Page 266 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Electrode Polarisations 243

9.4.7 Cathodic Activation Polarisation
Much of the early work on SOFC cathodes has been on Sr-doped LaMn03 (LSM),
which is a predominantly electronic conductor. In the case of electrolyte-
supported or anode-supported cells, powder of LSM is spread (screen-printed)
over the electrolyte (YSZ) surface, and fired at elevated temperatures to bond the
cathode onto the electrolyte. In most practical applications, the LSM used is of a
typical composition Lal..,Sr,MnO3, with x = 0.15 ... 0.25. These materials
exhibit a diffusion coefficient of oxygen D on the order of cm2/s and a
surface exchange coefficient k,,, of about cm/s at 1000°C in air [28]. In
such a case, the overall cathodic reaction occurs at the LSM-YSZ-gas phase TPB.
The effective TPB length that can be realised is generally <20.000 cm-l,
(equivalent to 50% surface coverage of 1 micron LSM particle size), and typically
< 5,000 cm-l, with the result that at temperatures below about 900°C. the
cathodic activation polarisation is usually large. which limits cell performance.
This limitation has now been well recognised, and the SOFC research and
development community has all but abandoned this approach and has shifted
focus to porous, MIEC electrodes, either single phase or composite.
The concept of porous, effectively MIEC electrodes, is not new [29]. It has been
extensively studied in aqueous electrochemistry. In aqueous electrochemistry, if
a porous electrode is used, the electrolyte fills the pores of the electrode. If the
rate-limiting step in the electrode reaction is the overall rate of charge transfer,
then increasing the electrolyte/electrode surface area should improve the rate.
Over the thickness of the porous electrode, transport of electrical charges occurs
in two phases - electronic through the matrix phase, and ionic through the
solution (electrolyte) phase. That is. over the porous electrode, transport is by
mixed ionic and electronic conduction (MIEC). In addition, convective or
diffusive transport of neutral reactive species occurs through the liquid
electrolyte in the pores. In this manner, the electrode reaction is spread out into
the porous part of the electrode. The theory of porous electrodes in aqueous
electrochemistry has been developed on this premise. In such a case, the
transport of ionic and neutral species occurs through the electrolyte filling up the
pores, and electron transport occurs through the solid part of the porous
electrode. In solid state electrochemistry also, an analogous porous electrode
should be capable of transporting both ions and electrons: that is, it must be a
mixed ionic electronic conducting (MIEC) material. In solid state
electrochemistry, in an analogous electrode, ion and electron transport occurs
through the solid part of the MIEC electrode, and neutral (gaseous) species
transport through the porous interstices [18,2 7,301.
Figure 9.4 shows a schematic of a porous MIEC electrode used in solid state
electrochemistry, in which the pathways for the various species are shown. The
MIEC characteristics can be realised in two ways: (1) Use of a single phase,
porous MIEC material, such as Sr-doped LaCo03 (LSC), or (2) Use of a composite,
two-phase porous mixture of an electronic conductor (e.g. LSM) and an ionic
conductor (e.g. YSZ). In the case of a composite, two-phase mixture, the MIEC
properties are realised globally (at the microstructural level, not at the atomistic