Adding a New Member to the MXene Family: Synthesis, Structure, and Electrocatalytic Activity for the Hydrogen Evolution Reaction of V₄C₃T_x

Two-dimensional transition-metal-based carbides (or nitrides), so-called MXenes, that can be derived from the three-dimensional MAX phases, have attracted considerable attention throughout the past couple of years. The particular structure together with their hydrophilic and metallic nature make them promising candidates for a plethora of applications, such as sensors, electrodes, and catalysts. Obviously, the respective chemical and physical properties are highly dependent on the chemical composition, stoichiometry, and surface structure of the MXene. Here, we introduce a new member of the MXene family, V₄C₃T_x (T representing the surface groups), based on the chemical exfoliation of the 413 MAX phase V₄AlC₃ by treatment with aqueous hydrofluoric acid. X-ray powder diffraction data together with scale-bridging electron microscopy studies prove the successful removal of aluminum from the MAX phase structure. The electrocatalytic activity for the hydrogen evolution reaction of this new MXene is tested in acidic solution over the course of 100 cycles. Interestingly, we find a significant improvement of the catalytic performance over time (i.e., the overpotential required to achieve a current density of 10 mA cm^-2 decreases by almost 200 mV) that we assign to the removal of an oxide species from the surface of the MXene, as shown by XPS measurements. Our study provides crucial experimental data of the electrocatalytic activity of MXenes together with the evolution of its surface structure that is also relevant for other transition-metal-based MXenes in the context of further potential applications.