Page 310 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

28 6 High Temperature Solid Oxide Fuel Cells: Fundamentals. Design and Applications

and OCV are observed, the OCV value may be used instead of the Emf of the inlet
gas only in case where it is in some way verified that the leak is evenly distributed
over the cell area.

10.7 Summary
This chapter has considered the main types of electrochemical tests which have
been applied to SOFCs and has outlined the main issues which require detailed
attention for obtaining meaningful test results.
One important aspect in electrode testing is to assure a correct geometry in
three-electrode set-ups, This is very difficult in practice in case of electrode-
supported cells with thin electrolytes. Unfortunately, even testing the individual
electrodes in sound geometry set-ups is not a perfect procedure either, because
the sum of the contributions from individual cell components to the cell
resistance does not add up to the actual measured total cell resistance. This is
probably due to differences in the fabrication of the special cells for electrode
characterisation and the practical cells.
Another important issue is that of gas leakage, which can cause significant
errors in performance data. Especially in the case of gas leakage, cells can easily
be at a higher temperature than their surrounding environment, causing cells to
give better apparent performance than individual electrodes tested under better
controlled conditions. Also, the gas composition at the electrodes (usually at the
anode) may be different from the intended composition in case of gas leakage. A
method for estimating the size of the gas leakage has been presented here.
It is recommended that cell test results be reported in a way that makes it easy
to derive area specific resistance (ASR) from the i-V curves. Sufficient
information should be provided so that the ASR values can be corrected for
effects of finite fuel utilisation. Also, the choice of fuel composition should
preferably reflect real cell operation conditions. The ASR should be derived using
the Emf and a cell voltage in the range of 0.5-0.7 V and its corresponding current
density. In case of a grossly non-linear I-V curve, a differential ASR value is of
little practical use.

References
[ 11 K. J. Vetter, Electrochemical Kinetics - Theoretical and Experimental Aspects,
Academic Press, New York, 1967.
[2] J. O’M. Bockris and A. K. N. Reddy, Modern Electrochemistry, Plenum
Press, New York, 19 70.
[3] Southampton Electrochemistry Group, Instrumental Methods in
Electrochemistry, Ellis Horwood, Chichester, 198 5,
[4] R. A. Rapp and D. A. Shores, in Physicochemical Measurements in Metal
Research, Part 2, ed. R. A. Rapp, Wiley Interscience, New York, 1970,
pp. 123-192.