Page 295 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

2 72 High Temperature Solid Oxide Fuel Cells: Fundnmentafs, Design and Applications

of the cell design, component materials, and fabrication processes, performance
stability with time, and performance variation with temperature. Often the
results of screening tests are reported as power density (W/cm2) figures, and
record values of maximum power density are extensively quoted. However, this
measure can be confusing because power density varies greatly with fuel
composition and with electrode polarisation. The i-V curves for SOFCs are often
linear and therefore allow an interpretation in terms of area-specific resistance
(ASR). Even though ASR has no generally accepted definition, it is much
less dependent on test conditions than power density, and it is preferable to use
it to compare screening test results. The concept of ASR is discussed in the
next section.

10.4 Area-Specific Resistance (ASR)

A fuel cell stack can be regarded as a ‘black box’ into which hydrogen (gas) and
oxygen (air) are inputs, and electricity and exhaust gases are outputs. For such a
stack, ASR is defined as:
Emf - U
ASR =
i

where Emf is the electromotive force with the inlet fuel and air, and U is the cell
voltage at the current density, i, at the design point. A possible design point, for
example, might be 0.6 V at 1000°C, a fuel utilisation of 85% and an air flow of 4
times the stoichiometric amount (‘4 stoichs’).
The cell voltage, U, should be measured independently of the leads carrying
the current, i.e. separate potential probes should be used. This ASR is in most
cases not very sensitive to small variations in cell voltage and fuel utilisation. By
determining the ASR at a few different temperatures, an apparent activation
energy, EA, may be derived. Thus, in a voltage interval (from say 0.5 to 0.7 V)
and a temperature interval (say from 650 to lOSOT), the cell may be
characterised, with fair approximation, by only two characteristic numbers,
namely ASR at one temperature and EA.
In case the i-V curve is concave, it may be tempting to use a differential ASR
(i.e. the tangent) at high current densities as this gives a nice low value. Such a
number has the drawback that it does not reflect the cell performance over the
full polarisation range as does the quantity defined by Eq. (1).
Often, cell tests are conducted with very low fuel and air utilisations because
these are easier to perform than the ‘realistic’ tests with high fuel utilisation. In
case of insignificant fuel and oxygen utilisations, the relevant definition of ASR is
again that of Eq. (l), but the insignificant utilisation makes this value
incomparable to ASR derived from experiments with high fuel utilisation,
because the concentration polarisation resistance due to the fuel and air
conversion can be a considerable fraction of the total cell resistance. Thus, it is,
in general, necessary to specify also the fuel and oxygen utilisation together with