Ideally polarised electrode

The calomel electrode Hg/HgjClj, KCl approximates to an ideal non-polarisable electrode, whilst the Hg/aqueous electrolyte solution electrode approximates to an ideal polarisable electrode. The electrical behaviour of a metal/solution interface may be regarded as a capacitor and resistor in parallel (Fig. 20.23), and on the basis of this analogy it is possible to distinguish between a completely polarisable and completely non-polarisable... 

The thermodynamic theory of the ideally polarised electrode has been extensively reviewed in the past few decades [1-5], and the relationship with the ideally non-polarisable interface has been derived in an elegant treatment by Parsons [6]. The starting point in all derivations is the Gibbs-Duhem equation which defines the relationship between the extensive thermodynamic variables. For a bulk phase this has the form ... 

Hence, the flux /, of any species i is constant through space. Since for an ideally polarisable electrode, the flux of any species into or out of the electrode must be zero (since the species does not absorb, adsorb or react at this boundary), Ji = 0 everywhere. Therefore from the Nernst-Planck equation... 

Important thermoelectrochemical insight has been obtained by investigating the temperature dependencies of a, C and p. The temperature coefficients of a at the electrocapillary maximum, of C at the capacitive minimum and of are important sources for information about the double layer stmcture. The latter depends on temperature dependent changes of physical solution properties (see Sect. 2.5). Classical investigations have been done at ideal polarizable electrodes , i.e. at electrodes where no charge transfer across the interface electrode/solution is occurring during polarisation. Very often, the mercury drop electrode has been used as an example of an ideal polarisable electrode. 

We first consider what happens when a surface of an ideally polarisable electrode is charged. The charging of the electrode happens when the electrolyte counterions balance the induced charge on the electrode surface. This accumulation of charge... 

The opposite extreme, the ideal polarisable electrode , is one through which no current would pass whatever potential were imposed on the electrode. This condition is approached within certain potential limits by electrode-solution combinations in which no reversible charge-transfer process is established. 

Sadkowski, A., On the ideal polarisability of electrode displaying CPE-type capacitance dispersion. Journal of Electroanalytical Chemistry, 2000, 481 pp. 222-226 Sadkowski, A, Response to the Comments on the ideal polarisability of electrodes displaying CPE-type capacitance by G. Ldng, K. E. Heusler. Journal of Electroanalytical Chemistry, 2000, 481 pp. 232-236... 

Zoltowski, P, Comments on the paper On the ideal polarisability of electrodes displaying CPE-type capacitance by A. Sadkowski. Journal of Electroanalytical Chemistry, 2000, 481 pp, 230-231... 

Electrochemical surface thermodynamics is a boundless science. For ideally polarisable (mercury-like) electrodes it is to a great extent fixed in textbooks and courses. On the contrary, for... 

An ideal non-polarisable electrode is a system through which large currents can pass in either direction without displacing the electrode potential from its equilibrium position. This is unattainable in practice, but electrode systems which have a high exchange current density (q.v.), such as the Ag, Ag electrode, can support large currents at quite a small overpotential (q.v.). 

An ideal reversible cell is characterised by an e.m.f. that remains constant irrespective of the rate of reaction in either direction, i.e. each interface constituting the cell must be so completely non-polarisable that it resists any attempt to change its potential. Although this is impossible to achieve in practice, a number of interfaces approximate to ideality providing the rate of reaction is maintained at a very low value. These reversible electrodes (or half-cells) are used as reference electrodes for determining the potential of a single electrified interface. 

A real system which approaches this ideal closely is that of a mercury electrode at a potential where it is perfectly polarised. In aqueous KCl solution, for example, there is a region extending to about --1.8V vs the Hg/Hg2Cl2 potential where the ideal condition of purely capacitance impedance holds good. 

At the effective property or continuum level, the simulation of electrode and cell performance basically requires only a parameterised electrochemical model. Such an electrochemical model is usually described as a current-voltage relation, or I-V curve, for a single cell, in terms of parameters that are effective cell properties and operational parameters. The I-V relation describes the voltage (potential) loss at a specified current with respect to the ideal thermodynamic performance, which is called overpotential or polarisation (q). This cell I-V curve is specific for the materials, structural characteristics, and operational parameters (gas compositions, pressure, temperature) of a given PEN element. 

A third distinct type of electrode model developed in response to the need for modelling the composite structure of SOFC electrodes more accurately is the Monte Carlo, or stochastic structure, model. This model is based on a random number-generated 2- or 3-D structure of electrode particles, electrolyte particles, and holes (for gas pores). It has been shown to represent the composite conductivity quite well and may be able to model polarisation behaviour adequately [56-58]. This is of Interest because microstructure, and in particular hard-to-control variations in local microstructure, may have an important effect on overall polarisation, perhaps more so than the intrinsic kinetic characteristics measured at an ideal interface. 

The major loss of fuel cell voltage is due to the ohmic and electrode polarisations. Eflfoits are made to reduce the polarisations, so that V approaches E and hence maximum efficiency is obtained. This can be done by improving the electrode stmcffire, using highly conducting electrol5die and better electrocatalysts. Hence, the actual and ideal potentials for a fuel cell are different due to various losses. 

W Wagner s DC polarisation cell. The negative electrode is a reversible electrode and the activity of M is unity at the MIP MX interface. A DC potential E is applied so that the activity of M at the P MXIion-blocking electrode interface is exp(-EFIRT) at steady state. Boltzmann statistics for an ideal solid solution are assumed for the electrons and electron holes in P MX. The concentration of electrons and electron holes are denoted by C and C respectively. The superscript zero denotes that the concentration in P /MX is at equilibrium with M. 

This technique allows one to distinguish between hole and electron transport and measure this across a broad range of fields and at different temperatures. As the optical properties of liquid crystals are commonly studied in liquid crystalline cells, which have electrodes, the cells make ideal ToF samples. It is worth noting that application of an electric field across a liquid crystalline cell may disrupt the orientation of the molecules in the mesophase particularly in low-viscosity, high-temperature nematic phases. It is necessary to verify, using polarised microscopy, that such switching is not occurring in samples used for mobility measurements. 

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