Structure-property correlation of cerium doped bismuth vanadate for energy application

Abstract

The demand for electricity is rapidly growing and electrochemical devices such as solid oxide fuel cells (SOFCs) have shown promising developments to mitigate this energy demand. SOFCs are devices that can convert chemical fuels such as CO, H2 and ethanol directly into electrical energy more efficiently than any combustion process. Bismuth vanadate (Bi2VO5.5) is commonly doped or co-doped with transitional metals to enhance the material ionic conductivity and function as a SOFC electrolyte. The study of Bi2VO5.5 doped with rare-earth metals is sparsely reported and this dissertation addresses the current literature gap. In this work, the impact of doping Bi2VO5.5 and Bi2Cu0.1V0.9O5.35 electrolytes with different Cerium (Ce4+) concentrations, using the soft chemistry (the citrate method) and the solid state method was investigated. This study focused on understanding how Ce4+ doping/substitution into Bi2VO5.5 and Bi2Cu0.1V0.9O5.35 affected these material properties such as thermal stability, phase transition behaviour and ionic conductivity. The Bi2VO5.5 electrolyte synthesized using the citrate method (cit-BiVO) was determined to be a multi-phase material exhibiting the α- and β-phases at room temperature and undergoes reversible phase transitions from α → β → γ after thermal cycles. It was discovered using the Rietveld method that by doping cit-BiVO with different concentrations of Ce4+ using the citrate method to produce Bi2CexV1-xO5.5 – δ electrolytes or cit-BiCexVO samples, the β-phase was the most stable phase in cit-BiCexVO samples at room temperature. Additionally, the ionic conductivity increased with increasing Ce4+ doping concentrations. However, a drop in conductivity was observed for cit-BiCe0.18VO sample. It was noticed that the PXRD peaks associated with the α-phase of cit-BiVO gradually disappeared with increasing Ce4+ doping concentrations. The Raman spectra measured at room temperature showed that Ce4+ ions substituted for V5+ lattice sites in the crystal structure. Overall, it was determined that doping cit-BiVO with Ce4+ at increasing concentrations improved cit-BiVO ionic conductivity but not the material phase stability since all doped samples had reversible phase transitions from α → β → γ phases as seen by the STA analyses