Unquestionably, one of the grand challenge for modernsociety is energy.
The demand for energy is increasing exponentially with thecontinued growth of world population, economy and living standards 1,2. Emissionof greenhouse gases (COx, NOx and SOx) from fossilfuels are intimidating to the modern society in terms of energy crisis, globalwarming, environmental pollution 3.Hence, sustainable energy is needed to address the above problems. Solidoxide fuel cells (SOFCs) are electrochemical devices which convert the chemicalenergy of fuel directly into electrical energy without combustion. SOFCs areconsidered as alternative to the conventional electric power systems due to itshigh energy conversion efficiency, wide fuel options (hydrocarbons) and lowpollutant emissions. However, the widespread commercialization of SOFCs arestill hindered due to its high operating temperature (HT,1073-1273K) 4. This HT leads to the high temperature oxidation, corrosion, phasetransition of a component materials and thermal expansion mismatch betweenvarious SOFC components 5,6.
Hence, researcher now looking for thedevelopment of intermediate temperature (IT, 873-1073K) SOFCs for commercialization. However, with decreasing the operatingtemperature, internal resistance of the cell increases tremendously whichdecreases the performance of cell. Therefore, how to decrease the internalresistance is the challenging for SOFC researchers.
Numerous factors lead tothe SOFC internal resistance: first and foremost is the large resistance of thecurrent electrolyte materials at IT. Next, the polarization resistances ofelectrodes (especially cathode) are magnified with the decrease of temperature.There are two possible ways to address these issues: the dimensional thicknessof the electrolyte can be reduced to decrease the area specific resistance ofthe fuel cell and/or developing an electrolyte material having improved ionicconductivity 7-9.SOFC consist of three major components: Porous cathode and anode separated by asolid 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 underelectrical load to the anode, where they react with H+ produced byhydrogen 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 componentsare listed in Table 1 10-12. Solidelectrolyte is the heart of SOFC through which the oxide ions move from cathodeto anode, where it reacts with fuel to form water and heat. The oxide ionconduction occurs via oxygen vacancy hopping mechanism, which is thermallyactivated process. An ideal SOFC electrolyte should have the followingcharacteristics 13-16:i) Thematerial must have appreciable oxygen ionic conductivity (~ 0.
1 S/cm) in theoperating temperature regime.ii) Thetransport number for oxygen ion conductivity must be close to unity i.e., itshould have negligible electronic conductivity. iii) The materials must be stable over a widerange of oxygen partial pressure.iv) Electrolyte must have good mechanicalstrength and good thermal shock resistancev) Compatibility with electrodes andinterconnect materials.vi) Chemically inert to the fuel cell gases.
vii) Below cost and environmentally benign. Examplesfor SOFCs electrolytes are: Fluorite structure-stabilized zirconia, doped ceria(rare earth or alkaline earth metal), d-Bi2O3, perovskitestructure-strontium/ magnesium doped lanthanum gallate (LSGM), Bi4V2O11and La2Mo2O9 based derivatives andpyrochlores. Variation of ionic conductivity of somesolid electrolytes with temperature is shown in Figure.
2 5. Atpresent, zirconia based i.e., yttria-stabilized ZrO2 (YSZ) electrolyteswidely used as electrolyte for commercial SOFCs due to its high ionicconductivity, stability in both oxidizing and reducing environment andcompatibility with electrode materials 17.
However, the ionic conductivity ofYSZ at IT is lower than that of lanthanum gallate and ceria based electrolytes.At HT, YSZ causes a thermal degradation, thermal expansion mismatch, interfacialreaction between the electrodes and electrolyte 18 and extensive growth of grainsizes after calcination at HT as shown in Figure.3 19. Also, the problem with LSGM based electrolyte includes possiblereduction and volatilization of gallium oxide, difficulty in the formation ofsingle phase structures, relatively high cost of gallium, significantreactivity with perovskite electrodes (under oxidizing conditions) as well aswith metal anodes in reducing conditions 20,21. Hence, to overcome from theproblems associated with YSZ and LSGM, in the present review we focused on theceria based electrolytes especially on transport properties.
The mainadvantages of ceria based oxygen ion conductors include a higher ionconductivity with respect to stabilized ZrO2 (particularly at lowtemperature) and a lower cost in comparison with LSGM and its derivatives 22.