Transients, chronoamperometric

Bard AJ, Denault G, Friesner RA, Domblaser BC, Tuckerman LS (1991) Scanning electrochemical microscopy - theory and application of the transient (chronoamperometric) SECM response. Anal Chem 63(13) 1282-1288. doi 10.1021/ac00013a019... 

Senthamarai R, Rajendran L (2008) Analytical expression for transient chronoamperometric current at ultramicroband electrode. Russ J Electrochem 44 1156-1161... 

FIGURE 5.1 Comparison of (A) simulated SECM transients with (B) transients corresponding to different electrode geometries (aU processes are diffusion controlled), (a) SECM transient for a conductive substrate, (b) two-electrode thin-layer cell, (c) microdisk, (d) planar electrode, (e) SECM with an insulating substrate, and (f) one-electrode thin-layer cell. Curves A, B, E, and F were computed with L = d/a=0.l. (Adapted from Bard, A.J., Denuault, G., Friesner, R.A., Dornblaser, B.C., and Tuckerman, L.S., Scanning electrochemical microscopy Theory and application of the transient (chronoamperometric) SECM response. Anal. Chem., 63, 1282-1288, 1991. Copyright 1991 American Chemical Society.)... 

A.J. Bard, G. Denault, R.A. Friesner, B.C. Domblaser, L.S. Tuckerman. Scanning electrochemical microscopy—Theory and apphcation of the transient (chronoamperometric) SECM response. Anal Chem. 63 1282(1991). 

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. 

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... 

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).

Additionally, it can often, however, be a good idea to perform chronoamperometric transients over a wide range of times and voltages to ascertain those experimental conditions which do indeed yield a linear Cottrell plot that passes through the origin, i.e. to ascertain those experimental conditions over which diffusion is indeed the sole form of mass transport. 

Figure 2.22 Chronoamperometric current transients (dotted lines) and probe-beam deflection transients (solid lines) during the oxidation-reduction cycles of (PAH-Os)t5(PSS)- 4 PEM film in 10 mM HCI solution. Panels A-D show consecutive oxidation-reduction steps with a wait time of 180 s at the end of each train in the reduced state. Taken from Ref. [28].

Deposition of mercury at boron-doped diamond (BDD) and platinum electrodes has also been studied [33]. Deposition and oxidation of mercury was performed by cyclic voltammetry from the solution of 1 mM Hg2 ( 104)2 in 1 M Na l04. In order to learn more about this deposition, it was carried out also under chronoamperometric conditions. The results obtained are shown in Fig. 2 in the form of dimensionless current-time transients. Experimental curves obtained at two different overpotentials were compared with the theoretical curves calculated for instantaneous and progressive nucleation. A good agreement of experimental plots with the instantaneous nucleation mechanism was... 

The chronocoulometric method offers some advantages over the chronoamperometric method, (a) The measured signal grows with time and hence the later part of the transient, that corresponds better with theory, suffers less from instrumental inaccuracy, (b) The act of integration smoothes random noise in the current signal and hence also improves the accuracy, (c) Even the long-time approximation, eqn. (37b), contains... 

Fig. 2.18 Chronoamperometric profiles showing oxidative faradaic transients of gold nanoparticles at potentials of (a) 0.8 V and (b) 1.1 V at a Glassy Carbon microelectrode of 11 pm of radius. Reproduced from reference [62] with permission...

Martin and Unwin simulated chronoamperometric feedback allowing for unequal diffusion coefficients of the oxidized and reduced forms of the redox mediator [63]. Unlike steady-state SECM response, the shape of the tip current transients is sensitive to the ratio of the diffusion coefficients, y = Do /Dr (Fig. 8). When Do = DR, the tip current attains a steady-state value much faster... 

The latest contribution to the theory of the EC processes in SECM was the modeling of the SG/TC situation by Martin and Unwin [86]. Both the tip and substrate chronoamperometric responses to the potential step applied to the substrate were calculated. From the tip current transient one can extract the value of the first-order homogeneous rate constant and (if necessary) determine the tip-substrate distance. However, according to the authors, this technique is unlikely to match the TG/SC mode with its high collection efficiency under steady-state conditions. 

Fig. 16.5. BIA chronoamperometric transient for the oxidation of 2mM lC Fe(CN)6 in 0.4 M K2S04 electrolyte injection of 100 p.L onto Pt disc electrode (d = 3.28 mm) at +0.6 V vs. SCE. Injection flow rate 75.3 (iL s micropipette tip internal diameter 0.47 mm. 1 = 0 corresponds to the start of the injection period.

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