Interfacial electrochemical methods

Although there are only three principal sources for the analytical signal—potential, current, and charge—a wide variety of experimental designs are possible too many, in fact, to cover adequately in an introductory textbook. The simplest division is between bulk methods, which measure properties of the whole solution, and interfacial methods, in which the signal is a function of phenomena occurring at the interface between an electrode and the solution in contact with the electrode. The measurement of a solution s conductivity, which is proportional to the total concentration of dissolved ions, is one example of a bulk electrochemical method. A determination of pH using a pH electrode is one example of an interfacial electrochemical method. Only interfacial electrochemical methods receive further consideration in this text. 

The largest division of interfacial electrochemical methods is the group of dynamic methods, in which current flows and concentrations change as the result of a redox reaction. Dynamic methods are further subdivided by whether we choose to control the current or the potential. In controlled-current coulometry, which is covered in Section IIC, we completely oxidize or reduce the analyte by passing a fixed current through the analytical solution. Controlled-potential methods are subdivided further into controlled-potential coulometry and amperometry, in which a constant potential is applied during the analysis, and voltammetry, in which the potential is systematically varied. Controlled-potential coulometry is discussed in Section IIC, and amperometry and voltammetry are discussed in Section IID. 

The electrode whose potential is a function of the analyte s concentration (also known as the working electrode). 

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

An electrode whose potential remains constant and against which other potentials can be measured. 

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The diversity of interfacial electrochemical methods is evident from the partial family tree shown in Figure 11.1. At the first level, interfacial electrochemical methods are divided into static methods and dynamic methods. In static methods no current passes between the electrodes, and the concentrations of species in the electrochemical cell remain unchanged, or static. Potentiometry, in which the potential of an electrochemical cell is measured under static conditions, is one of the most important quantitative electrochemical methods, and is discussed in detail in Section IIB. 

The classical electrochemical methods are based on the simultaneous measurement of current and electrode potential. In simple cases the measured current is proportional to the rate of an electrochemical reaction. However, generally the concentrations of the reacting species at the interface are different from those in the bulk, since they are depleted or accumulated during the course of the reaction. So one must determine the interfacial concentrations. There axe two principal ways of doing this. In the first class of methods one of the two variables, either the potential or the current, is kept constant or varied in a simple manner, the other variable is measured, and the surface concentrations are calculated by solving the transport equations under the conditions applied. In the simplest variant the overpotential or the current is stepped from zero to a constant value the transient of the other variable is recorded and extrapolated back to the time at which the step was applied, when the interfacial concentrations were not yet depleted. In the other class of method the transport of the reacting species is enhanced by convection. If the geometry of the system is sufficiently simple, the mass transport equations can be solved, and the surface concentrations calculated. 

Results are similar for films deposited on YSZ however, there appears to be a difference between films deposited on ceria vs YSZ in terms of interfacial electrochemical resistance. As shown previously in Figure 6c, LSC films on YSZ often exhibit a second high-frequency impedance associated with oxygen-ion exchange across the electrode/electrolyte interface.That this difference is associated with the solid—solid interface has been confirmed by Mims and co-workers using isotope-exchange methods. As discussed in greater detail in sections 6.1—6.3, this interfacial resistance appears to result from a reaction between the electrode and electrolyte, sometimes detected as a secondary phase at the interface. 

Li et al. [278] have studied adsorption of L-phenylalanine at Au(lll) electrodes using electrochemical and subtractively normalized interfacial FTIR methods. It has been found that the adsorbed molecules change their orientation with the electrode potential. At a negatively charged surface, the compound was predominantly adsorbed in the neutral form of the amino acid. At potentials positive with respect to pzc, L-phenylalanine was adsorbed predominantly as zwitterion with —GOO ... 

Kazarinov VE, Andreev VN, Mayorov AP. Investigation of the adsorption properties of the Ti02 electrode by the radioactive tracer method. J Electroanal Chem Interfacial Electrochem 1981 130 277-285. 

Non-electrochemical methods can and should be used for studying electrode surfaces and the interfacial region structure, particularly in situ in real time where this is possible. 

Refs. [i] Adamson AW, Cast AP (1997) Physical chemistry of surfaces, 6til edn. Wiley, New York [ii] Schmickler W (1996) Interfacial electrochemistry. Oxford University Press, New York [iii] Bard AJ, Faulkner LR (2001) Electrochemical methods, 2nd edn. Wiley, New York [iv] Wieck-owski A (ed) (1999) Interfacial electrochemistry. Theory, experiment, and applications. Marcel Dekker, New York... 

The cleanliness and single crystallinity of electrode surfaces are not assumed even if the preparative steps outlined above are followed. The verification or identification of initial, intermediate, and final interfacial stmctures and compositions is an essential ingredient in our studies. The interfacial characterization methods employed to date have been conveniently classified in terms of whether they are conducted under reaction conditions (in situ) or outside the electrochemical cell (ex situ). In situ methods here consisted of cychc voltammetry (CV), EC-STM and DBMS. Ex situ methods included LEED, AES, and HREELS. 

FIGURE 1.23 Communicating with a conducting polymer PPy/Cl in solution (a) cyclic voltammetry—a plot of current flow versus the electrical (potential) stimulus applied (b) the electrochemical quartz crystal mircobalance readout—mass polymer versus electrical (potential) stimulus applied (c) the resistometry readout—resistance of the polymer versus the electrical (potential) stimulus applied. (Printed with permission from Materials Science Forum, Vol. 189-190, Characterization of conducting polymer-solution interfacial processes using a new electrochemical method, A. Talaie, G. G. Wallace, 1995, p. 188, Trans Tech Publications, Switzerland.)... 

Experimental studies of ET at the ITIES by conventional electrochemical methods are scarce (8,34-36) and the data are often complicated by coupling of the interfacial ET and IT reactions and experimental artifacts (1,16). 

The quality of the Si/Si02 interface is crucial in MOS devices. The interfacial structure or the flatness on the atomic scale becomes very important as the demands for very thin oxide increases. The Si/Si02 interfacial structure has been studied by TEM, and by AFM/STM for the surfaces after the oxide layer is removed by chemical etching. Here, we report a novel electrochemical method for the evaluation of the interfacial structure, which can be applicable to a wide range of the thickness of the oxide layers. 

Among the six interfacial variables discussed in this section, the surface charge density oo, the surface potential (fo, and the potential at the OHP fd (usually called the diffuse layer potential), are most important in characterizing interfacial properties. The three remaining variables (i.e., ap, /p, and Od) can be estimated using Eqs. (5), (7), and (8) if oo, and /rf are known exactly. ao can be determined experimentally by the potentiometric titration method, and detailed explanation of the potentiometric titration is given, for example, by Yates [10]. The estimate of fo for the ceramic powder/aqueous solution interface is discussed in the next section, yd is perhaps the most important interfacial electrochemical parameter since it is closely correlated with the kinetic stability of a given colloidal suspension and it can be conveniently determined (approximately) experimentally. 

Conventional electrochemical methods provide a vast amount of kinetic and mechanistic information about heterogeneous redox processes. However, it is desirable to supplement this with the molecular structural information that can now be provided by several in-situ surface analytical techniques [1, 2]. Of the techniques available, infrared spectroscopy is well suited for this task since the spectral data can yield valuable information on the identity as well as the reactivity of the interfacial species. This is especially true when examining multistep reactions involving adsorbed intermediate. 

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