  The recently discovered surface-group-dependent superconductivity in
Nb$_2$C-MXene fabricated by the molten salts method is attracting wide
attention. However, regarding the superconductivity of Nb$_2$C-MXene with
functional F groups (Nb$_2$CF$_x$), there were some conflicting results in
experimental and theoretical studies. Herein, we systematically carried out
experimental and theoretical investigations on the superconductivity in
Nb$_2$C-MXene with the Cl functional group (Nb$_2$CCl$_x$) and Nb$_2$CF$_x$.
The experimental results of the Meissner effect and zero resistivity have
proved that Nb$_2$CClx is superconducting with the transition temperature (Tc)
~ 5.2 K. We extract its superconducting parameters from the temperature
dependence of resistivity and the field dependence of the magnetization. The
Ginzburg-Landau parameter K$_G$$_L$ is estimated to be 2.41, indicating that
Nb$_2$CClx is a typical type-II superconductor. Conversely, both magnetic and
electrical transport measurements demonstrate that Nb$_2$CF$_x$ is not
superconducting. The first-principles density functional theory (DFT)
calculations show that the Tc of Nb$_2$Cl$_x$ is ~ 5.2 K, while Nb$_2$CF$_x$ is
dynamically unstable with imaginary frequency in phonon spectrum, which is in
good agreement with the experimental results. Our studies not only are useful
for clarifying the present inconsistency but also offer referential
significance for future investigations on the superconductivity of MXenes.
