Intercalation of Transition Metals into MXenes: Impact on Electronic and Pseudocapacitive Properties

MXenes are two-dimensional transition metal carbides and nitrides characterized by versatile electronic and electrochemical properties. Herein, we investigate the electronic interactions between various redox-active transition metals (Ni, Co, Mn, and Zn) intercalated into the conductive Ti₃C₂T_x MXene host. Employing X-ray absorption spectroscopy (XAS) and Bader charge analysis, we reveal that the oxidation states of the intercalated ions remain unchanged upon insertion, whereas Ti atoms within the MXene layers become progressively oxidized with increasing intercalant concentration. Consequently, the electrical resistivity of the intercalated MXenes increases. Ab initio molecular dynamics (AIMD) and density functional theory (DFT) demonstrate distinct spatial arrangements and coordination environments of the intercalated cations, significantly influencing their electronic density of states and interactions with MXene surfaces. Pseudocapacitance measurements in 0.1 M NaOH show distinct behaviors: Co exhibits significant redox activity with less participation from Ti of MXene, while Ni ions show negligible oxidation state changes with predominant Ti redox involvement. Our findings reveal the complex electronic and redox behavior of transition metal-intercalated MXenes, guiding the targeted modification of 2D material properties through careful selection of intercalant species.