Electrodes polarizable

In addition, the potential of the electrode can be varied, resulting in a change in the stmcture of the interface. If no current is passed when the potential of the electrode changes, the electrode is called an ideally polarizable electrode, and can be described using thermodynamics. 

Even in the absence of Faradaic current, ie, in the case of an ideally polarizable electrode, changing the potential of the electrode causes a transient current to flow, charging the double layer. The metal may have an excess charge near its surface to balance the charge of the specifically adsorbed ions. These two planes of charge separated by a small distance are analogous to a capacitor. Thus the electrode is analogous to a double-layer capacitance in parallel with a kinetic resistance. 

Tanahashi, 1., Yoshida, A. and Nishino, A., Electrochemical characterization of activated carbon fiber cloth polarizable electrodes for electric double layer capacitors. J. Electrochem. Soc., 1990, 137(10), 3052 3056. 

Another progress in our understanding of the ideally polarizable electrode came from theoretical works showing that the metal side of the interface cannot be considered just as an ideal charged plane. A simple quantum-mechanical approach shows that the distribution of the electron gas depends both on the charge of the electrode and on the metal-solution coupling [12,13]. 

The structure and composition of the lithium surface layers in carbonate-based electrolytes have been studied extensively by many investigators [19-37], High reactivity of propylene carbonate (PC) to the bare lithium metal is expected, since its reduction on an ideal polarizable electrode takes place at much more positive potentials compared with THF and 2Me-THF [18]. Thevenin and Muller [29] found that the surface layer in LiC104/PC electrolyte is a mixture of solid Li2C03 and a... 

Electrometric titrations with two polarizable electrodes involving chelating agents. K. Stulik and F. Vydra, Chelates Anal. Chem. 1969, 2, 93-115 (70). 

If the concentration of the metal ion is not negligible at the potential of zero charge, the electrode potential varies linearly with log c according to Eq. (2) and there is no distinctive sign of the situation where the charge at the interface vanishes. The Nemst approach is obviously unsuitable for defining the nature and the amount of the charge at an interface. If the concentration of the metal ion at the pzc is small or very small, the behavior of the interface becomes that of a polarizable electrode. 

For an ideally polarizable electrode, q has a unique value for a given set of conditions.1 For a nonpolarizable electrode, q does not have a unique value. It depends on the choice of the set of chemical potentials as independent variables1 and does not coincide with the physical charge residing at the interface. This can be easily understood if one considers that q measures the electric charge that must be supplied to the electrode as its surface area is increased by a unit at a constant potential." Clearly, with a nonpolarizable interface, only part of the charge exchanged between the phases remains localized at the interface to form the electrical double layer. 

Equation (17) expresses the cell potential difference in a general way, irrespective of the nature of the electrodes. Therefore, it is in particular valid also for nonpolarizable electrodes. However, since

interfacial structure, only polarizable electrodes at their potential of zero charge will be discussed here. It was shown earlier that the structural details are not different for nonpolarizable electrodes, provided no specifically adsorbed species are present. 

According to Fig. 2, as M comes in contact with S,3 4 the electron distribution at the metal surface (giving the surface potential XM) will be perturbed X ) The same is the case for the surface orientation of solvent molecules (Xs + SXS). In addition, a potential drop has to be taken into account at the free surface of the liquid layer toward the air (xs). On the whole, the variation of the electron work function (if no charge separation takes place as assumed at the pzc of a polarizable electrode) will measure the extent of perturbation at the surfaces of the two phases, i.e.,... 

Data from many experiments64,71,72,74,287-289 indicate that the differential capacitance of an ideally polarizable electrode at ff jin nonideal... 

Another type of supercapacitor has been developed in whieh instead of ideally polarizable electrodes, electrodes consisting of disperse platinum metals are used at which thin oxide films are formed by anodic polarization. Film formation is a faradaic process which in certain cases, such as the further partial oxidation and reduction of these layers, occurs under conditions close to reversibility. 

In order to obtain a definite breakthrough of current across an electrode, a potential in excess of its equilibrium potential must be applied any such excess potential is called an overpotential. If it concerns an ideal polarizable electrode, i.e., an electrode whose surface acts as an ideal catalyst in the electrolytic process, then the overpotential can be considered merely as a diffusion overpotential (nD) and yields (cf., Section 3.1) a real diffusion current. Often, however, the electrode surface is not ideal, which means that the purely chemical reaction concerned has a free enthalpy barrier especially at low current density, where the ion diffusion control of the electrolytic conversion becomes less pronounced, the thermal activation energy (AG°) plays an appreciable role, so that, once the activated complex is reached at the maximum of the enthalpy barrier, only a fraction a (the transfer coefficient) of the electrical energy difference nF(E ml - E ) = nFtjt is used for conversion. 

The interfacial tension always depends on the potential of the ideal polarized electrode. In order to derive this dependence, consider a cell consisting of an ideal polarized electrode of metal M and a reference non-polarizable electrode of the second kind of the same metal covered with a sparingly soluble salt MA. Anion A is a component of the electrolyte in the cell. The quantities related to the first electrode will be denoted as m, the quantities related to the reference electrode as m and to the solution as 1. For equilibrium between the electrons and ions M+ in the metal phase, Eq. (4.2.17) can be written in the form (s = n — 2)... 

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 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. 

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