Tunable energy storage capacity of two-dimensional Ti3C2Tx modified by a facile two-step pillaring strategy for high performance supercapacitor electrodes.

This paper proposed to tailor the layer microstructures of two-dimensional Ti3C2Txvia a facile Li+ pre-pillaring and successive pillaring strategy by various cations. By ion exchange with pre-intercalated Li+, as well as "coulombic attraction" to electronegative Ti3C2Tx, various inexpensive and column-like K+, Ca2+, Mg2+, Al3+ and NH4+ cations were migrated into the Ti3C2Tx interlayers. By expanding the interlayer about 17.02%, as well as enhancing the electron density of Ti atoms and C atoms, the Al3+ intercalated MXene Ti3C2Tx sheets delivered the highest specific capacity of 175 F g-1 at 0.3 A g-1. Coupling the Ti3C2Tx-Al3+ working electrode with a platinum counter electrode and a Hg/HgO reference electrode, the single electrode (calculated from a three-electrode system) exhibited a high energy density of 87.5 W h kg-1 and a high power density (2100 W kg-1) with an ultra-long and stable cycling performance. The binding energies of the intercalated ions with hydroxyl groups, the Bader charge distribution and degree of electron localization were evaluated by density functional theory calculations to further validate the ultra-high energy storage capacity of Al3+ pillared-Ti3C2Tx in the present study.