Low temperature micro solid oxide fuel cells with proton-conducting ceramic electrolytes
Date of Issue2016-05-10
School of Mechanical and Aerospace Engineering
Solid oxide fuel cells (SOFCs) are highly efficient energy conversion devices which can convert chemical energy directly into electricity without combustion. The major hurdle to SOFCs is the requirement of high operating temperature (>800 °C), which leads to a technological challenge for materials development and thermal management. Thus reducing the operating temperature of SOFCs to intermediate temperature (500-800 °C) range or even low temperature range (<500 °C) would greatly benefit the wide application of SOFC technology. Reducing the electrolyte thickness and/or utilizing proton-conducting electrolytes are two efficient approaches to reduce the operating temperature of SOFCs, as well as maintain high output power density. This work is dedicated to achieve high performance SOFCs at low operating temperature of 300-500 °C by using micro-SOFCs (µ-SOFCs) with proton-conducting thin film electrolytes. First, the chemical stability of nanoscale Y-BaCeO3 (BCY) proton-conducting electrolyte under µ-SOFCs operating environments is investigated. BCY-based SOFC shows a maximum power density of 30 mW/cm2 and an OCV of 0.59 V at 400 °C. The OCV decreases dramatically as testing temperature increases. The low OCV value and poor fuel cell performance originated from the cracks formed during test. Y-BaCeZrO3 (BCZY) electrolyte is employed to enhance the chemical stability and performance of cerium-based ceramic electrolytes. Both the OCV and fuel cell performance of BCZY-based SOFC have great improvement compared with BCY SOFC. The maximum power density has increased to 89 mW/cm2 at 400 °C, which is nearly 3 times that of BCY-based SOFC. The OCV was maintained at around 1.0 V, showing that BCZY is a promising electrolyte candidate with essential ionic conductivity and chemical stability for low temperature SOFC application. In order to enhance the sluggish cathodic oxygen reduction reaction (ORR) at low temperature, a cathodic modification method was introduced. High performance µ-SOFC using chemical stable Y-BaZrO3 (BZY) electrolyte was obtained by utilizing an 8 nm thin film GDC cathode interlayer. The cathodic polarization resistance was effectively reduced by the additional GDC interlayer between the Pt cathode and BZY electrolyte. A high peak power density of 445 mW/cm2 was obtained at 425 °C from the µ-SOFC with the GDC cathodic interlayer. The mixed oxygen and proton conduction in the GDC interlayer has expanded the ORR sites from a 2-dimentional planar interface between Pt and BZY to the entire GDC interlayer. The cathodic modification was further investigated by incorporating nanostructured Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) thin film cathode into µ-SOFCs. The nanostructured BSCF thin film has increased total surface area exposed to oxygen reactions compared to the previously reported dense BSCF thin film cathode, which rendered significantly enhanced cathode performances in improving the sluggish oxygen reduction reactions. µ-SOFCs using the nanostructured BSCF cathode achieved a fuel cell performance with a peak power density of 55 mW/cm2 at 450 °C. Finally, we explored the feasibility of direct operation of ethanol fuel with µ-SOFC at low temperature (<500 °C). µ-SOFC with highly crystallized BZY thin film electrolyte, nanoporous Pd anode and Pt cathode exhibits a peak power density of 72.4 and 15.3 mW/cm2 with the fuels of hydrogen and ethanol at operating temperature of 400 °C.
DRNTU::Engineering::Mechanical engineering::Energy conservation