Cetyltrimethylammonium bromide (CTAB) and sodium hydroxide (NaOH) were purchased from Titan Technology Discovery Platform; stannous chloride dihydrate (SnCl₂-2H₂O) was purchased from Yatai United Chemical Co. Ltd. 2D VMT nanosheet dispersion (50 mg mL^–1) was purchased from Nano Functional Materials Co. Ltd.; and organic-based nylon microporous filter membranes were purchased from Derfiltration Technology. All chemicals, including sodium chloride (NaCl), potassium chloride (KCl), and lithium chloride (LiCl), were analytically pure.

As depicted in Supplementary Figure S1a, 4.52 g of SnCl₂-2H₂O, 1.28 g of CTAB, and 2.4 g of NaOH were introduced to 70 mL of deionized water, magnetically stirred for 1.5 h to ensure complete dissolution, and thereafter subjected to ultrasonic agitation for 1 h. The ultrasonicated solution was subsequently transferred to a 100 mL hydrothermal reactor, where it experienced a 12-h hydrothermal reaction at 180 °C. The product was extracted and then rinsed multiple times with deionized water and anhydrous ethanol once it reached room temperature. Upon cooling the hydrothermal reactor to ambient temperature, the product was extracted and subjected to numerous cross-washings with anhydrous ethanol and deionized water, respectively. The cleaned product was further annealed at 500 °C for 3 h at a rate of 5 °C min^–1 to yield SnO₂ nanosheets.

As illustrated in Supplementary Figure S1b, a dispersion of VMT nanosheets at a concentration of 5 mg mL^–1 was initially generated. Subsequently, 3 mL of VMT nanosheet dispersion was combined with 10%, 30%, and 40% of SnO₂ nanosheets, respectively. The two components were meticulously combined using continuous magnetic stirring for 8 h, followed by ultrasonication for 1 h. Following that, using vacuum filtration, the VMT nanosheet dispersion was amalgamated with 3 mL of VMT nanosheet dispersion. After ultrasound treatment, the VMT-10%SnO₂, VMT-30%SnO₂, and VMT-40%SnO₂ mixed dispersions were sequentially deposited via vacuum filtration onto the nylon microporous filter membrane of the organic system and then dried for 8 h at 60 °C, resulting in VMT-X%SnO₂ membranes, where X% indicates the mass percentage of SnO₂ nanosheets relative to VMT nanosheets in the membrane.

Scanning electron microscopy (SEM, FEI Quanta 200FEG, Netherlands, and Zeiss Sigma 300, Germany) was used to examine the membranes’ microstructure. A dynamic light scattering device (Brookhaven, NanoBrook Omni, United States) was used to test the dispersions’ zeta potential at a concentration of 0.5 mg mL^–1. Using Cu Kα irradiation and a scan rate of 5 min^–1, X-ray diffraction spectroscopy (XRD) was performed on a Rigaku D/MAX-2200PC X-ray diffractometer.

Using an electrochemical workstation (CS301M, Wuhan Kotai Instrument Co., Ltd.), the produced devices’ ion transport characteristics and osmotic energy conversion ability were assessed and tested. Ag/AgCl electrodes were utilized for the testing unless otherwise noted. Schematically depicted in Figure 2a, the test apparatus was composed of a composite membrane encased in epoxy resin. The composite membrane was inserted into the epoxy resin plate that separated the device’s two chambers.

Current-voltage (I-V) curves, open-circuit voltage (V _OC), and short-circuit current (I _SC) were measured across a voltage range of – 0.2 V–0.2 V at a 50-fold KCl concentration gradient (0.5 M for high concentration and 0.01 M for low concentration) to ascertain the maximum power density.

To determine the output power density, external resistors with diverse resistance values were applied to the current-time (I-T) curves at 50-fold concentration gradients of KCl, NaCl, and LiCl (0.5 M for high concentration and 0.01 M for low concentration). Throughout the scanning procedure, the voltage was established at 0 V.

The I-V curves documenting salinity gradients of 10, 50, 100, 500, and 1,000 times were evaluated to determine ion selectivity utilizing Ag/AgCl electrodes with saturated salt bridges within a voltage measurement range of −0.2–0.2 V and a scan rate range of 10 mVs^–1.

The conductivity of KCl solutions, ranging from 10⁻⁶ M–1 M without a concentration gradient, was assessed by scanning the I-V curves at a rate of 10 mV s^–1 across a voltage range of −0.2–0.2 V.

Using a 0.1 M KCl solution in both chambers, external bias voltages of +1 V and −1 V were sequentially applied to the device, and the I-T curves were recorded every 3,000 s in a cycle to assess the ion transfer stability of the device.

Figure 1 shows the microscopic characterization of VMT nanosheets, SnO₂ nanosheets and VMT-X%SnO₂ composite membranes. VMT nanosheets and SnO₂ nanosheets were initially analyzed microscopically. As shown in TEM images (Figures 1a,b), the VMT and SnO₂ nanosheets exhibit a distinct lamellar structure at the nanometer scale, with the diameters of the VMT nanosheets surpassing those of the SnO₂ nanosheets. The zeta potentials of the VMT and SnO₂ nanosheets are −27.47 mV and −23.45 mV, respectively (Figure 1c). The higher zeta potential of VMT (−27.47 mV) establishes a robust electrostatic driving force for cation attraction and anion repulsion, while the moderate zeta potential of SnO₂ (−23.45 mV) prevents excessive local charge accumulation that might otherwise impede cation transport through overcrowding. This balance prevents “charge saturation” in nanochannels, ensuring efficient cation migration while maintaining anion exclusion. Subsequently, we can calculate the surface charge density of VMT nanosheets and SnO₂ nanosheets by the value of zeta potential as −2.12 mC m^–2 and –1.81 mC m^–2, respectively (Supporting Note1). Thereafter, we microscopically analyzed the VMT-X%SnO₂ composite membrane. The XRD spectra (Figure 1d) indicate that the VMT-30%SnO₂ composite membrane preserves the (003) and (009) crystalline absorption peaks characteristic of VMT nanosheets (Zhang et al., 2023) and also exhibits the (110) and (101) crystalline absorption peaks related to SnO₂ nanosheets (Chen et al., 2014). The aforementioned phenomenon demonstrated that the VMT-X%SnO₂ composite membrane preserved the original lattice structure of both VMT and SnO₂ nanosheets. Analysis of the cross-sectional SEM images of the VMT membrane and the VMT-X%SnO₂ composite membrane (Figure 1e) reveals that both exhibit a somewhat regular laminar structure, with components organized in close proximity. This structure creates favorable conditions for the development of nano-channels, as well as nanofluidic ion transport (Zhang et al., 2022). The nanochannel dimensions of VMT-X%SnO₂ composite membranes exhibit a non-monotonic variation with increasing SnO₂ nanosheet additive ratio, demonstrating initial expansion followed by subsequent contraction. The VMT-X%SnO₂ composite membranes exhibit a relatively uniform laminar structure, as illustrated in Figure 1f, facilitating the development of nano-channels. The SnO₂ nanosheets are deposited and adhere to the planar framework formed by VMT nanosheets. This structure comprises aluminum tetrahedra, silicate tetrahedra, and magnesium octahedra in an orderly arrangement (Wang et al., 2024). Multiple nano-channels for ion transport were established between the SnO₂ nanosheets and the VMT nanosheets. In these nano-channels, the surface charges of the walls attract counterions of opposite charge, resulting in the formation of electric double layer (EDL) structures. When the channel dimensions are comparable to the Debye length, the electric double layers, comprising the Stern and diffusion layers, overlap within the channel (Supplementary Figure S2). In this situation, ions with identical surface charges within the channel experience repulsion, whereas counterions can traverse the channel (Chang et al., 2022).

The ion transport properties and osmotic energy conversion performance of the fabricated devices were assessed using an electrochemical workstation (CS301M, Wuhan Kotai Instrument Co., Ltd.). Unless stated otherwise, Ag/AgCl electrodes were employed for all experiments in this investigation. As illustrated in Supplementary Figure S3, the test device was constructed from a composite membrane encased in epoxy resin. The device comprises two chambers: the left chamber contains a low concentration salt solution, while the right chamber contains a high concentration salt solution. The composite membrane is situated within an epoxy resin plate utilized to partition the two chambers. Significantly, given that the test area of the VMT-X%SnO₂ composite membranes with varying additive ratios was 0.03 mm² (Figure 1d), and then enclosed them in devices using epoxy resin for further testing.

In the absence of an external electric field, ions diffuse spontaneously from regions of high concentration to low concentration due to the concentration gradient (Mohammadi Amin and Krühne, 2024). The VMT-X%SnO₂ composite membrane possesses a negative surface charge, and its internal nano-channels preferentially allow the passage of cations while repelling anions. The differential ion migration behavior, influenced by the nanolimited domain effect and the material’s surface chemistry, results in cation-preferential transport, leading to charge separation and a net current across both sides of the membrane (Feng et al., 2016). Initially, we should ascertain the optimal additive ratio for the composite membrane. An aqueous KCl solution has been introduced into the chambers on both sides of the device as an electrolyte salt solution. The membrane-forming properties of the VMT-X%SnO₂ composite membrane deteriorate as the mass percentage of added SnO₂ nanosheets increases. The composite membrane becomes brittle and hence challenging to test when the additive ratio of SnO₂ nanosheets exceeds 40%. Consequently, the additive ratio of SnO₂ nanosheets incorporated is limited to 40% or below. The values of V _OC and I _SC are derived by analyzing the I-V curves of VMT-X%SnO₂ composite membranes with differing additive ratios under a 50-fold KCl concentration gradient (high concentration of 0.5 M, low concentration of 0.01 M) across a voltage range of −0.2–0.2 V, as illustrated in Figure 2a. Supplementary Equation S3 can be employed to ascertain the power densities of the VMT-X%SnO₂ composite membranes with differing additive ratios under a 50-fold KCl concentration gradient subsequent to the deduction of the redox potential (Figure 2b). As the additive ratio of SnO₂ nanosheets increases, the power density initially ascends and thereafter declines. The power density reaches its maximum at a 30% additive ratio of SnO₂ nanosheets (1.07 W m^–2), representing a 55.3% improvement over the power density of pure VMT membrane (0.69 W m^–2). As the additive ratio of SnO₂ nanosheets grew, V _OC also exhibited an upward trend (Figure 2c). Nonetheless, as the additive ratio of SnO₂ nanosheets grew, the current density initially ascended and then declined. It reached a maximum additive ratio of 30%, surpassing that of the pure VMT membrane, consequently enhancing the power density (Figure 2d). The observed trend can be attributed to the following mechanism: Initially, with increasing additive ratio of SnO₂ nanosheet, the interlayer spacing of nanochannels expands, thereby facilitating enhanced cation transport through the nanochannels and consequently leading to increased current density. However, when the additive ratio reaches a critical threshold, SnO₂ nanosheet begin to aggregate within the nanochannels. This aggregation can lead to the blockage of the nanochannels, resulting in a decrease in effective nanochannels and a reduction in cation flux. Consequently, the current density shows a downward trend. The VMT-30%SnO₂ composite membranes was chosen for further testing, as the optimal power density was attained with a 30% additive ratio of SnO₂ nanosheets.

Nanofluidic membranes can provide electrical energy to externally powered devices. (Wang et al., 2021). Supplementary Equation S4 can be utilized to ascertain the output power densities of the VMT membrane and the VMT-30%SnO₂ composite membrane. At a 50-fold KCl concentration gradient (0.01 M/0.5 M), the output power density of the VMT-30%SnO₂ composite membrane was 1.059 W m^–2, surpassing that of the VMT membrane (Figure 3a). This can be attributed to the superior output current density of the VMT-30% SnO₂ composite membrane compared to the VMT membrane (Figure 3b). However, the output power density of the VMT membrane and the VMT-30%SnO₂ composite membrane reached reach their peak values under different external resistance values. The reason for this phenomenon can be attributed to the fact that the internal resistance of the VMT-30%SnO₂ composite membrane is lower than that of the VMT membrane. Subsequently, we evaluated the output power density of the VMT-30%SnO₂ composite membrane using various electrolytes. The output power density was 0.727 W m^–2 with NaCl as the electrolyte and 0.531 W m^–2 with LiCl, both under a 50-fold electrolyte concentration gradient (0.01 M/0.5 M) (Figure 3c). The addition of KCl, NaCl, and LiCl as electrolytes resulted in output current densities of 28.9 A m^–2, 21.4 A m^–2, and 15.9 A m^–2, respectively, while sustaining a 50-fold electrolyte concentration gradient (0.01 M/0.5 M) (Figure 3d). The 50-fold NaCl concentration gradient (0.01 M/0.5 M) can be regarded as representative of the concentration gradient between artificial river water and artificial seawater. This discrepancy arises from variations in the cation diffusion coefficients of the electrolytes. The hierarchy of cation diffusion coefficients for the three selected electrolytes is K⁺ > Na⁺ > Li⁺. Elevated cation diffusion coefficients yield increased output current densities, thereby enhancing output power densities (Wu et al., 2020).

Upon establishing the optimal additive ratio of the composite membrane, we utilized Ag/AgCl electrodes with saturated salt bridges to further examine the transmembrane ion transport performance. Maintaining the concentration of the low side KCl solution at a constant 10⁻³ M, the I-V curves of the VMT-30%SnO₂ composite membrane and the VMT membrane were examined over various KCl solution concentration gradients within the voltage range of −0.2 V–0.2 V (Figures 4a,b). Consequently, the V _OC values of VMT-30% SnO₂ composite membrane and VMT composite membrane were derived from the I-V curves at varying concentration gradients of KCl solutions, respectively. The absolute values of V _OC for the VMT-30% SnO₂ composite membrane and VMT membrane escalated with the concentration gradient. The V _OC of the VMT-30%SnO₂ composite membrane was approximately −50 mV at a concentration gradient of 10 times, and around −98 mV when the gradient increased to 1,000 times. Furthermore, the absolute values of the V _OC for the VMT membrane at varying concentration gradients were lower than those of the VMT-30%SnO₂ composite membrane, corroborating previous test results (Supplementary Figures S4a,b). Cation transfer number (t ₊ ) and energy conversion efficiency are critical metrics for assessing the ion-selective performance of the devices (Ji et al., 2017; Zhou and Jiang, 2020). When the value of t ₊ equals 1, it signifies that the composite membrane exhibits complete cation selectivity. The t ₊ values of the VMT-30%SnO₂ composite and VMT membranes can be ascertained using Supplementary Equation S5. The maximum value of t ₊ for the VMT-30% SnO₂ device is 0.910, surpassing the maximum value of t ₊ for the VMT device, which is 0.869. The aforementioned conclusion demonstrates that the ion selectivity of the composite membrane improved with the incorporation of SnO₂ nanosheets, while VMT-30%SnO₂ exhibited commendable cation selectivity. As the KCl concentration gradient intensifies, the t ₊ values exhibit a declining trend, attributable to the reduction in the Debye length and concentration polarization at elevated concentrations (Figure 4c). The energy conversion efficiencies of the VMT-30%SnO₂ composite membrane and VMT membrane can be obtained from Supplementary Equation S6. The energy conversion efficiency of the VMT-30%SnO₂ composite membrane (33.63%) surpasses that of the VMT membrane (27.23%), and the efficiency diminishes with an increasing concentration gradient (Figure 4d).

The ion transport characteristics of VMT-30%SnO₂ composite membrane and VMT membrane were further examined by assessing their conductivity. During the conductivity test, the concentration of KCl solution in the chambers on either side of the testing apparatus had to be uniform. The precise value of the nano-channel conductivity was ascertained using Supplementary Equation S7. As the concentration of KCl solution increased, conductivity rose, and the conductivity of the VMT-30%SnO₂ composite membrane surpassed that of the VMT membrane (Figure 5a). This phenomenon can be mechanistically ascribed to the expansion of interlayer spacing in nanochannels induced by the introduction of SnO₂ nanosheets. Such structural evolution effectively mitigates the steric hindrance during ion transport, thereby facilitating the permeation kinetics of ions through the nanochannel network. The conductance (S), as indicated by Supplementary Equations S7, 8, was predominantly influenced by the surface charge density at low electrolyte solution concentrations. Consequently, when the concentration of the KCl solution was in the low concentration range (below 10⁻³ M) the conductivity tended to stabilize on a logarithmic scale. The phenomenon was typical of nanofluidic ion transport behavior. The S of KCl solutions in the high concentration range (higher than 10⁻³ M) was predominantly influenced by the S of the aqueous KCl solution at varying concentrations. Therefore, the conductance trend in this region, when represented on a logarithmic scale, aligned with the linear variation of the intrinsic conductance of KCl.

Simultaneously, we acquired the Nyquist plots of the VMT membrane compared to the VMT-30%SnO₂ membrane by measuring with a KCl solution concentration of 10⁻³ M on both sides of the chamber (Figure 5b). We subsequently fitted the Nyquist curves using an EIS equivalent circuit (Figure 5c) to get the internal resistance values of the VMT membrane and the VMT-30%SnO₂ composite membrane (Table 1). The charge transfer resistance (R_ct) of the VMT-30% SnO₂ composite membrane was around 361.8 kΩ, marginally lower than the Rct of the VMT membrane at 397.1 kΩ SnO₂ nanosheets reduce R_ct through synergistic surface charge effects and optimized channel structures, minimizing energy loss and improving ion transport efficiency. The comparatively lower internal resistance was advantageous for output power density, resulting in the VMT-30%SnO₂ composite membrane exhibiting a greater output power density than the VMT membrane.

Ultimately, we assessed the ion transport stability of the composite membranes over 3,000 s by alternately applying +1 V and −1 V external bias voltages outside the testing apparatus. As illustrated in Figure 5d, both the VMT membrane and the VMT-30%SnO₂ composite membrane exhibited minimal fluctuations throughout the 3,000 s, indicating that they both demonstrated relatively robust ion transport stability.