Teflon type layers

We begin by pointing out that this concept of covering an electrode surface with a chemically selective layer predates chemically modified electrodes. For example, an electrode of this type, the Clark electrode for determination of 02, has been available commercially for about 30 years. The chemically selective layer in this sensor is simply a Teflon-type membrane. Such membranes will only transport small, nonpolar molecules. Since 02 is such a molecule, it is transported to an internal electrolyte solution where it is electrochemically reduced. The resulting current is proportional to the concentration of 02 in the contacting solution phase. Other small nonpolar molecules present in the solution phase (e.g., N2) are not electroactive. Hence, this device is quite selective. 

There is considerable interest in extending PEFC technology to the direct methanol and formaldehyde electro-oxidation (34, 35). This requires Pt-based bi-metallic catalysts. Tests have been conducted with gas diffusion type Vulcan XC-72/Toray support electrodes with Pt/Sn (0.5 mg/cm, 8% Sn) and Pt/Ru (0.5 mg/cm, 50% Ru). The electrodes have Teflon content of 20% in the catalyst layer. 

Previous reports 13] emphasized the importance of sample handling, and indeed because of the very volatile nature of the compounds measured in this type of analysis, sample collection deserves special consideration. In general, narrow mouth glass vials with a total volume in excess of 50 ml are acceptable. The bottles need not be rinsed or cleaned with organic solvents, but simply cleaned with detergent and water, rinsed with distilled water, air dried, and dried in a 105°C oven for one hour. The vials are carefully filled with sample to overflowing (zero head space) and a Teflon faced silicone rubber septum is placed Teflon face down on the water sample surface. The septa may be cleaned in the same manner as the vials, but should not be heated more than one hour because the silicone layer slowly degrades at 105°C. 

The two copper oxide layers can be considered as polymeric since the covalent character is in the same range as for the carbon fluoride bond in Teflon. Thus, the 123-superconductors consist of two types of polymeric copper oxide layers held together by ionic bonding metals such as barium and yttrium. This theme of polymeric layers held together by ionic bonding to metals is common in the silicates and other minerals. 

Figure 9.9 Assembly of sandwich-type optically transparent thin-layer electrochemical cell, a, Glass or quartz plates b, adhesive Teflon tape spacers c, minigrid working electrode d, metal thin-film working electrode, which may be used in place of (c) e, platinum wire auxiliary electrode f, silver-silver chloride reference electrode g, sample solution h, sample cup. [Adapted with permission from T.P. DeAngelis and W.R. Heineman, J. Chem. Educ. 53 594 (1976), Copyright 1976 American Chemical Society.]...

A simple thin-layer cell that is made by threading a platinum wire through a Teflon capillary has been described,119 as has a micrometer type thin-layer cell for estimating n values in electrochemical reactions (Figure 3.15).120... 

Laminates have been extensively studied by PA-FTIR and are the subject of extensive investigations by the new much more complex step-scan PA-FTIR techniques. Kapton film is often used to illustrate the analysis of this type of sample. Kapton 200FN919 film is a DuPont product which consists of a 25 pm layer of polyimide laminated between 12.7 pm layers of Teflon. If a high mirror velocity is used then a spectrum of only the Teflon layer is obtained. On the other hand if a slow mirror velocity is used a composite spectrum of the Teflon and polyimide layer is obtained. The difference spectrum (low mirror velocity spectrum minus high mirror velocity spectrum) represents that of the polyimide layer. As illustrated in Figure 2.16 the difference spectrum closely resembles the PA FTIR spectrum of polyimide except for the regions where the strong C-F bands of the Teflon are present. 

So far, essentially three different approaches have been reported for the preparation of zeolitic membranes [119]. Tsikoyiannis and Haag [120] reported the coating of a Teflon slab during a "regular" synthesis of ZSM-5 by a continuous uniform zeolite film. Permeability tests and catals ic experiments were carried out with such membranes after the mechanical separation of the coating from the Teflon surface [121]. Geus et al. [122] used porous, sintered stainless steel discs covered with a thin top layer of metal wool to crystallize continuous polycrystalline layers of ZSM-5. Macroporous ceramic clay-type supports were also applied [123]. 

As seen in Fig. 22, ME As based on such thin-film catalyst layers can be constructed using a decal process, in which the ink is cast onto Teflon blanks for transfer to the membrane by hot-pressing. A second approach is to cast the same type of ink (TBA+ form of the ionomer) directly onto the membrane [12]. The latter process has an advantage over the decal process in... 

Hydrogen gas fuel and air (O2) are fed to anode and cathode Pt catalyst powder layers, respectively. The Pt catalysts is Teflon-bonded to porous carbon sheets to form gas-diffusion electrodes, with a catalyst loading of about 1.0 mg/cm. The Pt anode and cathode are separated by a thin inert porous matrix that is filled with concentrated phosphoric acid. The cell operates at 200°C (to improve the electrode kinetics), with a cell voltage of about 0.67 V at a current density of 0.150 A/cm. Most voltage losses occur at the air cathode. The hydrogen gas must be pure because sulfur and carbon monoxide poison the Pt anode catalyst. This type of fuel cell is commercially available today, with more than 200 systems installed all over the world in hospitals, hotels, office buildings, and utility power plants. 

Gas-diffusion electrodes with these catalysts are prepared by a few methods metalceramic porous hydroclosing layer used to work with enhanced pressure, as well as to work on air hydroclosing layer is hydrophobic Teflon or hydrophobic soot. Thus three-phase boundary (reaction s zone) created by pressure difference between electrolyte and gas hollow or by gradient of layer s wetting. Nickel net 400 micron is current conductor for all types of electrodes. 

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