In-Situ Electrochemical Techniques to Determine Ionomer Coverage in PEFC Electrodes

Abstract

Many electrochemical devices such as low temperature fuels utilize ionomers to improve efficiency and overall performance. In polymer electrolyte fuel cells (PEFCs), perflurosulfonic acid ionomers improve ionic (protonic) conductivity and Pt utilization within the catalyst layer. However, the strong interaction between sulfonate (-SO3 -) functional groups and Pt nanoparticles have been linked lower catalytic rates and higher O2 transport resistances.1,2 There have been numerous strategies implemented to control the local distribution of ionomer and Pt/C catalyst particles, but limitations in the resolution of state-of-the-art characterization techniques make it difficult to assess how ink formulations and/or electrode fabrication techniques control ionomer-catalyst interactions and its impact device performance.3–5 Herein, the development and utilization of in situ electrochemical techniques designed to probe the local ionomer interactions will be utilized in conjunction with established testing protocols to relate ionomer coverage (on Pt and/or C support) with kinetic performance and O2 transport resistance of fully-conditioned PEFC electrodes. 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. Schuler, T. et al. Fuel-Cell Catalyst-Layer Resistance via Hydrogen Limiting-Current Measurements. J. Electrochem. Soc. 166, F3020–F3031 (2019).Subbaraman, R., Strmcnik, D., Paulikas, A. P., Stamenkovic, V. R. & Markovic, N. M. Oxygen Reduction Reaction at Three-Phase Interfaces. ChemPhysChem 11, 2825–2833 (2010).Berlinger, S. A., McCloskey, B. D. & Weber, A. Z. Inherent Acidity of Perfluorosulfonic Acid Ionomer Dispersions and Implications for Ink Aggregation. J. Phys. Chem. B 122, 7790–7796 (2018).Orfanidi, A., Rheinländer, P. J., Schulte, N. & Gasteiger, H. A. Ink Solvent Dependence of the Ionomer Distribution in the Catalyst Layer of a PEMFC. J. Electrochem. Soc. (2018). doi:10.1149/2.1251814jesKhandavalli, S. et al. Rheological Investigation on the Microstructure of Fuel Cell Catalyst Inks. ACS Appl. Mater. Interfaces (2018). doi:10.1021/acsami.8b15039