Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide

 

We recently produced a new 2-D material, viz. Ti₃C₂, by selectively etching aluminium from a MAX phase Ti₃AlC₂ and labelled it MXene. MXenes represents a large family of transition metal carbides and carbonitrides, not just a single phase. MXenes allow a variety of chemical compositions and are establishing themselves as a new class of two-dimensional materials. MXenes possess good in-plane conductivity, which in combination with their rich surface chemistry makes them attractive for electrical energy storage applications. However their use in electrochemical capacitors was only recently demonstrated (Lukatskaya et.al, Science, 2013).

Here, we report on the intercalation of Li⁺, Na⁺, Mg²⁺, K⁺, NH₄⁺, and Al³⁺ ions between the 2D Ti₃C₂T_x layers. In most cases, the cations intercalated spontaneously. The intercalation of some ions, notably Al³⁺, can be promoted electrochemically. We also report on intercalation-induced high capacitance of >300 Farads per cubic centimeter (much higher than that of porous carbons) of flexible Ti₃C₂T_x paper electrodes in aqueous electrolytes. Material showed no degradation of the capacitive properties after more than 10,000 cycles. Several different electrochemical techniques were employed to understand the mechanism of charge storage, such as in-situ spectroscopy, X-ray diffraction and other techniques.