Superior Thermoelectric Performance of Ordered Double Transition Metal MXenes: CrTiCT (T = -OH or -F).

Using SCAN-rVV10+U, we show CrTiC and CrTiCT (T = -F and -OH) MXenes are moderate band gap semiconductors mostly in the antiferromagnetic state. All investigated MXene structures show large Seebeck coefficients (>400 μV/K), especially CrTiC (>800 μV/K) and CrTiCF (>700 μV/K). The hole relaxation time of p-type CrTiC(OH) is found to be ∼8 ps, ensuring its superior electron transport properties in comparison to other investigated MXenes. It is also discovered that the surface functionalization could decrease the phonon thermal conduction and that CrTiC(OH) has the smallest lattice thermal conductivity (∼6.5 W/m·K) and the largest electron thermal conduction (>50 W/m·K with = 10 cm). We predict the ZT value of p-type CrTiC(OH) can reach 3.0 at 600 K with the maximum thermoelectric conversion efficiency of 20%. Overall, the thermoelectric property of Cr-based ordered double transition metal MXenes is far superior to that of any known two-dimensional structures in the MXene family.