Page 26 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

lntroductian to SOFCs 7

1.5. High-Quality Electrolyte Fabrication Processes

One of the main issues in slowing down the advances in SOFCs has been the
difficulty of making good cells. The electrolyte has to meet several criteria
for success:

0 It must be dense and leak-tight.
0 It has to have the correct composition to give good ionic conduction at the
operating temperatures.
0 It must be thin to reduce the ionic resistance.
0 It must be extended in area to maximise the current capacity.
0 It should resist thermal shocks.
0 It must be economically processable.

These requirements are not easily reconciled. Industrial ceramic processing
has traditionally focused on the pressing of dry powders in metal dies or in rubber
moulds to make spark plugs, for example. Although zirconia sensors have been
made by this technique, and although much academic research has used this
method, it is difficult to make thin-walled parts of large area in this way. A
stacked tubular design made by powder pressing had been demonstrated in
the 1960s but this proved to be expensive because of diamond grinding and of
high resistance due to the 500 pm thick electrolyte [SI. It was far better to
move towards the advanced ceramic processes such as chemical vapour
deposition, tape casting and extrusion (see Figure 1.4) to make the required thin
films of electrolyte.
In the late 1970s, electrochemical vapour deposition began to be used to
make tubular cells at Westinghouse [9,10]. A porous tubular substrate, around
15-20 mm in diameter, made originally from calcia-stabilised zirconia but
later from the cathode material, doped lanthanum manganite, was placed in a
low-pressure furnace chamber, and zirconium chloride plus yttrium chloride

\-b Zirconium chloride vapor

Moving polymer trlm
\
a) Support tube

extruded plastic paste

Barrel

Figure 1.4 Schematic of three electrolytefabrication processes: (a) electrochemical vapour deposition: (b)
tapecasting: (c) extrusion.