Working electrodes potential window

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). 

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. 

To minimize absorption from the solution, optical thin layer cells have been designed. The working electrode has the shape of a disc, and is mounted closely behind an IR-transparent window. For experiments in aqueous solutions the intervening layer is about 0.2 to 2 ftm thick. Since the solution layer in front of the working electrode is thin, its resistance is high this increases the time required for double-layer charging - time constants of the order of a few milliseconds or longer are common - and may create problems with a nonuniform potential distribution. 

Supporting Electrolyte The electrolyte that is added to the electrolytic solution to make it electrically conductive as well as to control the reaction conditions. The supporting electrolyte also works to eliminate the migration current that flows in its absence. It may be a salt, an acid, a base or a pH buffer, which is difficult to oxidize or to reduce. It is used in concentrations between 0.05 and 1 M, which is much higher than that of electroactive species (usually 10-5 to 10 2 M). The supporting electrolyte sometimes has a great influence on the electrode reaction, changing the potential window of the solution, the double layer structure, or react-... 

The electrochemical window of pure molten cryolite has not been expressly stated, but a voltammogram of purified cryolite recorded at a graphite working electrode exhibits very little residual current over the range of potentials extending from 0.4 to -1.9 V vs. a nickel wire quasi-reference electrode [7]. Physical property data for molten cryolite and phase equilibria for the AlF3-NaF melt system have been summarized [31,32]. The extremely high temperature of cryolite places severe constraints on the materials that can be used for cells. Platinum and boron nitride are the materials of choice. 

Molten salt Liquidus temperature (°C) Potential window (V)/ working electrode Reference electrode Ref... 

For electrochemical work it is important to know the limiting potentials that may be applied in oxidative, anodic, or reductive, cathodic, scans of solutions in which solutes can undergo redox reactions without the solvent being oxidized or reduced. These limits constitute the electrochemical window for the solvent. However, the breadth of this window, in terms of the applicable voltages, depends not only on the solvent itself, but also on the material of the working electrode involved, the reference electrode against which the potentials are measured, and the nature of the supporting electrolyte present. 

As the capacity of the two electrodes is different, even in a symmetric capacitor, Equation 8.1 indicates that the value of C is determined by the electrode with the smallest capacity value. Moreover, the later electrode operates in a larger potential window than the other one, which consequently reduces the voltage range of the device. This drawback can be circumvented by balancing the respective masses of the electrodes or by using, for each electrode, different materials working in their optimal potential range. 

The common approach which has been recently introduced for realizing asymmetric capacitors using any of these two materials, as negative electrode combined with an activated carbon, consists in extending the working potential window of the positive electrode [113-116], In this sense, Figure... 

To conclude with the primary electrode characteristics, we describe briefly the DLC electrodes. The data are scarce and partly contradictory, probably due to the differences in film preparation methods. According to Howe [60], even films as thin as 50 nm are quite stable against corrosion. However, in later works [61, 62] such thin films turned permeable for electrolytes. The penetration of the electrolyte to a substrate metal resulted in its corrosion and, ultimately, in film peeling. Thicker films (0.1 to 1 pm) were less subjected to damage. The current-potential curves in supporting electrolytes resemble those for crystalline diamond electrodes (see Figs. 7, 8) the potential window is narrower, however [63], Fluorination of a-C H enhances corrosion resistance of the films significantly [64],... 

Empirical formulas exist to correct for the temperature dependence of the reference potentials in aqueous solution. When one must work in nonaqueous solvents, because of their conveniently large "window," one must add a 0.1 M to 1.0 M salt (see Fig. 11.67) to help conduct current, but there can be a problem with referencing the working electrode potential to a standard electrode. SCE can be used in many nonaqueous solvents, but in some cases such a direct experiment does not work one must use the Ag Ag+ ion... 

Ideal polarized electrode — Working electrode in the situation when a large change of potential is accompanied by an infinitesimal increase of the current. The ideal polarizability is characterized by a horizontal region of a potentiostatic I-E curve (so-called potential window where the electrode can be used for measurements). See also -> electrode. 

The capacitance of carbon electrodes in acidic and alkaline solutions is quite high, yet the narrow electrochemical window of aqueous solutions is a main disadvantage when used in supercapacitors. Hence, it is possible to increase the -> energy density of supercapacitors by enlarging their working potential window. For that, polar aprotic solutions can be suitable. Indeed, many apro-... 

Figure 4.5 Electrochemical window of chloroaluminate system. The asterisk indicates the composition of the melt in RE of N — 0.60. Another case is U = 0.67. The potential difference between N = 0.60 and 0.67 (130 180 mV) is not considered. Working electrodes GC (glassy carbon) W (tungsten) Pt (platinum).

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