Current Step Chronopotentiometry

A current step applied to an electrode provokes a change in its potential. The flux of electrons is used first to charge the double layer, and then for the faradaic reactions. The study of the variation of the potential with 

The equations for a planar electrode with only O present initially are the same as for the potential step, (10.3), as well as boundary conditions (10.4). The differences reside in the choice of the last boundary condition. To this end we need to know the concentrations of O and R on the electrode surface, [O] and [R] respectively. 

Differentiating, and taking account of boundary conditions (10.4c) and 10Ad), after inversion we arrive at 

The final boundary condition is expressed by the Nernst equation. Substituting the Sand equation (10.39), together with (10.40) and (10.41) into the Nernst equation 

For an oxidation (10.43) also applies, but with a negative sign in place of the positive sign. 

Our third example, in which a current step is applied, could be called a galvanostatic experiment, in the sense that the current, rather than the potential, is the externally controlled parameter. 

The first five initial and boundary conditions of the diffusion equation remain unaltered, and it is again the sixth that must be changed, to make the result applicable 

Note that the heterogeneous rate constant feh is not part of this equation, because the reaction is forced to proceed at a rate determined by the applied current. Solving the diffusion equation, one obtains the concentration as a function of time 

This is the Sand equation, derived in 1901. The transition time x is the time taken for the surface concentration of the reacting species to be reduced to zero. 

For a slow reaction, the shape of the transient is different, but the transition time x is independent of the kinetics of the reaction, because application of a constant 

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The porous electrode model can also be applied to current step chronopotentiometry this has been done by Martin and coworkers. The expression for the variation of potential with time following application of a constant current i is obtained via the following expression ... 

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. 

Chronopotentiometry is the electrochemical technique in which a current step is impressed across an electrochemical cell containing unstirred solution [1-5]. The resulting potential response of the working electrode versus a reference electrode is measured as a function of time (thus, chronopotentiometry), giving a chronopotentiogram as shown in Figure 4.3. 

Chronopotentiometry — is a controlled-current technique (- dynamic technique) in which the - potential variation with time is measured following a current step (also cyclic, or current reversals, or linearly increasing currents are used). For a - nernstian electrode process,... 

Current step— The excitation signal used in controlled current techniques in which the potential is measured at a designated time [i]. See also - chronopotentiometry, -> cyclic chronopotentiometry, - staircase voltammetry. Ref. [i] Heineman WR, Kissinger PT (1984) In Kissinger PT, Heine-man WR (eds) Laboratory techniques in electroanalytical chemistry. Marcel Dekker, New York, pp 129-142... 

Chronopotentiometry closely resembles chronoamperometry with the exception that the role of current and potential are reversed. In chronopotentiometry the current is controlled and is the variable and the electrode potential is the observable. A single step of the current or a double step can be employed. The double step method called current reversal chronopotentiometry is more information-rich as in previous comparisons. Both the applied currenttime wave form and the potential-time response for a reversible electrode process are illustrated in Fig. 4. The current step from 0 to a predetermined value depending upon the experimental conditions is maintained until time... 

Similar situation arose in the system of HfCl4-NaCl-KCl. Two steps were recorded at higher concentrations of HfCl4. The plot of /r( ) had a positive slope for the first step, and negative for the second step. Only at high current densities (chronopotentiometry) and at high scan rates (LSV) did the plot indicate an uncomplicated diffusion process. It was concluded that the adsorption of either Hf(II) or Hf(I) species caused the inactivation of the intermediates. 

Nowadays, most chemists know the name "Sand" only because it appears attached to a partic ilar equation in texts that describe chronopotentiometry. This technique can be useful in the diagnosis of electrode reactions (l). A current step i is Impressed across an electrochemical cell containing iinstirred solution, the potential of the working electrode is measured with respect to time and the transition time x is noted. According to the Sand equation, the product ix /2 should be constant for an uncomplicated linear diffusion-controlled electrode reaction at a planar electrode. 

Chronoamperometry, chronocoulometry, and chronopotentiometry belong to the family of step techniques [1-4]. In chronoamperometry the current, while in chronocoulometry the charge is measured as a function of time after application of a potential step perturbation. In the case of chronopotentiometry, a current step is applied, and the change of the potential with time is detected. 

Chronopotentiometry is a controlled-current technique in which the potential variation with time (0 is measiued following a current step. Other ciurent perturbations such as linear, cyclic, or current reversals are also used [1,3,4,12]. For a reversible electrode reaction following a ciurent step, chronopotentiograms shown in Fig. 7 can be obtained. 

In chronopotentiometry, the potential responses (potential transient) to current steps are recorded. If the system is initially stationary or close to equilibrium, the potential will change rapidly due to charging of the electrode interface. This fast step corresponds to the current peak in chronoamperometry. After electrode charging, the potential becomes dominated by the reaction of electroactive species, the faradaic part. 

The possibility that adsorption reactions play an important role in the reduction of telluryl ions has been discussed in several works (Chap. 3 CdTe). By using various electrochemical techniques in stationary and non-stationary diffusion regimes, such as voltammetry, chronopotentiometry, and pulsed current electrolysis, Montiel-Santillan et al. [52] have shown that the electrochemical reduction of HTeOj in acid sulfate medium (pH 2) on solid tellurium electrodes, generated in situ at 25 °C, must be considered as a four-electron process preceded by a slow adsorption step of the telluryl ions the reduction mechanism was observed to depend on the applied potential, so that at high overpotentials the adsorption step was not significant for the overall process. 

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