X-ray Insights into Formation of -O Functional Groups on MXenes: Two-Step Dehydrogenation of Adsorbed Water.

Engineered MXene surfaces with more -O functional groups are feasible for realizing higher energy density due to their higher theoretical capacitance. However, there have been only a few explorations of this regulation mechanism. Investigating the formation source and mechanism is conducive to expanding the adjustment method from the top-down perspective. Herein, for the first time, the formation dynamics of -O functional groups on Mo 2 CT x are discovered as a two-step dehydrogenation of adsorbed water through in situ near-ambient-pressure X-ray photoelectron spectroscopy, further confirmed by ab initio molecular dynamics simulations. From this, the controllable substitution of -F functional groups with -O functional groups is achieved on Mo 2 CT x during electrochemical cycling in an aqueous electrolyte. The obtained Mo 2 CT x with rich -O groups exhibits a high capacitance of 163.2 F g -1 at 50 mV s -1 , together with excellent stability. These results offer new insights toward engineering surface functional groups of MXenes for many specific applications.