Carboxylic acid layer, current

The typical perfluorocarboxylic acid membrane developed by Asahi Chemical is a multilayer membrane prepared by chemical treatment. The structure of the membrane is optimized for high current efficiency and low electric resistance. The thickness of the carboxylic acid layer is in the range of 2 to 10 microns. The chemical structure of the membrane is as follows (72). 

Dziewinski et al. [71] have reported current efficiencies greater than 90% at 80°C for isopropanol, acetone, and acetic acid, whereas methanol, chloroform, and carbon tetrachloride were converted into carbon dioxide at room temperature with ca. 100% current efficiency. The predicted order of ease of oxidation is alcohols < aldehydes, ketones < carboxylic acids. Thus, the complete conversion to carbon dioxide, in particular for large organic molecules, is expected only at elevated temperatures. To avoid the use of a separator, Farmer et al. [72,75] employed C0SO4 in sulfuric acid. A scheme of the system is shown in Fig. 17. Both electrodes, the anode and the cathode, are usually of a Pt-containing surface layer, although other materials such as Au, steel, graphite, Ti, Nb, Ir, or tin oxide can also be used. H2 is evolved at the cathode because the deposition of the mediator takes place... 

There are three types of blisters, water, salt, and others. Water blisters are formed when there is high water transport across the layers. Salt blisters result finm localized heating of the membrane because of high local current density or nonuniform current distribution. Proper cell design can alleviate this problem (Chapter 5). The other type of blister arises when the acidity of the anolyte is high. When the anolyte pH is below 2, the carboxylate layer protonates to a nonconductive carboxylic acid. This will increase the voltage and the internal vapor pressure, and finally result in the formation of voids in the carboxylate layer. 

Figure 7.6 Total ion current profile of the acidic fraction of a sample from a linseed oil reference paint layer (Opificio delle Pietre Dure, Florence, Italy), after saponification and silylation of carboxylic and hydroxyl ic groups [9]...

Fig. 17.7 Influence of Na+ concentration at interface of sulphonic acid and carboxylic layers on current efficiency 3-4 N NaCI 1.0-5.5 kA rrT2 33% NaOH.

Strength (to amplify double-layer effects) showed an increase in current as the solution was acidified, and the corresponding analysis also gave pK = 5.6. The results indicated that each of the electrochemical reactions were controlled by a common acid-base equilibrium at an electrode surface functionality. One possibility suggested was a carboxylate with the pK raised by a neighbouring hydrogen-bond acceptor. 

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