Current step methods

Method Measured quantity Reference component Integration necessary Bandwidth requirement Influence of parasitics 

Sawyer Tower charge Q capacitor no moderate high 

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A related technique is the current-step method The current is zero for t < 0, and then a constant current density j is applied for a certain time, and the transient of the overpotential 77(f) is recorded. The correction for the IRq drop is trivial, since I is constant, but the charging of the double layer takes longer than in the potential step method, and is never complete because 77 increases continuously. The superposition of the charge-transfer reaction and double-layer charging creates rather complex boundary conditions for the diffusion equation only for the case of a simple redox reaction and the range of small overpotentials 77 [Pg.177]

Other Techniques - Other electrochemical techniques that could be employed in sensor technology would include potential-step methods (or chrono-amperometry, as current is recorded with time), current-step methods (or chronopotentiometry, as potential is recorded with time) and AC impedance. None of these techniques appear to have yet been applied to catalyst sensing in a systematic way. 

It is therefore useful to examine whether the current step method can be performed under simplifying conditions. These possibilities will be considered below. 

ASTM standards D-2776 (9) and G-59 (10) describe standard procedures for conducting polarization resistance measurements. Potentiodynamic (11), potential step, and current-step methods (12,13) have all been described to determine the linear E-i behavior of an electrode near Eco . The current step method has been cited to be faster than potentiodynamic methods and less susceptible to errors associated with drift in Econ. This issue will be discussed below. Regardless of the method used, independent determination of pa and pc is still required. 

Jones and Greene (38) proposed that a current step method was attractive because of measurement speed and ability to contend with OCP drift during the time taken to conduct the measurement. For negligible Rs, it has been shown that the potential transient during a current step is given by... 

Usually, the study of the kinetics of quasireversible electrode reactions by constant-current techniques (generally called the galvanostatic or current step method) involves small current perturbations, and the potential change from the equilibrium position is also small. When both O and R are initially present, the linearized current-potential-concentration characteristic, (3.5.33), can be employed. Combination with equations 8.2.13 and 8.2.18 (with the latter modified by an added term, Cr) yields... 

In constant current step methods (58-61), the treatment depends on the order in which the adsorbed and diffusing species are electrolyzed. If only adsorbed O is electrolyzed, then the transition time r follows the relation... 

I step E=m current step method (galvanostatic transient method)... 

One example of how to circumvent these problems is given in [PHL 09], whereby we are able to cope with the imperfection of the step. Nevertheless, the current step method will always be tricky, because it necessarily leads to a change in the point of operation, and therefore potentially to a variation in Rohm... 

Fig. 74. A schematic diagram for a bridge set-up for the current step method this set-up eliminates IR drop from the measured overpotential as read on the oscilloscope B a pulse generator C the cell V voltmeter for monitoring the current flow and Ri, equal fixed resistors R variable resistor 0 oscilloscope.

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