Electrochemical Models at the Electrode Level

Electrode-level models describe the performance of SOFC electrodes in detail. They take into account the distribution of species concentrations, electric potential, current, and even temperature in the electrode. Their purpose is to (i) interpret the performance (polarisation curve) of electrodes in terms of rate-limiting resistances such as kinetic (activation), mass transfer, and ohmic resistance and (ii) predict the local polarisation in full-scale cell and stack models. 

To predict the local polarisation in a full-scale cell or stack at any point, its dependence on composition, pressure, and temperature of the gas flowing in the gas channel contacting the electrode must be known. In a large cell, these bulk gas properties vary from one point to the next. Electrode polarisation or overpotential - the difference between the local potential of the electrode under load and the potential at open circuit (equilibrium potential) - is also a local quantity because it depends not only on the bulk gas composition but also on the current density. In a large cell the current is usually distributed nonuniformly, as discussed in Sections 11.2-11.5. Similar to Eq. 7, one can express the local cell voltage under load, i.e., when current is passed, as the thermodynamic cell potential minus three loss terms the ohmic loss, the cathode polarisation, and the anode polarisation  

As discussed in Section 11.2, the total polarisation of each electrode consists of two contributions, activation polarisation (due to electrode kinetic resistance) and concentration polarisation (due to mass transfer resistance), so 

For cell- and stack-level modelling it is necessary to have reliable values of the total polarisation of cathode, r)c, and anode, t a, as a function of local bulk gas composition, pressure, and temperature, as well as the local current density. 

An electrode model is especially advantageous if it can be used to relate the kinetic and mass transfer resistance to electrode geometry and microstructure for instance, to thickness, porosity, pore or particle size, contact areas of phases, and/or grain size of electrode and electrolyte materials. A well-tested and validated electrode model, therefore, may serve to assist in the design of optimised electrode structures or electrode/electrolyte interfaces to minimise polarisation loss. 

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