Nanoscale Variations in the Electrocatalytic Activity of Layered Transition-Metal Dichalcogenides

Layered transition-metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2), have attracted considerable interest as alternatives to platinum in hydrogen evolution reaction (HER) electrocatalysis. It is generally accepted that the edge planes of 2H phase MS2 (where M = Mo or W) are catalytically active, while the basal planes are said to be "catalytically inert", which has inspired the rational design/synthesis of defect-rich nanomaterials with an abundance of exposed edge sites. The intrinsic electrochemical properties of pristine MoS2/WS2 crystals have been largely overlooked in this material-driven approach. Herein, nanometer-resolved measurements using scanning electrochemical cell microscopy (SECCM) reveal electrochemical activity at the basal plane, including spatial variations attributed to the localized folding of the surface (e.g., mechanical strain) or variations in electronic structure (e.g., defect density) throughout the crystal. Such effects are particularly evident in synthetic WS2 compared to the natural crystal of MoS2. Catalytic activity for the HER is greatly enhanced at macroscopic surface defects on both materials, measured directly where the active edge plane is exposed (e.g., crevices, holes, cracks, etc.) with single-layer sensitivity. Aging the crystals under ambient conditions (i.e., exposed to the ambient atmosphere for 30 days) substantially decreases the HER activities of MoS2 and WS2, attributable to the presence of adventitious adsorbates or surface oxidation, which particularly affects at the active edge plane. Overall, this work presents previously unseen electrochemical phenomena at TMD electrodes, highlighting how subtle changes in sample source, structure, and history can alter the catalytic activity drastically and emphasizing the care that must be taken when interpreting conventional macroscopic electrochemical data. This study further demonstrates the advantage of probe-based electrochemical mapping for establishing structure-function relationships in electromaterials science.