Current density sweeping

Chronopotentiometry at a dme appeared to be impossible until Kies828 recently developed polarography with controlled current density, i.e., with a current density sweep. He explained the method as follows. The high current density during the first stage of the drop life results in the initiation of a secondary electrolysis process at a more negative electrode potential followed by a reverse reaction with rapid (reversible) systems because of the increase in the electrode potential. 

Fig. 8.21 Current density dilTerences between fast and slow sweep rate polarisation curves and stress corrosion cracking suspectiblity as a function of potential for a C-Mn steel in nitrate, hydroxide and carbonate-bicarbonate solutions...

Cyclic Voltammetry measurement of the current or current density as a function of the electrode potential by application of one or more potential sweep cycles. 

Figure 20. Pit-dissolution current density pit radius and ion concentration buildup AC in the pit electrolyte corresponding to the critical condition for growing pits on 18Cr-8Ni stainless steel to passivate at different repassivation potentials, EK, in 0.5 kmol m 3 H2S04 + 0.5 kmol m-3 NaCl during cathodic potential sweep at different sweep rates.7 (From N. Sato, J. Electrochem. Soc. 129,261,1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

Fig. 4 shows the current density over the supported catalysts measured in 1 M methanol containing 0.5 M sulfuric acid. During forward sweep, the methanol electro-oxidation started to occur at 0.35 V for all catalysts, which is typical feature for monometallic Pt catalyst in methanol electro-oxidation [8]. The maximum current density was decreased in the order of Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It should be noted that the trend of maximum current density was identical to that of metal dispersion (Fig. 2 and Fig. 3). Therefore, it is concluded that the metal dispersion is a critical factor determining the catalytic performance in the methanol electro-oxidation.

In an ideal case the electroactive mediator is attached in a monolayer coverage to a flat surface. The immobilized redox couple shows a significantly different electrochemical behaviour in comparison with that transported to the electrode by diffusion from the electrolyte. For instance, the reversible charge transfer reaction of an immobilized mediator is characterized by a symmetrical cyclic voltammogram ( pc - Epa = 0 jpa = —jpc= /p ) depicted in Fig. 5.31. The peak current density, p, is directly proportional to the potential sweep rate, v ... 

The basic theory of mass transfer to a RHSE is similar to that of a RDE. In laminar flow, the limiting current densities on both electrodes are proportional to the square-root of rotational speed they differ only in the numerical values of a proportional constant in the mass transfer equations. Thus, the methods of application of a RHSE for electrochemical studies are identical to those of the RDE. The basic procedure involves a potential sweep measurement to determine a series of current density vs. electrode potential curves at various rotational speeds. The portion of the curves in the limiting current regime where the current is independent of the potential, may be used to determine the diffusivity or concentration of a diffusing ion in the electrolyte. The current-potential curves below the limiting current potentials are used for evaluating kinetic information of the electrode reaction. 

The film electrodeposition process was studied by means of linear sweep voltammetry. The rate of electrochemical reaction was determined from current density (current-potential curves). The film deposits were characterized by chemical analysis, IR - spectroscopy, XRD, TG, TGA and SEM methods. 

As expected from the anisotropy of chemical etching of Si in alkaline solutions, the electrochemical dissolution reaction shows a strong dependence on crystal orientation. For all crystal orientations except (111) a sweep rate independent anodic steady-state current density is observed for potentials below PP. For (111) silicon electrodes the passivation peak becomes sweep rate dependent and corresponds to a constant charge of 2.4 0.5 mCcm-2 [Sm6]. OCP and PP show a slight shift to more anodic potentials for (111) silicon if compared to (100) substrates, as shown in Fig. 3.4. 

Figure 6.20. Experimental linear sweep voltammogram of carbon-supported high surface area nanoparticle electrocatalyst in oxygen-saturated perchloric acid electrolyte (room temperature). Solid curve pure Pt dashed curve Pt50Co50 alloy electrocatalyst. Inset a blow up of the kinetically controlled ORR regime. Inset b comparison of the specific (Pt surface area normalized) current density of the Pt and the Pt alloy catalyst for ORR at 0.9 V.

Fig. P6.3. I, Voltammograms (current density vs. potential) and isoconcs (coverage vs. potential) diagrams for several organic molecules adsorbed on platinum. I, Solid line, pure electrolyte (0.01 MHCI broken line, addition of organic species. Sweep rate, 50 mV s"1 298 K. II. Measurements taken with radiotracer (RT) FTIR spectroscopy, and ellipsometric (ELIP) techniques. (Reprinted from J. O M. Bockris and K. T. Jeng, J. Electroanal. Chem. 330 54, copyright 1992, Figs. 5 and 6, with permission of Elsevier Science.)...

What Would Make a Sweep Rate Too Fast It is clear that an influence of the condenser charging current C(dV/dt) must be avoided. If, therefore, the lowest current density to be measured meaningfully is, say, 10 pA cm-2, a necessary condition is one in which the (irrelevant) charging current is 10 pA cm-2. 

Thus, a sweep rate so slow that it will outrun the validity of 8, = (rcDt)in will be much less than the 55 mV s 1 calculated on the basis of a constant current density during the sweep. If one makes a gucstimatc that a typical sweep has a full limiting current for 5% (l/20th) of the sweep time, then the acceptable minimum sweep rate would be around 55/20 = 2.5 mV s I. 

As mustbc the case, the maximum and minimum sweeprates (2—20 mV s ) depend on assumptions (e.g., the minimum current density to be measured or the fraction of the time during a sweep in which the limiting current is reached). Depending on these variables, then, one can only conclude that the rate may vary according to the reaction characteristics and for likely current densities between about 1 and lOOmV s-1. 

The peak has thus been explained, albeit quasi-quantitatively. It is known empirically that, as, v, the sweep rate, increases, the peak occurs at a higher value of the current density. The reason for this is not difficult to grasp if one realizes that with a larger sweep rate, the potential will reach a higher (more positive) value in a fixed time, say, in 10 s. Now, after 10 s, 8t and hence the iL value will be the same, independent of the sweep rate. However, iF will be greater after 10 s at a higher than at a lower sweep rate. Since the maximum current occurs when... 

Calculate the lower limit and the upper limit for the sweep rate in a cyclic voltammetry. The double-layer capacitance is 50 pF/cm2 and the diffusion coefficient is 10-5 cm2/s. The measurable current density is 100 pA/cm2 and the sweep range is 10 V. (Kim)... 

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