(503c) Electrifying Membranes: Localized Joule Heating of Laser-Induced Graphene Membranes for Desalination of Hypersaline Brines

Membrane distillation (MD) is an emerging desalination technique that can treat hypersaline brine to produce pure distillate. It is a thermally driven membrane separation process, in which the partial vapor pressure difference between the feed and the permeate sides leads to the water vapor molecules permeation through a hydrophobic microporous membrane. MD has received substantial attention for desalination of hypersaline brine (>75000 ppm), owing to the independence of mass on the feed salinity. However, MD’s widespread application is yet hindered by low energy efficiency and scaling. In this presentation, I will report a novel, single-step fabrication method to develop electroconductive graphene membranes. The surface of these membrane can be heated using Joule heating. Thus, we can apply the heat at the interface of the mass transfer. This approach promises potential improved energy efficiency and reduced scaling by mitigating concentration and temperature polarization.

The electroconductive membranes developed in our work are fabricated by laser irradiation of a polyethersulfone (PES) substrate in raster mode lasing, which converts the surface of the PES substrate into a stacked multi layer of graphene sheets, also known as laser-induced graphene (LIG). Upon formation of the LIG on the membrane surface, it was coated with a thin SiO₂ @ polydimethylsiloxane (PDMS) layer to obtain hydrophobic surface for MD application. PES-LIG/SiO₂-PDMS membranes prepared were characterized to confirm the formation of graphene and protective hydrophobic coating. Results indicate the formation of a superhydrophobic (152°) surface on the PES-LIG membrane. For surface heating with the Joule heating technique, direct (DC) and alternating current (AC) were used with different input power ranging from 0.5 to 10W. The thermal response of PES-LIG membranes was recorded with an FLIR camera to find the optimum heating parameters (power and frequency for AC) to evaluate MD performance. Surface temperatures in the range of 50-350 °C were recorded for 0.5-10 W input power. The highest average distillate flux achieved was 8.74 L.m^-2.h^-1 (10W and 60 Hz) with AC and 6.34 L.m^-2.h^-1 (10W) with DC Joule heating. For both cases, more than 99% rejection of NaCl was observed, indicating the successful performance of the electroconductive PES-LIG membranes to sustain the mass transfer interface on the MD feed side. Our observation concluded that AC would be preferable over DC as polarity switching in AC significantly reduces electrode corrosion, which is essential for the long-term operation of MD.