  MXene transition-metal carbides and nitrides are of growing interest for
energy storage applications. These compounds are especially promising for use
as pseudocapacitive electrodes due to their ability to convert energy
electrochemically at fast rates. Using voltage-dependent cluster expansion
models, we predict the charge storage performance of MXene pseudocapacitors for
a range of electrode compositions. $M_3C_2O_2$ electrodes based on group-VI
transition metals have up to 80% larger areal energy densities than
prototypical titanium-based ( e.g. $Ti_3C_2O_2$) MXene electrodes. We attribute
this high pseudocapacitance to the Faradaic voltage windows of group-VI MXene
electrodes, which are predicted to be 1.2 to 1.8 times larger than those of
titanium-based MXenes. The size of the pseudocapacitive voltage window
increases with the range of oxidation states that is accessible to the MXene
transition metals. By similar mechanisms, the presence of multiple ions in the
solvent (Li$^+$ and H$^+$) leads to sharp changes in the transition-metal
oxidation states and can significantly increase the charge capacity of MXene
pseudocapacitors.
