Characteristics of an Ideal Electrolyte

Gas sensors — (b) Gas sensors with solid electrolytes — Figure 4. Voltage characteristics of an idealized hydrocarbon electrode vs. Pt-air reference electrode in propylene-containing mixtures... 

Fig. 4 shows the example of cyclic voltammetiy curves for an activated carbon at 5 mV s" using different electrolyte (aqueous, organic, ionic liquid), where it is well visible that the voltage range is imposed by the stability vrfndow of the electrolyte. These almost rectangular box like shape curves are characteristic of an ideal EDL capacitor, with low ESR. 

The dependence i = f V) is shown in Fig. 7.2. At small values of FVfAT, the dependence is close to linear. As FV/AT increases, the current density exponentially tends to im- Such a volt-ampere characteristic corresponds to the ideal electrolyte. Therefore no current greater than im can exist in an ideal electrolyte. This restriction is typical for quiescent electrolytes. If the electrolyte moves, for exam-... 

The activity coefficient of the solvent remains close to unity up to quite high electrolyte concentrations e.g. the activity coefficient for water in an aqueous solution of 2 m KC1 at 25°C equals y0x = 1.004, while the value for potassium chloride in this solution is y tX = 0.614, indicating a quite large deviation from the ideal behaviour. Thus, the activity coefficient of the solvent is not a suitable characteristic of the real behaviour of solutions of electrolytes. If the deviation from ideal behaviour is to be expressed in terms of quantities connected with the solvent, then the osmotic coefficient is employed. The osmotic pressure of the system is denoted as jz and the hypothetical osmotic pressure of a solution with the same composition that would behave ideally as jt. The equations for the osmotic pressures jt and jt are obtained from the equilibrium condition of the pure solvent and of the solution. Under equilibrium conditions the chemical potential of the pure solvent, which is equal to the standard chemical potential at the pressure p, is equal to the chemical potential of the solvent in the solution under the osmotic pressure jt,... 

The activity a2 of an electrolyte can be derived from the difference in behavior of real solutions and ideal solutions. For this purpose measurements are made of electromotive forces of cells, depression of freezing points, elevation of boiling points, solubility of electrolytes in mixed solutions and other characteristic properties of solutions. From the value of a2 thus determined the mean activity a+ is calculated using the equation (V-38) whereupon by application of the analytical concentration the activity coefficient is finally determined. The activity coefficients for sufficiently diluted solutions can also be calculated directly on the basis of the Debye-Hiickel theory, which will bo explained later on. 

In the liquid phase, the simplest option is an ideal liquid, with an activity coefficient equal to 1.0. That choice leads to Raoult s law, which may suffice for similar chemicals. Other models include regular solution theory using solubility parameters (although not in Aspen Plus), NRTL, Electrolyte NRTL, UNIFAC, UNIQUAC, Van Laar, and Wilson. Characteristics of the models are ... 

Most often, the tabulated values of formal potential are given with respect to the normal hydrogen electrode (NHE), which has the defined potential 0 V. However, in practise, a silver/silver chloride (Ag/AgCl) electrode or a plain metal surface (e.g. Au or Pt) is commonly used as a RE. An Ag/AgCl RE, having an internal electrolyte of saturated KCl, has a characteristic potential of 197 mV with respect to the NHE. A plain metal surface, on the other hand, does not have a characteristic potential that can be expressed in terms of NHE. Instead, its potential depends on the prevailing conditions, affected by the deposited species and the electrolyte. E.g., if a Au surface is used as an RE to adjust the potential of, for instance, another Au surface (WE), both the RE and WE are affected by the same conditions. The equilibrium potential between such electrodes is ideally 0 V and a poised potential is directly an overpotential with respect to the equilibrium potential. 

Table 2 gives the electrochemical rate constants for an ideal PG cell. For a species to react at a specified electrode within an electrochemical reaction, the ratio D/X (where X is one of the length parameters outlined in Table 1) must be smaller than the rate constant k. The opposite is true for a species to be lost via reaction in the electrolyte or diffusion to the opposite electrode. As B is required at the illuminated electrode, and as shown by Eq. 8, for an ideal PG cell it is necessary for reaction length to be smaller than cell length, B is less likely to diffuse to the dark electrode than it is to be destroyed by reaction with Y. Consequently, for component B the characteristic length is X/. As explained further down, the concentration of Y and Z is assumed to be larger due to the diffusion distance, so for component Y the characteristic length is X [12]. 

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. 

---