Electrode hydrophobic

Scott et al. [44] have designed two types of flow cells (1) parallel flow channel arrangement and (2) a spot design of a flow bed. These two types of flow cells are designed to test with electrodes made of Teflon and carbon with ruthenium and platinum as catalysts on electrodes. Teflon was used for the purpose of providing hydrophobic effect in the electrode. By making the electrode hydrophobic, the flow of methanol is improved. 

In our recent publications [4, 5] we discussed results concerning ion and molecule mobility in cationic montmorillonite film modified electrodes hydrophobized with aliphatic and aromatic quaternary ammonium compounds. We also described experiences with independent measurements by electrochemical and radiochemical methods on transport processes in humate containing montmorillonite and bentonite thin layers [6]. We interpreted the results as consequences of changes in structure which lead to changes in porosity and ditfu-sional transport [7, 8, 9]. The present paper approaches the role of film thickness, and the kinetics of the swelling of films (porodine xerogels) at different levels of hydrophobization, and also the kinetics of the penetration and release of probe molecules. 

Especially for HT-PEM MEAs, the higher operation temperature of 160 °C and the harsh oxidizing H3PO4 electrolyte inducts a very pronounced corrosion of carbon materials. This carbon corrosion phenomenon leads to the formation of surface oxides. The surface oxides are causing a decrease in hydrophobicity of the electrode material. In the case of PAFC or HT-PEM MEAs, which are based on liquid electrolytes, this decrease of electrode hydrophobicity causes an increase in electrolyte loss and in mass transport limitation due to flooding of the electrodes. 

An ion-selective electrode in which a chelating agent is incorporated into a hydrophobic membrane. 

Phosphoric Acid Fuel Cell This type of fuel cell was developed in response to the industiy s desire to expand the natural-gas market. The electrolyte is 93 to 98 percent phosphoric acid contained in a matrix of silicon carbide. The electrodes consist of finely divided platinum or platinum alloys supported on carbon black and bonded with PTFE latex. The latter provides enough hydrophobicity to the electrodes to prevent flooding of the structure by the electrolyte. The carbon support of the air elec trode is specially formulated for oxidation resistance at 473 K (392°F) in air and positive potentials. 

Enzyme electrodes for other substrates of analytical significance have been developed. Representative examples are listed in Table 6-1. Further advances in enzyme technology, and particularly the isolation of new and more stable enzymes, should enhance the development of new biocatalytic sensors. New opportunities (particularly assays of new environments or monitoring of hydrophobic analytes) derive from the finding that enzymes can maintain then biocatalytic activity in organic solvents (31,32). 

Monolayers of l-tert-bntyl-l,9-dihydrofullerene-60 on hydrophobized ITO glass exhibited three well-defined rednction waves at -0.55 V, -0.94 V, and -1.37 V (vs. satn-rated calomel electrode, SCE), with the first two stable to cycling [283]. Improved transfer ratios near nnity were reported. The peak splitting for the first two waves was 65-70 mV, mnch less than reported for the pnre C60-modified electrodes. The rednction and oxidation peak cnrrents were equal however, the peak currents were observed to be proportional to the sqnare root of the scan rate instead of being linear with the scan rate as normally expected for snrface-confined redox species. 

Most electrode materials are hydrophilic and readily wetted by aqueous solutions. Two methods are used to create and maintain an optimum gas/solution ratio in the electrode. The first method employs a certain excess gas pressure in the gas space. This causes the liquid to be displaced from the wider pores in finer pores the liquid continues to be retained by capillary forces. The second method employs partial wetproofing of tfie electrode by the introduction of hydrophobic materials (e.g., fine PTFE particles). Tfien the electrolyte will penetrate only those pores in the hydrophilic electrode material where the concentration of hydrophobic particles is low. 

A second surface modification has been reported by Yamamoto et al. These workers added stearic acid to their carbon paste mixture. This produced an electrode which was relatively insensitive to ascorbic acid and DOPAC relative to dopamine. It is theorized that this electrode works because of electrostatic repulsion of the anionic ascorbate and DOPAC by surface stearate groups. Ionic repulsion has also been employed by covering the surface of the working electrode with an anionic polymer membrane. Gerhardt et al. used Nafion, a hydrophobic sulfonated perfluoro-polymer, to make a dopamine selective electrode. This electrode exhibited selectivity coefficients as large as 250 1 for dopamine and norepinephrine over ascorbic acid, uric acid, and DOPAC. 

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