Cation Engineering on Ti ₃ C ₂ T _<i>x</i> MXene for Enhanced Supercapacitive Energy Storage

Ti3C2Tx, one kind of MXene, is the most widely investigated in supercapacitors, because of its high specific capacitance, excellent conductivity, and open two-dimensional ion-transfer channel. However, the relatively narrow variable valence state of Ti atoms in Ti3C2Tx is overlooked, which is not conducive to higher capacitance. This work reports a method of cation engineering to introduce heteroatoms with wide variable valence states into Ti3C2Tx to boost its capacitive behavior. Based on the analysis of density functional theory and solution-based thermal treatment experiment, Mo atom is screened for cation engineering. As a result, Mo atom and cluster with an optimal ratio of ∼7.5 at. % are doped into Ti3C2Tx through replacing the Ti sites and forming Mo–C bonds mostly. In addition, cation engineering introduces apparent wrinkles and ripples that would help resist the self-restacking of Ti3C2Tx. As used in the three-electrode configuration, Mo doped Ti3C2Tx exhibits a specific capacitance of 518 F g–1, greatly higher than that of 256 F g–1 of Ti3C2Tx. As used in asymmetric supercapacitors, the device delivers a high specific capacitance of 91 F g–1 based on the two electrodes. Therefore, the proposed cation engineering is an efficient method to boost the capacitive behavior of MXenes.