Page 285 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

262 High Temperature Soiid Oxide Fuel Cells: Fundamentais, Design and Applications

interpret the results. Therefore, extensive testing is usually conducted only on
cells and the various cell components, primarily electrodes. Nevertheless, tests
on stacks and complete systems have been performed, see e.g. [20-261, but no
general test methods can be derived from these reports. If an interconnect
material is used for current collection from the electrodes in a single cell test,
then the test is sometimes referred to as a ‘short stack’, a ‘stack element’ or a
‘stack unit’ test [2 71. It should be noted that the contact resistance between the
electrodes and the interconnect is usually significant [2 51, especially in cases
where the interconnect is made of stainless steel or other chromia forming alloys,
which have a tendency to form poorly conducting surface layers [28,29].
There are special problems in SOFC electrode testing. An electrode can only
be characterised electrochemically if it is part of a cell with at least one
reference electrode, that is, a cell with at least three electrodes in contact
with the same electrolyte. However, it is difficult to place a usable reference
electrode on a thin electrolyte film in an electrode-supported cell; this is discussed
in the next section.

10.2 Testing Electrodes
The main problem with characterising the individual electrodes in single cells
and short stacks is the insertion of the reference electrodes, which are used to
judge the performance of the individual components and interfaces in the cell.
Since a single cell stack is made up of five components, an electrolyte, two
electrodes and two interconnects, there are four interfaces at which reference
electrodes can be inserted. Unfortunately, such reference electrodes will not
work in the geometries that are normally employed.
Results of tests on cells with thin electrolyte layers using one or more reference
electrodes have been reported on many occasions [30-331 but the electrodes
under investigation appeared not to be polarised; the short explanation is that
the reference eIectrodes were not working. The geometric requirements for the
position of electrodes in three-electrode set-ups have been treated in detail by
several researchers [34-391, and there is general agreement among these
researchers. In spite of this, there still seems to be a great need in the SOFC
community for basic information on how to measure electrode potentials
properly; some of these details are given below.
Figures 10.1 and 10.2 illustrate the problem. An electrode-supported cell with
a ‘reference’ electrode is often sketched as shown in Figure 10.la. However, such
a sketch is very deceiving when it is used for an assessment of the current
distribution. For this purpose, the sketch should be drawn to scale, i.e. the
electrolyte thickness should be the relevant unit of length. When the correct
length scale is used, as in Figure lO.lb, it is evident that the gap between the
upper working electrode and the ‘reference’ electrode is huge. This means that
the current distribution around the right-hand edge of the working electrode
becomes very different from the even current distribution in the main part of the
cell. Furthermore, the current in the vicinity of the ‘reference’ electrode becomes