The effect of D-spacing on the ion selectivity performance of MXene membrane

2-D materials with nanofluidic channels have gained significant attention for their potential as an ion separation membrane. However, the fundamental understanding of the interactions between nanochannel sizes and ion selectivity and conductivity remains complex as experimentally controlling the free interlayer spacing in sub-nanometer scales is challenging. Herein, we utilize molecular dynamic (MD) simulations to tailor the free interlayer spacing between a model 2-D MXene membrane to understand their effects on ion transport behaviour. As a validation, the free interlayer spacing of the MXene nanosheets was altered by impregnating different type of ions, which is then used in an electrically driven ion separation system. The simulation result shows that as the free interlayer spacing increases from below to above 6 Å, the selectivity of monovalent Li⁺ and K⁺ compared to Mg²⁺ decreases due to the reduced entrance energy barrier for Mg²⁺; however, higher overall ionic conductivity can be achieved. The experimental data using a membrane with free interlayer spacing between 6 and 7 Å agrees well with the simulation study. The difference in the ion permeation of H⁺, K⁺, Na⁺, Li⁺, Ca²⁺, and Mg²⁺ was not only attributed to the nanochannel size but also considering the degree of ion dehydration and ions interactions to the –O binding site of the membrane. Further investigations demonstrated that ion transport mechanism through MXene nanochannels followed the surface-charge-governed behaviour in HCl and KCl solutions at different concentrations, as evident from significantly higher ionic and proton conductivity at low concentrations (<10⁻³ M) compared to the bulk solutions. This work leads to a better understanding of 2-D nanochannel design in ion transport applications.