Page 181 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

158 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Apphtions

the anode surface coverage of hydrogen under those conditions becomes less
sensitive to the partial pressure.
Similarly for thermal activation on the anode, Arrhenius plots can be
constructed from the logarithmic dependence of current density on reciprocal
temperature for the evaluation of the activation enthalpy of the oxidation
reaction. Here again, within the Tafel regime, the activation enthalpy falls from
130 kJ/mol below 840°C to 110 kJ/mol at higher temperatures. While at the
lower temperatures a charge-transfer controlled reaction dominates, other
limiting considerations emerge as the temperature increases. This is compatible
with the actual development strategy for advanced anodes in lower temperature
fuel cells where enhancement of charge-transfer behaviour through
electrocatalysis is the objective, a strategy less relevant at higher temperatures
where anode performance is already adequate through thermal activation.

6.6 Anode Behaviour Under Transients Near Equilibrium

Complementary to steady-state characterisation by continuous current methods
are the perturbation techniques, such as time-domain transients induced by
current or potential steps, or frequency-domain analysis of system behaviour
by electrochemical impedance spectroscopy (EIS) [ 171. The methods reveal
dissipative mechanisms contributing to overpotential losses on the basis of
response time, and distinguish them from the purely resistive effects.
Identification of the observed EIS features with specific physicochemical
processes can then be effected by variation of other system parameters such as
applied potential, temperature or gas environment. The physical system can
then be analysed in terms of an equivalent circuit, an electronic analogue
reproducing the frequency response of the actual device. Processes giving rise to
impedance spectral features are therefore not necessarily electrical, but any
process involving storage and dissipation effects contributing to the overall
polarisation loss will appear in the equivalent circuit through a corresponding
electronic model component.
At the high-frequency limit of the spectrum the intercept on the resistive axis
specifies the serial ohmic component in the measured system, since this element
does not introduce a phase shift. Similarly the low-frequency limit approaches
the steady-state condition corresponding to the d.c. characteristics of the ceI1.
Each spectral feature detected between these limits represents a dissipation
process with the specific time dependence indicated by the inverse of the
frequency at which it occurs. It should be noted, therefore that two processes
with similar time constants in the anodic system will not be distinguishable by
impedance spectroscopy.
In the case of high-temperature fuel cell anodes, conflicting evidence has been
presented on the number and significance of the dissipative mechanisms detected
as contributing to polarisation losses by impedance spectroscopy. At least three
features may be distinguishable, dependent on the particular anode structure and
the experimental conditions [17,19]. This variability of the impedance spectra is