Membrane Electrode surface wetting

Nanofibrous membranes have been primarily used as coating material to develop electrochemical sensors. In general, a piece of membrane (10-100 pm) can be simply placed over the electrode surface and kept tight thanks to an o-ring. Eventually, a drop of the solvent can be added to wet the membrane and improve its adhesion to the electrode surface. Figures 13.3 and 13.4 show a glassy carbon electrode coated with nanofibrous membranes. 

Permselective membranes exclude specific molecules from transport due to their size or other properties. With dialysis membranes, the molecular size is aitical. An important application is the exclusion of protein molecules which may contaminate the electrode surface. Lipophilic membranes block the transport of polar molecules, and in ion-exchanger membranes, only ions with a defined charge sign are mobile. Best known are Nafion membranes (Du Pont), which act as cation exchangers in wet state. Anions cannot penetrate the membrane. 

For water electrolysis, GE used Nafion for the electrolyte as well as for a separator, mechanically pressing both electrodes against the membrane. Takenaka and Torikai proposed an electroless method for the deposition of noble metals on the surface of Nafion under wet conditions based on the difPiision of reducing agent through the membrane. ... 

For polymer electrolyte membrane fuel cell (PEMFC) applications, platinum and platinum-based alloy materials have been the most extensively investigated as catalysts for the electrocatalytic reduction of oxygen. A number of factors can influence the performance of Pt-based cathodic electrocatalysts in fuel cell applications, including (i) the method of Pt/C electrocatalyst preparation, (ii) R particle size, (iii) activation process, (iv) wetting of electrode structure, (v) PTFE content in the electrode, and the (vi) surface properties of the carbon support, among others. ... 

These examples clearly show how the EIS measurements allow the estimation of the membrane electrical parameters (R and C however, they also indicate the possibility of obtaining qualitative information on the membrane structure, which can also be of great interest. In any case, it should be pointed out that impedance is an extensive magnitude (it depends on the sample area), and for that reason, comparisons of the type of curves and the concentration dependence instead of particular values are usually made. In addition, IS measurements with the dry and wet membranes, but without an electrolyte solution between the electrode and the membrane surface, can also be performed and complementary information, mainly related to the membrane material itself or the interfacial (electrode/membrane) effects, can be obtained. 

There are two basic methods for measuring the pH of a dry solid. The first requires about 1-hour contact of 20 to 50 ml of distilled water with 1 gram of finely divided sample. The sample is not filtered and the pH measurement is taken on the supernatant liquid. The other method is to take a pH reading after one or more drops of distilled water is placed on the surface of the sample. This measurement requires a flat bulb combination electrode to be placed on a wetted area sufficiently large to provide contact between the glass membrane and reference junction. 

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