Adjustable electrochemical properties of solid-solution MXenes

MXenes are promising pseudocapacitive materials with ultrahigh specific capacitance. Currently, more than 30 stoichiometric MXene compositions and about 20 solid solutions have been experimentally synthesized. However, most studies focus on Ti3C2Tx or a few other single-M MXenes, and little is known about the electrochemical properties of solid-solution MXenes. Herein, two sets of niobium-based solid-solution MXenes (Ti(2-y)NbyT(x) and V2-yNbyTx; 0 <= y <= 2) were synthesized and the dependence of their electrochemical properties on the ratio of M elements in the structure was investigated. Relationships between the chemistry and charge storage ability, including capacitive properties and cycling stability in aqueous protic electrolyte, were determined. There is an inverse relationship between the prominence of the redox peaks and cycling stability; the latter increases with the niobium content. For instance, the capacitance retention after 20,000 cycles is less than 1% for Ti2CTx, but 78% for Ti0.4Nb1.6CTx. This study shows that electrochemical properties of MXenes can be controlled by tuning the ratio of transition metals in the MXene structure.