The hydrogen evolution reaction (HER) is an important energy conversion process that underpins many clean energy technologies including water splitting. Herein, we report for the first time the application of two-dimensional (2D) layered transition metal carbides, MXenes, as electrocatalysts for the HER. Our computational screening study of 2D layered M 2 XT x (M = metal; X = (C, N); and T x = surface functional groups) predicts Mo 2 CT x to be an active catalyst candidate for the HER. We synthesized both Mo 2 CT x and Ti 2 CT x MXenes, and in agreement with our theoretical predictions, Mo 2 CT x was found to exhibit far higher HER activity than Ti 2 CT x . Theory suggests that the basal planes of Mo 2 CT x are catalytically active toward the HER, unlike in the case of widely studied MoS 2 , in which only the edge sites of the 2H phase are active. This work paves the way for the development of novel 2D layered materials that can be applied in a multitude of other clean energy reactions for a sustainable energy future.

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Hydrogen evolution reaction (HER) via electrocatalysis is one method of enabling sustainable production of molecular hydrogen as a clean and promising energy carrier. Previous theoretical and experimental results have shown that some two-dimensional (2D) transition metal carbides (MXenes) can be effective electrocatalysts for the HER, based on the assumption that they are functionalized entirely with oxygen or hydroxyl groups on the basal plane. However, it is known that MXenes can contain other basal plane functionalities, e.g., fluorine, due to the synthesis process, yet the influence of fluorine termination on their HER activity remains unexplored. In this paper, we investigate the role and effect of basal plane functionalization (T x ) on the HER activity of 5 different MXenes using a combination of experimental and theoretical approaches. We first studied Ti 3 C 2 T x produced by different fluorinecontaining etchants and found that those with higher fluorine coverage on the basal plane exhibited lower HER activity. We then controllably prepared Mo 2 CT x with very low basal plane fluorine coverage, achieving a geometric current density of −10 mA cm −2 at 189 mV overpotential in acid. More importantly, our results indicate that the oxygen groups on the basal planes of Mo 2 CT x are catalytically active toward the HER, unlike in the case of widely studied 2H-phase transition metal dichalcogenides such as MoS 2 , in which only the edge sites are active. These results pave the way for the rational design of 2D materials for either the HER, when minimal overpotential is desired, or for energy storage, when maximum voltage window is needed.