A simple new route to covalent organic/inorganic hybrid proton exchange polymeric membranes

The development of proton exchange membranes as electrolytes for polymer electrolyte fuel cells operating at intermediate temperatures has been achieved combining two approaches: the preparation of a cross-linked polymer and the formation of covalent organic/inorganic hybrids. A covalently cross-linked sulfonated polyetheretherketone with elevated degree of sulfonation (DS = 0.8) was prepared by reaction with HSO₃Cl (SOPEEK). No degradation of PEEK was detected upon sulfonation, but only cross-linking via sulfonic groups with formation of SO₂ moieties was observed. The extent of ramification was calculated by analyzing the ¹H nuclear magnetic resonance (NMR) spectra of the products and resulted in 20% of the total amount of -SO₃H groups present in the polymer. The solubility of the -SO₂Cl precursor in organic solvents allowed easy carrying out of functionalization reactions in homogeneous conditions by lithiation and subsequent reaction with SiCl₄, thus introducing covalenly linked silicon moieties (SOSiPEEK) at the ratios 1:0.25 and 1:0.50 per monomeric unit. The products were characterized by ¹H and ¹³C NMR, attenuated total reflectance Fourier transform infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry.