2,6-Azulene-based Homopolymers: Design, Synthesis, and Application in Proton Exchange Membrane Fuel Cells

Azulene-based homopolymers are of great interest from the point view of chemistry and material science. Herein, by means of Friedel–Crafts acylation to introduce solubilizing chains on the 1-position of azulene, we designed and synthesized two examples of 2,6-azulene-based homopolymers RP­(Az-AC16) and P­(Az-AC16). The arrangement of 2,6-azulene units is irregular for RP­(Az-AC16), while P­(Az-AC16) has head-to-head/tail-to-tail arranged 2,6-azulene units. Proton-responsive studies demonstrate that RP­(Az-AC16) and P­(Az-AC16) show reversible proton responsiveness in both solution and thin film. To utilize the dynamically reversible proton-responsive property of these polymers in thin films, RP­(Az-AC16) and P­(Az-AC16) were incorporated into a Nafion matrix as proton exchange membranes, wherein the Nafion/P­(Az-AC16) composite membrane exhibits significant increases in proton conductivity relative to the Nafion membrane at different temperatures of each relative humidity (RH), which further results in a 64% improvement in hydrogen fuel cell output power under 30% RH at 80 °C. Our studies have realized the first solution synthesis of 2,6-azulene-based homopolymers and the first application of azulene-based π-systems in hydrogen fuel cells.