Incorporating self-anchored phosphotungstic acid@triazole-functionalized covalent organic framework into sulfonated poly(ether ether ketone) for enhanced proton conductivity

Abstract The design of new types of proton-conductive materials with high proton conductivity has sparked tremendous interest. Covalent organic framework (COF), a new category of porous crystalline materials possessing well-defined porosity and tunable functionality, holds great potential as next-generation proton conductors. Here, triazole COF loaded with phosphotungstic acid (HPW@COF) has been incorporated into sulfonated poly(ether ether ketone) (SPEEK) matrix to fabricate SPEEK/HPW@COF composite membranes. The one-dimensional channels of COF lined with protonated triazole (triazolium cations) by hydroscopic HPW and neutral triazole as well as counter anions (PO40W123−) could act as proton-donors and -acceptors forming interconnected hydrogen-bonding networks, thus greatly facilitating proton conduction via Grotthuss-type mechanism particularly under low RH. Accordingly, SPEEK/HPW@COF displayed 35.5 times higher proton conductivity than the plain SPEEK at 65 °C, 40% RH. Moreover, the proton conductivity of SPEEK/HPW@COF remained constant during soaking in water for 15 days, indicating the excellent immobilization of HPW due to the electrostatic interaction between basic nitrogen sites and HPW as well as the confinement effect of microporous COF channels. These results demonstrated that N-heterocycles functionalized COFs gave great promise as effective hosts to encapsulate proton carriers.