Nanomechanical Study on Perfluorinated Electrolyte Thin Films for Fuel Cells
Authors: Watabe Hiroyuki , Nguyen Hung Quoc , Liang Xiaobin , Nakajima Ken 
Published: September 2023

In polymer electrolyte fuel cells (PEFC), perfluorinated polymers are mainly used as the electrolyte from the viewpoints of proton conductivity and chemical durability. The roles are divided into the electrolyte membrane, which prevents the mixing of hydrogen and oxygen, and the coating ionomer, which forms the reaction field by transferring reaction gas and protons to the catalyst in the catalyst layer. Electrolyte polymers used as coating ionomers are considered to function in a thin film state of about 10 nm. Recent intensive researches have revealed that thin films exhibit different physical properties from the bulk state due to interactions with catalysts and carbon supports. In the field of structural analysis, studies using cryo-TEM and grazing incidence SAXS (GISAXS) have revealed that the structure of hydrophilic domains shows anisotropy and a less well-defined phase separation due to confinement effects, and that molecular orientation is dependent on the hydrophilicity and hydrophobicity of the substrate material. As for functions that directly contribute to power generation properties of PEFC, measurements using quartz crystal microbalance (QNM), current sensing atomic force microscopy (cAFM), and specially designed electrodes have shown that thinner films reduce water content, which in turn reduces the transport properties such as proton transport and oxygen permeability of films. On the other hand, the coating ionomer also has an aspect as a constituent material that determines the structure of the catalyst layer. To control the strength and pore structure of the catalyst layer, it is also important to understand the mechanical properties such as elastic modulus and surface adsorption properties in the thin film state. In this regard, several studies have shown that the surface becomes superhydrophilic and the α transition temperature increases in thin films, and surface morphological changes such as roughness have also been observed by AFM. However, these studies have yet to quantify the mechanical properties of nanostructures and discuss their in-plane distribution.

In this study, elastic modulus mapping of thin films was performed using the peak force quantitative nano mechanics (QNM) method, and recently developed bimodal AFM method, namely, amplitude- and frequency-modulated (AM-FM) AFM. The AM-FM method applies high-frequency resonance vibration to the cantilever in addition to the common tapping mode measurement, which allows the elastic modulus of the sample to be measured with high sensitivity. The electrolyte polymer samples were a long side-chain type perfluorosulfonic acid polymer prepared by AGC Inc. Thin films were prepared by spin-coating dilute dispersions onto substrates. The dispersion was made with water and ethanol as solvents. The distribution of elastic modulus, adhesion and energy dissipation were measured at the nanoscale. In particular, the adhesion and energy dissipation images show microstructures of a few nm in size. It was found that the change in surface structure was not uniform with respect to the change in film thickness, and that roughness, elastic modulus, and adhesion force increased in the region around 10 nm in thickness. In nanometer-order thin films, the adsorption state between the polymer molecules and the substrate and the polymer particle structure in the solution are thought to affect the film structure. It can be presumed that the coexistence of areas of large adsorption with the substrate and areas of weak adsorption results in a large roughness. It can be considered that evidence of a film thickness-dependent state transition has been obtained.