Molecular-Scale Asymmetry Nanochannels for High-Efficiency Osmotic Energy Generation

Osmotic power, as an example of iontronics, can convert an ion gradient to electrical energy by the membrane-based reverse electrodialysis technique. However, its efficiency in harvesting osmotic energy is mostly dependent on ion permeability and selectivity during transmembrane diffusion. The two-dimensional (2D) heterogeneous interface establishes molecular-scale asymmetric structure and charge that is expected to exert a crucial effect on the ion permeability and selectivity but remains unexplored. Here, we designed a 2D nanofluidic membrane with molecular-level asymmetric channels that can achieve high cation selectivity while maintaining outstanding ion conductivity. When applied to osmotic energy generators, this membrane can exhibit a high cation selectivity coefficient of 0.985 and a superior energy conversion efficiency of up to 47.1%, coupled with an excellent output power density of over 20 W m-2 in mixing the artificial seawater and river water. The Na+ ions transport through a 2D heterostructured membrane via an interface-induced contiguous ion adsorption-diffusion mechanism is uncovered. The asymmetric pore structure and negative charge distribution enable highly selective adsorption of Na+ ions and subsequently fast transport in the molecular-scale asymmetric nanochannels. This work provides an in-depth understanding of ion transport in asymmetric nanochannels and further inspires their applications in other advanced energy-harvesting devices.