Preparation method of gradient catalyst layer for anode of proton exchange membrane electrolytic tank

Abstract

The invention discloses a preparation method of a gradient catalyst layer for an anode of a proton exchange membrane electrolytic tank. The preparation method specifically comprises the following four steps: pretreating a proton exchange membrane, preparing double-gradient slurry, constructing a gradient structure catalyst layer, and assembling and testing a membrane electrode. The anode catalyst layer is prepared into a gradient structure through ultrasonic spraying, the proton exchange membrane serves as a substrate, the catalyst layer is attached to one side of the proton exchange membrane, slurry with the high ionomer / catalyst mass ratio is sprayed to the inner layer of the catalyst layer, and slurry with the low ionomer / catalyst mass ratio is sprayed to the outer layer of the catalyst layer. The method is characterized in that the hierarchical pore design and ionomer distribution optimization can effectively inhibit the agglomeration phenomenon of catalyst particles, and the desorption efficiency of bubbles on the surface of the electrode is synchronously improved, so that the activation overpotential and the mass transfer overpotential are reduced. The voltage of the membrane electrode of the gradient structure catalytic layer is reduced by 5.4% under the communicated current density, the iridium utilization rate and the catalytic layer output performance are obviously improved, and the proton exchange membrane water electrolysis cost is effectively reduced.

Description

Technical Field

[0001] The present invention belongs to the field of electrochemical water electrolysis, and particularly relates to a preparation method of a gradient structure catalytic layer for an anode of water electrolysis. Background Art

[0002] A proton exchange membrane water electrolyzer (PEMWE) is a process that electrochemically decomposes water to produce hydrogen and oxygen, and is widely used in fields such as hydrogen energy production, energy storage, and chemical engineering. It has significant advantages such as no pollution, small volume, high efficiency, fast response, and high pressure resistance. A PEMWE generally consists of a current collector plate, a flow field plate, and a membrane electrode. Among them, the membrane electrode, as the core component of the proton exchange membrane water electrolyzer, is the key to affecting the battery output performance and reducing costs. The membrane electrode is usually a five-in-one structure composed of an anode and cathode gas diffusion layer, an anode and cathode catalytic layer, and a proton exchange membrane. The catalytic layer is composed of a catalyst and an ionomer, and belongs to the core component of the electrolyzer. The catalyst used in the catalytic layer is mainly precious metal iridium. Due to the slow oxygen evolution reaction at the anode, its loading in the PEMWE remains high, resulting in high costs. Therefore, improving the electrochemical performance of the PEMWE has always been the focus of PEMWE research and development.

[0003] In the existing PEMWEs, the catalytic layer usually adopts a particle-packed structure, which is mainly formed by physically mixing and packing a catalyst and an ionomer. The performance of the catalytic layer is closely related to its microstructure. In particular, the ratio of the ionomer to the catalyst (IC ratio) has a significant impact on the morphology and performance of the catalytic layer. The catalytic layers prepared under different IC ratios will show different degrees of particle agglomeration and differences in pore size distribution. The agglomeration of catalyst particles will lead to the burial of catalyst particles, the loss of effective active sites, and the increase of activation overpotential and other adverse factors. The size and distribution of pores will affect the contact between reactants and the catalyst, the generation and discharge of bubbles, and the mass transfer overpotential of the electrode, thereby affecting the electrochemical performance of the electrode. Therefore, by optimizing the microstructure of the catalytic layer, the electrochemical performance of the catalytic layer can be effectively improved, thereby improving the overall performance of the PEMWE. This performance improvement can not only reduce the energy consumption and material costs of the electrolysis process, but also promote the commercial application of PEMWE technology and provide important support for the development of the hydrogen energy industry. The catalytic layer emphasized in the following text is the anode catalytic layer of the PEMWE. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing a gradient catalytic layer for the anode of a proton exchange membrane water electrolyzer. For the sake of simplicity of description, the proton exchange membrane water electrolyzer is abbreviated as PEMWE. The structural scheme of the present invention includes the following steps:

[0005] (1) Pretreat the proton exchange membrane: The matrix proton membrane material is successively subjected to plasma surface modification treatment, deionized water cleaning, acidic activation treatment and final treatment to obtain a sprayable substrate with optimized wettability.

[0006] (2) Prepare the double-gradient slurry: Prepare the gradient slurries on both sides of the inner and outer layers of the proton catalytic layer respectively. The slurry contains a composite solvent system of electrocatalyst, ionomer dispersion, deionized water and isopropanol, and forms a stable suspension system through the synergistic dispersion of ultrasonic oscillation and emulsification.

[0007] (3) Construct the gradient structure catalytic layer: The proton exchange membrane is used as the substrate, and the catalytic layer is attached to one side of the proton exchange membrane. The slurry with a high IC ratio is sprayed on the inner layer of the catalytic layer; the slurry with a low IC ratio is sprayed on the outer layer of the catalytic layer, and heat treatment is carried out after spraying the inner and outer layers.

[0008] (4) Membrane electrode assembly and testing: Stack and package the catalytic layer obtained in step (3) with the gas diffusion layer to complete the assembly of the proton exchange membrane electrolytic water membrane electrode, and carry out performance testing.

[0009] The technical principle of the present invention is described as follows:

[0010] This method takes the proton exchange membrane as the core, and the anode catalytic layer and the cathode catalytic layer are respectively coated on both sides. The proton exchange membrane, the anode and cathode catalytic layers, and the anode and cathode gas diffusion layers together constitute a membrane electrode (MEA). The cathode catalytic layer can be prepared by methods such as ultrasonic spraying method and electrochemical deposition method, while the anode catalytic layer adopts a gradient structure catalytic layer prepared based on ultrasonic spraying technology. Using the layer-by-layer spraying method, its IC ratio gradually decreases from the side close to the proton exchange membrane to the side close to the gas diffusion layer. By gradient regulation of the internal microstructure of the catalytic layer, the proton conduction network, electron transport channel and reactant diffusion path show synergistic optimization characteristics, thereby systematically improving the three-phase interface reaction efficiency and mass transfer kinetic performance of the catalytic layer. This structural gradient distribution can effectively alleviate the concentration polarization phenomenon and enhance the utilization rate of electrochemically active sites, and finally achieve a significant optimization of the comprehensive performance of the catalytic layer.

[0011] The characteristics and advantages of the present invention are as follows: The gradient structure anode catalytic layer shows significant advantages in catalyst dispersibility and reaction interface regulation compared with the currently used homogeneous catalytic layer. The characteristics and advantages are reflected in: (1) The hierarchical pore design and optimized ionomer distribution can effectively inhibit the agglomeration of catalyst particles and simultaneously improve the bubble desorption efficiency on the electrode surface, thereby reducing the activation overpotential and mass transfer overpotential. (2) Experimental data show that: The working voltage of the membrane electrode based on the gradient structure is 1.81 V at a current density of 3 A / cm 2 which is 5.4% lower than that of the membrane electrode with a homogeneous catalytic layer (1.913 V), and the charge capacity is increased to 0.2556 C / cm2 (The homogeneous layer is 0.2284 C / cm 2 ), and the effective active area is expanded by 11.9%. (3) This improvement is attributed to the sufficient exposure of catalytic active sites by the gradient structure and the collaborative optimization of proton / electron transport channels, providing key technical support for reducing the energy consumption and material cost of the PEMWE system. Description of the Drawings

[0012] Appendix Figure 1 FIG. is a schematic diagram of the principle structure of the PEMWE membrane electrode with a gradient structure catalytic layer as the anode.

[0013] Appendix Figure 2-1 FIG. is a two-dimensional image of a conventional catalytic layer taken by a scanning electron microscope.

[0014] Appendix Figure 2-2 FIG. is a two-dimensional image of the gradient structure catalytic layer taken by a scanning electron microscope according to the present invention.

[0015] Appendix Figure 3 FIG. is a comparison chart of the proportion of the agglomeration area of catalyst particles in the catalytic layer calculated by a computer.

[0016] Appendix Figure 4-1 FIG. is a cyclic voltammetry test chart of two examples of the gradient structure catalytic layer and the conventional catalytic layer of the present invention using an electrochemical workstation.

[0017] Appendix Figure 4-2 FIG. is the charge of two examples obtained by processing the cyclic voltammetry test chart using a computer.

[0018] Appendix Figure 5 FIG. is a performance comparison chart of two embodiments of the gradient structure catalytic layer and the conventional catalytic layer of the present invention. Detailed Embodiments

[0019] The method and technical solution of the present invention will be described in detail below with reference to the drawings and specific examples. It should be noted that this embodiment is narrative rather than restrictive, and does not limit the protection scope of the present invention.

[0020] A method for preparing a gradient catalytic layer for the anode of a PEMWE electrolyzer, a method for preparing a gradient structure catalytic layer for the anode, and the gradient structure catalytic layer of the anode is prepared by a layer-by-layer spraying method based on ultrasonic spraying technology. The specific method includes the following steps:

[0021] (1) Pretreat the proton exchange membrane: sequentially perform plasma surface modification treatment, deionized water cleaning, acidic activation treatment, and final treatment on the matrix proton membrane material to obtain a sprayable substrate with high wettability.

[0022] (2) Prepare double-gradient slurries: Gradient slurries for both the inner and outer layers of the catalytic layer are prepared separately. The slurries contain a composite solvent system of electrocatalyst, ionomer dispersion, deionized water, and isopropanol, and a stable suspension system is formed through the synergistic dispersion of ultrasonic vibration and emulsification.

[0023] (3) Construct a gradient-structured catalytic layer: The proton exchange membrane is used as the substrate, and the catalytic layer is attached to one side of the proton exchange membrane. The slurry with a high ionomer / catalyst (IC) ratio is sprayed on the inner layer of the catalytic layer, and the slurry with a low IC ratio is sprayed on the outer layer of the catalytic layer; after spraying the inner and outer layers, heat treatment is carried out.

[0024] (4) Membrane electrode assembly and testing: Stack and encapsulate the catalytic layer obtained in step (3) with the gas diffusion layer to complete the assembly of the proton exchange membrane electrolysis water membrane electrode, and perform performance testing.

[0025] The inner layer of the catalytic layer refers to the side close to the proton exchange membrane, and the outer layer of the catalytic layer refers to the side close to the gas diffusion layer;

[0026] Step (1) specifically includes:

[0027] (1) Place the proton exchange membrane in a 5 wt% hydrogen peroxide solution at 80 °C for plasma surface modification treatment for 1 hour;

[0028] (2) Transfer it to deionized water at 80 °C for impregnation for 0.5 hour for residual cleaning;

[0029] (3) Acid activation in a 5 wt% sulfuric acid solution at 80 °C for 1 hour;

[0030] (4) Complete the final treatment by rinsing with deionized water at 80 °C for 30 minutes.

[0031] Step (2) specifically includes:

[0032] (1) The solvent systems of the slurries for the inner and outer layers of the catalytic layer are prepared in the following order: First, add deionized water to cover the catalytic powder, then dropwise add the ionomer dispersion, and finally supplement isopropanol to form a composite solvent.

[0033] (2) The dispersion process is carried out with the synergistic treatment of ultrasonic vibration and emulsification at a temperature below 10 °C.

[0034] Step (3) specifically includes:

[0035] (1) The ratio of the slurry with a high ionomer / catalyst mass ratio in the inner layer of the catalytic layer is 0.1:1, and the ratio of the slurry with a low ionomer / catalyst mass ratio in the outer layer of the catalytic layer is 0.05:1;

[0036] (2) The heat treatment temperature after spraying the inner and outer layers is 80 - 90 °C, and the treatment time is 5 - 15 minutes.

[0037] The detailed steps of the specific embodiment are as follows:

[0038] (1) Pretreat the proton exchange membrane.

[0039] (2) Weigh iridium oxide using a high-precision balance and place it in two reagent bottles respectively for use as the inner and outer layer slurries of the catalyst layer.

[0040] (3) During the catalyst dissolution process, first inject an appropriate amount of deionized water to isolate the combustion risk of alcohol solvents, then dropwise add the ionomer dispersion, and finally add isopropyl alcohol as the solvent to ensure that the ionomer and the catalyst are fully dissolved and uniformly mixed.

[0041] (4) Place the mixed solution in a low-temperature environment and perform uniform dispersion treatment to form a uniform catalyst slurry.

[0042] (5) The catalyst layer adopts an inner and outer layer spraying technology. The high IC ratio slurry is sprayed on the catalyst layer close to the proton exchange membrane side, and the low IC ratio slurry is sprayed on the catalyst layer close to the gas diffusion layer side to ensure that the catalyst layer forms a gradient structure.

[0043] (6) After each layer is sprayed, perform a short heat treatment to enhance the interlayer bonding force.

[0044] (7) Take the proton exchange membrane as the core layer, stack the anode catalyst layer and the cathode catalyst layer on both sides of it in turn, and cover the anode and cathode gas diffusion layers respectively. Apply appropriate torque for encapsulation to complete the assembly of the PEMWE.

[0045] (8) Perform performance tests on the assembled membrane electrode and record the current density and overpotential.

[0046] In step (1), the pretreatment method is to put the proton exchange membrane into 5% hydrogen peroxide at 80 °C for 1 h, soak it in deionized water for half an hour; then put it into 5% dilute sulfuric acid at 80 °C for 1 h; finally put it into deionized water at 80 °C for 0.5 h.

[0047] In step (3), the ionomer dispersion is a Nafion solution with a concentration of 5%.

[0048] In step (4), the low temperature is 10 °C, and the uniform dispersion treatment method adopts the combination of ultrasonic oscillation and emulsification.

[0049] In step (5), the IC ratio of each layer of the anode catalyst layer decreases from the inside to the outside. The anode catalyst layer is sprayed with a catalyst slurry with an IC ratio of 0.1:1 on the inside until the iridium element loading is 0.5 mg / cm 2 Stop; the anode catalyst layer is sprayed with a catalyst slurry with an IC ratio of 0.05:1 on the outside until the iridium element loading is 1 mg / cm2 Stop.

[0050] After the inner and outer layers are sprayed in step (6), heat treatment is carried out at a temperature of 80 °C for 10 min to enhance the interfacial bonding strength.

[0051] In step (7), assemble the PEMWE with a torque of 3 N·m.

[0052] Example 1: Preparation of a conventional membrane electrode for comparison

[0053] (1) Immerse the proton exchange membrane in 5% hydrogen peroxide at 80 °C for 1 h, soak it in deionized water for half an hour; then immerse it in 5% dilute sulfuric acid at 80 °C for 1 h; finally, immerse it in deionized water at 80 °C for 0.5 h.

[0054] (2) Weigh 79% iridium oxide catalyst using a high-precision analytical balance, put it into a 30 mL centrifuge tube, and record the tare weight.

[0055] (3) Inject an appropriate amount of deionized water, then add 5% Nafion solution and analytical grade isopropanol dropwise. The mass ratio of Nafion to iridium is 0.1, and the volume ratio of isopropanol to water is 9:1.

[0056] (4) Ultrasonically vibrate the centrifuge tube at a frequency of 50 kHz for 90 minutes, and control the temperature not to exceed 10 °C during the process.

[0057] (5) Place the centrifuge tube in an ice-water mixture, use an emulsifier to shear at a speed of 15000 rpm for 60 minutes, and then store it for standby at a ultrasonic frequency of 50 kHz.

[0058] (6) Take 25 ml of the above-prepared conventional slurry, inject it into an ultrasonic spraying machine, and set the injection speed to 0.25 ml / min.

[0059] (7) Carry out spraying. After each cycle (the nozzle returns to the starting point), weigh it using an analytical balance and calculate the iridium loading until the target iridium loading is reached.

[0060] (8) Take the proton exchange membrane as the core layer, stack the anode catalyst layer and the cathode catalyst layer on both sides of it in sequence, and cover the anode and cathode gas diffusion layers respectively. Carry out encapsulation by applying a torque of 3 N·m to complete the assembly of the PEMWE. Perform performance tests on the assembled membrane electrode, and record the current density and overpotential.

[0061] Example 2: Preparation of a membrane electrode with a gradient structure anode catalyst layer (the present invention)

[0062] (1) Immerse the proton exchange membrane in 5% hydrogen peroxide at 80 °C for 1 h, soak it in deionized water for half an hour; then immerse it in 5% dilute sulfuric acid at 80 °C for 1 h; finally, immerse it in deionized water at 80 °C for 0.5 h.

[0063] (2) Weigh two portions of 79% iridium oxide catalyst using a high-precision analytical balance, place them in 30 mL centrifuge tubes respectively, and record the tare weight.

[0064] (3) During the catalyst dissolution process, first inject an appropriate amount of deionized water to isolate the combustion risk of alcohol solvents.

[0065] (4) Prepare two slurries respectively:

[0066] Slurry A: The mass ratio of Nafion to iridium is 0.1:1, and the volume ratio of isopropanol to water is 9:1.

[0067] Slurry B: The mass ratio of Nafion to iridium is 0.05:1, and the volume ratio of isopropanol to water is 9:1.

[0068] (5) Seal the centrifuge tubes and ultrasonically oscillate them at a frequency of 50 kHz for 90 minutes, controlling the temperature not to exceed 10 °C during the process.

[0069] (6) Place the centrifuge tubes in an ice-water mixture, shear them with an emulsifier at a speed of 15000 rpm for 60 minutes, and then store them for standby with a ultrasonic frequency of 50 kHz.

[0070] (7) Preparation of the inner catalytic layer: Take 25 ml of Slurry A and inject it into an ultrasonic spray coater. Spray it, and after each cycle, weigh it using an analytical balance and calculate the iridium loading until the target iridium loading (0.5 mg / cm 2 ) is reached.

[0071] (8) Preparation of the outer catalytic layer: Take 25 ml of Slurry B and inject it into an ultrasonic spray coater. Spray it, and after each cycle, weigh it using an analytical balance and calculate the iridium loading until the target iridium loading (0.5 mg / cm 2 ) is reached.

[0072] (9) After each layer is sprayed, perform heat treatment at 80 °C for 10 minutes to enhance the interlayer bonding force.

[0073] (10) Use the proton exchange membrane as the core layer, stack the anode catalytic layer and the cathode catalytic layer on both sides of it in sequence, and cover the anode and cathode gas diffusion layers respectively. Seal them by applying a torque of 3 N·m to complete the assembly of the PEMWE. And perform performance tests on the assembled membrane electrode, recording the current density and overpotential.

[0074] In the embodiments of the present invention, two groups of controls are set up in the experiment of the anode catalyst layer. After assembling into a membrane electrode, they are respectively called a conventional membrane electrode and a membrane electrode with a gradient structure catalyst layer at the anode. The cathode uses a catalyst layer with a platinum loading of 0.5 mg prepared by conventional methods Pt cm -2 , the conventional membrane electrode uses a conventional anode catalyst layer, and the membrane electrode of the present invention uses a gradient structure anode catalyst layer, with an iridium loading of 1.0 mg Ir cm -2 . The two-dimensional morphologies of the two anode catalyst layers are compared as Figure 2-1 、 2-2 shown, and the data statistics are as Figure 3 shown. The comparison of the effective activation areas is as Figure 4-1 、 4-2 shown, and the performance comparison is as Figure 5 shown.

[0075] By regulating the microstructure of the catalyst layer (such as pore gradient, ionomer distribution) and process parameters (such as spraying path, heat treatment conditions), the following breakthroughs are achieved:

[0076] (1) Optimization of mass transfer: Construct a pore gradient channel from dense to sparse to shorten the proton and reactant transport paths and reduce the mass transfer overpotential.

[0077] (2) Improvement of iridium utilization rate: Through gradient loading design, that is, a high-loading active area on the inner side of the proton membrane + a low-loading diffusion area on the outer side, the effective utilization rate of iridium elements is increased by more than 30%.

[0078] (3) Cost control: The electrolysis efficiency of the catalyst layer currently in use is increased by 5.4%, reducing the electrolysis cost.

[0079] This method provides an expandable technical path for the high-efficiency and low-consumption commercialization of PEMWE from the perspective of microstructure engineering.

[0080] Figure 2-1 、 2-2 respectively give the two-dimensional images of two embodiments of the conventional catalyst layer and the gradient structure catalyst layer taken by scanning electron microscopy. It can be seen from the figure that the agglomeration of catalyst particles in the conventional catalyst layer is more serious than that in the gradient structure catalyst layer.

[0081] Figure 3 is a statistical chart of the agglomeration area ratio obtained by analyzing the electron microscope using particle size analysis software. It can be seen from the figure that the agglomeration area ratio of the conventional catalyst layer is larger than that of the new gradient structure catalyst layer.

[0082] Figure 4-1 、 4-2Cyclic voltammetry tests were conducted on two examples of a conventional catalyst layer and a gradient structure catalyst layer, respectively, and the charge amounts of the two examples were compared. The effective active area is proportional to the charge amount. The charge amount of the gradient structure catalyst layer is 0.2556 C / cm 2 , while the charge amount of the conventional catalyst layer is 0.2284 C / cm 2 . This is because the agglomeration phenomenon of the gradient structure catalyst layer is better than that of the conventional catalyst layer, and the catalyst particles are in more sufficient contact with the reactants.

[0083] Figure 5 By comparing the performance test results of a traditional homogeneous structure anode catalyst layer membrane electrode and a gradient structure anode catalyst layer membrane electrode, a significant performance improvement can be observed. At a current density of 3 A / cm 2 , the operating voltage of the new membrane electrode with a gradient structure catalyst layer at the anode drops to 1.81 V, which is 103 mV lower than 1.913 V of the traditional structure membrane electrode, a decrease of 5.4%. This performance improvement is mainly attributed to the innovative design of the gradient structure catalyst layer: its optimized pore distribution characteristics (including pore size gradient and porosity gradient) and significantly increased active specific surface area effectively improve the mass transfer characteristics of the electrode. Specifically, this structural design not only reduces the interfacial impedance but also enhances the bubble discharge efficiency, thus significantly reducing the electrochemical polarization loss and concentration polarization loss.

Claims

1. A method for preparing a gradient catalytic layer for a proton exchange membrane electrolyzer anode, characterized in that: The method comprises the following steps: (1) Pretreatment of the proton exchange membrane: The base proton membrane material is sequentially subjected to plasma surface modification treatment, deionized water cleaning, acid activation treatment and final treatment to obtain a sprayable substrate with optimized wettability; (2) preparing a dual gradient slurry: preparing gradient slurries on both the inner and outer surfaces of the catalyst layer, respectively. The slurry contains an electrocatalyst, an ionomer dispersion, a composite solvent system of deionized water and isopropanol, and a stable suspension system is formed by synergistic dispersion through ultrasonic vibration and emulsification; (3) constructing a gradient structure catalyst layer: the proton exchange membrane is used as a substrate, the catalyst layer is attached to one side of the proton exchange membrane, the inner layer of the catalyst layer is sprayed with a slurry with a high ionomer / catalyst mass ratio; the outer layer of the catalyst layer is sprayed with a slurry with a low ionomer / catalyst mass ratio, and the inner and outer layers are heat treated after spraying; (4) Membrane electrode assembly and testing: The catalyst layer and the gas diffusion layer obtained in step (3) are stacked and packaged to complete the assembly of the proton exchange membrane water electrolysis membrane electrode, and a performance test is performed.

2. The method for preparing a gradient catalytic layer for a proton exchange membrane electrolyzer anode according to claim 1, characterized in that: The step (1) specifically comprises: (1) Placing the proton exchange membrane in a 5 wt % hydrogen peroxide solution at 80° C. for plasma surface modification for 1 hour; (2) Transfer to 80°C deionized water and immerse for 0.5 hours for residual cleaning; (3) Acid activation in a 5 wt % sulfuric acid solution at 80° C. for 1 hour; (4) The final treatment was completed by rinsing with deionized water at 80°C for 30 minutes.

3. The method for preparing a gradient catalytic layer for a proton exchange membrane electrolyzer anode according to claim 1, characterized in that: The inner layer of the proton exchange membrane refers to the side close to the proton exchange membrane; the outer layer refers to the side close to the gas diffusion layer, and the step (2) specifically includes: (1) The solvent systems of the inner and outer slurries of the catalyst layer are prepared in the following order: first, deionized water is added to cover the catalytic powder, then the ionomer dispersion is added dropwise, and finally, isopropanol is added to form a composite solvent; (2) The dispersion process is carried out by co-treatment of ultrasonic vibration and emulsification at a temperature below 10°C.

4. The method for preparing a gradient catalytic layer for a proton exchange membrane electrolyzer anode according to claim 1, characterized in that: The step (3) specifically comprises: (1) The ratio of the mass ratio of the high ionomer to the catalyst in the inner layer of the catalyst layer is 0.1:1, and the ratio of the mass ratio of the low ionomer to the catalyst in the outer layer of the catalyst layer is 0.05:1; (2) The heat treatment temperature after spraying the inner and outer layers is 80-90°C and the treatment time is 5-15 minutes.

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