Pore-Scale Reconstruction and Multiphase Simulation of PEMFC Catalyst Layers

Abstract

In PEMFC (Proton Exchange Membrane Fuel Cell), reducing Pt loading, and hence the cost, is of primary importance to the commercialization of the technology [1-3, 5]. One of the feasible ways to achieve this goal is to improve the structure of cathode catalyst layers. In order to optimize the catalyst layers and improve the overall performance, it is essential to advance the fundamental understanding of the relation between the structural parameters and characteristics of transport and reaction. Pore-scale simulation [4-6] is a promising approach in this regard [4, 5]. In this approach, the microstructures of catalyst layers are reconstructed at the pore scale, and the transport and electrochemical reaction processes in such structures are simulated directly, without assumptions on the effects of the geometry. In this work, we present our recent developed methods in the computational framework for pore-scale simulations. First, we developed a method that stochastically reconstructs customized catalyst layer structures, by extending the sphere-based simulated annealing method [4]. The refined method can generate not only the conventional isotropic structures, but also anisotropic ones, which have not been numerically studied before. In addition, several structural parameters that influence the performance of the catalyst layers can be specified in advance. Second, we developed a non-isothermal multiphase pore-scale simulation model, which is based on a phase-field Lattice Boltzmann method [8, 9]. Liquid water generation, phase change, and transport in pores are coupled with other physical processes occurring in cathode catalyst layers, including oxygen transport in the ionomer and pores, electron transport in Pt/Carbon, and proton transport in the ionomer. Application of the proposed framework to the optimization of the catalyst layer structures will be presented. Reconstructed catalyst layer geometries and multiphase simulation results (Figs. 1 and 2) will be shown. For different structural parameters and operating conditions, quantitative results including the polarization curves and the saturation levels in the catalyst layers will be presented. It will be shown that at low Pt loading conditions, the microstructure, especially regarding the ionomer, has substantial impacts to the performance of catalyst layers. The effects of the content ratio among different components and of the distribution of each component in the catalyst layers will be analyzed. Acknowledgement The financial support of Honda R&D Co. Ltd., Japan, is gratefully acknowledged.