Effect of mixed surface terminations on the work function and quantum capacitance of Sc2CT2 monolayer

MXenes are two-dimensional transition metal carbides and nitrides, which are promising for supercapacitor applications. The surface of MXenes is usually covered by a mixture of O, OH, and F groups during etching. The effect of mixed termination on the quantum capacitance and work function (WF) of MXenes is investigated by first principles calculation. The WFs of Sc2C(OH)2-R are smaller than those of Sc2CO2-R and Sc2CF2-R when the OH ratio is larger than about 27 %. The quantum capacitances of Sc2C MXenes with different ratio of F, O, and OH groups (named as Sc2CF2-R, Sc2CO2-R, Sc2C(OH)2-R) are explored to predict the electrode type of materials. Negative binding energies confirm the stability of the studied mixed systems. The increasing ratio of O group can make the system have decreased TDOS near Fermi level, thus decreasing quantum capacitance. The top quantum capacitance of Sc2CO2-R and Sc2C(OH)2-R systems at negative bias increase with the increasing ratio of O/OH group. All systems have larger storage charge at negative potential, and are all potential cathode materials, especially for Sc2CO2-R and Sc2CF2-R MXene with much smaller |Qa|/|Qc| than 1 and larger Cc. The quantum capacitance under wide voltage is also considered.