X-ray Line Profile Analysis of Nanoparticles in Proton Exchange Membrane Fuel Cell Electrodes

Abstract

We present a method to extract X-ray diffraction patterns from a multiphase system and analyze the particle size distribution of each phase. The method is demonstrated for crystalline nanoparticles in the electrodes of proton exchange membrane fuel cells (PEMFCs), where it is particularly useful to determine particle size distributions without destroying the device. The structure of the electrodes has a considerable influence on the power and durability of a fuel cell and can be further optimized, for example with respect to the durability of the cell. Since the membrane electrode assembly (MEA) contains multiple and partially X-ray transparent layers, the individual catalyst signals from the anode (platinum-ruthenium alloy) and the cathode (platinum) can be extracted from the diffraction patterns recorded of either side of the MEA using the technique presented in this article. By analysis of the platinum (220) reflection by fitting a pseudo-Voigt function, the individual particle size distributions are determined for the anode and the cathode. The catalyst surface area loss due to particle growth is studied in long-term experiments during the operation of a single model cell for 2100 h and, for comparison, during the storage in different gas atmospheres (Ar, H2, and O2) for 6500 h. With respect to the single cell operation, approximately one-third of the surface is lost in the storage experiment with a slight influence from the gas atmosphere and the catalyst type. The comparison with transmission electron micrographs shows that the size distributions have a similar shape and width but differ in absolute sizes.