Phosphoric acid-doped Gemini quaternary ammonium-grafted SPEEK membranes with superhigh proton conductivity and mechanical strength for direct methanol fuel cells

Abstract

Proton exchange membranes (PEMs) with excellent proton conductivity, high mechanical strength, and low methanol permeability are essential for their application in direct methanol fuel cells (DMFCs). Herein, flexible alkyl side chains containing Gemini quaternary ammonium (QA) ions were grafted onto sulfonated poly(ether ether ketone) (SPEEK) to obtain novel amphiphilic polymers (S-DMEAx-BPT), and then doped with phosphoric acid (PA) to prepare S-DMEAx-BPT/PA membranes. PA immobilized on flexible side chains can possess more mobility and higher proton conduction ability when compared with PA bound to rigid main chains, thus contributing to the extremely high proton conductivity of S-DMEAx-BPT/PA (295.4 mS cm-1, 80 °C, 100% R.H) even at low PA doping levels. Moreover, with the help of abundant ionic crosslinking, the membranes had excellent dry and wet mechanical properties. Benefiting from the negligible PA leaching, these membranes could be used as the electrolyte in DMFCs and output the peak power density of 110.7 mW cm-2, which was 2.1 times that of Nafion 212 (52.8 mW cm-2). Moreover, the open-circuit voltages (OCV) retention remained at 82.4% after 140 h. Therefore, the strategy combining Gemini QA cations grafting, flexible side chains, and low-level PA doping provides a new methodology for fabricating high-performance PEMs for DMFCs.