Polyoxometalate-Based Ionic-Liquid-Functionalized Sulfonated Poly(ether ether ketone) Composite Membranes with High Proton Conductivity over a Wide Operating Temperature Range

The design and preparation of proton exchange membranes (PEMs) with wide working temperature ranges play a crucial role in expanding their application, but most of the reported PEMs are difficult to meet the demand for operating at subzero temperatures. Herein, a series of composite membranes have been successfully fabricated by functionalizing a sulfonated poly(ether ether ketone) (SPEEK) membrane with phosphotungstic acid-ionic liquid (HPW-IL) and mesoporous graphite phase carbon nitride (mpg-C3N4). Notably, the optimal composite membrane SPEEK/HPW-IL/mpg-C3N4-1.0 exhibits attractive antifreeze property (<−75 °C) and great proton conductivity ranging from −40 °C (7.2 × 10–4 S cm–1, ambient humidity) to 85 °C (2.2 × 10–1 S cm–1, 95% RH) attributing to the formation of continuous proton transport channels induced by the double acid–base pairs (one is HPW and IL, the other is SPEEK and mpg-C3N4) and abundant hydrogen-bonding interactions among various components. Meanwhile, its antiswelling property has been greatly improved compared to the SPEEK membrane, thanks to the incorporation of mpg-C3N4. Given its excellent proton conductivity, a full-cell performance test was performed, achieving a maximum output power density of up to 230.3 mW cm–2, an open-circuit voltage of up to 0.85 V, and a long-term stability for 25 h at 80 °C, 100% RH. This work provides broad vision into the design of PEMs with remarkable performances that can be used in a wide temperature range.