By Michael Klein, The University of Texas at Austin, Electrochemical Energy Laboratory and MRS Student Chapter
Water desalination lies at a critical junction of the energy and water economies. It is a vital necessity as populations grow in arid regions around the world and climate, water, and land use changes drive desertification of populated regions. However, existing commercial desalination technologies are inefficient consumers of energy, both thermal distillation techniques and existing reverse osmosis (RO) technologies. Membrane technologies hold promise, however, as next-generation membranes with high-permeability and excellent salt rejection could greatly reduce the energy requirements and capital costs of RO water desalination. In a paper in a recent issue of Nano Letters, David Cohen-Tanugi and Jeffrey Grossman of MIT perform molecular dynamics simulations to evaluate the mechanical robustness of one of these candidate membranes, nanoporous graphene (NPG).
The authors use 5 MPa as a benchmark osmotic pressure needed to be withstood by any RO membrane used to desalinate seawater and examine how a number of factors influence NPG’s ability to withstand these pressures. Among the factors examined are the pore radius, pore separation, and porosity of the NPG; the pore radius and porosity of the substrate; and the effect of wetting and grain boundaries in the NPG. Additionally the authors simulate the effect of pressure on the desalination performance of the NPG membrane.
The examination of the effect of substrate pore properties is particularly interesting as it illustrates how important design of the full membrane is—an NPG membrane could be designed nearly ideally to perform desalination, but would still fail if laid down on a substrate with pores in excess of 10 microns. The authors point out the significance of this going forward: existing porous substrates, particularly thin-film polysulfone, can have large fluctuations in pore sizes, raising the likelihood that NPG membranes on these substrates could ultimately fail in a desalination application. The results of these simulations remain to be tested by experiment, but should provide useful direction for the design of experimental NPG membranes.
[Figure: diagrams showing the effect of substrate porous radius for two NPG pore configurations (left) and the combined effects of substrate pore radius and porosity (righ) on the maximum pressure the NPG/substrate membranes can withstand. Reprinted with permission from D. Cohen-Tanugi and J.C. Grossman, Nano Lett. 14(11), 6171-6178 (2014). Copyright 2014 American Chemical Society.]