High performance nanocomposite proton exchange membranes based on the nanohybrids formed by chemically bonding phosphotungstic acid with covalent organic frameworks

Abstract

The performance deterioration of the proton exchange membranes (PEMs) under low humidity is one of the key issues hindering the further development of PEM fuel cells (PEMFCs). Covalent organic frameworks (COFs) offer the possibility of facilitating proton transport due to their structural advantages such as ordered pore networks and tunable functionalities. In this work, we have applied a hydrothermal method to synthesize the phosphotungstic acid (HPW) functionalized covalent organic framework (COF) nanohybrids (P–COF), in which the HPW is immobilized within the cavities of COF through the chemical bonds. The strong acidity of HPW and the ordered structure of P–COF facilitates the fast and efficient proton conduction in the Nafion/P–COF nanocomposite membranes, resulting in superior proton conductivity along with outstanding performance in H2/O2 PEMFCs under low humidity. At 25 °C under 55% RH, the proton conductivity of Nafion nanocomposite membrane with 4 wt% P–COF (Nafion/P–COF-4) reaches a proton conductivity of 0.061 S cm−1, which is 307% higher than that of the recast Nafion. The membrane electrode assembly (MEA) based on Nafion/P–COF-4 membrane exhibits excellent performance in a H2/O2 fuel cell, achieving the peak power density of 724 mW cm−2 at 80 °C and 50% RH, 138% higher than the MEA based on the recast Nafion.