Morphology of Hydrated As-Cast Nafion Revealed through Cryo Electron Tomography

Nafion is an ion-containing random copolymer used as a solid electrolyte in many electrochemical applications thanks to its remarkable ionic conductivity and mechanical stability. Understanding the mechanism of ion transport in Nafion, which depends strongly on hydration, therefore requires a complete picture of its morphology in dry and hydrated form. Here we report on a nanoscale study of dry versus hydrated as-cast 100 nm Nafion membranes using analytical transmission electron microscopy (TEM) and cryogenic TEM tomography, respectively. For the dry membrane, spherical clusters similar to 3.5 nm in diameter corresponding to the hydrophilic sulfonic-acid-containing phase are identified. In contrast, cryo TEM tomography of the hydrated membrane reveals an interconnected channel-type network, with a domain spacing of similar to 5 nm, and presents the first nanoscale 3D views of the internal structure of hydrated Nafion obtained by a direct-imaging approach.