Page 296 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Testing of EZectrodes, Cells and Short Stacks 273

temperature and apparent activation energy. Furthermore, using almost dry
hydrogen, as is the common practice, it is not easy to conduct experiments with a
real negligible fuel utilisation, since even small current densities will create
enough water to change significantIy the Emf of the hydrogen/water fuel gas
versus air, e.g. if the inlet gas contains 0.1% H20 and the fuel utilisation is 0.1%,
this changes the HzO/Hz ratio by a factor of 2, which in turn changes the Emf by
34 mV at 850°C. Therefore, and in order to be able to compare results for
different fuel utilisations, the ASR value should be corrected for the effect of fuel
utilisation. Before describing how this may be done. various contributions to the
total ASR are examined below.
ASR may be divided into ohmic resistance, R,, and electrode polarisation
resistance, Rp. The ohmic resistance originates from the electrolyte, the
electrodes materials and the current collection arrangement. This is very
much dependent on geometric factors such as thickness of the cell components
and the detailed geometry of the contact between current collection and
electrodes, and between electrodes and electrolyte as current constrictions may
be important [4 11. The electrode polarisation resistance is further divided into
contributions from the various rate-limiting steps. Thus, ASR can be broken
dom7n in five terms:

where is the electrolyte resistance calculated from the measured specific
conductivity and the thickness: RcOnnect = R, - ReJyt is the resistance due to non-
optimised contact and current collection; RpVelchem is the electrode polarisation
originating from all the limiting chemical and electrochemical processes on the
electrode surfaces, in the bulk electrode material and on the electrolyte/electrode
interfaces; Rp,dm is the contribution from the gas phase diffusion; and Rp,,n,, is
the contribution due to gas conversion, i.e. fuel oxidation and oxygen reduction.
This division of ASR is based on what is possible to measure and calculate
reliably rather than on any physical or electrochemical basis. Some terms in
Eq. (2) can therefore be thought of as ‘equivalent resistances’, e.g. the Emf drop
due to changes in gas composition resulting from the fuel utilisation is translated
to an equivalent resistance. Depending on the exact type of electrode, different
types of contributions are possible as derived from more basic electrochemical
point of view. For example, current constriction may be important if the
electrode has coarse porous structure but of less or no importance in case of a fine
structured electrode. In one type of cathode, the surface diffusion may be
important, but in another the diffusion of oxide ions (and electrons) through the
electrode particles may cause the main polarisation loss. Values for ‘the Eq. (2)’
ASR contributions for an anode-supported cell (short stack) with a 1 mm thick
support fed with hydrogen (with 3% steam) are given in Table 10.1 for 5 and
85% fuel utilisations (FU).
It is seen that the contribution from the concentration polarisation, Rp,dif +
Rp.conver is dominating. In an electrode-supported cell, the limitation of gas
diffusion through the support is a cell-relevant resistance, whereas Rp,conver