Tailoring Ti₃C₂ MXene into Ti₃C₂T_x, T_x = NO and alloying with M = Al, Ga, In, Tl into MTi₃C₂NO as electrode materials for super-capacitor devices: Perspective from first-principles density functional theory

Theoretical calculations using a density functional theory (DFT) approach has been utilized to explore the structural, electronic, and optical properties of Ti₃C₂, Ti₃C₂T_x, T_x = NO, and its alloys with Group IIIA elements (Al, Ga, In, Tl) as electrode materials for supercapacitor devices. The results show good crystal stability and an increase in the metallic property of the functionalized/alloyed MXenes (Ti₃C₂NO, AlTi₃C₂NO, GaTi₃C₂NO, InTi₃C₂NO, and Tl Ti₃C₂NO) making them better candidates for application as electrode materials. The band structures of the studied compounds revealed a direct band interaction and a progressive increase in the overlap of the bands upon introducing the terminating group (NO) and when alloyed with Al, Ga, In, and Tl. The partial and total density of states of the studied compounds confirmed their metallic behaviour and conductivity. The spin-polarized band structures and spin-polarized DOS of the compounds studied have revealed their antiferromagnetic properties, while TlTi₃C₂NO showed an interesting non-collinear spin-polarized structure, which is indicative of a great Nerst effect. Our density of states calculations demonstrates a good interaction was formed in the valence orbitals of the studied compounds with a Fermi level of 0 eV, which is indicative of a good conductor and a metallic character. The quantum capacitance of the studied compounds (MXenes) revealed an improved QC upon surface functionalization and alloying with the obtained MXenes QC values as follows: Ti₃C₂ = 97.5 μF/cm², AlTi₃C₂NO = 141.8 μF/cm², InTi₃C₂NO = 142.5 μF/cm², and TlTi₃C₂NO = 100.5 μF/cm², which are good QC values for potential electrode materials, while GaTi₃C₂NO = 7.5 μF/cm². Our optical studies revealed the good interaction between the MXenes and electromagnetic waves. The MXenes exhibit good dielectric permeability, as the real parts Ꜫ_r(ω) of the dielectric function is ≥ 19. The imaginary parts Ꜫ_z(ω) of the dielectric function revealed peaks with intensity >100, which are indicative of inter-band transition.