Chronoamperometric current transients

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 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].

The increased current tailing at longer times along with a shift of the current peak to longer times found in kMC simulations with low CO surface mobility, cf. Figure 2.4a, is characteristic for experimental transients on small nanoparticles ( 1.8 nm). Overall, the simulated transients capture all the essential features of experimental current transients. Analogous as for large nanoparticles, the model fits chronoamperometric current transients for various potentials and, thereby, explore effects of particle size and surface structure on rate constants, Tafel parameters (transfer coefficients), and equilibrium potentials. Due to the stochastic nature of the MC approach, systematic optimization of the fits is a much more delicate task. 

FIGURE 1.30. Typical chronoamperometric current transients for redox switching obtained for a poIy(aniIine)-coated ultramicro Pt electrode (10-/M. diameter) in 1M HCl. In each case the film was initially reduced at -300 mV for 10 minutes before applying the potential pulse. The step amplitude is indicated in the diagram. 

The active site model was solved for COad monolayer oxidation, and results agree with chronoamperometric current transients that had been measured at various... 

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. 

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

FIGURE3.il (a) Fits (dashed grey lines) of experimental (thin black lines) current transients for CO electro-oxidation at Pt nanoparticles, with a size distribution centered around 3.3 nm at the indicated values of the electrode potential (versus SHE), (b) Tafel plots for kinetic rates obtained from the analysis of chronoamperometric transients at a range of electrode potentials. Extracted values of the equilibrium potential of oxidative formation from water... 

Fig. 37.8 Chronoamperometric data recorded for the growth of polypyrrole onto 1500 A thick, c-axis-oriented YBa2Cu307 s films on MgO(lOO). A 0.15 M pyrrole solution with 0.1 M Et4NBp4 in acetonitrile was used, and the resulting current transients were recorded following a potential step from 0.0 to 1.0 V vs. SCE. Data are shown for (A) a bare superconductor and (B) the superconductor coated with a monolayer of />/-(3-aminopropyl)pyrrole.

Martin and Unwin simulated chronoamperometric feedback allowing for unequal diffusion coefficients of the oxidized and reduced forms of the redox mediator [6]. Unlike steady-stale SECM response, the shape of the tip current transients is sensitive to the ratio of the diffusion coefficients, y=DJD (Figure 5.3). When Dq-D, the tip current attains a steady-state value much faster than for any y 1. At y < 1, a characteristic minimum appears in the short-time region, which is quite unusual for potentiostatic transients. Ghilane et al. [7] carried out similar simulations for a very broad range of y (0.001 ionic liquids, where the Dq and Z)r values may be strikiugly different. 

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. 

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... 

Diffusion current is usually measured in the case of several transient electrochemical methods, e.g., in chronoamperometric and chronocoulometric experiments (see chronoamperometry, -> chronocoulometry, -> Cottrell equation) as well as it determines the shape of... 

Figure 25. (A) Vectorial electron transfer in the two-heme-reconstituted de novo protein molecules organized as a monolayer at an electrode surface. (B) Transient current recorded with the two-heme reconstituted de novo protein monolayer during the double-potential step chronoamperometric experiment. The potential steps from -0.2 to -0.5 V (vs. SCE) to reduce the hemes in the protein, and after 70 ms the potential steps back, from —0.5 to —0.2 V, to oxidize the reduced hemes. The experiment was performed in 0.1 M phosphate buffer, pH 7.0, under argon.

It is always interesting to plot the effect of the chronoamperometric transient at a certain prefixed potential (Figure 12.3) and to check the stability of the modified layer with a chronopotentiometric transient (Figure 12.4) by the open circuit potential response. The break of the (3-oxide growth on the platinum is observed after 250 s (Figure 12.4) when the current density becomes stable (Figure 12.3). 

---