Towards the understanding of transport limitations in a proton-exchange membrane fuel cell catalyst layer: Performing agglomerate scale direct numerical simulations on electron-microscopy-based geometries

Improving the cathode catalyst layer design requires understanding the sources of transport limitations in proton-exchange membrane fuel cells. For the purpose, a framework consisting on an electron microscopy characterization setup in couple with a numerical modeling software is proposed. The latter integrates high-performing geometry building capabilities which ensure full phase discretization (carbon, platinum, ionomer and pore phase) and freedom when designing the structure morphology and meshing. The 3D structure of the carbon phase is extracted from a focused ion beam scanning electron microscopy analysis having a 2 nm isotropic resolution. The platinum phase is built according to a nanoparticle size histogram determined from high angle annular dark field scanning transmission electron microscopy images. To add the ionomer phase, the thickness of the layer is measured on high resolution transmission electron microscopy images. The multi-physics model includes gas transport in the pores, and gas and ionic transport in the Nafion. A 4-step reaction mechanism is used to solve the electrochemistry. Numerical simulations are performed on two catalyst layer portions. The results show that structural heterogeneities can deeply impact performance. Such impact is mainly linked to oxygen diffusion limitations through the Nafion film and, to a certain extent, to interparticle competition effects. [Display omitted] •3D cathode catalyst layer geometry is built.•Cathode catalyst layer agglomerate is modeled.•Oxygen mass transport limitations are observed at the agglomerate scale.•Effect of local heterogeneities on local performance is analyzed.