The Effect Of Transition And Non-transition Metal Oxides On Oxygen Reduction Reaction At Cathodes For IT-SOFCs

Solid oxide fuel cells(SOFCs)are considered to be one of the most promising technologies for future energy conversion.They have the advantage of being operable with a wide range of fuels including HZ and hydrocarbon fuels.However,their high cost of materials and fabrication procedure limits their usage.In order to reduce their cost,the SOFC operating temperature must be reduced to below 800?,thus allowing the use of inexpensive materials and mediating a quick start-up procedure,and extending the SOFC durability.The lower SOFC operating temperature,however,shall significantly reduce the electrochemical performance of the SOFC by increasing the ohmic resistance of electrolyte and the polarization resistances of electrodes.The ohmic resistance can be reduced by decreasing the thickness of the electrolyte or by utilizing an electrolyte that has a high ionic conductivity.The polarization resistance can be reduced by the improvisation and use of novel microstructured electrodes or by the surface modification of the existing electrodes.In this work,transition and non-transition metal oxides have been applied to modify the well-known cathodes,lanthanum strontium cobalt ferrite(LSCF)and lanthanum strontium ferrite(LSF),in order to reduce their polarization resistance and,hence,enhance their electrochemical performance in intermediate temperature SOFCs.In the first chapter,the solid oxide fuel cell has been introduced with a brief description of its working principle and components.In addition,the designs for SOFCs have also been discussed.In the second chapter,nickel oxide(NiO)has been investigated as a possible synergistic catalyst for the oxygen reduction reaction(ORR)on the LSCF surface.The effect of NiO particles on the effective oxygen chemical surface exchange coefficient has been revealed with the electrical conductivity relaxation(ECR)technique.At 800?,the coefficient increases from 3.48 x l0-5 to 6.65 x 10-5 cm s-1 and 6.9x 10-4 cm s-1 when NiO particles are deposited using the sputter and drop coating methods,respectively.Adding 5 wt.%NiO to LSCF reduces the area specific interfacial polarization resistance(ASR),for example,from 0.108 to 0.082 ?cm2 at 700 ?,as demonstrated by impedance spectroscopy on symmetric cells using samaria-doped ceria as the electrolyte.Adding NiO to the LSCF cathode can also improve the performance of anode-supported button cells,increasing the peak power density from 0.731 to 1.031 W cm-2 at 800?.In the third chapter,the effect of the PbO present in the Ag current collector was investigated on the ORR catalytic activity of the LSCF and LSF cathodes.The solution deposition of PbO particles on the bar surfaces of LSCF and LSF significantly boosts their ORR kinetics by improving the oxygen surface exchange process.Upon PbO addition,the chemical oxygen surface exchange coefficients of LSCF and LSF increase by a factor of?15 and?6,respectively.When 5.88 and 5.67 wt.%PbO are introduced into the porous backbones of LSCF and LSF electrodes,at 650?,the ASR decreases from 0.257 to 0.141Q ?cm2 and from 1.33 to 0.373?cm2,respectively.In the fourth chapter,the effect of B2O3,A12O3,Ga2O3,and In2O3 addition on the ORR catalytic activity of the LSCF cathodes has been investigated.Amongst these oxides,only In2O3 increases the electrochemical performance of cells with the LSCF,as well as LSF,electrodes.When 13.73 and 7.88 wt.%In2O3 is impregnated into the porous backbones of LSCF and LSF electrodes,at 650?,the ASR decreases from 0.41 to 0.27?cm2 and from 0.999 to 0.399 ?cm2,respectively.The solution deposition of In2O3 particles on to the bar surfaces of LSCF and LSF significantly boosts their ORR kinetics by improving the oxygen surface exchange process.Upon In2O3 addition,the chemical oxygen surface exchange coefficients of LSCF and LSF increase by a factor of?6 and?4,respectively.