Research ArticlePHYSICS

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

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Science Advances  03 Mar 2021:
Vol. 7, no. 10, eabe9924
DOI: 10.1126/sciadv.abe9924
  • 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 (Rm) decreases by ~9.3% when the UiO-66-NH2 layer faced a concentrated solution. (C) Diffusion potential (Vdiff) and diffusion current (Idiff) 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 (Isc) 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.

Supplementary Materials

  • Supplementary Materials

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

    Yi-Cheng Liu, Li-Hsien Yeh, Min-Jie Zheng, Kevin C.-W. Wu

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