Molecular modelling approach on comparison of Pt-based PEMFC cathode-catalysts

Molecular modelling is a novel approach in the development of new catalyst materials. Method was used to investigate Pt-based binary and ternary alloy catalysts utilized in PEM (Proton Exchange Membrane) fuel cells. The focus was on the development more effective cathode materials, the functionality of which are understood also on the molecular level. Further, the aim in the development of new catalysts was to minimize the amount of Pt needed and to reach high specific activity. In order to compare the laboratory-made, commercial and new non-synthesized Pt-based catalysts, the optimized crystal and surface structures of catalysts were defined using the molecular modelling techniques. The Pt structure was modified by Cr, Co and Ir with an atomic ratio of 3:1, 2:1:1 and 1:1. The modelling results were used in order to choose catalysts for experimental analyses. The catalysts were prepared on carbon carrier by reduction of metal salts. The catalyst content in the materials was 20 wt.%. In order to obtain required crystal structure, the catalysts were heat-treated at temperatures 300 - 1000 °C. Transmission electron micrographs show that the average particle size increases as a function of heat treatment temperature. The stability of the alloy catalysts was tested by electrochemical (voltammetric) measurements by determining the border potential where the modifier metals are starting to desorb from the surface of the metal alloy. The experimental surface analyses are consistent with the modelling results. After the determination and comparison of structural and electrostatic properties of catalysts, the oxygen reduction mechanism was calculated on the Pt and Pt2CoCr (001) surfaces. Based on the calculations, the reaction occurs by a four-electron pathway, and the limiting factor of the reaction is the dissociation of hydrogen superoxide (HO2-) intermediate. The dissociation rate of hydrogen superoxide can be supposed to have an effect on the durability of membrane electrode assemblies (MEA). The research indicated that the oxygen reduction reaction can be controlled by optimizing the atomic structure of Pt-based catalysts