Surface-Mediated Construction of Ultrathin Free-standing Covalent Organic Framework Membrane for Efficient Proton Conduction.

As a new class of crystalline porous organic materials, covalent organic frameworks (COFs) have attracted considerable attention in proton conduction due to their regular channels and tailored functionality. However, most COFs are insoluble and unprocessable, which makes the membrane preparation for practical usage a challenge. Herein, we employed surface-initiated condensation polymerization of trialdehyde and phenylenediamine in the synthesis of sulfonic COF (SCOF) coatings. The COF layer thickness can be finely tuned from ten to one hundred nanometers by controlling the polymerization time. Moreover, free-standing COF membranes were facilely obtained by sacrificing the bridging layer without any decomposition of the COF structure. Benefiting from the abundant sulfonic acid groups in the COF channels, the proton conductivity of SCOF membrane reached to 0.54 S cm -1 at 80 °C in pure water. To our knowledge, this is one of the highest values for pristine COF membrane in the absence of additional additives. Our approach would pave the way for the potential application of COF materials in proton conducting membranes.