Feedback mode of SECM operation

Feedback theory has been the basis for most quantitative SECM applications reported to date. Historically, the first theoretical treatment of the feedback response was the finite-element simulation of a diffusion-controlled process by Kwak and Bard (1), but we will start from a more general formulation for a quasi-reversible process under non-steady-state conditions and then consider some important special cases. 

General Theory of SECM Feedback for a First-Order Heterogeneous Reaction at the Tip/Substrate 

In the feedback mode experiment, both the tip and the substrate are immersed in a solution containing an electrolyte and a redox species (e.g., a 

The considerable complexity of SECM theory is due to the combination of a cylindrical diffusion to the ultramicroelectrode (UME) tip and a thin-layer-type diffusion space. The time-dependent diffusion problem for a simple quasireversible reaction in cylindrical coordinates is as follows (2,3)  

The assumption of equal diffusion coefficients (D0 = DR = D) allows the problem to be described in terms of a single species (O). The boundary conditions are of the form  

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The feedback mode of SECM operation is most suited for probing heterogeneous charge-transfer reactions. Electron transfer at the metal-solution interface was the first chemical reaction probed by SECM. An important advantage of this technique for studies of charge transfers at... 

Figure 9.29 Feedback modes of SECM operations. The UME tip is poised at a potential such that R will oxidize to O. (a) UME is freely hanging in the redox solution, ij = (b) UME...

In this section, we describe corrosion-related applications involving the most common modes of SECM operation (i.e., an amperometric microelectrode in either feedback or generator-collector mode). Traditionally, most of these applications have involved investigations of corrosion on bare metal and metal alloy substrates. But there is also much interest in the further development of coatings for corrosion prevention of metals, and a growing number of reports have appeared describing SECM studies related to these applications. 

Until a few years ago, the TG/SC mode of SECM operation was the most common way to image an enzyme that catalyzes oxygen reduction. TG/SC mode is well suited for imaging activity of surfaces with morphological features because it is relatively insensitive to changes in the tip-substrate distance [14]. The main difference between this mode and classical FB mode is that the feedback diffusion process is not required for TG/SC mode, which enables a direct measurement of activity in acidic solutions. This mode is the converse of SG/TC mode used for the anode catalysts. TG/SC mode has been applied to the study of the kinetics of oxygen reduction reaction (ORR) [14], evaluation of catalytically active nonprecious metal alloy compositions [57,58], optimization of Cu(II) biomimetics [59], thermodynamics-based design of catalysts [60], and analysis of wired enzyme architectures [61]. 

Figure 2. Feedback mode of the SECM operation, (a) The UME tip is far from the substrate, (b) Positive feedback species R is regenerated at the substrate, (c) Negative feedback Diffusion of R to the tip is hindered by the substrate.

Unlike feedback mode of the SECM operation, where the overall redox process is essentially confined to the thin layer between the tip and the substrate, in SG/TC experiments the tip travels within a thick diffusion layer produced by the large substrate. The system reaches a true steady state if the substrate is an ultramicroelectrode (e.g., a microdisk or a spherical cap) that generates or consumes the species of interest. The concentration of such species can be measured by an ion-selective (potentiometric) microprobe as a function of the tip position. The concentration at any point can be related to that at the source surface. For a microdisk substrate the dimensionless expression is [74, 75]... 

The development of SECM, which began in the late 1980s, is mainly credited to A. Bard and his coworkers who first described the technique, coined its name, and also developed the early modes of its operation, namely the feedback and the generation-collection modes, which will be described later. In the course of its nearly 30 years of existence, SECM has established itself as the tool of choice for studying spatially resolved local electrochemical reactivity of surfaces, including the quantitative study of the kinetics of both electrochemical and chemical reactions from microscopic to submicroscopic scales [2, 7]. Indeed, the ability of SECM to... 

The treatment of the two-phase SECM problem applicable to immiscible liquid-liquid systems, requires a consideration of mass transfer in both liquid phases, unless conditions are selected so that the phase that does not contain the tip (denoted as phase 2 throughout this chapter) can be assumed to be maintained at a constant composition. Many SECM experiments on liquid-liquid interfaces have therefore employed much higher concentrations of the reactant of interest in phase 2 compared to the phase containing the tip (phase 1), so that depletion and diffusional effects in phase 2 can be eliminated [18,47,48]. This has the advantage that simpler theoretical treatments can be used, but places obvious limitations on the range of conditions under which reactions can be studied. In this section we review SECM theory appropriate to liquid-liquid interfaces at the full level where there are no restrictions on either the concentrations or diffusion coefficients of the reactants in the two phases. Specific attention is given to SECM feedback [49] and SECMIT [9], which represent the most widely used modes of operation. The extension of the models described to other techniques, such as DPSC, is relatively straightforward. 

Figure 4. Charge-transfer processes at the liquid-liquid interface, (a) Probing ET at the liquid-liquid interface with the SECM. The kinetics of ET between two redox couples confined to different immiscible liquid phases can be measured with the SECM operating in the conventional feedback mode. Electroneutrality is maintained by transfer of the common ion (shown as an anion) across the interface (IT). Adapted with permission from Ref. [38]. Copyright 1995, American Chemical Society, (b) Schematic diagram of facilitated ion transfer reaction studied by SECM.

FIG. 18 Schematic representation of the SECM operating in the facilitated (A) and simple (B) IT feedback mode. (A) Potassium ions are transferred from the pipet into DCE by interfacial complexation with DB18C6 [Eq. (26)] and from DCE to... 

To study the kinetics of electron transfer, SECM can be operated in two modes a feedback mode (53, 69) and a substrate generation-tip collection mode (53). In the feedback mode, a mediator redox couple is required to probe the interface. The tip is poised to a potential well above Eredox- The steady-state current observed due to hemispherical diffusion of O toward the tip is expressed as (Figure 9.29a)... 

In addition to the amperometric feedback mode described above, other amperometric operation modes are also possible. For example, in the substrate generation/tip collection (SG/TC) mode, ip is used to monitor the flux of electroactive species from the substrate and vice versa for the tip generation/substrate collection (TG/SC) mode. These operation modes will be described in Section 12.3.1.3 and are useful in studies of homogeneous reactions that occur in the tip-substrate gap (see Section 12.4.2) and also in the evaluation of catalytic activities of different materials for useful reactions, e.g., oxygen reduction and hydrogen oxidation (see Section 12.4.3). In addition to the amperometric methods, other techniques, e.g., potentiometric method is also applicable for SECM and will be discussed in Section 12.3.2. We will also update the techniques suitable for the preparation of SECM amperometric tips in Section 12.3.1.1 and potentiometric probes in Section 12.3.2.2. 

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