Page 264 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

EZectrode Polarisations 241

which is the Tafel equation [2,26]. The above equations describing the
activation polarisation are from the extensive work on aqueous
electrochemistry. These equations are often used to describe activation
polarisation in solid state electrochemistry also. In aqueous electrochemistry,
the process of charge transfer occurs across the entire liquid electrolyte/solid
electrode interface. In solid state electrochemistry with the presence of gaseous
species, however, this charge transfer process either involves three phases: the
electrolyte, the electrode (electrocatalyst), and the gas phase, in the case of
purely electronic conducting electrodes or two phase MIEC electrodes: or two
phases: namely a single phase MIEC electrode and the gas phase, in the case of
single-phase MIEC electrodes. If the transport of ions is restricted to the
electrolyte, that of electrons through the electrocatalyst, and of the gaseous
species through the porous interstices, the charge transfer reaction is presumed
to occur at (or near) the three-phase boundary (TPB) where the three phases
meet. This TPB is characterised by a line, extending along the electrolyte surface,
and has the dimensions of cm/cm2 or cm-l. This suggests that the electrode
kinetics must depend upon the TPB length, in addition to fundamental physical
parameters, such as electrocatalytic activity of the electrocatalyst, and the
partial pressure of the reactant (Le. oxygen). That is, the exchange current
density. $, must depend upon electrode microstructure, such as the size and the
number of electrocatalyst particles per unit area of the electrolyte surface. Thus,
in such a case

i: = f(TPB, partial pressure of oxygen in the atmosphere, oxygen
vacancy concentration in the electrolyte, oxygen vacancy
(21)
mobility in the electrolyte, electron concentration in the
electrocatalyst, and temperature)

Figure 9.3(a) shows a schematic of such a charge transfer reaction. In a single-
phase MIEC electrode, on the other hand, the charge transfer reaction is not
restricted to the linear feature (e.g. TPB), but can occur over the entire electrode/
gas phase interface. In such a case, the exchange current density is given by
ii = f(partia1 pressure of oxygen, oxygen vacancy concentration
in the MIEC, oxygen vacancy mobility in the MIEC,
electronic defect concentration in the MIEC, (22)
and temperature)

Figure 9.3(b) shows a schematic of such a charge transfer reaction. In so far as
single phase, essentially electronically conducting materials as cathodes are