Overpotential electrochemical experiments

The electrochemical redox reaction of a substrate resulting from the heterogeneous electron transfer from the electrode to this substrate (cathodic reduction) or the opposite (anodic oxidation) is said to be electrochemically reversible if it occurs at the Nernstian redox potential without surtension (overpotential). This is the case if the heterogeneous electron transfer is fast, i.e. there must not be a significant structural change in the substrate upon electron transfer. Any structural change slows down the electron transfer. When the rate of heterogeneous electron transfer is within the time scale of the electrochemical experiment, the electrochemical process is fast (reversible). In the opposite case, it appears to be slow (electrochemically irreversible). Structural transformations are accompanied by a slow electron transfer (slow E), except if this transformation occms after electron transfer (EC mechanism). 

Nanopyramidal, nanorod-like, and spherical gold nanostructures were also used to study the ORR in 0.5 M KOH [85]. They were synthesized on polyciystalline gold substrates through electrochemical overpotential deposition by manipulating the deposited potentials and concentrations of HAuCLj. X-ray diffractimi and electrochemical experiments showed that the pyramidal structures were dominated by 111 facets, and therefore due to the lowest amount of 100 sites, the activity toward ORR was the lowest. The reduction peak current increased and the peak potential shifted positively in the following order nanopyramids < nanorods < nanospheres. 

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

Evidence is presented for continuous tuning of the band-filling between y - 0.00 and 0.50. In comparison, electrochemical oxidation of monoclinic /)-Ni(Pc) under the same conditions is also accompanied by a significant overpotential in forming tetragonal Ni(Pc)-(BF4)0.48- However, electrochemical undoping produces the monoclinic 7-Ni(Pc) phase with far less band structure tunability than in the silicon polymer. Experiments with tosylate as the anion indicate that tetragonal [Si(Pc)0](tosylate)y n can be tuned continuously between y = 0.00 and 0.67. For the anions PFg,... 

The best way to search for the existence of an inverted region (if any) would be to use a single electrochemical electron transfer reaction in one solvent medium at a particular electrode and determine the effect of high overpotential on the reaction rate or the current density. Many experiments were carried out at organic spacer-covered ( 2.0 nm thick) electrodes to search for the inverted region for the outer-sphere ET reactions however, no inverted region was observed." ... 

Correspondingly, a typical value for AG°/ES [cf Eq. (9.3)] is 0.5 so that (0r /3 In i) = (2RT/1.5F) = 1.3(RT/F). Although observed values of this coefficient vary from RT/4F to 2RT/F, and sometimes above this, the figure for the majority of electrochemical reactions is very near 2RT/F and thus the formation of the rate— overpotential relation to which this Weiss-Marcus harmonic energy variation theory gives rise is not consistent with experiment (Fig. 9.26). 

Next we consider the case where a low overpotential exists at some point in the electrochemical switching process, as is necessarily the case in cyclic voltammetric or chronopotentiometric experiments. Now, four additional mechanisms become possible on the time scale of the electron/ion transfer process. This is so because the electron/ion transfer step, E, is no longer constrained to be the first. Consequently, either one or both of the chemical steps may precede the coupled electron/ion transfer. During the oxidation of R to Of, if solvation of Ra is more rapid than its reconfiguration, the two mechanisms, CEC and CC E may occur. If reconfiguration of R is more rapid than its solvation, two additional mechanisms, C EC and C CE, become possible. 

Unlike ethanol oxidation, chronopotentiometric experiments of methanol and glycerol oxidation on Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C electrodes show a remarkable increase of the overpotential with time, the oxygen discharge potential being attained after 2.5 and 1.5 h with methanol and glycerol, respectively. In addition to decreasing the electrochemical stability of the electrodes, the oxidation of methanol and glycerol on Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C is also much slower tlm that of ethanol under comparable experimental conditions as shown by the CVs illustrated in Fig. 23. 

These first experiments were based on symmetric cells with an electrode diameter of 10 mm, so-called button cells. The electrolyte was of the yttria-stabihzed zirconia (YSZ) type, 8YSZ based on 8mol% yttria-stabilized zirconia, with a thickness between 130 and 150 pm. On both sides, the electrode was applied by screen-printing or wet powder spraying (WPS) with a thickness of 50 pm, and the porosity was about 30%. A schematic view of a cross-section of the cell geometry is depicted in Figure 9.2. Various types of cathodes were screened by potentiodynamic current-potential measurements. Comparison of the electrochemical behavior in relation to material composition was based on the measured current density at an overpotential t] of—0.1 V. 

---