In Situ Growth of Imine-Bridged Anion-Selective COF/AAO Membrane for Ion Current Rectification and Nanofluidic Osmotic Energy Conversion

Facing the energy crisis, using the salinity gradient between seawater and freshwater for osmotic energy conversion is a direct way to obtain energy. So far, most nanofluidic membranes utilized for osmotic energy generation are cation-selective. Given that both anion- and cation-selective membranes have the identical importance for energy conversion devices, it is of great significance to develop anion-selective membranes. Herein, an anion-selective membrane is synthesized by in situ growth of imine-bridged covalent organic framework (COF) on ordered anodic aluminum oxide (AAO) at room temperature. The imine groups and residual amino groups of COF can combine with protons in neutral solution, enabling the COF positively charged and efficiently transport of anions. Particularly, due to the asymmetry in the charge and structure of COF/AAO, the as-prepared membrane exhibits excellent ionic current rectification property, which can inhibit ion concentration polarization effectively and possess high ion selectivity and permeability. Using the present COF/AAO membrane, salinity gradient energy can be successfully harvested from solutions with high salt content, and the output power density reached 17.95W m⁻² under a 500-fold salinity gradient. The study provides a new avenue for construction and application of anion-selective membranes in the smart ion transport and efficient energy conversion.     

Anomalous Channel‐Length Dependence in Nanofluidic Osmotic Energy Conversion

Recent advances in materials science and nanotechnology have lead to considerable interest in constructing ion‐channel‐mimetic nanofluidic systems for energy conversion and storage. The conventional viewpoint suggests that to gain high electrical energy, the longitudinal dimension of the nanochannels (L) should be reduced so as to bring down the resistance for ion transport and provide high ionic flux. Here, counterintuitive channel‐length dependence is described in nanofluidic osmotic power generation. For short nanochannels (with length L < 400 nm), the converted electric power persistently decreases with the decreasing channel length, showing an anomalous, non‐Ohmic response. The combined thermodynamic analysis and numerical simulation prove that the excessively short channel length impairs the charge selectivity of the nanofluidic channels and induces strong ion concentration polarization. These two factors eventually undermine the osmotic power generation and its energy conversion efficiency. Therefore, the optimal channel length should be between 400 and 1000 nm in order to maximize the electric power, while balancing the efficiency. These findings reveal the importance of a long‐overlooked element, the channel length, in nanofluidic energy conversion and provide guidance to the design of high‐performance nanofluidic energy devices.     

Osmotic Power Generation with Positively and Negatively Charged 2D Nanofluidic Membrane Pairs

In nature, hierarchically assembled nanoscale ionic conductors, such as ion channels and ion pumps, become the structural and functional basis of bioelectric phenomena. Recently, ion‐channel‐mimetic nanofluidic systems have been built into reconstructed 2D nanomaterials for energy conversion and storage as effective as the electrogenic cells. Here, a 2D‐material‐based nanofluidic reverse electrodialysis system, containing cascading lamellar nanochannels in oppositely charged graphene oxide membrane (GOM) pairs, is reported for efficient osmotic energy conversion. Through preassembly modification, the surface charge polarity of the 2D nanochannels can be efficiently tuned from negative (?123 mC m^?2) to positive (+147 mC m^?2), yielding strongly cation‐ or anion‐selective GOMs. The complementary two‐way ion diffusion leads to an efficient charge separation process, creating superposed electrochemical potential difference and ionic flux. An output power density of 0.77 W m^?2 is achieved by controlled mixing concentrated (0.5 m ) and diluted ionic solutions (0.01 m ), which is about 54% higher than using commercial ion exchange membranes. Tandem alternating GOM pairs produce high voltage up to 2.7 V to power electronic devices. Besides simple salt solutions, various complex electrolyte solutions can be used as energy sources. These findings provide insights to construct cascading nanofluidic circuits for energy, environmental, and healthcare applications.     

On the Origin of Ion Selectivity in Ultrathin Nanopores: Insights for Membrane‐Scale Osmotic Energy Conversion

Nanopores in ultrathin or atomically thin membranes attract broad interest because the infinitesimal pore depth allows selective transport of ions and molecules with ultimate permeability. Toward large‐scale osmotic energy conversion, great challenges remain in extrapolating the promising single‐pore demonstration to really powerful macroscopic applications. Herein, the origin of the selective ion transport in ultrathin nanopores is systematically investigated. Based on a precise Poisson and Nernst–Planck model calculation, it is found that the generation of net diffusion current and membrane potential stems from the charge separation within the electric double layer on the outer membrane surface, rather than that on the inner pore wall. To keep the charge selectivity of the entire membrane, a critical surface charged area surrounding each pore orifice is therefore highly demanded. Otherwise, at high pore density, the membrane selectivity and the overall power density would fall down instead, which explains the giant gap between the actual experimental achievements and the single‐pore estimation. To maximize the power generation, smaller nanopores (pore diameter ≈1–2 nm) are appropriate for large‐scale osmotic energy conversion. With a porosity of ≈10%, the total power density approaches more than 200 W m^‐2, anticipating a substantial advance toward high‐performance large‐scale nanofluidic power sources.     

High-Strength,Thin PBO Nanofiber Membrane with Long-Term Stability for Osmotic Energy Conversion

Ion-selective membrane embedded in a reverse electrodialysis system can achieve the conversion of osmotic energy into electricity. However, the ingenious design and development of pure polymer membranes that simultaneously satisfy excellent mechanical strength, long-term stability, high power density, and increased testing area is a crucial challenge. Here, high-strength, thin PBO nanofiber membranes (PBONM) with 3D nanofluidic channels and a thickness of 0.81 µm are prepared via a simple vacuum-assisted filtration technology. The thin PBONM exhibits excellent mechanical properties: stress of 235.8 MPa and modulus of 16.96 GPa, outperforming the state-of-the-art nanofluidic membranes. The obtained PBONM reveals surface-charge-governed ion transport behavior and high ion selectivity of 0.88 at a 50-fold concentration gradient. The PBO membrane-based generator delivers a power density of 7.7 and 40.2 W m⁻² at 50-fold and 500-fold concentration gradient. Importantly, this PBONM presents excellent stability in response to different external environments including various saline solutions, pH, and temperature. In addition, the maximum power density of PBONM reaches up to 5.9 W m⁻² under an increased testing area of 0.79 mm², exceeding other membrane-based generators with comparable testing areas. This work paves the way for constructing high-strength fiber nanofluidic membranes for highly efficient osmotic energy conversion.     

Fabrication of Bio‐Inspired 2D MOFs/PAA Hybrid Membrane for Asymmetric Ion Transport

Biological ion channels are known as membrane proteins which can turn on and off under environmental stimulus to regulate ion transport and energy conversion. Rapid progress made in biological ion channels provides inspiration for developing artificial nanochannels to mimic the structures and functions of ion transport systems and energy conversion in biological ion channels. Due to the advantages of abundant pore channels, metal–organic frameworks (MOFs) have become competitive materials to control the nanofluidic transport. Herein, a facile in situ synthesis method is developed to prepare hybrid nanochannels constructed by 2D MOFs and porous anodic aluminum (PAA). The introduction of asymmetries in the chemical composition and surface charge properties gives the hybrid outstanding ion current rectification properties and excellent ion selectivity. A power density of 1.6 W m^?2 is achieved by integrating it into a salinity‐gradient‐driven device. With advantages of facile fabrication method and high ion selectivity, the prepared 2D MOFs/PAA hybrid membrane offers a promising candidate for power conversion and water desalination.