In this presentation, it is shown how a simple meniscus imaging probe based on an easily-fabricated nanopipet can be deployed in the scanning electrochemical cell microscopy (SECCM) format as a fast, versatile and robust method for the microscopic functional imaging of electrocatalytic interfaces. The technique is applied to study the local electrochemical activity of crystalline molybdenum disulfide (2H-MoS2) and amorphous molybdenum sulfide (a-MoSx) thin films, which have shown promise as highly-active, earth-abundant and stable hydrogen evolution reaction (HER) electrocatalysts. Both materials possess significant structural heterogeneity at the (sub)microscale, which results in highly spatially-dependent electrocatalytic activity, not visible through the lens of conventional “bulk” electrochemistry. The basal plane of freshly-exfoliated 2H-MoS2 is uniformly active on the μm lengthscale, while macroscopic surface defects exhibit greatly accelerated HER kinetics, scaling with the dimensions of the exposed edge plane (i.e. step height). Performing high-resolution mapping with probes as small as ≈30 nm reveals previously unseen complexity, with localized folding of the surface (mechanical strain), variations in electronic structure (defect density) and/or surface aging all modulating local HER activity. In contrast, a-MoSx possesses spatially heterogeneous electrocatalytic activity on the tens-of-μm scale, which is not attributable to microscopic variations in elemental composition or chemical structure (Mo/S bonding environments), but rather local variations in the electrochemical surface area (ECSA) resulting from the nanoscale porosity of the thin film. All-in-all, this body of work provides a roadmap for future studies in electrochemical science, in which the activity of electrode materials can be viewed as quantitative maps and movies, readily obtained, to reveal active sites directly and unambiguously.

High-resolution electrochemical imaging of molybdenum sulfide electrocatalysts with scanning electrochemical cell microscopy (SECCM)

ACS Spring 2022

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