Page 280 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Electrode Polarisations 2 5 7

9.6 Summary

Electrode polarisations cause large voltage losses in SOFCs and need to be
reduced to low levels for increased efficiency. The three polarisations described in
this chapter are ohmic polarisation, concentration polarisation, and activation
polarisation. The ohmic contribution stems from resistance to electron and ion
flows in the materials, and is generally dominated by the electrolyte resistance,
with the consequence that SOFCs employing thick (>lo0 micron) YSZ as
electrolyte have high ohmic losses at temperatures below about 900°C. Now that
thinner electrolytes are being used in electrode-supported cells, this resistance
has dropped and it is possible to use YSZ down to about 700°C.
Concentration polarisation is caused by the resistance to mass transport
through the electrodes and interfaces and is generally largest at the cathode,
particularly when thick, cathode-supported cells are employed. The voltage drop
is large at high current densities under conditions such that the electrolyte/
electrode interface is starved of fuel (anode) or oxidant (cathode) when gaseous
species cannot diffuse fast enough through the porous interstices of electrodes. In
the case of anode, diffusion is generally rapid due to the presence of low
molecular weight hydrogen. This means anode-supported cells usually exhibit
low concentration polarisation, even with relatively thick anodes.
Activation polarisation is the voltage drop due to the sluggishness of
reactions occurring at the electrode-electrolyte interfaces. Several processes are
necessary for electron transfer to take place, especially at the cathode. Because
LSM has little ionic conductivity, these processes are localised at the TPBs.
Recently, it has become common to use MIEC (composite or single-phase)
cathodes to spread the TPB and extend the reaction zones: this has had a
beneficial effect on reducing the activation polarisation and allowed better SOFC
performance at lower temperatures.
Bulk of the studies reported to date show that with the materials that have
been researched to date, the largest contribution to polarisation is from the
cathodic reaction. The kinetics of the reduction process is governed by the
composition of materials as well as by the microstructure of the cathode.
Minimisation of the electrode polarisations is possible by choosing appropriate
materials, their compositions and morphology. From a microstructural
standpoint, activation polarisation is lower if the electrode structure is fine in the
immediate vicinity of the electrolyte. For minimising concentration polarisation,
by contrast, electrode structure should be coarse with large amount of porosity.
For this reason, an ideal electrode structure is graded, fine near the electrolyte to
minimise activation polarisation, and coarse in regions away from the
electrolyte to minimise concentration polarisation.

References

[l] D. R. Gaskell, Introduction to Thermodynamics of Materials, Taylor and
Francis, 1995.