Electroregulation of graphene-nanofluid interactions to coenhance water permeation and ion rejection in vertical graphene membranes

Abstract

Graphene oxide (GO) membranes with nanoconfined interlayer channels theoretically enable anomalous nanofluid transport for ultrahigh filtration performance. However, it is still a significant challenge for current GO laminar membranes to achieve ultrafast water permeation and high ion rejection simultaneously, because of the contradictory effect that exists between the water-membrane hydrogen-bond interaction and the ion-membrane electrostatic interaction. Here, we report a vertically aligned reduced GO (VARGO) membrane and propose an electropolarization strategy for regulating the interfacial hydrogen-bond and electrostatic interactions to concurrently enhance water permeation and ion rejection. The membrane with an electro-assistance of 2.5 V exhibited an ultrahigh water permeance of 684.9 L m^-2 h^-1 bar^-1, which is 1-2 orders of magnitude higher than those of reported GO-based laminar membranes. Meanwhile, the rejection rate of the membrane for NaCl was as high as 88.7%, outperforming most reported graphene-based membranes (typically 10 to 50%). Molecular dynamics simulations and density-function theory calculations revealed that the electropolarized VARGO nanochannels induced the well-ordered arrangement of nanoconfined water molecules, increasing the water transport efficiency, and thereby resulting in improved water permeation. Moreover, the electropolarization effect enhanced the surface electron density of the VARGO nanochannels and reinforced the interfacial attractive interactions between the cations in water and the oxygen groups and π-electrons on the VARGO surface, strengthening the ion-partitioning and Donnan effect for the electrostatic exclusion of ions. This finding offers an electroregulation strategy for membranes to achieve both high water permeability and high ion rejection performance.

Keywords: electroregulation; graphene oxide; interfacial interaction; membrane.

Fig. 1.

Structure of VARGO membrane and SEM characterization. (A) Structural schematics of the VARGO membrane and nanofluid transport through the VARGO nanochannels. (B) Photographs of linear and annular VARGO structures. (C–E) Top views of linear VARGO membrane imbedded in the epoxy resin matrix. The VARGO membrane exhibits a lamellar structure with a width of approximately 1.2 μm. (F and G) Cross-sectional views of the linear VARGO membrane possessing a membrane thickness of approximately 80 μm. Inset: magnified view of the cross-section of the VARGO membrane (dashed box in part G). (H) Low- and (I) high-resolution SEM images of the annular VARGO surface. (J) Cross-section morphologies of the annular VARGO membrane.

Fig. 2.

Performance of VARGO membranes and mechanism of water permeation and ion rejection. (A) SEM images of the section morphologies of the VARGO membranes with different thicknesses. (B) Pure water permeances of the VARGO membranes with different thicknesses. (C) Pure water permeances and water flow velocities (Inset) of the VAGO membrane and VARGO membranes with the RGO reduced at 30, 60, and 90 °C. (D) Snapshots of the MD simulations for water molecule transports through the VAGO and VARGO nanochannels at 400 ps. (E) Time evolution of the number of water molecules passed through the VAGO and VARGO nanochannels. (F) Simulated water flow rates through the VAGO and VARGO membranes. The inset shows the schematics of water transport through the VAGO and VARGO90 nanochannels. (G) NaCl rejection rates of the VAGO membrane and the VARGO membranes with the RGO reduction temperatures of 30, 60, and 90 °C. (H) Rejection rates of the VARGO90 membrane at 0.8 bar for different salt solutions at a salt concentration of 5 mM. (I) Top and side views of the optimized adsorption configurations of Na⁺ on the RGO sheet (i) and graphene sheet (ii). The C, O, and H atoms are denoted as gray, red, and white spheres, respectively. The purple sphere denotes Na⁺.

Fig. 3.

Water permeation of the electroregulated VARGO membranes and electrically enhanced permeation mechanism. (A) Electro-conductivities of VAGO, VARGO30, VARGO60, and VARGO90 membranes. (A, Inset): the VARGO90 membrane served as a connecting conductor in the electric circuit to light up the LED lamp. (B) Pure water permeances of the VARGO90 membrane under different applied voltages. (C) Variations of WCA of the membranes under different voltages over time. A 5 mM NaCl solution serves as the test solution. (D) Cyclic voltammetry curves and charge densities (Inset) of the membranes under different voltages. The membrane served as the cathode and a titanium mesh was employed as the anode in a two-electrode system. A 5 mM NaCl solution served as the electrolyte solution and the scanning rate was 10 mV s⁻¹. (E) Schematic of electroregulated water permeation. (F) MD simulation model demonstrating the entry of water molecules into a graphene nanochannel with an interlayer spacing of 9 Å. (G) Time evolution of the number of water molecules entering the graphene nanochannel under different electric densities of 0, −0.5, −1.0, and −2.0 e nm⁻² on the graphene nanochannel. (H) Simulated transport rates of water molecules entering the graphene nanochannel under different electric densities. (I) Distributions of water molecules in the graphene nanochannel under different electric densities.

Fig. 4.

Rejection performance and enhancement mechanism of electroregulated VARGO membranes. (A) NaCl rejection rates of the VARGO90 membrane under voltages ranging from 0 to 2.5 V, with a feed of 5 mM NaCl and the transmembrane pressure of 0.8 bar. (B) Comparison of water permeance and NaCl rejection performance of the VARGO90 membrane (without/with an external voltage of 2.5 V) with the reported graphene-based membranes and commercial membranes (, , –60) (SI Appendix, Table S7). (C) Electron density differences of Na⁺ adsorptions on the RGO sheet (i), RGO sheet with a charge of −2.0 e (ii), graphene sheet (iii), and graphene sheet with a charge of −2.0 e (iv). The blue and yellow areas denote the electron density accumulation and depletion, respectively, with an isosurface of 0.01 e Å⁻³. (D) In situ ATR-FTIR spectra of the membrane at different voltages ranging from 0 to 2.5 V in a 5 mM NaCl solution. The VARGO90 membrane served as the cathode and a platinum plate functioned as the anode. (D, Inset): comparison of the ATR-FTIR spectra in the purple dashed box. (E) Number density profile of Na⁺ ions along the z-axis of the simulation box. To determine the effect of the nanochannel wall on the ion distribution, an RGO nanochannel with a width of approximately 40 Å was used for the simulation. The black curve in the figure denotes the number density profile of the Na⁺ ions between the two RGO sheets without electric charge density, and the blue curve denotes the number density profile of the Na⁺ ions between the two RGO sheets with electric charge density. RGO (−) denotes the negatively charged RGO sheet with an electric charge density of −2.0 e nm⁻², and RGO (+) denotes the positively charged RGO sheet with an electric charge density of 2.0 e nm⁻². (E, Inset): snapshot of the simulation. The simulation box is 34.5 × 39.4 × 48.0 Å³. The negatively charged C, positively charged C, O, and H atoms are denoted by the gray, yellow, red, and white spheres, respectively. The purple and green spheres denote Na⁺ and Cl⁻, respectively. (F) Snapshots of the MD simulations for the NaCl solution transporting through the VARGO nanochannels (i) without electric charge density and (ii) with an electric charge density of 2.0 e nm⁻². (G) Mean square displacements (MSD) of H₂O, Na⁺, and Cl⁻, through the VARGO nanochannels as a function of time. (H) Time evolution of the number of H₂O and NaCl passed through the VARGO nanochannels. Dashed lines: without electric charge density; solid lines: with an electric charge density of 2.0 e nm⁻². The Inset in H shows the number of H₂O and NaCl passed through the VARGO nanochannels at 200 ps.