Two-electrode cells

The electrical conductivity measurement using a two-electrode cell is very sensitive to the calibration technique, since every minor change in the cell constant affects the results 

A two-electrode cell made of two bright platinum discs 5 mm in diameter at a distance of 12 mm was used in the conductivity measurement of molten fluoride systems by Matiasovsky et al. (1970). These authors showed that most of the Rs = plots 

A cell similar to that used by MatiaSovsky et al. (1970) was used for conductivity measurements of chlorides and fluorides also by Winterhager and Werner (1956). These authors obtained constant Rs values between 20 and 50 kHz. The distinctly lower limiting frequencies are probably caused by the use of larger platinized electrodes, i.e. electrodes with a much larger effective surface area. This results in greater double-layer capacitance and higher a values, so that Rg = Re at lower frequencies, according to Eq. (8.64). 

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The second electrode in a two-electrode cell that completes the circuit. 

The ionic conductivity of a solvent is of critical importance in its selection for an electrochemical application. There are a variety of DC and AC methods available for the measurement of ionic conductivity. In the case of ionic liquids, however, the vast majority of data in the literature have been collected by one of two AC techniques the impedance bridge method or the complex impedance method [40]. Both of these methods employ simple two-electrode cells to measure the impedance of the ionic liquid (Z). This impedance arises from resistive (R) and capacitive contributions (C), and can be described by Equation (3.6-1) ... 

Electrochemical measurements usually concern not a galvanic cell as a whole but one of the electrodes, the working electrode (WE). However, a complete cell including at least one other electrode is needed to measure the WE potential or allow current to flow. In the simplest case a two-electrode cell (Eig.l2.1a) is used for electrochemical studies. The second electrode is used either as the reference electrode (RE) or as an auxiliary electrode (AE) to allow current to flow. In some cases these two functions can be combined for example, when the surface area of the auxiliary electrode is much larger than that of the working electrode so that the current densities at the AE are low, it is essentially not polarized and thus can be used as RE. 

In the classical version one uses a two-electrode cell with DME and a mercury AE (the pool) at the bottom of the cell (see Fig. 23.2). The latter, which has a large surface area, is practically not polarized. The current at the DME is low and causes no marked ohmic potential drop in the solution and no marked polarization of the AE. Hence, to change the DME potential, it will suffice to vary the external voltage applied to the cell. During the measurements, 7 vs. % rather than 7 vs. E curves are recorded. 

Kashiwazaki67 has fabricated a complementary ECD using plasma-polymerized ytterbium bis(phthalocyanine) (pp—Yb(Pc)2) and PB films on ITO with an aqueous solution of 4M KC1 as electrolyte. Blue-to-green electrochromicity was achieved in a two-electrode cell by complementing the green-to-blue color transition (on reduction) of the pp—Yb(Pc)2 film with the blue (PB)-to-colorless (PW) transition (oxidation) of the PB. A three-color display (blue, green, and red) was fabricated in a three-electrode cell in which a third electrode (ITO) was electrically connected to the PB electrode. A reduction reaction at the third electrode, as an additional counter electrode, provides adequate oxidation of the pp Yb(Pc)2 electrode, resulting in the red coloration of the pp—Yb(Pc)2 film. 

This value of capacitance is well consistent with the experimental value of 192 F/g measured in a two-electrode cell. Hence, one must be extremely careful that the values of capacitance reported for ECPs in literature are generally obtained from three-electrode cell measurements. The above example shows clearly that in the case of a real two-electrode capacitor the capacitance values are always smaller than in a three-electrode cell configuration. 

Figure 3. Galvanostatic discharge curves (three-electrode cell) at 2 mA ofaPPy/CNTs pellet electrode (m=6.7 mg) before (2) and after (1) galvanostatic cycling in a symmetric capacitor (two electrode cell) at U=0.8 V. After the discharge (1), the electrode was charged up to 0.2 V (curve 3) and then discharged (curve 4).

The real potential of epr studies in electrochemistry was demonstrated by the work of Maki and Geske (1959) who employed a two-electrode cell based... 

In 1983, Yakushi et ai obtained UV-visiblc spectra of polypyrrole at various doping levels via the second approach described above and the results are shown in Figure 3.71. The authors employed a two-electrode cell and hence the potentials quoted are the potentials between the working and counter electrodes prior to removing the film. 

GZO films were electrodeposited from an electrodeposition bath containing 0.29-g gadolinium halide and 0.1-g zirconium halide dissolved in a 150-mL electrolyte solution. Electrodeposition was performed at a current density of 1 mA/cm2 and under constant stirring in a vertical two-electrode cell configuration. The average rate of deposition was about 25 nm/min. 

Figure 3. ip TLc/ F.hsph vs- or conical Plana two electrode cell (A) and hemisplfierical-planar two electrode cell (B). Calculated using Eqs. 2 and 3 (64). 

Research related to the use of vanadium phosphates or V2O5 as oxidants of gases such as CO and SO2 in commercial processes shows that solid vanadyl sulfate can serve as a gas-permeable solid-phase electrolyte [102]. Two reversible redox features are observable at slow scan rates (20-150 mV s ) by CV in a gas-tight two-electrode cell packed with powdered VOSO4 3H2O between a 10-mm carbon disk and a 3-mm glassy carbon electrode. The V(IV/V) couple was observed at 0.55 V versus C, and the V(IV/III) couple was observed at —0.97 V. Unlike in aqueous solution where vanadyl sulfate is reduced to [V(H20)6] ", the V=0 bond in the solid remains intact. The oxidation of CO(g) can be observed when it is introduced into this cell. 

A practical use of a microelectrode The advantages of microelectrode techniques are especially pronounced for systems with limited conductivity (e.g., polymer electrolytes), which are popular candidates for state-of-the-art lithium ion batteries, normally with room-temperature conductivities K - 10-4 S/cm. (a) A researcher is evaluating a newly synthesized lithium polymer electrolyte. He uses a two-electrode cell in which an electrolyte disk of 0.1 cm X 1.0 cm is... 

Fig. 5.40 Circuits for conductivity measurements with two-electrode cell (a) and four-electrode cell (b). In (a), S AC voltage source D detector I, II, III bridge elements. In (b), S constant-current source POT potentiometer Rs variable resistor C and C electrodes for current flow P and P electrodes for voltage measurement.

Figure 17-7 E(cathode) becomes more negative with time when electrolysis is conducted in a two-electrode cell with a constant voltage between the electrodes.

In electrogravimetric analysis, analyte is deposited on an electrode, whose increase in mass is then measured. With a constant voltage in a two-electrode cell, electrolysis is not very selective, because the working electrode potential changes as the reaction proceeds. 

Figure 5.3 Classical impedance bridge for a two-electrode cell.

Some salient features of the apparatus are summarized in this column. The basic code is a three-character alphanumeric, of which the first character is a digit that describes the cell, the second is a letter that indicates whether and how dissolved oxygen was removed, and the third is a digit that identifies the technique of data acquisition. By means of a list of abbreviations, the typical entry "0A0" may be decoded to find that a two-electrode cell was used, that deaeration was accomplished with a stream of inert gas, and that the data were recorded on a pen-and-ink recorder. Inert gases are not identified because that would, In our judgment, consume more space than it would be worth, but they do not include hydrogen because hydrogen is sometimes not inert ... 

Fig. 1.6 Equivalent circuit for a two-electrode cell. A single interface is usually represented by the elements in the dashed rectangle. Cdh RP, and Rs denote the double-layer capacitance, the Faradaic resistance, and the solution resistance, respectively...

In fact, it has been shown that the capacitance values for the composites with PANI and PPy strongly depend on the cell construction [92], With chemically deposited ECP, extremely high values of specific capacitance can be found—from 250 to 1100F g 1—using a three-electrode cell, whereas smaller values of 190F g 1 for MWNT/PPy and 360F g 1 for MWNT/PANI have been measured in a two-electrode cell. It highlights the fact that only two-electrode cells allow the materials performance to be well estimated in electrochemical capacitors. 

FIGURE 8.29 Comparative voltammograms of two-electrode cells built with a-Mn02/carbon black and a-Mn02/CNT electrodes, respectively, in 1 mol L Na2S04. Carbon black or CNTs loading 15 wt%. (Adapted from Raymundo-Pinero, E., et al., J. Electrochem. Soc., 152, A229, 2005.)... 

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