Functionalization of MXenes [Ti3C2(=O/OH/F)X] with highly mesoporous NH2-activated biochar for permselective salty ions removal by capacitive deionization

Abstract

Capacitive deionization (CDI) is a cutting-edge solution for efficient saltwater desalination. In this study, the gas-hydrolyzed nanoflake MXenes (OMX) was hydrothermally synthesized and then coupled with amine-charged activated biochar (cAB) to form the asymmetric CDI for enhanced salty ion removal. The ratio of negatively charged –OH/=O/-F terminal group of Ti₃C₂T_X MXenes was optimized for better ion intercalation. Concurrently, the positively-charged cAB was used as an anode to eliminate the need for expensive ion-exchange membranes. The cAB structure creates a multi-porous network ideally for ion accommodation, which can retain 80 % of its original surface area after amination. Moreover, the interlayer spacing of OMX can expand from 9.3 to 13.4 Å after hydrolyzation at 120 °C. Electrochemical tests of OMX exhibits a specific capacitance of 185.0 F g⁻¹ at 5 mV/s, while the strong interaction between amine functional groups and Na₂SO₄ electrolyte enhances the capacitance of cAB up to 255 F g⁻¹ at 5 mV/s and retains 75 % of the original value at 100 mV/s. The asymmetric cAB||OMX hybrid CDI can minimize the co-ion repulsion, resulting in the enhancement of CDI efficiency. The specific electrosorption capacity of cAB||OMX can achieve 49.3 mg g⁻¹ at 5000 mg L⁻¹ NaCl and at 1.2 V. The Ragone plot of SEC and SAR in cAB||OMX highlights the minimal requirement of IEM on gas-hydrolyzed MXene hybrid CDI system, which can open the avenue to enhance the desalination performance for diverse water-energy nexus applications