Pure the oxygen ion conductivity. The defect reactions

Pure CeO2 is basically poor oxygen ionic
conductor (? ~ 10?5 S cm?1 at 873 K) having a fluorite
structure with space group Fm3m over the temperature range from room
temperature to its melting point and oxygen vacancies (

) are the predominant ionic defects 23. Some of the
physical properties of pure CeO2 are listed in Table 2. 24-28.
Rare earth (RE) or alkaline earth metal (AE) ion doped ceria are the common
electrolyte materials used for IT-SOFCs due to their high ionic conductivity
and low polarization resistance as well as low cost when compared to YSZ and LSGM
29.

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            Partial
substitution of tetravalent cerium by trivalent RE (RE = Y3+, Gd3+,
Sm3+, Nd3+, La3+)/ divalent AE (AE = Sr2+,
Ca2+) metal ion dopants results in the formation of

 and thus increases the
oxygen ion conductivity. The
defect reactions can be written in Kröeger-Vink notation as:

                       

                                         

                       

                                                    

The
symbols here denote the usual meanings of the Kröger-Vink notations i.e., each
pair of RE3+/ AE2+ cations create one

defect for the charge compensation.
The

/

and

 point defects tend to cluster due to static
electrical attraction and the tendency increases at higher doping level,
leading to the formation of larger (RE¢-

)/ (AE¢¢-

) pairs of defect aggregation 30-32. Ionic conductivity of ceria can be
increased by increasing the dopant concentration. However, at higher dopant
levels defect association occurs between cation and vacancies. Once an associate
is formed, the oxygen vacancy is not free to participate in the conduction
process and thus conductivity is lowered 33. The crystal structure of doped
ceria is shown in Figure. 4.

Design of the ceria based electrolytes for IT-SOFCs
mainly depends on dopant ion, dopant concentration, oxygen vacancy
concentration, defect association energy and local defect structure. All of
these strongly influence on the homogeneity, stability and electrical
conductivity of the solid solutions 34. As shown in Figure. 5, ionic conductivity of RE/ AE metal ion doped ceria at
1273 K increases with the increase in dopant size (even beyond the ionic radius
of Ce4+, 0.97 Å) reaches a maximum value at Sm3+ (1.08 Å)
and then decrease. Hence, the ionic conductivity of doped ceria electrolyte was
influenced not only by the ionic radius and valence of dopant cations, but also
on concentration of the dopant. The conduction in ceria based electrolyte
materials occurs via oxygen vacancy diffusion mechanism, thus the ionic
conductivity is observed to increase with increase of oxygen vacancies i.e.,
increase of doping level. In CeO2, trivalent dopants exhibit much
higher conductivity than the divalent dopants (Figure. 5) due to the lower ionic size mismatch between the
trivalent dopants and host Ce4+ ions. In doped CeO2, the
conductivity increases as concentration of dopant increases and reaches a
maximum at a certain doping level and then decrease, which is due to the low
mobility of defect associates. These defect associate formation increases with
increasing the ionic radii mismatch between the dopant cations and host cation
35,36.

Kim 40 introduced the concept of critical ionic
radius (rc) to explain the ionic conductivity of doped ceria.
Accordingly, the dopant with ionic radius equal to rc will not
change the lattice parameter of fluorite structure i.e., expansion due to
bigger trivalent ionic radius and contraction due to vacancy creation will be
counterbalanced. The critical ionic radius for divalent and trivalent dopant
cations in CeO2 was calculated to be 1.106 Å and 1.038 Å,
respectively. As the ionic radius of Gd3+ (1.05 Å) and Sm3+
(1.08 Å) are close to rc, the gadolinium doped ceria (GDC) and
samarium doped ceria (SDC) exhibits the highest ionic conductivity. But the
ionic conductivity of yttria doped ceria (YDC) was found to be lower as
compared to SDC and GDC, even though the ionic radius of Y3+ (1.03
Å) is closer to rc. This indicates that the structure-conductivity
relationship based on rc is not sufficient to explain the ionic
conductivity behavior in doped ceria 41.

Mori et al., 42,43 originally introduced
the term “effective index, Ei”
to explain the composition which exhibits the high ionic conductivity in doped
ceria taking into the account of the ionic radii mismatch between the dopant
cation and host cation and the amount of oxygen vacancies that are expected to
produce by the dopant substitution in an idealized crystallographic site. The Ei is given by the formula

where,
avg.rc, rd and rh are the average ionic radius of the cation, dopant
and host ion (Ce4+) respectively. The eff ro is the effective oxygen ion radius, which is
given by the formula

where,
1.4 Å is the ionic radius of O2- in oxides and ? is the level of oxygen vacancies in CeO2 based oxides.
As per the Eq. (3), the doped cerium oxide will have an idealized non-distorted
fluorite structure when the value of Ei
becomes unity or close to unity.

            The
total ionic conductivity of ceria based electrolytes depends on the
contribution from both grain and grain boundary (GB). The oxide ion
conductivity in crystallographically mismatched GBs depends on many parameters
44 viz., sample preparation, extent of lattice mismatch, space charge layer,
dopants segregating at GB, amount of SiO2 contamination and
microcracks. In fact, all of these vary from one source to another.
Furthermore, the positive potential of GB core renders the depletion of oxygen
vacancy near the GB region, which makes the GBs highly resistive in the
fluorite structured oxides. Therefore, one of the major problems associated
with the application of RE/ AE ions doped ceria as an electrolyte in SOFC is
the ceramic GB resistance, which is usually larger than or in the same range as
the bulk resistance, although the GB areas are expected to be thin. This
implies that GBs blocks the ionic charge transport and they indeed often act as
Schottky barriers 45. This is an extra contribution to the total resistance
exhibited by the electrolyte. Hence, the reduction of the GB resistance is of
uppermost importance. This blocking phenomenon can lead to contradictions in
the measurement of total conductivity.

            Several
attempts have been made to reduce the GB resistance. Among them, preparation
method of solid solution can influence both electrical properties and also
structural /micro structural properties. The precursors synthesized by chemical
methods (sol-gel, hydrothermal, combustion and co-precipitation) are
advantageous over the ceramic methods because of high purity, homogeneity,
lower sintering temperature/ time to produce ultrafine structures.

            Silica (a commonly present impurity
in most of the ceramics) has a detrimental effect on ionic conductivity 46.
Several researchers report that, increase in GB resistivity is due to the
presence of SiO2, which either segregates along the GB region or if
present in large amount forms a secondary phase of poor conductivity 47. To
overcome the problem of SiO2 in CeO2 based electrolytes,
a small amount of additives such as Al2O3 48, Fe2O3
49, TiO2 50, CaO 51 and CoO 52 are added so that they have a
scavenging effect on silica impurities and increase the conductivity. This is
done by gathering silica containing phase into a discrete configuration and
dewetting of inter granular phase by crystallization 53.

            During
the high temperature sintering process, the dopant cations diffuse from the
grain interiors and segregate near the GBs. The presence of such impurity
centers and depletion of oxygen vacancy near the GB region enhance the
resistance offered by GBs to the oxide ion conduction across the GBs 54. Much
work has been devoted on limiting the impurities segregation at GBs and thus
enhancing the ionic conductivity. It has been shown that by adding certain
additives, it is possible to lower the detrimental effect of impurity
segregation at GBs on the conductivity. Sintering temperature can be reduced
from 1573 K to 1173 K without compromising the sample density (³ 94% theoretical density) by the addition
of Mn2O3, Fe2O3, Bi2O3,
Cr2O3, CoO, CuO and NiO 55-62.
Such additives must be carefully chosen in such a way that it has negligible
effect on both ionic and electronic performance. Kleinlogel et al., 55
reported that the addition of 1 mol% of CoO on GDC lowers the sintering
temperature from 1523 K to 1173 K and increases the densification rate. Figure. 6 shows the scavenging effect
of transition metal oxides in doped CeO2.

The additive effect of various RE and
AE ions as dopants in ceria on the ionic conductivity has been studied by many
investigators 37-39,64,65.
The ionic conductivities of ceria doped with 10 mol% RE and AE ions at 1073 K
are plotted against the radius of the dopant as in       Figure.
5. The doping of Gd3+/ Sm3+ ion among the RE metal
ions and Ca2+ ion among the AE metal ions with an ionic radius of
about 1.10 Å gives the maximum ionic conductivity. This is due to the similar
ionic radius as the host ion resulting in the minimum association enthalpy
between dopant ion and oxygen vacancy. The ionic conductivities of ceria doped
with Mg2+ and Ba2+ ions are exceptionally low (Figure. 5), which may be ascribed to
the insufficient solubility of these ions in ceria.

The ionic conductivity of most widely
used ceria based electrolyte i.e., GDC at different temperature is shown in Figure. 7 66-68.
In GDC conductivity increases as concentration of Gd3+ increases and
reaches a maximum at 20 mol% and then decrease, which is due to the low
mobility of defect associates at higher concentration. Unlike gadolinium and
samarium, other RE metal ion dopants viz., lanthanum 69, yttrium 70,
neodymium 71, praseodymium 72 and dysprosium 73 for ceria decreases the
ionic conductivities. Similarly, among the AE metal ion dopants, the highest
conductivity occurs for calcium doped ceria (CDC) 74 but not for other ions
viz., strontium 75, magnesium 76 and barium 36.

The main disadvantage in using doped ceria as SOFC
electrolyte arise from the partial reduction of Ce4+ to Ce3+
at HT under reducing condition leading to:

(i)                
n-type electronic conductivity that lowers
the open circuit potential due to internal electronic short circuit in a cell,

(ii)             
nonstoichiometry related expansion of the
lattice resulting mechanical stress and failure.

Above problems can
be solved by combination of doped ceria with other solid electrolytes such as
YSZ or LaGaO3 in multilayer cells. But the performance of multilayer
cells is relatively poor with low conductivity due to the formation of reaction
products at the solid/ electrolyte interface as well as difference in thermal
expansion of the electrolytes resulting in micro cracks 77-79.
Even though the reduction of Ce4+ to Ce3+ in doped CeO2
is a critical issue, but the thermal expansion coefficient matches with that of
stainless steel and hence suits well for metal supported IT-SOFCs in the real
system applications. Also, doped CeO2 is somewhat chemically inert
towards the electrode materials and renders high power density (~
400 mW/ cm2) at IT.

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