Solid Oxide Fuel Cells for Intermediate Temperature Operation

Solid oxide fuel cells (SOFCs) efficiently convert chemical energy directly to electricity, with only water as product. SOFCs are typically built in large scale and for stationary applications, due to the high operating temperature (600 to 1000C). Several efforts have focused on reducing the operating temperature, either with thinner electrolyte or enhancement of catalyst/electrolyte interfaces. In particular, thin film electrolyte significantly reduces the Ohmic resistance of SOFCs and allows intermediate temperature operations (300 to 500C). In order to justify the functionality and characterize nano thin film SOFCs, fabrication methods for nano thin film electrolyte SOFCs are presented in this dissertation. In the first, thin-film SOFC structures containing electrolyte membranes of only 50 nm thickness were fabricated with sputtering, lithography, and etching. The micro SOFCs were made of yttria-stabilized zirconia (YSZ) electrolyte and 80 nm thick porous Pt as cathode and anode. The peak power density at 350C was 131 mW/cm2. The high power densities achieved are not only due to the reduction of electrolyte thickness but also to the high charge-transfer reaction rates at the interfaces between the nanoporous electrodes (cathode and/or anode) and the nanocrystalline thin electrolyte. To further increase the surface area density of previously developed SOFC, an SOFC array structure with 10 time of surface area utilization ratio was developed. Also, a low temperature micro SOFC with corrugated electrolyte membrane was developed and tested. To increase the electrochemically active surface area, yttria-stabilized zirconia membranes with thickness of 70 nm were deposited onto pre-patterned silicon substrates. Fuel cell performance of the corrugated electrolyte membranes released from silicon substrate showed an increase of power density relative to membranes with planar electrolytes. Maximum power densities of the corrugated fuel cells of 677 mW/cm2 and 861 mW/cm2 were obtained at 400 and 450C, respectively. Finally, a design and fabrication of micro-SOFC array with high surface area density is presented. The structure consists of a corrugated nano thin film electrolyte and a silicon supporting layer on a two-stage wafer through hole. The goal is to maximize the surface area within a given volume to obtain high absolute power output. An array of total 500 thousands fuel cells with 65 nm thick electrolyte was tested in parallel without a single membrane failure.