Towards Ultrahigh Osmotic Energy Harvesting By Covalent-Organic Framework Based Ionic Diode Membranes

The chemical energy stored between seawater and river water, the so-called osmotic energy (or blue energy), can be harvested by an ion-selective membrane. However, previously reported membranes suffer from insufficient ion selectivity and inferior transmembrane ionic flux (low conductance), thus impeding practical application. For example, the output osmotic power density reported by most of existing ion selective membranes is typically below the commercial benchmark (5 W/m2). Taking the inspiration from electrocytes in electric eel, which consist of a large number of sub-nanoscale rectified ion channels that allow unidirectional ion transport with amplified flux, we engineered a sub-2 nm-scale covalent-organic framework (COF)-based ionic diode membrane (IDM) for highly efficient osmotic energy harvesting (Fig. 1). We show that the proposed IDM can rectify ionic current even in high salt concentration, which was supported by our simulations based on the Poisson-Nernst-Planck model. We also probe application of this membrane in harvesting energy from salinity gradients. Notably, in addition to ultrahigh ion selectivity, these sub-2 nm-scale ionic diode membranes can achieve an unprecedented power density, higher than the commercial benchmark bandgap. Impressively, the exploited IDM can produce an ultrahigh power density as high as 27.8 W/m2 by mixing artificial salt-lake water and river water, outperforming all the state-of-the-art ion selective membranes under the same testing condition. Our work would open up new avenues of using pinhole-free COF membranes towards next-generation highly selective and ultrahigh-performance sustainable energy harvesting.