Layered transition-metal dichalcogenides have very interesting electrochemical properties which depend on their elemental composition, crystal structure, size, and defects. Their electrochemistry is strongly anisotropic and largely driven by the edge sites. These edge sites display rapid electron transfer kinetics toward various redox probes, such as Fe(CN)₆^4-/^3- and Ru(NH₃)₆²⁺/³⁺; specifically, MoS₂ has been demonstrated to be electrochemically active for the detection of multiple biomarkers. More recently, there is also increasing knowledge on their inherent electrochemistry, whereby the metals can undergo electrochemical reduction or be oxidized to higher valence states, both of which alter their properties. Such reductive or oxidative pretreatments can tailor the electrochemical activity of TMDs toward the oxidation/reduction of electrochemically active substances in solution. Significant efforts have also been carried out to create catalytic systems for hydrogen evolution with the eventual aim of replacing platinum catalysts for such reactions. We discussed various strategies for activation and improving HER on TMDs, including maximizing the exposure of active sites, fabricating TMD hybrids on a catalyst support, and doping TMDs. Given the fact that TMDs are highly versatile materials in terms of chemical composition and crystal structure, one can expect that efforts toward further enhancing the electrochemical properties will lie in this direction. One can envision that doping and chemical modifications of the surface/edges of TMDs will lead to its tailored properties for further applications. In summary, we believe that the core of future development in the electrochemistry of TMDs lies with achieving greater control over their chemical and physical properties, such as (1) exploiting their unique anisotropic structures for electrochemical sensing and catalysis, (2) improved control of transitions between their semiconducting and metallic phases as both phases hold promise for very distinct types of applications, and (3) achieving a better understanding of inherent electrochemical properties of TMDs such that we may limit their negative effects or even go further to manipulate them toward catalytic applications such as the HER.

http://pubs.acs.org/doi/full/10.1021/acs.chemrev.5b00287