Electrolytes capacitance measurements

The differential capacitance method cannot be used for reactive metals, such as transition metals in aqueous solutions, on which the formation of a surface oxide occurs over a wide potential re ion. An immersion method was thus developed by Jakuszewski et al. 3 With this technique the current transient during the first contact of a freshly prepared electrode surface with the electrolyte is measured for various immersion potentials. The electrode surface must be absolutely clean and discharged prior to immersion.182-18 A modification of this method has been described by Sokolowski et al. The values of obtained by this method have been found to be in reasonable agreement with those obtained by other methods, although for reactive metals this may not be a sufficient condition for reliability. 

The experimental data bearing on the question of the effect of different metals and different crystal orientations on the properties of the metal-electrolyte interface have been discussed by Hamelin et al.27 The results of capacitance measurements for seven sp metals (Ag, Au, Cu, Zn, Pb, Sn, and Bi) in aqueous electrolytes are reviewed. The potential of zero charge is derived from the maximum of the capacitance. Subtracting the diffuse-layer capacitance, one derives the inner-layer capacitance, which, when plotted against surface charge, shows a maximum close to qM = 0. This maximum, which is almost independent of crystal orientation, is explained in terms of the reorientation of water molecules adjacent to the metal surface. Interaction of different faces of metal with water, ions, and organic molecules inside the outer Helmholtz plane are discussed, as well as adsorption. 

It may be noted that the statement made above—that the surface potential in the electrolyte phase does not depend on the orientation of the crystal face—is necessarily an assumption, as is the neglect of S s1- It is another example of separation of metal and electrolyte contributions to a property of the interface, which can only be done theoretically. In fact, a recent article29 has discussed the influence of the atomic structure of the metal surface for solid metals on the water dipoles of the compact layer. Different crystal faces can allow different degrees of interpenetration of species of the electrolyte and the metal surface layer. Nonuniformities in the directions parallel to the surface may be reflected in the results of capacitance measurements, as well as optical measurements. 

The electrode roughness factor can be determined by using the capacitance measurements and one of the models of the double layer. In the absence of specific adsorption of ions, the inner layer capacitance is independent of the electrolyte concentration, in contrast to the capacitance of the diffuse layer Q, which is concentration dependent. The real surface area can be obtained by measuring the total capacitance C and plotting C against Cj, calculated at pzc from the Gouy-Chapman theory for different electrolyte concentrations. Such plots, called Parsons-Zobel plots, were found to be linear at several charges of the mercury electrode. ... 

A solution of brain lipids was brushed across a small hole in a 5-ml. polyethylene pH cup immersed in an electrolyte solution. As observed under low power magnification, the thick lipid film initially formed exhibited intense interference colors. Finally, after thinning, black spots of poor reflectivity suddenly appeared in the film. The black spots grew rapidly and evenutally extended to the limit of the opening (5, 10). The black membranes have a thickness ranging from 60-90 A. under the electron microscope. Optical and electrical capacitance measurements have also demonstrated that the membrane, when in the final black state, corresponds closely to a bimolecular leaflet structure. Hence, these membranous structures are known as bimolecular, black, or bilayer lipid membranes (abbreviated as BLM). The transverse electrical and transport properties of BLM have been studied usually by forming such a structure interposed between two aqueous phases (10, 17). 

Cjnn is the capacitance due to the inner layer, which can be experimentally obtained from the plot of 1/Ld (with Cd being the capacitance measured at a given charge density) for several electrolyte concentrations versus the calculated l/LG-ch at a constant surface charge density (Parsons and Zobel plot) [2]. If this plot is not linear, this is an indication that specific adsorption occurs. 

The following data were obtained during interfacial capacitance measurements of a single-crystal n-Ti02 electrode in 0.1 MTBAP (tributylammonium phosphate) + CHjCN at a frequency of 500 Hz. Calculate the flatband potential on n-Ti02 in this electrolyte and the concentration ofmajority carriers. Assume e = 86. 

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 polysiloxanes were characterized by Fourier transform-IR (FTIR) spectroscopy, H and Si NMR spectrometry, and by GPC. AC conductivities of the polymer electrolytes were measured under dry helium by using an automatic capacitance bridge (General Radio Corporation). Glass transition (Tg) and melt (TJ temperatures were recorded on a differential scanning calorimeter (Perkin Elmer DSC-4). More detailed experimental procedures are published elsewhere (9, 12). 

Figure 17. The double layer capacitance measured with two different metals (Ag and Hg) in an aqueous electrolyte solution as a function of the charge density at the surface. The influence of the nature of the metal phase on the total capacitance is clearest around the point of zero charge. The results are taken from W. Schmickler, Chem. Rev. 96, 3177 (1996).

As with metals, the Helmholtz layer is developed by adsorption of ions or molecules on the semiconductor surface, by oriented dipoles or, especially in the case of oxides, by the formation of surface bonds between the solid surface and species in solution. Recourse to band-edge placement can be sought through differential capacitance measurements on the semiconductor-redox electrolyte interface [29j. 

Capacitance measurements are generally regarded as the most reliable method for determination of the band edge positions at a sensitized semiconductor-electrolyte interface [14]. The Mott-Schottky relationship, Eq. 7 ... 

The use of a small electrolyte covered resist area around the microelectrode is essential when capacitance measurements are performed, as the resist capacitance is parallel to the electrode capacitance. With a specific resistance of a = 1012 LI cm and a dielectric constant of e = 1.5 for the resist, and assuming a typical electrode capacitance of 10pFcm 2 with a 50 tm electrode, an error of 5% is obtained, if the electrolyte covered surface is 10 3cm2 (/=1000Hz) [88]. Thus, for capacitance measurements, the use of nanoliter droplets is essential. 

Figurel.44 CyclicvoltammogramofoxideformationforAl/Al203 and corresponding capacitance measurements recorded simultaneously with the voltammograms. Electrolyte acetate buffer, dU/dt = 20mVs-1 [21].

The surface state capacitance for t i CdTe-electrolyte interface is plotted as a function of electrode potential in Fig. 16 (the minimum was taken as the value at 0.2V NHE). The surface state capacitance decreases in the cathodic direction in the region -0.56 to -2.26V (NHE). Capacitance measurements at cathodic potentials less negative than -0.56V could not be carried out because of the onset of a C02 independent anodic dark current. Assuming (in consistence with other examples of pseudo capacitance behavior) that the capacitance-potential curve is symmetrical with respect to a maximum at -0.66V, the number of surface states was calculaed using the above equation. The number of surface states as a function of electrode potential, on the basis of this assumption, is shown in Fig. 17. Geometric area of the electrode was used to calculate the surface state density. Real surface area may be larger. 

FIGURE 4.16 Instrumentation scheme for impedance measurements 1 generator, 2, 4, and 6 amplifiers 3 attenuator 5 filter 7 zirconia oxygen sensor 8 osdlloscope C capacitance and Z electrochemical sensor impedance. (From Zhuiykov, S., In-situ diagnostics of solid electrolyte sensors measuring oxygen activity in melts by developed impedance method, Meas. Sci. Technol. 17 (2006) 1570-1578. With permission.)... 

Information about the PZC and the nature of the solid/electrolyte interface can be obtained from capacitance measurements with scrupulous care in electrode surface preparation and solution purity (35). For example, capacitance curves for Ag (100) at different concentrations of two electrolytes, KPF6 and NaF, are shown in Figure 13.4.5. The essential inde-... 

Further important detectors utilize electrochemical as well as conductivity and capacitance measurements. Electrochemical detectors are absolute while conductime-ters with extremely small measuring cells are very sensitive and rather specific detectors for gel chromatography of (poly)electrolytes. 

Electrochemical measurements with the thiol-coated gold surface acting as an electrode can provide very important information about the integrity and order of the monolayer . Two main phenomena are usually used for monolayer characterization capacitance measurements (with cychc voltammetry or impedance spectroscopy) and heterogeneous electron transfer (cychc voltammetry). The electrode in contact with solution acts as a capacitor whose capacitance strongly depends on the distance between the electrolyte and the metal surface, i.e. the thickness of the monolayer . Therefore, for well-packed monolayers which are impermeable for electrolyte, the measured capacitance can be used to calculate the thickness of a monolayer. These data are usually in good agreement with the thickness measured by other methods. 

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