Page 43 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

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

porcelain. Buff [8] reproduced the results, which turned out very differently
depending on the position of the contacts in oxidising or reducing regions of
flames, and he interpreted the voltages as a mixture of thermoelectric forces and
voltages which he had previously observed between bare platinum wires in
flames [ll]. However, Gaugain investigated his experiment, which at first was
constructed of two tubes of glass, platinum wires, air and alcohol vapour, in
more detail [12]. He observed the delivery of current, the polarity of the
electrodes and their behaviour when the electrode metals or the gas supply were
changed. He also noted the large voltage alteration when different gases
were mixed with oxygen beyond a certain proportion (known today as the jump
at the stoichiometric point), and phenomena associated with an iron/air cell
which convinced him of the decisive role of oxygen in the electrode reaction.
Although restricted by the lack of sensitivity of the available measuring device
(a leaf electroscope) so that small voltage differences could not be detected,
Gaugain nevertheless found that ‘the new source of electricity possesses a11 the
characteristic features of an aqueous-electric cell’, and thus he discovered in
18 53 galvanic solid electrolyte gas cells.
Towards the end of the nineteenth century the term ‘solid electrolyte’ was in use,
and many facts were known about the behaviour of these materials. The Science of
Electricity by Wiedemann (1893-98) includes the chapters ‘Conductivity of
Solid Salts’ and ‘Determination of the Electromotive Force -Two Metals and Solid
Electrolytes’ and ‘Electrolysis of Solid Electrolytes’ [ 131. However, in Ostwald’s
textbook on general chemistry, solid electrolytes are not mentioned [14].
Technological interest in solid ion conductors first arose in connection with
the development of electric lighting devices. Early carbon filament lamps
manufactured since about 1880 could not compete with the existing gas
incandescent light. In 1897, Nernst suggested in a patent [15] that a solid
electrolyte in the form of a thin rod could be made electrically conducting by
means of an auxiliary heating appliance and then kept glowing by the passage of
an electric current. At first Nernst mentioned only ‘lime, magnesia, and those
sorts of substances’ as appropriate conductors. Later investigations stimulated
by experiences with gas mantles led to his observation ‘that the conductivity of
pure oxides rises very slowly with temperature and remains relatively low,
whereas mixtures possess an enormously much greater conductivity, a result in
complete agreement with the known behaviour of liquid electrolytes’ [16]. He
pointed out that, for example, the conductivity of pure water and pure common
salt is low but that of an aqueous salt solution is high. In a short time many of
the mixed oxides which exhibit high conductivity at elevated temperatures,
including the particularly favourable composition 85% zirconia and 15%
yttria [17], the so-called Nernst mass [18,19], were identified. The thesis of
Reynolds [20], inspired by Nernst and presented in 1902, expanded this field by
measuring the conductivity in the range 800-1400°C of numerous binary and
ternary systems, among others, formed by ZrOz with the oxides of the elements
La, Ce, Nd, Sm, Ho, Er, Yb, Y, Sc, Mg, Ca, Th and U, including investigations on
the role of composition, concentration, direction of temperature alteration
(hysteresis) and other phenomena.