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Nano select: construction method of gas effective diffusion coefficient in catalytic layer of fuel cell based on pore size distribution

wallpapers News 2020-12-15

proton exchange membrane fuel cell is a clean efficient energy conversion device. It has a series of advantages such as zero emission fast start low noise high energy density so on. It is an ideal vehicle power source. With the improvement of people's pursuit of lightweight low cost of vehicle power system the working current density power density of fuel cell are also increasing. The mass transfer ability of porous electrode is the key factor to determine whether the reaction gas can be efficiently transported to the catalyst surface for electrochemical reaction the limiting working current density of fuel cell is mainly affected by the mass transfer ability of electrode. Therefore how to accurately reflect the effective diffusion coefficient of reactive gas in porous catalytic layer is the key to guide the design of electrode structure improve the accuracy of battery performance simulation. At present the effective gas diffusion coefficient of catalytic layer of fuel cell is mostly calculated by Bruggeman formula according to the porosity of catalytic layer the volume diffusion coefficient of gas but it can not accurately reflect the gas diffusion phenomenon in the micropores below micron. Especially when the pore size is close to the average free path of gas molecules the collision probability between molecules pore wall will increase significantly This phenomenon is called Knudsen effect. Although a few researchers have introduced the Knudsen effect into the effective diffusion coefficient of the catalytic layer they usually calculate it according to the fixed pore size the pore size of the actual catalytic layer can cover the whole range from nanometer to micron. The pore size distribution is an important factor to be considered concerned when calculating the effective diffusion coefficient of the catalytic layer. To solve this problem a calculation method of effective gas diffusion coefficient covering the full-scale pore size of catalytic layer of fuel cell was proposed by Professor Qin Yanzhou of State Key Laboratory of internal combustion engine of Tianjin University. The pore size distribution curve of Pt / C catalyst layer was measured by bet. The pore size distribution curve was divided into enough intervals by fractal method. In each interval the Knudsen diffusion coefficient was calculated according to the average pore size. The gas Knudsen diffusion coefficient of the whole catalytic layer is obtained by weighted average of the gas Knudsen diffusion coefficients of all pore sizes according to the volume fraction then the effective gas diffusion coefficient of the catalytic layer is obtained by modifying the volume diffusion coefficient. At the same time the new effective diffusion coefficient of catalytic layer gas is used to calculate the cathode oxygen diffusion battery performance compared with the traditional calculation results of Bruggeman gas effective diffusion coefficient. The results show that the oxygen diffusion ability of catalytic layer considering the Knudsen effect of pore size distribution will be significantly reduced resulting in the maximum current density reduced by 42% It is verified that the nusson effect of pore size distribution has an important influence on the mass transfer in the catalytic layer the performance of the fuel cell. Although

in this paper are based on the pore size distribution curve of the specific catalytic layer of the fuel cell the construction method of the effective diffusion coefficient in the micropore is universal. It can be used to calculate the effective gas transport coefficient in a wide range of pore size ranging from nanometer to micron. It can be used for pore structure design related numerical simulation of porous media aiming at gas mass transfer.


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