Influence of processing and fabrication parameters on microstructural properties of anode-supported solid oxide fuel cells

Abstract

Power generation under environmentally friendlier and more efficient cycles has caused a great interest in alternative fuels and power sources. Currently, electrical power is mainly provided by traditional power generation cycles which generate air pollutant such as sulfur and noxious compounds. A solid oxide fuel cell (SOFC) is an energy conversion device that has the potential to efficiently generate electricity in an environmentally-friendly manner. The SOFCs generally operate between 600°C and 1000°C by oxidizing hydrogen or other fuels using air as the oxidant. A typical anode-supported SOFC is made from a dense yttria-stabilized zirconia (YSZ) electrolyte, a porous lanthanum strontium manganate cathode, and a porous nickel/YSZ cermet anode. Surveying the published works on SOFCs especially during the last couple of decades shows that the main challenges for anode-supported SOFCs are in finding suitable fabrication methods and in tailoring the desired microstructural properties. The aim of this research is to investigate both fabrication and microstructural properties of SOFC anodes in order to quantify the influential processing parameters using both experimental and modeling approaches. Controlling the rheological behavior of ceramic slurries is an important step in the casting process to achieve improved micro-structural properties. The rheological behavior of ceramic slurries containing nickel oxide (NiO)/YSZ was investigated in a colloidal casting method (so-called drying-free casting method) in order to study the effects of solid loading fraction and dispersant concentration on the viscosity of anode slurries. The drying-free casting method offers more flexibility in controlling the microstructure of ceramics and eliminates defects compared with conventional casting processes. A new viscosity correlation for ceramic suspensions was proposed to predict the relative viscosity data, showing excellent agreement with the experimental data from this study and with reported data in literature for other ceramic systems. In addition, microstructural features and functional properties of the anodes significantly affect the electrochemical performance of the anode-supported SOFCs. The SOFC anode characteristics and properties including porosity and pore size distribution, sintering shrinkage, mechanical strength, and electrical conductivity were investigated using an integrated experimental approach by adding carbon micro-spheres (CMSs) powders as the pore-former. The anode porosity was modeled to differentiate the impacts of processing parameters such as fabrication pressure (under uniaxial compaction condition), pore-former loading fraction, and particle size ratio. The results of this study can be used to interpret the physical basis of pore formation and also to assist in fabrication of SOFC porous ceramic substrates with desired microstructural characteristics. To evaluate the effect of anode microstructural properties on the cell electrochemical behavior, a number of anode-supported SOFCs were fabricated by depositing YSZ electrolyte thin film using electrophoretic deposition (EPD) and integrating LSM/YSZ cathode layer using slurry brush-painting method. The applied EPD technique was theoretically formulated and compared with the experimental results that showed a linear deposition behavior for YSZ especially at initial deposition period. The cell polarization behavior was also investigated by electrochemical impedance spectroscopy (EIS) analysis over the fabricated NiO/YSZ-YSZ bi-layers. The results were used to determine the ohmic, polarization, and area specific resistance (ASR) behavior of the fabricated half cells. The SOFCs fabricated by controlling anode porosity to be in the range of 35% to 40% showed higher electrochemical performance.