Sensing electrode electrochemical potential

Elumalai, P., Plashnitsa, V.V., Ueda, T. and Miura, N. (2008) Sensing characteristics of mixed potential- type zirconia-based sensor using laminated-oxide sensing electrode. Electrochem. Commun., 10 (5), 745-8. 

The steady-state potential of this sensing electrode (mixed potential) and the corresponding EMF of the sensor are established when the rates of the two electrochemical reactions are equal [13]. In order to estimate the mixed potential, one should consider the absolute values of the cathodic and anodic currents, expressed by the Butler-Volmer equation, taking also into account the rate from the catalytic reaction which determines the amounts of adsorbed species at the three-phase boundaries. A detailed analysis [8] concluded either a logarithmic or linear dependence for the concentration of reacting gas on mixed potential. The former... 

Electrochemical Reaction/Transport. Electrochemical reactions occur at the electrode/electrolyte interface when gas is brought to the electrode surface using a small pump. Gas diffuses through the electrode structure to the electrode/electrolyte interface, where it is electrochemically reacted. Some parasitic chemical reactions can also occur on the electrocatalytic surface between the reactant gas and air. To achieve maximum response and reproducibility, the chemical combination must be minimized and controlled by proper selection of catalyst sensor potential and cell configuration. For CO, water is required to complete the anodic reaction at the sensing electrode according to the following reaction ... 

It follows from Equation 6.12 that the current depends on the surface concentrations of O and R, i.e. on the potential of the working electrode, but the current is, for obvious reasons, also dependent on the transport of O and R to and from the electrode surface. It is intuitively understood that the transport of a substrate to the electrode surface, and of intermediates and products away from the electrode surface, has to be effective in order to achieve a high rate of conversion. In this sense, an electrochemical reaction is similar to any other chemical surface process. In a typical laboratory electrolysis cell, the necessary transport is accomplished by magnetic stirring. How exactly the fluid flow achieved by stirring and the diffusion in and out of the stationary layer close to the electrode surface may be described in mathematical terms is usually of no concern the mass transport just has to be effective. The situation is quite different when an electrochemical method is to be used for kinetics and mechanism studies. Kinetics and mechanism studies are, as a rule, based on the comparison of experimental results with theoretical predictions based on a given set of rate laws and, for this reason, it is of the utmost importance that the mass transport is well defined and calculable. Since the intention here is simply to introduce the different contributions to mass transport in electrochemistry, rather than to present a full mathematical account of the transport phenomena met in various electrochemical methods, we shall consider transport in only one dimension, the x-coordinate, normal to a planar electrode surface (see also Chapter 5). 

In electrochemistry we frequently refer to a technical term electrode potential. The electrode potential means in its physical sense the energy level, i.e. the electrochemical potential, of electrons in an electrode. It is however convenient, as described in the foregoing (Eqs. 9.3 and 9.4), to define the electrode potential in terms of the real potential ae rather than the electrochemical potential r/s of electrons in the electrode. 

According to the definition of electrochemical potential given in Eq. (13), it does not make sense to talk about absolute potential values because only differences in potential can be measured. Values of potentials are reported and tabulated with respect to a reference electrode. The potential of the reference electrode, by definition, is zero (there is no potential difference between two electrodes of the same type). The primary reference electrode by convention is the standard hydrogen electrode (SHE) Pt/H2 [14]... 

The standard electrode potential is a relative quantity in the sense that it is the potential of an electrochemical cell in which the reference electrode (left-hand electrode) is the standard hydrogen electrode, whose potential has been assigned a value of 0.000 V. 

Elumalai, P. and Miura, N., Influence of annealing temperature of NiO sensing electrode on sensing characteristics of YSZ-based mixed-potential-type NO sensor, in Chemical Sensors VI Chemical and Biological Sensors and Analytical Methods, Eds. C. Bruckner-Lea et. al., PV 2004—08, The Electrochemical Soc. Proc. Series,... 

FIGURE 3.11 Eg versus correlations for the device using a three-electrode structure in air, 200 ppm NO and 200 ppm NO, at 550°C. (Reprinted from Zhuiykov, S. et al., Potenti-ometric NO sensor based on the stabilised zirconia and NiCr204 sensing electrode operating at high temperatures, Electrochem. Comm. 3 (2001) 97-101, with permission from Elsevier... 

Miura, N. et al., NO sensing characteristics of mixed-potential-type zirconia sensor using NiO sensing electrode at high temperatures, Electrochem. Solid-State Lett. 8... 

Amperometry is one of a family of electrochemical methods in which the potential applied to a sensing electrode is controlled instrumentally and the current occurring as a consequence of oxidation/reduction at the electrode surface is recorded as the analytical signal. In its simplest form, the applied potential is stepped to and then held at a constant value and the residting current is measured as a function of time. When amperometric detection is used in conjimction with separation techniques such as capillary electrophoresis or Uquid chromatography, the sensing... 

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