Page 262 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Electrode Polarisations 2 39

point some distance away from the TPB. Thus, many possibilities exist
wherein there are a number of possible parallel steps to the net charge
transfer reaction. Experimental work combined with continnum
modeling has been conducted in order to analyse various reaction steps
[15,17,20,2 11; experimental methods used in these studies include
steady-state current-voltage characteristics and electrochemical
impedance spectroscopy. The importance of the role of TPB in
Lal,(Sr,Ca),Mn03 and Lal-$r,Mn03 cathodes is documented in
appended references [14,20,22,23]. These studies showed that oxygen
reduction predominantly occurs at TPB’s when these materials are used
as cathodes. However, bulk transport of oxygen through Lal-,Sr,MnO3
has been reported under high applied overpotentials [9]. A wide variety of
reaction mechanisms have been proposed in the literature, even using
nominally similar electrode materials. This lack of consistency is
apparently due to the fact that electrode processes and morphology are
closely interrelated, and also that the fundamental mechanisms are not
fully understood at the present time. Considering that the current and
potential distribution in the 3-dimensional porous system and the
available reaction zone both depend on the microstructure and the
presence of secondary phases and impurities at the interface, it is clear
that a simple, unified reaction mechanism is unrealistic. But this in large
part is also due to a lack of a quantitative characterisation of electrode
microstructure, and a Iack of quantitative analysis of the relationship
between microstructure and cathode performance. Such a quantitative
study has been reported in only a limited number of studies, such as those
by Zhao et al. [24] and Weberet al. [25].
Above reactions 1) to 4) describe a number of possible, series steps, and in the
simplest model, the slowest one is the rate-determining step4. The remaining
steps then can be assumed to be close to equilibrium. Many of the above steps are
generally thermally activated. The rate of cathodic reaction is directly
proportional to the net current density: or more precisely, the net current density
is proportional to the cathodic reaction rate. Associated with the reaction rate, or
the passage of current, is a loss in voltage, which is the activation polarisation or
overpotential; the terminology being derived from the thermally activated
nature of the reaction. The relationship between the cathodic activation
polarisation, Y&~, and the current density is usually nonlinear, except at very low
current densities. In general,

r&t = f(materia1 properties, microstructure, temperature,
atmosphere, current density) (16)
It is to be emphasised that it is not necessary that there be a single rate-determining step. If two or
more steps exhibit similar kinetic barriers, it is quite possible that a simple dependency of the so-called
rate-determining step on a given parameter (e.g. oxygen partial pressure) will not be reflected in the
overall measured effect. That is, there can be more than one step away from equilibrium The analysis of
data in such a case can be particularly difficult.