Polarization losses

Whenever energy is transformed from one form to another, an iaefficiency of conversion occurs. Electrochemical reactions having efficiencies of 90% or greater are common. In contrast, Carnot heat engine conversions operate at about 40% efficiency. The operation of practical cells always results ia less than theoretical thermodynamic prediction for release of useful energy because of irreversible (polarization) losses of the electrode reactions. The overall electrochemical efficiency is, therefore, defined by ... 

Fig. 6. Discharge behavior of a battery where is the open circuit voltage (a) current—potential or power curve showing M activation, ohmic, and M concentration polarization regions where the double headed arrow represents polarization loss and (b) voltage—time profile.

Reactions (3.9) to (3.11) proceed rapidly to equilibrium in most anodic solid oxide fuel cell (SOFC) environments and thus H2 (Eq. 3.8) rather than CH4 is oxidized electrochemically resulting in low polarization losses. Upon doubling the stoichiometric coefficients of equation (3.8), summing equations (3.8) to (3.11) and dividing the resulting coefficients by two one obtains ... 

The total output photovoltage must exceed the thermodynamic potential difference for water splitting (1.229 V at 25°C), the energy level mismatches for the anodic and cathodic processes, and the polarization loss or overvoltages due to kinetic, diffusion, and IR potential losses in the bulk of electrolyte. 

Figure 3.5 [36], For the 02 reduction reaction on freshly prepared LSM electrodes, the initial polarization losses are very high and decrease significantly with the cathodic polarization/current passage (see Figure 3.5b). Consistent with the polarization potential, the impedance responses at open circuit decrease rapidly with the application of the cathodic current passage. For example, the initial electrode polarization resistance, RE, is 6.2 Qcm2 and after cathodic current treatment for 15 min RK is reduced to 0.7 Qcm2 see Figure 3.5 (a). The reduction in the electrode polarization resistance is substantial. The analysis of the impedance responses as a function of the cathodic current passage indicates that the effect of the cathodic polarization is primarily on the reduction in the low-frequency impedance [10]. Such activation effect of cathodic polarization/current on the electrochemical activity of the cathodes was also reported on LSM/YSZ composite electrodes [56-58], Nevertheless, the magnitude of the activation effect on the composite electrodes is relatively small.

The activation polarization loss is dominant at low current density. At this point, electronic barriers have to be overcome prior to current and ion flow. Activation losses show some increase as current increases. Ohmic polarization (loss) varies directly with current, increasing over the whole range of current because cell resistance remains essentially constant. Gas transport losses occur over the entire range of current density, but these losses become prominent at high limiting currents where it becomes difficult to provide enough reactant flow to the cell reaction sites. 

In most modern practical batteries, a major part of polarization loss at moderately high current densities is due to ohmic potential drop. Considerable attention is therefore given during the design of a battery to ... 

Polarization losses associated with an electrode process are termed overvoltage . The overvoltage 1] is defined as... 

The oxygen electrode suffers from considerable polarization losses on discharge, largely due to mass transport limitations. Metal-air cells have... 

Such a comparative study has been made by Byakov and his collaborators.29 255 They have shown that in the case of water the main contribution to the loss rate given by formula (6.3) comes from excitation of intramolecular vibrations rather than from dipole relaxation. This is all the more so in nonpolar media where the main channel of continuous losses is not the relaxation of constant dipole moments (which are zero) but the polarization losses due to the electron-inducing dipole moments in molecules. The possible exceptions are the media consisting of molecules with a high degree of symmetry, such as methane and neopentane, which have no active vibrations in the IR region. 

One of the characteristics of radiation considered in radiation chemistry and in radiobiology is the linear energy transfer (LET). For fast charged particles the LET practically equals the ionization losses (or polarization losses, in condensed media) and is given by the formulas for the stopping power presented in Section V.A. 

Arrhenius plots of conductivity for the four components of the elementary cell are shown in Fig. 34. They indicate that electrolyte and interconnection materials are responsible of the main part of ohmic losses. Furthermore, both must be gas tight. Therefore, it is necessary to use them as thin and dense layers with a minimum of microcracks. It has to be said that in the literature not much attention has been paid to electrode overpotentials in evaluating polarization losses. These parameters greatly depend on composition, porosity and current density. Their study must be developed in parallel with the physical properties such as electrical conductivity, thermal expansion coefficient, density, atomic diffusion, etc. 

Concentration polarization losses are sometimes expressed as a function of the limiting current, U, which is usually taken as a measure of the maximum rate at which a reactant can be supplied to an electrode ... 

Compared with a Teflon -bonded commercial electrode, the composite electrode showed lower polarization losses at high current densities, even though the composite material did not contain Pt. The ohmic and mass transfer resistances were lower in the composite electrode than in the commercial electrode. The sintered contacts and interlocked networks formed in the composite structure permitted better electrical and physical contact between the carbon fibres and metal fibres, leading to a composite electrode with a high void volume and large macroscopic porosity, which increased the accessibility of carbon to the reactants [22],... 

Fig. 13. Vh20, measured in aq. saturated or molten NaOH, at 1 atm.90 CO2 is excluded by argon purge. The molten electrolyte is prepared from heated, solid NaOH with steam injection. O2 anode is 0.6-cm2 Pt foil. IR and polarization losses are minimized by sandwiching 5 mm from each side of the anode, two interconnected Pt gauze (200 mesh, 50 cm2 = 5 cm x 5 cm x 2 sides) cathodes. Inset At 25 °C, 3 electrode values at 5 mV/s versus Ag/AgCl, with either 0.6-cm2 Pt or Ni foil, and again separated 5 mm from two 50-cm2 Pt gauze acting as counter electrodes.

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