Chronoamperometric

The chronoamperometric technique illustrates the principle that analytically useful current responses depend critically on the efficiency of analyte mass transport within the solution. The analyte mass transport in turn depends on the efficiency with which an appHed voltage can maintain the surface concentrations of oxidized and reduced species at values specified by the Nemst equation. It is generally the case in chronoamperometry that the bulk concentration of one of the species is zero whereas the surface concentration of the other species is forced to zero by the appHed potential, but this is not always so. 

FIGURE 3-1 Chronoamperometric experiment (a) potential-time waveform (b) change of concentration profiles with time (c) the resulting current—time response. 

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). 

FIGURE 4-29 Cottrell plot of the chronoamperometric response for 1 x 1(T3M Ru(NH3)63 + at a Kel-F/gold composite electrode. Points are experimental data, the solid line is the least-squares fit to theory. Dashed lines are theoretical Cottrell plots for a macroelectrode with active area equal to the active area of the composite (curve a) and to the geometric area of the composite (curve b). (Reproduced with permission from reference 87.)... 

Figure 9. Chronoamperometric curves for the growth of a polythiophene film on a stationary platinum disk electrode, from 0.1 M thiophene and 0.1 M LiC104 acetonitrile solutions, at different water contents (---) 0.04%, (--------) 0.14%,...

Figure 22. Chronoamperometric responses obtained when a bilayer was submitted to step potentials from 200 mV to different anodic potentials in the 600 to 2000-mV range in 0.1 M LiC104 aqueous solution. (Reprinted from T. F. Otero and J. Rodriguez, in Intrinsically Conducting Polymers An Emerging Technology, M. Aldissi, ed., pp. 179-190, Figs. 1, 2. Copyright 1993. Reprinted with kind permission of Kluwer Academic Publishers.)...

Figure 38. Evolution of the proposed surface aspect of a polypyrrole film during an oxidation reaction initiated from high cathodic potentials (E < -800 mV vs. SCE). The chronoamperometric response is shown at the bottom. Experimental confirmation can be seen in the pictures in Ref. 177. (Reprinted from T. F. Otero and E. Angulo, Oxidation-reduction of polypyrrole films. Kinetics, structural model, and applications. Solid State Ionics 63-64, 803, 1993, Figs. 1-3. Copyright 1993. Reprinted with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055, KV Amsterdam, The Netherlands.)...

The anodic overpotential r controls both the rate and degree of oxidation, which means that the opening of the compacted structure is faster the greater the anodic potential, and oxidation is not completed until a steady state is attained at every anodic potential. This overpotential is also included in the constant a, with a subsequent influence on the two terms of the chronoamperometric equation. Both experimental and theoretical results in Fig. 43 show good agreement. 

Figure 46. Separation of the overall oxidation curve into its two components a relaxation curve, responsible for the initial slope and the position of the chronoamperometric maximum, and a diffusion curve that controls the overall shape of the chronoampero-gram. (Reprinted from T. F. Otero, H.-J. Grande, and J. Rodriguez, J. Phys. Chem. 101, 3688, 1997, Figs. 1, 3, 6, 7, 13. Copyright 1997. Reprinted with permission from the American Chemical Society.)...

After a critically sized nucleus is formed, it starts to grow. Under the same hypothesis stated above for chronoamperometric growth, and taking into account that the radial growth of each cylinder occurs at the same time that the overpotential (tj) rises owing to the potential sweep (o) ... 

Otero and co-workers208,212 have visually observed nuclei of oxidized polymer in thin polypyrrole films on electrodes. They attribute these to sites of counter-ion and solvent ingress. A nucleation model based on the growth of ionically conductive zones provides good agreement with experimental chronoamperometric responses. 

Traore M, Moddo R, Vittori O (1988) Electrochemical behaviour of tellurium and silver teUuride at rotating glassy carbon electrode. Hectrochim Acta 33 991-996 Ngac N, Vittori O, Quarin G (1984) Voltammetrie and chronoamperometric studies of tellurium electrodeposition of glassy carbon and gold electrodes. J Electroanal Chem 167 227-235... 

For the individual types of transient measuring techniques, special names exist but their terminology lacks uniformity. The potentiostatic techniques where the time-dependent current variation is determined are often called chronoamperometric, and the galvanostatic techniques where the potential variation is determined are called chronopotentiometric. For the potentiodynamic method involving linear potential scans, the term voltammetry is used, but this term is often used for other transient methods as well. 

Figure 13. (a) Cyclic voltammograms, (b) chronoamperometric curves for oxidation of methanol catalyzed by the Pt-porous nano-... 

To improve the selectivity of chronoamperometric in vivo analysis, a differential measurement ta hnique has been employed Instead of a single potential pulse, the potential is alternately pulsed to two different potentials giving rise to the name double chronoamperometry. This waveform is shown in Fig. 15 B. Because the current contributions of individual electroactive components add linearly to produce the observed current output, the difference in current response at the two potentials is the current due to only those compounds which are oxidized at the higher potential and not oxidized at the lower potential. This system provides two responses, the current due to easily oxidized compounds and the current due to harder to oxidize compounds. This gives greater selectivity than the direct chronoamperometric method. 

CO Stripping Chronoamperometiy Before discussing experimental results, let us examine what the LH mechanism predicts for the chronoamperometric response of an experiment where we start at a potential at which the CO adlayer is stable and we step to a final potential E where the CO adlayer will be oxidized. We will also assume that the so-called mean field approximation applies, i.e., CO and OH are well mixed on the surface and the reaction rate can be expressed in terms of their average coverages dco and qh- The differential equation for the rate of change of dco with time is... 

Chronoamperometric transients for CO stripping on polycrystalline platinum were measured by McCallum and Fletcher [1977], Love and Lipkowski [1988] were the hrst to present chronoamperometric data for CO stripping on single-crystalline platinum. However, they interpreted their data on the basis of a different model than the one discussed above. Love and Lipkowski considered that the oxidation of the CO adlayer starts at holes or defects in the CO adlayer, where OH adsorbs. These holes act as nucleation centers for the oxidation reaction, and the holes grow as the CO at the perimeter of these holes is oxidized away by OHads- This nucleation and growth (N G) mechanism is fundamentally different from the mean held model presented above, because it does not presume any kind of mixing of CO and OH [Koper et ah, 1998]. Basically, it assumes complete surface immobility of the chemisorbed CO. 

In order to assess the role of the platinum surface structure and of CO surface mobility on the oxidation kinetics of adsorbed CO, we carried out chronoamperometry experiments on a series of stepped platinum electrodes of [n(l 11) x (110)] orientation [Lebedeva et al., 2002c]. If the (110) steps act as active sites for CO oxidation because they adsorb OH at a lower potential than the (111) terrace sites, one would expect that for sufficiently wide terraces and sufficiently slow CO diffusion, the chronoamperometric transient would display a CottreU-hke tailing for longer times owing to slow diffusion of CO from the terrace to the active step site. The mathematical treatment supporting this conclusion was given in Koper et al. [2002]. 

Figure 6.2a shows chronoamperometric transients for CO oxidation recorded on three different stepped electrodes for the same final potential. Clearly, the electrode with the higher step density is more active, as it oxidizes the CO adlayer in a shorter period of time. Figure 6.2b shows a fit of a transient obtained on a Pt(15, 15, 14) electrode (terrace 30 atoms wide) by both the mean field model [(6.5), solid line] and the N G model [(6.6), dashed line]. The mean field model gives a slightly better fit. More importantly, the mean field model gives a good fit of all transients on all electrodes. 

CO Stripping Voltammetry As discussed in the previous section, the chronoamperometric transients can be modeled using the LH mechanism. Using the... 

Similar studies have been carried out with Pt(l 11) and stepped surfaces with (111) terraces [Angelucci et al., 2007a, b]. The voltammetric profiles of these surfaces agree qualitatively with those depicted in Fig. 6.9. For the stepped surfaces, the potentials Ex and 2 depend linearly on the step density for terraces wider than 5 atoms. This hnear dependence is a consequence of the dependence of the oxidation rate on the step density, as was observed in the chronoamperometric CO stripping experiments. In H2SO4... 

Figure 6.11 Stripping chronoamperometric current transients on stepped Rh single-crystal surfaces, (a) Full transients obtained on the four different surfaces at F = 0.65 V (with respect to an RHE), in 0.5 M H2SO4. (b) Initial parts of the transients in (a).

Figure 6.20 Extrapolated current density at t = 0 obtained from chronoamperometric experiments for Pt(l 11), Pt(lOO), and Pt(l 10) electrodes in 0.2 M HCOOH + 0.5 M H2SO4 on electrode. The straight lines show the regions where the Tafel behavior is observed. (Data taken from Herrero et al. [1994].)...

Henero E, Franaszczuk K, Wieckowski A. 1994. Electrochemistry of methanol at low index crystal planes of platinum An integrated voltammetric and chronoamperometric study. J Phys Chem 98 5074-5083. 

Housmans THM, Koper MTM. 2003. Methanol oxidation on stepped Pt[u(lll) x (110)] electrodes A chronoamperometric study. J Phys Chem B 107 8557-8567. 

For the SECMIT mode the tip current response is governed primarily by K, Kg, y, and the dimensionless tip-substrate distance, L. Here, we briefly examine the effects of these parameters on the chronoamperometric and steady-state SECMIT characteristics. All chronoamperometric data are presented as normalized current ratio versus in order to emphasize the short-time characteristics, for the reasons outlined previously [12,14-16]. Steady-state characteristics, derived from the chronoamperometric data in the long-time limit, are considered over the full range of tip-substrate separations generally encountered in SECM. 

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