The manufacture and testing of anode supported Ni-10Sc1CeSZ SOFCs for intermediate temperature operation

Developing solid oxide fuel cell (SOFC) systems that operate in lower temperature regimes improves system stability, widens materials selection and lowers performance degradation issues previously observed with higher temperature cells. In this work, the development of an intermediate temperature SOFC (IT-SOFC) based on a Ce-doped Scandia-stabilised Zirconia (ScSZ) structure manufactured via screen-printing is outlined. In this thesis we report on the successful manufacture of anode supported 8YSZ baseline cells and cells containing Ni-10Sc1CeSZ anodes supporting 10Sc1CeSZ electrolytes both prepared via die pressing, screen-printing and co-sintering. Commercial 8YSZ and 10Sc1CeSZ reference cells were also tested. This research aims to demonstrate the viability of 10Sc1CeSZ within an IT-SOFC cell structure and examine the effectiveness of 10Sc1CeSZ on lowering the cell operating temperature. Results for the tested cells on H$_2$ at 800$^o$C show the lab grade and commercial grade YSZ cells obtained OCV values of 1.06V and 1.04V and maximum power density values of 392 mW cm$^-$$^2$ and 466 mW cm$^-$$^2$ respectively. The commercial 10Sc1CeSZ cells exhibited the highest OCVs > 1.10V of all tested cells with the lowest area specific resistance of 0.496 Ω cm$^2$ obtained for the lab grade 10Sc1CeSZ cells. Peak power densities of 68.24 mW cm$^-$$^2$ and 9.12 mW cm$^-$$^2$ at 800$^o$C were achieved for the biogas fuelled lab grade YSZ and 10Sc1CeSZ cells respectively

Influence of reduction conditions of NiO on its mechanical and electrical properties

Yttria stabilized zirconia with a nickel catalyst (Ni-YSZ) is the most developed, widely used cermet anode for manufacturing Solid Oxide Fuel Cells (SOFCs). Its electro-catalytic properties, mechanical durability and performance stability in hydrogen-rich environments makes it the state of the art fuel electrode for SOFCs. During the reduction stage in initial SOFC operation, the virgin anode material, a NiO-YSZ mixture, is reduced to Ni-YSZ. The volume decrease associated with the change from NiO-YSZ to Ni-YSZ creates voids and causes structural changes, which can influence the physical properties of the anode. In this work, the structural, mechanical and electrical properties of NiO samples before and after reduction in pure H2 and a mixture of 5 vol. % H2-Ar were studied. The NiO to Ni phase transformations that occur in the anode under reducing and Reduction-Oxidation (RedOx) cycling conditions and the impact on cell microstruc-ture, strength and electrical conductivity have been examined. Results show that the RedOx treatment of the NiO samples influence on their properties controversially, due to structural transformation (formation of large amount of fine pores) of the reduced Ni. It strengthened the treated samples yielding the highest mechanical strength values of 25.7 MPa, but from another side it is resulting in lowest electrical conductivity value of 1.9×105 S m-1 among all reduced samples. The results of this investigation shows that reduction conditions of NiO is a powerful tool for influence on properties of the anode substrate

Composite Cathode for High-Power Density Solid Oxide Fuel Cells

Reduction of solid oxide fuel cell (SOFC) operating temperature will play a key role in reducing the stack cost by allowing the use of low-cost metallic interconnects and new approaches to sealing, while making applications such as transportation more feasible. Reported results for anode-supported SOFCs show that cathode polarization resistance is the primary barrier to achieving high power densities at operating temperatures of 700 C and lower. This project aims to identify and develop composite cathodes that could reduce SOFC operating temperatures below 700 C. This effort focuses on study and use of (La,Sr)(Co,Fe)O{sub 3} (LSCF) based composite cathodes, which have arguably the best potential to substantially improve on the currently-used, (La,Sr)MnO{sub 3}-Yttria-stabilized Zirconia. During this Phase I, it was successfully demonstrated that high performances can be achieved with LSCF/Gadolinium-Doped Ceria composite cathodes on Ni-based anode supported cells operating at 700 C or lower. We studied electrochemical reactions at LSCF/Yttria-stabilized Zirconia (YSZ) interfaces, and observed chemical reactions between LSCF and YSZ. By using ceria electrolytes or YSZ electrolytes with ceria diffusion barrier layers, the chemical reactions between LSCF and electrolytes were prevented under cathode firing conditions necessary for the optimal adhesion of the cathodes. The protection provided by ceria layer is expected to be adequate for stable long-term cathode performances, but more testing is needed to verify this. Using ceria-based barrier layers, high performance Ni-YSZ anode supported cells have been demonstrated with maximum power densities of 0.8W/cm2 at 700 C and 1.6W/cm{sup 2} at 800 C. Ni-SDC anode supported cells with SDC electrolytes yielded >1W/cm{sup 2} at 600 C. We speculate that the power output of Ni-YSZ anode supported cell at 700 C and lower, was limited by the quality of the Ceria and Ceria YSZ interface. Improvements in the low-temperature performances are expected based on further development of barrier layer fabrication processes and optimization of cathode microstructure