Open circuit electrode kinetic equation

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. 

Oxidation of CO is also important in fuel cell applications. By combining the half reaction for the CO2/CO couple, equation (a), with electrochemical reduction of O2, fuel cells may achieve a maximum open circuit potential given by IlSlfCOj/CO) - (02/H20) = 1.34 . In practice, electrocatalysts are required to lessen the normally high kinetic overpotentials for electrodic CO oxidation. An example of a CO/O2 fuel cell which operates at the relatively low T of 80°C by employing [Rh(CO)2Br2] as the electrocatalyst and the Rh couple to mediate CO oxidation is shown below . 

The rapid temperature change of the electrode perturbs the equilibrium at the electrode-solution interface and causes a change in the potential of the electrode measured with respect to a reference electrode. The change in the open-circuit potential, A t, and its relaxation with time are used to obtain kinetic information about the electrode reaction. A number of different phenomena come into play to cause the potential shift with temperature (e.g., temperature dependence of the double-layer capacitance and the Soret potential arising from the temperature gradient between the electrode and the bulk electrolyte), but the response can be treated by a general master equation (40) ... 

The electrochemical kinetic behavior described in Equations (27.19)-(27.21) are easily discernible and therefore provide a basis for diagnostics for the establishment of self-discharge mechanisms after recording the dependence of potential or voltage of an electrode or cell in an open-circuit voltage on time. The presence of... 

In electrode kinetics, as empirically represented by Tafel s equation, a basic feature is the potential-dependence of the reaction rate (current-density). This effect arises in Gurney s representation in a fundamental and general way as the electric potential V, of the electrode metal is changed by AV relative to that of the solution (in practice, measured relative to the potential of a reference electrode at open-circuit), the effective value of the electron work function 4> of the metal is changed according to... 

For the measurement of gas components like hydrocarbons (HC) or nitric oxides (NOx) in non-equilibrated gas phases kinetically determined sensors are used (Fig. 19.2 middle).Depending on the electrode material, the gas components do not equilibrate on the measuring electrode at temperatures <700 °C. Thus gas components which are not thermodynamically stable are electrochemically active. In an HC- and 02-containing gas, for example, at least two electrode reactions can take place the electrochemical reduction of oxygen and the electrochemical oxidation of hydrocarbons. The measured open-circuit voltage does not obey the Nemst equation. Therefore such electrode behaviour is often referred to non-Nernstian electrodes (or mixed potential sensors). The cell voltage depends logarithmically on the concentrations of the hydrocarbons ... 

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