Expanded polytetrafluoroethylene functionalized with free radical scavengers and hydrophilic groups for superior chemical stability of proton exchange membranes

Abstract

Introduced free radical scavengers and decreased gas crossover have a significant impact on a proton exchange membrane's (PEM's) chemical stability and performance. A novel PEM containing expanded polytetrafluoroethylene (ePTFE) functionalized with free radical scavengers and hydrophilic groups was designed and fabricated to enhance the PEM's electrochemical performance and chemical stability. ePTFE with unsaturated bonds was prepared by a chemical modification reaction using benzoin and potassium tert-butoxide; hydrogen bromide acted as a halogenating agent, and 3-mercapto-1,2,4-triazole worked as a free radical scavenger grafted onto the ePTFE molecular chains. The ePTFE was functionalized by hydrophilic groups, and radical scavenger groups led to perfluorosulfonic acid (PFSA) ionomers that had better compatibility with the modified ePTFE. The prepared membrane's gas permeability, proton conductivity, and electrochemical performance were thereby significantly improved. In addition, the generated free radicals were quickly eliminated by the free radical scavenger groups, thus enhancing the PEM's chemical stability. The microstructure, morphology, and performance of the PEM at various degradation levels were systematically evaluated. This work reveals that expanded polytetrafluoroethylene functionalized with free radical scavengers and hydrophilic groups not only reduced the hydrogen peroxide and active free radicals generated but also effectively eliminated the free radicals, thereby mitigating chemical degradation of the PEM. This paper provides a promising way to improve the proton conductivity and chemical stability of PEMs, offering excellent application potential for boosting fuel cell lifetime and performance.