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Phosphotungstic acid (HPW)-filled composite proton exchange membranes possess high proton conductivity under low relative humidity (RH). However, the leaching of HPW limits their wide application. Herein, we propose a novel approach for anchoring water soluble phosphotungstic acid (HPW) by polydopamine (PDA) coated graphene oxide and halloysite nanotubes (DGO and DHNTs) in order to construct hybrid three-dimensional proton transport networks in a sulfonated poly(ether ether ketone) (SPEEK) membrane. The introduction of PDA on the surfaces of the hybrid fillers could provide hydroxyl groups and secondary amine groups to anchor HPW, resulting in the uniform dispersion of HPW in the SPEEK matrix. The SPEEK/DGO/DHNTs/HPW (90/5/5/60) composite membrane exhibited higher water uptake and much better conductivity than the SPEEK membrane at low relative humidity. The best conductivity reached wass 0.062 S cm−1 for the composite membrane, which is quite stable during the water immersion test.

In this work, sulfonated poly(ether ether ketone) (SPEEK) nanocomposite membranes were prepared with high loading of phosphotungstic acid (HPW) immobilized by polydopamine coated halloysite nanotubes (DHNTs). XPS and FTIR analysis revealed the formation of acid-base interactions between HPW and DHNTs in the nanocomposite membranes, indicating the successful immobilization of HPW on the surface of high aspect-ratio DHNTs, which helped shorten the distance of proton hopping and create more continuous proton transport pathways along the high aspect-ratio nanotubes. The activation energy for the proton transfer in the nanocomposite membranes was greatly reduced and membrane proton conductivity was significantly enhanced. Compared with the SPEEK control membrane, the nanocomposite membrane with 42.9 wt% HPW loading achieved 114% increase in proton conductivity, up from 0.048 S cm−1 to 0.117 S cm−1, and the membrane water uptake remained almost the same. Under the long term water immersion test, the SPEEK/DHNTs/HPW nanocomposite membrane exhibited much slower decrease in proton conductivity than the SPEEK/HPW membrane without DHNTs, indicating the strong HPW-binding effect from DHNTs on slowing down the leakage of HPW.