Surface functionalization effect on physical properties and quantum capacitance of Ca2C MXenes

Effects of surface terminal groups on the structural, mechanical, and electronic properties, as well as the quantum capacitance of Ca2CT2 (T = F–, O–, Cl–, OH–) MXenes are studied by first-principles electronic structure computations. Ca2C(OH)2 is determined to be the most stable structure compared with Ca2C and Ca2CT2 (T = F–, Cl–) MXenes, while Ca2CO2 is mechanically unstable. The surface terminations have a strong influence on the work function of MXene as they can alter the Fermi level and the associated electron transfer. Analyzing the atom projected density of states shows the existence of localized electron states at and around the Fermi level, generating a high charge density close to the Fermi level and resulting in relatively high quantum capacitance. The quantum capacitance of Ca2CCl2 is the highest (152 μF/cm2) among the studied cases, while Ca2C(OH)2 has the lowest quantum capacitance. The observed variations in quantum capacitance are mainly attributed to the creation and annihilation of new electronic states and the shift of Fermi level in the studied MXenes. Also, presence of surface terminations of Ca2C MXenes considerably changes the electrode quantum capacitance and Ca2CCl2 is the most promising one among the studied cases.