Phase engineering of two-dimensional transition-metal dichalcogenides (TMDs) has been the subject of considerable interest as it represents a promising strategy for a highly active hydrogen evolution reaction (HER). However, various types of active sites on the basal planes and edges of TMDs have shown complicated mechanisms of the HER in TMDs, hindering the systematic engineering of the catalytic activity of TMDs. Here, we report the intrinsic basal-plane activity of a series of TMDs, Mo1−xWxTe2, whose phases can be engineered from semiconducting to metallic states by adjusting the stoichiometric ratio of tungsten atoms (x). Three forms of 2H- (semiconducting) and 1T′-(metallic) Mo1−xWxTe2, bulk, powder, and exfoliated flakes, as well as microreactors, were used to investigate the HER process of the phase-engineered TMDs. The catalytic activity of Mo1−xWxTe2 exhibits the best performance at the phase-transition boundary (i.e., x = 0.09) with a hydrogen conversion rate of 0.692 s−1, which is 10–20 times higher than that of other 2H and 1T′ samples with different x values. Our study provides a novel approach, using the phase-transition boundary, to modify the catalytic activity of polymorphic nanomaterials.

Hydrogen evolution reaction (HER) is a promising clean and sustainable energy source with zero carbon emissions. Numerous studies have been conducted with versatile low dimensional materials, and the development of highly active electrochemical catalysts for HER is one of the most important applications of the materials in these studies. Despite such extensive research, the physical origin of the active catalytic performance of low dimensional materials remains unclear, and is distinguished from that of classical transition metal‐based catalysts. Here, recent studies on the intrinsic catalytic activity of 2D semimetals are reviewed, particularly among transition metal dichalcogenides (TMDs), highlighting promising strategies for the design of materials to further enhance their catalytic performance. One attractive approach for active HER involves fabricating single‐atom catalysts in the framework of TMDs. The electrochemical reaction at a catalytic atom for hydrogen evolution has typically been described by the Sabatier principle. Recent studies have focused on optimizing the Gibbs free energy for hydrogen adsorption via down‐sizing, alloying, hybridizing, hetero‐structuring, and phase boundary engineering, mostly with TMDs. The unique advantages of TMDs and their derivatives for HER are summarized, suggesting promising research directions for the design of low dimensional electrochemical catalysts for efficient HER and their energy applications.