Electrode area chronoamperometry

Chronoamperometry has proven useful for the measurement of diffusion coefficients of electroactive species. An average value of it1/2 over a range of time is determined at an electrode, the area of which is accurately known, and with a solution of known concentration. The diffusion coefficient can then be calculated from it1/2 by the Cottrell equation. Although the electrode area can be physically measured, a common practice is to measure it electrochemically by performing the chronoamperometric experiment on a redox species whose diffusion coefficient is known [6]. The value of A is then calculated from it1/2. Such an electrochemically measured surface area takes into account any unusual surface geometry that may be difficult to measure geometrically. 

Figure 16.2 Double-potential-step chronoamperometry of 4.6 mM [Fe(CO)2(r)5-Cp)]2 in 0.1 M Bu4NPF6/propionitrile at -43°C. Step time Texp = 0.1 s. (A) Current transients for potential step from -0.8 to -1.9 V in blank electrolyte solution (thin line) and with added [Fe(CO)2(rj5-Cp)]2 (dark line). (B) Ratio of experimental currents -i(t + texp)/i(t) for 0 < t < Texp versus normalized time (t/texp) compared to theory (solid line) for kobs = 10.5 s 1. Electrode area = 0.0032 cm2. [Reprinted with permission from E.F. Dalton, S. Ching, and R.W. Murray, Inorg. Chem. 30 2642 (1991). Copyright 1991 American Chemical Society.]...

In practice two methods are used for stationary planar electrodes in quiescent solution chronoamperometry and chronopotentiometry. By use of an electroactive species whose concentration, diffusion coefficient, and n value are known, the electrode area can be calculated from the experimental data. In chronoamperometry, the potential is stepped from a value where no reaction takes place to a value that ensures that the concentration of reactant species will be maintained at essentially zero concentration at the electrode surface. Under conditions of linear diffusion to a planar electrode the current is given by the Cottrell equation [Chapter 3, Eq. (3.6)] ... 

The imposition of a rapid, controlled change in an electrode s interfacial potential, a requirement of many electrochemical techniques (e.g., chronoamperometry and cyclic voltammetry [42]), is limited by the system s RC time constant, aR C, where a (units cm ) is the electrode area, (units ohms) is the uncompensated solution resistance, and Q (units F/cm is the interfadal capacitance. 

In cases where electrolysis of the solution bulk is not desired (as in voltammetry or chronoamperometry), the working electrode area is kept reasonably small (nominal dimensions of square millimeters). Such an electrode is termed a microelectrode. On the other hand, in electrolysis procedures (as in thin-layer cells or coulometry), the ratio of electrode area to solution volume (A/V) must be maximized. Large-area working electrodes [grids or porous electrodes such as reticulated vitreous carbon (RVC)j are used in such cases. 

Chronoamperometry is often used for measuring the diffusion coefficient of electroactive species or the surface area of the working electrode. Analytical applications of chronoamperometry (e.g., in-vivo bioanalysis) rely on pulsing of the potential of the working electrode repetitively at fixed tune intervals. Chronoamperometry can also be applied to the study of mechanisms of electrode processes. Particularly attractive for this task are reversal double-step chronoamperometric experiments (where the second step is used to probe the fate of a species generated in the first step). 

The chronoamperometry curves at 500 mV in 3 M sulfuric acid with 1M methanol are shown in Fig. 4-9 and Fig. 4-10 for smooth and high area Pt-Ru electrodes, respectively. Although Pt-Ru electrodes showed less current than the pure platinum at first, they showed much less decay and higher sustained current. Even the electrode with higer coverage of ruthenium (I.IV for 15 s), which showed more than ten fold smaller current than pure platinum at first, gave higher ciirrent after 40s. In the case of... 

Fig. 4-18 Chronoamperometry of a smooth and a hogh area Pt-Sn electrodes in 3 M H2S04 with 1M CHSOH at SOOmV. 

Fig. 4-24 Chronoamperometry of a high area Pt-Mo electrode in 3 M H2S04 with 1M CH3OH at SOOmV. 

Actually, chronoamperometry is not a commonly performed electroanalytical technique. Probably its most common application is to determine the electrochemical area of an electrode if the concentration and diffusion coefficient of the analyte are already known. 

The product D0 (dCo/dx)x=0 t is the flux or the number of moles of O diffusing per unit time to unit area of the electrode in units of mol/(cm2 s). (The reader should perform a dimensional analysis on the equations to justify the units used.) Since (3Co/3x)x=01 is the slope of the concentration-distance profile for species O at the electrode surface at time t, the expected behavior of the current during the chronoamperometry experiment can be determined from the behavior of the slope of the profiles shown in Figure 3. IB. Examination of the profiles for O at x = 0 reveals a decrease in the slope with time, which means a decrease in current. In fact, the current decays smoothly from an expected value of oo at t = 0 and approaches zero with increasing time as described by the Cottrell equation for a planar electrode,... 

Cottrell equation — Consider a large planar - electrode, of surface area A, initially at rest, in contact with a semiinfinite layer of unstirred solution containing excess electrolyte and some small amount of electroactive species R with bulk concentration Cr. At the instant t = 0, the potential of the electrode is suddenly changed (see -> chronoamperometry) to a value at which the reaction... 

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. 

In most chronoamperometry, with measurement times of 1 ms to 10 s, the diffusion layer is several micrometers to even hundreds of micrometers thick. These distances are much larger than the scale of roughness on a reasonably polished electrode, which will have features no larger than a small fraction of a micrometer. Therefore, on the scale of the diffusion layer, the electrode appears flat the surfaces connecting equal concentrations in the diffusion layer are planes parallel to the electrode surface and the area of the diffusion field is the geometric area of the electrode. When these conditions apply, as in Figure 5.23a, the geometric area should be used in the Cottrell equation. 

Figure 5.2.4 Evolution of the diffusion field during chronoamperometry at an electrode with active and inactive areas on its surface. In this case the electrode is a regular array such that the active areas are of equal size and spacing, but the same principles apply for irregular arrays, (a) Short electrolysis times, (b) intermediate times, (c) long times. Arrows indicate flux lines to the electrode.

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