Pore-Scale Simulation of Tortuosity in the Catalyst Layer of Proton Exchange Membrane Fuel Cells

Abstract

The porous structures of the catalyst layer (CL) for proton exchange membrane (PEM) fuel cells were reconstructed using the sphere-based simulated annealing (SA) method, in which different carbon sphere diameters, carbon phase fractions, ionomer loadings, and porosities were achieved to simulate different samples. The pore-scale simulation model based on the random walk algorithm was developed to evaluate the tortuosity in the CL. The results show that the carbon phase fraction, particularly the carbon sphere diameter, significantly influences the tortuosities in both pores and ionomer. Larger carbon spheres contribute to tightening the solid phase and enlarging the pore size within the CL, thereby affecting the pore structure and the ionomer distribution. The tortuosity obtained from the random walk algorithm aligns well with those from pore-scale diffusion and flow simulations using the Lattice–Boltzmann method (LBM). The random walk algorithm has the highest efficiency of the three methods in calculating the tortuosity of the CL when the porosity is less than 0.2, demonstrating its wider applicability. This study further proposes formulas for predicting the tortuosities in both pores and ionomer, which are closer to actual CLs than empirical equations. In addition, the present work confirms that the structural randomness resulting from the reconstruction method used in this work has little effect on the tortuosity, indicating that a small number of reconstructed samples suffice for investigating transport processes in the CL. Overall, our research contributes to the prediction of the properties of real porous media in energy engineering. This manuscript reports on the reconstruction of the catalyst layer in proton exchange membrane fuel cells using the simulated annealing method under some appropriate assumptions. A pore-scale simulation using a random walk algorithm was performed to compute the tortuosity both in the pores and in the ionomer, which is compared with pore-scale diffusion and flow simulations using the Lattice–Boltzmann method. A parametric study for several geometric parameters was carried out. Some conclusions are drawn from the results of the pore-scale simulations, and several formulas are proposed accordingly. This study helps to predict the tortuosity in the porous structure closer to the actual porous medium than the empirical equations. Meanwhile, the proposed tortuosity formulas improve the accuracy of the macroscopic models compared to using the assumed transmission parameter values. In addition, structural randomness analysis is explored, which will further reduce the workload in actual engineering calculations and improve computational efficiency.