Efficient Osmotic Energy Conversion Enabled by Self-Standing COF Membranes With Varied Sulfonic Acid Group Density

The Gibbs free energy generated from the mixing of seawater and freshwater across a salinity gradient is considered one of the most significant yet underutilized renewable energy sources. Membrane-based reverse electrodialysis (RED) enables direct electricity generation from osmotic energy by harnessing the net ion flux driven by concentration gradients across ion-selective membranes. However, entropy generation caused by non-selective ion mixing significantly limits the power density of RED systems. Therefore, enhancing membrane ion selectivity is critical. 2D covalent organic frameworks (COFs) demonstrate remarkable potential for osmotic energy conversion due to their aligned 1D nanochannel, high porosity, and organized ionic groups. Herein, we present a strategy leveraging electrostatic repulsion to controllably fabricate TpPa-(SO₃H)_X COF (X = 0.5, 1, 1.5, 2) membranes with varied ionic group density. Via stoichiometric modulation during COF synthesis, we achieved variation in sulfonic acid group density within nanochannels, enabling optimized charge-governed ion selectivity. Under salinity gradients mimicking seawater/freshwater conditions (0.5 m/0.01 m, NaCl), the device delivered an exceptional power output density of 24.53 W m^-2, representing a 4.9-fold enhancement over commercial benchmarks (5 W m^-2). This study presents a novel method and strategy for the design and application of ion-selective membranes in mass transport and efficient energy conversion.