Ultraefficient Li+/Mg2+ separation with MXene/CNT membranes under electric field assistance

Similar physicochemical properties render the separation of Li+/Mg2+ from salt lake brine difficult for two-dimensional (2D) membranes. Herein, ultraefficient Li+/Mg2+ separation was achieved under the influence of an electric field using MXene/carbon nanotube (CNT) membranes developed on tubular ceramic substrates via thermal crosslinking. Crosslinking reactions between CNTs and MXene yielded well–intercalated CNTs between MXene nanosheets, forming antiswelling MXene/CNT membranes (MCMs) (interlayer spacing: 8.04 Å) suitable for Li+/Mg2+ separation. At 2.8 V applied voltage, the MCMs exhibited Li+ flux of 0.0491 mol m−2h−1 and high Li+/Mg2+ selectivity of 54.5, which outperform existing state–of–the–art 2D membranes. Specifically, under a positive electric field, Li+ with a small hydration diameter and low hydration energy permeated through the membrane nanochannels more easily than Mg2+, increasing and decreasing Li+ and Mg2+ flux, respectively. Additionally, Mg2+ retention was enhanced via the complexation of enriched Mg2+ with –OH and –COOH on the membrane surface. The MXene membrane surface was thus reconfigured to be positively charged, inducing effective Mg2+ rejection. This study offers a novel and practical approach for efficiently separating Li+/Mg2+ as well as monovalent/multivalent ions.