Voltammogram transient

Figure 5.2. NEMCA and its origin on Pt/YSZ catalyst electrodes. Transient effect of the application of a constant current (a, b) or constant potential UWR (c) on (a) the rate, r, of C2H4 oxidation on Pt/YSZ (also showing the corresponding UWR transient)3 (b) the 02 TPD spectrum on Pt/YSZ4,7 after current (1=15 pA) application for various times t. (c) the cyclic voltammogram of Pt/YSZ4,7 after holding the potential at UWR = 0.8 V for various times t.

Structural changes on surfaces can often be treated as first-order phase transitions rather than as adsorption process. Nucleation and growth of the new phase are reflected in current transients as well as dynamic STM studies. Nucleation-and-growth leads to so-called rising transients whereas mere adsorption usually results in a monotonously falling transient. In Fig. 10 are shown the current responses to potential steps across all four current peaks in the cyclic voltammogram of Fig. 8a [44], With the exception of peak A, all structural transitions yield rising current transients sug-... 

Fig. 19. Sampled-current voltammogram constructed from the current-time transients that resulted from a series of potential-step experiments at a stationary Pt electrode in a 35.0 x 10 3 mol L-1 solution of Ni(II) in the 66.7 m/o AlCl3-EtMeImCl melt ( ) total current, ( ) partial current for the electrodeposition of Ni, (O) partial current for the electrodeposition of Al. The total current was sampled at 3 s after the application of each potential pulse. Adapted from Pitner et al. [47] by permission of The Electrochemical Society. 

FIGURE 1.11. Convolution of the cyclic voltammetric current with the function I j Jnt, characteristic of transient linear and semi-infinite diffusion. Application to the correction of ohmic drop, a —, Nernstian voltammogram distorted by ohmic drop , ideal Nernstian voltammogram. b Convoluted current vs. the applied potential, E. c Correction of the potential scale, d Logarithmic analysis. 

Values of A , and k may be extracted from the polarographic data, although the treatment is complex. Examples of its use to measure the rate constants for certain redox reactions are given in Refs. 339 and 340 which should be consulted for full experimental details. The values obtained are in reasonable agreement with those from stopped-flow and other methods. The technique has still not been used much to collect rate constants for homogenous reactions. The availability of ultramicroelectrodes has enabled cyclic voltammograms to be recorded at speeds as high as 10 Vs". Transients with very short lifetimes (< ps) and their reaction rates may be characterised. ... 

Herrero and Abruna [25] have also studied the kinetics and mechanism of Hg UPD on Au(lll) electrodes in the presence and absence of bisulfate, chloride, and acetate ions. In the absence of the interacting anions (in perchloric acid), the Hg UPD was significantly controlled by gold-mercury surface interactions. In sulfuric acid solutions, the kinetics of the initial and final stages of mercury deposi-tion/dissolution was altered. The presence of two well-ordered structures at potentials below and above mercury deposition led to the formation of two pairs of sharp spikes in cyclic voltammograms. In the chloride medium, the voltammetric profile exhibited two sharp peaks and thus it was very similar to that obtained in sulfuric acid solution. Neither nucleation, nor growth kinetics mechanism was found to be linked to the process of formation/disruption of the mercury chloride adlayer. The transients obviously deviated from the ideal Langmuir behavior. 

Another example of a transient array is the set of microelectrodes on which cyclic voltammograms are recorded and a suitable pattern recognition technique is used to analyze it. Clearly, the boundaries of information acquisition can be greatly expanded by the inclusion of time and by careful analysis of the transient signals. 

The dotted line in Fig. 6.7 marks the maximum values of the ratio (A(JQq) for which a transient behavior is achieved, i.e., a well-defined peak is obtained in CV (with the relative difference between the peak and plateau currents being greater than 5 %). From the data shown in this figure, it is concluded that values of (Ag/Q ) < 0.4 are required to observe a transient behavior, whatever the electrode radius. For (Aq/Qq) > 0.5, the stationary CV response is obtained (for example, for i + 2 = 10s 1, the stationary voltammogram corresponds to scan rates below 500 mV s-1). Under these conditions, the expression of the voltammogram at disc electrodes is given by... 

The forward and reverse currents i/rf and i//( of the square wave voltammograms corresponding to Fig. 7.5c are shown in Fig. 7.6a for microelectrodes of the four electrode geometries considered. From these curves, it can be seen that both currents present a sigmoidal shape and they are separated by 2Esw in the case of spheres and discs. This behavior clearly shows that the steady state has been attained. On the other hand, in the case of cylinders and bands, y/f and i/// show a transient behavior under these conditions. From Fig. 7.6b, c, it can be verified that a decrease in the radius, ((w/2) = rc = 0.1 pm) and that of both radius and frequency (Fig. 7.6c, (w/2) = rc = 0.1 pm and/= 10 Hz) do not lead to a stationary SWV response at cylinder and band microelectrodes. 

D. Hardacre, C. Seddon, K. R. Compton, R. G. Voltammetry of oxygen in the room-temperature ionic liquids l-ethyl-3-methylimida-zolium bis(triflyl)imide and HexEt3N+ TfiN one-electron reduction to form superoxide. Steady-state and transient behavior in the same cyclic voltammogram resulting from widely different diffusion coefficients of oxygen and superoxide. J. Phys. Chem. A 2003,... 

Figure 5 represents an ideal reversible one-electron transfer process in the absence of drop or capacitative charging current, although in real experiments contributions to the response from both these terms are unavoidable. Figure 6 shows the effect of uncompensated resistance for both transient and steady-state voltammograms, whilst Fig. 7 shows the influence of double layer capacitance on a cyclic voltammetric wave. Note that for steady-state voltammetric techniques only very low capacitative charging... 

Figure 32 shows a typical microelectrode voltammogram for an electro-chemically reversible system under near steady-state conditions. Of course at very fast scan rates the behaviour returns to that of planar diffusion and a characteristic transient-type cyclic voltammetric response is obtained as the mass transport changes from convergent to linear diffusion. 

Figure 23. Cyclic voltammetry, (a) Imposed potential versus time variations, (b) Resulting transient current-potential curve for a simple electron transfer. The concentration profiles of the reactant R and product P are indicated at various characteristic potentials of the voltammogram. Epc and Epa, cathodic and anodic peak potentials, (c) Schematic change of the cyclic voltammogram as a function of the chemical stability of the product.

Kolb et al. [3.155, 3.224, 3.225] studied experimentally the system Au(lll)/Cu, H, so/. Along with observed discontinuous-like q E) isotherms (Fig. 3.50), non-monotonous current transients were potentiostatically measured (Fig. 3.51). These findings were explained in terms of first order phase transitions related to the peaks in the cyclic voltammograms. 

Jung, K.-N., and Pyun, S.-I. 2006b. The cell-impedance-controlled lithium transport through LiMn2O4 film electrode with fractal surface by analyses of ac-impedance spectra, potentiostatic current transient and linear sweep voltammograms. Electrochimica Acta 51, 4649 658. 

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