Dipole Polarization-Induced High Proton Conduction and Aligned Molecular Structures of Nafion Blend Membranes with PVDF and PVDF-Trfe for Enhanced Pemfcs
Authors: Lee Iksu , Kim Seungbin , Lee Dongjun , Kim Daeun , Bae Insung 
Published: December 2024

Proton exchange membrane fuel cells (PEMFCs) have emerged as promising energy generation solutions in various applications, owing to their eco-friendly operation. Perfluorosulfonated polymers like Nafion and Aquivion are preferred choices for PEMs due to their high proton conductivity and excellent stability. However, the bicontinuous structure of Nafion membranes limits proton conductivity, necessitating strategies for enhancement.

Methods such as mechanical stretching and magnetic field alignment have been explored to align PEM nanostructures. Mechanically stretched Nafion membranes exhibit increased proton conduction due to enhanced alignment of molecular structures.1-4 Additionally, capillary force lithography has shown promise in hierarchically aligning Nafion molecules, improving power generation and proton conduction at the membrane-electrode interface.5

To further improve ionic conduction, ferroelectric polymers like poly(vinylidene fluoride) (PVDF) have been investigated. PVDF's molecular arrangement influences its polymorphic crystalline structures, with the β-phase exhibiting superior dipole polarization. Moreover, its copolymer, poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE), exhibits the β-phase crystalline structure with high dipole polarization.6 While PVDF has been utilized in PEMs for fuel cells, the focus has been primarily on mechanical properties rather than proton conductivity enhancement.

In this presentation, Nafion blend membranes incorporated with PVDF and PVDF-TrFE exhibit nanostructure alignment achieved through mechanical uniaxial stretching and melt-recrystallization process. This process distinctly forms phase-separated morphology in Nafion molecules, creating enhanced connectivity in hydrophilic proton channels and thus boosting proton conductivity. Moreover, the stretching induces a successful transformation of PVDF molecules from the α to β phase, aligning fluorine and hydrogen atoms in a trans (TTTT) structure. The synergistic effect of the aligned nanostructure and dipole polarization of β-phase PVDF and PVDF-TrFE significantly enhances proton conduction in the membranes for proton exchange membrane fuel cells (PEMFCs). Atomic force microscopy and small-angle X-ray scattering confirm the controlled structures of the blend membranes. Additionally, the improved proton conductivity enhances proton conduction at the membrane-catalyst layer interface in the membrane-electrode assembly, ultimately elevating the power generation of PEMFCs.

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