A catalytic layer structure for improving electrochemical reaction efficiency

Abstract

The present invention proposes a cathode catalyst layer structure for improving the electrochemical reaction efficiency of a fuel cell. The catalyst layer includes three layers, namely, layer I close to a proton exchange membrane, layer III close to a diffusion layer, and layer II located between layer I and layer III. The content of active components in the catalyst of layer II is higher than those in layer I and layer III, the catalyst loading of layer II is higher than that in layer I and layer III, the EW value of the resin in layer III is higher than that in layer I and layer II, and the I / C ratios of layers I, II, and III decrease successively. Through the above-mentioned improvements, the present invention reduces the active metal concentration in layer I and improves proton conduction, increases the active metal concentration in the central region of the catalyst layer, i.e., layer II, and improves proton conduction, and reduces the active metal concentration in layer III and improves gas transmission.

Description

Technical Field

[0001] The present invention belongs to the field of fuel cells, and in particular relates to a membrane electrode catalyst layer structure for improving electrochemical reaction efficiency. Background Art

[0002] As a key member of the renewable energy family, hydrogen energy has seen its status gradually rise in recent years. Fuel cells, the most efficient energy conversion device for hydrogen utilization, have also seen rapid technological development. The commercialization of fuel cell vehicles by Toyota in Japan has ignited a global boom in fuel cell technology.

[0003] Hydrogen fuel cells are composed of hundreds of membrane electrode and bipolar plates connected in series. The membrane electrode, a key component of the fuel cell, is where the entire electrochemical reaction occurs. It typically consists of a seven-in-one structure consisting of a proton exchange membrane, anode and cathode catalyst layers, cathode and cathode gas diffusion layers, and sealing materials. The performance, efficiency, and reliability of the membrane electrode are crucial to fuel cells. During fuel cell operation, hydrogen diffuses through the anode gas diffusion layer into the catalyst layer, where it undergoes electrochemical oxidation to generate hydrogen ions. The hydrogen ions then pass from the anode through the membrane to the cathode catalyst layer, where they undergo an electrochemical reduction reaction with oxygen that has passed through the cathode gas diffusion layer and reached the catalyst layer.

[0004] Currently, commercial membrane electrode catalyst layers generally adopt a uniform structure, resulting in low catalyst utilization in areas close to the proton membrane and the diffusion layer where the electrochemical speed is slow. Although relevant patents have proposed a multi-layer structure design to improve the reaction efficiency of the catalyst layer, for example, the catalyst layer is divided into an inner layer and an outer layer, the inner layer is a hydrophilic catalyst layer, and the outer layer is a hydrophobic catalyst layer, the purpose is to provide a catalyst layer structure with strong gas mass transfer capacity, a large electrochemical reaction three-phase interface, and a small amount of Pt; there are also methods that divide the catalyst layer into two or more layers, each layer using different catalysts and different component ratios to achieve a gradient distribution of porosity and pore size in the vertical direction of the catalyst layer, thereby improving the mass transfer resistance of the catalyst layer, but these technical solutions are more concerned with reducing the gas mass transfer resistance in the catalyst layer, hoping to improve the reaction efficiency by improving the mass transfer process. The hydrophobicity gradient distribution inevitably requires the introduction of a hydrophobic agent in the catalytic layer. The hydrophobic agent is an electronically insulating and proton-insulating material that will increase the ohmic resistance of the catalytic layer, which in turn will affect the battery performance. The pore gradient distribution is mainly achieved by regulating the proportion of resin. In this structure, the active sites in the catalytic layer are more concentrated in the catalytic layer close to the membrane side, while the resin content in the catalytic layer away from the membrane gradually decreases. This only considers the gradient distribution of gas in the catalytic layer, but does not consider the gradient distribution of protons in the catalytic layer, so it is not conducive to achieving optimal performance. Summary of the Invention

[0005] Based on the above background technology, the present invention takes into account the concentration difference of gas in the vertical direction of the catalyst layer, and also takes into account the concentration difference of protons in the vertical direction. During the operation of the fuel cell, the reaction in the cathode catalyst layer is that hydrogen ions, oxygen molecules and electrons generate water. The concentration of hydrogen ions gradually decreases from the proton membrane to the diffusion layer along the thickness of the catalyst layer, and the concentration of oxygen molecules gradually decreases from the diffusion layer to the proton membrane along the thickness of the catalyst layer, resulting in a slowdown in the electrochemical reaction rate near the proton membrane and the diffusion layer, while the electrochemical reaction rate in the middle area is faster, which is the main area where the electrochemical reaction occurs. Based on the above ideas, a three-layer catalytic layer structural design is proposed. The noble metal concentration of the catalytic layers on both sides is reduced to improve gas diffusion and proton conduction; the noble metal concentration of the middle catalytic layer is increased to improve the reaction rate of this layer.

[0006] The specific scheme of the present invention is as follows:

[0007] A fuel cell cathode catalyst layer for improving electrochemical reaction efficiency is provided. The cathode catalyst layer is divided into three layers, and each layer uses a different slurry. The slurries differ in catalyst type, slurry composition, slurry ratio, etc., thereby reducing the active metal concentration in the catalyst layer near the proton exchange membrane and improving proton conduction, increasing the active metal concentration in the central region of the catalyst layer and improving proton conduction, and reducing the active metal concentration in the catalyst layer near the diffusion layer and improving gas transmission. The cathode catalyst layer structure is as follows: the catalyst layer includes three layers, namely, layer I near the proton exchange membrane, layer III near the diffusion layer, and layer II located between layer I and layer III. The content of active components in the catalyst of layer II is higher than that of layer I and layer III, the catalyst loading amount of layer II is higher than that of layer I and layer III, the EW value of the resin in layer III is higher than that of layer I and layer II, and the I / C (ion exchange resin to carbon carrier in the catalyst) ratio of layer I, layer II, and layer III decreases in sequence.

[0008] Based on the above scheme, preferably, the layer I, layer II and layer III are independently prepared by one of the methods such as ultrasonic spraying, electrostatic spraying, electrostatic spinning, screen printing, slit coating, etc., and the layer I, layer II and layer III are prepared layer by layer in sequence on the proton exchange membrane or gas diffusion layer.

[0009] Based on the above scheme, preferably, the catalyst type in layer I close to the proton exchange membrane is Pt / C, with a Pt content of 20-40%; the EW value of the ion exchange resin is 700-900 (the low EW value is selected here to ensure the ion conduction speed of the catalytic layer close to the membrane); the ratio of ion exchange resin to carbon support in the catalyst is I / C 0.8-1.1; the Pt loading is 0.01-0.05 mg / cm 2 .

[0010] Based on the above scheme, preferably, the catalyst type in the middle catalytic layer, i.e., layer II, is one of Pt / C, PtPd / C, PtCo / C, and PtIrCo / C, with an active metal content of 50-70%; the ion exchange resin EW value is 700-900; I / C is 0.7-0.8; and the Pt loading is 0.1-0.2 mg / cm 2 ;

[0011] Based on the above scheme, preferably, the catalyst type in layer III close to the diffusion layer is Pt / C, with a Pt content of 20-40%; the ion exchange resin EW value is 900-1100; I / C is 0.5-0.7; and the Pt loading is 0.01-0.05 mg / cm 2 .

[0012] Beneficial effects

[0013] (1) The present invention is based on the fact that the concentration of hydrogen ions gradually decreases from the proton exchange membrane to the diffusion layer along the thickness direction of the cathode catalyst layer; and the concentration of oxygen molecules gradually decreases from the diffusion layer to the proton exchange membrane along the thickness direction of the catalyst layer. The reaction zone in the catalyst layer can be divided into three regions: the region near the proton membrane, the middle region and the region near the diffusion layer. The middle region is a region with high concentrations of hydrogen ions and oxygen molecules, and is the main region where the electrochemical reaction occurs. The region near the proton membrane and the region near the diffusion layer have low concentrations of oxygen molecules or hydrogen ions, so the electrochemical reaction occurs slowly. Therefore, the present invention adopts a three-layer catalyst structure design and adjusts conditions such as precious metal loading, I / C ratio and EW value of the resin to reduce the active metal concentration in layer I near the proton exchange membrane and layer III near the gas diffusion layer, and increase the active metal concentration in layer II in the central region of the catalyst layer, so as to improve the reaction efficiency of the entire catalyst layer, improve the catalyst utilization rate and reduce the amount of precious metal used.

[0014] (2) The present invention uses a resin with a low EW value in the area close to the proton membrane and the middle area to improve the proton conductivity, and uses a resin with a high EW value in the area close to the diffusion layer, thereby reducing costs while ensuring performance.

[0015] (3) The I / C ratio in layers I, II, and III of the present invention decreases successively, thereby increasing the proton conductivity of layer I, increasing the porosity of layer III, and improving the reaction efficiency of the entire catalytic layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Schematic diagram of oxygen concentration and ion concentration in the cathode catalyst layer;

[0017] Figure 2 Comparison of polarization curves of single cells in Example 1;

[0018] Figure 3Comparison of single cell polarization curves in Example 2. DETAILED DESCRIPTION

[0019] Unless otherwise specified, the products used in the following examples are all commercially available conventional products.

[0020] Example 1

[0021] Four catalyst slurries were prepared respectively, and the components were as follows:

[0022]

[0023]

[0024] The above slurries 1, 2, and 3 are ultrasonically sprayed onto an area of 50 cm 2 The cathode catalyst layer is formed on the surface of the proton exchange membrane (Gore 18 micron thick composite membrane). The slurry utilization rate during the spraying process is about 75%, and the Pt loading of the three-layer catalyst layer is 0.06 mg / cm 2 , 0.21mg / cm 2 , 0.06mg / cm 2 Ensure that each sprayed layer is completely dry before spraying the next layer. Then spray the formed slurry 4 onto the other side of the membrane to form the anode catalyst layer.

[0025] The CCM coated with the catalytic layer on both sides was further hot-pressed with the gas diffusion layer to form a membrane electrode. The effective area of the assembly was 50 cm 2 The performance of single cell 1# was evaluated.

[0026] Comparative Example 1

[0027] In order to compare with Example 1, a membrane electrode with a single cathode structure catalyst layer was prepared at the same time, and the effective area of the assembly was 50 cm 2 The performance of single cell 2# was evaluated. The Pt loading in the cathode catalyst layer of single cell 2# was 0.33 mg / cm 2 , cathode and anode slurries are as follows.

[0028]

[0029] Comparative Example 2

[0030] A multilayer membrane electrode with a porosity gradient distribution and a cathode multilayer membrane electrode with a hydrophilicity gradient distribution were prepared. The effective area of the assembly was 50 cm 2 The performance of cells 3# and 4# was evaluated. In the cathode catalyst layer of cell 3#, the porosity increased along the membrane to the diffusion layer, while in the cathode catalyst layer of cell 4#, the hydrophobicity increased along the membrane to the diffusion layer.

[0031] The cathode and anode slurries for single cell 3# are shown in the table below.

[0032]

[0033] The cathode and anode slurries for single cell 4# are shown in the table below.

[0034]

[0035] The performance curves of four membrane electrodes were tested, such as Figure 2 As shown, it can be seen that under almost the same Pt load, the performance of the catalytic layer (1#) of the three-layer structure of the present invention is significantly better than the catalytic layer (2#) of the single structure, and the reaction efficiency of the catalytic layer is improved. Further, compared to the three-layer catalytic layer with the same catalyst porosity gradient distribution, the performance is better, and also better than the three-layer catalytic layer with the same catalyst hydrophilicity gradient distribution. Analyzing the reason, during operation, the hydrogen ion concentration of the cathode catalytic layer gradually decreases from the proton membrane to the diffusion layer along the thickness direction of the catalytic layer, and the oxygen molecule concentration gradually decreases from the diffusion layer to the proton membrane along the thickness direction of the catalytic layer, resulting in the electrochemical reaction speed near the proton membrane and near the diffusion layer slowing down, while the electrochemical reaction speed in the middle area is faster. The three-layer structure in the present invention has the highest utilization rate of the catalyst under the same total Pt load, so it embodies higher performance.

[0036] Example 2

[0037] Four catalyst slurries were prepared respectively, and the components were as follows:

[0038]

[0039] The above slurries 1, 2, and 3 are sequentially slit-coated to an area of 50 cm 2 The cathode catalyst layer is formed on the surface of the proton exchange membrane (N211), ensuring that each layer is completely dry before applying the next layer. The resulting slurry 4 is then applied to the other side of the membrane to form the anode catalyst layer. The slurry utilization rate during the coating process is approximately 80%.

[0040] The CCM formed above was further hot-pressed with the gas diffusion layer to form a membrane electrode, and the assembled effective area was 50 cm 2 The performance of the single battery was evaluated.

[0041] Comparative Example 3

[0042] In order to compare with Example 2, a membrane electrode with a cathode single-structure catalyst layer was prepared at the same time. The cathode and anode slurries were as follows.

[0043]

[0044] The performance curves of two membrane electrodes were tested, such as Figure 3 As shown in the figure, the structure of the three-layer catalytic layer is shown in the figure. Although the total Pt loading is from 0.36 mg / cm 2 (Battery 6#) reduced to 0.25mg / cm 2 (The three-layer loadings of battery 5# are 0.018, 0.2115, and 0.018, respectively), but there is almost no performance degradation, achieving the purpose of reducing Pt loading.

Claims

1. A fuel cell cathode catalyst layer, characterized in that: The catalytic layer comprises three layers, namely, layer I close to the proton exchange membrane, layer III close to the diffusion layer, and layer II located between layer I and layer III. The active component content of the catalyst in layer II is higher than that in layer I and layer III. The catalyst loading in layer II is higher than that in layer I and layer III. The EW value of the resin in layer III is higher than that in layer I and layer II. The I / C ratios of layers I, II, and III decrease in sequence. The catalyst of layer I is Pt / C with a Pt content of 20-40 wt%; the catalyst of layer II is one of Pt / C, PtPd / C, PtCo / C, and PtIrCo / C with an active metal content of 50-70 wt%; the catalyst of layer III is Pt / C with a Pt content of 20-40 wt%; The EW value of the ion exchange resin in layer I is 700-900; the EW value of the ion exchange resin in layer II is 700-900; the EW value of the ion exchange resin in layer III is 900-1100; The I / C (ion exchange resin and carbon support in the catalyst) in layer I is 0.8-1.1; the I / C in layer II is 0.7-0.8; and the I / C in layer III is 0.5-0.

7.

2. The cathode catalyst layer according to claim 1, wherein The layer I, layer II and layer III are independently prepared by one of ultrasonic spraying, electrostatic spraying, electrostatic spinning, screen printing and slit coating. The layer I, layer II and layer III are prepared layer by layer in sequence on the proton exchange membrane or gas diffusion layer.

3. The cathode catalyst layer according to claim 1, wherein: The Pt loading of layer I is 0.01-0.05 mg / cm 2 ; The Pt loading of layer II is 0.1-0.2 mg / cm 2 ; The Pt loading of layer III is 0.01-0.05 mg / cm 2 .

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