Electrolysis potential-current curves

Figure 6.19.4 Potential-current curves of electrolysis of a solution of NaCI by the diaphragm process with a 3 M NaOH solution at the cathode. Adapted from Hamann and Vielstich (2005).

The potential-current curve for an electrolyzer can be described nsing a similar equation with slightly different parameters due to different materials and designs nsed in fuel and electrolytic cells. Certainly, Ep, should be used instead of EQ when this equation is employed for electrolysis, and a small correction with a sign should be made taking into account the sign convention adapted in this book and described in Chapter 2. 

Theoretically, at a low Uappl the counteraction would be expected to result in full polarization of the electrodes, i.e., would become equal to Eappl, so no current will be passed however, the actual pc,2 at the electrode surface is continuously diminished by diffusion of the Cl2 gas into the solution and so there results a residual current, i = (2 appl - E fR. The amount of the latter increases more or less gradually with increasing Uappl, because the actual pC 2 increases until it finally becomes 1 atm, where Cl2 gas starts to escape from the solution. In the meantime, the anode has been completely covered with Zn metal, so that [Zn] has become unity. In fact, E has now attained a constant maximum value, the so-called decomposition potential, where electrolysis really breaks through. Any further increase in app) would, according to first expectations, cause a linear current increase, i = ( app, - Edecomp )IR. However, Fig. 3.2 shows that the experimental current curve deviates more and... 

Since in the course of measurement of the anode and cathode potentials the influence of the electrolyte resistance is eliminated the current curves during the ideal reversible course of the electrolysis should rise vertically upwards after transgressing the cathode or anode equilibrium potentials. Actually, however, this is never exactly the case as electrolysis is always accompanied by polarization phenomena, at least by concentration polarization. Owing to this polarization the rising parts of the current curves are always somewhat inclined to the voltage axis. From the magnitude of their slope it can be found... 

The distance between the current curves of two metals may sometimes be controlled by temperature as the value of the overvoltage varies under its effect. The depositi m potentials of nickel and zinc on electrolysis of their salts in aqueous ammonia solution at 20 °C differ only slightly because the high overvoltage for nickel reduces the difference between the respective reversible potentials, thus making simultaneous deposition of both metals quite possible. On the other hand overvoltage for nickel at 90 °C is considerably lower so that the difference between the deposition potentials increases and simultaneous deposition of nickel and zinc is no l nger possible. 

Under potentiostatic conditions, the electrode potential is set on the plateau of the A —> B transition. During the electrolysis, the current will decrease with the concentration of A and reach zero (curve d). The process is quite selective, as the other electrode process cannot take place. Note that this electro synthetic procedure requires the use of a potentiostat with three electrodes (working, reference, and auxiliary). 

Potential-Time Curves in Constant-Current Electrolysis... 

POTENTIAL-TIME CURVES IN CONSTANT-CURRENT ELECTROLYSIS... 

Potentiometric stripping analysis is carried out in several stages. After electrochemical generation of the mercury film on a graphite substrate, the elements to be determined are accumulated by electrolysis at constant potential. The next stage is the oxidation of the deposited elements by the oxidant present in the. solution. For this, the current circuit is disconnected. The deposited analytes are stripped in the order of their electrochemical jxitentials. Anodically deposited precipitates can similarly be stripped by chemical reduction. In all cases, potential - time curves with transition times proportional to concentration result [39]-[41]. 

In Fig. 6.10, instantaneous variations of electrolysis current are given for a coarse potential staircase with overlaid heat pulses. Diagrams of this type are useful to understand how TPV curves are coming about. The diagrams contain pieces of information according to the variables potential, current, temperature and time. They are more descriptive than three-dimensional pictures would be, with the variables current, voltage and temperamre. Presentations like that one sketched in Fig. 6.10 have some characteristics of spectra, thereby sometimes they have been... 

Minimizing Electrolysis Time The current-time curve for controlled-potential coulometry in Figure 11.20 shows that the current decreases continuously throughout electrolysis. An exhaustive electrolysis, therefore, may require a long time. Since time is an important consideration in choosing and designing analytical methods, the factors that determine the analysis time need to be considered. 

Coulometry. If it can be assumed that kinetic nuances in the solution are unimportant and that destmction of the sample is not a problem, then the simplest action may be to apply a potential to a working electrode having a surface area of several cm and wait until the current decays to zero. The potential should be sufficiently removed from the EP of the analyte, ie, about 200 mV, that the electrolysis of an interferent is avoided. The integral under the current vs time curve is a charge equal to nFCl, where n is the number of electrons needed to electrolyze the molecule, C is the concentration of the analyte, 1 is the volume of the solution, and F is the Faraday constant. 

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