Role of Surface Terminations for Charge Storage of Ti₃C₂T_x MXene Electrodes in Aqueous Acidic Electrolyte

In this study, we used 2-Dimmensionnal Ti₃C₂ MXene as model materials to understand how the surface groups affect their electrochemical performance. By adjusting the nature of the surface terminations (Cl-, N/O-, and O-) of Ti₃C₂ MXene through a molten salt approach, we could change the spacing between MXene layers and the level of water confinement, resulting in significant modifications of the electrochemical performance in acidic electrolyte. Using a combination of techniques including in-operando X-ray diffraction and electrochemical quartz crystal microbalance (EQCM) techniques, we found that the presence of confined water results in a drastic transition from an almost electrochemically inactive behavior for Cl-terminated Ti₃C₂ to an ideally fast pseudocapacitive signature for N,O-terminated Ti₃C₂ MXene. This experimental work not only demonstrates the strong connection between surface terminations and confined water but also reveals the importance of confined water on the charge storage mechanism and the reaction kinetics in MXene.