Effect of vacancies on the electrochemical behavior of Mo-based MXenes in aqueous supercapacitors

Abstract

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 Mo1.33CTzi-MXene to that of Mo2CTz (with much less vacancies) in a 1 M sulfuric acid (H2SO4) or 15 M of lithium bromide (LiBr) electrolyte. The Mo1.33CTz exhibits higher volumetric capacitances and energy densities, but at the cost of a higher SD rate. Specifically, the Mo1.33CTz 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 Mo2CTz 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.