Double potentiostat

To operate an RRDE, one needs a four-electrode potentiostat, often referred to also as a "double" potentiostat. Now, we started this book with a simple two-electrode cell, then went on to the three-electrode configuration. How does the fourth electrode fit into this scheme The... 

The electrical double-layer structure of a Pt/DMSO interface has been investigated using the potentiostatic pulse method.805 The value of C at E = const, as well as the potential of the diffuse layer minimum, have been found to depend on time, and this has been explained by the chemisorption of DMSO dipoles on the Pt surface, whose strength depends on time. Eg=Q has been found11 at E = -0.64 V (SCE in H2O). 

On the basis of experimental findings Heinze et al. propose the formation of a particularly stable, previously unknown tertiary structure between the charged chain segments and the solvated counterions in the polymer during galvanostatic or potentiostatic polymerization. During the discharging scan this structure is irreversibly altered. The absence of typical capacitive currents for the oxidized polymer film leads them to surmise that the postulated double layer effects are considerably smaller than previously assumed and that the broad current plateau is caused at least in part by faradaic redox processes. 

On the basis of this argument, the mechanism for the current oscillation and the multilayer formation can be explained as follows. First note that U is kept constant externally with a potentiostat in the present case. In the high-current stage of the current oscillation, the tme electrode potential (or Helmholtz double layer potential), E, is much more positive than U because E is given hy E=U —JAR, where A is the electrode area, R is the resistance of the solution between the electrode surface and the reference electrode, andj is taken as negative for the reduction current. This implies that, even if U is kept constant in the region of the NDR, is much more... 

Under potentiostatic conditions, the photocurrent dynamics is not only determined by faradaic elements, but also by double layer relaxation. A simplified equivalent circuit for the liquid-liquid junction under illumination at a constant DC potential is shown in Fig. 18. The difference between this case and the one shown in Fig. 7 arises from the type of perturbation introduced to the interface. For impedance measurements, a modulated potential is superimposed on the DC polarization, which induces periodic responses in connection with the ET reaction as well as transfer of the supporting electrolyte. In principle, periodic light intensity perturbations at constant potential do not affect the transfer behavior of the supporting electrolyte, therefore this element does not contribute to the frequency-dependent photocurrent. As further clarified later, the photoinduced ET... 

The double-pulse potentiostatic method (Fig. 5.18C) is suitable for studying the products or intermediates in electrode reactions, formed in the A pulse by means of the B pulse. For example, if an electroactive substance is reduced in pulse A and if pulse B is sufficiently more positive than pulse A, then the product can be reoxidized. The shape of the I-t curve in pulse B can indicate, for example, the degree to which the unstable product of the electrode reaction is changed in a subsequent chemical reaction. 

Fig. 5.18 Potentiostatic methods (A) single-pulse method, (B), (C) double-pulse methods (B for an electrocrystallization study and C for the study of products of electrolysis during the first pulse), (D) potential-sweep voltammetry, (E) triangular pulse voltammetry, (F) a series of pulses for electrode preparation, (G) cyclic voltammetry (the last pulse is recorded), (H) d.c. polarography (the electrode potential during the drop-time is considered constant this fact is expressed by the step function of time—actually the potential increases continuously), (I) a.c. polarography and (J) pulse polarography...

The simplest case occurs when a number of nuclei is formed at the beginning of the process and does not change with time instantaneous nucleation). This is the case when the electrolysis is carried out by the double-pulse potentiostatic method (Fig. 5.18B), where the crystallization nuclei are formed in the first high, short pulse and the electrode reaction then occurs only at these nuclei during the second, lower pulse. A second situation in which instantaneous nucleation can occur is when the nuclei occupy all the active sites on the electrode at the beginning of the electrolysis. 

Emersion has been shown to result in the retention of the double layer structure i.e, the structure including the outer Helmholtz layer. Thus, the electric double layer is characterised by the electrode potential, the surface charge on the metal and the chemical composition of the double layer itself. Surface resistivity measurements have shown that the surface charge is retained on emersion. In addition, the potential of the emersed electrode, , can be determined in the form of its work function, , since and represent the same quantity the electrochemical potential of the electrons in the metal. Figure 2.116 is from the work of Kotz et al. (1986) and shows the work function of a gold electrode emersed at various potentials from a perchloric acid solution the work function was determined from UVPES measurements. The linear plot, and the unit slope, are clear evidence that the potential drop across the double layer is retained before and after emersion. The chemical composition of the double layer can also be determined, using AES, and is consistent with the expected solvent and electrolyte. In practice, the double layer collapses unless (i) potentiostatic control is maintained up to the instant of emersion and (ii) no faradaic processes, such as 02 reduction, are allowed to occur after emersion. 

Emersion of an electrode from electrolyte with its double layer intact is now a widely accepted phenomenon and technique. Not only is it a phenomenon which deserves careful consideration and study, but also a process which opens up a new set of experimental methods to the study of the electrochemical double layer. Electrode emersion involves the careful removal of an electrode from electrolyte under potentiostatic control, usually hydrophobically 11-5). When fairly concentrated electrolyte parts ("unzips") from the electrode surface during hydrophobic emersion, the double layer remains essentially intact on the electrode surface and no electrolyte outside the double layer remains. This phenctnenon is not due to the presence of organics or other impurities as seme have suggested. The emersion process works well with rigorously clean electrode surfaces (5). 

FIGURE 2.45. Equivalent circuit for the cell and instrument. WE, RE, and CE, working, reference, and counter electrodes, respectively iph, photocurrent ij/, double-layer charging current Q, double-layer differential capacitance Rc, Ru, cell compensated (by the potentiostat) and uncompensated resistances, respectively Rs, sampling resistance RP, potentiostat resistance E, potential difference imposed by the potentiostat between the reference and working electrodes Vpu, photo-potential as measured across the sampling resistor. Adapted from Figure 1 of reference 51, with permission from Elsevier. 

A polarizable Interface is represented by a (polarizable) electrode where a potential difference across the double layer is applied externally, i.e., by applying between the electrode and a reference electrode using a potentiostat. At a reversible interface the change in electrostatic potential across the double layer results from a chemical interaction of solutes (potential determining species) with the solid. The characteristics of the two types of double layers are very similar and they differ primarily in the manner in which the potential difference across the interface is established. 

Potentiostatic techniques, 787, 1115, 1118 and impurities on electrodes. 1120 potential interval measurements, 1121 p-polarized light, 802 Potentiodynamic techniques, 1423, 1438 vs. potentiostatic techniques, 1426 Potentiostatic transients, 1414 difficulties in, 1415 double layer charging, 1416 radicals in, 1416 IR drop in, 1416 Prandtl layer, 1228... 

Fig. 7.42. A potentiostatic transient. The current (A-B) ascends almost vertically after being switched on, because all of it goes to charge the double layer. In B-C, the current is increasingly used in the form of electrons crossing the double layer. After C the current should decline slowly as diffusion control sets in. In reality, at solid polycrystalline electrodes, in reactions involving adsorbed intermediates, there is often some further variation of /, owing to, e.g., surface crystalline rearrangements and the effect of impurities from the solution.

Figure 8.9 shows a somewhat idealized potentiostatic relation. Here, a poten-tiostat controls the electrode potential and one observes the current as a function of time. At higher times, the current becomes transport controlled. How may one eliminate this influence and get back to apart of the (t)v—f relation influenced by neither double-layer charging nor diffusion control so that the exchange current and rate constant of the electrode reaction can be obtained (Bockris)... 

The net effect of the presence of the solution resistance on potential excitation methods is that the potential seen by the electrode solution interface is different from the potential applied by the potentiostat. This difference is current-dependent and the current is itself potential-dependent. The resistance also makes it more difficult to separate current components arising from the double-layer capacitance from the faradaic process. Similar complications arise for current excitations. 

An equivalent circuit of the three-electrode cell discussed in Chapters 6 and 7 is illustrated in Figure 9.1. In this simple model, Rr is the resistance of the reference electrode (including the resistance of a reference electrode probe, i.e., salt bridge), Rc is the resistance between the reference probe tip and the auxiliary electrode (which is compensated for by the potentiostat), Ru is the uncompensated resistance between the reference probe and the working-electrode interphase (Rt is the total cell resistance between the auxiliary and working electrodes and is equal to the sum of Rc and Ru), Cdl is the double-layer capacitance of the working-electrode interface, and Zf is the faradaic impedance of the electrode reaction. 

The steady-state method has advantages in its freedom from double-layer charging and in the simplicity of light and current measurements. The transient method is mechanically simpler and does not require a dual potentiostat (Chap. 6). Moreover, the rate of electrolysis is smaller, and hence solutions may suffer more slowly from the buildup of contaminants arising from side reactions. 

The potentiostatic transient can be divided into three time intervals. At the beginning, the first time interval, the current decays during the process of nucleation and growth. This is the double-layer charging current, iAX. In the second... 

There are other advantages of microelectrodes as compared to the macroelectrodes. Because the current is small, on the order of nA to pA, the voltage drop due to the electrical resistance of the medium is negligible. For this reason, it is possible to perform electrochemical experiments in media that would be otherwise unsuitable for macroelectrodes, such as resistive hydrocarbon solvents and solid electrolytes, without a potentiostat. Secondly, the double-layer capacitance is very small because the area of the electrode is small. This means that very fast modulation experiments... 

Ohmic drop in three-electrode circuits. In modem coulometry and voltammetry the use of a potentiostat and a three-electrode configuration is the routine practice. The three electrodes are usually called the working, reference, and counter (or auxiliary) electrodes (see Figure 5.2). The cell current passes between the working electrode immersed in the test solution and the counter electrode, which may be in the test solution but is usually isolated from it by a single- or double-junction glass frit. 

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