Page 42 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Chapter 2

History

Hans-Heinrich Mobius

2.1 The Path to the First Solid Electrolyte Cas Cells
Starting in 1800, Davy carried out many investigations into the electrolysis of
water and aqueous solutions. Experiments using more and more concentrated
solutions of alkali hydroxides led to melting flux electrolysis and in 1807 to the
discovery of alkali metals [l]. Davy observed that dry solid alkali compounds
were non-conductors but became electrically conducting through just a little
moisture. For Faraday it seemed important that many electrically conducting
liquids lost their conductivity during solidification [2]. In his continuing
investigations, Faraday introduced the basic terminology of electrochemistry,
and with the aid of many results concerning the concept ‘electrolyte’ in 1834 he
classified substances into first and second types of conductors, metallic and
electrolytic [3]: the first type now recognised as electronic and the second type as
ionic conductors.
Faraday encountered problems with the classification of silver sulphide, which
exhibited conductivities comparable to metals in the high-temperature range,
but, in contrast to metals, lost its conductivity upon cooling [2]. Hittorf (185i)
devoted himself to this special problem and proved that AgzS is electrolytically
decomposable [4]. The generation of a counter voltage (polarisation by chemical
precipitation) during the passage of a current was recognised as a characteristic
feature of electrolytic conductivity of solids [4,5], and this led to the discovery of
an increasing number of solid conductors of the second type (ionic).
As early as 1774, Cavendish [6] had observed an increase in the conductivity
of glass on heating. The electrolytic nature of this conduction was discovered by
Beetz [7] and Buff [8] in 18.54. Using mercury, zinc amalgam, various solid
metals, carbon, and pyrolusite (MnOz) as electrodes, Buff demonstrated galvanic
cells and batteries free of water ‘in which glass takes over the role of the moist
conductor’, and he investigated the associated voltage and polarisabilfty.
A short period before, Gaugain [9] and Bequerel [lo] had published
experiments on the thermoelectricity between metal contacts on glass and