Unlocking osmotic energy harvesting potential in challenging real-world hypersaline environments through vermiculite-based hetero-nanochannels

Abstract

Nanochannel membranes have demonstrated remarkable potential for osmotic energy harvesting; however, their efficiency in practical high-salinity systems is hindered by reduced ion selectivity. Here, we propose a dual-separation transport strategy by constructing a two-dimensional (2D) vermiculite (VMT)-based heterogeneous nanofluidic system via an eco-friendly and scalable method. The cations are initially separated and enriched in micropores of substrates during the transmembrane diffusion, followed by secondary precise sieving in ultra-thin VMT laminates with high ion flux. Resultantly, our nanofluidic system demonstrates efficient osmotic energy harvesting performance, especially in hypersaline environment. Notably, we achieve a maximum power density of 33.76 W m^-2, a 6.2-fold improvement with a ten-fold increase in salinity gradient, surpassing state-of-the-art nanochannel membranes under challenging conditions. Additionally, we confirm practical hypersaline osmotic power generation using various natural salt-lake brines, achieving a power density of 25.9 W m^-2. This work triggers the hopes for practical blue energy conversion using advanced nanoarchitecture.

Fig. 1. Fabrication and characterization of the VMT nanosheets and VMT membrane.

a Schematic illustration of the preparation of the large-scale VMT nanosheets. b AFM image and height profile of the nanosheets corresponding to the AFM image. Scale bar, 2 μm. c Optical microscopy image of the large-scale VMT nanosheets. Scale bar, 20 μm. d SEM image of the cross-sectional morphology of the VMT membrane. Scale bar, 3 μm. e Stress−strain curve of VMT membrane. f AFM image of the VMT membrane and height profile of the VMT membrane corresponding to the AFM image. Scale bar, 1 μm. g Optical image of a VMT membrane.

Fig. 2. The ionic transmembrane properties of the VMT-based membrane.

a Zeta potential of VMT nanosheet colloidal suspension at different pH. b Representative I–V curves of the VMT membrane. c Conductivity of VMT membrane as a function of salt concentration. d Classical molecular dynamics simulation snapshot that showed the distribution of ions. e The ratio of cation (η) in nanochannel. f I−T curves of VMT membrane in 0.1 M KCl at pH 7 with an external bias alternating between +1 and −1 V. g I–V curves of the VMT-PVDF membrane measured in electrolyte solutions with different concentrations. The applied transmembrane voltage was between −0.5 V and +0.5 V. h Ionic rectification ratio under different concentrations calculated from the I–V curves. i Theoretical calculation of ionic concentration distribution in nanochannel at −1 V and +1 V. Error bars indicated the standard deviations from three different samples.

Fig. 3. Selective ion transport behavior in VMT-based membrane.

a Schematic illustration of dual-separation mechanism. b Two configurations under a 50-fold concentration gradient with C_high = 1 M KCl, C_low = 0.02 M KCl. c Numerical simulations depicting the cation concentration profiles on both sides of the VMT layer, which were plotted along the dashed line near the orifice of the VMT channel (Supplementary Fig. 21). d The open-circuit potential and short-circuit current of the VMT-PVDF membrane with increasing concentration gradient. The high-concentration side of the electrolyte solution was fixed as 1 M on the PVDF side and the low-concentration solution on the VMT side varied from 0.002 M to 0.5 M. Error bars indicated the standard deviations from three different samples.

Fig. 4. High-performance osmotic energy conversion of the VMT-based membrane.

a, b Current density and output power density of VMT-based membrane as functions of load resistance under three salinity gradients. The low-salinity solution was placed in the VMT side and fixed at 0.01 M NaCl. High-salinity solution was tunable from 0.05 to 5 M NaCl. c The long-term stability of the energy conversion at 500-fold salinity gradient. d Comparison with previous studies (details in Supplementary Table 2). The ratio of P_max at 500-fold and 50-fold salinity gradient (P_max−500/P_max-50) and power densities of the membranes. e Output power density under different types of electrolytes. f The AIMD snapshots that showed the distribution of ions in VMT nanochannel. The interlayer spacing was 1.53 nm, which was the same as the XRD results (Supplementary Fig. 8). g The MSD curves of ions. The inset showed the MSD curves of water. The value of diffusion coefficient was proportion to the slope of MSD. h Power density under three different highly concentrated natural brines. Error bars indicated the standard deviations from three different samples.