Photo-controllable Ion-Gated Metal-Organic Framework MIL-53 Sub-nanochannels for Efficient Osmotic Energy Generation.

Abstract

By closing and opening ion channels, electric eels are able to convert ion concentration gradients into electricity. Inspired by electric eels, considerable artificial sub-nanoscale ion channels with high ion selectivity and transportation efficiency have been designed for harvesting the osmotic energy between ionic solutions of different salinities, but constructing smart ion-gated sub-nanochannels for effective ion transport is still a huge challenge. Herein, photo-controllable sub-nanochannels of metal-organic framework (MOF) NH₂-MIL-53 encapsulated with spiropyrans (SP-MIL-53) were fabricated by a facile in situ growth strategy. Interestingly, the highly ordered sub-nanochannels of SP-MIL-53 were switched on and off to efficiently regulate the ion flux by the light-driven isomerization of SP, which made it a smart ionic gate with a high on-off ratio of 16.2 in 10 mM KCl aqueous solution via UV irradiation. Moreover, the ion-gated sub-nanochannel membrane yielded a high power density of 8.3 W m^-2 under a 50-fold KCl concentration gradient in the open state. Density functional theory calculations revealed that K⁺ ions in SP-MIL-53 sub-nanochannels had a higher mobility constant (3.61 × 10^-2) with UV irradiation than without UV illumination (2.33 × 10^-22). This work provides an effective way to develop smart ion-gating sub-nanochannels for capturing salinity gradient power.