Exchange current density, Arrhenius

Figure 7. Arrhenius plot of the exchange current density of the Fe2+/Fe + reaction for various concentrations.

The activation overpotentials for both electrodes are high therefore, the electrochemical kinetics of the both electrodes can be approximated by Tafel kinetics. The concentration dependence of exchange current density was given by Costamagna and Honegger.The open-circuit potential of a SOFC is calculated via the Nernst equation.The conductivity of the electrolyte, i.e., YSZ, is a strong function of temperature and increases with temperature. The temperature dependence of the electrolyte conductivity is expressed by the Arrhenius equation. 

The exchange current density is the electrode reaction rate at the equilibrium potential (identical forward and reverse reaction rates) and depends on the electrode properties and operation. The typical expression for determining the exchange current density is the Arrhenius law (3.23), where the constant A depends on the gas concentration. Costamagna et al. [40] provide the following expressions for the anodic and cathodic exchange current density, respectively ... 

Based upon the electrodics, the exchange current density is related to the activation energy (16. pp.ll52). For an Arrhenius relation for the effect of temperature upon corrosion rate, AG and AG j are analogous to activation energy and equal 6.44 kcal/mol and 4.30 kcal/mol, respectively. The literature indicates activation energies for mass transfer limited processes range between 1 and 3 kcal/mol and for reaction limited between 10 and 20 kcal/mol (12). Based upon this criteria, corrosion of the 304 S.S. in pure water in the experimental system may lie between the mass transfer and reaction rate limited cases. 

Fig. 36 Arrhenius plots of the exchange current densities (i o) for the HER/HOR on Pt(hkl) in acid solution. (Reprinted with permission from Ref. 117,...

The exchange current density is also temperature dependent. The relationship between exchange current density and temperature follows the Arrhenius equation,... 

Similar to the case for the HOR, both the exchange current density (tQ /Hjo) and the electron transfer coefficient (0 20) are temperature dependent. The temperature dependency of ioj/HjO expressed as in the Arrhenius form ... 

The temperature dependence of the exchange current density jo follows the Arrhenius equation analogous to the relationship given above for k. 

Exchange Current Density Exchange current density io is a very important parameter that has a dominating influence on the kinetic losses. It appears from Eq. (4.35) that activation polarization should increase with temperature. However, io is a highly nonlinear function of the kinetic rate constant of reaction and the local reactant concentration and can be modeled with an Arrhenius form as... 

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