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. To reduce this overpotential, modifications must be made to the structure through mechanistic studies to increase the reaction rate of the proton/electron coupled transfer steps. Here, we use
in-situ/ operandoRaman spectroelectrochemistry to probe the HER mechanism and environmental changes of the Ti3C2 electrocatalyst during HER. In acidic media, we observed a shift in the termination group from O to OH, consistent with a protonation step followed by a deprotonation step to form hydrogen gas. This mechanism was further corroborated by density functional theory (DFT) calculations. DFT calculations show that protonation of the O functional groups followed by binding of a Ti OH* with a TiH* is the rate determining step, resulting in a large HER overpotential. In neutral media, we observe an overcharging of the structure allowing for the adsorption of water prior to the HER process, which is rate limiting and causes a large HER overpotential. Through DFT analyses, we found that the reduction process in both electrolytic systems substantially increases the TiC bond strength for the outer Ti atoms accompanied by a shift of the Ti d-band center away from the Fermi level indicating a much lower reactivity during HER. By combining the experimental and computational results, reaction mechanisms were found for both electrolytic systems. These fundamental insights enable for proper modifications to be made for MXene catalysts to enhance their electrocatalytic performance.