MXene-Based Quantum Dots Optimize Hydrogen Production via Spontaneous Evolution of Cl- to O-Terminated Surface Groups

MXene quantum dots (MQDs) offer wide applications owing to the abundant surface chemistry, tunable energy-level structure, and unique properties. However, the application of MQDs in electrochemical energy conversion, including hydrogen evolution reaction (HER), remains to be realized, as it remains a challenge to precisely control the types of surface groups and tune the structure of energy levels in MQDs, owing to the high surface energy-induced strong agglomeration in post-processing. Consequently, the determination of the exact catalytically active sites and processes involved in such an electrocatalysis is challenging because of the complexity of the synthetic process and reaction conditions. Herein, we demonstrated the spontaneous evolution of the surface groups of the Ti2CTx MQDs (x: the content of O atom), i.e., replacement of the -Cl functional groups by O-terminated ones during the cathode reaction. This process resulted in a low Gibbs free energy (0.26 eV) in HER. Our steady Ti2COx/Cu2O/Cu foam systems exhibited a low overpotential of 175 mV at 10 mA cm(-2) in 1 M aq. KOH, and excellent operational stability over 165 h at a constant current density of -10 mA cm(-2).