Pore-scale study of pore-ionomer interfacial reactive transport processes in proton exchange membrane fuel cell catalyst layer

Understanding interactions between constituent distributions and reactive transport processes in catalyst layer (CL) of proton exchange membrane fuel cell is crucial for improving cell performance and reducing cell cost. In this study, high-resolution porous structures of cathode CL are reconstructed, where all the constituents in CLs are resolved. A pore-scale model based on the lattice Boltzmann method is developed for oxygen diffusion in pores and ionomer, as well as electrochemical reaction at the Pt surfaces. Particularly the model considers the pore-ionomer interfacial transport processes with distinct characteristics of sharp concentration drop, large diffusivity ratio and interfacial dissolution reaction. After validated by interfacial transport processes with analytical solutions, the pore-scale model is applied to reactive transport processes inside complex CL nanoscale structures. Pore-scale results reveal that pore-ionomer interfacial transport processes generate extremely high local transport resistance, significantly reducing the total reaction rate. As volume fraction of carbon increases, the value of the optimum ionomer content generating the best cell performance decreases, while the value of the optimum ionomer content resulting in the lowest performance loss under low Pt loading reduces. The two values generally are different. The pore-scale model helps to understand reactive transport processes and to optimize the CL structures.