Novel proton exchange membranes based on structure-optimized poly(ether ether ketone ketone)s and nanocrystalline cellulose

摘要

Graphical abstractSchematic diagram of proton transport mechanism in composites.Display OmittedHighlights•Two sulfonated fluorenyl-containing poly(ether ether ketone ketone)s (SFPEEKKs) were synthesized.•Nanocrystalline cellulose was inserted into SFPEEKKs to obtain nanocomposite proton exchange membranes.•Nanocomposite membranes presented excellent properties.AbstractTwo sulfonated fluorenyl-containing poly(ether ether ketone ketone)s (SFPEEKKs) were synthesized as the matrix of composite proton exchange membranes by directly sulfonating copolymer precursors comprising non-sulfonatable fluorinated segments and sulfonatable fluorenyl-containing segments. Surface-modified nanocrystalline cellulose (NCC) was produced as the “performance-enhancing” filler by treating the microcrystalline cellulose with acid. Two families of SFPEEKK/NCC nanocomposite membranes with various NCC contents were prepared via a solution-casting procedure. Results revealed that the insertion of NCC at a suitable ratio could greatly enhance the proton conductivity of the pristine membranes. For example, the proton conductivity of SFPEEKK-60/NCC-4 (SFPEEKK with 60% fluorenyl segments in the repeating unit, and inserted with 4% NCC) composite membrane was as high as 0.245Scm−1at 90°C, which was 61.2% higher than that of the corresponding pure SFPEEKK-60 membrane. This effect could be attributed to the formation of hydrogen bond networks and proton conduction paths through the interaction between −SO3H/–OH groups on the surface of NCC particles and −SO3H groups on the SFPEEKK backbones. Furthermore, the chemically modified NCC filler and the optimized chemical structure of the SFPEEKK matrix also provided good dimensional stability and mechanical properties of the obtained nanocomposites. In conclusion, these novel nanocomposites can be promising proton exchange membranes for fuel cells at moderate temperatures.