Prepolarizing

This indicates a constant expansion rate for each cylinder during the polarization time. The expansion rate decreases with increasing cathodic potential of prepolarization, decreasing anodic potentials, or decreasing step temperatures, which is in good agreement with experimental results, as will be shown later. 

Influence of the Cathodic Potential of Prepolarization and Closing of the Structure... 

This nucleation potential will approach its maximum value asymptotically as the potential of prepolarization is shifted to more cathodic values. In order to check the experimental validity of this prediction, this equation can be rearranged ... 

This describes a semilogarithmic dependence between the overpotential for the opening of the polymeric structure (tjN) and the cathodic overpotential (ffc) at which it was closed. The experimental results (Fig. 56) fit Eq. (53). This equation also contains an asymptotic approach to the opening potential (rjN) when the cathodic potential of prepolarization increases. 

Figure 57. Evolution of the peak potential (Ep) as a function of the cathodic potential of prepolarization (Ec). (Reprinted from T. F. Otero, H.-J. Grande, and J. Rodriguez, J. Phys. Chem. 101, 8525, 1997, Figs. 3-11,13. Copyright 1997. Reproduced with permission from the American Chemical Society.)...

Figure 25. Effect of corrosion and prepolarization on (a) PMC voltage and (b) photocurrent voltage dependence. Left n-Si (covered with Pt particles) in contact with a 5 M HBr/0.05 M Br2 aqueous solution. A comparison is made of the PMC peak during the first and the third potential sweeps. Right n-WSe2 in contact with an aqueous 0.05 M Fe2+/3+ solution. The effect of cathodic prepolarization on position and height of the PMC peak is shown.

The overpotential three-dimensional nucleation process is affected by the change of prepolarization potential in the underpotential deposition region of Pb on Ag. When A is low (potential is close to feq), the rate of the... 

In order to understand this, consider that in an FFC experiment the amplitudes of the acquired signals are approximately proportional to the signal acquisition field Ba- For example, in the case of the basic prepolarized sequence (described in Section VIII.C.), one can show (65) that... 

The effect of switching times on the two factors can be inferred from Pig. 23. For non-polarized sequences, we have v p = [mex(oo) — 7nex(0)]/mex(oo) which is always smaller than the theoretical value of 1. This is also true for prepolarized sequences, since it can be easily seen that the experimental value Ypp = [Mex(0) — Mex(oo)]/Mex(0) is again smaller than the theoretical value. 

Fig. 28. FFC Inversion Recovery sequence. In the upper case the sample is first prepolarized in a filed Bp, then switched to the acquisition field Ba where the first RF pulse of 180° is applied and the sample magnetization is inverted. The field is then switched to B,. and the sample is allowed to relax for the variable time t. Finally, the field is switched again to the acquisition value and the magnetization is sampled by any of the sample-detection methods (here, a simple FID following a 90° RF pulse). Notice that, as shown in the lower diagram, in the special case when Bp = Ba it is possible to neatly avoid the extra switching interval prior to the inversion pulse.

The authors [373] have studied the effect of prolonged cathodic polarization at negative potentials (E < 0.00 V) using both gold wire and rotating disc electrodes. They have shown that Au electrode prepolarized at —0.60 V for 20 min reveals in... 

A common observation in most cases is that the surface of amorphous alloys, especially those containing Ti, Zr and Mo, is largely covered with inactive oxides which impart low electrocatalytic properties to the material as prepared [562, 569, 575], Activation is achieved by removing these oxides either by prepolarization or, more commonly and most efficiently, by leaching in HF [89, 152, 576]. Removal of the passive layer results in a striking enhancement of the electrocatalytic activity [89], but surface analysis has shown [89, 577] that this is due to the formation of a very porous layer of fine particles on the surface (Fig. 32). A Raney type electrode is thus obtained which explains the high electrocatalytic activity. Therefore, it has been suggested [562, 578] that some amorphous alloys are better as catalyst precursors than as catalysts themselves. However, it has been pointed out that the amorphous state appears to favor the formation of such a porous layer which is not effectively formed if the alloy is in the crystalline state [575]. 

It is of particular importance to emphasize that the shape of the hysteresis curve changes with frequency, amplitude, shape, and relaxation time between prepolarization and recording pulses of the excitation signal. Therefore the extracted characteristic values differ for two... 

The pulses have trapezoid shape or triangular shape with similar rise times of pulses to measure a closed hysteresis loop. The retention is measured by pulses instead of a standard hysteresis loop since the excitation has to be modified to get the unknown initial polarization state, but compare it to a reference value, e.g. polarization with 1 second delay. In principle this could also be measured using the hysteresis measurement as described above, and monitoring the polarization during the prepolarization pulse (pulse no. 1 in Figure 3.6). The... 

The reverse reaction, dissolved O2 reduction in alkaline and acid solutions, was also studied [117, 118]. The reduction was found to be highly inhibited, probably due to the lack of adsorption sites for oxygen and/or reduced intermediates. The reaction is hypothesized to proceed mainly at sp3-sites the non-diamond sp2 carbon was deactivated by anodic prepolarization. The kinetic parameters were found as a = 0.24, k° = 7 10-5 cm s-1 in alkaline medium. It is significant that, owing to its high overvoltage, the dissolved O2 reduction does not interfere with other reactions, which thus can be studied on diamond electrodes without air removal from the cell this could be advantageous in certain types of analytical applications. 

They find that the anode potential determines the effect. The first reaction which occurs predominatingly at iron and palladium electrodes, requires the lowest potential. With platinized platinum electrodes the potential lies higher the oxidation action can exceed the evolution of oxygon and with a particularly high potential, which is obtained by prepolarizing the platinized anode,1 ethane is produced. With polished platinum and iridium anodes the potential is still higher than with prepolarized platinized platinum anodes. Thus the production of ethane predominates over the oxidation of acetic ester. 

Activation (of noble metal electrodes) — Noble metal electrodes never work well without appropriate pretreatment. Polycrystalline electrodes are polished with diamond or alumina particles of size from 10 pm to a fraction of 1 pm to obtain the mirror-like surface. The suspensions of polishing microparticles are available in aqueous and oil media. The medium employed determines the final hydrophobicity of the electrode. The mechanical treatment is often followed by electrochemical cleaning. There is no common electrochemical procedure and hundreds of papers on the electrochemical activation of -> gold and platinum (- electrode materials) aimed at a particular problem have been published in the literature. Most often, -> cyclic and - square-wave voltammetry and a sequence of potential - pulses are used. For platinum electrodes, it is important that during this prepolarization step the electrode is covered consecutively by a layer of platinum oxide and a layer of adsorbed hydrogen. In the work with single-crystal (- monocrystal) electrodes the preliminary polishing of the surface can not be done. 

To achieve a steady-state environment, some actions must be taken before starting each impedance measurement for a PEMFC system. For example, Wagner [23] prepolarized the cell for at least 15 min at the measuring potential. The current densities before and after measurement were taken to prove the stability of the cell during measurement times. Guo et al. [38] operated a fuel cell at 0.6 V for 20 h to reach its steady-state operating current. Pickup et al. [34, 39] ran a H2/02 fuel cell for 30 min at 0.5 V before the impedance measurements were performed. In Gode et al. s work [40], the cell was mn galvanostatically for 1 h prior to the impedance measurement. 

A markedly increased bandwidth of heteronuclear Hartmann-Hahn transfer for a given average rf power can be achieved with the MGS-1 and MGS-2 sequences developed by Schwendinger et al. (1994) (see Fig. 33G and H). The sequences are MLEV-4 and MLEV-8 expansions of new composite pulses R, which consist of square pulses with rf phases of 0 or 180° and different rf amplitudes that are separated by delays (see Fig. 34). Figure 35 shows HCCH-COSY spectra of a fully C-labeled protein using DIPSI-2 and MGS-2 for the initial polarization transfer from H to (prepolarization) as well as for back-transfer from to H (Majumdar et al., 1993). Note that the absolute bandwidth of MGS-2 is markedly increased compared to DIPSI-2, even though the average power... 

Fig. 3 Potentiodynamic current transients measured on cathodically prepolarized tungsten electrode in 0.5 M H2SO4 at several potentials indicated on the plot (vs. Ag/AgCl)...

Fig. 7. Cyclic-voltammetric i-E profiles of the Ti02 electrode, obtained in the dark in deaerated (for curves a and c) and in Oj-saturated (for curves b and d) 0.1 M aqueous NaOH. Curves c and d are for the electrode prepolarized for 2 min at 0.57 V under UV illumination (k > 335 nm). Delay of 2 min between the hght-off and starting of the cathodic sweep. S = 50 mV s"...

Fig. 16. Spectral photoresponses of a polycrystalline Ti02 electrode polarized at 0.8 V (vs. RHE) in 0.1 M aqueous NaOH. Curve A is for the electrode prepolarized at 0.8 V under the X = 340 nm illumination...

The prepolarization of the TiOz electrode, leading to the steady-state coverage by the surface-bonded peroxides, is shown to cause a perceptible decrease of the quantum efficiency of the photocurrent (curve A). Curve B in Fig. 16 has been obtained under identical conditions and for the same TiOz electrode made practically free from the peroxo species. As at this relatively positive potential (about 0.8 V more positive than the flat-band potential of anatase) the surface electron-hole recombination becomes negligible, the observed diminution of the photocurrent (attaining, in Fig. 16A, 15 per cent) is to be ascribed as a whole to the increase of the photoanodic overvoltage. 

Figure 5.7 Current-time record following a voltage pulse excitation on a quasi-perfect cubic face prepolarized at a subcritical overvoltage of //growth = - 4 mV in the standard system Ag (100)/AgNO3. Current scale 10 nA div time scale 0.5 s div" //nuc = - 10 mV, pulse duration fnuc = 80 ps. Electrode areaA = 2.2 x 10 cm. The current-time integral gives an electricity amount of one monolayer.

The ideal capacity or surface conductivity curves as presented in Figs. 5.9 and 5.10, have only been obtained after anodic prepolarization of the Ge electrodes and during a fast potential scan from anodic toward cathodic potentials. This result indicates that there are other factors which influence the surface properties of Ge electrodes. Before discussing these phenomena it should be mentioned that the capacity and the surface conductivity curves depend on the pH of the solution. The corresponding minima as determined after anodic prepolarization are given by curve a in Fig. 5.11 as has been measured by several authors [17-19]. The slope of this curve is 64 mV/pH above pH 4. According to this result the electrode potential at which the minimum of the capacity curve occurs (A[Pg.95]

As already mentioned, each of these results was obtained after anodic prepolarization. If the same type of measurements are done after cathodic polarization then the capacity curve and its minimum occur at more cathodic potentials, as illustrated in the upper part of Fig. 5.12. The pH-dependence of this minimum is also given in Fig. 5.11 (curve b). According to this result, the potential across the Helmholtz layer A0h is different... 

Fig. 5.11 Potentials of capacity minima after anodic (a) and cathodic (b) prepolarization of intrinsic Ge electrodes with dependence on the pH of the solution. (After ref. [18])...

Fig. 5.12 a) Space charge capacity (right scale) of intrinsic Ge vs. electrode potential after anodic and cathodic prepolarization at pH2. b) Current-potential curve. Both measurements were performed at a scan rate of 0.35 V s" (After ref. [22])... 

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