Uncontrolled mixing remains the primary hurdle impeding the practical application of reverse electrodialysis (RED) to harvest Gibbs free energy in the form of salinity gradients. Improving the permselectivity of membranes is therefore essential, with ionic density being one of the most critical factors. Herein, we systematically investigate how the charge population in nanofluidic membranes affects the ionic charge separation and consequently the accompanying power density. To establish this relationship, we decouple the effect of the ionic density from the impact of pore structure using a multivariate strategy to construct covalent-organic-framework-based membranes, in which the content of ionic sites can be precisely manipulated from 0 to 0.18 C m<sup>-2</sup>, a range that has rarely been experimentally explored. Beyond the region reported (0.002-0.06 C m<sup>-2</sup>), wherein increasing pore surface charge density of the membrane enhances permselectivity and leads to a greater osmotic voltage, a sharp volcano-like curve is observed. The optimal membrane affords record-high power outputs among membrane systems, one order of magnitude higher than the value set for commercialization. Our study provides insights into the impact of ionic density of the membrane on osmotic energy harvesting that can guide RED stack design to advance sustainable energy generation from natural salinity gradients.

Manipulating charge density in nanofluidic membranes for optimal osmotic energy production density

ACS Spring 2022

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