KINETICS OF HOMOGENEOUS REACTIONS COUPLED TO HETEROGENEOUS ELECTRON TRANSFER

KINETICS OF HOMOGENEOUS REACTIONS COUPLED TO HETEROGENEOUS ELECTRON TRANSFER 

The origins of SECM homogeneous kinetic measurements can be found in the earliest applications of ultramicroelectrodes (UMEs) to profile concentration gradients at macroscopic (millimeter-sized) electrodes (1,2). The held has since developed considerably, such that short-lived intermediates in electrode reactions can now readily be identified by SECM under steady-state conditions, which would be difficult to characterize by alternative transient UME methods, such as fast scan cyclic voltammetry (8). 

1 Principles of (a) the feedback (TG/SC) modes and (b) the SG/TC mode for measuring follow-up chemical reactions of electrogenerated species. Species B, produced from a solution precursor species, A, at a generator electrode undergoes decomposition during transit to a collector electrode. Note that in the case of the feedback (a) and SG/TC (b) modes, only the tip current is generally measured, whereas for the TG/SC mode (a), the tip and substrate currents are of interest. The coordinate system and notation for the SECM geometry are shown. The schematic is not to scale typically, rs a 10a and d a. 

In the SG/TC mode (Fig. lb), species B is electrogenerated at the substrate electrode and the reverse process, in which B is oxidized to A, occurs at the tip. In experiments hitherto, the substrate electrode has generally (1,2,6,10), but not exclusively (4,11), been macroscopic (millimeter dimensions) with a disk of micrometer dimensions serving as the collector electrode. The substrate response in the SG/TC mode is thus typically transient (2,4,6). As with the feedback and TG/SC modes discussed above, the tip/ substrate separation is a key experimental variable. Since the substrate electrode is generally large, high-resolution positioning is only required in the direction perpendicular to the surfaces of the tip and substrate. 

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Scanning electrochemical microscopy (SECM) and related ultramicroelectrode (UME) methods have proven powerful for measuring the kinetics of homogeneous reactions coupled to heterogeneous electron transfer. For this type of investigation, the tip and substrate are both electrodes and one can usefully consider the tip/substrate electrode configuration as a variable gap ultrathin layer cell. In essence, the gap thickness determines the diffusional transit time of chemical species between the tip and substrate and hence the range of timescales that can be studied. 

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