The proton-exchange membrane (PEM) is a central, and often performance-limiting, component of all-solid H2/O2 fuel cells. Nafion (R) , the most widely used PEM, consists of a perfluorinated polymer that combines a hydrophobic Teflon-like backbone with hydrophilic ionic side groups. It stands out among polymer materials for its high, selective permeability to water and small cations.

The long elusive nanometer-scale structure that underlies many of these outstanding properties of Nafion has now been conclusively determined by a quantitative analysis of small-angle scattering data, using a novel approach based on numerical Fourier transformation. Nafion is riddled with ~2.5-nm diameter water channels lined with hydrophilic side groups. In this nearly one-dimensional confinement, proton conductivity by a relay (Grotthuss) mechanism can proceed even faster than vehicular transport of other ions, as observed experimentally. The channels, which are locally parallel to their neighbors and can be considered as cylindrical inverted micelles, are stabilized by the rigid polymer backbones proven by nuclear magnetic resonance (NMR). The chains are anchored in small crystallites, which thus act as physical crosslinks. This definitive structural model finally provides a valid target for the design of other, cheaper ionic polymers that could replace Nafion.