Materials development for intermediate temperature fuel cells
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The work in this thesis mainly focuses on the preparation and optimization of materials for intermediate temperature fuel cells (ITFCs) with the aim of achieving high fuel cell performance as well as good stability. The fuel cell fabrication was also studied in order to develop a cost-effective fabrication process. Methods such as solid state reaction, combustion and carbonate co-precipitation were adopted for the synthesis of the materials. The densification temperature of Ce0.8Gd0.05Y0.15O1.9 (GYDC) electrolyte was greatly reduced by the carbonate co-precipitation synthesis and subsequently a simple one-step co-press-sintering fabrication process was developed. LiNO3 as sintering additive further reduced the densification temperature of GYDC and up to 96% relative density was achieved at 800 °C. Lithiated NiO was employed as cathode for IT-SOFCs and demonstrated good electrocatalytic activity. In addition, lithiated NiO was also investigated as both anode and cathode for IT-SOFCs and its stability was studied. Oxide-carbonate composites have demonstrated very high ionic conductivity as the melting of carbonates greatly enhanced the mobility of ions in materials. High power densities up to 670 mW cm-2 at 550 °C were achieved for the composite electrolytebased ITFCs. However, the traditional lithiated NiO cathode can gradually dissolve into the carbonate melt and scanning electron microscopy studies found obvious morphology change nearby the cathode/electrolyte interface which may be due to the dissolution of nickel ions. Perovskite oxide Sm0.5Sr0.5Fe0.8Cu0.2O3-δ (SSFCu) has been demonstrated to be a compatible and stable cathode for the composite electrolyte based ITFCs, as a stable current output of about 0.4 A cm-2 was observed under a constant voltage of 0.7 V during a cell test lasting 100 h. Instead of GYDC, BaCe0.5Zr0.3Y0.16Zn0.04O3-δ (BCZYZn) was also employed as substrate material for the carbonate composite electrolyte and SrFe0.7Mn0.2Mo0.1O3-δ (SFMMo) was developed and used as cathode.