  Two-dimensional transition metal carbides and nitrides (MXenes) are an
emerging class of atomically-thin superconductors, whose characteristics are
highly prone to tailoring by surface functionalization. Here we explore the use
of hydrogen adatoms to enhance phonon-mediated superconductivity in MXenes,
based on first-principles calculations combined with Eliashberg theory. We
first demonstrate the stability of three different structural models of
hydrogenated Mo- and W-based MXenes. Particularly high critical temperatures of
over 30 K are obtained for hydrogenated Mo$_2$N and W$_2$N. Several mechanisms
responsible for the enhanced electron-phonon coupling are uncovered, namely (i)
hydrogen-induced changes in the phonon spectrum of the host MXene, (ii)
emerging hydrogen-based phonon modes, and (iii) charge transfer from hydrogen
to the MXene layer, boosting the density of states at the Fermi level. Finally,
we demonstrate that hydrogen adatoms are moreover able to induce
superconductivity in MXenes that are not superconducting in pristine form, such
as Nb$_2$C.
