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Scanning electrochemical microscopy (SECM) operating as a variable gap ultra-thin layer twin-working electrode cell, has long been recognised as a powerful technique for investigating fast kinetics (heterogeneous electron transfer and homogeneous reactions coupled to electron transfer) as a consequence of high mass transport rates between the working electrodes when biased to promote redox shuttling. Recently, SECM has advanced technically and nanogap cells with dimensions on the 10s of nm scale have been reported. In this paper, we consider double layer effects on voltammetric measurements in this configuration, outlining a comprehensive model that solves the Nernst-Planck equation and Poisson equation with charged interfaces. For supporting electrolyte concentrations that have been used for such measurements (50 mM and 100 mM), it is shown that for typical electrode charges and charge on the glass insulator that encases the ultramicroelectrode (UME) tip used in SECM, there are profound effects on the voltammetric wave-shapes for redox reactions of charged redox couples, in the common modes used to study electron transfer kinetics, namely the tip-voltammetry (feedback) mode and substrate-voltammetry (substrate-generation/tip-collection and competition) modes. Using the reduction and oxidation of a singly charged redox species to a neutral and doubly charged species, respectively, as exemplar systems, it is shown that the charge on the electrodes can greatly distort the voltammetric wave-shape, while charge on the glass that surrounds the UME tip can affect the limiting current. Analysis of SECM voltammograms using methods that do not account for double layer effects will thus result in significant error in the kinetic values derived and tip-substrate distances that have to be estimated from limiting currents in SECM. The model herein provides a framework that could be developed for further studies with nanogap-SECM (e.g. consideration of alternative models for the electrical double layer, other supporting electrolyte concentrations, potential of zero charge on the electrodes and charges on the redox couples). The model results presented are shown to qualitatively match to SECM voltammetric features from experimental data in the literature, and are further supported by experimental data for redox processes of tetrathiafulvalene (TTF), namely the TTF/TTF+ and TTF+/TTF2+ redox couples. This serves to demonstrate the immediate practical application of some of the ideas presented herein. For future applications of SECM, the use of different supporting electrolyte concentrations and a range of tip-substrate separations may allow the determination of both electron transfer kinetics and double layer properties.

A major challenge in electrocatalysis is to understand the effect of electrochemical processes on the physicochemical properties of nanoparticle or nanocluster (NC) ensembles, especially for complex processes, such as the oxygen reduction reaction (ORR) considered herein. We describe an approach whereby electrocatalysis at a small number of well-defined mass-selected Pt NCs (Pt923±37, diameter, d ≈ 3 nm), deposited from a cluster beam source on carbon-coated transmission electron microscopy (TEM) grids, can be measured by a scanning electrochemical cell microscopy (SECCM) setup, in tandem with a range of complementary microscopy and spectroscopy techniques. The SECCM setup delivers high mass transport rates and allows the effects of transient reactive intermediates to be elucidated for different Pt surface coverages (NC spacing). A major observation is that the ORR activity decreases during successive electrochemical (voltammetric) measurements. This is shown to be due to poisoning of the Pt NCs by carbon-/oxygen-containing moieties that are produced by the reaction of reactive oxygen intermediates (RIs), generated by the ORR, with the carbon support. The effect is most prominent when the Pt surface coverage on the carbon support is low (<6%). Furthermore, the NC deposition impact energy drastically affects the resulting Pt NC stability during electrochemistry. For lower impact energy, Pt NCs migrate as a consequence of the ORR and are rearranged into characteristic groups on the support. This previously unseen effect is caused by an uneven flux distribution around individual NCs within the ensemble and has important consequences for understanding the stability and activity of NC and nanoparticle arrays.

Nanoelectrochemistry is an important and growing branch of electrochemistry that encompasses a number of key research areas, including (electro)catalysis, energy storage, biomedical/environmental sensing, and electrochemical imaging. Nanoscale electrochemical measurements are often performed in confined environments over prolonged experimental time scales with nonisolated quasi-reference counter electrodes (QRCEs) in a simplified two-electrode format. Herein, we consider the stability of commonly used Ag/AgCl QRCEs, comprising an AgCl-coated wire, in a nanopipet configuration, which simulates the confined electrochemical cell arrangement commonly encountered in nanoelectrochemical systems. Ag/AgCl QRCEs possess a very stable reference potential even when used immediately after preparation and, when deployed in Cl– free electrolyte media (e.g., 0.1 M HClO4) in the scanning ion conductance microscopy (SICM) format, drift by only ca. 1 mV h–1 on the several hours time scale. Furthermore, contrary to some previous reports, when employed in a scanning electrochemical cell microscopy (SECCM) format (meniscus contact with a working electrode surface), Ag/AgCl QRCEs do not cause fouling of the surface (i.e., with soluble redox byproducts, such as Ag+) on at least the 6 h time scale, as long as suitable precautions with respect to electrode handling and placement within the nanopipet are observed. These experimental observations are validated through finite element method (FEM) simulations, which consider Ag+ transport within a nanopipet probe in the SECCM and SICM configurations. These results confirm that Ag/AgCl is a stable and robust QRCE in confined electrochemical environments, such as in nanopipets used in SICM, for nanopore measurements, for printing and patterning, and in SECCM, justifying the widespread use of this electrode in the field of nanoelectrochemistry and beyond.