Relationship between Electric Double-Layer Structure of MXene Electrode and Its Surface Functional Groups

MXenes are emerging electrode materials intended for electric double-layer capacitors because of their large specific capacitance of more than 300 F/g. Recent advances in synthesis methods have enabled a decrease in surface functional groups and chemical control of their design, but the influence of surface functional groups on capacitive properties is still unclear. Here, we applied density functional theory combined with effective screening medium and reference interaction site model calculations to systematically investigate the atomic-scale double-layer structure of Ti₃C₂T₂ MXene electrodes depending on their terminated halogen elements. The termination with halogen atoms having larger atomic numbers (I > Br > Cl > F) increased the electric double-layer capacitance. The increased capacitance originates from the smaller valence electron numbers of the terminating atoms with lower electronegativity that facilitate the electrostatic accumulation of electrons at the electrode surface. Such a solid trend provides a basis for consideration in designing MXene surfaces with larger capacitance.