Electrochemical and Mechanical Performance of Solid Oxide Fuel Cells Supported on Ni or NiFe Metal Thick-Film

DoctorSolid oxide fuel cell (SOFC) its considered as a promising energy-conversion system due to its high electrical efficiency and environmentally friendly nature. Compared to the other types of fuel cells, SOFCs have superior properties in conversion efficiency, power density, stability, life time, and multi-fuel capability. However, current SOFCs are very weak against mechanical and thermal shocks because they have drawbacks, such as ceramic-inherent brittleness and low thermal conductivity. Current unit cell of SOFCs have been commonly fabricated either electrolyte-supported type or anode-supported type. Recently, metal-supported SOFC (MS-SOFC) has received great attentions, since it can overcome the weakness of ceramics due to the good thermal and mechanical properties of metallic support. The commonly used materials used for the supporting metal are FeCr alloys including ferritic stainless steels (STSs), since they show good oxidation resistance, similar thermal-expansion coefficient (TEC) with YSZ (yittria-stabilized zirconia) electrolyte along with low price. The MS-SOFCs, in many cases, are fabricated by a high-temperature co-firing process in reducing atmosphere to prevent the metal oxidation. However, during the process, the Ni particles in anode are easily coarsened and react with a Cr in STS support. The coarsening and the reaction problems result in the poor catalytic activity and the destruction of single cells. Furthermore, the metal supports generally had very thick thickness (1~2mm) which made the cell very rigid and massive. Thin Ni or NiFe support can be one of the possible solutions because they do not have Cr-poisoning problems with Ni-based anode. Thus, in this study, metal-supported SOFCs were developed using Ni and NiFe as a supporting metal. In order to make the cell with light weight and mechanically flexible, thin metal support was fabricated by tape casting method. Ni-supported single cell with total thickness of ~230μm was successfully fabricated. The three-layered cell had proper microstructure in each componentmacro-porous support, micro-porous anode, and dense electrolyte. Electrochemical performance of metal-supported single cell with LSCF cathode was tested at 600, 700, and 800oC, supplying wet H2 as a fuel gas and open air as an oxidant gas, respectively. The single cell showed a maximum power density of ~ 1.3W/cm2 at 800oC. This was the highest value among the metal-supported SOFCs based on YSZ electrolyte. It was mainly due to the absence of Cr-poisoning and fine microstructure of anode obtained by two step firing process (Chapter 3). Although the Ni-supported cell showed an excellent performance, Ni support had a high mismatch in TEC with YSZ electrolyte. Although the Ni-supported cell showed an excellent performance, Ni support had a high mismatch in TEC with YSZ electrolyte. Therefore, 50Ni/50Fe alloy was used as a supporting metal because it has a TEC value similar to that of YSZ electrolyte. The Ni/Fe-supported three-layered cell with a thickness of ~230μm could be fabricated by co-firing and metal reduction. The single cell performance was a little lower (~0.9W/cm2) than the Ni-supported cell at 800oC due to the diffused Fe into the anode during sintering. Thermal cycling characteristics for Ni- and Ni/Fe-supported single cells were also tested using a heating and cooling rate of 20oC/min between room temperature and 800oC. For both Ni- and Ni/Fe-supported cells, the OCV values were well maintained during first 10 cycles, where Ni/Fe-supported cell showed more stable in polarization resistance. For example, the Ni/Fe-supported cell showed only 3% drop in power density, while the Ni-supported one had ~15% after thermal cycles, as explained in Chapter 4. In order to confirm the mechanical flexibility of thin MS-SOFC, a simple 3-point bending test was carried out. When the load-displacement characteristics of MS-SOFC were compared to those for anode- and electrolyte-supported type cells, MS-SOFC showed much larger displacement before rupture than the others, which proved a good mechanical flexibility, as explained in Chapter 5.As a conclusion, we have successfully fabricated the thin metal-supported SOFCs with a good electrochemical performance, mechanical flexibility and thermal cycling properties, where the anodic microstructure was found to be very critical for achieving the high power density