Surface area from electrode polarization

Large electrodes directly into the measuring cell will disturb ionic current flow pattern, and polarization will occur on the metal surface. The electrode metal should not be in direct contact with the electrolyte, but should be recessed (Figure 7.27). The electrolyte is contained in a tube with isolating walls, and if a part of this wall is substituted by electrode metal, the current will prefer the high-conductivity path of the metal. The current lines will deviate from the path parallel to the tube walls, and in one part of the area the current will enter, in the other part it will leave. Thus the electrodes are polarized, but not by a current in the external leads. The polarization may not be uniform over the electrode surface area, and the polarization will occur according to local current direction and polarization admittance. When the metal is recessed, the current will also deviate into the electrolyte of the bridge path, but the current will not pass any metal surface, and no polarization will occur. 

Fig. 16.3 Setup for high-frequency impedance measurements WEI working electrode, WE2 high-surface-area Ft electrode. Potential is applied and ctnrent measured by FRA one electrode is polarized by potentiostat (From Ref. [686], copyright (2012), with permission fiom Elsevier)...

There are limits to the reduction of the electrode surface area. Small surface area polished electrodes demonstrate significant polarization potentials, which decrease pacing efficiency. In theory, extremely small electrodes may be designed like arrowheads therefore they may be more likely to penetrate or perforate the myocardial wall. This problem can be overcome by placing a protective soft polymer collar around the base of the electrode to prevent penetration (Fig. 1.12). Such a protective collar, however, may prevent the electrode from making contact with the endocardium. Despite these criticisms, there is no evidence after more than a decade of clinical experience, of any unique problems, associated with very small surface area electrodes. 

In practice, the suitability of a reaction system is determined by the kinetics of the reaction, which depends on temperature, pressure of gases, electrode polarization, surface area of electrodes, and presence of a catalyst. A fuel cell that is thermodynamically and kinetically feasible must be considered from an econonuc viewpoint before it is accepted. Thus, since hydrogen, hydrazine, and methanol are too expensive for general application, their use in fuel cells has been limited to special cases. Hydrogen has been used for fuel cells in satellites and space vehicles, in which reliability and lightness are more important than cost. Hydrazine fuel cells have been used in portable-radio power supplies for the United States Army because of their truly silent operation. Methanol fuel cells have been used to power navigation buoys and remote alpine television repeater stations because such power systems are comparatively free from maintenance problems over periods of a year or more. The polarization at the electrodes of a fuel cell is the most important single factor that limits the usefulness of the cell. The various polarization characteristics for a typical fuel cell are plotted separately as a function of current density in Fig. 9.11. 

When a battery produces current, the sites of current production are not uniformly distributed on the electrodes (45). The nonuniform current distribution lowers the expected performance from a battery system, and causes excessive heat evolution and low utilization of active materials. Two types of current distribution, primary and secondary, can be distinguished. The primary distribution is related to the current production based on the geometric surface area of the battery constmction. Secondary current distribution is related to current production sites inside the porous electrode itself. Most practical battery constmctions have nonuniform current distribution across the surface of the electrodes. This primary current distribution is governed by geometric factors such as height (or length) of the electrodes, the distance between the electrodes, the resistance of the anode and cathode stmctures by the resistance of the electrolyte and by the polarization resistance or hinderance of the electrode reaction processes. 

Pines and coworkers70 showed the feasibility of spin polarization transfer by SPINOE from laser-polarized 129Xe to surface 13C nuclei on low surface area materials in high-resolution solid-state NMR experiments. This technique provides the basis for novel surface 13C NMR investigations, e.g. of surface coatings, supported catalysts and electrode materials. 

Fig. 8-7. Cathodic and anodic polarization curves observed for a transfer reaction of redox electrons of hydrated Ti /Ti particles at a mercury electrode in 1 M H28O4 solution containing 0.17 M and 0.03 M Ti 4 at 25°C electrode surface area = 0.15 cm. [From Vetter, 1967.]...

Activation polarization arises from kinetics hindrances of the charge-transfer reaction taking place at the electrode/electrolyte interface. This type of kinetics is best understood using the absolute reaction rate theory or the transition state theory. In these treatments, the path followed by the reaction proceeds by a route involving an activated complex, where the rate-limiting step is the dissociation of the activated complex. The rate, current flow, i (/ = HA and lo = lolA, where A is the electrode surface area), of a charge-transfer-controlled battery reaction can be given by the Butler—Volmer equation as... 

Figure 6.21 shows the AC impedance spectra for the cathodic ORR of the cell electrodes prepared using the conventional method and the sputtering method. It can be seen that the spectra of electrodes 2 and 3 do not indicate mass transport limitation at either potentials. For electrode 1, a low-frequency arc develops, due to polarization caused by water transport in the membrane. It is also observable that the high-frequency arc for the porous electrode is significantly depressed from the typical semicircular shape. Nevertheless, the real-axis component of the arc roughly represents the effective charge-transfer resistance, which is a function of both the real surface area of the electrode and the surface concentrations of the species involved in the electrode reaction. 

Enoch et al. [90] used electrodialysis reversal (EDR) to prepare boiler makeup water for Dutch power stations from several types of surface waters. EDR uses automatic reversal of electrode polarity at regular time intervals to minimize membrane scaling. The EDR unit contained 200 anion and cation exchange membrane pairs, each with a surface area of 0.47 m. Polarity reversal occurred every 15, 20, or 25 min. Samples of surface water were desalted by 96% at an energy consumption of 1 kWh/m of product water and at a current density (8.3 A/cm ) that was 80% of the limiting current density (current density when the surface water cation concentration at the membrane surface drops to zero). 

The method is schematically illustrated in figure 2. The operational amplifier A, working in the follower configuration, is used to apply, between the points D and T, the potential difference V present at the terminals of the generator G, which is assumed to be positive. This hypothesis simplifies the analytic expression of the potential difference V, because its polarity determines the behaviour of the two electrodes. The differential-input voltmeter Q determines the intensity of the current that, flowing through the electrochemical cell C from D to T, polarizes the electrodes Wi and W. These electrodes, made of the same material, are identical. Their surface areas are equal to S and, conventionally, they are polarized anodically and cathodically. 

A pyroelectric detector consists of a thin single crystal of pyroelectric material placed between two electrodes. It acts as a temperature-dependent capacitor. Upon exposure to IR radiation, the temperature and the polarization of the crystal change. The change in polarization is detected as a current in the circuit connecting the electrodes. The signal depends on the rate of change of polarization with temperature and the surface area of the crystal. These crystals are small they vary in size from about 0.25 to 12.0 mm. ... 

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