Not only in electrochemistry but also in biology and in membrane transport, localized processes at solid-liquid or liquid-liquid interfaces play an important role at defect sites, pores, or individual cells, but are difficult to characterize by integral investigation. Scanning electrochemical microscopy is suitable for such investigations. After two decades of development, this method is based on a solid theoretical foundation and a large number of demonstrated applications. It offers the possibility of directly imaging heterogeneous reaction rates and locally modifying substrates by electrochemically generated reagents. The applications range from classical electrochemical problems, such as the investigation of localized corrosion and electrocatalytic reactions in fuel cells, sensor surfaces, biochips, and microstructured analysis systems, to mass transport through synthetic membranes, skin and tissue, as well as intercellular communication processes. Moreover, processes can be studied that occur at liquid surfaces and liquid-liquid interfaces.

The solid electrolyte interphase (SEI) in lithium-ion batteries separates the highly reductive lithiated graphite from reducible electrolyte components. It is critical for the performance, durability, and safe operation of batteries. In situ imaging of the SEI is demonstrated using the feedback mode of scanning electrochemical microscopy (SECM) with 2,5-di-tert-butyl-1,4-dimethoxy benzene as mediator. The formation of the SEI is indicated by a decrease of the mediator regeneration rate. Prolonged imaging of the same region revealed fluctuation of the passivating properties on time scales between 2 min and 20 h with an inhomogeneous distribution over the sample. The implications of the approach for in situ assessment of local SEI properties on graphite electrodes are discussed with respect to studying the influence of mechanical stress on SEI reliability and the mode of action of electrolyte additives aiming at improving SEI properties.

The substrate-generation/tip-collection mode of scanning electrochemical microscopy was used to detect hydrogen peroxide formed as an intermediate during oxygen reduction at various electrodes. The experiment is conceptually similar to rotating ring-disk experiments but does not require the production of a ring-disk assembly for the specific electrode material in question. In order to limit the extension of the diffusion layer above the sample, the sample electrode potential is pulsed while the Pt ultramicroelectrode probe (UME) is held at a constant potential for oxidative amperometric detection of hydrogen peroxide. The signal at UME is influenced by the sample region within the diffusion length of hydrogen peroxide during the pulse of 2.5 s. The method is tested with three model electrodes showing different behavior with respect to the oxygen reduction reaction (ORR) in acidic solution. Simple analytical models were used to extract effective rate constants for the most important reaction paths of ORR at gold and palladium-cobalt samples from the chronoamperometric response of the UME to a reduction pulse at the sample electrode.

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A linear array of eight individual addressable microelectrodes has been developed in order to perform highthroughput scanning electrochemical microscopy (SECM) imaging of large sample areas in contact regime. Similar to previous reports, the soft microelectrode array was fabricated by ablating microchannels on a polyethylene terephthalate (PET) film and filling them with carbon ink. Improvements have been achieved by using a 5 µm thick Parylene coating that allows for smaller working distances, as the probe was mounted with the Parylene coating facing the sample surface. Additionally, the application of a SECM holder allows scanning in contact regime with a tilted probe, reducing the topographic effects and assuring the probe bending direction. The main advantage of the soft microelectrode array is the considerable decrease in the experimental time needed for imaging large sample areas. Additionally, soft microelectrode arrays are very stable and can be used several times, since the electrode surface can be regenerated by blade cutting. Cyclic voltammograms and approach curves were recorded in order to assess the electrochemical properties of the device. An SECM image of a gold on glass chip was obtained with high resolution and sensitivity, proving the feasibility of soft microelectrode arrays to detect localized surface activity. Finite element method (FEM) simulations were performed in order to establish the effect of diffusion layer overlapping between neighboring electrodes on the respective approach curves.Scanning electrochemical microscopy (SECM) is a scanning probe microscope technique that is being extensively employed for the spatial characterization of chemical reactivity at different interfaces (i.e., liquid/solid, liquid/liquid, and liquid/gas) with a high resolution and high sensitivity. [1][2][3][4] Instrumentally, SECM comprises a (bi)potentiostat for recording the current generated at a microelectrode, which is horizontally (x, y) and vertically (z) scanned over a substrate by using a fine positioning system. For surface reactivity imaging, the microelectrode is biased at a potential where diffusion-limited electrolysis of a redox mediator occurs and the changes on the steady-state current at the scanning microelectrode are associated with local reaction sites on the specimen surface. In addition, quantitative kinetic information can be extracted, by comparing experimental data with numerical simulations of coupled heterogeneous kinetics and mass transport phenomena. [5][6][7][8][9][10][11][12] As a consequence, SECM has found several applications on diverse fields ranging from the study of living cells, 13,14 localized corrosion, 15,16 imaging human fingerprints, 17 screening electrocatalysts, 18 patterning surfaces, 19 and investigating kinetics of very fast heterogeneous reactions. 20,21 For a rather long time, instrumental developments have aimed at increasing the lateral resolution and providing complementary information such as topography and local reactivity from complex samples. For i...