Electrode parameters

In general, low level detection is masked by the noise level inherent in any measuring device. Electrochemical methods are susceptible to electrical interference from external sources, variations in reference electrode parameters resulting from aging or contamination, and interference from redox... 

Effect of Carbon Black and Graphite Conductive Additives on the Electrode Parameters... 

First, when external current sources are used, the power consumption may be unpractically large. It all depends on the electrodic parameters of the electronation reaction—the larger its exchange-current density and the lower its Tafel slope, the larger will be the external protection current that must be used to achieve protection. 

Fig. 7.21 Influence of edge rounding radius on the secondary current density distribution along the surface of plate electrode (parameter x) between location a and b (kinetics obtained from regional tap water study, cti = 47 ppm, average current load —50 A m 2 calculated cell voltage for all radius nearly constant value of 6.04 V, electrolyte conductivity 667 p,S cm-1).

The potential benefit of impedance studies of porous GDEs for fuel applications has been stressed in Refs. 141, 142. A detailed combined experimental and theoretical investigation of the impedance response of PEFC was reported in Ref. 143. Going beyond these earlier approaches, which were based entirely on numerical solutions, analytical solutions in relevant ranges of parameters have been presented in Ref. 144 which are convenient for the treatment of experimental data. It was shown, in particular, how impedance spectroscopy could be used to determine electrode parameters as functions of the structure and composition. The percolation-type approximations used in Ref. 144, were, however, incomplete, having the same caveats as those used in Ref. 17. Incorporation of the refined percolation-type dependencies, discussed in the previous section, reveals effects due to varying electrode composition and, thus, provides diagnostic tools for optimization of the catalyst layer structure. 

The mixed potential developed is a function of various electrode parameters including, morphology, adsorption, catalytic, and electrocatalytic properties [54], To get a measurable potential difference between two electrodes, there must be asymmetry between them. Therefore, in most of the mixed-potential sensors the RE is usually Pt and the SE is oxide and/or an oxide mixture [13]. As a result, depending on the nature of the SE, it is possible that both reducible and oxidizable gases can be analyzed by the single sensor having a simple design. 

Current-time curves may be fitted to experimental current-time data using appropriate least-squares procedures. In this way unknown sensor parameters, such as membrane thickness, d, or the kinetic rate constant, k, can be determined (Fig. 34). The quality of the parameters essentially depends on the exactness of the model and the given electrode parameters. Unique fitting of the current-time curves to experimental data is only possible with maximally three free parameters. 

Fig. 34. Simulated current-time behavior of a GOD electrode. The electrode parameters were determined by least-squares fits of experimentally measured current-time data to be Ds = 1.5310-4 mm2/s, Dp = 3.56TCT4 mm2/s, k = 0.95 s-1, d = 0.094 mm. Glucose concentration O 0.071 mmol/1, A 0.142 mmol/1, 0.285 mmol/1. (Redrawn from Schulmeister and Scheller, 1985a).

Fig. 35. Concentration profiles of a model maltose sequence electrode. Parameters Ds - 7.52-KT5 mm2/s, Dz = 1.5310 mm2/s, Z P = 2.46DZ) ki = 0.06 s k2 = 0.95 s 1, d = 0.141 mm. Profiles are given for t = 5 s and the stationary case (with har). Maltose (S), glucose (2), and hydrogen peroxide (P) are rendered dimensionless with d and S°, respectively. (Redrawn from Schulmeister and Scheller, 1985b).

Fig. 36. Concentration profiles of a cyclic lactate-sensing electrode. Parameters ki = 1.0 s-1, k2 - 1.1 s 1,D = 9.0-10-5 mm2/s, d = 0.1 mm, n = 4. Profiles of lactate (a) and pyruvate (b) are given for t = 1,10,60 s and the stationary case. Lactate and pyruvate are rendered dimensionless with S°. (Redrawn from Schulmeister, 1987b).

Fig. 6 Relation between geometrical gold electrode parameters for suppressing spurs for AT-and BT cut quartz crystals...

We desire an expression for the dependence of the current on flow velocity and electrode parameters. The total internal area of the electrode, which encompasses the sum of the areas of all of the pores, is a (cm ), and the total electrode volume is LA (cm ). Porous electrodes are frequently characterized by their specific area, s, given by... 

In these equations n and D are characteristic constants for the electroactive species and m and t are electrode parameters which can be kept constant. Therefore the limiting diffusion current is proportional to the concentration of the electroactive species in the solution. Thus the limiting diffusion current measured as the wave... 

The technical cause is due to a deficiency of the Eh calculation method from redox-couples. It is noteworthy that the greatest scatter is associated with anion redox-couples whose components easily enter into the composition of complex compounds. Ignoring complex compounds-associated anions naturally results in distorted Eh values. Anderson (2005) stated that platinum electrodes react well to Fe and Mn only at their sufficiently high concentrations. So, electrode parameters reflect first of all ratio Fe /Fe +. 

It is possible to determine the ideal ablation factor by using the results of numerical solution of equations (13.73) and (13.74). It depends on three dimensionless parameters h, which describes the relation between electric and hydrodynamic forces acting on the drop Re, which characterizes the flow structure and d, which includes the mesh electrode parameters. The dependences Ki on these three parameters are shown in Fig. 13.23. 

Eor the OMF KIEL model, o = ao, A , and = ci values were all calculated from the CNLS fit parameters, leading to estimates of about 233,118, and 115, respectively. They thus agree less well with the CKIEL and CKOEL fit results. The above A/(co) fit results used proportional weighting, but A/"(co) NLS fits with either proportional or unity weighting led to closely similar estimates. The Pw and ysc values estimated for these fits were about 0.604 and 0.607, respectively. Einally, an OME KIEL fit of the ( >) part of the data, with electrode parameters fixed at their KIEL A/(co)-fit values, led to b, A , and estimates of about 175, 156, and 18.5, respectively. The last value is clearly an estimate of the CKl ci quantity here. Further, the Pw estimate was 0.338, very close to the fixed value of 1/3 for the CKIEL fitting. The stark inconsistency between the OMF A/(co) and <7 ((o) Pw estimates, also observed in all other such published comparisons, is a clear indication of the failure of the OMF to take proper account of d . Therefore, it is a particularly inappropriate fitting model and should not be used. 

Equations (31) and (32) can be used to analyze impedance spectra without knowledge of structural electrode parameters (thickness, density, etc). However, we need this information in order to transform the ohmic parameters obtained by a fit into specific electrochemical parameters. In particular, this information can be used to calculate the effective surface area of the particles. Particles used in practical batteries can usually be treated either as thin plates (Levi and Aurbach [1997]) or as pseudospherical in shape (Barsoukov [2003]), and have a narrow size distribution due to sieving. Particle size values are provided by material manufacturers. The number of particles in a given volume can be estimated from the ratio of their crystallographic density of particles, Op, to the density of the composite-electrode film, a. This allows one to calculate the electrochemically active surface area for a composite electrode for thin-plate particles as 5 = xAdalUOp] and for spherical particles as 5 = 3xAdal[rCp. Here x is the fraction of active material in the composite A is the geometric area of the electrode d is the thickness of the composite electrode <7 is the density of the composite electrode Op is the true density of particles and I and r are the thickness of the plate and radius of spherical particles, respectively. 

In practice, imperfections in the planarity of the electrode blocks limit the dimensions of the gaskets that can be employed. Increasing the surface area and the flow-rates will raise the background current, whilst in many cases flow-rates are limited by the chromatographic system. Typical electrode parameters are therefore an area of 15-20 mm with gaskets of 20-100 pm giving a cell volume of less than 1 pL and capable of working with flow-rates of ca 1 mL min ... 

Hi) Frequency response methods. This method superimposes low-amph-tude, high-frequency (about 1000 Hz) AC signals over the DC potential supplied to the electrode. The response of the resulting AC component of current to changes in the frequency is analyzed to give information on a variety of electrode parameters including the ohmic correction. For details of the principle and its application, consult Refs. 24 and 25. 

Equation (7.24) indicates that both the real and imaginary components can be affected by the electrode parameters such as Rg r, Cdij Q/ and In theory, these five parameters can be simulated based on both the experimental impedance and proposed ECs similar to those shown in Figure 7.10. Flowever, with too many parameters, a simulation will become difficult and the simulated values may become arbitrary. The effects of the magnitudes of these parameters can be seen in Figure 7.11. 

Mamlouk M, Scott K (2010) The effect of electrode parameters on performance of a phosphoric acid-doped PBI membrane fuel cell. Int J Hydrogen Energy 35 784-793... 

In the nonideal case, one has to evaluate the interplay of three effective electrode parameters, viz. volumetric exchange current density P, proton conductivity ap, and oxygen diffusivity D. Instead of considering these parameters, it is more insightful to evaluate and compare three corresponding characteristic current densities. These include the current conversion capability f = PIcl, defined above, the characteristic current density due to proton transport... 

Typical Electrode Parameters and the Resulting Newman s Dimensionless Reaction Penetration Depth e, the Typical Dimensionless Current Density jo, and the Non-Newman Dimensionless RPD for Various Electrodes. 

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