Controlled current-potential relation during

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. 

Pyun et al. started " their exploration of the anomalous current response with the CTs obtained from Lii.sCoOj which is the cathode material of almost all commercially available rechargeable lithium batteries today. They reported that the CTs obtained from U1.SC0O2 composite and thin fitm " electrodes hardly exhibit a typical trend of diffusion controlled lithium transport, i. e. Cottrell behavior. Furthermore, they have found that the current-potential relation obeys Ohm s law during the CT experiments. They thus suggested that lithium transport at the interface of the electrode and the electrolyte is mainly limited by the internal cell resistance, and not by lithium diffusion in the bulk electrode. This concept is called the cell-impedance controlled lithium transport. 

Coulometric Analysis, Fig. 1 Dimensionless current (electric charge)-time relation during controlled - potential electrolysis, (a) i-t (b) log (i)-t (c) Q-t [2]... 

The objective of controlled-potential electroanalytical experiments is to obtain a current response that is related to the concentration of the target analyte. This objective is accomplished by monitoring the transfer of electrons) during the redox process of the analyte ... 

Controlled-potential coulometry involves nearly complete reduction or oxidation of an analyte ion at a working electrode maintained at a constant potential and integration of the current during the elapsed time of the electrolysis. The integrated current in coulombs is related to the quantity of analyte ion by Faraday s law, where the amps per unit time (coulomb) is directly related to the number of electrons transferred, and thus to the amount of analyte electrolyzed. 

The control error in a fast experiment may be a transient problem existing only during brief periods of high current flow. Consider a step experiment on the equivalent circuit shown in Figure 15.6.1a, in which the working interface has only a capacitance representing the double layer. Even if an ideal control circuit exists so that e f is instantaneously stepped (from e.g., 0 V), there will be a lag in the true potential, because iR is nonzero while the double layer is charging. The actual relation (see Problem 15.8) is... 

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