Mixed MXenes: Mo1.33CTz and Ti3C2Tz freestanding composite films for energy storage

MXenes are a class of 2D materials with outstanding properties, including high electronic conductivity, hydrophilicity, and high specific capacitance. In particular, Mo1.33CTz MXene has a high specific capacitance, whereas films of Ti3C2Tz MXene possess high flexibility and high electronic conductivity. The fabrication of composite materials based on these two MXenes is therefore motivated, taking advantage of combining their good properties. In this article, we introduce a one-step approach to prepare composite MXene films using pristine Mo1.33CTz and Ti3C2Tz MXenes. The composite films display superior flexibility and electronic conductivity, as well as high capacitance, up to 1380 F cm(-3) (460 F g(-1)), in 1 M H2SO4. A capacitance retention of 96% is obtained after 17,000 cycles. In addition, the capacitance retentions are about 56% and 25% at scan rates of 200 mV s(-1) and 1000 mV s(-1), respectively. A significant rise in the capacitance at high rates, 875 F cm(-3) (282 F g(-1)) at a current density of 20 A g(-1), is achieved by using a 3 M H2SO4 solution. The use of composite MXene as negative electrodes for asymmetric supercapacitor devices, as well as lithium-ion batteries, is also discussed. This work suggests new pathways for the use of MXene composites with double transition metals (Mo and Ti) in energy storage devices.