the amount of liquid water produced. When the liquid water increases, the driving pressure increases, and the reactant transport rate decreases accordingly while the drainage capacity increases, thus achieving a dynamic balance between mass transport and reaction inside the battery. However, the gas content in the MPL of Example 2 model was 12.5%, -14.7%, and -16.6% higher than that of Comparative Example 1 model at 10 kPa, 15 kPa, and 20 kPa, respectively. The difference between Example 2 and Example 1 lies only in the arrangement of macropores, which shows that the positive gradient macroporous structure is a key factor in improving reactant gas transport.

[0060] In summary, the arrangement of macropores is a key factor, and Example 1 is the best; the ordered positive gradient porous electrode can regulate the balance of liquid water inside the electrode under different humidity conditions, improve the transport of substances, and thus enhance the electrochemical performance of the fuel cell.

[0061] References

[0062] [1]

[0063] [2] Nagai, Y., J. Eller, T. Hatanaka, S. Yamaguchi, S. Kato, A. Kato, F. Marone, H. Xu & F. N. Büchi (2019). Improving water management in fuel cells through microporous layer modifications: Fast operando tomographic imaging of liquidwater. Journal of Power Sources, 435, 226809.

Claims

1. A method for preparing a single cell of a fuel cell, characterized in that, This is achieved through the following steps: (1) Dissolve the carbon precursor in methanol to prepare a carbon precursor solution; the carbon precursor is a carbon-containing compound; (2) Nanoscale polystyrene microspheres and micron-scale polystyrene microspheres are uniformly dispersed in an ethanol solution to prepare a first layer of polystyrene microsphere template on a silica substrate; (3) The dried first layer of polystyrene microsphere template is immersed in carbon precursor solution to fill the pores between microspheres with carbon precursor solution to obtain a combination of polystyrene microspheres and carbon precursor. (4) After drying the obtained conjugate, it is soaked in tetrahydrofuran to remove the template by dissolving the polystyrene microspheres, thereby forming microsphere-like pores; (5) After cleaning and drying the obtained sample, it is placed in a tube furnace for high-temperature carbonization to obtain a porous carbon layer. (6) Using the porous carbon layer prepared above as a substrate, use micron-sized microspheres of different sizes layer by layer to repeat steps (2)-(5) to obtain a ladder porous structure, which is a positive or negative gradient. (7) Transfer the sample from the silica substrate to the gas diffusion layer and hot-press it with other components into a single cell.

2. The method according to claim 1, characterized in that, The dispersion method described in step (2) is ultrasonic dispersion; The diameter of the nanoscale polystyrene microspheres is 100nm~500nm; the diameter of the micron-scale polystyrene microspheres is 10μm~100μm.

3. The method according to claim 1, characterized in that, The preparation method of the first layer of polystyrene microsphere template in step (2) is spin coating; the spin coating speed used in the spin coating method is 500. rpm~ 3000 rpm The spin coating method uses a spin coating time of 30s-60s.

4. The method according to claim 1, characterized in that, The template described in step (3) is soaked in the carbon precursor solution for 3 hours.

5. The method according to claim 1, characterized in that, The template removal in step (4) includes a drying and soaking process; the drying temperature is 50°C; and the soaking time is 24 hours.

6. The method according to claim 1, characterized in that, The drying temperature in step (5) is 90°C; the high-temperature carbonization temperature is 750°C; the high-temperature carbonization time is 2 hours; and the protective gas for the high-temperature carbonization is any one of nitrogen, helium, argon, and neon.

7. The method according to claim 1, characterized in that, In step (6), the substrate is changed to the porous carbon layer prepared previously during the repeated steps (2)-(5).

8. The method according to claim 1, characterized in that, In step (7), the sample transfer involves chemical etching to remove the silicon dioxide substrate and transferring it to the gas diffusion layer; the gas diffusion layer is made of carbon paper.

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