Page 67 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

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

All SOFC concepts would benefit if the cell stacks could be operated at
intermediate temperatures of around 700-8 50°C, especially by using better
oxide conductors. At first the highly conductive mixed oxides with cerium were
totally excluded because of their electronic conductivity and mechanical
instability between oxidising and reducing gases [40]. After the results of
Takahashi in the 1960s, progress was made by theoretical treatment of the
transport processes of ions and electrons in mixed conductors with the result
that the electronic current decreases faster with decreasing load voltage than
predicted by the equivalent circuit, and doped ceria remains a viable solid
electrolyte, particularly for lower operation temperatures [138, 1391. Coatings
of doped zirconia on the surface of doped ceria and dispersions of ceria and
zirconia electrolyte particles were investigated. Recently, 500°C has been
proposed as the optimum operation temperature of cells with Ceo.9Gdo.101.95
(25 pm) electrolyte [140,141].
In 1994, new solid electrolytes with high conductivity at low temperatures
were found in the form of doped lanthanum gallates [142]. Since the self-
diffusion coefficients of cations are apparently fundamentally larger in the
perovskite-type oxides than in fluorite oxides, lanthanum gallate electrolyte and
associated electrode materials tend to react too easily at the temperatures of
fabrication and operation of cells.
At low operation temperatures, polarisation losses and the importance of
catalysis of the electrode reactions increase. At the cathode, mixed potentials can
arise when traces of combustible substances determine the electrode potential in
competition with oxygen, an effect, mentioned near the end of Section 2.3,
whose cause was recognised by Hartung in 1981 [143], today the basis of the
development of hydrocarbon sensors. For the anodes growing interest is directed
to materials which accelerate the electrochemical oxidation of CO and
hydrocarbons and is stable against fuel impurities.
Many new cell and system ideas are currently being pursued. These
include internal or in situ reforming of natural gas at the SOFC anode
(1991); the HEXIS concept with stacks of circular cells arranged between
plates of chromium alloys and without tight seals: a combination of
electricity production, heat exchange and afterburning (1 9 9 1): microtubular
cells with high thermal shock resistance suitabIe for rapid start-up (1994);
and high-efficiency hybrid SOFC/gas turbine power plants (1995) with SOFC
operating under pressure. Many of these ideas are discussed elsewhere in
this book.

References
[ 11 H. Davy, Elelrtrochemische Untersuchungen, Vorlesung am 19.1 1.1807,
Verfahrensarten, um die feuerbestandigen Alkalien zu zersetzen,
in Ostwalds Klassiker der exakten Naturwissenschaften, Nr. 45 (ed. W.
Ostwald),Leipzig, 1893, pp. 52-55.