The Effects of Catalyst Layer Microstructure and Water Saturation on the Effective Diffusivity in PEMFC

Abstract

The effective diffusivity of oxygen in a cathode catalyst layer of PEMFC depends on both the catalyst layer microstructure and the changes in oxygen pathways due to liquid water. In this paper, various aspects of the effective diffusivity are investigated using pore-scale simulation, with emphasis on the effects of Knudsen diffusion. Based on microstructures reconstructed by the spherebased simulated annealing method, both the higher-order lattice Boltzmann method (LBM) and the continuum diffusion equation with the Bosanquet approximation are used to evaluate the effective diffusivity considering Knudsen diffusion. Results show that the continuum diffusion equation reproduces the results from the higher-order LBM that simulates the effects of Knudsen diffusion based on the kinetic theory, when the mean pore size is evaluated using the erosion-dilation method or the chord length distribution. It is also shown that the change of the pore size with water saturation levels should be considered to predict the effects of Knudsen diffusion accurately. The dependence of the effective diffusivity on the agglomerate size is explained by the effects of Knudsen diffusion. Derjaguin's correction for Knudsen diffusion is found to have negligible effects on the effective diffusivity for the present cases. The empirical correlations of various parameters are presented.

Numerical investigation on the nano/microscale transport processes in proton exchange membrane fuel cells: A review

“…由此可见, 合理优化CL的亲疏水性, 有助于 进一步优化CL的水管理并提高CL性能. Zheng和Kim [43] 开展孔尺度两相流模拟, 研究了ORR产生的液态水在 CL孔隙中的积聚, 以及液态水对氧气输运过程的影响. 结果表明, 随着饱和度的增加, 氧气输运的有效孔径 减小, 气体扩散的努森效应越来越显著.…”

Progress in 3D electrode microstructure modelling for fuel cells and batteries: transport and electrochemical performance

“…The porous GDL, commonly carbon paper or woven carbon fabrics [4], are placed on either side of the membrane in a PEMFC to ensure that reactants effectively diffuse to the catalyst layer, and assist electron conduction to and from the CL [66]. Under high current density conditions, oxygen transport loss in the cathode can account for a substantial portion of the performance loss [17]. The oxygen transport loss is triggered by the electrode flooding when the produced liquid water blocks pores in the GDL and covers the electrochemically active sites in the CL, thereby increasing surface overpotential.…”

“…In addition to GDL, LBM is used to study cathode CL which is another critical component of PEMFCs [181,[263][264][265][266][267][268]. Shin et al [268] studied the mass transport properties and catalyst utilization of the CL on the performance of PEMFC using a 3D LBM model based on the quasi-random nanostructural model.…”

“…Shin et al [268] studied the mass transport properties and catalyst utilization of the CL on the performance of PEMFC using a 3D LBM model based on the quasi-random nanostructural model. Based on microstructures reconstructed by the sphere-based simulated annealing method, the LBM was used to evaluate the effects of the CL structural parameters, water saturation and Knudsen diffusion on the effective oxygen diffusivity in the CL [265]. Molaeimanesh and Akbari [267] proposed a 2D LBM agglomerate model to simulate electrochemical reaction in the catalyst layer.…”

Quantification of the Knudsen Effect on the Effective Gas Diffusion Coefficient in Partially Saturated Pore Distributions

“…[ 6,27,28 ] The Knudsen effect is then increased and reduces the effective gas diffusion significantly. [ 21,29 ] In addition, swelling of the material due to interlayer water further reduces the free space for molecular diffusion. [ 26 ] In cement‐based materials, gas diffusion coefficients at dry state of m 2 s −1 are reduced to m 2 s −1 at high saturation levels above 80%.…”