Overvoltage and current density

In anodic reactions, activation polarization is often dominating (except when passivation effects occur). Then, the relationship between overvoltage and current density is, as previously shown ... 

Conversely, in cathodic reactions (reduction processes), both types of polarization have to be taken into account. The relationship between overvoltage and current density will therefore be as follows ... 

In low- and medium-temperature fuel cells, activation overvoltage is the most important irreversibility and cause of voltage drop. The relation between activation overvoltage and current density is described by flie Tafel equation, e.g.. 

The D s designate the diffusion coefficients, and 3 is the thickness of the respective diffusion layer. The correlation between overvoltage and current density is ... 

Truly first equation of electrochemical kinetics, Tafel equation, links overvoltage and current density as ... 

For activation polarization, relationship between overvoltage and current density... 

Fig. 118. Variation of overvoltage with current density (Bowden and Rideal)...

The results recorded below for the variation of the overvoltage with current density at a mercury cathode in dilute sulfuric acid at 25 have been estimated from the data of Bowden and Rideal [ Proc. Roy. Soc. 120A, 59 (1928)]. 

Fig. 1.17 The primary and secondary current distribution along the cathode of an arbitrary electrochemical system is shown. Due to overvoltages the current density is more uniform. Zero and infinite current densities no longer exist.

FIGURE 4.3.12. Plot of overvoltage vs current density in the low current density region for graphite in saturated NaCl solutions (pH ca.0.5) at 50°C and pci2 = ca.l atm [45]. (With permission from The Electrochemical... 

For activation polarization, the relationship between overvoltage 17 and current density i is... 

Activation losses are the main cause of voltage drop (mainly at cathode) for the low- and medium-temperature fuel cells. In 1905, Tafel obtained a pattern for the verification of overvoltage against current density as shown in Fig. 2.2. 

The reorganization free energy /.R represents the electronic-vibrational coupling, ( and y are fractions of the overpotential r] and of the bias voltage bias at the site of the redox center, e is the elementary charge, kB the Boltzmann constant, and coeff a characteristic nuclear vibration frequency, k and p represent, respectively, the microscopic transmission coefficient and the density of electronic levels in the metal leads, which are assumed to be identical for both the reduction and the oxidation of the intermediate redox group. Tmax and r max are the current and the overvoltage at the maximum. 

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