Localized electrochemical impedance spectroscopy: visualization of spatial distributions of the key parameters describing solid/liquid interfaces.

Acquisition of localized electrochemical impedance spectra as a function of spatial coordinates combined with novel approaches of data analysis brings a key for visualization of two-dimensional distributions of important parameters describing solid/liquid interfaces. They include the capacitance of the electric double layer, the resistance of the interfacial charge transfer, capacitances of adsorption, or other parameters depending on the properties of the system. Additionally, the proposed approach eliminates many common methodological problems of localized electrochemical impedance microscopies related to the frequency dependence of the actual pictures and difficulties with raw data interpretation. Thus, it offers a unique insight into the localized processes at the interface which is not possible to achieve using classical techniques.

4D shearforce-based constant-distance mode scanning electrochemical microscopy.

4D shearforce-based constant-distance mode scanning electrochemical microscopy (4D SF/CD-SECM) is designed to assess SECM tip currents at several but constant distances to the sample topography at each point of the x,y-scanning grid. The distance dependent signal is achieved by a shearforce interaction between the in-resonance vibrating SECM tip and the sample surface. A 4D SF/CD-SECM measuring cycle at each grid point involves a shearforce controlled SECM tip z-approach to a point of closest distance and subsequent stepwise tip retractions. At the point of closest approach and during the retraction steps, pairs of tip current (I) and position are acquired for various distances above the sample surface. Such a sequence provides x,y,I maps, that can be compiled and displayed for each selected data acquisition distance. Thus, multiple SECM images are obtained at known and constant distances above the sample topography. 4D SF/CD-SECM supports distance-controlled tip operation while continuous scanning of the SECM tip in the shear-force distance is avoided. In this way, constant-distance mode SECM imaging can be performed at user-defined, large tip-to-sample distances. The feasibility and the potential of the proposed 4D SF/CD-SECM imaging is demonstrated using on the one hand amperometric feedback mode imaging of a Pt band electrode array and on the other hand the visualization of the diffusion zone of a redox active species above a microelectrode in a generator/collector arrangement.

Electrocatalytic activity of spots of electrodeposited noble-metal catalysts on carbon nanotubes modified glassy carbon.

A strategy for the screening of the electrocatalytic activity of electrocatalysts for possible application in fuel cells and other devices is presented. In this approach, metal nanoclusters (Pt, Au, Ru, and Rh and their codeposits) were prepared using a capillary-based droplet-cell by pulsed electrodeposition in a diffusion-restricted viscous solution. A glassy carbon surface was modified with carbon nanotubes (CNTs) by electrophoretic accumulation and was used as substrate for metal nanoparticle deposition. The formed catalyst spots on the CNT-modified glassy carbon surface were investigated toward their catalytic activity for oxygen reduction as a test reaction employing the redox competition mode of scanning electrochemical microscopy (RC-SECM). Qualitative information on the electrocatalytic activity of the catalysts was obtained by varying the potential applied to the substrate; semiquantitative evaluation was based on the determination of the electrochemically deposited catalyst loading by means of the charge transferred during the metal nanoparticle deposition. Qualitatively, Au showed the highest electrocatalytic activity toward the oxygen reduction reaction (ORR) in phosphate buffer among all investigated single metal catalysts which was attributed to the much higher loading of Au achieved during electrodeposition. Coelectrodeposited Au-Pt catalysts showed a more positive onset potential (-150 mV in RC-SECM experiments) of the ORR in phosphate buffer at pH 6.7. After normalizing the SECM image by the charge during the metal nanocluster deposition which represents the mass loading of the catalyst, Ru showed a higher electrocatalytic activity toward the ORR than Au.

Alternating current techniques in scanning electrochemical microscopy (AC-SECM).

Alternating current scanning electrochemical microscopy (AC-SECM) is a growing branch within the variety of SECM methods. This review covers publications involving AC-SECM from its beginning to date. The findings of several research groups are thematically structured along with the specific experimental procedures. This should enable researchers to rationally choose purposeful parameters for their AC-SECM experiments.

Frequency-dependent alternating-current scanning electrochemical microscopy (4D AC-SECM) for local visualisation of corrosion sites.

For a better understanding of the initiation of localised corrosion, there is a need for analytical tools that are capable of imaging corrosion pits and precursor sites with high spatial resolution and sensitivity. The lateral electrochemical contrast in alternating-current scanning electrochemical microscopy (AC-SECM) has been found to be highly dependent on the frequency of the applied alternating voltage. In order to be able to obtain data with optimum contrast and high resolution, the AC frequency is swept in a full spectrum at each point in space instead of performing spatially resolved measurements at one fixed perturbation frequency. In doing so, four-dimensional data sets are acquired (4D AC-SECM). Here, we describe the instrument set-up and modus operandi, along with the first results from the imaging of corroding surfaces. Corrosion precursor sites and local defects in protective organic coatings, as well as an actively corroding pit on 304 stainless steel, have been successfully visualised. Since the lateral electrochemical contrast in these images varies with the perturbation frequency, the proposed approach constitutes an indispensable tool for obtaining optimum electrochemical contrast.

Frequency dependence of the electrochemical activity contrast in AC-scanning electrochemical microscopy and atomic force microscopy-AC-scanning electrochemical microscopy imaging.

Alternating current mode scanning electrochemical microscopy (AC-SECM) enables local detection of electrochemical surface activity without any redox mediator present in solution. Z-approach curves toward the substrate result in a negative feedback curve of the ac signal for insulating samples. On conducting samples, however, the shape of the feedback curve was found to be dependent on the ac perturbation frequency. Approach curves over a wide range of frequencies were performed, and the results were applied to interpret laterally resolved frequency-dependent measurements obtained with combined atomic force microscopy-AC-SECM (AFM-AC-SECM). For the first time, this frequency dependence of the signal was utilized to fine-tune the electrochemical contrast in lateral imaging in AC-SECM. An array of gold microelectrodes embedded in silicon nitride displaying significant changes in electrochemical activity as well as in topography was investigated using a bifunctional AFM-SECM tip with an integrated recessed ring microelectrode. Due to the unique geometrical conditions the electrochemical contrast between the conducting gold spots and the insulating SixNy is reversed, crosses zero, and inverts as a function of the applied ac frequency.

Redox competition mode of scanning electrochemical microscopy (RC-SECM) for visualisation of local catalytic activity.

In order to locally analyse catalytic activity on modified surfaces a transient redox competition mode of scanning electrochemical microscopy (SECM) has been developed. In a bi-potentiostatic experiment the SECM tip competes with the sample for the very same analyte. This leads to a current decrease at the SECM tip, if it is positioned in close proximity to an active catalyst site on the surface. Specifically, local catalytic activity of a Pt-catalyst modified sample with respect to the catalytic reduction of molecular oxygen was investigated. At higher local catalytic activity the local 02 partial pressure within the gap between accurately positioned SECM tip and sample is depleted, leading to a noticeable tip current decrease over active sites. A flexible software module has been implemented into the SECM to adapt the competition conditions by proper definition of tip and sample potentials. A potential pulse profile enables the localised electrochemically induced generation of molecular oxygen prior to the competition detection. The current decay curves are recorded over the entire duration of the applied reduction pulse. Hence, a time resolved processing of the acquired current values provides movies of the local oxygen concentration against x,y-position. The SECM redox competition mode was verified with a macroscopic Pt-disk electrode as a test sample to demonstrate the feasibility of the approach. Moreover, highly dispersed electro-deposited spots of gold and platinum on glassy carbon were visualised using the redox competition mode of SECM. Catalyst spots of different nature as well as activity inhomogeneities within one spot caused by local variations in Pt-loading were visualised successfully.