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Several classes of sol-gel derived, fuel-cell-type gas sensors are described. The electrochemical sensors are comprised of a hydrophobically modified silicate network and a dispersion of carbon powder and inert metal (palladium or platinum in the zero valent state) or organometallic (e.g., cobalt porphyrin) catalyst modifier. The carbon powder provides conductivity, the Ormosil network assures high porosity, which is pertinent for gas permeation through the back of the electrodes, and the hydrophobic functional group assures that only a thin layer at the outermost section of the electrode is wetted by the electrolyte, thus providing fast dynamic response.

Palladium-modified carbon ceramic composite gas electrodes (Pd/CCE) are presented and characterized. The electrodes are comprised of graphite powder percolating through hydrophobically modified silica xerogels. The electrodes were prepared by mixing Pd salt and carbon powder with the sol-gel precursors and reduction of palladium ions after xerogel formation. The morphology and surface characteristics of the composite materials were examined by electron microscopy, X-ray microanalysis, nitrogen adsorption and water wetting angle measurements. Oxygen reduction was used as a model reaction to probe the characteristics of the wetted section of the Pd/CCEs. Voltammetric studies and polarization curves were employed to elucidate the size and structure of the active layer as a function of palladium content.

A new class of sol-gel-derived electrochemical gas sensors, comprised of a homogeneous dispersion of catalyst-modified carbon powder in porous, organically modified silica, is introduced. The porous structure of the electrode material permits high gas permeability. Their hydrophobic surface rejects water, leaving only a very thin layer at the outermost surface in contact with the electrolyte and thus minimizing effects of liquid side mass transfer. The carbon powder provides electric conductivity, and the catalyst guarantees selectivity and sensitivity. Heat-treated Co tetramethoxymesoporphyrin modified composite ceramic-carbon electrodes exemplify this new class of gas sensors. Detection of several gases by anodic (sulfur dioxide) or cathodic (carbon dioxide and oxygen) reactions is demonstrated. Metrological characteristics of a prototype oxygen sensor are presented.

The electrochemical mechanism of pyrolyzed carbon supported Co-tetramethoxymeso-porphyrin (Co-TMMP) catalysis of oxygen reduction is studied by a fuel-cell-type hydrophobic ceramic-carbon composite electrode (CCE), made of sol-gel-derived graphite-silica material. Only a thin layer at the outermost surface of the hydrophobic porous CCE is in contact with the electrolyte, thus minimizing effects of electrode structure on the current density. This property makes CCEs most useful for elucidation of kinetic mechanisms of gas electrodes. At low cathodic overpotentials, oxygen reduction in acidic solution on heat-treated Co-TMMP supported graphite powder exhibits first-order kinetics with respect to adsorbed di-oxygen, which obeys Langmuir adsorption isotherm. The activation energy of oxygen reduction equals 7.5 {+-} 0.4 kcal/mol. Half-order oxygen dependence is observed at high overpotentials, and the reaction is limited by the rate of hydrogen peroxide conversion.

Sol-gel derived indicator ceramic carbon electrodes (CCEs) made of graphite powder entrapped in hydrophobically modified silica gel are presented. Voltammetric characteristics of the electrodes are compared with other classes of graphite electrodes. Background current, double layer capacitance and metrological characteristics, including renewal repeatability, interelectrode reproducibility and operational stability for various solvents are presented. The indicator CCEs exhibit better stability than carbon paste electrodes and their renewal is easier and more reproducible than that of glassy carbon electrodes.

A new class of composite electrodes made of sol-gel derived carbon-silica materials is introduced. Modifed porous composite carbon-silica electrodes can exhibit hydrophobic or hydrophilic surface characteristics and can serve as an indicator (inert) electrode, as a potentiometric (selective or reference) electrode, and in amperometric sensing and biosensing. The composite (carbon) ceramic electrodes (CCP) are rigid, porous, easily modified chemically and have a renewable external surface. The electrodes otter higher stability than carbon paste electrodes, and they are moreamenable to chemical modification than monolithic and (organic) composite carbon electrodes

Palladium-modified carbon ceramic composite gas electrodes (Pd/CCE) are presented and characterized. The electrodes are comprised of graphite powder percolating through hydrophobically modified silica xerogels. The electrodes were prepared by mixing Pd salt and carbon powder with the sol-gel precursors and reduction of palladium ions after xerogel formation. The morphology and surface characteristics of the composite materials were examined by electron microscopy, X-ray microanalysis, nitrogen adsorption and water wetting angle measurements. Oxygen reduction was used as a model reaction to probe the characteristics of the wetted section of the Pd/CCEs. Voltammetric studies and polarization curves were employed to elucidate the size and structure of the active layer as a function of palladium content.

Sol-gel derived composite silica-carbon electrodes exhibit favorable electrochemical characteristics. The electrodes benefit from the conductivity and electrochemical advantages of the carbon powder, from the favorable properties of the ceramic network and from the versatility of the sol-gel process. Hydrophobie composite electrodes reject water, only their outermost surface is wetted and they exhibit good signal to background currents. A comparison of several types of carbon powders reveals that higher carbon loading and larger surface area electrodes can be attained by incorporation of dense graphite powder. When high surface area, small size carbon-black powder is used, a homogeneous distribution of microelectrodes, separated by insulating modified silica is formed. This ensemble of microelectrodes increases the sensitivity of the CCEs by more than two orders of magnitude as compared to glassy carbon electrode and graphite CCEs.