Resistance electrodes

Because of the very large resistance of the glass membrane in a conventional pH electrode, an input amplifier of high impedance (usually 10 —10 Q) is required to avoid errors in the pH (or mV) readings. Most pH meters have field-effect transistor amplifiers that typically exhibit bias currents of only a pico-ampere (10 ampere), which, for an electrode resistance of 100 MQ, results in an emf error of only 0.1 mV (0.002 pH unit). 

Control electrode resistance too high Check connections, measure resistance and potential of electrode, possibly renew... 

Measurement of resistance As previously mentioned, the four-electrode resistivity meters can be used to measure resistances. For this purpose the... 

Vitanov and Popov et al.156 660-662 have studied Cd(0001) electrolyti-cally grown in a Teflon capillary in an aqueous surface-inactive electrolyte solution. The E is independent of ce) and v. The capacity dispersion is less than 5%, and the electrode resistance dispersion is less than 3%. The adsorption of halides increases in the order Cl" < Br" < I".661 A comparison with other electrodes shows an increase in adsorption in the sequence Cd(0001) < pc-Cd < Ag( 100) < Ag(l 11). A linear Parsons-Zobel plot with /pz = 1.09 has been found at a = 0. A slight dependence has been found for the Cit a curves on ce, ( 5%) in the entire region of a. Theoretical C, E curves have been calculated according to the GCSG model. 

Figure 21. (a) PMC potential and (b) cathodic photocurrent-potential curves for a p-Si (111) electrode (resistivity, 10 ft cm). Electrolyte, 1 M NH4F light intensity 1 mW cm-2. Sweep toward negative potentials. 

This means the equivalent series resistance of SC should notably be reduced in order to increase their specific power. Hence, as can be shown, the electrode resistivity should be reduced significantly - down to 0.5... 

Ohm.cm2 (of visible surface area) or even less. Some visible ways to lower the electrode resistivity are (a) fabricating thin electrodes, (b) increasing the conductivity of electrolyte and electrode material, (c) matching the electrode porosity with the size of species in the electrolyte in order to facilitate the ion transport along the pores, and (d) assembling bipolar devices. 

In order to see how the electrode thickness might be optimized in order to provide the lowest electrode resistivity, we have developed a theoretical model to describe the charge/discharge processes in porous carbon electrodes. As a first approximation, let us consider an electrode having two sets of cylindrical pores, namely, nanopores (NP) of less than 3 nm in diameter and transport channels (TC) of more than 20 nm in diameter, with each nanopore having an exit to only one TC. ... 

However, it is not so easy to derive a similar functional relationship between the electrode resistance and its thickness that is why a computer simulation has been carried out to study the dependence of electrode resistance on its thickness. In the calculations, Rm was chosen as a unit resistance, and the distance between the NP tiers (4r) was chosen as a unit thickness. Figure 3 illustrates the results of calculations at various ratios between Re, R and Rm, namely, R, = Re = 0.002Rm (curve I ) / , = 0.004/ ,. R,. = 0.000 li m (curve 2). In these and other cases checked, the plots of resistance vs. thickness display a sharp drop followed by a slower rise after reaching a minimum. Let us consider these regions in more detail. 

In the equation (5) the inner resistance Rin includes not only the electrode resistance, but also the separator resistance f s, defined as Rs =... 

At the same time, the cathodic polarization of the electrode is accompanied by the formation of a nonconducting phase, leading to an increase in electrode resistance. Thus, at increased current densities, the region of stable potential value cannot be obtained as a result of electrode resistance increase and ohmic drop, as is shown by curve 2 in Figure 2. 

It is frequently important to determine electrode resistance to account for any contributions to iR drop and circuit rise time due to the electrode itself. Two general strategies are available for this purpose. One strategy for accurately determining p is to use a four-point probe (see Fig. 11.2) for the measurement [6]. This device supplies a controlled current to be passed through the film via... 

The standard electrode potential and its temperature coefficient are found in the literature.36 Kinetic parameter values were measured in-house for HOR,33 ORR,34 OER,35 and COR.12 22 Table 2 gives cell component materials and transport properties. The membrane and electrode proton conductivity in Table 2 are based on the measured membrane and electrode resistance,42,43 which is a strong function of relative humidity (RH). In what follows next, we will describe the... 

FIGURE 7 Primary current distribution on a wavy electrode. Resistance to current flow is represented schematically by the size of the resistors. 

When an a.c. voltage is applied to a perfect capacitor, no energy is dissipated. However, a real capacitor dissipates energy because of lead and electrode resistances, d.c. leakage resistance and, most importantly, dielectric losses. These account for the capacitor s dissipation factor or loss tangent tan 3. It is sometimes convenient to regard the lossy capacitor as an ideal capacitor shunted by a resistance Rp or in series with a resistance rs, as shown in Fig. 5.5. 

For example, for a 500kW 500pF unit with p s = 4 x 10-7Q Hz-1 2, Pe is 0.02W at 0.1 MHz and 7kW at 500 MHz. Therefore it is evident that below 1 MHz the major contribution to heat generation is dielectric loss, whilst at higher frequencies a significant proportion is due to electrode resistance, and that, because of the skin effect, this resistance cannot be reduced by making the electrodes or leads thicker than a small fraction of a millimetre. However, the thicker the electrodes and leads are the better is the heat transfer from the capacitor. 

If the reference electrode is correctly placed in the same equipotential as the working electrode, ohmic drop in the electrolyte may be negligible, but this may not be the situation in the bulk of the electrode. Quantitative determination of the electrode resistance is difficult when using voltammetry only. It is much easier using galvanostatic cycling. 

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