Controlled current change during

The largest division of interfacial electrochemical methods is the group of dynamic methods, in which current flows and concentrations change as the result of a redox reaction. Dynamic methods are further subdivided by whether we choose to control the current or the potential. In controlled-current coulometry, which is covered in Section IIC, we completely oxidize or reduce the analyte by passing a fixed current through the analytical solution. Controlled-potential methods are subdivided further into controlled-potential coulometry and amperometry, in which a constant potential is applied during the analysis, and voltammetry, in which the potential is systematically varied. Controlled-potential coulometry is discussed in Section IIC, and amperometry and voltammetry are discussed in Section IID. 

By the template technique using anodic oxide films and pyrolytic carbon deposition, one can prepare monodisperse carbon tubes. Since the length and the inner diameter of the channels in an anodic oxide film can easily be controlled by changing the anodic oxidation period and the current density during the oxidation, respectively, it is possible to control the length and the diameter of the carbon tubes. Furthermore, by changing the carbon deposition period, the wall thickness of the carbon tubes is controllable. This template method makes it possible to produce only carbon tubes that are not capped at both ends. Various features of the template method are summarized in Table 10.1.1 in comparison with the conventional arc-discharge method. 

In electrogravimetry [19], the analyte, mostly metal ions, is electrolytically deposited quantitatively onto the working electrode and is determined by the difference in the mass of the electrode before and after the electrolysis. A platinum electrode is usually used as a working electrode. The electrolysis is carried out by the con-trolled-potential or the controlled-current method. The change in the current-potential relation during the process of metal deposition is shown in Fig. 5.33. The curves in Fig. 5.33 differ from those in Fig. 5.31 in that the potentials at i=0 (closed circles) are equal to the equilibrium potential of the M +/M system at each instant. In order that the curves in Fig. 5.33 apply to the case of a platinum working electrode, the electrode surface must be covered with at least a monolayer of metal M. Then, if the potential of the electrode is kept more positive than the equilibrium potential, the metal (M) on the electrode is oxidized and is dissolved into solution. On the other hand, if the potential of the electrode is kept more negative than the equilibrium potential, the metal ion (Mn+) in the solution is reduced and is deposited on the electrode. 

When metal ion M"+ is deposited by the controlled-current method, the electrode potential during the electrolysis changes in the order T, 2, 3, 4, 5, 6 in Fig. 5.33 and the next reduction process occurs near the end of the electrolysis. If the solution is acidic and the next reduction process is hydrogen generation, its influence on the metal deposition is not serious. However, if other metal is deposited in the next reduction process, metal M is contaminated with it. In order that two metal ions M"1+ and M "21 can be separated by the controlled-current method, the solution must be acidic and the reduction of hydrogen ion must occur at the potential between the reductions of the two metal ions. An example of such a case is the separation of Cu2+ and Zn2+ in acidic solutions. If two metal ions are reduced more easily than a hydrogen ion (e.g. Ag+ and Cu2+), they cannot be separated by the controlled-current method and the controlled-potential method must be used. 

Fig. 13.10 Change in space-time yield during the electrochemical HDH of 200 mM DBP in paraffin oil media using a Nation 117 membrane reactor. Ratios of the waste volume (cm3) to the cathode geometric surface area (cm2) are indicated in figure. Cathode Three-layer Ti mesh-supported Pd (25 cm2, 2 mg Pd cm-2). Anode Three-layer Ti mesh-supported Pt (25cm2, 2mgPtcm 2). Controlled current density 10 mAcm-2. Catholyte 200 mM DBP in paraffin oil (50-1,000cm3). Anolyte O.5MH2SO4 aqueous solution (50-1,000cm3). Flow rate 100 ml min-1. Temperature 18.5 0.5°C...

Figure 9. Typical discharge (lithiation) voltage profile of the Li/11.7%Cu-graphite cell at 50 °C in 1 1 EC DEC (1 MLiPF LP-40). Inset is an expanded region showing the voltage relaxation change during current interruption at about 0.08 V of the Li/11,7%Cu-graphite and Li/graphite control cell.

The power supply drives the movement of ionic species in the medium and allows adjustment and control of either the current or the voltage. In more sophisticated units, the power may be controlled as well and conditions may be programmed to change during electrophoresis. Capillary systems use power suppfies capable of providing voltages in the kilovolt range. 

Figure 22-9 Changes in cell potential (A) and current (B) during a controlled-potential deposition of copper. The cathode is maintained at — 0.36 V (vs. SCE) throughout the experiment. (Data from J. J. Lingane, Ana/. Chim. Acta, 1948,2, 590.)...

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