Possibility of bare and functionalized niobium carbide MXenes for electrode materials of supercapacitors and field emitters

Abstract

To explore applicability of MXenes in supercapacitors and electronic devices, the quantum capacitances and work functions of Nbn+1CnZ2 (n=1, 2, 3 and 4, and Z=bare, F, O, OH, OCH3), were investigated using ab initio density functional theory. We demonstrate that both bare and functionalized niobium carbides break through the quantum capacitance limitation of graphene-based electrodes of supercapacitors. Although the functional groups substantially decrease the quantum capacitances of bare MXene sheets at positive electrodes, they have a relatively small influence at negative electrodes except for the Nb2C sheet. The theoretical quantum capacitance of bare MXene sheets at the positive electrode decreases with the increase of the layer thickness. The introduction of F and O atoms makes the work function increase and both OH and OCH3 groups lower the work function of the MXene sheets. Our work demonstrates that bare Nb2C MXene sheet is a promising material for electrodes of supercapacitors with theoretical quantum capacitances of 1828.4 and 1091.1F/g at positive and negative electrodes, respectively, and OCH3-functionalized Nbn+1Cn MXene sheets are suitable for the field emitters with an ultralow work function of 1.0eV.