The Fabrication Of High-throughput Graphene Oxide Membrane And Its Osmotic Energy Conversion

To address the challenge of global energy crisis and environmental deterioration,renewable energies have become an urgent demand for the sustainable development of human society.Salinity difference energy stored as the form of salinity difference between seawater and freshwater,is an important renewable clean energy.Reverse electrodialysis(RED)is a conventional method for the harvesting of salinity difference energy,but its application has been limited by the performance of ion selective membranes for the past half century.The recent rise of 2D materials provide an encouraging solution because the fast ion transport was observed in the interstitial space between restacked 2D nanosheets combined with high ion selectivity.Thus,the 2D materials,such asgraphene oxide(GO)and Mxene,become an important candidate of the high-throughput membranes,due to the abundant surface charge and extremely thin lamella thickness.Up to now,the factors affecting the transport properties of graphene oxide membrane,including the interlayer spacing and the physical and chemical properties of the flake surface,have been extensively studied.However,the orientation of 2D lamella in membranes is still a long-overlooked element in the existing literatures.In this paper,graphene oxide sheets(GO)are prepared by the modified Hummers,and the chemical modification method is used to regulate the surface charge state of the nanosheets.By vacuum filtration,the horizontally stacked graphene oxide membrane(H-GOM)was prepared.Afterward,the process of encapsulation,dicing and thinning was employed to fabricate vertically oriented graphene oxide membrane(V-GOM).Through these two systems,the influence of the orientation of 2D lamella towards the performance of 2D porous membranes is identified.The differences of ion transport and osmotic energy conversion between H-GOM and V-GOM were compared.Benefit from the vertically-oriented unidirectional layers,the ions have ultra-fast permeation in V-GOM,with three orders of magnitude higher than that in H-GOM.Simultaneously,the V-GOM and H-GOM have the similar charge selectivity.Thus,the orientation of 2D nanosheet plays a crucial role in high ion permeation.Under the salt energy conversion between artificial river water and seawater,the V-GOM could achieve an extremely high output power density of 10.6 W/m2,exceeding the critical value of 5.0 W/m2 for industrial development requirement.In addition,the power density can be further enhanced by optimizing the membrane thickness,pH value,temperature,and electrolyte solution.From the theoretical aspect,the ion transport in V-GOM and H-GOM was analyzed under the continuum framework,composed of the Navier-Stokes(NS)equations and the Poisson-Nernst-Planck(PNP)equations.The theoretical analysis and simulation results reveal the mechanism of the ultrafast transport in V-GOM:extremely short ion path,low barrier to enter membrane and larger inlet area.These findings provide inspiration for the design of high-performance nanoporous membrane.