Gas diffusion layer of proton exchange membrane fuel power cell -Lithium - Ion Battery Equipment
In order to realize the large-scale production of fuel power cell technology in China, it is necessary to solve the problem of gas diffusion layer of fuel power cell core material.
At present, China mainly relies on imports to solve the problem of gas diffusion layer. Generally speaking, gas diffusion layer is also a bottleneck technology in the development of fuel power cell technology in China.
Wang Haijiang pointed out that the hydrogen energy and fuel power cell research team of the South University of Science and Technology has optimized the gas diffusion layer structure of fuel power cells, developed the gas diffusion layer production process, and realized the small batch production of the gas diffusion layer in the past two years.
From the perspective of fuel power cell single cell structure, the fuel power cell stack is composed of multiple single cells in series. The fuel power cell single cell has a seven-layer structure, with a proton exchange membrane in the middle, a catalytic layer on both sides of the proton exchange membrane, and two gas diffusion layers on both sides of the catalytic layer. The fuel power cell single cell structure is formed by adding bipolar plates.
In addition, the single cell of fuel power cell needs two gas diffusion layers. Although the price of gas diffusion layer is cheaper than that of proton exchange membrane in the market at present, the amount of gas diffusion layer is relatively large, and it needs two, so it has a great impact on the price of fuel power cell.
The US Department of Energy has made some calculations based on the existing fuel power cell technology. In small batches, the price basically accounts for 21%, which is more expensive than the proton exchange membrane.
With the increase of volume, the price of gas diffusion layer will decrease, but even in mass production, it will also account for a certain proportion. When the scale reaches 500000 vehicles, there will still be 6% of the price, so the gas diffusion layer has a great impact on the price of fuel power cells.
Wang Haijiang said that the gas diffusion layer can affect the mass transfer, heat transfer and ohmic resistance of fuel power cells, because it is very useful for water management, so it indirectly affects the dynamics of fuel power cells.
In addition, because the gas diffusion layer affects the performance of fuel power cells, it indirectly affects the cost of fuel power cells. Especially now, the development of fuel power cell technology is in the direction of high current operation, and the mass transfer is very important during high current operation.
So, how can we have a good gas diffusion layer material?
Wang Haijiang pointed out that the first step is to analyze the role of gas diffusion layer in fuel power cells. First of all, the first purpose is the use of mass transfer. The gas diffusion layer should evenly diffuse the gas to the catalyst for reaction, and at the same time, the product should be taken out. This is the first purpose of the gas diffusion layer.
At the same time, the heat generated by the reaction in the fuel power cell will also be carried out through the gas diffusion layer. In addition, the current generated by the gas diffusion layer is also led to the external circuit through the gas diffusion layer. Mass transfer, heat transfer and conductivity are the important functions of the gas diffusion layer in the fuel power cell.
In addition, the gas diffusion layer should have a certain mechanical strength to support the membrane electrode. For example, the membrane electrode is not strong enough for CCM. There should be a support. This is the four functions of the gas diffusion layer. Because of such a functional requirement, there are certain requirements for the material selection of the gas diffusion layer.
First of all, the gas diffusion layer must be a porous medium with good thermal conductivity, good conductivity and certain mechanical strength.
In addition, because the gas diffusion layer works in the fuel power cell, it should have good chemical stability. Because of this requirement, there are not many choices of gas diffusion layer. In the early days, some people used metal mesh or metal foam as the gas diffusion layer. Because of its low chemical stability, there were basically only two choices of materials for the gas diffusion layer, either carbon cloth or carbon paper. The mechanical strength of carbon cloth was not particularly high, so almost all companies used carbon paper as the gas diffusion layer later.
The structure of the gas diffusion layer material of the proton exchange membrane fuel power cell has a great influence on the performance of the gas diffusion layer. How to solve the problem of gas diffusion layer structure?
Wang Haijiang said that the hydrogen energy and fuel power cell research team of the South University of Science and Technology optimized the structure of the gas diffusion layer using simulation methods. Using some existing gas diffusion layers, the structure of the gas diffusion layer was obtained by CT scanning method, and then the structure of the gas diffusion layer was put into the computer for two-phase simulation to optimize the structure of the gas diffusion layer. After the simulation, some properties of the gas diffusion layer are also measured by experiments.(Lithium - Ion Battery Equipment)
The most important thing about the gas diffusion layer is that the gas should go from it. The diffusion coefficient of the gas in the porous medium is the key index to determine the performance of the fuel power cell.
Wang Haijiang pointed out that the research team of hydrogen energy and fuel power cell at the South University of Science and Technology used one to measure the diffusion coefficient. There were two chambers, one containing nitrogen, one containing oxygen, and the other containing a ball valve. When the ball valve opened, the oxygen would go through the sample to the other chamber, simulating the concentration of oxygen in the other chamber, The diffusion coefficient of oxygen in porous media can be calculated. With the measurement of diffusion coefficient, the performance of gas diffusion layer structure can be optimized by measuring samples with different structures.
In addition, the research team of hydrogen energy and fuel power cell at the South University of Science and Technology also conducted a study on water management in the gas diffusion layer, using a test similar to that of a single cell, but instead of normal MEA in the single cell, it placed samples of the gas diffusion layer and a proton exchange membrane. The anode side is water flowing inside, the membrane is always completely wet, and the cathode side is in accordance with different air flow, Measure the speed of water coming out under different air flow rates, so as to get the relationship between the speed of water passing through the gas diffusion layer and the air flow rate.
Wang Haijiang said that because the water flows through the gas diffusion layer at a very high speed, the phenomenon that the inner membrane of the fuel power cell is easy to dry is caused. At the lower side of the dotted line, the speed of water is relatively small, which causes water flooding. With such a measurement, we can calculate what kind of gas diffusion layer is the most effective for water management in fuel power cells. (The article is from Wang Haijiang, a professor of the South University of Science and Technology and an academician of the Canadian Academy of Engineering, who collated the shorthand of his speech at the China Electric Vehicle Centennial, and has not been reviewed by himself)
At present, China mainly relies on imports to solve the problem of gas diffusion layer. Generally speaking, gas diffusion layer is also a bottleneck technology in the development of fuel power cell technology in China.
Wang Haijiang pointed out that the hydrogen energy and fuel power cell research team of the South University of Science and Technology has optimized the gas diffusion layer structure of fuel power cells, developed the gas diffusion layer production process, and realized the small batch production of the gas diffusion layer in the past two years.
From the perspective of fuel power cell single cell structure, the fuel power cell stack is composed of multiple single cells in series. The fuel power cell single cell has a seven-layer structure, with a proton exchange membrane in the middle, a catalytic layer on both sides of the proton exchange membrane, and two gas diffusion layers on both sides of the catalytic layer. The fuel power cell single cell structure is formed by adding bipolar plates.
In addition, the single cell of fuel power cell needs two gas diffusion layers. Although the price of gas diffusion layer is cheaper than that of proton exchange membrane in the market at present, the amount of gas diffusion layer is relatively large, and it needs two, so it has a great impact on the price of fuel power cell.
The US Department of Energy has made some calculations based on the existing fuel power cell technology. In small batches, the price basically accounts for 21%, which is more expensive than the proton exchange membrane.
With the increase of volume, the price of gas diffusion layer will decrease, but even in mass production, it will also account for a certain proportion. When the scale reaches 500000 vehicles, there will still be 6% of the price, so the gas diffusion layer has a great impact on the price of fuel power cells.
Wang Haijiang said that the gas diffusion layer can affect the mass transfer, heat transfer and ohmic resistance of fuel power cells, because it is very useful for water management, so it indirectly affects the dynamics of fuel power cells.
In addition, because the gas diffusion layer affects the performance of fuel power cells, it indirectly affects the cost of fuel power cells. Especially now, the development of fuel power cell technology is in the direction of high current operation, and the mass transfer is very important during high current operation.
So, how can we have a good gas diffusion layer material?
Wang Haijiang pointed out that the first step is to analyze the role of gas diffusion layer in fuel power cells. First of all, the first purpose is the use of mass transfer. The gas diffusion layer should evenly diffuse the gas to the catalyst for reaction, and at the same time, the product should be taken out. This is the first purpose of the gas diffusion layer.
At the same time, the heat generated by the reaction in the fuel power cell will also be carried out through the gas diffusion layer. In addition, the current generated by the gas diffusion layer is also led to the external circuit through the gas diffusion layer. Mass transfer, heat transfer and conductivity are the important functions of the gas diffusion layer in the fuel power cell.
In addition, the gas diffusion layer should have a certain mechanical strength to support the membrane electrode. For example, the membrane electrode is not strong enough for CCM. There should be a support. This is the four functions of the gas diffusion layer. Because of such a functional requirement, there are certain requirements for the material selection of the gas diffusion layer.
First of all, the gas diffusion layer must be a porous medium with good thermal conductivity, good conductivity and certain mechanical strength.
In addition, because the gas diffusion layer works in the fuel power cell, it should have good chemical stability. Because of this requirement, there are not many choices of gas diffusion layer. In the early days, some people used metal mesh or metal foam as the gas diffusion layer. Because of its low chemical stability, there were basically only two choices of materials for the gas diffusion layer, either carbon cloth or carbon paper. The mechanical strength of carbon cloth was not particularly high, so almost all companies used carbon paper as the gas diffusion layer later.
The structure of the gas diffusion layer material of the proton exchange membrane fuel power cell has a great influence on the performance of the gas diffusion layer. How to solve the problem of gas diffusion layer structure?
Wang Haijiang said that the hydrogen energy and fuel power cell research team of the South University of Science and Technology optimized the structure of the gas diffusion layer using simulation methods. Using some existing gas diffusion layers, the structure of the gas diffusion layer was obtained by CT scanning method, and then the structure of the gas diffusion layer was put into the computer for two-phase simulation to optimize the structure of the gas diffusion layer. After the simulation, some properties of the gas diffusion layer are also measured by experiments.(Lithium - Ion Battery Equipment)
The most important thing about the gas diffusion layer is that the gas should go from it. The diffusion coefficient of the gas in the porous medium is the key index to determine the performance of the fuel power cell.
Wang Haijiang pointed out that the research team of hydrogen energy and fuel power cell at the South University of Science and Technology used one to measure the diffusion coefficient. There were two chambers, one containing nitrogen, one containing oxygen, and the other containing a ball valve. When the ball valve opened, the oxygen would go through the sample to the other chamber, simulating the concentration of oxygen in the other chamber, The diffusion coefficient of oxygen in porous media can be calculated. With the measurement of diffusion coefficient, the performance of gas diffusion layer structure can be optimized by measuring samples with different structures.
In addition, the research team of hydrogen energy and fuel power cell at the South University of Science and Technology also conducted a study on water management in the gas diffusion layer, using a test similar to that of a single cell, but instead of normal MEA in the single cell, it placed samples of the gas diffusion layer and a proton exchange membrane. The anode side is water flowing inside, the membrane is always completely wet, and the cathode side is in accordance with different air flow, Measure the speed of water coming out under different air flow rates, so as to get the relationship between the speed of water passing through the gas diffusion layer and the air flow rate.
Wang Haijiang said that because the water flows through the gas diffusion layer at a very high speed, the phenomenon that the inner membrane of the fuel power cell is easy to dry is caused. At the lower side of the dotted line, the speed of water is relatively small, which causes water flooding. With such a measurement, we can calculate what kind of gas diffusion layer is the most effective for water management in fuel power cells. (The article is from Wang Haijiang, a professor of the South University of Science and Technology and an academician of the Canadian Academy of Engineering, who collated the shorthand of his speech at the China Electric Vehicle Centennial, and has not been reviewed by himself)
