High O₂ Permeability Ionomers for Improved Fuel Cell Performance

In a proton exchange membrane (PEM) fuel cell, the local oxygen transport across the ionomer film in the catalyst layer has a significant impact on electrode performance especially at high current density.1 It is therefore crucial to use ionomers that have higher oxygen permeability than the baseline Nafion. In this work, novel ionomers with increased oxygen permeability have been synthesized by copolymerization of perfluoro-2,2-dimethyl-1,3-dioxole (PDD) with perfluoro(4-methyl-3,6-dioxaoct-7-ene) sulfonyl fluoride (PFSVE) and a ter-monomer. PDD is the main source of higher permeability due to its bulky structure, PFSVE provides ionic conductivity and the amount of the ter-monomer is adjusted to achieve high yields without compromising the equivalent weight (EW). The ring structure of PDD creates additional open space within the ionomer structure for improved gas permeability. Some of the newly developed ionomers have up to five times higher permeability than Nafion, which should result in a significant improvement of fuel cell performance, mainly at high current densities. Ionomers with different PDD content and equivalent weight have been studied to establish the correlation between ionomer properties and MEA performance. Local oxygen resistance, ionomer sheet resistance, ionomer coverage, and SO3 - group coverage will be evaluated and correlated to electrode performance. The performance and durability of the electrodes using high permeability ionomer will be correlated to the ionomer interaction with catalyst particle. This work will provide a comprehensive understanding of interactions among Pt, carbon, ionomer and their impact on the electrode structure and fuel cell performance and durability. The attained information will be used to improve fuel cell electrode design. Acknowledgement: The project is financially supported by the Department of Energy’s Fuel Cell Technology Office under the Grant DE-SC0018597. References: 1. Baker et al, J. Electrochem. Soc. 156, B991 (2014).