Materials Development for the Fabrication of Metal-Supported Solid Oxide Fuel Cells by Co-sintering

Solid Oxide Fuel Cell (SOFC) is an upcoming technology seen with great expectations for the production of electrical energy with good efficiency and minimal environmental impact. Successful commercialization of SOFCs has however been hindered despite the optimistic promises made by some developers. This slackened commercialization of SOFCs technology is mainly due to the high cost associated with SOFC production and its limited long term stability. The long term stability of conventional Anode Supported-Solid Oxide Fuel Cell (AS-SOFC) with Ni based anode is tested by its limited tolerance towards redox cycling and rapid thermal cycling. The introduction of new generation SOFC, the so called Metal Supported- Solid Oxide Fuel Cell (MS-SOFC) has shown to overcome the drawbacks associated with the conventional AS-SOFC. Thus, MS-SOFC is looked upon as the potential candidate for the rapid commercialization of SOFC technology. In MS-SOFC design, the cell is supported on a porous metal substrate instead of expensive and non-reliable anode as in AS-SOFC. In this design the thickness of the functional layers (anode, cathode and electrolyte) is kept thin as possible (in the order of 10-50m) just necessary for electrochemical activities while the support being provided by the metal substrate. Although MS-SOFC can be fabricated by different routes, co-sintering of metal/anode/electrolyte multilayers in non-oxidizing atmosphere at high temperatures (1300 to 1400oC) is the most promising as far cost efficiency and industrial scale up is concerned. The cathode is usually applied after high temperature processes and sintered in situ during operation in this route. This fabrication approach however has some drawbacks associated with it. This work is basically on the development of materials and optimization of the multilayer design for the production of MS-SOFC by cost-effective co-sintering approach. YSZ (Y2O3 stabilized ZrO2), Ni-YSZ cermet, and ferritic stainless steel are considered for the electrolyte, anode and the support respectively. The anode and electrolyte were modified with the help of suitable dopants and the multilayer design was also altered in order to facilitate the co-sintering, preventing or reducing the generally encountered issues in this fabrication route. Coarsening of Ni in Ni-YSZ anode cermet and over-sintering of anode during high temperature co-sintering is a well-known issue. Ni coarsening reduces the number of triple phase boundaries (TPB) thereby affecting electrochemical performance. The electrical conductivity of the anode also degrades due to Ni coarsening. In current work, the effect of Al doping on Ni-YSZ anode sintering in Metal Supported-Solid Oxide Fuel Cell (MS-SOFC) was studied. It was found that, the addition of Al into the anode accounts for a finer microstructure if compared to undoped Ni-YSZ anode material. The electrical conductivity of the Al-doped anode was also found to increase considerabely and such result may be attributed to the fine microstructure caused by the segragation of Al2O3 formed during the course of sintering on the grain boundaries of both Ni and YSZ, thus inhibiting the sintering. 5wt% Al-doped NiO used for Ni-YSZ anode material gave the finest microstucture and the highest electrical conductivity at room temperature although it showed the lowest bulk density. Overall, Al-doped Ni-YSZ anode material was found to be a suitable material for the anode in MS-SOFC produced by co-sintering. The modification of the reduction kinetic of NiO and the interaction between the anode and steel during the fabrication of Metal Supported Solid Oxide Fuel Cells (MS-SOFC) is also studied in the present work. With the aim to limit NiO reduction under inert atmosphere at high temperature, doping elements such as Al and Ce were considered for NiO powders modification and anode production. In order to simulate the reactions at the metal/anode interface, NiO/YSZ/steel composites were prepared with pure and Al-doped NiO. A sudden volume expansion above 10000C followed by substantial shrinkage above 12000C was observed for the composites when sintered in Ar at 14000C. Such volume expansion can be related to the oxidation of steel due to the RedOx reaction between NiO and steel. Moreover, it was found that the volume expansion, i.e. the steel oxidation, can be minimized to a good extent when Al-doped NiO is used. Hence it is proposed that Al-doped NiO is a promising candidate material to be used for anodes in high temperature sintering of MS-SOFC. Other problems encountered during co-sintering of multilayers for MS-SOFC include delamination, cracking, bending, and interdiffusion of Fe,Cr and Ni between anode and the substrate. In another section of work, green multilayers were produced by tape-casting for the fabrication of MS-SOFC half-cell by co-sintering. The binder loss step during co-sintering was optimized so as to prevent the cracking of the multilayers due to the binder loss events. Intermediate layers (layer between metal support and rest of the layers) composed of metal-ceramic powder composite were also investigated to prevent delamination and to inhibit interdiffusion of the elements.CeO2-steel, YSZ-steel, and LDC(La doped Ceria)-steel powder composites were considered for investigation to use as intermediate layer. Out of the different multilayer design considered for investigation, YSZ/(Al-NiO)-YSZ/LDC-steel/steel multilayer design was found to be a good compromise so as to give a half-cell, with good bonding between the layers, which is camber free, and with moderate interdiffusion of elements between the substrate and the anode. It was however found in all the designs that complete densification of YSZ electrolyte could not be obtained. In order to address the issue of limited densification of YSZ electrolyte during co-sintering, Fe was considered for doping YSZ. A comparative study was done on Fe doped YSZ samples for sintering in air and argon atmosphere, with the aim to analyze the effect of Fe as sintering aid under MS-SOFC fabrication by co-sintering conditions. Samples showed enhanced densification with increasing Fe concentration in both the sintering atmospheres thus concluding that Fe can be used as a sintering aid for YSZ even in argon. The samples sintered in argon atmosphere were however characterized by larger lattice parameter, density and grain size. The increase in lattice parameter can be attributed to the oxygen vacancies generated under low p(O2) in argon atmosphere. The microstructural analysis of the samples showed the presence of small amount of secondary phase, and the concentration of such phase was seen to be higher in the argon sintered samples. Comparison of colors of argon and air sintered samples indicates the reduction and/or precipitation of Fe dopant in samples sintered in argon. Gas tight dense electrolyte could be obtained for MS-SOFC fabricated by co-sintering when Fe doped YSZ is employed for electrolyte, although the performance of the cell was quite poor.