The Structural Aspects of PFSA Ionomers As Determined By STEM and Simulations

The need for next generation polymer electrolyte membrane fuel cells (PEMFCs) has driven considerable research effort into the development of novel membrane materials that are highly proton conducting at low temperatures. At present, perfluorosulfonic acid (PFSA) ionomers are the electrolyte of choice in current PEMFCs. PFSA ionomers including the benchmark Nafion™, consist of a hydrophobic poly-tetrafluoroethylene (PTFE) backbone with pendant perfluorinated vinyl ether side chains each terminated with a sulfonic acid group (i.e., –SO₃H). The chemistry and molecular structure of the side chains and backbone in PFSAs play a crucial role in determining the hydrated morphology and transport properties especially in the presence of water. Under hydrated conditions, the mesoscale phase separation occurs due to the hydrophobic and hydrophilic segments of the macromolecules. This is mainly due to the aggregation of ionic clusters and the reorganization of the PTFE backbones, which have been investigated by a variety of experimental and modeling approaches.

We have recently examined the morphology of dry and hydrated perfluorosulfonic acid (PFSA) ionomers at cryo and room temperature using TEM/STEM with EELS capability. Z-contrast imaging was utilized to identify the micro-phase separation of the hydrophilic side chains containing water and the hydrophobic polytetrafluoroethylene (PTFE) backbones. The results compare very favorably with hydrated morphologies obtained through mesoscale dissipative particle dynamics (DPD) simulations. The cryo-STEM images of plunge-frozen samples was also found to agree with morphologies based on SAXS experiments. Chain conformations of the perfluorosulfonic acid (PFSA) ionomers: Nafion™ and Aquivion^® were investigated with electron energy-loss spectroscopy (EELS) on a 200 kV transmission electron microscope (TEM) equipped with a monochromator. The results were compared with polytetrafluoroethylene (PTFE) to evaluate the effect of the pendant perfluoroether side chains of the ionomers on the structure of the PTFE backbone. Several unique spectroscopic features corresponding to conformational changes were identified in the low-loss region and the fine structures of the carbon K-edge. Results obtained from high-level density function theory (DFT) based electronic structure calculations confirm the conformational dependence of the EEL spectra of the PFSA ionomers. Comparison with the spectra obtained from the experiments revealed the correlation between the specific side chain chemistry and backbone conformation. This spectroscopic information will allow us to further explore the morphological properties of these materials when combining with additional imaging techniques.