  Using Density functional theory (DFT) in conjunction with a solvation model
we have investigated the phenomenon of eletrode-electrolyte interaction at the
electrode surface and its consequences on the electrochemical properties like
the charge storage and total capacitance of doped and substituted oxygen
functionalised Ti$_{3}$C$_{2}$ supercapcitor electrode. We have studied
nitrogen doped, nitrogen substituted and molybdenum substituted Mxenes in
acidic electrolyte H$_{2}$SO$_{4}$ solution. By considering nitrogen doping at
different sites, we found that the greatest capacitance is obtained for doping
at functional sites. Our results agree well with the available experiment. We
also found that the enhancement in capacitances due to nitrogen doping is due
to amplifications in the pseudocapcitances. We propose that the primary
mechanism leading to the enhanced value of the capacitances due to nitrogen
doping is surface redox activity. The performances for substituted systems, on
the other hand, are degraded in comparison to the pristine ones. This suggests
that better storage capacities in Ti$_3$C$_{2}$O$_{2}$ electrode can be
obtained by doping only. We provide insights into the reasons behind
contrasting behaviour in doped and substituted systems and suggest ways to
further improve the capacitances in doped system.
