Origins of Water State and Ionic Cluster Morphology for High Proton Conductivity of Short Side-Chain Perfluorinated Sulfonic Acid Membranes

The property of perfluorinated sulfonic acid (PFSA) membranes depends not only on the ion exchange capacity (IEC), but also on the chemical structure of the functional side-chain and the phase-separation morphology. Two PFSA membranes, the long side-chain (LSC) and the short side-chain (SSC), have been investigated to study the structure–property relationship, covering the ionic domain structure and the proton transport. The proton conductivity of the SSC PFSA membrane is 143 and 209 mS/cm at 30 °C and 80 °C in water, which is 30–40% higher than that of the LSC PFSA membrane (103 and 161 mS/cm). The bound-to-free water ratio in the hydrated membranes was analyzed by differential scanning calorimetry and Raman spectroscopy, which show that a higher ratio accounts for the improved proton conductivity of the SSC PFSA membrane. The chain mobility was analyzed by solid-state nuclear magnetic resonance, which reveals that the side chain of the SSC membrane more readily self-assembles. This result was verified by the morphology from transmission electron microscopy. The small-angle X-ray scattering results show that the SSC PFSA membrane exhibits smaller domain spacing between the ionic clusters in dehydrated membranes. These observations, a larger ionic cluster and smaller domain spacing in the dehydrated SSC membrane, indicate a reduced size of the hydrophobic assembly feature domains, and the ionic channel connectivity is better in the SSC, which can be another key issue for its improved proton conductivity, in addition to the higher IEC and higher proton mobility.