Microstructure, state of water and proton conductivity of sulfonated poly(ether ether ketone)

•52% sulfonated PEEK have been characterized electrochemically and structurally.•Proton conductivity has been correlated to membrane morphology and state of water.•High degree of nano phase separation gives improved proton conductivity.•Freezable water molecules are responsible for high conductivity.•Proton conductivity of 0.1 S/cm was reached at water content as high as 69%.•Ionic domains in s–PEEK are smaller than the ionic domains in the Nafion.

A detailed study of the microstructure, the transport properties and the state of water for fluorine-free sulfonated poly(ether ether ketone) (s-PEEK) membranes with a degree of sulfonation of about 50% is reported as a function of the membrane water content. SAXS curves show how 50% s-PEEK membranes at low and intermediate water contents are characterized by a poor hydrophobic/hydrophilic phase-separation and narrow ionic domains (< 2 nm). For membrane water contents of up to 40%, all the water molecules contained in the membranes appear to be strongly bounded to the sulfonic groups and water molecules are unable to freeze. When the membranes are treated in water at temperatures higher than 80 °C, membrane swelling starts and for water contents above 40–50% a better ionic microstructuring inside the membranes occurs. In highly swollen membranes part of the water molecules is in a less associated freezable bounded state. The presence of more mobile water molecules is responsible for the increase in the membrane proton conductivity and conductivity of the order of 0.1 S/cm has been recorded for s-PEEK with 69% of water content.