Page 23 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

4 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and App!ications

cathode current collector joined together into one sheet, thus combining the two
components. Additionally, the interconnector can contain gas channels which
supply fuel to the anode and oxidant t.o the cathode as well as electrically
connecting the anode of one cell to the cathode of the next.
It turned out that there were several problems with flat plate stacks as they
were made larger to generate increased power, including sealing around the
edges and thermal expansion mismatch which caused cracking. Consequently,
tubular designs have had greater success in recent years. However, the
configuration of Figure 1.2 has been prominent in zirconia sensors, discussed in
the next section, which are now manufactured in large numbers.

1.3 Zirconia Sensors for Oxygen Measurement
An SOFC in reality already exists on every automobile: it is the oxygen sensor
device which sits in the exhaust manifold in order to control the oxygen content
of the effluent mixture entering the exhaust catalyst. The composition of the
effluent mixture must be controlled to near stoichiometric if the catalyst is to
operate at its optimum performance. Yttria-stabilised zirconia (YSZ) is generally
used as the electrolyte because this uniquely detects oxygen, and platinum
is normally painted on its surface using proprietary inks to provide the electrodes.
In the original design, which was used from the 1970s the configuration was
similar to that of Baur and Preis [5]. A thimble of YSZ, containing typically 8 wt%
yttria, was pressed from powder and fired to 1500°C to densify it. Platinum
electrodes were applied and the unit then fixed in a steel plug which could be
screwed into the car exhaust manifold, so that the YSZ +anode was protruding
into the hot gases. Air was used as the oxygen reference on the cathode side. A
wire connection supplied the voltage from the inner electrode to the engine
management system, while the other electrode was grounded to the chassis.
Once the exhaust warmed up, above 600"C, the voltage from the sensor
reflected the oxygen concentration in the exhaust gas stream. This voltage
varied with the logarithm of oxygen level, giving the characteristic 1-shaped
curve of voltage versus oxygen concentration, hence the name 'lambda sensor'.
The control system then used the oxygen sensor signal to manage the engine so
that the exhaust composition was optimised for the catalyst. Various
improvements have been made to this basic system over the years; for example, a
heating element can be built within the thimble, in order to obtain a rapid
heating sensor.
The major improvement introduced by Robert Bosch GmbH in 1997 was to
redesign the zirconia sensor and to manufacture it by a different method. Instead
of pressing a thimble from dry powder, a wet mix of zirconia powder with
polymer additives was coated and dried like a paint film on a moving belt in a
tape-casting machine. The film dried to a thickness of around 100 pm and could
be screen printed with the platinum metallisation before pressing three or four
sheets together to form a planar sensor array which was fired and then sectioned
to size before inserting in the metal boss which screwed into the engine manifold.