Preparation and investigation on the low temperature proton exchange membranes with the enhanced proton conductivity at subzero temperature

This research focused on the preparation and investigation on the low temperature proton exchange membranes (LTPEMs) based on the polymers of sulfonated poly(ether ether ketone) (SPEEK) and poly(vinyl alcohol) (PVA). The compact structure of SPEEK/PVA was constructed owing to the intermolecular hydrogen bonds between sulfonated groups of SPEEK and hydroxyl groups of PVA. The target of the enhanced proton conductivity at subzero temperature was achieved while the phosphoric acid (PA) molecules were doped in SPEEK/PVA-based membranes through the intermolecular hydrogen bonds. The formed SPEEK/PVA/PA membranes possessed the high and stable proton conductivity at subzero temperature. Specifically, SPEEK(94%)/PVA(60%/40%)/PA membranes exhibited the proton conductivities of 8.28 × 10−2 S/cm at 30 °C and 3.86 × 10−2 S/cm at −30 °C. The membrane could show the values of 1.31 × 10−1 S/cm at 30 °C and 5.30 × 10−2 S/cm at −30 °C even it underwent the seven temperature cycle test processes of −30 °C–30 °C. Besides that, the proton conductivity retained 3.33 × 10−2 S/cm at −30 °C while the experiment lasted 980 h. Furthermore, the proton conduction mechanisms during the heating/cooling process have been discussed and the results were indicative of PA and water molecules participating into the proton conduction processes. The prepared SPEEK/PVA/PA membranes were expected to cater the requirements on structure stability, proton conduction and mechanical strength as LTPEMs, which could be applied in proton exchange membranes fuel cell (PEMFC) at subzero temperature.