Anode-Supported Solid Oxide Fuel Cells Fabricated by Single Step Reduced-Temperature Co-Firing

Abstract

Anode-supported solid oxide fuel cells (SOFCs) consisting of NiO-Y 0.16 Zr 0.92 O 2-δ (YSZ) anode support layer, NiO-YSZ anode functional layer, YSZ electrolyte and (La 0.8 Sr 0.2 ) 0.98 MnO 3-δ (LSM)-YSZ cathode were successfully fabricated by single-step cofiring at 1250 • C. Cells were prepared by tape casting, with Fe 2 O 3 sintering aid used to obtain a nearly dense YSZ electrolyte. Scanning electron microscope (SEM)-energy dispersive X-ray spectroscopy (EDS) showed no evidence of reactions or interdiffusion between layers during co-firing. The cells yielded area specific resistance of 0.44 cm 2 and a maximum power density of 0.91 W/cm 2 at 800 • C. Impedance spectroscopy measurements showed that the LSM-YSZ cathode polarization resistance was higher for the co-fired cathodes than for a cathode that was fired separately at an optimized temperature of 1175 • C. However, reducing the cell co-firing time decreased cathode polarization resistance and increased cell power output. Analysis of SEM images showed that co-firing caused more sintering and coarsening than in the optimally-fired LSM-YSZ, reducing three-phase boundary density and explaining the increased cathode resistance. Anode-supported solid oxide fuel cells (SOFCs) with yttriastabilized zirconia (YSZ) electrolytes are usually fabricated using at least two high-temperature firing steps: co-firing of the anode and electrolyte followed by application and firing of the cathode.1,2 This is based primarily on differences in desired firing temperatures -the YSZ electrolyte is densified at ∼ 1400• C -whereas cathode materials, typically based on (La 0.8 Sr 0.2 ) 0.98 MnO 3-δ (LSM), are usually fired at 1200 • C. Also, the two-step process reduces possible reactions between cathode and electrolyte during firing. On the other hand, there are a number of reasons why single-step fired SOFCs are desirable. First, it simplifies the process, reducing the processing time and input energy, important given the general need to reduce SOFC cost in order to improve commercial viability.3-7 Second, single-step co-firing is a requirement for fabrication strategies where the entire stack, including cells, gas channels and interconnectors, are fired together.8 Third, it allows the fabrication of cells with a cathode-side mechanical support instead of an anode support.In order to achieve single-step firing, the temperature should be reduced to allow a good cathode microstructure, while still densifying the YSZ electrolyte. YSZ electrolyte densification as low as 1200• C has been demonstrated when utilizing an appropriate sintering aid, e.g. Fe 2 O 3 .9,10 Recently, anode-supported SOFCs with YSZ/GDC bi-layer electrolytes have been produced by co-firing with Fe 2 O 3 sintering aid at 1250• C, but with the cathode added in a second lower-temperature firing step.11 These temperatures are still above the ideal firing temperature for LSM-YSZ cathodes of ∼ 1175• C. 12 Also, the shrinkage that occurs during single-step co-firing will exacerbate cathode densification, further reduci...

Single‐step fabrication of an anode supported planar single‐chamber solid oxide fuel cell

“…At present, anode‐supported SC‐SOFCs are fabricated using at least two sintering steps: co‐sintering anode and electrolyte, followed by a separate deposition and sintering of the cathode . The rationale for a two‐step sintering is that the materials for anode, electrolyte, and cathode require different sintering temperatures to achieve the expected microstructures.…”

“…Furthermore, thermal expansion coefficient (TEC) mismatch between each layer becomes a dominant factor when materials have higher viscosity that is, during cooling process . Therefore, in order to achieve the single‐step co‐sintering of an anode supported SC‐SOFC, the sintering temperature was decreased to obtain a preferable cathode microstructure, while still densifying the electrolyte . In addition, the sintering temperature of the cathode is generally less than that of the anode and electrolyte .…”

“…[6][7][8] At present, anode-supported SC-SOFCs are fabricated using at least two sintering steps: co-sintering anode and electrolyte, followed by a separate deposition and sintering of the cathode. [9][10][11] The rationale for a two-step sintering is that the materials for anode, electrolyte, and cathode require different sintering temperatures to achieve the expected microstructures. Though a two-step sintering process decreases possible reactions/interactions between electrolyte and cathode, 11 there are several reasons why single-step co-sintering of SC-SOFCs are desirable, mainly pertaining to simplifying the process along with decreasing the processing time and input energy.…”

“…11,17,18 In addition, the sintering temperature of the cathode is generally less than that of the anode and electrolyte. 11,19,20 Therefore, the cathode particle size was chosen to be greater than that of the anode and cathode, so as to retard the sintering kinetics of the cathode and thereby suppressing quicker densification, as well as to balance the final density of materials during the co-sintering process. 8 Lowering sintering temperature is also helpful in reducing the TEC mismatch between each component and thereby mitigating possible crack or delamination in the cell during sintering.…”

“…The rationale for a two‐step sintering is that the materials for anode, electrolyte, and cathode require different sintering temperatures to achieve the expected microstructures. Though a two‐step sintering process decreases possible reactions/interactions between electrolyte and cathode, there are several reasons why single‐step co‐sintering of SC‐SOFCs are desirable, mainly pertaining to simplifying the process along with decreasing the processing time and input energy . These benefits of single‐step co‐sintering of SC‐SOFCs reduce further the cost of SC‐SOFCs fabrication and thereby improve commercial viability if employed in commercial scale.…”