Manipulating Hofmeister Effect for Fabricating Freestanding Ionic Covalent Organic Framework Membranes

Interfacial assembly of ionic covalent organic framework (iCOF) membranes remains a significant challenge. In this study, we report the fabrication of freestanding iCOF membranes by manipulating Hofmeister effect during interfacial polymerization (IP) process. Different kinds of inorganic salts are introduced into the aqueous phase to regulate the dissolution and crystallization dynamics of TpPa-SO₃H. We discover that Li₂SO₄ provides the optimal ion-specific regulation by combining a strongly hydrated anion that effectively dehydrates the growing iCOF and a cation that interacts only weakly with the sulfonate framework, enabling TpPa-SO₃H to enrich and stay at the interface. This ion-specific synergy facilitates the subsequent π-π stacking and ordered assembly into highly crystalline, intact freestanding membranes. The resulting membrane demonstrates a high proton conductivity of 334.7 mS cm^-1 at 100°C and 98% RH.