Nature of the Current-Voltage Curve

We have, at this point, examined some rather complex features of the current-voltage (i-V) curve. As you are now aware, it is affected by the nature and concentration of the electroactive species, by the electrode material, and by the mode of mass transport. The general equation that describes the current density j as the function of applied voltage E is 

The symmetry coefficient a is important in electrode kinetics, but less important for the operation of electrochemical sensors. The discussion of its meaning can be found in standard electrochemical textbooks (e.g. Koryta et al., 1993). Its value typically lies between 0.3 and 0.7. 

Let s now increase the sensitivity on the current axis such that Curve A appears to cross the zero current line at only one point. Now we are looking specifically at the round insert area of Fig. 5.1. We label the two curves recorded at this magnification as A and B, but otherwise everything is the same as in the first experiment. 

The crossing point for Curve B also corresponds to the equilibrium because no net current flows through the interface. Therefore, it is possible to write (5.6) as two equally fast reactions. 

The same scale applies also to Curve A but we note that  

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Misono et al. have reported ° similar reactions in diglyme (1,2-dimethoxyethane) at a mercury cathode with Bu4NBr electrolyte and added water. In a divided cell, substantial yields of 1,4-dihydrobenzene and 2,5-dihydrotoluene are produced from benzene and toluene respectively. No 1,4-dihydrotoluene or 1,2-dihydrobenzene were found although a little cyclohexene was detected. The current-voltage curves are essentially unaffected by the presence of the aromatic substance in the system, indicating the intermediacy of the solvated electron and the relatively slow nature of the reduction process. 

The phenomenon of decreasing the surface coarseness of a metal upon anodic dissolution under certain conditions is defined as electropolishing. In cases when polishing occurs, the current-voltage curve was found to exhibit a plateau characteristic for diffusion cmitrol of the dissolution process. Some facts point to the complex nature of the phenomenon of electropolishing. 

Regardless of the nature of the surface state it is clear that it can capture an electron from the conduction band producing cathodic current. This cathodic current balances the anodic current produced when the photoexcited holes produced the oxidized surface state. The net result of these two processes is electron-hole recombination leading to no net current. This recombination process is what controls the voltage of photocurrent onset as can be seen in curve 2 of Figure 5. 

Actually, the electrochemistry of diamond dates back to the paper [11], A current-voltage curve of crystalline diamond electrode was first taken there, as well as the differential capacitance measured at the diamond/electrolyte solution interface. The diamond electrodes turned out to be photosensitive, and their photo-electrochemical behavior was compared with their semiconductor nature. 

The hydrogen over-voltage on lead at a given current density depends on the time of polarization and the nature of the anions in the solution [28]. Figure 2.14 shows the 77 vs Ig I c relationship at two different polarization rates. It can be seen that the Tafel curve displays hysteresis phenomena which depend on the speed with which the polarization is changed [28]. The observed hysteresis has been attributed (a) to adsorption of anions on the Pb surface which alter both the distribution of the surface charges and the surface H concentration [28,29] or (b) to dissolution of a certain amount of hydrogen in the Pb [30,31]. 

As seen in previous sections, the response to a potential step is a pulse of current, which decreases with time as the electroactive species near the electrode surface is consumed and consists of a faradaic, /f, and a capacitive contribution, Iq. The advantage of most pulse techniques results from the measurement of the current flow near the end of the pulse when the faradaic current has decayed, often to a diffusion-limited value but when the capacitive current is insignificant. Pulse widths, tp, are adjusted to satisfy this condition and the additional condition that time has not been allowed for natural convection effects to influence the response. There is a greatly improved signal-to-noise ratio (sensitivity) compared to steady state techniques and in many cases, greater selectivity. Detection limits are of the order of 10 M. Furthermore, for analytical purposes, most current-voltage profiles from the pulse techniques are faster to interpret than those of dc voltammograms, because they are peak-shaped rather than the typical step curve of conventional voltammet-ric methods. 

Current-voltage curves. Nature of electrode is given, the name polarography has been definitely set aside for the mercury-dropping electrode case. 

In principle, all of the parameters in this equation are measurable so it should be possible to determine the mobility from the voltage dependence of the current. However, the actual behaviour of cells of this type is critically dependent on the nature of the interface. The currents obtained can also depend on the history of the cell. One example of this and of another way that ppm levels of ionic impurities can cause problems is provided by the field anneal phenomenon [25, 26]. If the sample is heated into the isotropic phase and a DC current applied to the cell for a period of time ionic impurities become adsorbed on the electrode surface where they help to facilitate charge injection. Thereafter the I/V characteristics of the Col phase are found to be totally different and, in extreme cases, the measured conductivity can (apparently) be orders of magnitude higher Because of the extreme sensitivity of sandwich cell measurements to small difference in the electrode surface and, because experimentally the voltage dependence of the current often deviates quite markedly from the expected (V-Vc) behaviour, reliable determination of mobilities from the I/V curves is difficult and this approach is rarely used [27]. 

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