Voltage polarization

As can be seen from Eigure 11b, the output voltage of a fuel cell decreases as the electrical load is increased. The theoretical polarization voltage of 1.23 V/cell (at no load) is not actually realized owing to various losses. Typically, soHd polymer electrolyte fuel cells operate at 0.75 V/cell under peak load conditions or at about a 60% efficiency. The efficiency of a fuel cell is a function of such variables as catalyst material, operating temperature, reactant pressure, and current density. At low current densities efficiencies as high as 75% are achievable. 

Equation (2-38) is valid for every region of the surface. In this case only weight loss corrosion is possible and not localized corrosion. Figure 2-5 shows total and partial current densities of a mixed electrode. In free corrosion 7 = 0. The free corrosion potential lies between the equilibrium potentials of the partial reactions and U Q, and corresponds in this case to the rest potential. Deviations from the rest potential are called polarization voltage or polarization. At the rest potential = ly l, which is the corrosion rate in free corrosion. With anodic polarization resulting from positive total current densities, the potential becomes more positive and the corrosion rate greater. This effect is known as anodic enhancement of corrosion. For a quantitative view, it is unfortunately often overlooked that neither the corrosion rate nor its increase corresponds to anodic total current density unless the cathodic partial current is negligibly small. Quantitative forecasts are possible only if the Jq U) curve is known. 

Polarization Voltage different between OCV and //f-free CCV (closed-circuit voltage) vs. Hg/HgO (9 mol L"1 KOH). 

FIGURE 7.2 Schematic of a DNA sensor based on a capacitive EIS structure. For operation, a DC (direct current) polarization voltage (VG) is applied via the reference electrode (RE) to set the working point of the EIS sensor, and a small AC (alternating current) voltage (E ) is applied to the system in order to measure the capacitance of the sensor. ssDNA - single-stranded DNA, cDNA - complementary DNA, dsDNA - double-stranded DNA. 

Figure 16.8 Polarization (voltage versus log current density) curve for a typical stainless steel in water (cf. Fig. 15.5).

Knowledge of the amount by which the voltage of a cell, delivering a particular level of current, deviates from its equilibrium value is of central importance in assessing the performance of a practical battery system. This polarization voltage, Ep0], can be associated with two principal causes ... 

The electric field which actually affects the charge transfer kinetics is that between the electrode and the plane of closest approach of the solvated electroactive species ( outer Helmholtz plane ), as shown in Fig. 2.2. While the potential drop across this region generally corresponds to the major component of the polarization voltage, a further potential fall occurs in the diffuse double layer which extends from the outer Hemlholtz plane into the bulk of the solution. In addition, when ions are specifically absorbed at the electrode surface (Fig. 2.2c), the potential distribution in the inner part of the double layer is no longer a simple function of the polarization voltage. Under these circumstances, serious deviations from Tafel-like behaviour are common. 

The 02 electrode comprises a platinum cathode and a silver anode in an electrolyte of half-saturated KC1. When a polarizing voltage (-0.6 V) is applied the following reactions occur ... 

The phenomenon of limiting current is marked by a nonohmic increase of voltage as the current is increased. A rather simple study of the ratio of current to voltage will reveal this increase. The voltage is the sum of the electrode voltage, Ve, whatever polarization voltages are present, Vp, and the IR drop ... 

Fig. 1.1 Schematic of the origin of polarization (voltage) losses in a SOFC.

Experiments using electrodes under monotonous potential distribution and constant voltage combined with altemating-current-impedance measurements give us the opportunity to study the potential area of electrode activity for synthesized products i.e. to optimize conditions of their electrochemical synthesis. Moreover, the length of the electrode part immersed in the electrolyte is correlated to the polarizing voltage amplitude. This allows us to pinpoint the limits of the electrode processes reversibility. 

Figures 11.3 and 11.4 show the frequency spectra of 2600F, 0.5mQ BCAP0010 DLC capacitance and series resistance for three different polarization voltages. It is interesting to observe the low-capacitance value when there is no voltage polarization. This phenomenon should be studied as a function of the electrode thickness. If ionic depletion was the cause, a thick electrode should display a more pronounce effect.

FIGURE 11.3 Maxwell Technologies BCAP0010 capacitance as a function of the frequency for three different values of the polarization voltage. The data have been obtained with an impedance spectrometer with a DC bias voltage option. 

FIGURE 8.16 Schematic fuel cell polarization voltage [V] vs. current density [A/cm2] curve. 

Fig. 15. The relation of current I flowing through an electrolyzer to the voltage of the external source of current in a system with constant polarization voltage Ep.

From this it follows that Ea has the same magnitude as the polarization voltage of the system at the beginning of the continuous electrolysis. 

Apply 0.8 V polarizing voltage to the electrode and activate the recorder. 

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