Tuning Ionic Transport in 2D Ionic COFs through Ultrasound-Driven Morphology Engineering

Abstract

Controlling ionic transport in crystalline covalent organic frameworks (COFs) is hindered by long diffusion paths and tortuous pore geometries, limiting their efficacy in ion separations such as sulfate removal from water. Herein, we introduce an ultrasound-assisted synthesis of guanidinium-functionalized ionic COFs, yielding uniform nanospheres (15-120 nm) embedded in larger aggregates with radially accessible pores, in contrast to the fibrous morphologies obtained via conventional hydrothermal methods. This non-equilibrium sonochemical approach, conducted under ambient conditions, affords frameworks with comparable crystallinity and porosity but superior morphological control. The nanospherical COFs exhibit enhanced sulfate adsorption performance, achieving a capacity of 102 mg g⁻¹, rapid equilibration within 20 min (~2.2× faster initial rates than hydrothermal analogs), and a sulfate-chloride selectivity coefficient of 3.6 in mixed solutions. Mechanistic investigations, supported by ball-milling controls, density functional theory calculations (revealing a ~5.5-fold stronger binding energy for SO₄²⁻ over Cl⁻), and comparative isotherms, demonstrate that the compact morphology minimizes interlayer dislocations and shortens diffusion pathways, enabling monotonic uptake and homogeneous pore accessibility for hydrated multivalent ions. This work establishes a green, energy-efficient strategy for morphologyengineered ionic COFs, providing a generalizable framework to overcome diffusion bottlenecks in 2D porous materials. The insights pave the way for advanced applications in water purification, ion-selective membranes, and electrochemical systems where structure-dynamics coupling is critical.