Electrode potential window

Demaiconny L, Raymundo-Pinero E, Beguin F (2011) Adjustment of electrodes potential window in an asymmetric carbon/Mh02 supercapacitor. J Power Sources 196 580-586... 

The reduction of hexavalent to tetravalent uranium is an important step in the reprocessing of spent nuclear fuels. Using an electrochemical technique instead of a chemical reducing agent avoids having to remove a radioactive chemical reagent. The model described below was developed to predict the performance of pilot plant cells. A model should enable one to describe an electrode-potential window (see below) for satisfactory operation of the cell. 

It is important to choose an electrolyte with a wide electrochemically stable range. For a solvent, the selection seems difficult due to its intrinsic electrochemical stability. For example, for an aqueous solution, the electrochemical disassociation window of water is around 1.23 V at room temperature. If water is used as a supercapacitor electrolyte solvent, the maximum cell voltage will be around 1.23 V if acetonitrile is the solvent, the electrode potential window is around 2.0 V with an ion liquid, the electrode potential window can be as high as 4.0 V. Therefore, different solvents have different potential windows. Table 2.2 lists several common solvents and their potential windows for supercapacitors. 

The change of the electronic resistance of PPy as a function of electrode potential in solutions of various compositions is shown in Figure 60. Because of the selected electrode potential limits, onfy a reversible increase/decrease was found no evidence of overoxidation was observed [73]. With a polymer transistor manufactured with a film of PPy as the active material, Ofer et al. [645] found a limited electrode potential window wherein PPy has a high electronic conductivity. 

As shown in Figure 3.6-1, GC and Pt exhibit anodic and cathodic potential limits that differ by several tenths of volts. However, somewhat fortuitously, the electrochemical potential windows for both electrodes in this ionic liquid come out to be 4.7 V. What is also apparent from Figure 3.6-1 is that the GC electrode exhibits no significant background currents until the anodic and cathodic potential limits are reached, while the Pt working electrode shows several significant electrochemical processes prior to the potential limits. This observed difference is most probably due to trace amounts of water in the ionic liquid, which is electrochemically active on Pt but not on GC (vide supra). 

FIGURE 4-5 Accessible potential window of platinum, mercury, and carbon electrodes in various supporting electrolytes. 

The limited anodic potential range of mercury electrodes has precluded their utility for monitoring oxidizable compounds. Accordingly, solid electrodes with extended anodic potential windows have attracted considerable analytical interest. Of the many different solid materials that can be used as working electrodes, the most often used are carbon, platinum, and gold. Silver, nickel, and copper can also be used for specific applications. A monograph by Adams (17) is highly recommended for a detailed description of solid-electrode electrochemistry. 

S.2.2 Carbon Electrodes Solid electrodes based on carbon are currently in widespread use in electroanalysis, primarily because of their broad potential window, low background current, rich surface chemistry, low cost, chemical inertness, and suitability for various sensing and detection applications. In contrast, electron-transfer rates observed at carbon surfaces are often slower than those observed at metal electrodes. The electron-transfer reactivity is strongly affected by the origin... 

While electroanalytical techniques are inherently quite sensitive, the resolution of a mixture of electroactive compounds is very difficult. Practical considerations limit the usable potential window to no more than 3 V and typically around 1.5 V. This is because at more extreme potentials the medium or the electrode itself begin to oxidize or reduce. In addition, the electrochemical response of compounds as a function of applied potential is fairly broad so that at least a 200-400 mV difference in half-wave potentials is required for adequate resolution. This typically limits electrochemical resolution of mixtures to no more than three or four electroactive compounds. 

Because of the close distance between electrode and window the concentration of methanol in the thin electrolyte layer diminishes at positive potentials and can only slowly be supplied by diffusion. In order to have measurable quantities of formic acid (or methyl formate) one has to work with methanol concentrations in the order of 1 M or more. 

Before in situ external reflectance FTIR can be employed quantitatively to the study of near-electrode processes, one final experimental problem must be overcome the determination of the thickness of the thin layer between electrode and window. This is a fundamental aspect of the application of this increasingly important technique, marking an obstacle that must be overcome if it is to attain its true potential, due to the dearth of extinction coefficients in the IR available in the literature. In the study of adsorbed species this determination is unimportant, as the extinction coefficients of the absorption bands of the surface species can be determined via coulometry. 

In the frequency region where the i/(0H) vibrations of interfacial H20 are observed, the normal Raman scattering from the bulk solution can obscure the SERS of interfacial H20 if appropriate precautions are not taken. In the studies reported here, the SERS of interfacial H20 was acquired with the electrode surface positioned as close to the electrochemical cell window as possible to minimize contributions from the bulk solution. When altering the electrode potential to deposit Pb onto the Ag electrode surface, the electrode was pulled away from the window several mm, the surface allowed to equilibrate at the new conditions, and the electrode repositioned near the cell window for spectral acquisition. 

If the nonlinear character of the kinetic law is more pronounced, and/or if more data points than merely the peak are to be used, the following approach, illustrated in Figure 1.18, may be used. The current-time curves are first integrated so as to obtain the surface concentrations of the two reactants. The current and the surface concentrations are then combined to derive the forward and backward rate constants as functions of the electrode potential. Following this strategy, the form of the dependence of the rate constants on the potential need not be known a priori. It is rather an outcome of the cyclic voltammetric experiments and of their treatment. There is therefore no compulsory need, as often believed, to use for this purpose electrochemical techniques in which the electrode potential is independent of time, or nearly independent of time, as in potential step chronoamperometry and impedance measurements. This is another illustration of the equivalence of the various electrochemical techniques, provided that they are used in comparable time windows. 

The Butler-Volmer rate law has been used to characterize the kinetics of a considerable number of electrode electron transfers in the framework of various electrochemical techniques. Three figures are usually reported the standard (formal) potential, the standard rate constant, and the transfer coefficient. As discussed earlier, neglecting the transfer coefficient variation with electrode potential at a given scan rate is not too serious a problem, provided that it is borne in mind that the value thus obtained might vary when going to a different scan rate in cyclic voltammetry or, more generally, when the time-window parameter of the method is varied. 

Table 1 Potential windows (V, vs. SCE) for a few common non-aqueous solvents on platinum and mercury electrodes...

Figure 9 Potential windows for platinum electrodes in non-aqueous solvents...

It is easy to show [124] that the polypyrrole present in these electrodes contributes 8.85 X 10 mAh to the total experimental discharge capacity. Furthermore, it is known that over the potential window used here, LiMn204 has a theoretical (maximum) capacity of 148.3 mAh g [124]. Correcting the experimental capacities in Fig. 23 for the polypyrrole contribution and dividing by the mass of the LiMn204 used shows that in the nanotubular electrode 90% (133.8 mAh g ) of the theoretical capacity is utilized, whereas in the control electrode only 37% (54.9 mAh g ) of the capacity is used. These data clearly show that the nanotubular electrode is superior to the control LiMn204 electrode. 

The reason for this loss in capacity with increasing scan can be clearly seen in the voltammograms in Fig. 30A and 30A. The peak separation, discussed in detail above, becomes larger as the scan rate is increased. The result of this enhanced distortion of the voltammetric wave is the inability to utilize the capacity of the electrode over the useful potential window of the electrode (3.0 to 1.5 V). As would be expected (see above), this distortion is less for the microtubular electrode, and this should result in higher capacities for this electrode. 

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