(340a) Janus-like Meso-Porous Hybrid Frameworks for Super-Efficient and Cost-Competitive Water Desalination By Membrane Evaporation

In contrast to other desalination techniques such as nanofiltration and reverse osmosis, ME offers a potentially low energy and high rejection route to the desalination of highly dirty or salty waters. However, to become competitive with other desalination technologies, specific membrane materials must be developed and tailored to allow for high flux, low fouling tendency and long term stability.

Specifically, in order to achieve efficient vapour transport from the hot to the cold side, a ME membrane must be highly porous, as thin as possible and not wettable by the process liquids. The pores need therefore to be large enough to facilitate vapour transport, while having sufficiently small dimensions to avoid liquid wetting and formation of a direct liquid bridge between the feed and the permeate sides. In addition, the membrane material must be heat conductive to allow for direct heating and evaporation of water in direct contact with the material surface. Most previous studies have investigated low porosity membrane materials formed from sintered stainless steel powders. While these materials are highly hydrophobic, they are also expensive and difficult to process. It is consequently important to investigate other alternatives as well as techniques for improving the process efficiency by modifying the membrane properties and structure.

Here, novel metal based scaffolds were decorated for the first time with ultra-thin layers of graphene materials by a direct electroplating. The micron-sized pores across the metal scaffolds were found by micro-Raman analysis to be entirely covred with controllable thicknesses of graphene, which latter was used for seeding specific functionalization and provide a strong interface as well as a corrosion protection barrier against chloride anions present in water. Both sides of the membranes were then specifically modified to alter the water evaporation and vapour diffusion and condensation characteristics. A Janus structure was prodcued by (i) grafting on one side of the membrane fluorosilane chains, used to generate a super-hydrophobic interface, while (ii) the second side of the membrane was coated with a nanoscale but yet thermally insulating and semi-hydrophilic alumina layer by Atomic Layer Deposition (ALD). The macro-properties of the materials, including their wettability, thermal conductivity, mechanical strength, as well as their surface charge and morphologies were systematically investigated and correlated to the electroplating conditions. The performance of the membranes for desalting model 3.5 wt% NaCl solutions were also linked to process parameters, including the liquid velocity within the module and the temperature difference between both sides of the membranes, as well as to the materials properties, fluorosilane functional groups densities and morphology of the alumina layer deposited.

In short, the membranes were found to exhibit Janus behaviours in nearly all aspects of their behaviours. The water contact angle values ranged from 160^o to 60^o, while the thermal conductivity of the alumina-coated side was found to be up to 25 times lower than that of the fluoro-sialne grafter graphene side. In addition, >99.99% desalination performance were achieved for flux as high as 145 LMH for a feed to permeate temperature difference of only 50^oC. The hybrid Janus membranes were also found to be extremely stable and desalination tests for up to 5 consecutive days were performed without loss of performance. This work demonstrates the feasabiity of hybrid materials with controlled porosities and pore distributions for the competitive desalination of brines.