Electrically conductive laser-induced graphene engraved Janus membrane for membrane distillation

Abstract

Electrically conductive membranes (ECMs) show significant potential in overcoming wetting and pore-blocking challenges in membrane distillation (MD) processes. In this study, a novel Janus ECM has been developed by laser-induced graphene (LIG) engraving technique on a polyamide-imide (PAI) membrane, complemented by a silica-infused PVDF layer on the opposite side to create a unique 'sandwich' structure. The Janus structure integrates the hydrophilicity of the LIG layer with the hydrophobicity of the PVDF layer, facilitating efficient vapor flux. The optimized membrane achieved electrical conductivity of 142 ± 1.5 S/cm, water vapor flux of 11.3 ± 0.56 LMH, and 99.77 ± 0.1 % salt rejection. Its underwater oleophobicity effectively resisted wetting by surfactants and oil-water emulsions. Furthermore, applying a cathodic potential of 3V enhanced the electrostatic interactions between organic foulants and the membrane surface, reducing fouling and achieving a minimal flux decline of 2.45 %/hr over 6 h. Experiments on mineral scaling mitigation revealed that AC electric fields outperformed DC fields. A square waveform at 5Hz frequency achieved over 99 % salt rejection with minimal flux decline for monovalent and multivalent ion solutions, attributed to enhanced electrophoretic mixing under the AC field. A long-term test with the monovalent salt solution under AC/1V/5Hz(square waveform) exhibited excellent flux with a minimum flux decline of 1.35 %/hr after 18 h of experiment with no significant change in the membrane LIG structure. The in-situ LIG engraving process and the membrane's adaptability to varying electric field applications highlight its potential for large-scale use.