Nafion’s Nanoscale Morphology Revealed

For decades, DuPont’s perfluorosulfonic acid polymer Nafion has been a standard material used to make membranes in fuel cells, electrolyzers, and other applications—it has impressive ionic conductivity and mechanical and chemical stability. Yet 50 years after first using Nafion, researchers continue to puzzle over its ion-transport mechanism. Even the microscopic morphology of the material, which likely holds the key to understanding that mechanism, remains controversial. Frances I. Allen and coworkers of the University of California, Berkeley, and Lawrence Berkeley National Laboratory may have laid that controversy to rest. The team imaged Nafion for the first time in the hydrated state—which is required for high ionic conductivity—via cryo electron tomography, a technique that provides a three-dimensional view of internal microstructures (ACS Macro Lett. 2014, DOI: 10.1021/mz500606h). By comparing results from dry and frozen hydrated samples, the team found that in the dry state Nafion’s hydrophilic sulfonic acid phase forms spherical clusters 3.5 nm in diameter. Hydrating the polymer transforms the isolated clusters to an interconnected 3-D nanoscale channel network that propagates throughout the membrane.