Perfluorinated Ionomers for Alkaline Membrane Fuel Cells

Perfluorinated ionomers have significant advantages over hydrocarbon-based ionomers due to the higher oxygen permeability, hydrophobicity, less catalyst poisoning, ideal three-phase interface and ability to create a porous electrode structure. There have been a few attempts to prepare perfluorinated polymers for the use of alkaline applications mostly in the late 1980s; however, none have been stable and all rapidly hydrolyzed under high pH conditions.²

We have synthesized stable hydroxide conducting perfluorinated ionomers through resonance structure formation upon functionalization of perfluorinated polymer.³ The synthesis of phenyl pentamethyl guanidinium functionalized perfluorinated ionomer was accomplished with a three-step procedure, i) attachment of a 4-fluoroaniline spacer to carboxylic acid Nafion precursor ii) functionalization of tetramethyl gunanidinium⁴ and iii) methylation with dimethyl sulfate. The alkaline membrane fuel cells (AMFCs) using the perfluorinated copolymer in the catalyst layers exhibited the peak power density of 577 mW/cm², which was more than 2x greater than a state-of-the-art polyaromatic polymer electrolyte⁵ (335 mW/cm²). The performance decay rate of the perfluorinated polymer cell was 225 mV/dec/h at 80°C, comparable to the state-of-the-art polyaromatic alkaline polymer cell (183 mV/dec/h at 60°C).

The interfacial dynamics of the perfluorinated ionomers using Pt disk microelectrodes indicate that the perfluorinated ionomers have two major benefits over the conventional hydrocarbon-based alkaline ionomers: i) cation desorption capability at an intermediate electrode potential range, ca. 0.2 to 0.6 V and ii) much faster oxygen permeability.⁶

The oxygen reduction reaction (ORR) of the platinum microelectrodes between liquid and polymer electrolytes are compared using 0.1 M guanidinium aqueous solution and the guanidinium functionalized perfluorinated ionomer. The ORR behavior of platinum micro-electrodes with the guanidinium solution exhibited an “electrostatic-defying” adsorption which limited the access of O₂reactants into the Pt surface while the ORR behavior of platinum with guanidinium functionalized ionomer did not show the cation adsorption probably due to the conformation change of the cation by the polymer-cation linkage. Due to the absence of guanidinium adsorption at high potential, the ORR activity of platinum at the ionomer interface substantially improved. 

Acknowledgment

U.S. Department of Energy Fuel Cell Technologies Program (Technology Development Manager: Dr. Nancy Garland) funded this research. We thank the Korea Institute of Energy Research for supporting the Visiting Scholar Program.

References

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