Unquestionably, power systems due to its high energy conversion

Unquestionably, one of the grand challenge for modern
society is energy. The demand for energy is increasing exponentially with the
continued growth of world population, economy and living standards 1,2. Emission
of greenhouse gases (COx, NOx and SOx) from fossil
fuels are intimidating to the modern society in terms of energy crisis, global
warming, environmental pollution 3.
Hence, sustainable energy is needed to address the above problems.

            Solid
oxide fuel cells (SOFCs) are electrochemical devices which convert the chemical
energy of fuel directly into electrical energy without combustion. SOFCs are
considered as alternative to the conventional electric power systems due to its
high energy conversion efficiency, wide fuel options (hydrocarbons) and low
pollutant emissions. However, the widespread commercialization of SOFCs are
still hindered due to its high operating temperature (HT,
1073-1273
K) 4. This HT leads to the high temperature oxidation, corrosion, phase
transition of a component materials and thermal expansion mismatch between
various SOFC components 5,6. Hence, researcher now looking for the
development of intermediate temperature (IT, 873-1073
K) SOFCs for commercialization. However, with decreasing the operating
temperature, internal resistance of the cell increases tremendously which
decreases the performance of cell. Therefore, how to decrease the internal
resistance is the challenging for SOFC researchers. Numerous factors lead to
the SOFC internal resistance: first and foremost is the large resistance of the
current electrolyte materials at IT. Next, the polarization resistances of
electrodes (especially cathode) are magnified with the decrease of temperature.
There are two possible ways to address these issues: the dimensional thickness
of the electrolyte can be reduced to decrease the area specific resistance of
the fuel cell and/or developing an electrolyte material having improved ionic
conductivity 7-9.
SOFC consist of three major components: Porous cathode and anode separated by a
solid oxygen ion (O2?) conducting electrolyte. At the cathode,
oxidant normally oxygen from the air is supplied, which gets reduced to O2?.
Then the O2? are transported through the solid electrolyte under
electrical load to the anode, where they react with H+ produced by
hydrogen fuel to form water. Thus, the final products of SOFC are electricity,
heat and water. The schematic diagram of SOFC is shown in the Figure. 1 5. Materials for SOFC components
are listed in Table 1 10-12.

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            Solid
electrolyte is the heart of SOFC through which the oxide ions move from cathode
to anode, where it reacts with fuel to form water and heat. The oxide ion
conduction occurs via oxygen vacancy hopping mechanism, which is thermally
activated process. An ideal SOFC electrolyte should have the following
characteristics 13-16:

i)       The
material must have appreciable oxygen ionic conductivity (~ 0.1 S/cm) in the
operating temperature regime.

ii)     The
transport number for oxygen ion conductivity must be close to unity i.e., it
should have negligible electronic conductivity.

iii)  
The materials must be stable over a wide
range of oxygen partial pressure.

iv)  
Electrolyte must have good mechanical
strength and good thermal shock resistance

v)     
Compatibility with electrodes and
interconnect materials.

vi)  
Chemically inert to the fuel cell gases.

vii) Be
low cost and environmentally benign.

 

            Examples
for SOFCs electrolytes are: Fluorite structure-stabilized zirconia, doped ceria
(rare earth or alkaline earth metal), d-Bi2O3, perovskite
structure-strontium/ magnesium doped lanthanum gallate (LSGM), Bi4V2O11
and La2Mo2O9 based derivatives and
pyrochlores. Variation of ionic conductivity of some
solid electrolytes with temperature is shown in Figure. 2 5.

            At
present, zirconia based i.e., yttria-stabilized ZrO2 (YSZ) electrolytes
widely used as electrolyte for commercial SOFCs due to its high ionic
conductivity, stability in both oxidizing and reducing environment and
compatibility with electrode materials 17. However, the ionic conductivity of
YSZ at IT is lower than that of lanthanum gallate and ceria based electrolytes.
At HT, YSZ causes a thermal degradation, thermal expansion mismatch, interfacial
reaction between the electrodes and electrolyte 18 and extensive growth of grain
sizes after calcination at HT as shown in Figure.
3 19. Also, the problem with LSGM based electrolyte includes possible
reduction and volatilization of gallium oxide, difficulty in the formation of
single phase structures, relatively high cost of gallium, significant
reactivity with perovskite electrodes (under oxidizing conditions) as well as
with metal anodes in reducing conditions 20,21. Hence, to overcome from the
problems associated with YSZ and LSGM, in the present review we focused on the
ceria based electrolytes especially on transport properties. The main
advantages of ceria based oxygen ion conductors include a higher ion
conductivity with respect to stabilized ZrO2 (particularly at low
temperature) and a lower cost in comparison with LSGM and its derivatives 22.