Synthesis and investigation of sulfonated poly(p-phenylene)-based ionomers with precisely controlled ion exchange capacity for use as polymer electrolyte membranes

Abstract

To achieve precise control of sulfonated polymer structures, a series of poly(p-phenylene)-based ionomers with well-controlled ion exchange capacities (IECs) were synthesised via a three-step technique: (1) preceding sulfonation of the monomer with a protecting group, (2) nickel(0) catalysed coupling polymerisation, and (3) cleavage of the protecting group of the polymers. 2,2-Dimethylpropyl-4-[4-(2,5-dichlorobenzoyl)phenoxy]benzenesulfonate (NS-DPBP) was synthesised as the preceding sulfonated monomer by treatment with chlorosulfuric acid and neopentyl alcohol. NS-DPBP was readily soluble in various organic solvents and stable during the nickel(0) catalysed coupling reaction. Sulfonated poly(4-phenoxybenzoyl-1,4-phenylene) (S-PPBP) homopolymer and seven types of random copolymers (S-PPBP-co-PPBP) with different IECs were obtained by varying the stoichiometry of NS-DPBP. The IECs and weight average molecular weights (M_ws) of ionomers were in the range of 0.41–2.84 meq. g⁻¹ and 143 000–465 000 g mol⁻¹, respectively. The water uptake, proton conductivities, and water diffusion properties of ionomers exhibited a strong IEC dependence. Upon increasing the IEC of S-PPBP-co-PPBPs from 0.86 to 2.40 meq. g⁻¹, the conductivities increased from 6.9 × 10⁻⁶ S cm⁻¹ to 1.8 × 10⁻¹ S cm⁻¹ at 90% RH. S-PPBP and S-PPBP-co-PPBP (4 : 1) with IEC values >2.40 meq. g⁻¹ exhibited fast water diffusion (1.6 × 10⁻¹¹ to 8.0 × 10⁻¹⁰ m² s⁻¹), and were comparable to commercial perfluorosulfuric acid polymers. When fully hydrated, the maximum power density and the limiting current density of membrane electrode assemblies (MEAs) prepared with S-PPBP-co-PPBP (4 : 1) were 712 mW cm⁻² and 1840 mA cm⁻², respectively.