Sensing electrode polarization

When the cell responds to the sample gas, both reactions should take place on the sensing electrode to form a local cell, which results in a mixed potential of the electrode. This has been well confirmed by the measurements of respective polarization curves. 

This type of bulk property detector monitors the conductivity of the eluent. All ions from the analyte and from the buffer contribute to produce a signal. Detector response is linear over a wide range. Cell resistance is inversely proportional to electrolyte concentration. Since AC voltages must be used to avoid polarization of the sensing electrodes, the physical quantity measured is impedance, not resistance. 

A new cholesterol FIA biosensor based on a luminol/H202 ECL reaction induced by a glassy carbon electrode polarized at +425 mV versus a Pt pseudo j reference was introduced. The cholesterol biosensor sensing layer used in this method is based on cholesterol oxidase (COD) immobihzed on either UltraBind membrane or Immunodyne membrane [117]. Exploration to other literature shows that heroin was determined employing Zeolite Y sieves by immobilizing... 

However, the water electrolysis is not excluded in this case. The two reactions determine the decay both of pH and ionic strength of solution at the PPy electrode. The change of local pH causes the sample contraction. The change of electrodes polarity causes the sense modification of the two above mentioned reactions, and the corresponding change of pH causes sample swelling. 

Because Ag-AgCl electrodes are thermodynamically reversible, they exhibit (after stabilization) low noise and theoretically zero electrode polarization impedance effects. They do produce a steady electrode potential, however, which produces a dc offset in direct-coupled systems. This requires compensation and is discussed in Chapter 7. This situation may cause problems in the sensing of low-level dc potentials. Because of the electrochemical nature of these electrodes, each one assumes an absolute potential. When two such electrodes are used as a sensing pair, a dc potential difference exists (frequently of the order of a few millivolts). If this potential difference remains constant, any measurement of a bioelectric potential is unaffected, except for a steady baseline elevation. In the usual case, however, the resting potentials of the two electrodes change unequally with time and environmental temperature. This results in objectionable baseline drift in experimental determinations. 

In a broad sense similar effective double layers can be formed via gaseous adsorption or evaporation (e.g. Na evaporated on Pt electrodes deposited on fT-A Ch has been shown to behave similarly to electrochemi-cally supplied Na). In other cases, such as the effective double layer formed upon anodic polarization of Pt deposited on YSZ, the electrochemically created effective double layer appears to be unique and cannot be formed via gaseous oxygen adsorption at least under realistic (<300 bar) oxygen pressure conditions. 

This presentation reports some studies on the materials and catalysis for solid oxide fuel cell (SOFC) in the author s laboratory and tries to offer some thoughts on related problems. The basic materials of SOFC are cathode, electrolyte, and anode materials, which are composed to form the membrane-electrode assembly, which then forms the unit cell for test. The cathode material is most important in the sense that most polarization is within the cathode layer. The electrolyte membrane should be as thin as possible and also posses as high an oxygen-ion conductivity as possible. The anode material should be able to deal with the carbon deposition problem especially when methane is used as the fuel. 

The term electrode potential is often used in a broader sense, e.g. for the potential of an ideally polarized electrode (Chapter 4) or for potentials in non-equilibrium systems (Chapter 5). 

If a piezoelectric plate (Fig. 6.1), polarized in the direction indicated by P, carries electrodes over its two flat faces, then a compressive stress causes a transient current to flow in the external circuit a tensile stress produces current in the opposite sense (Fig. 6.1(a)). Conversely, the application of an electric field produces strain in the crystal, say a negative strain reversal of the field causes a positive strain (Fig. 6.1(b)). The changes in polarization which accompany the direct piezoelectric effect manifest themselves in the appearance of charges on the crystal surface (see Eq. (2.71)) and, in the case of a closed circuit, in a current. 

Similar to chemical kinetics, the mechanism of electrochemical reactions regularly requires a series of physical, chemical, and electrochemical steps, comprising charge-transfer and charge-transport reactions. The velocity of these individual steps controls the kinetics of the electrode reactions and, thus, of the cell reaction. In this sense, three diverse kinetics effects for polarization must be taken into account ... 

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