Proton transport of porous triazole-grafted polysulfone membranes for high temperature polymer electrolyte membrane fuel cell

Abstract

Introduction of porous structure to high temperature polymer electrolyte membranes is one of effective pathways to increase their proton conductivity under elevated temperature. However, the effect of the porous structure on the proton diffusion mechanism of these membranes is still unclear. In this work, the proton transport behaviour of a series of porous triazole-polysulfone (PSf) membranes under elevated temperature is comprehensively investigated. The functional triazole ring in the framework of porous triazole-PSf acts a proton acceptor to form acid-base pair with phosphoric acid (PA). In addition, the proton diffusion coefficient and proton conductivity of PA-doped porous triazole-PSf is an order of magnitude higher than that of the PA-doped dense triazole-PSf membrane. Percolation theory calculation convinces that the high proton conductivity of PA-doped porous triazole-PSf is due to the formation of continuous long-range proton diffusion channels under high pore connectivity and porosity. On the contrary, excessive pore connectivity also results in high gas permeability, leading to decrease of the open circuit voltage and cell performance of the single cell. Consequently, the optimum porosity for the PA-doped porous triazole-PSf membrane is 75% for fuel cell operating with the maximum peak power density of 550 mW·cm−2 and great durability for 120 h under 140 °C.