Temperature dependence electrolyte effects

The obvious question then arises as to whether the effective double layer exists before current or potential application. Both XPS and STM have shown that this is indeed the case due to thermal diffusion during electrode deposition at elevated temperatures. It is important to remember that most solid electrolytes, including YSZ and (3"-Al2C)3, are non-stoichiometric compounds. The non-stoichiometry, 8, is usually small (< 10 4)85 and temperature dependent, but nevertheless sufficiently large to provide enough ions to form an effective double-layer on both electrodes without any significant change in the solid electrolyte non-stoichiometry. This open-circuit effective double layer must, however, be relatively sparse in most circumstances. The effective double layer on the catalyst-electrode becomes dense only upon anodic potential application in the case of anionic conductors and cathodic potential application in the case of cationic conductors. 

According to Equation 6.6, the velocity of the EOF is directly proportional to the intensity of the applied electric held. However, in practice, nonlinear dependence of the EOF on the applied electric held is obtained as a result of Joule heat production, which causes the increase of the electrolyte temperature with consequent decrease of viscosity and variation of all other temperature-dependent parameters (protonic equilibrium, ion distribution in the double layer, etc.). The EOF can also be altered during a run by variations of the protonic concentration in the anodic and cathodic electrolyte solutions as a result of electrophoresis. This effect can be minimized by using electrolyte... 

The calorimetric measurements in metal oxide-aqueous electrolyte solution systems are, beside temperature dependence of the pzc measurements, the method for the determination of the enthalpy of the reaction in this system. Because of the low temperature effects in such systems they demand very high precision. That is why these measurements may be found only in a few papers from the last ten years [89-98]. A predominant number of published measurements were made in the special constricted calorimeters (bath type), stirring the suspension. The flow calorimeters may be used only for sufficiently large particles of the solid. A separate problem is the calculation of the enthalpy of the respective reactions from the total heat recorded in the calorimeter. A total thermal effect consists of the heat of the neutralization in the liquid phase, heat connected with wetting of the solid, heat of the surface reaction and heat effects caused by the ion solvation changes (the ions that adsorb in the edl). Considering the soluble oxides, one should include the effects connected with the transportation of the ions from the solid to the solution... 

Temperature variations. Essentially all kinetic phenomena are temperature dependent ion diffusion (in both electrolyte and active materials), electron transfer, desolvation, adsorption, etc. Additionally, thermodynamic equilibrium constants are temperature dependent, so any temperature variations within the cell will produce uneven plating and stripping, electrode shape change effects and uneven utilization again leading to compromised performance. 

The paper by Van den Vlekkert et al. [120] presented the measurements of temperature dependence of the surface potential t/>o at the 7 AI2O3 electrolyte interface. According to these authors, such investigations are of a double significance. Firstly, because this device can be used as miniature sensors of pH measurements, e.g. in the clinical measurement of blood acidity. Secondly, the function has some relations to the thermal effects accompanying adsorption of one proton or two protons on the oxide surface. Knowing these values, one can determine indirectly the values of these heats. The heat values obtained in this way can be compared with those determined from suitable analysis of calorimetric experimental data. 

Rudziriski. W. et al., Calorimetric effects and temperature dependence of simple ion adsorption at oxide/electrolyte interfaces The systems in which PZC and CIP do not coincide, J. Colloid Interf. Sci.. 226, 353, 2000. 

Rudziriski, W. et al.. Estimation of enthalpic effects of ion adsorption at oxide/ electrolyte interfaces from temperature dependence of adsorption data. Colloids Surf. A, 152, 381, 1999. 

If data are lacking on the system of interest, it is often a fair approximation (especially at low and moderate concentrations) to use a model-substance approach. The behavior of an electrolyte is assumed to be similar to that of a known electrolyte of the same charge type. For example, NaCl is a model substance for 1 1 salts. This approach is particularly useful in estimating the temperature dependence of activity and osmotic coefficients when these coefficients are known only at 25°C, the model-substance approach may be used to estimate the effect of temperature. 

Values closer to 2 12 are found if allowance is made for dissociation in calculating M. This would indicate that no change in association is produced by the presence of the electrolyte in the water. The Eotvos constants of binary mixtures seem to depend on the concentration and temperature.6 The effect of temperature is either (i) normal, when d[a(Mv)2l3]jdt is about 2 1, or (ii) abnormal, when this coefficient is less than 2 1 but increases with temperature from these results, conclusions have been drawn as to the molecular weights of dissolved substances.7 Light (including ultraviolet) has no influence on the surface tension of solutions.8... 

It should be noted that pKw is temperature-dependent and can affect the calculation. Table 2.7 presents the effect of pH on the ionization of weak electrolytes (acids and bases) and is taken from Doluisis and... 

Contrary to outer sphere electron transfer reactions, the validity of the Butler-Volmer law for ion transfer reactions is doubtful. Conway and coworkers [225] have collected data for a number of proton and ion transfer reactions and find a pronounced dependence of the transfer coefficient on temperature in all cases. These findings were supported by experiments conducted in liquid and frozen aqueous electrolytes over a large temperature range [226, 227]. On the other hand, Tsionskii et al. [228] have claimed that any apparent dependence of the transfer coefficient on temperature is caused by double layer effects, a statement which is difficult to validate because double layer corrections, in particular their temperature dependence, depend on an exact knowledge of the distribution of the electrostatic potential at the interface, which is not available experimentally. Here, computer simulations may be helpful in the future. Theoretical treatments of ion transfer reactions are few they are generally based on variants of electron transfer theory, which is surprising in view of the different nature of the elementary act [229]. 

Having expressions for the temperature and pressure effects on C° and V°, straightforward, if somewhat lengthy, integration gives expressions for AS°, AH°, and AG°, which can refer either to an ion j or an electrolyte k, depending on the fit parameters used in the expression. Thus... 

---