Recent studies of the high energy‐conversion efficiency of the nanofluidic platform have revealed the enormous potential for efficient exploitation of electrokinetic phenomena in nanoporous membranes for clean‐energy harvesting from salinity gradients. Here, nanofluidic reverse electrodialysis (NF‐RED) consisting of vertically aligned boron‐nitride‐nanopore (VA‐BNNP) membranes is presented, which can efficiently harness osmotic power. The power density of the VA‐BNNP reaches up to 105 W m−2, which is several orders of magnitude higher than in other nanopores with similar pore sizes, leading to 165 mW m−2 of net power density (i.e., power per membrane area). Low‐pressure chemical vapor deposition technology is employed to uniformly deposit a thin BN layer within 1D anodized alumina pores to prepare a macroscopic VA‐BNNP membrane with a high nanopore density, ≈108 pores cm−2. These membranes can resolve fundamental questions regarding the ion mobility, liquid transport, and power generation in highly charged nanopores. It is shown that the transference number in the VA‐BNNP is almost constant over the entire salt concentration range, which is different from other nanopore systems. Moreover, it is also demonstrated that the BN deposition on the nanopore channels can significantly enhance the diffusio‐osmosis velocity by two orders of magnitude at a high salinity gradient, resulting in a huge increase in power density.

most intensively studied research areas in the development of high-performance separation membranes.Most 2D materials typically have a layered geometry where the atoms are linked by strong in-plane covalent bonds, while the two adjacent layers are held together by van der Waals forces. [7] Exfoliated nanosheets typically show a high surface-area-tothickness ratio, which is instrumental in improving the adsorption capacity or the ion selectivity, both of which can lead to a better separation performance. In addition to highly effective ion-rejection properties, 2D membranes are also known to possess excellent water permeability. Due to these properties, many 2D materials such as nm-thick graphitic carbon nitride (g-C 3 N 4 ) nanosheets, [8] MoS 2 sheets, [9] WS 2 nanosheets, [6] graphene, [10] Mxene nanosheets, [11] and graphene oxide (GO), [12] have been recently used as the building blocks for fabricating ultrathin layered membranes for separation applications. These 2D membranes have shown highly efficient size-selective ion separation and high water permeance.Hexagonal boron nitride (h-BN), so-called "white graphene," is however one of the promising 2D materials that has not been utilized to its full potential in ion-separation applications. For efficient ion separation, the membrane should show high ion selectivity, which is primarily dependent on the channel size and surface charge on the membrane. The h-BN material is known to have a high surface charge density resulting from the adsorption of the hydroxyl ions on the surface defect sites. [13] It also shows excellent oxidation and corrosion resistance, which is an important property for wastewater treatment applications. The high chemical stability of h-BN also enables it to be resistant to chemical cleaning, which is frequently needed during separation processes. Qin et al. reported high ionic conductivities for h-BN nanofluidic channels, prepared by the one-step BN exfoliation method and amine functionalization. [14] More recently, Chen et al. developed a 2D h-BN membrane by functionalizing h-BN flakes (h-BNF) with amino groups to overcome its poor water dispersibility. The functionalized h-BNF membranes demonstrated fast solvent transport and good ion-rejection properties, based on molecular sieving mechanism. [15] However, these membranes do not show charge-based (Donnan) exclusion, due to the amine functionalization of the 2D layered nanomaterials have attracted considerable attention for their potential for highly efficient separations, among other applications. Here, a 2D lamellar membrane synthesized using hexagonal boron nitride nanoflakes (h-BNF) for highly efficient ion separation is reported. The ion-rejection performance and the water permeance of the membrane as a function of the ionic radius, ion valance, and solution pH are investigated. The nonfunctionalized h-BNF membranes show excellent ion rejection for small sized salt ions as well as for anionic dyes (>97%) while maintaining a high water permeability, ≈1.0 × 10 −3 L m m −2 h −1 bar −1...

The alignment of hexagonal boron-nitride nanotubes (BNNTs) in aqueous KCl solutions under spatially uniform electric fields was examined experimentally, using direct optical visualization to probe the orientation dynamics of individual BNNTs for different electric-field frequencies. Different from most previously studied nanowires and nanotubes, BNNTs are wide-bandgap materials which are essentially insulating at room temperature. We analyze the electro-orientation of BNNTs in the general context of polarizable cylindrical particles in liquid suspensions, whose behavior can fall into different regimes, including alignment due to Maxwell-Wagner induced dipoles at high frequencies, and alignment due to fluid motion of the electrical double layer around the particles at lower frequencies. For BNNTs, the variation of the crossover frequencies in the electro-orientation spectra was studied in electrolytes of different conductivity. The effect of BNNT surface charge on electro-orientation was further studied by changing the pH of the aqueous solution. We find that the electric-field alignment of the BNNTs in the low-frequency regime is associated with the charging and motion of the electrical double layer around the particle. However, as BNNTs are non-conducting particles, the reasons for the formation of the electrical double layer are likely to be different than that of conducting particles. We discuss two possible mechanisms for the double-layer formation and alignment of 1D dielectric particles, and make comparison to those for the more commonly studied conducting particles.

Application of the abundant salinity-gradients in the environment, motivated by the Gibbs free energy of mixing fresh and salt water, has been under intensive investigation for clean-energy harvesting. Globally, there is a potential of at least 2.6 TW of power that could be generated at coastal estuaries from the ion-concentration gradient. Recent investigation on an energy conversion efficiency of single boron nitride nanotube (BNNT) has revealed the huge potential of an efficient exploitation of electrokinetic phenomena in nanofluidic systems for energy harvesting. Here, for the first time, we demonstrate rationally designed nanostructured vertically aligned boron-nitride-nanopore membranes (VA-BNNP) which can efficiently harness osmotic power from salinity gradient. A thin hexagonal boron nitride (hBN) layer was uniformly deposited within the pores of anodized alumina substrates by low-pressure chemical vapor deposition which produced first-ever macroscopic VA-BNNP membrane with high nanopore density, up to ~108 pores/cm2. Cross-sectional scanning-electron-microscope (SEM) images show the hBN layer (~35 nm) was uniformly deposited along the pores without excessive hBN on the top surface, ensuring that most pores remained open. In addition, the results of scanning confocal Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) showed the high quality of the hBN layers in the AAO pores. We investigated the power generation of the macroscopic VA-BNNP membranes at different pH and salinity concentrations. The power generation per unit pore area increased as the salt concentration and pH increased. The highest power density of the membrane was up to ~100 W/m2 which is two orders of magnitude higher than that of other macro-scale, salinity gradient driven, power generation system reported so far. In addition, we also elucidate the fundamental ion transport mechanism in BN nanopore using the molecular dynamic simulations to support to the experimental power density values. These findings indicate the great potential of large-area VA-BNNP membranes as next-generation nanostructured membranes for renewable energy harvesting.

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