MXenes have shown great promise as electrocatalysts for the hydrogen evolution reaction (HER), but their mechanism is still poorly understood. Currently, the benchmark Ti3C2 MXene suffers from a large overpotential....

Current climate issues can be partially remedied through the inclusion of renewable energy sources. However, these energy sources suffer from the need for highly efficient energy storage systems. To this end, studies have been conducted on developing energy storage materials that can provide high energy and power densities. Two-dimensional (2D) carbide and nitride MXenes have the potential to provide both if their mechanism of charge storage is understood. Here, we use in situ/operando Raman spectroelectrochemistry to investigate in real time the charge storage mechanism of the benchmark Ti 3 C 2 MXene in acidic and neutral media. We found that during charge/discharge cycling in acidic media, protons are pulled towards the surface, leading to a reversible reaction with the surface termination groups leading to a switch from À OÀ to À O(OH) to À OÀ chemistry, indicative of a pseudocapacitive mechanism. In neutral media, no pseudocapacitive behavior is observed and it is found that the material exhibits an electrostatic double layer charge storage mechanism through attracting sodium ions towards the surface for transient adsorption processes. Taking into consideration the mechanism we propose, the Ti 3 C 2 MXene can exhibit a pseudocapacitance of 358 F/g within a voltage window of 1.35 V. Ultimately, these fundamental insights can be used to design electrode materials with both high energy and power densities.

Two-dimensional (2D) carbide and nitride MXenes possess properties that are desirable for a broad range of electrocatalytic applications including the hydrogen evolution reaction (HER). These properties include high surface area, hydrophilicity, heterogeneity of redox-active transition metals, and tunable surface functionalities allowing for low HER overpotentials. In this paper, we report on the cathodic etching and À O/À OH functionalization of hybrid Ti 3 CN upon the application of an external potential for improved HER performance and show that the active sites for HER on this MXene catalyst are located primarily on the À OÀ and À OH functional groups. The overpotential for the hybrid Ti 3 CN improves by 350 mV upon in-situ À O/À OH functionalization and etching, reaching À 0.46 V vs. RHE at a current density of 10 mA cm À 2 , much lower than those reported for the benchmark Ti 3 C 2 carbide MXene. These results provide a path forward to tuning the electrocatalytic activity of MXenes and related electrocatalysts for water splitting.