Development Of Non-doped Ceria-based Heterostructure Composites For LT-SOFCs

Solid oxide fuel cells(SOFCs)are widely regarded as a promising energy conversion technology due to their high conversion efficiency,low pollution,and fuel flexibility.However,the current development of SOFCs is still hampered by the high operating temperatures,as the common electrolytes such as doped zirconia and doped ceria require high temperatures to run the cells,resulting in high cost,performance degradation,and technique complexity.To address this challenge,extensive efforts have been made to developing new alternative electrolytes to enable low-temperature SOFCs(LT-SOFCs).Among them,an unique ionic conductor,non-doped ceria(Ce O₂)with fast ionic transport at 400-550? has been proposed recently without using conventional doping method,indicating a new way to develop LT electrolytes.In this thesis,to further exploit the potential of non-doped ceria,three types of functional composites based on non-doped ceria are developed via various heterostructure approaches and applied in LT-SOFCs to evaluate their electrolyte functionality.In the first part of the thesis,the non-doped Ce O₂ electrolyte is synthesized via precipitation method and its surface morphology is modified in terms of using different precipitation agents(Na₂CO₃,NH₄HCO₃,KOH).It is found the Ce O₂ prepared by Na₂CO₃ precipitant achieves an attractive fuel cell power density of 706 m W cm^-2 at 550? along with>85 hours stability,which are superior to other two samples.Material characterizations reveal the existence of slight amount of Na₂CO₃ in the best-performing sample as an ultra-thin coating layer on the Ce O₂ particles,forming a heterostructure,while the other two samples prepared by NH₄HCO₃ and KOH are pure Ce O₂.Benefiting from the heterostructure,the best-performing non-doped Ce O₂ can be protected from being reduced by H₂ to guarantee stability and gain a high ionic conductivity of 0.14 S cm^-1at 550? by virtue of the ceria/carbonate interface.In the second part of the thesis,considering the ion-conducting and catalytic properties of Ce O₂and Cu O,a new type of semiconductor-ionic heterostructure composite,Ce O₂-Cu O is proposed to realize the dual functionalities of electrolyte and anode in LT-SOFCs.It is shown the 7Ce O₂-3Cu O electrolyte exhibits fuel cell power densities of 554-845 m W cm^-2 at 500-550?,and the 3Ce O₂-7Cu O anode can support two types of SOFCs to achieve high open-circuit voltages(OCVs)of 1.0-1.05 V at 500-550? with good power outputs,confirming the dual electrolyte and anode functionalities of the designed Ce O₂-Cu O composites.Further studies in terms of polarization curve and AC impedance analysis manifest that the 7Ce O₂-3Cu O electrolyte gets enhanced ionic conductivity via forming heterstructure and gains hybrid H⁺/O^2-conducting capability,while the3Ce O₂-7Cu O anode shows good catalytic activity comparable to a typical triple-conducting anode.On basis of these findings,a bulk heterojunction effect based on Ce O₂/Cu O pn junction is proposed to interpret the electronic suppression in the 7Ce O₂-3Cu O electrolyte.In the third part of the thesis,the non-doped Ce O₂ is considered as a coating material to modify a layer-structure Na Fe O₂ for electrolyte uses.A heterostructure composite made of Ce O₂ coated Na Fe O₂ is prepared by wet-chemical route and applied in SOFCs for comparison with the pure Na Fe O₂.It is found the introduction of Ce O₂ coatings brings about reduced grain-boundary resistance to Na Fe O₂ as a result of fast ionic transport at heterointerface,leading to improved fuel cell performance.The SOFC based on 10 mol%Ce O₂ coated Na Fe O₂ demonstrates a high peak power density of 727 m W cm^-2 and OCV of 1 V at 550? as well as a promising stability for 40h.Besides,a preliminary follow-up study has also been carried out to combine non-doped Ce O₂with another layer-structure oxide Na Co Fe O₂ to develop heterostructure composites,exhibiting an improved fuel cell power output of 1006 m W cm^-2 at 550°C and long-term stable operation for160 hours.Overall,three types of non-doped ceria based heterostructure composites have been developed in this thesis,with promising electrolyte properties to enable high performance of LT-SOFCS.These works indicate a new material system and three different heterointerface approaches to develop advanced materials for LT-SOFCs.