Multi-scale coarse structure gas diffusion film and preparation method thereof

Abstract

The invention discloses a multi-scale rough structure gas diffusion film and a preparation method thereof. The gas diffusion membrane comprises a supporting layer and a microporous layer, wherein the supporting layer is formed by carrying out fusion molding on polyvinylidene fluoride and NaCl particles of a first size and removing NaCl, and the microporous layer is arranged on the supporting layer and formed by carrying out fusion molding on polyvinylidene fluoride, multi-walled carbon nanotubes and NaCl particles of a second size and removing NaCl. The surface of the gas diffusion film is subjected to solvent heat treatment to form a scaly rough structure, the gas diffusion film has super-hydrophobic performance, the water contact angle is larger than 160 degrees, the rolling angle is smaller than 5 degrees, and the CO2 flux is larger than 1.8 mL * cm <-2 > * min <-1 > * Pa <-1 >. According to the method, a sacrificial template method is combined with solvent heat treatment, the gas diffusion film with multi-scale pore channels and a super-hydrophobic surface is constructed, the gas mass transfer efficiency, the electrolyte infiltration resistance and the long-term operation stability of the gas diffusion film in an electrochemical CO2 reduction reaction are remarkably improved, and the gas diffusion film is suitable for a CO2 electrochemical reduction system under the condition of high current density.

Description

TECHNICAL FIELD

[0001] The present application relates to a multi-scale rough structure gas diffusion membrane and a preparation method thereof, and belongs to the technical field of electrochemical materials. BACKGROUND

[0002] Greenhouse effect as a major challenge to human society, global warming caused by it is the main reason for climate change and loss of biodiversity. As the main greenhouse gas CO2, due to the imbalance of carbon cycle caused by fossil fuel combustion and forest degradation, leading to the continuous accumulation of CO2, and causing sea level rise, ocean acidification and glacial melting and other serious ecological crisis. The international community has established a collaborative approach to emission reduction, which is based on energy structure transformation (development of renewable energy), carbon capture and storage and CO2 resource utilization. Current CO2RR technology covers electrochemical catalysis, thermal catalysis, photocatalysis and biological catalysis, etc. Among them, thermal catalysis is limited by harsh reaction conditions and secondary emissions, biological catalysis is affected by the lack of enzyme activity and strict anaerobic requirements, and photocatalysis faces the bottleneck of light energy utilization caused by material band gap defects, while electrochemical method has significant commercialization potential due to its mild reaction conditions, controllable reaction process and high product generation rate.

[0003] Most of the current research on CO2RR is still in the laboratory stage, and the commonly used reactor is the traditional three-electrode H-type electrolytic cell (H-Cell). However, this system is limited by the relatively long mass transfer distance and the low solubility of CO2 in aqueous electrolyte solution, which makes the H-Cell system mainly based on hydrogen evolution reaction at high current density, limiting the reaction efficiency of CO2RR. Compared with the mass transfer of CO2 in the liquid phase system, the diffusion coefficient of CO2 in the gas phase is ~0.1 cm 2 s -1) about 4 orders of magnitude higher than that of CO2 in the liquid phase, which makes it more suitable to use CO2 in the gas phase as the reactant source of CO2RR. Since there is no aqueous solution as a medium, the gas-phase CO2 itself cannot participate in the electrochemical reaction, and for this reason, researchers focus on the development of a gas-liquid-solid three-phase interface system of CO2RR based on the GDE flow-cell derived from fuel cell technology. The development of a three-phase interface system shortens the mass transfer distance of CO2 in the aqueous electrolyte solution from about 50 μm to about 50 nm, significantly improves the gas-phase mass transfer, and to some extent avoids the limitation of poor solubility of CO2 in the electrolyte solution. The catalytic current density of the Flow-Cell system is also significantly improved compared with the H-Cell, which benefits from its support for high-pH electrolyte to optimize the CO2RR electron transfer kinetics, realize the direct contact of gaseous CO2 with the catalyst, and effectively suppress the occurrence of the hydrogen evolution side reaction. Therefore, the use of Flow-Cell is a necessary condition for the industrial-scale application of CO2RR technology to realize industrial-grade CO2 electrolysis. A large number of existing studies focus on the electrolyte effect (cation / anion effect, local pH, electrolyte type and concentration) of the catalyst, the morphology effect (tip effect, area effect) and the surface modification (hydrophobicity regulation, chemical / electronic state modification), but the effect of improving the stability and durability of the electrode is still limited. In recent years, research has found that the reduction of the catalytic performance of GDE is also closely related to the interface characteristics change of GDL in the process of electrocatalysis, but due to the commercial gas diffusion membrane basically meets the performance required by GDL, the comprehensive and in-depth exploration of the interface microenvironment of GDL is still relatively scarce. It is worth noting that most of the commercial gas diffusion membranes tested at present are designed for fuel cells, and there are often problems of insufficient stability and durability in the CO2RR process (especially for the system generating ethylene and ethanol). Therefore, in order to optimize the catalytic performance (activity, selectivity, stability) of GDE, the basic research on the interface microenvironment of GDL should be as important as the research on the inherent characteristics of the catalyst, and the construction of GDL suitable for CO2RR is a necessary condition for promoting the development of CO2RR research.

[0004] In view of the fact that the current commercial GDL process is complex and cannot meet the requirements of the anti-permeability of the electrolyte solution of the membrane electrode in the long-term flushing process, the long-term stability and the durability under high cathode potential, the present application is proposed. SUMMARY

[0005] The purpose of the present application is to provide a gas diffusion membrane with a multi-scale rough structure. The present application uses a more easily processed hydrophobic material PVDF as the substrate of GDL, and multi-walled carbon nanotubes with excellent conductivity as the conductive filler. A composite gas diffusion membrane with a multi-scale rough structure is constructed by a sacrificial template method and solvent thermal induction.

[0006] The unique multi-scale rough structure of the gas diffusion membrane provided by the application significantly enhances the stability and durability of the GDE in the CO2RR process. In addition, the application also determines the change rule of the hydrophobicity of the GDL under the application of external voltage and the long-term flushing of the electrolyte solution through the electrowetting experiment and the flushing durability experiment, and reveals the change of the electrolyte solution flow direction in the GDL flooding process. This has very important significance for the subsequent design of the GDL and the stable super-hydrophobic properties of the GDL in the large-scale application of the future CO2RR technology.

[0007] The multi-scale rough structure gas diffusion membrane provided by the application comprises: a support layer, a three-dimensional through skeleton structure formed by melt forming polyvinylidene fluoride and NaCl particles of a first size and removing NaCl; a microporous layer, disposed on the support layer, a conductive and hydrophobic layer formed by melt forming polyvinylidene fluoride, multi-walled carbon nanotubes and NaCl particles of a second size and removing NaCl; The surface of the gas diffusion membrane is subjected to a solvothermal treatment to form a fish scale-like rough structure, and has super-hydrophobic properties.

[0008] Preferably, the particle size of the NaCl particles in the support layer ranges from 150 to 300 μm; The particle size of the NaCl particles in the microporous layer ranges from 50 to 100 μm.

[0009] Preferably, in the microporous layer, the mass ratio of the polyvinylidene fluoride, the multi-walled carbon nanotubes and the NaCl particles is 1:0.05-0.4:7, preferably 1:0.3:7.

[0010] Preferably, the treatment liquid used in the solvothermal treatment comprises an alcohol solvent, a cosolvent and water.

[0011] The alcohol solvent is an alcohol with 3-7 carbon atoms, preferably pentanol; When the size of the gas diffusion membrane is 2.5×2.5 cm 2 , in the treatment liquid, the content of the alcohol solvent is 3-5.5 mL, preferably 4 mL, the cosolvent is dilute HCl, the content is 12-17 mL, and the content of the water is 15 mL.

[0012] Preferably, the treatment temperature of the solvothermal treatment is 100-200℃, and the treatment time is 1-10 hours, preferably 5 hours at 150℃.

[0013] The water contact angle of the gas diffusion membrane of the application is greater than 160°, the rolling angle is less than 5°, and the CO2flux is greater than 1.8 mL·cm -2·min - 1·Pa - 1.

[0014] The application also provides a preparation method of the gas diffusion membrane, comprising the following steps: polyvinylidene fluoride is mixed with NaCl particles of a first size, and after melt molding, the NaCl is removed by water washing to form a support layer; polyvinylidene fluoride and multi-walled carbon nanotubes are mixed with NaCl particles of a second size, and after melt molding, the NaCl is removed by water washing to form a microporous layer on the support layer, thereby obtaining a composite membrane; The composite membrane is subjected to solvothermal treatment, cleaning and drying to obtain a gas diffusion membrane with a fish scale-like surface structure.

[0015] The gas diffusion membrane of the application is a high-performance gas diffusion layer, which is mainly applied in a flow electrolysis cell for gas-phase CO2 electrochemical reduction reaction (CO2RR) and is a core component of an electrode. In the CO2RR flow electrolysis cell, the gas diffusion membrane is directly used to construct a gas diffusion electrode (GDE), and its specific role and application mode are as follows: Construct a stable gas-liquid-solid three-phase reaction interface: one side of the membrane is exposed to a gas-phase CO2 flow, the other side is in contact with a liquid electrolyte, and the surface is loaded with a catalyst. Its unique multi-scale pore structure provides an efficient channel for CO2 gas transmission from the flow channel to the catalyst surface.

[0016] The super-hydrophobic properties of the surface (contact angle > 160°, rolling angle < 5°) can effectively prevent the penetration of liquid electrolyte to submerge the catalyst sites, thereby maintaining a stable, thin-layer three-phase reaction interface on the catalyst surface. This is the key to achieving high current density and high efficiency CO2RR.

[0017] Improve reaction efficiency and product selectivity: by ensuring the rapid mass transfer of CO2 gas, the reaction rate decline and side reactions (such as hydrogen evolution reaction, HER) competition caused by insufficient CO2 supply are avoided. The stable three-phase interface ensures the smooth transmission of reactants and products (such as ethylene, ethanol, etc.), which helps to improve the faradic efficiency of the target product.

[0018] Enhance the long-term stability and durability of the electrode: in the industrial-grade CO2RR process, the electrode needs to withstand high voltage, strong alkaline electrolyte and long-term operation. The excellent mechanical properties and electrolyte penetration resistance of the GDL of the application can effectively resist the pressure changes caused by potential fluctuations and product generation during operation, prevent structural collapse and performance degradation. Its solid structure and stable hydrophobicity make it have a longer service life than many commercial GDLs under the harsh conditions of generating C2+ products (such as ethylene, ethanol).

[0019] Compared with the prior art, the present application has the following remarkable advantages: Excellent superhydrophobic performance: through the synergistic effect of sacrificial template method and solvothermal induction, a multi-scale rough structure is successfully constructed on the film surface, realizing superhydrophobic characteristics, with a water contact angle as high as 167.3° and a rolling angle as low as 4.2°, which can effectively block electrolyte solution and prevent electrode liquid overflow.

[0020] Excellent gas mass transfer capacity: the multi-scale hierarchical pore structure (the large pores of the support layer ensure fast gas transmission, and the small pores of the microporous layer provide high Laplace pressure to block liquid) makes the CO2 membrane flux far exceed that of various commercial carbon papers / carbon cloths (such as Sigracet 22 BB, HCP330P, AvCarb P75T).

[0021] Good mechanical and conductive properties: by optimizing the template size and filler ratio, the gas diffusion membrane has sufficient mechanical strength (the tensile strength can reach 6.35×10 -2 MPa) and excellent conductivity (the conductivity can reach 0.29 S / m), ensuring its structural stability and electronic transmission efficiency in the electrochemical environment.

[0022] Significant improvement in stability: the gas diffusion membrane is specially designed for the harsh environment of CO2RR, and its solid structure and stable superhydrophobic surface make it show better durability than commercial GDL under long-term operation and high current density, providing a reliable electrode material basis for the industrial application of CO2RR technology. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is the surface morphology of the GDL prepared by sieving 50 mesh (a), 100 mesh (b), 150 mesh (c), 300 mesh (d) NaCl particles, sieving 50 mesh (e), 100 mesh (f), 150 mesh (g), 300 mesh (h) NaCl particles.

[0024] Figure 2 is the change in the infrared spectrum of PVDF before and after melting.

[0025] Figure 3 is the influence of fillers on the microporous layer morphology, with multi-walled carbon nanotubes (MWCNT) (a), graphene (b), CB (c), and stone (d) as fillers.

[0026] Figure 4 is the influence of filler type on the water contact angle of the microporous layer.

[0027] Figure 5Effect of mPVDF, mMWCNT and mNaCl ratio on the morphology of microporous layer: (a) 1:0.4:7, (b) 1:0.3:7, (c) 1:0.2:7, (d) 1:0.1:7, (e) 1:0.05:7.

[0028] Figure 6 Effect of filler amount on the water contact angle (a), resistivity and conductivity (b), tensile strength (c) of microporous layer.

[0029] Figure 7 Effect of alcohol carbon chain length on the surface morphology of PM-M: (a) 4 mL of propanol, (b) 4 mL of butanol, (c) 4 mL of pentanol, (d) 4 mL of hexanol, (e) 4 mL of heptanol.

[0030] Figure 8 Effect of alcohol carbon chain length on the surface wettability, membrane flux, resistivity and conductivity of PM-M: (a) Effect of alcohol carbon chain length on the surface wettability of PM-M; (b) Effect of alcohol carbon chain length on the flux of PM-M; (c) Effect of alcohol carbon chain length on the resistivity and conductivity of PM-M.

[0031] Figure 9 Effect of pentanol content on the surface morphology of PM-M: (a) 3 mL, (b) 3.5 mL, (c) 4 mL, (d) 4.5 mL, (e) 5 mL, (f) 5.5 mL.

[0032] Figure 10 Effect of pentanol content on the surface wettability, membrane flux, resistivity and conductivity of PM-M: (a) Effect of pentanol content on the surface wettability of PM-M; (b) Effect of pentanol content on the flux of PM-M; (c) Effect of pentanol content on the resistivity and conductivity of PM-M.

[0033] Figure 11 Effect of cosolvent content on the surface morphology of PM-M: (a) 12 mL, (b) 13 mL, (c) 14 mL, (d) 15 mL, (e) 16 mL, (f) 17 mL.

[0034] Figure 12 Effect of cosolvent dilute HCl content on the surface wettability, membrane flux, resistivity and conductivity of PM-M: (a) Effect of cosolvent dilute HCl content on the surface wettability of PM-M; (b) Effect of cosolvent dilute HCl content on the flux of PM-M; (c) Effect of cosolvent dilute HCl content on the resistivity and conductivity of PM-M.

[0035] Figure 13are the LSV curves of SPM-M, Sigracet 22 BB, AvCarb P75T and HCP330P.

[0036] Figure 14 are the CO2RR product distributions at different cathode potentials: (a) SPM-M; (b) Sigracet 22 BB; (c) AvCarb P75T; (d) HCP330P.

[0037] Figure 15 are the product faradaic efficiencies of GDEs at different voltages: (a) H2product faradaic efficiency; (b) total C product faradaic efficiency; (c) total C 2+ product faradaic efficiency. DETAILED DESCRIPTION

[0038] The experimental methods used in the following examples are conventional methods unless otherwise specified.

[0039] The materials, reagents, etc. used in the following examples can be obtained commercially unless otherwise specified.

[0040] The raw materials used in the following examples are as follows: Polyvinylidene fluoride powder (PVDF) was purchased from Zhongcheng Plastic. MWCNT was purchased from Shenzhen Suiheng Graphene Technology Co., Ltd. Propanol, butanol, pentanol, hexanol, heptanol, sodium chloride (NaCl), potassium hydroxide (KOH), potassium bicarbonate (KHCO3), potassium carbonate (K2CO3), potassium chloride (KCl), isopropanol and nano-copper powder were all of analytical purity and purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Carbon dioxide (CO2, 99.999%) was purchased from Beijing Huatong Jingke Gas Chemical Co., Ltd. Ion exchange membrane (3PK-130) was purchased from Gaoss Union Company. 5 wt.% Nafion solution was purchased from DuPont. Sigracet 22 BB (22BB) hydrophobic carbon paper, AvCarb P75T (P75T) hydrophobic carbon paper and HCP330P (330P) hydrophobic carbon cloth were purchased from SCI Materials Hub.

[0041] Preparation of PM-M: Firstly, 7 g of PVDF powder and a mixture of NaCl particles of certain size were filled into a 75 mm glass dish as the GDL support layer; then 3 g of PVDF powder, conductive filler and a mixture of NaCl particles of certain size were taken as the GDL microporous layer, heated and melted at 200 ℃ for 30 min, and then soaked in water for 24 h to remove NaCl, thereby constructing a hydrophobic composite GDL with a multi-scale pore structure. For convenience of description, the GDL without filler is referred to as a PVDF composite gas diffusion membrane, abbreviated as PM, and the GDL with MWCNT is referred to as a PVDF / PVDF-MWCNT composite gas diffusion membrane, abbreviated as PM-M.

[0042] SPM-M modification: A treatment solution containing dilute HCl, deionized water and a certain alcohol was prepared in a high-temperature and high-pressure kettle with a PTFE liner. A piece of 2.5 x 2.5 cm 2 PM-M was immersed in the solution. Solvothermal treatment was carried out in an oven at a certain temperature for a certain time. Then, the PM-M was taken out of the high-pressure kettle and then rinsed with ethanol and deionized water three times in an ultrasonic cleaner to remove the residual treatment solution. Finally, it was dried in an oven at 60 ℃ for 24 h.

[0043] Example 1, selection of gas diffusion layer template size The reaction time was controlled at 30 min, the reaction temperature was controlled at 200 ℃, PVDF with a molecular weight of 500,000 was selected as the base of the support layer, NaCl particles were used as the template, and the PVDF to NaCl ratio was fixed at 1:7. The effects of different template sizes on the GDL were investigated.

[0044] To clarify the effect of template size on the surface morphology of GDL, the effects of NaCl templates of different sizes on the surface morphology and pores of GDL were analyzed by SEM, and the results are shown in Figure 1 Figure 1 (a)-(d) are diagrams of NaCl templates of different sizes screened by sieves after ball milling of NaCl particles. Figure 1 (a) is the NaCl template screened by a 50-mesh sieve, with a particle size greater than 300 μm; Figure 1 (b) is the NaCl template screened by a 100-mesh sieve, with a particle size between 300 and 150 μm; Figure 1 (c) is the NaCl template screened by a 150-mesh sieve, with a particle size between 150 and 100 μm; Figure 1 (d) is the NaCl template screened by a 300-mesh sieve, with a particle size between 100 and 50 μm. Figure 1 ​(e)-(f) are the GDL surface morphologies synthesized by NaCl templates with 50 mesh, 100 mesh, 150 mesh and 300 mesh sieves, respectively, from Figure 1 It can be seen that, after mixing and melting of NaCl templates with different sizes and PVDF, the PVDF can form a three-dimensional skeleton structure stacked by spherical shell structures and penetrating each other. In addition, by comparing the pores formed on the membrane surface of GDL prepared by different mesh numbers, it can be found that the pore structure formed by GDL is almost consistent with the size of the template, so it is judged that the size of the GDL gas diffusion channel can be effectively controlled by adjusting the size of the template. At the same time, from Figure 1 (e)-(h), it can also be seen that, as the size of the template decreases, the surface roughness gradually increases, which will inevitably affect its macroscopic wetting properties.

[0045] From Figure 1 It can be seen that, although the size of the selected template can form a GDL with a three-dimensional skeleton structure penetrating each other, its macroscopic properties are not clear. For GDL, its structure can be divided into a microporous layer and a support layer. For the support layer, it plays a role of supporting the catalyst and the microporous layer, and providing a stable gas transmission channel for CO2. For the microporous layer, it plays a role of enhancing the electronic conductivity and optimizing the gas-liquid transmission channel at the three-phase interface. Therefore, the support layer of GDL needs to have certain mechanical strength and hydrophobicity, and the microporous layer needs to have excellent electrical conductivity and hydrophobicity. Therefore, the present application respectively measures the water contact angle, tensile strength and deformation of GDL with different templates, and the results are shown in Table 1.

[0046] From Table 1, it can be seen that the water contact angle of the GDL surface gradually increases with the decrease of the size of the template. It can be known that PVDF itself is a hydrophobic material, and the difference in hydrophobicity is because the decrease of the size of the template increases the roughness of the GDL surface. By measuring the GDL with different template sizes by the tensile mode of the texture analyzer, it is found that the tensile strength and deformation of the GDL first increase and then decrease with the decrease of the size of the template. When the NaCl particles sieved by 300 mesh are used as the template, the tensile strength of the GDL is only 0.72×10 -2 MPa, which is far lower than the tensile strength of the gas diffusion membrane prepared by other sizes of templates. This is because the NaCl particles sieved by 300 mesh have smaller size and larger specific surface area, so that the spherical shell structure formed by PVDF on the surface of the NaCl particles is thinner, so that the GDL will be broken under the action of lower tensile stress. With the increase of the size of the template, the tensile strength of the GDL gradually increases, and when the NaCl with a mesh number of 100 is used as the template, the tensile strength of the GDL reaches the maximum, and at this time the tensile strength is 6.35×10 -2MPa. When the NaCl particles with 50 mesh were used as the template, the tensile strength of the GDL gradually decreased, which was speculated to be caused by the lower cross-linking degree between the spherical shell structures formed by the larger NaCl particles. Meanwhile, it was found that the deformation of the GDLs prepared by using the NaCl particles with the mesh number of 300 and above was above 0.08%, which meant that the GDL prepared by using the PVDF had a certain deformation capacity, which ensured that the GDL could offset the pressure fluctuation caused by the consumption of CO2 and the generation of the gaseous product in the CO2RR process through deformation, thereby improving the durability of the GDL in use.

[0047] Table 1 Influence of pore size on water contact angle and tensile strength of the support layer

[0048] The existence of the dense small pores on the surface of the GDL plays a very important role in regulating the gas-liquid transmission and inhibiting the occurrence of the hydrogen evolution reaction. The increase in the surface roughness caused by the small pores plays a key role in ensuring the hydrophobicity of the GDL surface, preventing the electrolyte solution from flooding the electrode and maintaining the stable existence of the three-phase interface in the CO2RR process. The role of the dense small pores on the surface of the GDL in preventing the electrolyte solution from flooding the electrode and preventing the occurrence of liquid flooding can be well explained by the Laplace pressure difference formula, as shown in formula (1).

[0049] (1) wherein ΔP is the pressure of the solution entering the pores of the GDL, γ is the interfacial tension of the solution, r is the pore radius of the GDL and θ is the contact angle of the solution. For the pressure of the solution breaking through the pores of the GDL, since the hydrophobic material PVDF is used as the substrate, this makes the contact angle of the GDL to the solution greater than 90°, and the construction of the microporous layer is conducive to increasing the pressure that needs to be overcome by the solution to break through the pores of the GDL, thereby playing a role in preventing the electrolyte solution from flooding the electrode. Therefore, for the microporous layer, the contact angle of the solution to the GDL is greater than 90°, which is conducive to preventing the electrolyte solution from flooding the electrode. Figure 1 According to the SEM results, the NaCl particles with the smallest size range after ball milling (NaCl with a mesh number of 300) were selected as the template for the microporous layer. Since the tensile strength of the GDL prepared by using the NaCl particles with a mesh number of 300 is far lower than that of the GDL prepared by using the NaCl particles with other sizes, in order to enhance the mechanical strength of the GDL, the NaCl particles with a mesh number of 100 after ball milling were used as the template for the support layer, and the conductive GDL with a multi-scale pore structure was constructed. In the preparation of the GDL, the PVDF needs to be spread on the surface of the NaCl through high temperature, but high temperature melting often causes changes in the molecular structure of the polymer material, and therefore Figure 2The changes in functional groups of PVDF before and after melting were compared by infrared spectroscopy. The figure shows that the peak positions of the infrared spectrum of PVDF before and after melting are basically unchanged. This further verifies that no chemical reaction occurred in PVDF before and after melting, and the difference in hydrophobicity of GDL surface is determined by roughness.

[0050] Example 2: The effect of filler on the microporous layer I. The Influence of Packing Type on Microporous Layers The reaction time was controlled at 30 min and the reaction temperature at 200 ℃. PVDF with a molecular weight of 500,000 was selected as the substrate of the support layer. NaCl particles with a mesh size of 300 after ball milling were used as templates. The ratio of PVDF, conductive filler and NaCl was fixed at 1:0.3:7. The influence of the type of conductive filler on the surface wettability, conductivity and micromorphology of GDL microporous layer was analyzed.

[0051] Figure 3 (a)-(d) show the effects of conductive fillers MWCNT, graphene, carbon black (CB), and graphite on the surface morphology of the microporous layer, respectively. Figure 3 As can be observed in (a), when the conductive filler is MWCNT, a large number of intertwined MWCNTs are found on the surface of the GDL microporous layer, encapsulated by PVDF, forming a loose network structure. When the conductive filler is graphene, CB, or graphite, such as Figure 3 As shown in (b)-(d), compared with the loose network structure formed by MWCNT and PVDF, graphene, CB and graphite form sheet-like stacks, granular stacks and block stacks on the surface of the GDL microporous layer, respectively, which is obviously not conducive to electron transfer. Therefore, it can be preliminarily inferred that MWCNT as a conductive filler for the microporous layer has better conductivity than other fillers.

[0052] To further clarify the influence of conductive filler type on the conductivity of GDL microporous layer, the resistivity and conductivity of four different types of conductive fillers were measured using a KDY-1 four-probe resistivity meter. The results are shown in Table 2. Table 2 shows that when the fillers are carbon black and graphite, the resistance exceeds the range of the four-probe resistivity meter. When the filler is graphene, the resistivity of the GDL microporous layer is 5118.38 Ω and the conductivity is 0.0195 S / m, indicating a certain level of conductivity. When the filler is MWCNT, the conductivity is the best compared to the other four fillers, with a resistivity of 947.16 Ω and a conductivity of 0.1056 S / m. The excellent conductivity of the GDL microporous layer after adding MWCNT is mainly due to the high length-to-diameter ratio of MWCNT, which allows it to form a three-dimensional conductive network with a lower filler content compared to other conductive fillers, thus giving the microporous layer excellent conductivity.

[0053] Table 2. Effect of filler type on the resistivity and conductivity of the microporous layer

[0054] Finally, the effect of conductive filler on the surface wettability of the microporous layer was evaluated, and the results are shown in [the table below]. Figure 4 .like Figure 4 As shown, the water contact angle of the GDL microporous layer surface increased to some extent after the addition of conductive fillers. Compared with the GDL microporous layer without fillers, the average water contact angle of the GDL microporous layer with graphite filler increased from 136.60° to 138.76°, an increase of 2.16°; the average water contact angle of the microporous layer with carbon black filler increased from 136.60° to 138.50°, an increase of 1.90°; the average water contact angle of the microporous layer with graphene oxide filler increased from 136.60° to 141.16°, an increase of 4.56°; and the average water contact angle of the microporous layer with MWCNT filler increased from 136.60° to 142.90°, an increase of 6.30°. The increase in the average water contact angle is due to the addition of fillers increasing the surface roughness of the gas diffusion membrane. This result is consistent with... Figure 3 The observed effects of conductive fillers on the surface morphology of the microporous layer are consistent. In summary, the addition of conductive fillers enhances both the conductivity and hydrophobicity of the GDL microporous layer. Among the three conductive fillers, the GDL microporous layer prepared using MWCNT as the conductive filler shows the greatest increase in both conductivity and hydrophobicity. Considering the overall performance of the GDL microporous layer, and prioritizing conductivity, MWCNT is selected as the conductive filler for the system.

[0055] II. The Influence of Filler Content on Microporous Layers The reaction time was controlled at 30 min, and the reaction temperature at 200 ℃. PVDF with a molecular weight of 500,000 was selected as the substrate for the support layer. NaCl particles with a sieve size of 300 mesh after ball milling were used as templates. MWCNTs were selected as the conductive filler for the system. PVDF m MWCNT and m NaCl The ratios were set to 1:0.05:7, 1:0.1:7, 1:0.2:7, 1:0.3:7, and 1:0.4:7, respectively, to analyze the effects of conductive filler content on the surface wettability, conductivity, mechanical strength, and microstructure of the GDL microporous layer. Figure 5 (a)-(e) show the effect of conductive filler content on the morphology of the microporous layer. Figure 5It can be observed that as the amount of MWCNT filler increases, the exposure of MWCNTs on the GDL surface gradually increases, and the integrity of the spherical shell structure formed on the GDL surface becomes increasingly poor. This is because MWCNTs are nanomaterials, and like other nanomaterials, they will agglomerate due to van der Waals forces. At the same time, the properties of one-dimensional materials cause them to intertwine and entangle, making it difficult to uniformly disperse them with simple grinding. As a result, PVDF cannot encapsulate them, thus exposing MWCNT agglomerates on the spherical shell surface. The formation of MWCNT agglomerates inevitably affects the surface wettability and mechanical strength of the GDL microporous layer.

[0056] Then as Figure 6 The effects of MWCNT addition on the surface wettability, conductivity, and tensile strength of the GDL microporous layer were evaluated, as shown in the figures. Figure 6 (a) The effect of MWCNT addition on the water contact angle of the GDL microporous layer surface. As can be observed from the figure, the water contact angle of the GDL microporous layer surface gradually increases with the increase of the conductive filler content in the microporous layer. When the MWCNT ratio in the microporous layer increases from 0.05 to 0.4, the water contact angle of GDL increases from 139.56 ± 1.96° to 147.66 ± 3.18°. This result is due to the gradual increase in the exposure of MWCNT on the surface of the spherical shell structure. To further clarify the effect of MWCNT addition on the conductivity of the GDL microporous layer, a four-probe resistivity meter was used to analyze the effect of filler type on the conductivity of the GDL microporous layer, such as... Figure 6 As shown in (b), the conductivity of the GDL microporous layer gradually increases with the increase of conductive filler content, while the resistivity gradually decreases with the increase of conductive filler content. When the proportion of MWCNT in the microporous layer increases from 0.05 to 0.4, the conductivity of GDL increases from 1.00 × 10⁻⁶. -4 As the S / m increased to 11.5 S / m, the resistivity of GDL decreased from 266974.03 Ω to 8.64 Ω, indicating that the conductivity of the GDL microporous layer gradually increased with the increase of MWCNT addition. Finally, a texture analyzer was used to evaluate the effect of MWCNT addition on the tensile strength of the GDL microporous layer, such as... Figure 6As shown in (c), the tensile strength of the GDL microporous layer first increases and then gradually decreases with the increase of the filler content ratio. When the proportion of MWCNTs in the microporous layer is less than 0.05, MWCNTs are easier to disperse in the PVDF matrix. The fibrous network structure of the stacked MWCNTs creates better stress transfer between the filler and the matrix, which improves the tensile strength of the microporous layer to a certain extent. When the proportion of MWCNTs in the microporous layer is greater than 0.05, the tensile strength of the GDL microporous layer gradually decreases with the increase of the filler content ratio, from 0.05 MPa when the MWCNT proportion is 0.1 to less than 0.03 MPa when the MWCNT proportion is 0.4. This is because the increase of filler increases the possibility of MWCNTs agglomerating during dispersion. Under external pressure, the fibrous agglomerates of MWCNTs are more likely to break than the molten PVDF matrix, resulting in a decrease in overall strength. Considering the comprehensive performance of the GDL microporous layer, under the condition of prioritizing the conductivity and certain mechanical properties of the microporous layer, the optimal conditions are selected, and m is chosen. PVDF m MWCNT and m NaCl The ratio of 1:0.3:7 was used as the GDL microporous layer.

[0057] Example 3: Effect of alcohol on PM-M performance I. The effect of alcohol carbon chain length on PM-M performance The contents of alcohol, dilute HCl (a co-solvent), and deionized water in the treatment solution were fixed at 4 mL, 15 mL, and 15 mL, respectively. By changing the type of alcohol in the treatment solution, alcohols with different chain lengths (C15) were added. n H 2n+2 O(n=3, 4, 5, 6, 7), the PM-M prepared in Chapter 2 were cut into pieces with a size of 2.5 × 2.5 cm. 2 PM-M was modified further, with the modification temperature set at 150 ℃ and the reaction time at 5 h, to study the effect of alcohol carbon chain length on the morphology and properties of PM-M.

[0058] First, the effect of alcohols with different carbon chain lengths on the surface morphology of PM-M was analyzed by SEM, such as... Figure 7 (a)-(e) show the effects of propanol, butanol, pentanol, hexanol, and heptanol on the surface morphology of PM-M, respectively. Figure 7 It was found that alcohols of different chain lengths, after heat treatment, could all form a fish-scale-like rough structure on the PM-M surface. The formation of this fish-scale structure can be attributed to the combined effect of the solvent and heat treatment. This treatment causes the PM-M to swell only on the film surface, resulting in asymmetric shrinkage deformation and the formation of the fish-scale structure. Experiments showed that the development of this fish-scale structure on the PM-M surface becomes increasingly incomplete with increasing alcohol carbon chain number. Figure 7As shown in (a)-(b), when the alcohol used in the treatment solution is propanol or butanol, the fish-scale-like structures formed on the PM-M surface vary in size, and this change becomes more pronounced when propanol is used in the treatment solution. Figure 7 As shown in (a), the fish-scale-like structure on the PM-M surface has undergone severe deformation, and the three-dimensional framework structure has collapsed. This phenomenon is caused by the fact that shorter-chain alcohols have smaller molecular volumes than longer-chain alcohols, making them more likely to penetrate the PVDF interior, exacerbating the swelling of the PVDF. This leads to severe deformation of the PM-M during cooling and shrinkage, causing the collapse of the PM-M three-dimensional framework. This inevitably affects the surface hydrophobicity and membrane flux of the GDL. When using pentanol with a chain number of 5, as... Figure 7 As shown in (c), the three-dimensional framework structure of PM-M remains almost unchanged, with the fish-scale structure densely arranged along the surface of the PM-M three-dimensional framework. Further increasing the chain number of the alcohol used, PM-M was modified with hexanol and heptanol, as shown... Figure 7 (d)-(e) It can be observed that as the number of alcohol chains increases, the fish-scale rough structure on the PM-M surface gradually decreases. This is because alcohol molecules with longer chains have larger volumes and are difficult to penetrate into the interior of PVDF during the swelling process, resulting in an insufficient swelling process. Consequently, it is difficult to develop a relatively complete fish-scale structure during the subsequent cooling and shrinkage process.

[0059] The surface wettability, membrane flux, and conductivity of PM-M were measured using a contact angle meter, a membrane flux testing device, and a four-probe resistivity meter, respectively. The results are as follows: Figure 8 As shown. By Figure 8 (a) It can be seen that by modifying PM-M with alcohols of different carbon chain lengths, when the number of carbon chains n in the alcohol is less than 5, the water contact angle of the modified PM-M surface increases with the increase of the number of carbon chains in the alcohol, from 159.00 ± 2.30° for propanol to 167.30 ± 3.77° for pentanol, while the roll-off angle decreases from 8.11 ± 0.38° for propanol to 4.15 ± 0.82° for pentanol; when the number of carbon chains n in the alcohol is greater than 5, the water contact angle of the modified PM-M surface decreases with the increase of the number of carbon chains in the alcohol, from 167.30 ± 3.77° for pentanol to 154.70 ± 1.78° for heptanol, while the roll-off angle increases from 4.15 ± 0.82° for pentanol to 10.66 ± The reason for this phenomenon is mainly due to the irregular fish-scale structure formed on the surface of PM-M when the number of carbon chains of the alcohol is small. When the number of carbon chains of the alcohol increases, the fish-scale structure on the surface of PM-M is not fully developed. Both of these factors will reduce the hydrophobicity. Therefore, compared with the other four alcohols, pentanol has the best effect on hydrophobic modification of PM-M as a treatment solution. Figure 8 (b) shows the effect of alcohol carbon chain length on PM-M membrane flux. Figure 8As can be seen in (b), the membrane flux of PM-M gradually increases with the increase of alcohol carbon chain length, from 0.81 mL cm⁻¹ for propanol. - 2 min -1 Pa -1 Increased to 1.82 mL cm -2 min -1 Pa -1 The main reason for this is that alcohols with different carbon chain numbers exhibit varying degrees of swelling of PVDF. Compared to alcohols with higher carbon chain numbers, alcohols with lower carbon chain numbers have a stronger swelling effect on PVDF, making them more prone to causing the collapse of the PM-M three-dimensional framework and resulting in a decrease in membrane flux during shrinkage. For PM-M, the swelling and shrinkage process not only leads to changes in membrane flux but also affects the conductivity of PM-M. Figure 8 (c) shows the effect of alcohol carbon chain length on the conductivity of PM-M, from Figure 8 (c) It can be seen that the conductivity of PM-M gradually decreases with the increase of the number of carbon atoms in the alcohol, but the overall conductivity of the modified PM-M is improved compared with that of the unmodified PM-M (the resistivity of the unmodified PM-M is 947.16 Ω and the conductivity is 0.11 S / m). The resistivity of the modified PM-M increased from 171.29 Ω with propanol as the treatment solution to 444.22 Ω with heptanol as the treatment solution; the conductivity of PM-M decreased from 0.58 S / m with propanol as the treatment solution to 0.23 S / m with heptanol as the treatment solution. The reason for this phenomenon is that the asymmetric shrinkage after swelling of the membrane surface changes the arrangement of the filler MWCNT, forcing the MWCNT to rearrange along the stress shrinkage direction. This optimizes the conductive network formed by MWCNT and promotes the increase of the conductivity of PM-M. In summary, the hydrophobicity of the PM-M surface first increases and then decreases with the increase of the number of alcohol carbons, the membrane flux increases with the increase of the number of alcohol carbons, and the conductivity decreases with the increase of the number of alcohol carbons. Considering the surface wettability, CO2 membrane flux, and conductivity of PM-M, pentanol is selected to prepare the subsequent PM-M heat treatment solution.

[0060] II. Effect of alcohol content on PM-M performance The contents of the co-solvent dilute HCl and deionized water in the treatment solution were fixed at 15 mL and 15 mL, respectively. The amount of pentanol added was varied by adding 3 mL, 3.5 mL, 4 mL, 4.5 mL, 5 mL, and 5.5 mL of pentanol to a sample with a size of 2.5 × 2.5 cm. 2 PM-M was modified at a temperature of 150 °C for 5 h to investigate the effect of pentanol content on the surface morphology and properties of PM-M. Figure 9 (a)-(d) show the effect of pentanol content on the electron microscopy morphology of PM-M surface.Figure 9 As can be seen from (a)-(b), when the pentanol content in the treatment solution is 3 mL and 3.5 mL, the fish-scale rough structure on the PM-M surface is not fully developed. As the pentanol content increases, the fish-scale rough structure on the PM-M surface becomes more and more complete. When the pentanol content is higher than 4 mL, such as Figure 9 As shown in (e)-(f), the fish-scale structure of PM-M begins to break down, transforming from a three-dimensional fish-scale structure into a nanosheet structure attached to the surface of MWCNT. The reason for this phenomenon is consistent with the effect of using alcohols with smaller carbon chain numbers on the surface morphology of PM-M. The increase in pentanol content exacerbates the swelling of PVDF, causing its surface to break down during the subsequent cooling and shrinkage process.

[0061] The effects of pentanol content on PM-M surface wettability, film flux, resistivity, and conductivity are as follows: Figure 10 As shown. By Figure 10 (a) It can be seen that by using heat treatment solutions with different pentanol contents to modify PM-M, the hydrophobicity of the PM-M surface after modification is greater than 150°, indicating that modification can effectively improve the hydrophobicity of the PM-M surface. At the same time, the hydrophobicity of the PM-M surface first increases and then decreases with the increase of pentanol content. When the pentanol content in the treatment solution increased from 3 mL to 4 mL, the water contact angle of the PM-M surface increased from 155.00 ± 1.42° to 167.30 ± 3.77°, and the roll-off angle decreased from 7.89 ± 0.87° to 4.15 ± 0.82°. When the pentanol content in the treatment solution increased from 4 mL to 5.5 mL, the water contact angle of the PM-M surface decreased from 167.30 ± 3.77° to 152.40 ± 1.41°, and the roll-off angle increased from 4.15 ± 0.82° to 11.07 ± 0.23°. This is mainly due to the difference in the development of the fish-scale-like structure on the PM-M surface in treatment solutions with different pentanol contents. Figure 9 As shown in (a), when the pentanol content in the treatment solution is less than 4 mL, the fish-scale structure on the PM-M surface is not fully developed, resulting in a significantly lower roughness of the modified PM-M compared to the PM-M modified with 4 mL of pentanol. When the pentanol content in the treatment solution is 4 mL, the fish-scale structure on the PM-M surface is fully developed, and the hydrophobicity of the PM-M surface is improved. When the pentanol content in the treatment solution is greater than 4 mL, such as... Figure 9 As shown in (e)-(f), the PM-M surface gradually transforms from a three-dimensional fish-scale structure to a nanosheet structure attached to the MWCNT surface, which reduces the hydrophobicity of the PM-M surface.

[0062] Figure 10(b) shows the effect of pentanol content on PM-M membrane flux. The experiment found that as the pentanol content increased, the PM-M membrane flux gradually decreased, from 2.49 mL / cm² when the pentanol content was 3 mL. -2 min -1 Pa -1 Decreased to 1.86 mL cm⁻¹ (4 mL) -2 min -1 Pa -1 The flux then stabilized. Similar to the effect of reducing the carbon chain length of alcohol in the treatment solution on PM-M membrane flux, the decrease in PM-M membrane flux was due to the increased alcohol content in GDL exacerbating PVDF swelling, leading to the collapse of the PM-M three-dimensional framework during shrinkage. Figure 10 (c) shows the effect of pentanol content on the conductivity of PM-M. Figure 10 (c) shows that the resistivity of PM-M decreased from 667.40 Ω with 3 mL of pentanol to 346.03 Ω with 4 mL of pentanol, while the conductivity increased from 0.15 S / m with 3 mL of pentanol to 0.29 S / m with 4 mL of pentanol. The increased conductivity of PM-M at 4 mL is mainly due to the shrinkage of PM-M after solvent heat treatment, which forced the MWCNTs to rearrange along the stress direction. Further increasing the pentanol content slightly reduced conductivity, presumably because the increased pentanol content caused some MWCNTs to detach during the swelling and shrinkage process. Considering the surface wettability, CO2 membrane flux, and conductivity of PM-M, the pentanol content of the subsequent PM-M heat treatment solution was determined to be 4 mL.

[0063] Example 4: Effect of cosolvent content on PM-M performance The contents of pentanol and deionized water in the treatment solution were fixed at 4 mL and 15 mL, respectively. The content of the co-solvent dilute HCl in the treatment solution was varied by adding 12 mL, 13 mL, 14 mL, 15 mL, 16 mL, and 17 mL of dilute HCl to a sample with a size of 2.5 × 2.5 cm. 2 PM-M was modified at a temperature of 150 °C for 5 h. The effect of the co-solvent content of dilute HCl on the morphology and properties of PM-M was first investigated. Figure 11 (a)-(f) show the effect of the amount of flux on the electron microscopy morphology of the PM-M surface. Figure 11 As can be seen from (a)-(d), when the content of the co-solvent dilute HCl is less than 15 mL, the fish-scale-like rough structure on the PM-M surface develops more completely with the increase of the co-solvent content. When the content of the co-solvent dilute HCl is greater than 15 mL, such as Figure 11As shown in (e)-(f), the fish-scale-like rough structure on the surface of PM-M is gradually destroyed. This is because the increase in the content of the co-solvent dilute HCl exacerbates the swelling effect of PVDF on the surface of PM-M, resulting in severe deformation of PM-M during cooling and shrinkage.

[0064] Figure 12 To investigate the effect of co-solvent content on the surface wettability, film flux, and conductivity of PM-M, the following was conducted. Figure 12 (a) It can be seen that modifying PM-M with heat treatment solutions containing different amounts of co-solvents can improve the hydrophobicity of the PM-M surface. The hydrophobicity of the PM-M surface first increases and then decreases with the increase of co-solvent content. The water contact angle increases from 158.30 ± 3.64° with a co-solvent content of 12 mL to 167.30 ± 3.77° with a co-solvent content of 15 mL, and then decreases from 167.30 ± 3.77° with a co-solvent content of 15 mL to 151.4 ± 4.25° with a co-solvent content of 17 mL. The roll-off angle decreases from 9.61 ± 0.53° with a co-solvent content of 12 mL to 4.15 ± 0.82° with a co-solvent content of 15 mL, and then increases from 4.15 ± 0.82° with a co-solvent content of 15 mL to 11.07 ± 0.34° with a co-solvent content of 17 mL. The reason for this phenomenon is that when the cosolvent content is less than 15 mL, the fish-scale rough structure on the surface of PM-M gradually develops completely, causing the surface roughness of PM-M to gradually increase. When the cosolvent content is greater than 15 mL, the swelling of PVDF on the surface of PM-M intensifies, which leads to the destruction of the fish-scale rough structure on the surface of PM-M during the shrinkage process, resulting in a decrease in the surface roughness of PM-M.

[0065] Figure 12 (b) The effect of co-solvent content on PM-M membrane flux, from Figure 12 (b) It can be seen that the CO2 membrane flux of PM-M gradually decreases with the increase of cosolvent content. The membrane flux of PM-M increases from 2.69 mL cm⁻¹ when the cosolvent content is 12 mL. -2 min -1 Pa -1 The concentration decreased to 0.079 mL cm⁻¹ of 17 mL cosolvent. -2 min -1 Pa -1 This is because the increased content of the cosolvent leads to a greater degree of swelling of PM-M, which causes severe deformation of PM-M during the subsequent cooling and shrinkage process, resulting in the collapse of the three-dimensional skeleton structure of PM-M. Figure 12 (c) The effect of co-solvent content on the conductivity of PM-M, by Figure 12(c) It can be seen that when the content of the co-solvent increases from 12 mL to 17 mL, the resistivity of PM-M decreases from 397.79 Ω to 247.21 Ω, and the conductivity of PM-M increases from 0.25 S / m to 0.40 S / m. This indicates that different amounts of solvent added to the heat treatment solution all contribute to the increase of PM-M conductivity. The reason for the enhanced conductivity of PM-M is also due to the asymmetric shrinkage after the PM-M surface swelling, which changes the arrangement of MWCNTs, forcing the MWCNTs to rearrange along the stress shrinkage direction, thus optimizing the conductive network formed by the MWCNTs. Considering the surface wettability, CO2 membrane flux, and conductivity of PM-M, the co-solvent content of the subsequent PM-M heat treatment solution is determined to be 15 mL.

[0066] Example 5: Electrocatalysis Experiment 20 mg of copper nanoparticles were used as a catalyst and dispersed in a mixed solvent containing 70 μL isopropanol, 430 μL ultrapure water, and 30 μL 5% Nafion solution. The mixture was sonicated for 45 min until a uniform ink was formed. The copper nanoparticle ink was then uniformly drop-coated onto the surface of an SPM-M electrode and dried under N2 to prepare the working electrode. The three-electrode system constructed in Flow-Cell was measured using an electrochemical workstation with an IT assay. A 1 mol / L KOH solution was used as the electrolyte solution; 3PK-130 was used as the ion exchange membrane separating the anode and cathode chambers; an Ag / AgCl electrode (potassium chloride salt, salt bridge) was used as the reference electrode; the counter electrode was a 3 × 1 cm iridium oxide mesh; the working electrode used the diffusion electrode prepared in this invention and the membrane flux of three other commonly used GDLs, where Sigrette 22 BB was a hydrophobic carbon paper with a microporous layer on its surface, AvCarb P75T was a hydrophobic carbon paper without a microporous layer on its surface, and HCP330P was a PTFE-modified hydrophobic carbon cloth. The flow rates of CO2 and electrolyte were consistent with the parameters set for the high-salt solution scouring durability test. Gas-phase products generated on the GDL surface were sampled at fixed time intervals (15 min) and quantitatively analyzed by gas chromatography. Electrochemical reaction data were collected using a CHI760E electrochemical workstation, and the Faradaic efficiency of the liquid and gas products was calculated based on the collected charge information. All electrochemical tests were IR compensated, and all potentials were converted to reversible hydrogen electrode (RHE) values ​​using formula (2): (2) Among them, E RHE E is the RHE potential. Ag / AgCl The potential of Ag / AgCl was determined by linear sweep voltammetry (LSV) in the potential range of -0.2 V to -1.2 V (vs RHE) at 10 mV / s. -1The sample was scanned at a certain rate. Gaseous products were continuously injected and analyzed in the gas chromatograph. Gaseous products were analyzed every 15 minutes, and the Faraday efficiency of a certain gaseous product was calculated based on equation (3): (3) Where ν (vol %) is the volume concentration of the gaseous product (GC data); V is the gas flow rate measured by a flow meter; I is the steady-state total current of the Flow-Cell, n is the number of electrons transferred in the products; and F is the Faraday constant, 96485 C mol. -1 P is one atmosphere, 1.013 × 10⁻⁶ 5 Pa.

[0067] from Figure 13 As can be seen, the onset potential of SPM-M is -0.47 V, which is significantly higher than that of AvCarb P75T, Sigracet 22 BB, and HCP330P. Meanwhile, when the potential is less than -0.7 V, the current density of SPM-M is significantly lower than that of Sigracet 22 BB and HCP330P. This is because SPM-M has a slightly higher resistance, meaning that at the same potential, the electrochemical kinetics of SPM-M are weaker than those of Sigracet 22 BB and HCP330P. Therefore, SPM-M requires more energy to overcome the system resistance in the GDE composed of SPM-M components. This amplifies the effect of this internal resistance on the current density at low potentials. When the cathode potential is greater than -0.7 V, the current density of SPM-M is higher at the same potential. This is because SPM-M not only provides a stable three-phase interface, but its abundant pore structure and high membrane flux ensure rapid CO2 mass transfer in the GDE, promoting CO2RR.

[0068] To compare the catalytic performance of GDEs constructed from four different GDLs, the product distribution of the four different GDEs at different cathode potentials was determined by electrochemical workstation coupled with gas chromatography. The results are shown below. Figure 14 The Faraday efficiency of GDE for H2, CO, CH4, C2H4 and C2H6 was tested respectively.

[0069] Figure 14In Figure (a), the CO2RR product distribution of the GDE constructed by SPM-M under different cathode potentials is shown. For SPM-M, within the potential range of -0.6 V to -2 V (vs. RHE), the H2 Faradaic efficiency of the GDE constructed by SPM-M is less than 35%, and the generated C products are mainly CO and C2H4, while CH4 and C2H6 account for a small proportion of the overall carbon product distribution. When the applied voltage is -0.6 V (vs. RHE), the Faradaic efficiency of C2H4 is 27.07%, which is significantly lower than that of CO. As the voltage increases, the CC coupling path becomes dominant, the Faraday efficiency of CO decreases, and the Faraday efficiency of C2H4 increases. When the applied voltage increases to -1.4 V (vs. RHE), the selectivity of the GDE constructed by SPM-M for C2H4 reaches its maximum, at which point the Faraday efficiency of C2H4 is 43.49%. Thereafter, as the voltage increases, the total Faraday efficiency of C products decreases, while the Faraday efficiency of H2 products gradually increases until flooding occurs when the voltage increases to -2.2 V (vs. RHE). Figure 14 (b) CO2RR product distribution of GDE constructed for Sigrette 22 BB at different cathode potentials, such as Figure 14 As shown in (b), the product distribution trend of the diffusion electrode constructed with Sigracet 22 BB carbon paper is basically consistent with that of the diffusion electrode constructed with SPM-M. The selectivity for C2H4 reaches its highest when the applied voltage is increased to -1.2 V (vs. RHE). After that, the Faraday efficiency of H2 products gradually increases with increasing voltage, while the total Faraday efficiency of C products continuously decreases. However, compared with the GDE constructed with SPM-M, the stability of the GDE constructed with Sigracet 22 BB at high voltage is worse. Flooding occurs when the voltage is increased to -1.8 V (vs. RHE). Figure 14(c) and (d) show the CO2RR product distribution of the GDE constructed with HCP330P carbon cloth and AvCarb P75T carbon paper, respectively. It is worth noting that in the potential range of -0.6 V to -1.2 V (vs. RHE), there is significant CH4 formation in the C product of the GDE constructed with HCP330P. In contrast, the Faraday efficiency of C2H4 remains at around 17%, indicating that the GDE constructed with HCP330P carbon cloth has better CH4 selectivity than the GDE constructed with SPM-M and Sigracet 22 BB. At the same time, the CH4 selectivity of the GDE constructed with HCP330P gradually increases with increasing voltage. When the voltage rises to -1.2 V (vs. RHE), the Faraday efficiency of CH4 is 27.78%, at which point the CH4 selectivity reaches its highest level. The GDE constructed with AvCarb P75T exhibits a minimum Faraday efficiency of 42.12% in the potential range of -0.6 V to -1.2 V (vs. RHE), which is significantly higher than the other three GDEs. Meanwhile, the GDE constructed with AvCarb P75T carbon paper and HCP330P carbon cloth shows significantly lower stability at high voltages compared to the GDEs constructed with SPM-M and Sigracet 22 BB. When the voltage is increased to -1.4 V (vs. RHE), the GDE constructed with AvCarb P75T carbon paper and HCP330P carbon cloth rapidly undergoes flooding.

[0070] To further investigate the effect of GDL on the catalytic performance of CO2RR, the Faradaic efficiency of GDE products under different voltages was calculated. The results are shown in [Figure number missing]. Figure 15 .from Figure 15 It can be seen that within the range of -0.6 V to -1.2 V (vs. RHE), the AvCarb P75T carbon paper exhibits the highest Faraday efficiency for H2 products, consistently maintaining above 40%. 2+ The Faradaic efficiency of the product and total C product was significantly lower than that of the other three GDLs. This is mainly due to the lack of hydrophobic reinforcement on the surface of the AvCarb P75T carbon paper, which leads to the disruption of the three-phase interface due to electrowetting when voltage is applied. The electrolyte solution then floods the catalyst and comes into contact with the carbon fibers on the carbon paper surface, resulting in severe hydrogen evolution. HCP330P carbon cloth, Sigrette 22 BB carbon paper, and SPM-M all have excellent hydrophobicity, and compared to AvCarb P75T carbon paper, they all exhibited lower Faradaic efficiency for H2 products and higher Faradaic efficiency for C products during the catalytic process. However, despite this, from... Figure 15In (a), it can be observed that the Faraday efficiency of H2 products of HCP330P carbon cloth increases rapidly with increasing voltage. Therefore, although superhydrophobicity is the key to ensuring the three-phase interface on the GDL surface, the lack of a microporous layer still results in its ability to block electrolytes and prevent flooding being far lower than that of Sigracet 22 BB carbon paper and SPM-M. Figure 15 As shown in (c), it is worth noting that although HCP330P carbon cloth exhibits a high total C product Faradaic efficiency, as Figure 15 In (b), we can find its C. 2+ The product's Faraday efficiency is significantly lower than that of Sigracet 22 BB carbon paper and SPM-M. 2+ The product's Faraday efficiency remained at approximately 15%, further indicating that the microporous layer controlled the carbon content. 2+ It also plays an important role in the selectivity of gaseous products. The presence of the microporous layer increases the contact area between CO2 and the catalyst layer, thereby promoting mass transfer and ultimately promoting the production of C. 2+ Product formation.

[0071] from Figure 15 It can be seen that both Sigracet 22 BB carbon paper and SPM-M exhibit excellent selectivity at low voltages. Within the potential range of -0.6 V to -1.6 V (vs. RHE), the product Faradaic efficiency changes of Sigracet 22 BB carbon paper and SPM-M are essentially consistent. 2+ The Faraday efficiency of both the product and the total C product showed a trend of first increasing and then decreasing. Although the Faraday efficiency of the H2 product gradually increased with increasing voltage, it remained below 30% within this range, indicating that the three-phase interface could exist stably. With further increases in cathode potential, SPM-M exhibited significantly better stability at higher cathode potentials than Sigracet 22 BB carbon paper. When the cathode potential exceeded -1.6 V (vs. RHE), Sigracet 22 BB carbon paper rapidly flooded, while SPM-M only experienced flooding when the cathode potential exceeded -2.0 V (vs. RHE). This demonstrates that SPM-M's high-potential stability is significantly superior to that of Sigracet 22 BB carbon paper. The excellent high-voltage stability of SPM-M is mainly due to its unique multi-scale rough structure and multi-scale pore distribution. The multi-scale rough structure on the SPM-M surface effectively suppresses the influence of external applied voltage and electrolyte solution slippage on the electrowetting of the SPM-M surface. At the same time, the multi-scale channel structure forms a microporous layer on the SPM-M surface, increasing the pressure required for the solution to break through GDL. This promotes mass transfer while effectively blocking the electrolyte solution and preventing flooding. The combined effect of these two factors significantly improves the stability of the three-phase interface and catalytic performance during the CO2RR process.

Claims

1. A multi-scale rough structure gas diffusion membrane, comprising: The support layer is a three-dimensional through-frame structure formed by melting polyvinylidene fluoride and NaCl particles of the first size and then removing the NaCl. A microporous layer, disposed on the support layer, is a conductive and hydrophobic layer formed by melting and molding polyvinylidene fluoride, multi-walled carbon nanotubes and second-sized NaCl particles and then removing the NaCl. The surface of the gas diffusion membrane is subjected to solvent thermal treatment to form a fish-scale-like rough structure, which gives it superhydrophobic properties.

2. The gas diffusion membrane according to claim 1, characterized in that: The particle size range of the NaCl particles in the support layer is 150–300 μm; The NaCl particles in the microporous layer have a particle size range of 50–100 μm.

3. The gas diffusion membrane according to claim 1 or 2, characterized in that: In the microporous layer, the mass ratio of the polyvinylidene fluoride, the multi-walled carbon nanotubes, and the NaCl particles is 1:0.05-0.4:

7.

4. The gas diffusion membrane according to any one of claims 1-3, characterized in that: The solvent heat treatment uses a treatment solution including alcohol solvents, co-solvents, and water.

5. The gas diffusion membrane according to claim 4, characterized in that: The alcohol solvent is an alcohol with 3-7 carbon atoms; In the treatment solution, the content of the alcohol solvent is 3-5.5 mL, the content of the co-solvent is dilute HCl, which is 12-17 mL, and the content of water is 15 mL.

6. The gas diffusion membrane according to any one of claims 1-5, characterized in that: The solvent heat treatment is performed at a temperature of 100-200℃ for 1-10 hours.

7. The gas diffusion membrane according to any one of claims 1-6, characterized in that: The gas diffusion membrane has a water contact angle greater than 160°, a roll-off angle less than 5°, and a CO2 flux greater than 1.8 mL·cm⁻¹. -2 ·min -1 ·Pa -1 .

8. A method for preparing the gas diffusion membrane according to any one of claims 1-7, comprising the following steps: Polyvinylidene fluoride is mixed with NaCl particles of the first size, melted and molded, and then washed with water to remove NaCl, forming a support layer. Polyvinylidene fluoride, multi-walled carbon nanotubes and second-size NaCl particles are mixed, melted and shaped, and then washed with water to remove NaCl, forming a microporous layer on the support layer to obtain a composite membrane. The composite membrane is subjected to solvent heat treatment, cleaned and dried to obtain a gas diffusion membrane with a fish-scale-like surface structure.

9. The application of the gas diffusion membrane according to any one of claims 1-7 as a gas diffusion layer in an electrochemical CO2 reduction reaction.

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