Resistance, microelectrode

Photoresist Microelectrodes The local cyclic voltammograms on photo-resist-microelectrode prepared on grains 1 and 2 are shown in Figure 1.33a. Clearly, the shape of the curves is different for each case. On grain 2, the measured anodic... 

Svaetichin, G., 1951, Low resistance microelectrode, Acta Physiol. Scand. 24, Suppl. 86 3-13. 

A microelectrode has been used by Uchida et al. to study lithium deposition in order to minimize the effect of solution resistance [41], They used a Pt electrode (10-30 jum in diameter) to measure the lithium-ion diffusion coefficient in 1 mol L 1 LiC104/PC electrolyte. The diffusion coefficient was 4.7 x 10-6 cm2 s at 25 °C. 

To better understand the diffusion-limited school of thought mentioned above, it is worth digressing momentarily on another noble -metal electrode system silver on YSZ. Kleitz and co-workers conducted a series of studies of silver point-contact microelectrodes, made by solidifying small (200—2000 //m) silver droplets onto polished YSZ surfaces. Following in-situ fabrication, the impedance of these silver microelectrodes was measured as a function of T (600-800 °C), P02 (0.01-1.0 atm), and droplet radius. As an example. Figure 9a shows a Nyquist plot of the impedance under one set of conditions, which the authors resolve into two primary components, the largest (most resistive) occurring at very low frequency (0.01—0.1 Hz) and the second smaller component at moderately low frequency ( 10 Hz). 

Stripping is the most sensitive form of voltammetry. In anodic stripping polarography, analyte is concentrated into a single drop of mercury by reduction at a fixed voltage for a fixed time. The potential is then made more positive, and current is measured as analyte is reoxidized. In cyclic voltammetry, a triangular waveform is applied, and cathodic and anodic processes are observed in succession. Microelectrodes fit into small places and their low current allows them to be used in resistive, nonaqueous media. Their low capacitance... 

Cyclic voltammetric and related techniques are particularly valuable for determining Ea values in cases where one member of the redox couple is unstable. At a microelectrode in electrolytes of low resistivity, cyclic voltammograms can be recorded at scan rates up to about 100 V s-1 and at low temperatures. This allows the detection of reversibility when the unstable partner has a life-time of the order of a millisecond or so. Ultra-microelectrode techniques promise to lower this limit even further.1-3... 

The simplest specific example to examine is for a microelectrode with a spherical geometry [53]. The resistance (R) of a spherical microelectrode was given in Chapter 7. However, for a sphere whose radius is much smaller than the distance from the sphere to the counter electrode, the resistance is... 

Advantages are also found with microelectrodes used at short time scales where time-dependent currents are obtained, as the example in Figure 12.4 shows [56]. Time-dependent currents are proportional to the electrode area. Thus, for the case of a sphere, combination of the expression for the time-dependent current with the resistance shows that the ohmic drop is directly proportional to the radius. Thus, ohmic drop is always minimized by the use of a smaller electrode. 

Several examples of the use of microelectrodes in highly resistive media exist. The first reported measurements were an examination of the reduction of aromatic hydrocarbons such as perylene in benzene containing tetrahexylammonium perchlorate [57]. Although this electrolyte is presumably in a quite associated state in benzene (or toluene [58]), it does impart sufficient conductivity for electrochemistry to be observed. In subsequent work, this result was confirmed and extended to other low-dielectric-constant solvents [59]. Even voltammetry in hexane has been shown to be possible with a microelectrode [60]. In this sol-... 

The magnitude of the ohmic drop at a microelectrode can be evaluated quite readily for case 1 from a knowledge of the specific solution resistance (obtained from conductivity measurements such as in Table 12.1) and the expressions for the voltammetric current for the specific microelectrode employed. Case 2 is also straightforward if the free concentration of ions exceeds that of the electroactive species. However, the situation is somewhat more complicated for the third class. In this case, and in case 2 for fully associated electrolyte, migration as well as diffusion can affect the observed voltammetric signals. In all three cases, the situation may be further complicated by a change in structure of the double layer. However, this is ignored for now, and is considered in the section on very small electrodes. 

There is, of course, a limit on the amount of solution resistance that can be overcome even with microelectrodes. Attempts have been made to perform electrochemistry in the gas phase [79] or in supercritical C02 180] with microelectrodes. In each case, however, the conduction path was shown to be not through the bulk phase, but rather across the insulating surface between the microelectrode and the counter electrode. This mechanism enables electrochemical detection in highly unusual media for voltammetry and illustrates that only very small conduction pathways are required to obtain well-defined electrochemical behavior. 

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