Study On Ceria-based Composite Electrolyte And Electrode Materials For Low Temperature Ceramic Fuel Cells

Solid oxide fuel cell （SOFC） or ceramic fuel cell （CFC）, an electrochemicaldevice that converts the chemical energy in fuels directly into electricity, has attractedgrowing attention because of its distinct advantages: high efficiency, lowenvironmental impact and fuel flexibility. The current SOFC research trend is todecrease the working temperature with the purpose of reducing the system costs andimproving the reliability. While the lowering operational temperature requires highionically conductive electrolyte and super-performance electrode materials, whichhave become barriers to significantly hinder the commercialization process of CFCs.The development of doped ceria oxide-carbonate composite electrolyte has effectivelyenhanced the ionic conductivity and solved some problems related with the single-phase electrolyte materials, which has demonstrated a new approach to realize thewide application of CFCs.In this dissertation, based on the ceria-（Li/Na）₂CO₃composite, we prepared andinvestigated the effects of the different SDC precursors on composites’ electricalproperties, developed and studied the performance of two kinds of novel electrodematerials-Pr₂NiO₄-Ag and lithiated transition metal oxides composites and applied amodified Wagner polarization method to study the complex multi-ionic conductiveproperties. The electrical properties, such as the ionic conductivity, mixed oxygenion/proton conduction behavior, the possible ionic conduction mechanism of SDC-carbonate composite and fuel cell performance for composite electrolyte CFCs withdifferent electrode materials are detailed analyzed. The main results are:The electrical properties of SDC-（Li/Na）₂CO₃composite electrolytes werehighly dependent on the SDC precursors which were prepared by nitrate-citriccombustion, solid-state reaction and modified nanocomposite （NANO） approaches,respectively. Among the composites, composite electrolyte by NANO approachexhibited the highest ionic conductivity and fuel cell performance. A maximum powerdensity of839mW·cm^-2was achieved at600℃under H₂/air atmosphere.The new developed Pr₂NiO₄（PNO） perovskite cathode catalyst showed a goodchemical compatibility with SDC-carbonate composite at and below650℃. Singlecell with PNO cathode delivered a peak power output of652mW·cm^-2at600℃withhydrogen as fuel and air as the oxidant, which was comparable with the common usedlithiated NiO cathode, while much higher in comparison with other perovskite oxides. The introduction of10wt%of Ag active catalyst into surface of PNO by infiltrationway not only improved the charge transfer rate but also raised the oxygen surfaceexchange kinetics, leading to a higher catalytic activity and subsequent an increasedelectrochemical performance. It is expected that the optimizations of the impregnationprocess and the electrode composition could further enhance the fuel cellperformance.The compositing or doping of ZnO reduced the particle size and the electricalconductivity while improved the oxygen reduction electro-catalytic activity oflithiated NiO. Peak power densities of600and859mW·cm^-2were achieved at500and600℃, respectively, in H₂/air condition. Moreover, single cell with modifiedcathode showed a stable performance during5h test under a constant externalresistance condition, indicating great promise for practical application.In SDC-carbonate composite, the ionic polarization process varied under oxygenand hydrogen atmospheres, which was attributed to the intrinsic and extrinsic ionicconducting properties, especially for oxygen ion and proton conduction. The ACimpedance spectroscopy measurements in different applied gas atmospheres offeredan indirect evidence for the mixed oxygen ion/proton conduction properties incomposite electrolyte, while the modified Wagner DC polarization approach clearlyshowed the oxygen ion and proton conduction in the SDC-carbonate composite. Themodified Wagner polarization method is also proposed as a universal approach tostudy the materials’ complex multi-ionic conduction properties. The results show thatthe proton conductivity in the SDC-carbonate composite is higher than oxygen ionicconductivity. Moreover, the oxygen ionic conductivity in composite electrolyte isenhanced compared with the single-phase electrolyte. The mixed oxygen ion andproton transports co-contribute the high ionic conductivity of the SDC-carbonatecomposite and subsequent high fuel cell electrochemical performance at the reducedtemperature.