i-MXenes, a new family of 2D transition metal carbides with in-plane ordered vacancies, have shown great potential in aqueous supercapacitor (SC) applications due to their high volumetric capacitances and energy densities. However, how vacancies affect their electrochemical performance, in general, and their self-discharge (SD) behavior in particular, remains unexplored. Herein, we compare the electrochemical performance of vacancy rich, ordered Mo_1.33CT_z i-MXene to that of Mo₂CT_z (with much less vacancies) in a 1 M sulfuric acid (H₂SO₄) or 15 M of lithium bromide (LiBr) electrolyte. The Mo_1.33CT_z exhibits higher volumetric capacitances and energy densities, but at the cost of a higher SD rate. Specifically, the Mo_1.33CT_z symmetric SCs deliver an energy density as high as 25.4 mWh cm(-3) at 152.4 mW cm^-3, with 65.4% voltage retention after 10 h in 15 M LiBr. In comparison, the Mo₂CT_z symmetric SCs have a maximum energy density of 20.8 mWh cm^-3 at 124.9 mW cm^-3, with 73.1% voltage retention after 10 h in the same electrolyte. The SD rates in the H₂SO₄ electrolyte are quite rapid.