Charge-transfer processes at the

Controlled-potential (potentiostatic) techniques deal with the study of charge-transfer processes at the electrode-solution interface, and are based on dynamic (no zero current) situations. Here, the electrode potential is being used to derive an electron-transfer reaction and the resultant current is measured. The role of the potential is analogous to that of the wavelength in optical measurements. Such a controllable parameter can be viewed as electron pressure, which forces the chemical species to gain or lose an electron (reduction or oxidation, respectively). 

The intercept should reflect the unchanging activation polarization at the two interfaces, as well as some other effects (presence of a film before anodization, time lag in attainment of the steady state, etc.). Nevertheless, the fact that it is small or negligible indicates that charge transfer processes at the interfaces are fast and that the kinetics of the growth are entirely transport controlled. 

At the copper electrode, reduction of Cu2+ is favoured and oxidation of Cu atoms is restricted, so that net cathodic flow occurs. Finally, to prevent a build up of Zn2+ ions near the zinc/electrolyte interface and of SO42-counter ions near the copper, a flux of ions must take place in the electrolytic phase to balance the charge transfer processes at the interfaces. To maintain the flux continuity condition, the applied voltage difference becomes distributed in such a way that ... 

In many PEC systems the chemical kinetics for the primary charge transfer process at the interface are not observed at the light intensities of interest for practical devices and the interface can be modeled as a Schottky barrier. This is true because the inherent overpotential, the energy difference between where minority carriers are trapped at the band edge and the location of the appropriate redox potential in the electrolyte, drives the reaction of interest. The Schottky barrier assumption breaks down near zero bias where the effects of interface states or surface recombination become more important.(13)... 

Figure 9.2 shows the short-term transient behavior of a fuel cell as obtained from a dynamic model derived from experimental electrochemical impedance studies (Qi et al., 2005). Figure 9.2a shows the cell voltage versus time due to two different resistive load changes (a resistance increase and decrease). The inset shows the existence of three distinct process timescales. The first, A VRn. is an immediate response in the cell voltage which results from pure resistive elements within the cell. The second, A VRrl. is also relatively fast (circa sub-millisecond), that results from the time it takes a charge transfer process at the electrode-electrolyte interface to... 

Figure 4. Charge-transfer processes at the liquid-liquid interface, (a) Probing ET at the liquid-liquid interface with the SECM. The kinetics of ET between two redox couples confined to different immiscible liquid phases can be measured with the SECM operating in the conventional feedback mode. Electroneutrality is maintained by transfer of the common ion (shown as an anion) across the interface (IT). Adapted with permission from Ref. [38]. Copyright 1995, American Chemical Society, (b) Schematic diagram of facilitated ion transfer reaction studied by SECM.

For the detection of dopamine, controlled-potential (potentiostatic) techniques, which are concerned with the study of charge transfer processes at the electrode-solution interface, are favored due to a number of advantages. These include high sensitivity, selectivity towards electroactive species, wide linear range, portability and low cost of instrumentation, speciation capability and a wide range of electrodes which allow assays of unusual environments [29]. 

Charge Transfer Processes at the Semiconductor-Liquid Interface... 

---