Nanoporous Perfluoroalkyl and Sulfonic Acid-Functionalized Covalent Organic Frameworks with Enhanced Proton Conduction for Proton-Exchange-Membrane Fuel Cells

Abstract

The proton-conducting materials serve as the key components of proton-exchange membrane fuel cells, significantly influencing their overall practical performance. Nafion is the predominant commercial proton-conducting material, known for its superior proton conductivity. However, its practical use is limited by high costs, disordered nanoscale pores, and restricted thermal stability. In this contribution, we developed a bottom-up self-assembly strategy to construct perfluoroalkyl- and sulfonic acid-functionalized covalent organic frameworks (COFs) using hydrazine ligation. The effects of fluorine chains of varying lengths on hydrophobicity and proton conductivity were systematically investigated. COF-Fx-SO₃ exhibits a Nafion-like structure, enabling proton conduction through its nanoscale ordered channels. At a 97% relative humidity (RH) and 90 °C, the highest intrinsic proton conductivity is 7.7 × 10^–3 S cm^–1, which significantly surpasses that of nonfluorinated COFs. Moreover, the anhydrous proton conductivity of fluorinated COFs at 140 °C reaches 1.93 × 10^–2 S cm^–1 after doping with phosphoric acid, which is 1 orders of magnitude higher than that of nonfluorinated COFs. This work illustrates the practical feasibility of the nanoconfined effect in enhancing proton conduction and highlights the potential of Nafion-like COFs as proton-conducting materials.