Electrodes water vapor

Aluminum Oxide Moisture Sensor. This type of sensor is a capacitor, formed by depositing a layer of porous aluminum oxide onto a conductive substrate, and then coaling the oxide with a thin film of gold The conductive base and the gold layer become the capacitor s electrodes. Water vapor penetrates the gold layer and is absorbed by the porous oxidation layer The number of water molecules absorbed determines the electrical impedance of the capacity, which is. m turn, a measure of water vapor pressure. 

Phillips (19) formed a reference electrode by coating a noble metal with a Langmuir-Blodgett multilayer of calcium palmitate and used it to detect the presence of a polar vapor in dry atmospheres by its adsorption on an uncoated metal electrode. Water vapor and oxygen were carefully excluded from the surrounding atmosphere, since either gas penetrated the multilayer coating on the reference electrode. This report describes the preparation, properties, and several applications of a more general and useful type of reference electrode. 

This technique can be used to measure displacement where, in effect, the two electrodes are connected to the two bodies. It has also other applications (for example, in moisture meters where the presence of water vapor between the electrodes causes the capacitance change). 

A 13 liter, semispherical sampling vessel with a flow rate of 0.5 /min was used. The electrode (38 mm diameter) in front of the ZnS(Ag) scintillator was placed in the center of the bottom and was set at -3,000 V relative to the vessel wall. Since the ERM is sensitive to water vapor (Porstendorfer et al, 1980 Dalu et al., 1983), the air sample was passed through a dehumidefier to maintain the relative humidity in the chamber less than 2.9 %. 

Yoon el al. [112] reported an all-solid-state sensor for blood analysis. The sensor consists of a set of ion-selective membranes for the measurement of H+, K+, Na+, Ca2+, and Cl. The metal electrodes were patterned on a ceramic substrate and covered with a layer of solvent-processible polyurethane (PU) membrane. However, the pH measurement was reported to suffer severe unstable drift due to the permeation of water vapor and carbon dioxide through the membrane to the membrane-electrode interface. For conducting polymer-modified electrodes, the adhesion of conducting polymer to the membrane has been improved by introducing an adhesion layer. For example, polypyrrole (PPy) to membrane adhesion is improved by using an adhesion layer, such as Nafion [60] or a composite of PPy and Nafion [117],... 

Fig. 3-42 Methanol oxidation current of Nafion SFE Pt electrode after 2 min. cf potential holding at 500 mV or 600 mV atTff C. Methanol was fed throu gas phase at various partial pressures, water vapor pressure 0, electrolyte 3 M H2S04...

The electrode reactions are comprised of the oxidation of hydrogen on Ihe anode (the negative electrode) to hydrated protons with the release ol electrons and on the cathode (the positive electrode) the reaction of oxygen with protons la I unn water vapor with the consumption of electrons. Electrons llow from the anode through the external load to the cathode and the circuit is closed by ionic current transport through the electrolyte. In an acid cell, the current is carried by protons. 

As for the reoxidation of reduced heteropoly compounds in the solid state, few reliable studies have been reported. It was reported that the reoxidizability increases with an increase in standard electrode potentials of countercations (108). In the case of reoxidation by O2 of le -reduced CsxHj - PMo 12O40, the rates divided by the surface area show a monotonic variation (Fig. 53e) as in Figs. 53c and d, indicating a surface reaction. A similar variation was observed for the Na and K salts. The presence of water vapor sometimes accelerates the migration of oxide ion, probably in the form of OH- or H20, and makes surface-type reactions more like bulk type II reactions (266). 

There are several things that need to be pointed out on this design. First, the upper electrode is porous, either because it is very thin (10-20 nm) or because it is deposited under such conditions that it cracks. In any case, the polymer beneath it comes into contact with the gas or vapor. Obviously, it is difficult to make robust electrical connections to the top electrode. Fortunately, it is not necessary because it forms an electrically floating plate, which is common to the two capacitors one between the Cr, Ni, Au plate (Ci) and the other between the top and the Ta plate (C2). The corresponding leakage resistances are R and R2. The response of this sensor to water vapor is shown in Fig. 8.14. 

McBride and Hall (37,38) reported the first observation of a controlled catalytic reaction on alumina using IETS. They studied the catalytically induced transfer hydrogenation from water vapor to unsaturated hydrocarbon chains chemisorbed on alumina at both ends of the chain. They absorbed muconic acid ( trans-trans-1,3 butadiene 1,4 dicarboxylic acid, HOOC-CH=CH-CH=CH-COOH ) onto oxidized aluminum strips using the liquid doping technique. The samples were returned to the vacuum system, and in the presence of 0.3 torr of D2O vapor, heated to up to 400° C by passing current through a heater strip evaporated on the back of the glass slide. The films were then allowed to cool and the junctions completed by evaporation of the Pb counter electrode. 

During cell/stack operation, water content in the membrane is affected by the local intensive variables, such as local temperature, water vapor concentration in the gas phase, gas temperature and velocity in the channel, and the properties of the electrode and gas diffusion media. The power fluctuation can result in temperature variation inside the cell/stack, which will subsequently change the local membrane water content. As the water content in the membrane tends to be non-uniform and unsteady, this results in operation stresses. When the membrane uptakes water from a dry state, it tends to expand as there is no space for it to extend in plane and it can wrinkle up as schematically shown in Fig. 4 when the membrane dries out, the wrinkled part may not flatten out, and this ratcheting effect can cause the pile up of wrinkles at regions where membrane can find space to fold. The operation stress is typically cyclic in nature due to startup-shutdown cycles, freeze-thaw cycles, and power output cycles. 

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