Synthesis and characterization of carbonitride MXenes and their electrochemical performance
The family of two-dimensional transition metal carbides/nitrides/carbonitrides, MXenes, has been growing rapidly since the discovery of first MXene in 2011. They are typically produced by selectively etching atomically thin metal layers from layered ceramics called MAX phases, which have a composition of Mn+1AXn; M is an early transition metal, A is an element from group IIIA or IVA, X is either C and/or N and n = 1-4. This family now has more than 55 members, most of which are carbides with very few nitrides and carbonitrides. Despite Ti3CNTx (Tx stands for mixed surface terminations of O, OH and F) exhibiting unique properties and behavior compared to its carbide counterpart in applications including energy storage, electromagnetic shielding, optics, and sensing, it had been the only carbonitride MXene for more than 10 years until recently, when we reported on Ti2C0.5N0.5Tx. This is, in part, because of the challenging synthesis due to the lower stability of nitride MXenes compared to carbides. Herein, I report on the synthesis, characterization and applications of titanium carbonitride MXenes series from MAX phases with different n, viz. n =1, 2, and 3. By tuning the synthesis conditions using potassium fluoride and hydrochloric acid, we realized Ti2C0.5N0.5Tx MXene from Ti2AlC0.5N0.5. By adjusting this etching technique for other carbonitrides MAX phases, I produced a total of seven new MXenes, viz. Ti2C0.25N0.75Tx, Ti2C0.75N0.25Tx, Ti4C0.8N2.2Tx, Ti4C0.6N2.4Tx, Ti4C0.4N2.6Tx, and Ti4C0.2N2.8Tx. These carbonitrides were characterized using various microscopy and spectroscopy techniques, and their electrochemical performance as electrode materials for energy storage and conversion was investigated. When tested as electrode material for sodium ion batteries, Ti2C0.5N0.5Tx exhibited the highest specific capacity of 182 mAh g−1 at 20 mA g−1 among all reported multilayer MXenes. Carbonitride MXenes outperformed their carbide counterparts when tested as electrocatalyst for hydrogen evolution reaction. For example, a promising low onset overpotential of 56 mV was measured for Ti3CNTx compared to 649 mV for Ti3C2Tx.