development and human activities, giving rise to the need for a method to obtain clean water resources. Membrane filtration, one of the processes to separate contaminants and salts from water, has attracted much attention as a way to secure clean water in an energy efficient manner. [2,3] As a result, various kinds of membrane materials, especially polymers, [4] have been widely studied for desalination and water purification. [5][6][7][8] However, the availability of polymer membranes in a large-scale process is unsuitable due to a poor mechanical and chemical stability, along with fouling issues. [9] It is crucial to find other potential material candidates to tackle these problems.Graphene derivatives are emerging candidates for efficient water treatment membranes, attributed to their unique nanochannel network [10] as well as the robust chemical and mechanical stability; [11][12][13] these chemical and physical stability are originated from their graphitic structure. [14,15] In particular, graphene oxide (GO) is regarded as a versatile platform for separating ions or contaminants due to its tunability and scalability. Furthermore, the distinctive nanochannel of the GO membranes formed by vacuum-filtration allows the GO membrane not only to block unwanted solutes, but also to achieve a high water permeability. However, it is extremely difficult to maintain the rejection performance of the GO membrane in a long term since the oxygen functional groups interact with water molecule via hydrogen bonding, resulting in membrane swelling in aqueous solution. For instance, Zheng et al. found that the d-spacing of a GO membrane significantly increased in an aqueous solution compared to its dry state-swelling up to 7 nm at equilibrium-while limiting the mobility of water molecules. [16] Additionally, to explain and predict the separation performance of multilayered GO membrane, several research groups have studied the water flow through such graphene-based layers and the influence of the degree of oxidation using molecular simulations. [17][18][19][20][21][22][23] They observed a swelling effect depending on the oxygen functional groups and an irregular structure of water molecules when passing through GO nanochannels.In order to effectively suppress the permeation of ions through the membrane for desalination, it is necessary not only to design the GO nanopathway more precisely, but also to maintain the fine nanochannels over a long period of time.Graphene-based membranes are a promising candidate for separating pollutants and ions. In particular, graphene oxide (GO) membranes are widely studied due to their unique nanochannels. The characteristic nanochannels of GO membranes can be manipulated via intercalation of cations, inhibiting the transport of other ions in the diffusion process. To maintain the tailored nanochannel during a pressure-assisted filtration procedure, it is essential to retain such inserted cations. Here, dibenzo-18-Crown-6 molecules (DB18C6) tightly binding to potassium ions are intercalated into G...

No abstract

Layered-stacked graphene oxide (GO) membranes have shown ultrafast water permeability and ion selectivity by virtue of well-defined nanochannels and intrinsic surface charge properties; however, accumulating water molecules in GO capillaries...

Reverse electrodialysis (RED) directly harvests renewable energy from salinity gradients, and the achievable potential power heavily relies on the ion exchange membranes. Graphene oxides (GOs) are considered a solid candidate for the RED membrane because the laminated GO nanochannels with charged functional groups provide an excellent ionic selectivity and conductivity. Yet, a high internal resistance and poor stability in aqueous solutions limit the RED performance. Here, we develop a RED membrane that concurrently achieves high ion permeability and stable operation based on epoxy-confined GO nanochannels with asymmetric structures. The membrane is fabricated by reacting epoxy-wrapped GO membranes with ethylene diamine via vapor diffusion, overcoming the swelling properties in aqueous solutions. More importantly, the resultant membrane exhibits asymmetric GO nanochannels in terms of both channel geometry and electrostatic surface charges, leading to the rectified ion transport behavior. The demonstrated GO membrane exhibits the RED performance up to 5.32 W·m–2 with >40% energy conversion efficiency across a 50-fold salinity gradient and 20.3 W·m–2 across a 500-fold salinity gradient. Planck–Nernst continuum models coupled to molecular dynamics simulations rationalize the improved RED performance in terms of the asymmetric ionic concentration gradient within the GO nanochannel and the ionic resistance. The multiscale model also provides the design guidelines for ionic diode-type membranes configuring the optimum surface charge density and ionic diffusivity for efficient osmotic energy harvesting. The synthesized asymmetric nanochannels and their RED performance demonstrate the nanoscale tailoring of the membrane properties, establishing the potentials for 2D material-based asymmetric membranes.

Two-dimensional (2D) membranes enable ion-sieving through well-defined subnanoscale channels. In particular, graphene oxide (GO), a representative 2D material with a flexible structure, can be manufactured into various types of membranes, while defects such as pores and wrinkles are readily formed through self-aggregation and self-folding during membrane fabrication. Such defects provide a path for small ionic or molecular species to be easily penetrated between the layers, which deteriorates membrane performance. Here, we demonstrate the effect of shear-induced alignment with continuous agitation on GO membrane structure during pressure-assisted filtration. The shear stress exerted on the GO layers during deposition is controlled by varying the agitation rate and solution viscosity. The well-stacked 2D membrane is obtained via the facile shear-controlled process, leading to an improved salt rejection performance without additional physicochemical modifications. This simple approach can be extensively utilized to prepare the well-ordered structure of other 2D materials in various fields where the defect control is required.