Page 145 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

122 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

and x < 0.4 in Lal-,Sr,Mn03, the oxygen content exhibits two plateaus in
its oxygen potential dependence; one is around the oxygen-excess (3 + d) at high
oxygen partial pressures (Region I in Figure 5.2a), the other being the
stoichiometric one (around 3, Region 111 in Figure 5.2a) at the lower oxygen
partial pressures. Particular interest has focused on the oxygen-excess region
because the perovskite lattice does not allow interstitial oxygen and therefore it
is a challenge in defect chemistry to explain the ‘oxygen excess’ region.
Many attempts have been made to take into account the thermogravimetric
results by considering the formation of metal vacancies in the A- and the B-sites.
Sometimes, the ‘oxygen excess’ can be discussed together with the A-site
deficiency in the structure, because there is an expectation that the metal
vacancies can be easily formed on the A sites compared with the B sites. The
validity of defect models is usually checked by examining the reproducibility of
thermogravimetric results. Here, however, care must be taken; good
reproducibility does not necessarily mean that the adopted defect mode1 is
appropriate. For example, RoosmaIen and Cordfunke [ 1 7-20] tried to interpret
the defect structure of the oxygen-excess composition by the following
assumptions: [Mnh,] (=[Mn4’]) is constant independent of the Sr content, and
oxygen nonstoichiometry and oxygen incorporation in the oxygen-excess region
involves the oxidation of Mn,,(= &In2+) to Mn&,(=Mn3+):
3
-02 + 6MnM, - LaMnOs + vLa + V;,, + 6Mn&,
2

Results of their calculations can reproduce the thermogravimetric results
fairly well. However, their assumption that [MI&,] (=[Mn4+]) is constant may
not be accurate. Their assumption implies that with decreasing oxygen
potential, the concentration of Mn3+ becomes lower than that of Mn4+. This does
not happen in a system in which equilibration proceeds reasonably. This is
therefore due to their inappropriate consideration of possible reactions among
defects. Recently, rapid progress has been made in computer calculations so that
complicated chemical equilibria calculations can be made without difficulty. In
fact, Yokokawa et al. [25] made an attempt to explain the oxygen
nonstoichiometry of Lal-,Mn03-d with different lanthanum deficits as a
function of oxygen potential and succeeded in reproducing those
thermogravimetric results by a single model. The concentrations of the
respective manganese ions derived from this model show a reasonable oxygen
potential dependence as expected in accordance with normal chemical
understanding. Recently, Mizusaki et al. [15] have proposed a defect clustering
model to explain the fact that the oxygen excess region disappears for x > 0.4 in
Lal-,SrXMn03+d. In addition to metal vacancies, they propose a ‘vacancy
excluding space’ that is needed for each cation vacancy to exist stably without
having the vacancy in its neighbouring space: for the metal vacancy (V:a), this