Electrocatalytic reduction reactions (i.e., the hydrogen evolution reaction (HER) and oxygen reduction reaction) at individual, faceted Au nanocubes (NCs) and nano-octahedra (ODs) expressing predominantly {100} and {111} crystal planes on the surface, respectively, were studied by nanoscale voltammetric mapping. Cyclic voltammograms were collected at individual nanoparticles (NPs) with scanning electrochemical cell microscopy (SECCM) and correlated with particle morphology imaged by electron microscopy. Nanoscale measurements from a statistically informative set of individual NPs revealed that Au NCs have superior HER electrocatalytic activity compared to that of Au ODs, in good agreement with macroscale cyclic voltammetry measurements. Au NCs exhibited more particle-to-particle variation in catalytic activity compared to that with Au ODs. The approach of single-particle SECCM imaging coupled with macroscale CV on well-defined NPs provides a powerful toolset for the design and activity assessment of nanoscale electrocatalysts.

Scanning ion conductance microscopy (SICM) has emerged as a versatile tool for studies of interfaces in biology and materials science with notable utility in biophysical and electrochemical measurements. The heart of the SICM is a nanometer-scale electrolyte filled glass pipette that serves as a scanning probe. In the initial conception, manipulations of ion currents through the tip of the pipette and appropriate positioning hardware provided a route to recording micro-and nanoscopic mapping of the topography of surfaces. Subsequent advances in instrumentation, probe design, and methods significantly increased opportunities for SICM beyond recording topography. Hybridization of SICM with coincident characterization techniques such as optical microscopy and faradaic electrodes have brought SICM to the forefront as a tool for nanoscale chemical measurement for a wide range of applications. Modern approaches to SICM realize an important tool in analytical, bioanalytical, biophysical, and materials measurements, where significant opportunities remain for further exploration. In this review, we chronicle the development of SICM from the perspective of both the development of instrumentation and methods and the breadth of measurements performed. CONTENTS

The conversion of CO 2 into value-added products is a compelling way of storing energy derived from intermittent renewable sources and can bring us closer to a closed-loop anthropogenic carbon cycle. The ability to synthesize nanocrystals of well-defined structure and composition has invigorated catalysis science with the promise of nanocrystals that selectively express the most favorable sites for efficient catalysis. The performance of nanocrystal catalysts for the CO 2 reduction reaction (CO 2 RR) is typically evaluated with nanocrystal ensembles, which returns an averaged system-level response of complex catalyst-modified electrodes with each nanocrystal likely contributing a different (unknown) amount. Measurements at single nanocrystals, taken in the context of statistical analysis of a population, and comparison to macroscale measurements are necessary to untangle the complexity of the ever-present heterogeneity in nanocrystal catalysts, achieve true structure−property correlation, and potentially identify nanocrystals with outlier performance. Here, we employ environment-controlled scanning electrochemical cell microscopy to isolate and investigate the electrocatalytic CO 2 RR response of individual facet-defined gold nanocrystals. Using correlative microscopy approaches, we conclusively demonstrate that {110}-terminated gold rhombohedra possess superior activity and selectivity for CO 2 RR compared with {111}-terminated octahedra and high-index {310}-terminated truncated ditetragonal prisms, especially at low overpotentials where electrode kinetics is anticipated to dominate the current response. The methodology framework described here could inform future studies of complex electrocatalytic processes through correlative single-entity and macroscale measurement techniques.

Single entity electrochemical (SEE) studies that can probe activities and heterogeneity in activities at nanoscale require samples that contain single and isolated particles. Single, isolated nanoparticles are achieved here with...

Nanoelectrode ensembles (NEEs), prepared by Au template synthesis, are presented as a proof-of-concept sample platform to study individual electrodeposited materials by scanning electrochemical cell microscopy (SECCM). With this platform, the non-conductive membrane support does not contribute to the electrocatalytic activity recorded at each electrode. Use of low-density template membranes results in electrodes that are isolated because initial membrane pores are typically separated by significant (microscale) distances. Electrodeposition of catalytic nanoparticles onto the electrodes of the array and observation of electrocatalytic activity are demonstrated to be suitable for correlative SECCM voltammetric mapping and electron microscopy. Suitability of NEEs for studies of surface Au oxidation, hydrazine oxidation, and hydrogen evolution (hydrogen evolution reaction, HER), and at Pt particles on NEEs (Pt-NEEs) for HER is demonstrated.