Syntheses and electrochemical properties of (Nb1−yTiy)4C3Tx solid solution MXenes†

MXenes are emerging as highly promising materials for pseudocapacitive applications, offering exceptionally high specific capacitance. To date, over 30 distinct MXene compositions and nearly 20 solid solutions have been synthesized in the lab. However, research predominantly centers on Ti₃C₂T_x and a handful of other single-metal MXenes, with scant attention paid to solid-solution MXenes. Herein, a series of solid solution MAX phases (Nb_1−yTi_y)₄AlC₃ and corresponding (Nb_1−yTi_y)₄C₃T_x MXenes have been synthesized. The maximum proportion of Ti that can be incorporated into the M site has been examined. We found that the rate performance of MXenes improves with increasing Ti content. Especially, (Nb_0.7Ti_0.3)₄C₃T_x and (Nb_0.6Ti_0.4)₄C₃T_x retain 62.92% and 68.38% of their capacitances, when the scan rate is increased from 5 mV s⁻¹ to 1000 mV s⁻¹, respectively, compared to Nb₄C₃T_x's 13.88% retention. (Nb_0.7Ti_0.3)₄C₃T_x shows a specific capacitance of 311.06 F g⁻¹ under a current density of 1 A g⁻¹ and good cycling stability. This research demonstrates that the electrochemical characteristics of (Nb_1−yTi_y)₄C₃T_x solid solution MXenes can be manipulated by adjusting the ratio of transition metals.