Fast proton-selective transport through covalent organic frameworks in aqueous phase

Covalent organic frameworks (COFs) are expected to be promising candidates as the building block for proton-exchange membranes (PEMs) due to their ordered well-defined nanopore structures with high density and tunable sizes. However, the mechanism of proton-selective transport through COF membranes is still unraveled. We herein investigate the proton-selective transport behaviors through four types of COFs in the aqueous phase. Proton can transport through COF membranes whose effective pore size >8 Å with low free energy barriers of <10 kJ mol−1. Furthermore, molecular analyses manifest that nucleophilic sites on the pore-wall can provide additional pathways for proton and promote the vehicular and Grotthuss transports via increasing the quantities and durability of in-pore hydrogen bonds. This dramatically reduces the energy barrier from ∼42 to ∼12 kJ mol−1 for proton transport especially under confinement (effective pore size <4 Å). Meanwhile, all COF membranes show relatively high methanol penetration barriers. Therefore, COFs with nucleophilic sites or relatively large pores can provide both high proton conductivity (16–60 S m−1) and selectivity (>1 × 104). The revealed mechanisms in this work are expected to not only increase our understandings of proton transport, but also help to design next-generation PEMs with exceptional permselectivity.