Water shortages are a growing challenge for coastal and inland regions, with high salinity brines (e.g., produced water or reverse osmosis waste brines) presenting a particularly challenging problem. The high-salinity water requires the use of thermally driven treatment processes (e.g., membrane distillation (MD)) which suffer from high complexity and cost. Herein, we demonstrate that frequency control of applied alternating current at high potentials (20 Vpp) to a porous thin-film carbon nanotube (CNT)/polymer composite Joule heating element can prevent CNT degradation in ionizable environments (e.g., high salinity brines). We demonstrate via electrical impedance spectroscopy that the electrooxidation of CNTs is a strong function of the applied frequency and ion concentrations in the CNT’s immediate environment. The application of high frequencies (&gt;100 Hz) leads to electrons preferentially charging the electrical double layer (EDL), while at low frequencies electrons in the fully charged EDL can participate in CNT-degrading electrochemical reactions. Operation at sufficiently high frequencies allows for direct immersion and operation of these porous thin-films in highly-ionizable environments and their use as flow-through heating elements. Furthermore, we demonstrate that the porous CNT/polymer composites can be used as a self-heating membrane for desalination making it possible to achieve exceptionally high single-pass recoveries in MD that approach 100%, far exceeding the classical MD theoretical limit of 6.5%.

Porous CNT Joule heaters in ionizable environments and their use in desalination: Frequency dependent stability and application in MD

253rd National Meeting (2017)

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