(560cs) Effect of Ionomer Coverage on Pt Electrocatalyst Performance in PEFCs

Authors: 
Van Cleve, T., University of Michigan
Mooney, M., Ohio State University
Wang, G., National Renewable Energy Laboratory
Kabir, S., National Renewable Energy Laboratory
Neyerlin, K. C., National Renewable Energy Laboratory
Polymer electrolyte fuel cells (PEFCs) are an attractive technology for mobile and stationary electric power generation that holds several benefits over conventional internal combustion engines, which are less efficient and emit harmful exhaust. Despite their significant promise, commercially available fuel cell vehicles currently utilize high platinum metal loadings to overcome cell voltage losses. Commonly, perfluorosulfonic acid ionomers such as Nafion are incorporated in the catalyst layer to improve ionic (protonic) conductivity and Pt site utilization thereby improving ORR performance in both RDE and MEA studies. However, high ionomer loadings can be detrimental to performance since the strong interaction between sulfonate (-SO3-) functional groups and Pt nanoparticles has been linked lower catalytic rates and higher O2 transport resistances.

It is essential to understand the local distribution of ionomer near catalytic sites in order to accurately assess whether improvements result from increased accessibility or higher intrinsic activity on catalytic sites. Current ex situ characterization techniques (USAXS, STEM/EDS, and Nano-CT) cannot provide necessary resolution to determine ionomer coverage on Pt sites so it is difficult to determine how different ink formulations (catalyst, solvent, and ionomer) impact cell performance.

Here, the development and utilization of in situ diagnostics designed to probe the local ionomer interactions will be utilized in conjunction with established testing protocols to relate ionomer coverage (on Pt and/or support) with kinetic performance and O2 transport resistance. Specifically, CO displacement chronoamperometry and electrochemical impedance spectroscopy techniques will be described to determine local ionomer coverages across a series of Pt/C MEAs. These results will promote a deeper understanding of ionomer-catalyst interactions which in turn will inform future materials development and integration necessary to achieve high performance PGM (e.g. Pt/C and Pt alloy/C) electrocatalyst-based electrodes.