Ion-Conductive Properties of Polymeric Ionic Liquid-Coated Core-Shell Nanoparticles for Anhydrous Proton Exchange Membrane

Polymer electrolyte fuel cells (PEFC) have attracted attention as next-generation energy sources for vehicles and distributed power sources. The operating environment of PEFC is expected to expand from the current low-temperature range (&lt; 90^oC and high-humidified environment) to the high-temperature range (&gt; 100^oC and low-humidified environment). Especially, the high-temperature range operating PEFC are classified as HT-PEFC. However, conventional perfluorosulfonic acid electrolytes, such as Nafion, are unsuitable for HT-PEFC because the proton-conduction mechanism is dependent on hydrogen bonding between water molecules. Thus, there is an urgent need to develop proton exchange membranes suitable for HT-PEFC that conduct protons in high-temperature and low-humidity environments. In a previous study, it was discovered that high proton conductivity can be achieved by constructing nanosized proton-conductive channels and using ionic liquids as proton-conducting materials. However, a facile method for fabricating proton exchange membranes with no anisotropy in the direction of proton conduction has not yet been developed.<br/>In this paper, we have proposed a procedure for constructing isotropic three-dimensional (3D) controlled ion-conductive channels employing polymeric ionic liquid (PIL)-coated core-shell nanoparticles. A PIL is an ion-conducting material containing an ionic liquid with a repeating polymer structure. In this system, we employed SiO₂ (silica) nanoparticles as the core nanoparticles and poly(1-vinylimidazole)/bis(trifluoromethanesulfonyl)imide (P1VIm/TFSI) as the PIL. The PIL-coated nanoparticles were then assembled, resulting in the 3D ion-conductive channels have been constructed by the contact of the PIL layers on the surface of the nanoparticles between adjacent particles. The PIL-coated nanoparticles were prepared using a unique precipitation polymerization technique applying precipitation polymerization.<br/>The progress of the PIL coating onto the surface of the silica nanoparticles was determined using SEM-EDS mapping analysis. We have observed sulfur and fluorine elements originating from TFSI anion from the same position as silicon elements derived from silica nanoparticles, indicating that the PIL was successfully coated onto the surface of silica nanoparticles. The ionic conductivity of the PIL-coated nanoparticles was measured in a pelletized state prepared by simply pressing the nanoparticles. The pelletized PIL-coated nanoparticles exhibited ionic conductivity even under high-temperature and anhydrous conditions, with a maximum conductivity of approximately 10^-4 S cm^-1 (160^oC). Subsequently, to evaluate the construction of 3D ion-conductive channels, we have measured the anisotropy of the ionic conductivity of the pellets. Notably, there was no anisotropy in the ionic conductivity, and both vertical and horizontal directions achieved similar ionic conductivities. Furthermore, the ion-conduction mechanism was clarified by using variable-temperature ¹H solid-state MAS NMR spectroscopy. The imidazole N-H resonances in the PIL structure were observed at 9–12 ppm assigned to “mobile” protons which is contributing to the proton conduction. As this phenomenon was not observed with the PIL homopolymer, we have concluded that the coating of PIL onto the surface of the silica nanoparticles and the accumulation of PIL-coated nanoparticles increased the number of mobile protons.<br/>In conclusion, we have achieved the facile construction of 3D ion-conductive channels by the accumulation of PIL-coated core-shell nanoparticles. Moreover, the ion-conduction mechanism was discussed using solid-state NMR spectroscopy, and the fabricated PIL-coated nanoparticles contained mobile protons. Thus, the PIL-coated core-shell nanoparticles and their assembly process are effective for developing a proton exchange membrane for HT-PEFC.