Double layer charging potential-step chronoamperometry

Overlapping of Double-Layer Charging and Faradaic Currents in Potential Step and Double Potential Step Chronoamperometry. Oscillating and Nonoscillating Behavior... 

TABLE 6.3. Double-Layer Charging and Ohmic Drop in Potential Step Chronoamperometry Characteristic Function f(s) and /(f) in the Laplace and Original Spaces0... 

This last point, which has been ignored until now, in fact imposes limitations on all transient techniques. Essentially, in addition to the faradaic current flowing in response to a potential perturbation, there is also a current due to the charging of the electrochemical double-layer capacitance (for more details see Chapter 5). In chronoamperometry this manifests itself as a sharp spike in the current at short times, which totally masks the faradaic current. The duration of the double layer charging spike depends upon the cell configuration, but might typically by a few hundred microseconds. Since It=o cannot be measured directly it is necessary to resort to an extrapolation procedure to obtain its value, and whilst direct extrapolation of an /Vs t transient is occasionally possible, a linear extrapolation is always preferable. In order to see how this should be done we must first solve Pick s 2nd Law for a potential step experiment under the conditions of mixed control. The differential equations to be solved are... 

Potential-step method is an electrochemical technique in which the potential of the working electrode is either held at constant or stepped to a predetermined value, and the resulting current due to Faradaic processes and double-layer charging processes occurring at the electrode (caused by the potential step) is monitored as a function of time. Especially for practical electrochemical sensor in real-world application, chronoamperometry is preferred due to its simplicity and low cost of instrumentation. Besides its wide use in most electrochemical sensor systems, chronoamperometry has been used in the understanding of the kinetics of the electrochemical processes as well. 

Chronoamperometry (or potential step) can be regarded as an extreme fast potential sweep of CV which is especially beneficial for the practical IL-based sensor development due to its unique double-layer stmcture as discussed in the earlier section. However, as with all pulsed voltammetric techniques, chronoamperometry generates high charging currents, which decay exponentially with time as in any Randles circuit. As shown in our early work [90], the time-dependent oxygen reduction currents, i(t), in ILs, can be described as the sum of the Faradaic current for EC reaction, if, and the double-layer charging current, ic, on the electrode as shown in Eq. (2.4) ... 

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