Highly selective and high-performance osmotic power generators in subnanochannel membranes enabled by metal-organic frameworks

Abstract

The electric organs of electric eels are able to convert ionic gradients into high-efficiency electricity because their electrocytes contain numerous "subnanoscale" protein ion channels that can achieve highly selective and ultrafast ion transport. Despite increasing awareness of blue energy production through nanochannel membranes, achieving high-performance energy output remains considerably unexplored. Here, we report on a heterogeneous subnanochannel membrane, consisting of a continuous UiO-66-NH₂ metal-organic framework (MOF) and a highly ordered alumina nanochannel membrane. In the positively charged membrane, the angstrom-scale windows function as ionic filters for screening anions with different hydrated sizes. Driven by osmosis, the subnanochannel membrane can produce an exceptionally high Br^-/NO₃ ^- selectivity of ~1240, hence yielding an unprecedented power of up to 26.8 W/m² under a 100-fold KBr gradient. Achieving ultrahigh selective and ultrafast osmotic transport in ion channel-mimetic MOF-based membranes opens previously unexplored avenues toward advanced separation technologies and energy-harvesting devices.

Fig. 1. Schematic depiction of the electric eel–inspired heterogeneous membrane, UiO-66-NH₂@ANM, with subnanoscale channels.

The electric organ of electric eels has densely packed array of highly ion selective cell membranes known as electrocytes. An ionic concentration difference between the cell membranes can be converted into electricity by controlling the ionic fluxes with numerous asymmetric subnanoscale protein ion channels. Inspired by this, the continuous and pinhole-free UiO-66-NH₂ membrane with numerous ordered subnanochannels was fabricated onto the alumina nanochannel membrane (ANM) support, named as UiO-66-NH₂@ANM.

Fig. 2. Preparation and characterization of UiO-66-NH₂@ANM.

(A) Schematic of the fabrication process of UiO-66-NH₂@ANM. (i) The residual aluminum layer on the anodized ANM was removed with CuCl₂ and HCl mixed solution. (ii) The barrier layer of ANM was etched by 5 wt % H₃PO₄. (iii) NH₂-functionalized ANM was obtained by surface modification of 3-aminopropyltriethoxysilane (APTES). (iv) The continuous UiO-66-NH₂ membrane was grown onto ANM by solvothermal reaction. (B) Top view and (C) cross-sectional view SEM images of UiO-66-NH₂@AMM fabricated, indicating that a continuous and pinhole-free UiO-66-NH₂ layer with thickness of ~750 nm was densely grown on the top of the ANM support. Inset in (B) represents the amplified SEM image. (D) Contact angle measurements of the UiO-66-NH₂ and ANM sides of the membrane. (E) Nominal pore size distributions of UiO-66-NH₂ calculated based on the N₂ adsorption/desorption isotherms by using the nonlocal density functional theory model.

Fig. 3. Surface charge–governed ion transport in UiO-66-NH₂@ANM.

(A) Schematic of the (i) heterogeneous UiO-66-NH₂@ANM. (ii) Illustrated lattice structure of the UiO-66-NH₂ membrane, which has (iii) ordered window size of 6 to 7 Å. (B) Dynamic current measurements of UiO-66-NH₂@ANM recorded in 0.01 M KCl solution with an external bias alternating between +1 and −1 V. (C) Transmembrane ionic conductance of UiO-66-NH₂@ANM as a function of the KCl concentration varying from 10⁻⁶ to 3 M. It indicates that the membrane conductance starts to deviate from the bulk value (gray line) at a high concentration of 1 M (corresponding to a Debye length of ~0.3 nm; inset), showing the apparent surface charge–governed ion transport phenomenon even in high saline condition owing to the subnanoscale channels of UiO-66-NH₂.

Fig. 4. Osmotic energy conversion of UiO-66-NH₂@ANM.

(A) Schematic of our osmotic energy-harvesting device under a salinity gradient. (B) I-V curves of UiO-66-NH₂@ANM recorded under two opposite configurations of 1000-fold KCl gradient, where the redox potential contribution has been subtracted. The internal resistance (R_m) decreases by ~9.3% when the UiO-66-NH₂ layer faced a concentrated solution. (C) Diffusion potential (V_diff) and diffusion current (I_diff) as a function of concentration gradient. The lower concentration in contact with the ANM side was fixed at 1 mM. (D) Current density (open symbols) and power density (solid symbols) harvested under various KCl concentration gradients. The maximum output power densities achieved were ~2.19, 4.93, and 7.12 W/m² under 5-, 50-, and 500-fold concentration gradients, respectively.

Fig. 5. Highly selective and high-performance osmotic power of UiO-66-NH₂@ANM.

The effect of anion salt types on the (A) current density, (B) power density, and (C) short-circuit current (I_sc) of UiO-66-NH₂@ANM generated in 1000 mM/10 mM concentration gradient. The maximum power densities with Br⁻, Cl⁻, NO₃⁻, and SO₄²⁻ were ~26.8, 11.0, 0.0216, and 0.0497 W/m², respectively. (D) Comparison of the output osmotic powers between the subnanoscale UiO-66-NH₂@ANM and the nanoscale ANM (with pore diameter of 25 nm) in different types of salts. (E) The selectivity ratios of the subnanoscale UiO-66-NH₂@ANM and the nanoscale ANM were calculated on the basis of their output powers in various salt systems shown in (D). UiO-66-NH₂@ANM can exhibit an unprecedented Br⁻/ NO₃⁻ selectivity of ~1240. (F) Schematic depiction of the anion-selective property of subnanoscale UiO-66-NH₂@ANM based on the size exclusion effect.

Fig. 6. Stability of UiO-66-NH₂@ANM in aqueous solution.

(A) Top view SEM images and (B) XRD patterns of UiO-66-NH₂@ANM tested after being soaked in aqueous solution for 30 days at room temperature. a.u., arbitrary units. (C) Short-circuit current of UiO-66-NH₂@ANM recorded in 500 mM/10 mM KCl gradient for continuous 12 hours. (D) Output power density of UiO-66-NH₂@ANM recorded in 1000 mM/10 mM KBr gradient for continuous 1 week.