Fundamentals and Strategy of Electrode-Level Models

The objective of an electrode model is to analyse the point-to-point distribution of the reaction in an SOFC electrode, leading to current, potential, and species concentration distributions. The result of the analysis is a prediction of the polarisation of the electrode due to (i) kinetic resistance, (ii) mass transfer resistance, and (iii) ohmic resistance. 

The analysis includes a whole set of material properties and structural parameters. In principle it is based on the same fundamental laws used in full-scale cell analysis. Thus, mass transfer is subject to mass balances (Eqs. (1), (2)), heat flow to energy balances (Eqs. (5), (6)), and fluid flow to Eqs. (3), (4), but it is usually negligible in the pores of the electrodes. In addition, current flow is 

In a similar manner, the species mass balance equations, Eqs. (1), (2), may be coupled with the electrochemical rate at each point of the reaction zone (at or near the TPB). In the continuum-level modelling discussed in Section 11.2, the concentration polarisation of the electrode, riconc was related to a limiting current of the reactant, e.g., Eq. (9). A more fundamental and general expression for the concentration overpotential (the term overpotential denotes exclusively the local polarisation) at any point of the electrode reaction zone is the so-called Nernst equation for example 

Once the local concentration overpotential is known, the activation overpotential, ria, is obtained by subtracting Tjc from total Tj. The local activation overpotential is the actual driving force of the electrochemical reaction. It is related to the local current density at any point of the reaction zone by an electrochemical rate equation such as the Butler-Volmer equation (Eq. (10a)). Therefore, the rate equation, the Nernst equation (Eq. (37)), and the potential balance in combination couple the electric field with the species diffusion field. In addition, the energy balance applies also at the electrode level. Although this introduces another complication, a model including a temperature profile in the electrode is very useful because heat generation occurs mainly by electrochemical reaction and is localised at the reaction zone, while the 

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