Multiscale tomography of nanoporous carbon-supported noble metal catalyst layers

Abstract

Noble metal catalysts are a scarce, non-renewable resource, and yet are required in a wide range of industrial applications, including in polymer electrolyte membrane fuel cells (PEMFCs). The effectiveness of PEMFCs depends not only on the size, active surface area, and distribution of the Pt catalyst nanoparticles, which affects reaction kinetics, but also on the porous structure of the carbon support, which affects mass transport. Unfortunately, the very different size scales – several nm for the Pt catalyst vs. several μm for the porosity features – cannot be characterized by a single method. Here, we present a novel approach for integrating information from both of these size scales to build a single geometrical model. Focused Ion Beam – Scanning Electron Microscope tomography (SEMt) was carried out on a commercial PEMFC cathode catalyst layer to characterize porosity, connectivity as well as pore-size and grain-size distribution. Transmission Electron Microscopy tomography (TEMt) was used to analyze volume and surface area distributions of nanometer sized platinum catalyst particles. Further, we propose an up-scaling approach to translate the information obtained from TEMt to SEMt. Knowledge of catalyst particle locations within the solid support matrix will be critical in enabling the analysis of limiting transport processes in PEMFC CCLs.