A Critical Revision of the Nano-Morphology of Proton Conducting Ionomers and Polyelectrolytes for Fuel Cell Applications

A few aspects of the nano-morphology of hydrated Nafion and other ionomers and polyelectrolytes in their acid form are revisited by examining the evolution of small angle X-ray scattering (SAXS) data which are recorded for a wide range of water volume fractions (phi(water) approximate to 7-56 vol%). A consistency check with the recent parallel cylinder model discloses that this is most likely biased by a large uncertainty of the experimentally determined water content. We rather find our data to be consistent with locally flat and narrow (around 1 nm) water domains. The formation of relatively thin water films is suggested to be a common feature of many ionomers and polyelectrolytes, and the underlying driving force is most likely electrostatics within these highly dissociated systems. The water films may act as a charged (e.g., with positive protonic charge carriers) glue, keeping together the oppositely charged polymer structures. While this interaction tends to produce flat morphologies, the formation process is suggested to be constraint by limited conformational degrees of freedom of the corresponding polymer and the interactions between polymer backbones. This may leave severe tortuosities on larger scales which depend on the sample history (including swelling, de-swelling, aging, stretching, and pressing).