Role of the Supporting Electrolyte

This section has been devoted to discussing the factors to be taken into account in choosing the solvent and supporting electrolyte. The dual role of the supporting electrolyte is established and in particular its role in ensuring diffusion control as the dominant mechanism for mass transport of analyte to the electrode illustrated. The marginal effect of temperature has been explained. 

The role of the supporting electrolyte is, however, more complex. It decreases the cell resistance, strongly influences the double-layer structure - at high concentration of supporting electrolyte the charging and faradaic processes can be separated, which allows simplification of the modeling of the cell impedance and the mathematical treatment - and, in analytical applications, may decrease matrix effects. 

A.4.1 - Highlighting the role of the supporting electrolyte in mass transport... 

The active role of the supporting electrolyte cation in the reduction of organohalogen compounds was mentioned by Levin as early as 1952 [69]. Examples of how a small change in the characteristics of the molecule brings about a sharp change in the course of the reaction right up to a change in the final product can be found with Feoktistov and Gol din [70]. 

The liquid medium (the solution to be studied) is mainly non- or poor-conductive (H20, organic solvents), which makes it practically impossible to pass electrical current between electrodes immersed in that solution. So, the first role of a supporting electrolyte is to provide the solution with some conductive properties by adding an electrolyte. 

Besides the activation overpotential, mass transport losses is an important contributor to the overall overpotential loss, especially at high current density. By use of such high-surface-area electrocatalysts, activation overpotential is minimized. But since a three-dimensional reaction zone is essential for the consumption of the fuel-cell gaseous reactants, it is necessary to incorporate the supported electrocatalysts in the porous gas diffusion electrodes, with optimized structures, for aqueous electrolyte fuel-cell applications. The supported electrocatalysts and the structure and composition of the active layer play a significant role in minimizing the mass transport and ohmic limitations, particularly in respect to the former when air is the cathodic reactant. In general, mass transport limitations are predominant in the active layer of the electrode, while ohmic limitations are mainly due to resistance to ionic transport in the electrolyte. For the purposes of this chapter, the focus will be on the role of the supported electrocatalysts in inhibiting both mass transport and ohmic limitations within the porous gas diffusion electrodes, in acid electrolyte fuel cells. These may be summarized as follows ... 

This section will provide you with the information that will allow you to select the solvent and the supporting electrolyte for a particular analysis. The nature and role of this supporting electrolyte will be explained. The effect of temperature is mentioned and together with the knowledge gained from Sections 1.2 and 1.3 you will be in a position to design a dc polarography experiment. 

The ability of the supporting electrolyte anion to stabilize the intermediate carbo-cations in only moderately polar solvents such as CH2CI2 or THF may play an important role for product formation. The 0104 ion is known to stabilize car-bocations in ether [9] and in alcoholic solutions [10], whereas the tosylate anion (TsO ) has been suggested to add to reactive carbocations [11] (see Sect. 14.3.1). Itis unfortunate that in many cases Cl04 has proven to be superior in EGA-catalyzed reactions, since the known safety risks of organic perchlorates (explosive) prevent the general use of metal perchlorates as the supporting electrolyte. 

One more question of principle on the role played by ionic atmosphere and micropotential in the electrochemical kinetics was considered/ Later these results were generalized for the case when there is no equilibrium with respect to the reacting ions in the presence of the excess quantity of the supporting electrolyte (Dogonadze and Kuznetsov ). 

An important role is also played by the nature of the support electrolyte and, in particular, of the anion. As an example, tosylate greatly favors the electrodeposition of PP but is detrimental for PT for the latter, innocent anions, such as perchlorate, tetrafluoroborate, or hexafluorophosphate, are the best choice. 

With the increase in cathodic polarization, due to a shift of corresponding equilibria, pH at the electrode surface (pH ) increases. Its changes are especially significant in slightly acidic media. Due to this, the shape of polarization curve depends on the nature of the supporting electrolyte. If the latter (such as SO ) is able to interact with protons, it plays a role of buffer and reduces pH changes to a certain extent (cf. continuous and dotted curves in Figure 4.4). 

This study, in conjunction with that discussed in 12.2.1.2, show that when using aqueous electrolytes or Nafion saturated with H20, the induction of NEMCA on finely dispersed noble metal catalysts is rather straightforward. The role of the electronically conducting porous C support is only to conduct electrons and to support the finely dispersed catalyst. The promoting species can reach the active catalyst via the electrolyte or via the aqueous film without having to migrate on the surface of the support, as is the case when using ceramic solid electrolytes. 

---