Ultramicroelectrodes, solution resistance

This discussion and presentation of ohmic drop problems should lead to the conclusion that, with maybe an exception for ultramicroelectrodes, the resistivity of electrochemical solutions must be decreased as much as possible. 

The second category of electrode response for an ultramicroelectrode occurs at high V. Under these conditions, linear diffusion is operative and the response does not differ from that of conventional electrodes with surface diameters in the 0.1 to 2 mm range. However, the effects of double-layer charging (see Chapter 1) and solution resistance are considerably reduced owing to the small size of the electrode. The transition from low to high V is shown in Fig. 22 for an electrode with r= 1pm at r>=0.03, 10, 500, and... 

An understanding of the operation of the SECM and an appreciation of the quantitative aspects of measurements with this instrument depends upon an understanding of electrochemistry at small electrodes. The behavior of ultramicroelectrodes in bulk solution (far from a substrate) has been the subject of a number of reviews (17-21). A simplified experimental setup for an electrochemical experiment is shown in Figure 1. The solution contains a species, O, at a concentration, c, and usually contains supporting electrolyte to decrease the solution resistance and insure that transport of O to the electrode occurs predominantly by diffusion. The electrochemical cell also contains an auxiliary electrode that completes the circuit via the power supply. As the power supply voltage is increased, a reduction reaction, O + ne — R, occurs at the tip, resulting in a current flow. An oxidation reaction will occur at the auxiliary electrode, but this reaction is usually not of interest in SECM, since this electrode is placed sufficiently far from the UME... 

This approach has been employed, for example, in determining the steady-state uncompensated resistance at an ultramicroelectrode (28) and the solution resistance between an ion-selective electrode tip and a surface in a scanning electrochemical microscope (29, 30). It also is sometimes possible to model the mass transport and kinetics in an electrochemical system by a network of electrical components (31, 32). Since there are a number of computer programs (e.g., SPICE) for the analysis of electric circuits, this approach can be convenient for certain electrochemical problems. 

The cell in Figure 2 is a typical apparatus used in LL studies. However, recently small interfaces, called here microinterfaces, were shown to have some experimental advantage. The purpose of this modification was to use the same advantage that the ultramicroelectrodes have. Ultramicroelectrodes help to overcome solution resistance difficulties that originate from a potential shift due to an uncompensated iR drop. As the interfacial area becomes smaller, the diffusion geometry becomes a spherically symmetric process, which means that the ratio of charge transport current versus solution resistance increases and, ultimately, renders the iR drop minimal. In ITIES studies, restriction of the interfacial area and use of a current amplifier for voltammetric studies is a viable alternative to a four-electrode potentiostat. 

The correlation of anodic peak potentials (at — 70 °C methane 5.1 V to n-octane 3.97 V vs see) with gas-phase ionization potentials and calculated HOMO energies shows that useful data can be obtained even at potentials exceeding 5.0 V vs see. Because of the low solubility of CsAsFg appreciable solution resistance is observed. This presents difficulties when large electrodes are employed, but ultramicroelectrodes create no problems. 

As discussed in detail by Newman [29], the solution resistance for a disc-shaped ultramicroelectrode is inversely proportional to the electrode radius. 

The particular behavior of a single miaoelectrode or an ensemble of millions of microelectrodes is discussed in Section 14.3. Since a nanoparticle is the ultimate case of an ultramicroelectrode, it is appropriate to discuss some of the properties of nanoparticles employing the equations developed for microelectrodes, in order to calculate the increased rate of diffusion towards an isolated nanoparticle and the corresponding decrease in solution resistance. 

As usual, a numerical example might help to illustrate the advantage of ultramicroelectrodes, from the point of view of solution resistance. In Section 14.3.2 we obtained a limiting current density of 0.8 A cm for an electrode having a radius of 0.25 pm, in a lOmM solution of the reactant. If we assume a specific conductivity of K = 25mScm the solution resistance Rs according to Eq. (14.49), is 1 X 10 Q cm. We assumed here a solution of medium specific conductivity, and... 

Garcia et al. [77,78] reported an electron transfer percolation threshold in highly resistive oil-continuous microemulsions. The Faradaic electron transfer is modulated by the amount of cosurfactant present in AOT-toluene-water microemulsions. Below a certain threshold concentration of the cosurfactant, the electron transfer between electroactive solutes in the water droplets and ultramicroelectrode is retarded or blocked. Electron transfer becomes facilitated, and a sharp increase in Faradaic current is observed above the threshold concentration. This effect was demonstrated for ruthenium hexamine reduction [77,78], ferrocyanide oxidation [77,78], acrylamide oxidation [77], and allQ lamide oxidation [77,79] with acrylamide, alkylamides, and acetonitrile as cosurfactants in AOT microemulsions. NMR results [80] suggest that there is an interfacial packing transition of the surfactant (AOT) at about the same cosurfactant concentration as the threshold transition observed electrochemically. 

One of the key barriers to the more widespread adoption of voltammetric techniques was the limited range of media in which analysis could traditionally be performed, i.e., aqueous solutions containing a relatively high concentration of supporting electrolyte. This restriction arose because resistance between the working or sensing electrode and the reference electrode limited the precision with which the applied potential could be controlled. However, microelectrodes, also commonly known as ultramicroelectrodes, whose critical dimension is in the... 

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