Effect of Morphology of Nanoscale Hydrated Channels on Proton Conductivity in Block Copolymer Electrolyte Membranes

Hydrated membranes with cocontinuous hydrophilic and hydrophobic phases are needed to transport protons in hydrogen fuel cells. Herein we study the water uptake and proton conductivity of a model fuel cell membrane comprising a triblock copolymer, polystyrenesulfonate-block-polyethylene-block-polystyrenesulfonate (S-SES), as a function of water activity in both humid air and liquid water. We demonstrate that the water uptake and proton conductivity of S-SES membranes equilibrated in liquid water are fundamentally different from values obtained when they were equilibrated in humid air. The morphological underpinnings of our observations were determined by synchrotron small-angle X-ray scattering and cryogenic scanning transmission electron microscopy. A discontinuous increase in conductivity when nearly saturated humid air is replaced with liquid water coincides with the emergence of heterogeneity in the hydrated channels: a water-rich layer is sandwiched between two polymer-rich brushes. While the possibility of obtaining heterogeneous hydrated channels in polymer electrolyte membranes has been discussed extensively, to our knowledge, this is the first time that direct evidence for the formation of water-rich subdomains is presented.