Interface non-polarizable

This chapter will include equilibria at non-polarizable interfaces for a metal or semiconductor phase-electrolyte system (a galvanic cell in the broadest sense) and for two electrolytes (the solid electrolyte-electrolyte solution interface, or that between two immiscible electrolyte solutions). 

The liquid metal mercury-solution interface presents the advantage that it approaches closest to an ideal polarizable interface and, therefore, it adopts the potential difference applied between it and a non-polarizable interface. For this reason, the mercury-solution interface has been extensively selected to carry out measurements of the surface tension dependence on the applied potential. In the case of other metal-solution interfaces, the thermodynamic study is much more complex since the changes in the interfacial area are determined by the increase of the number of surface atoms (plastic deformation) or by the increase of the interatomic lattice spacing (elastic deformation) [2, 4]. 

This equation will be applied to an electrochemical cell formed by a polarizable and non-polarizable interface in line with the scheme... 

At the non-polarizable interface the electrochemical equilibrium holds (with the... 

The above discussion applies to a non-polarizable interface with a common electrolyte MX distributed in two immiscible phases a and P ... 

Liquid/liquid interfaces have been classified as ideal-polarizable interfaces and non-polarizable interfaces (25-27). Let us first discuss the system in which a strongly hydrophilic 1 1 electrolyte BjAi is dissolved in water (e.g., LiCl in water) and a strongly hydrophobic 1 1 electrolyte BjAj is dissolved in an organic solvent [e.g., tetrabutylam-moniiim tetraphenylborate (TBATPB) in NB], as shown in Cell 1 ... 

Figure 17.3.4 Mixed solvent model. (A) KCl/TBATPB system (polarizable interface) (B) TBACl/TBATPB system (non-polarizable interface). (Reprinted with permission from (54). Copyright 1985 Elsevier Science).

The difference between polarizable and nonpolarizable interfaces can be easily understood in terms of this equivalent circuit. A high value of Rp is associated with a polarizable interface, whereas a low value of Rp represents a non-polarizable interface. 

For an ideally polarizable interface with negligible solution resistance, the phase angle is —90°. For an ideally non-polarizable interface the phase angle is zero. Real systems do not behave ideally of course. The actual phase angle will, therefore, be somewhere in between, and it will depend on frequency in most cases. The phase-sensitive voltmeter can measure the absolute value of the impedance vector Z and the phase angle simultaneously... 

A good and well investigated example of a non-polarizable interface is that between silver iodide and aqueous solutions. At this iriterface one may distinguish chaises of ionic concentrations in the aqueous layer and a certain excess or defect of one of the kinds of lattice ions. Experimentally one determines the total adsorption of certain ions (cf 2, p 116) and thus at a certain adsorption of Ag+-ions it remains uncertain whether this is an adsorption in the lattice or in the liquid layer ( 4 f. 2, p. 139). Usually, however, the concentration of Ag- or I ions in the aqueous solution is very low and the adsorption in the liquid layer may be neglected (or a small correction applied for it). 

The reference electrode-solid electrolyte interface must also be non-polarizable, so that rapid equilibration is established for the electrocatalytic charge-transfer reaction. Thus it is generally advisable to sinter the counter and reference electrodes at a temperature which is lower than that used for the catalyst film. Porous Pt and Ag films exposed to ambient air have been employed in most previous NEMCA studies.1,19... 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

Hydrogen electrodes are approximately non-polarizable, which implies that the solution and the interface are in equilibrium. This simplifies the task of maintaining a constant reference potential. In an ideally non-polarizable electrode, the electrode... 

A non-polarizable electrode-solution interface is a reversible electrode. Therefore, the potential is determined by the composition of the solution based on the Nemst equation (given by Eq. 1.36). So, for example, for the copper electrode in a solution of CuS04 the potential is given by... 

In this section, the subtractive multipulse techniques DMPV and SWV are applied to reversible ion transfer across different liquid-liquid systems with one or two polarizable interfaces. These electrochemical techniques allow the accurate and easy determination of standard potentials directly from the peak potentials of the current-potential curves since non-faradaic and background currents are minimized [12, 35-40]. 

Karraker and Radke [18], accounting only for the interactions between the individual ions and the water and air continua (thus ignoring the term A33 in Eq. (11)), concluded that the dispersion interactions always repel the ions from the water/air interface, with the exception of the non-polarizable H+, for which Bu = °. 

Our calculations can be viewed as an extension of the work of Bryk and Haymet."" They studied the behavior of the Na and Cf ions at the static ice/water interface on the nanosecond timescale. We were interested in the solidification process itself and in the expulsion of the ions into the remaining liquid. This required using longer simulation times (hundreds of nanoseconds). For the purpose of our research, we used the SPC/E water potential.Potential parameters for sodium and chloride ions were taken as the non-polarizable set from Ref. 41. 

If reaction (8.8.7) is driven in either the forward or reverse direction by passing a current, the potential sAi < ) does not change provided there is no significant change in the ionic activity a,-. For this reason, the electrode solution interface is termed non-polarizable. In practice, the forward and reverse reactions of the equilibrium (8.8.7) occur at finite rates. 

The metal solution interface both in its polarizable and non-polarizable forms is extremely important in electroanalysis and in practical electrochemical devices. The discussion in this section has focused on its fundamental electrical properties. These systems are considered in much more detail in chapter 9, which deals with electrochemical equilibria, and chapter 10, which is concerned with polarizable interfaces and the electrical double layer. 

In examining the properties of the metal solution interfaee, two limiting types of behavior are found, namely, the ideal polarizable interfaee and the ideally non-polaiizable interface. In the former case, the interface behaves as a capacitor so that charge can be placed on the metal using an external voltage source. This leads to the establishment of an equal and opposite charge on the solution side. The... 

The thermodynamic properties of an ideally polarizable interface are most easily examined by considering an electrochemical cell with one polarizable electrode and one non-polarizable electrode. An example of such a system is... 

The above analysis shows how the GAI is applied to the simplest polarizable interface in contact with a 11 electrolyte. Other more complicated situations have been analyzed for systems with more complex electrolytes and molecular solutes. More details can be found in reviews by Mohilner [1] and Parsons [G4]. The essential feature of these analyses is that an equation is derived which relates the change in interfacial tension to the change in the potential of the polarizable electrode with respect to that of a non-polarizable electrode, and to the chemical potentials of the components of the solution. 

The above result may raise questions, because the properties of adsorbed monolayers at charged interfaces should be governed by the long-range coulombic interactions. However, it can be easily explained if we take into account that, when we adopt an adsorption mechanism like that represented by Eq. (2), in fact, we model the adsorbed layer as a mixture of adsorbate A molecules and solvent clusters Sa with dimensions equivalent to A. That is, the adsorbed layer consists of species with dimensions greater than 0.25 nm and therefore the (hstance of the closest approach between two adsorbed dipoles cannot fall below 0.5 - 0.6 nm. Thus when we model the adsorbed layer as a mixture of adsorbate A molecules and solvent clusters Sa, the coulombic interactions stop to play the dominant role regarding the properties of this layer. This result is independent of whether we have polarizable or non-polarizable adsorbed molecules and, in fact, verifies the use of... 

Linear spectroscopy Reflection - TIR, ATR, Non-Polarizable ER Interface water/dodecane 35, 36... 

In practice the main requirement of a reference electrode is that it has a stable potential and that this is not substantially changed during the experiment. This is the case with the hypothetical, completely non-polarizable electrode, the potential of which is unaffected when electric current flows across the metal-solution interface. For practical conditions this means that the exchange current must be large compared with any net current that it is required to pass in use. Ideally "no" current flows through the reference electrode (in a three electrode system) if a high imp ance (>10Mfl) voltmeter is used. 

In all cases such interfaces are non-polarizable, since the only way to change the interfacial potential difference is to modify the bulk properties of the phase(s), for example, the degree of doping or stoichiometry for semiconductors. 

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