Symmetric/asymmetric hourglass nanochannel in covalent organic framework for osmotic power generation

Abstract

A two-dimensional covalent organic framework (COF) is a suitable material for reverse electrodialysis membrane in osmotic energy harvesting due to its consistent pore size, well-aligned structure, and chemical stability, which facilitates effective ion transport. To enhance membrane efficiency, several studies have been conducted to induce ion rectification by fabricating membranes using two types of COFs with different pore sizes or charge characteristics. In this study, we produced an hourglass-type COF structure and further incorporated polyelectrolytes into the membrane to improve ion transport efficiency. Alkaline treatment on both sides of membrane was carried out to partially break imine bonds constituting the COF, eliminating hydrogen bonds in COF. This process increases hydrophilicity and pore size, thereby improving ion conductivity of the symmetric hourglass membrane. Additionally, polyelectrolytes with a strong negative charge and high hydrophilicity were introduced onto one side of the COF, which converts it into an asymmetric hourglass-type membrane. It is demonstrated that unique symmetric/asymmetric structures govern the direction of ion diffusion and enhance the membrane performance. This study explains the effective ion transport in an hourglass COF membrane, controlling the channel symmetry. Furthermore, it offers a novel method of fabricating a polymer-incorporated composite membrane based on COF and elucidates the role of polymers in the membrane.