Proton transfer efficiency enhanced over wide relative humidity: Etidronic acid-modified polyphosphazenes cross-linked polybenzimidazoles membranes

Abstract

To enhance the proton conductivity and oxidative stability of the proton exchange membrane, an effective strategy to construct robust high-temperature PEMs (HT-PEMs) is proposed. The polyoxy(etidronic acid)phazenes cross-linked m-polybenzimidazole membranes (POHP-mPBI) are prepared by solution casting, thermal-crosslinking and acidification. Polyoxy(etidronic acid sodium)phazenes (Na–POHP) is cross-linkable proton conductors by partially phosphonic acid functionalized polyphosphazenes, which ensures proton transport while retaining the cross-linked groups. After covalent cross-linking between Na–POHP and mPBI, the cross-linked macromolecular structure stably locks the proton conductor in the polymer matrix, which ensures the construction of the proton transfer pathway at wide relative humidity (RH). The molecular dynamics simulation was conducted to verify the hydrogen bonds network with the POHP-mPBI membrane. The POHP-mPBI cross-linked membranes exhibit good oxidative stability, mechanical strength, proton conductivity and low fuel transmembrane permeability. At 180 °C and different RH (100% RH, 50% RH, 30% RH and 0 RH), the proton conductivity of POHP(50)-mPBI membrane is 0.150, 0.076, 0.055 and 0.048 S cm−1, respectively. After the water washing for 96 h, the durability of proton conductivity and weight loss shows a rare decline. The incorporation of proton conductors (POHP) into the PBI matrix by cross-linking is a perspective method to enhance the comprehensive performance of HT-PEM.