N-substitute polyphosphazenes cross-linked hydroxyl-terminated SPEEK high temperature proton exchange membrane to achieve high proton conductivity over low relative humidity

Abstract

Improving the proton conductivity of high temperature proton exchange membrane (HTPEM) over low-medium relative humidity (RH) still remains a challenge. The poly(triazoles-phosphazenes) (PTPP) containing triazoles can function as basic sites and cross-linker. The hydroxyl-terminated SPEEK (SOPEEK) contains abundant –SO3H to be applied as proton conductor and bears cross-linking sites with PTPP. The PTPP-SOPEEK composite membranes are obtained by covalent cross-linking between PTPP and SOPEEK, which is expected to realize effective proton conduction. The prepared PTPP-SOPEEK membranes show good physical and chemical stability, dimensional stability, high proton conductivity over low-to-medium RH, and low fuel permeation. At 140 °C and 100% RH, the proton conductivity of PTPP(15)-SOPEEK can reach to 0.223 S cm−1, while at 180 °C, the proton conductivity is 0.086 S cm−1 at 50% RH, 0.062 at 30% RH, and 0.045 S cm−1 at 0% RH, respectively. After washing with water for 96 h, the conductivity durability of PTPP(15)-SOPEEK shows almost no decay. The gas (H2 & O2) and liquid (methanol) fuel permeability coefficient of PTPP(15)-SOPEEK membranes is 1–2 orders of magnitude lower than the Nafion membrane. Therefore, the PTPP-SOPEEK membranes can act as promising alternatives for the application in HTPEM.