Enhancing Ionic Selectivity and Osmotic Energy by Using an Ultrathin Zr-MOF-Based Heterogeneous Membrane with Trilayered Continuous Porous Structure.

Designing a nanofluidic membrane with high selectivity and fast ion transport property is the key towards high-performance osmotic energy conversion. However, most of reported membranes can produce power density less than commercial benchmark (5 W/m 2 ), due to the imbalance between ion selectivity and permeability. Here, we report a novel nanoarchitectured design of a heterogeneous membrane with an ultrathin and dense zirconium-based UiO-66-NH 2 metal-organic framework (MOF) layer and a highly aligned and interconnected branched alumina nanochannel membrane. The design leads to a continuous trilayered pore structure of large geometry gradient in the sequence from angstrom-scale to nano-scale to sub-microscale, which enables the enhanced directional ion transport, and the angstrom-sized (~6.6-7 Å) UiO-66-NH 2 windows render the membrane with high ion selectivity. Consequently, the novel heterogeneous membrane can achieve a high-performance power of ~8 W/m 2 by mixing synthetic seawater and river water. The power density can be largely upgraded to an ultrahigh ~17.1 W/m 2 along with ~48.5 % conversion efficiency at a 50-fold KCl gradient. This work not only presents a new membrane design approach but also showcases the great potential of employing the zirconium-based MOF channels as ion-channel-mimetic membranes for highly efficient blue energy harvesting.