Depression of electro-oxidation of Ti3C2Tx MXene Joule heater by alternating current for Joule heating membrane distillation

Abstract

Membrane distillation (MD) is a promising technology with great potential to mitigate the crisis of water shortage. However, it still suffers from the low productivity due to that water vaporization on the membrane would reduce the transmembrane temperature and hence the permeate flux. Herein, an electrothermal membrane that can heat the feed solution on the membrane and hence enables the MD operation under a low electrical input was developed. This was achieved by coating a commercial PET-PTFE membrane with MXene nanosheets through a filtration method. The resultant MXene/PET-PTFE composite membrane could heat the surroundings (both air and brine) efficiently, showing a conversion efficiency of 73% at 1 W. Furthermore, it was shown that the composite membrane, while degrades rapidly in highly ionizable brine under a direct current (d.c.) input, could work stably over a long duration at high potential under high-frequency alternating current (a.c.) input. Systematic electrochemical impedance spectroscopic investigation suggested that the reciprocal vibration of ions under a high-frequency a.c. could inhibit the formation of electrical double layer, and thus depresses the electrochemical oxidation of MXene and water splitting. Furthermore, the composite membrane enabled efficient in-situ heating in a Joule heating MD (JHMD) system, delivering a permeate flux of 1.8 kg m−2h−1 at an input power of 3 W, which is 333% higher than that based on the conventional MD process. Theoretical simulation of the temperature distribution by a finite element method (FEM) confirmed a higher transmembrane temperature in the JHMD system than in the conventional MD, which is responsible for the performance enhancement.