Balance of the Proton Conductivity and Dimensional Stability of Sulfonated Poly(ether ether ketone) Membranes through Ionic–Covalent Cross-Linking

To enhance proton transfer within the proton exchange membrane, a combination of ionic and covalent cross-linking strategies was utilized to fabricate sulfonated poly(ether ether ketone) (SPEEK) composite membranes. Side chains of SPEEK were grafted, and then, the ionic cross-linking intermediates were mixed with ionic liquid (IL) [AMIM][Cl] and graphene oxide (GO). Covalent cross-linking was subsequently achieved through the Menshutkin reaction. By adjusting the proportions of IL and GO, SPEEK ionic–covalent cross-linking composite membranes (C-SPEEK/IL/GO) with enhanced performance were synthesized. The establishment of the ionic cross-linking network and the presence of covalent cross-linking within the composite membranes were confirmed using Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS), respectively. The intricate microscopic network structure within the C-SPEEK/IL/GO facilitates rapid proton transport. Consequently, the proton conductivity of C-SPEEK/IL/GO-1% attained a remarkable 47.43 mS·cm^–1 at 120 °C. The ionic–covalent cross-linking network within the C-SPEEK/IL/GO combined membrane endows it with a dense architecture, which constricts the hydrophilic channels, thereby enhancing the membrane’s dimensional stability. Additionally, the thermal stability of the C-SPEEK/IL/GO composite membrane has been significantly enhanced compared to that of the pristine SPEEK.