Interpreting Ionic Conductivity for Polymer Electrolyte Fuel Cell Catalyst Layers with Electrochemical Impedance Spectroscopy and Transmission Line Modeling

Abstract

An optimal cathode catalyst layer with uniform ionomer distribution is critical to reduce the platinum loading and increase the utilization in polymer electrolyte fuel cells (PEFCs). Having well distributed ionomer with low ionomer to carbon (I/C) ratio is desirable, as it will also minimize local oxygen transport resistance. In this work, we used electrochemical impedance spectroscopy (EIS) method and transmission line model (TLM) to measure and evaluate ionic conductivity of pseudo catalyst layers (PCLs) comprised of Vulcan XC72 carbon black and 3M 825 EW ionomer with I/C ratios of 0.3, 0.6, 1 and 1.4 at a relative humidity (RH) range of 50 to 120 %. These results were compared with our previous hydrogen pump (HP) measurements, where PCL was sandwiched between the two membranes and protons had to transport through the entire PCL to be counted towards current. EIS effective ionic conductivity results reported here are higher than the results of HP because in the HP set-up ionic pathways must percolate all the way through the PCL to be effectively counted, whereas in EIS measurement, ionomer segments that are in contact with the membrane but not percolating all the way through the PCL can be detected. Additionally, three effects were studied: 1) hot press, 2) platinum content and carbon black support, and 3) ionomer content or lack of ionomer. Ionic conductivity increased significantly by hot pressing PCL onto the membrane, however, it was independent with respect to the hot press conditions, such as pressure and temperature. Hot pressing with low I/C ratio had more impact than high I/C ratio because at high I/C ratio even without hot press there is a sufficient amount of contact points between catalyst layer and membrane. At high RHs, catalyst layer containing Pt has higher ionic conductivity compared to PCL because of Pt particles being hydrophilic, which helps to distribute the ionomer more uniformly. To investigate the effect of condensed water, PTFE dispersion was used as a binder for Pt/C layer instead of ionomer, resulting ionic conductivity decreased with increasing applied potential at 100% and 120% RH conditions. At lower RHs, ionic conductivity is independent of the applied potentials.