Nanoscale morphological investigation into surface coverage of platinum catalysts by ionomers on vertically aligned carbon nanotube fuel cell catalyst layers

Abstract

The nanoscale morphology and configuration of carbon nanotube supports and ionomers are investigated statistically, with the aim of improving catalyst utilization for advanced vertically aligned carbon nanotube (VACNT)-based fuel cell catalyst layers. A series of three-dimensional catalyst layers of resolution 4 nm are generated by a comprehensive stochastic modeling approach known as the quasi-random nano-structural modeling method, to reflect the anisotropy and irreproducibility of the catalyst layer structures. The statistical investigation reveals that the morphology and configuration of CNTs and the consequential formation of ionomer layers on the surface of CNTs significantly affect the surface coverage by ionomers and the utilization of Pt catalysts, implying the existence of the optimal weight ratio of ionomer to carbon. The formation of ion transport paths through ionomers is likely to improve the catalyst utilization of the VACNT catalyst layers. However, excessive formation of ionomer layers leads to poor catalyst utilization by reducing the amount of gas-exposed Pt catalysts that can be conveniently accessed by reactants. Furthermore, an increment in the CNT diameter can affect the catalyst utilization directly. This may result in undesirable excessive formation of ionomer layers and consequential underutilization of Pt catalysts.