Page 304 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

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

705.7 Non-activated Contributions to the Total Loss
Evident causes of the big difference between EA,cell and EA,catho~e are
contributions to ASR which are fairly constant with temperature such as Rp,conoer
and contributions with a metallic-type dependence on temperature such as from
Ni/YSZ or metallic connections. These would be most significant at higher
temperatures where the thermally activated contributions are smallest. This is
part of the explanation for the declining EA observed for Risa cells (as stated
under Table 10.2). A similar tendency is seen in the analysis of Allied-Signal data
[3 81. Conservatively, such non-activated contributions can be estimated as the
difference between measured and calculated ASR at high temperatures where
the non-activated contributions prevail.
The data in Table 10.2 also indicate a slight increase in EA with
increasing current i. Part of the explanation is the relative importance of
R,,,,,,, plus Rp,difi Correction for this effect would increase EA at low i.
From Tables 10.2 and 10.3, the difference between EA for the calculated
ASR (the sum of the individual contributions, Table 10.3) and ASR as measured
on practical cells (Table 10.2), is about a factor of 2 for the three cells plotted
in Figure 10.1 1; it is not likely that this effect could be due to uncertainties and
non-linearities.

70.5.2 Inaccurate Temperature Measurements
Another reason for the cell EA values being much smaller than EA of the
electrodes and the electrolyte could be the differences between actual and
assumed cell temperatures. Measurement of cell temperature during testing is
not trivial. A cell larger than a few cm2 in area is likely to experience
temperature gradients due to cooling by feed gases and heating by current. For
this reason, measurements of temperature at multiple points on the cell itself
are recommended; measurements of furnace temperature or gas stream
temperature will typically be misleading. Even sheathed thermocouples in
contact with the cell surface or contacting structure may influence the actual
cell temperature by heat conduction. Using such thermocouples (diameter =
1.5 mm), a temperature rise of 20°C has been observed at 1 A/cm2 at 860°C
[46]. Within the first 3 s, a rise of < 1°C was observed, and within 20 s the
temperature was up by 4°C. The total rise occurs over 2-3 min. This means
that in testing, significant heating of the cell is avoidable for only a few seconds,
and that i-V curves should be recorded with this in mind, or corrected for the
observed T(i) effect. The difference in measured and calculated ASR, at, for
example, 750"C, for Risa thick-electrolyte cells, if assumed to be (Figure 10.11)
entirely due to an inaccurate temperature measurement, would correspond to
an over-temperature of about 70°C which is highly unlikely given the
measured temperature increases under load. Hence, the difference between the
measured EA for cells and the value expected from the known activation
energies of the electrode processes and the electrolyte does not seem to be due to
erroneous temperature measurement.