Structure-Property Relationships of PFSA-Based Ionomer Thin Films

Abstract

In energy conversion devices (e.g. fuel cells), the ionomer layer within the heterogeneous catalyst layers (CLs) exhibits increased mass transport resistance, hindering device performance. This increased resistance is attributed to confinement effects of the ionomer to nanoscale thicknesses and greater influence of the ionomer-catalyst interactions under confinement. Chemical modifications to the ionomer side chains have the potential to improve transport and/or alter ionomer-catalyst interactions to overcome the performance limitations. To understand the impact of side chain and end group modifications, we investigate the structure-property relationships of perfluoro sulfonic acid (PFSA)-based ionomers thin films (20 to 100 nm) synthesized by 3M. Thin film water uptake and conductivity are determined via spectroscopic ellipsometry and electrochemical impedance spectroscopy (EIS), respectively, under controlled humidity and correlated with structural changes during hydration characterized via in situ grazing-incidence x-ray scattering (GIXS). The collected data are analyzed to draw structure-hydration-conductivity maps for various film chemistries and used to identify the chemical and structural factors controlling ionomer functionality. The results provide insight into the improvements and limitations on performance attainable by these PFSA-based chemistries. Furthermore, the impact of side chain chemistry on structure-property relationships can be used to guide ionomer design for improved device performance.