Page 261 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

2 3 8 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

proposed reaction mechanisms for oxygen reduction, it is virtually impossible to
select any one over another as definitive [9-201. This is also due to the fact that
the reaction mechanisms are surely material and microstructure-dependent.
Although it is generally difficult to isolate a single rate-determining step by
virtue of the presence of a number of series steps, it is usuaIly possible to describe
the overall process in a phenomenological framework. The following describes
the plausible reaction steps for one of the possible reaction schemes; it is to be
emphasised that the following is only a generic sequence of steps and by no
means one that has been demonstrated with any degree of certainty.
Surface adsorption3 of oxygen molecules on the electronic conductor,
which also is the electrocatalyst,
1 1
02 (gas) + 02nds (electrocatalyst)

Dissociation of adsorbed oxygen molecules into adsorbed atoms,
1
- OZnds (electrocatalyst) + Oaas (electrocatalyst)
2

Surface diffusion of adsorbed oxygen atoms to a three phase boundary
(TPB) between the electrocatalyst (e.g. LSM) - electrolyte (e.g. YSZ) -
gas phase,
O,ns(electrocatalyst) --+ O,~s(electrocalayst/electrolyte TPB)

Formation of oxide ions by electron transfer with incorporation of these
ions into the electrolyte
Onds(electrocatalyst/electrolyte TPB) + 2e’(electrocatalyst)
+ (oxygen vacancy/electrolyte) + Oz(e1ectrolyte)

where Kroger-Vink notation has been assumed. Central to the above
scheme is the occurrence of the charge transfer reaction at or near a TPB;
in-situ l80 exchange experiments under cathodic polarisation and
subsequent SIMS-analysis, have confirmed the occurrence of the charge
transfer reaction at a TPB [19]. Several variations of the above scheme
are possible; such as, for example, the occurrence of the charge transfer
reaction on the electrocatalyst surface to form an oxide ion, followed by
surface diffusion of the oxide ion to a TPB, and its incorporation into the
solid electrolyte at the TPB, or further surface diffusion of the oxide ion on
the electrolyte surface, and its incorporation into the electrolyte at a

Surface adsorption of oxygen molecules may also occur on the surface of the electrolyte, YSZ,
foltowed by its dissociation into oxygen atoms and their surface diffusion to aTPB.