No abstract

Electrochemical interfaces used for sensing, (electro)catalysis, and energy storage are usually nanostructured to expose particular surface sites, but probing the intrinsic activity of these sites is often beyond current experimental capability. Herein, it is demonstrated how a simple meniscus imaging probe of just 30 nm in size can be deployed for direct electrochemical and topographical imaging of electrocatalytic materials at the nanoscale. Spatially resolved topographical and electrochemical data are collected synchronously to create topographical images in which step-height features as small as 2 nm are easily resolved and potential-resolved electrochemical activity movies composed of hundreds of images are obtained in a matter of minutes. The technique has been benchmarked by investigating the hydrogen evolution reaction on molybdenum disulfide, where it is shown that the basal plane possesses uniform activity, while surface defects (i.e., few to multilayer step edges) give rise to a morphology-dependent (i.e., height-dependent) enhancement in catalytic activity. The technique was then used to investigate the electro-oxidation of hydrazine at the surface of electrodeposited Au nanoparticles (AuNPs) supported on glassy carbon, where subnanoentity (i.e., sub-AuNP) reactivity mapping has been demonstrated. We show, for the first time, that electrochemical reaction rates vary significantly across an individual AuNP surface and that these single entities cannot be considered as uniformly active. The work herein provides a road map for future studies in electrochemical science, in which the activity of nanostructured materials can be viewed as quantitative movies, readily obtained, to reveal active sites directly and unambiguously.

Nanostructured electrochemical interfaces (electrodes) are found in diverse applications ranging from electrocatalysis and energy storage to biomedical and environmental sensing. These functional materials, which possess compositional and structural heterogeneity over a wide range of length scales, are usually characterized by classical macroscopic or “bulk” electrochemical techniques that are not well-suited to analyzing the nonuniform fluxes that govern the electrochemical response at complex interfaces. In this Perspective, we highlight new directions to studying fundamental electrochemical and electrocatalytic phenomena, whereby nanoscale-resolved information on activity is related to electrode structure and properties colocated and at a commensurate scale by using complementary high-resolution microscopy techniques. This correlative electrochemical multimicroscopy strategy aims to unambiguously resolve structure and activity by identifying and characterizing the structural features that constitute an active surface, ultimately facilitating the rational design of functional electromaterials. The discussion encompasses high-resolution correlative structure–activity investigations at well-defined surfaces such as metal single crystals and layered materials, extended structurally/compositionally heterogeneous surfaces such as polycrystalline metals, and ensemble-type electrodes exemplified by nanoparticles on an electrode support surface. This Perspective provides a roadmap for next-generation studies in electrochemistry and electrocatalysis, advocating that complex electrode surfaces and interfaces be broken down and studied as a set of simpler “single entities” (e.g., steps, terraces, defects, crystal facets, grain boundaries, single particles), from which the resulting nanoscale understanding of reactivity can be used to create rational models, underpinned by theory and surface physics, that are self-consistent across broader length scales and time scales.

In this work, we report the first spatially-resolved voltammetric measurements of the hydrogen evolution reaction on natural crystals of molybdenite, unequivocally demonstrating enhanced catalytic activity on the edge plane relative to the basal plane.