The influence of different functional groups on quantum capacitance, electronic and optical properties of Hf2C MXene

Density functional theory was used to explore the electronic properties, optical properties, and quantum capacitance of bare Hf2C and functionalized Hf2CT2 (T = -O, -S, -Se, -F, -Cl, –OH) MXenes. The introduction of functional group improves the stability of the system because of larger binding energies and negative formation energies. The introduction of O groups makes the system undergo the transition from metal to semiconductor, while the introduction of other groups doesn’t change the metal character. All the functionalized Hf2CT2 MXenes have higher optical conductivity than Hf2C in deep UV region, and Hf2CSe2 has the strongest absorption and optical conductivity in infrared region among all the studied systems. The introduction of functional groups can effectively modulate the quantum capacitance and the type of electrode materials. Hf2CCl2, Hf2C(OH)2, Hf2C, and Hf2CF2 are potential anode electrode materials, while Hf2CO2, Hf2CS2, and Hf2CSe2 are potential cathode electrode materials. The wide voltage doesn’t change the type of electrode materials except Hf2C MXene. The work function and Bader charge are further analyzed. In addition, H2C MXene with mixed termination is further explored.