Following defect passivation in molybdenum disulfide (MoS2) electrocatalysts using identical-location nanoscale electrochemical microscopy.

Molybdenum disulfide (MoS2) is a promising hydrogen evolution reaction (HER) electrocatalyst whose activity is strongly influenced by structural defects such as edge sites and sulfur vacancies. However, directly probing the contribution of these defect populations remains challenging because conventional electrochemical techniques average over large areas, masking the influence of individual defect-rich regions. Herein, identical-location scanning electrochemical cell microscopy (SECCM) is used to map HER activity across individual monolayer 2H-MoS2 nanocrystals before and after sequential passivation with 3-octylthiophene (3-OT) and bis(trifluoromethanesulfonyl)imide (HTFSI). Comparison of low-pressure and atmospheric-pressure chemical vapour deposition-grown MoS2 reveals substantial differences in morphology and HER activity, consistent with differences in defect populations. Successive identical-location SECCM mapping of low-pressure-grown (defective) MoS2 demonstrates that passivation progressively suppresses HER activity. Local outlier factor (LOF) analysis reveals that 3-OT produces a spatially non-uniform passivation response, increasing local electrochemical heterogeneity, whereas subsequent treatment with HTFSI results in more uniform suppression of HER activity across the nanocrystal surface. These results demonstrate how identical-location SECCM combined with controlled surface treatment (chemical passivation, herein) can provide new insight into structure-activity relationships in heterogeneous electrocatalytic materials and establish a general framework for investigating the electrochemical consequences of controlled surface modifications at the nanoscale.