Facile one-step fabrication of sulfonated polyhedral oligomeric silsesquioxane cross-linked poly(ether ether ketone) for proton exchange membranes

Abstract

A series of sulfonated polyhedral oligomeric silsesquioxane (POSS) cross-linked poly(ether ether ketone) hybrid membranes (C-PEEK-sPOSS) were facilely synthesized, for which the processes of sulfonation and cross-linking were achieved in one-step. The cross-linked hybrid C-PEEK-sPOSS membranes were fabricated using poly(ether ether ketone) with phenoxyl side groups (PEEK-OH), epoxy-POSS and 4-phenolsulfonic acid sodium via a nucleophilic addition reaction. Because the modified cross-linked structure contained a controllable degree of sulfonation and functionalized POSS, a good micro-phase separation for enhanced proton transport was observed by transmission electron microscopy (TEM). Furthermore, the cross-linked hybrid membranes were evaluated by the water uptake, proton conductivity, thermal properties, and so on. For example, C-PEEK-sPOSS-3 membranes possessed prominent properties, such as adequate thermal stability, superior mechanical properties and excellent oxidation resistance. Compared with Nafion 117, these membranes also exhibited higher proton conductivity (0.198 S cm⁻¹vs. 0.160 S cm⁻¹), greater selectivity (67.03 × 10⁴ Ss cm⁻³vs. 4.19 × 10⁴ Ss cm⁻³), lower methanol permeability (1.82 × 10⁻⁷ cm² s⁻¹vs. 18.6 × 10⁻⁷ cm² s⁻¹), and low swelling ratio at 80 °C under fully hydrated conditions (28.4%). The single cell test exhibited superior current and power densities of 1.7 A cm⁻² and 706 mW cm⁻² at 80 °C, respectively.