Bio-Inspired Salinity-Gradient Power Generation With UiO-66-NH2 Metal-Organic Framework Based Composite Membrane

Abstract

Salinity-gradient directed osmotic energy between seawater and river water has been widely considered as a promising clean and renewable energy source, as there are numerous river estuaries on our planet. In the past few decades, reverse electrodialysis (RED) technique based on cation-selective membranes has been used as the key strategy to convert osmotic energy into electricity. From this aspect, developing high-efficiency anion-selective membranes will also have great potential for capturing osmotic energy, however, remains systematically unexplored. In nature, electric eels can produce electricity from ionic gradients by using their "sub-nanoscale" protein ion channels to transport ions selectively. Inspired by this, here we developed a UiO-66-NH₂ metal-organic framework (MOF) based anion-selective composite membrane with sub-nanochannels, and achieved high-performance salinity-gradient power generation by mixing artificial seawater (0.5 M NaCl) and river water (0.01 M NaCl). The UiO-66-NH₂ metal-organic framework based composite membranes can be easily and economically fabricated with dense structure and long-term working stability in saline, and its performance of power generation can also be adjusted by pH to enhance the surface charge density of the MOF sub-nanochannels. This study will inspire the exploitation of MOFs for investigating the sub-nanochannel directed high-performance salinity-gradient energy harvesting systems based on anion-selective ion transport.

Keywords: biomimetics; energy conversion; ion transport; metal-organic frameworks; nanofluidic; salinity gradient.

FIGURE 1

Schematic illustration of the sub-nanometer protein ion channels of an electric ell.

FIGURE 2

Preparation and characterization of the UiO-66-NH₂ composite membranes. (A) The fabrication process for the UiO-66-NH₂ composite membranes. Representative (B) top and (C) cross-sectional SEM images of the secondary-growth UiO-66-NH₂ membrane. Representative (D) top and (E) cross-sectional SEM images of the single-growth UiO-66-NH₂ membrane.

FIGURE 3

Surface charge-governed ion transport in UiO-66-NH₂ composite membrane. (A) Schematic illustration of the UiO-66-NH₂ composite membrane with sub-nanometer sized channels. (B) Representative I-V curves obtained with three different NaCl concentration. (C) Ionic conductance of the UiO-66-NH₂ membranes at different electrolyte concentration. When the salt concentrations were <1 M, the ionic conductance values of the UiO-66-NH₂ membranes (red square) deviate significantly from the bulk value (black curve), demonstrating the surface charge governed ion transport behavior.

FIGURE 4

UiO-66-NH₂ composite membranes for salinity-gradient osmotic energy conversion. (A) Schematic illustration of the proposed energy harvesting device. (B) As the concentration gradient was increased, the V _diff gradually increased and the J _diff gradually decreased. The high NaCl concentration was fixed at 0.5 M. (C) Current density and (D) power density of the as-prepared UiO-66-NH₂ composite membranes as a function of the external resistance under three different NaCl concentration gradients. For three salinity gradients, the measured current densities all gradually decrease with increasing external resistance. The maximum power density values were ∼0.8, 1.47, and 0.42 W/m² for 5-, 50-, and 500-fold NaCl concentration gradients, respectively.

FIGURE 5

Long-term stability of the as-prepared UiO-66-NH₂ composite membranes. (A) Representative SEM images of the UiO-66-NH₂ composite membranes, showing no obvious change after immersion in deionized water for 1 month. (B) Under 50-fold NaCl concentration gradient, the output power density of the UiO-66-NH₂ composite membranes showed strong stability examined in 1 week.