Microelectrodes transient studies

Platinum, glasslike carbon, and tungsten are often used as inert working electrodes for the fundamental electrochemical studies in the ionic liquids. For such transient electrochemical techniques as cyclic voltammetry, chronoamperometry, and chronopotentiometry, it is safer to use the working electrode with a small active area. This is because most of the ionic liquids will have low conductivity, and this often causes the ohmic drop in the measured potentials by the current flowing between the working and counter electrode. Microelectrodes may be useful for the electrochemical measurements in the case of handling low conductive media. 

Table IK shows the strength of fast-transient methods in the study of electrode reactions. Their limitations, both from the experimental and the theoretical points of view, are discussed shortly. Table 1 also includes a comparison with microelectrodes, to show the potential of transient techniques. The basis for this comparison is discussed in detail in Section 23.5.

From the foregoing discussion, it can be seen that the kinetics of electrochemical systems can be investigated in a very simple manner by making steady-state measurements at microelectrodes with a very minimal amount of equipment. As a comparison with the capabilities of other techniques, Table 1 shows the values of the rate constant of coupled first-order chemical reactions that are accessible to a range of transient and other techniques including steady-state microelectrode measurements. In principle, microelectrodes can be used to study very fast reactions indeed, the only real limitation being the fabrication of a suitably small electrode. 

An example of this latter application is that of Pletcher and his coworkers [29] into the deposition of lithium in ether solvents. This system is of particular interest in view of its relevance to lithium battery studies. At conventional electrodes, it is difficult to study transient behaviour because the electrode solution is so resistive but, as shown, in Fig. 16 studies at microelectrodes are very effective. This figure shows a series of current-time transients for the deposition of lithium on 40 /zm radius copper disc electrodes and no problems were encountered from either iRu drop or charging currents. Some might question the relevance of microelectrode studies at very low currents to technological systems such as batteries but it must be remembered that, whilst the currents are small, the current densities involved are, in fact, quite large. 

The possibilities for the use of microelectrodes for the study of fast kinetics by transient methods are exemplified by the work of Howell and Wightman [30], which showed that cyclic voltammograms could be obtained at microelectrodes at sweep rates as high as 106 V s 1 without the need for iRu drop correction. Figure 17, for example, shows some cyclic voltammograms obtained for the reduction of anthracene whilst Fig. 18 shows how closely... 

It can be seen that there is a steady-state and a transient component of the current. It is of importance that Ic is proportional to v, and this is so not only in the case of the double-layer capacitance, but also for any pseudocapacitance (electrochemically active surface layer, absorbed atoms, etc.) as well as for thin-layer cells. It follows that during cyclic voltammetric experiments the capacitive current may exceed the faradaic current - which is proportional to - at high sweep rates that causes problems in the study of fast kinetics by microelectrodes. 

Whatever the origin of defects in a passive film, a local loss of passivity can only occur when the exposed metal surface does not immediately repassivatc. Indeed, it has been observed that already well below the critical pitting potential depassivation and repassivation events may occur. These can be seen particularly well when working with electrodes of small surface area (microeleetrodes), because they contain relatively few defects that lead to breakdown events. Individual events therefore can be studied more easily. The results of Figure 6.41 illustrate the described behavior. It presents potentiostatic transients observed in the passive potential region on an iron-chromium alloy in NaCl using a microelectrode [40]. Each individual current peak represents a... 

The viability of the impact approach has been demonstrated in authentic seawater media. Figure 8.14a shows oxidative current-time transients of citrate-capped silver nanoparticles (13 2 nm in radius) dispersed in seawater measured at a carbon microelectrode of radius ca. 6 pm whilst Fig. 8.14b shows the size distribution of the nanoparticles in terms of the number of atoms making up each nanoparticle inferred from the charge passed during transients such as those in Fig. 8.14a. From the independently measured average nanoparticle size of 13 nm radius it can be estimated that a single silver nanoparticle of this size contains ca. 5.4 X 10 atoms. It is evident from Fig. 8.14 that the nanoparticles must be extensively aggregated in the seawater (on the timescale studied which was ca. 40 min from the addition of the nanoparticles) deconvolution of the distribution... 

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