Page 348 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

324 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

reforming anode as a function of pore size. In extremely wide pores, the
overpotential is negative over a large part of the pore wall (away from
the interface with the electrolyte), indicating that the electrochemical
process may be reversed away from the interface and hydrogen generation
occurs not only by reforming but electrochemically if pore dimensions are
not optimised.

17.8.4 Monte Carlo or Stochastic Electrode Structure Model
A third distinct type of electrode model developed in response to the need for
modelling the composite structure of SOFC electrodes more accurately is the
Monte Carlo, or stochastic structure, model. This model is based on a random
number-generated 2- or 3-D structure of electrode particles, electrolyte particles,
and holes (for gas pores). It has been shown to represent the composite
conductivity quite well and may be able to model polarisation behaviour
adequately [56-581. This is of interest because microstructure, and in particular
hard-to-control variations in local microstructure, may have an important effect
on overall polarisation, perhaps more so than the intrinsic kinetic characteristics
measured at an ‘ideal’ interface.
The Monte Carlo-type electrode model is also called the particle connectivity
model because its physics is straightforwardly based on Kirchhoff‘s law for an
electrical network, with particle resistance and interconnection resistances
defined by a set of rules to mimic the current flow and electrochemical current
generation within the microstructure. The electrochemical process is
considered to take place with a constant resistance in agreement with
intuitive notions about the mechanism. Variants of this concept attach
correlated values to the resistances in the network to model polarisation more
closely according to a percolation concept of active sites and passive
connections [5 91. Other specialised types of electrode models are mentioned
briefly below.

11.8.4.7 Electrode or Cell Models Applied to Ohmic Resistance-Dominated Cells
These models start from solving Laplace’s equation (Eq. (3 6)) with appropriate
boundary conditions, sometimes including polarisation. The most important
application is the correct design of test cells with reference electrodes because
small deviations in reference electrode placement may cause appreciable
deviations in polarisation readings [60-631.
17.8.4.2 Diagnostic Modelling of Electrodes to Elucidate Reaction Mechanisms
Because the electrode kinetics of both anode and cathode and their dependence
on microstructure are so important for performance, much attention has been
given to elucidating reaction mechanisms based on independent
electrochemical measurements (usually with respect to a reference electrode).
AC impedance measurements are particularly favoured. The interpretation of
these measurements requires specialised models that reflect in part the
hypothesised kinetics and in part the electrode structure. It seems certain that