Modifications of nanoporous silver cathode with enhanced performance and thermal stability for low temperature solid oxide fuel cells by additive manufacturing
Date of Issue2018
School of Mechanical and Aerospace Engineering
Low temperature solid oxide fuel cells (LT-SOFCs) operating at 300 to 500 °C allow extended material selection, reduced manufacturing cost, and wide applications such as portable power source. However, the increased electrode polarization resistance, especially for oxygen reduction reaction at cathode, caused by lowering of the temperature will result in a reduction of overall electrochemical performance. Metallic electrode, such as platinum (Pt) and silver (Ag), provides an alternative choice for LT-SOFCs due to its high electro-catalytic activity and electrical conductivity as compared to the conventional mixed oxide. Nevertheless, the metallic electrode has poor thermal stability due to the agglomeration of the microstructure at elevated temperature, resulting in performance degradation. Therefore, optimizing the microstructure of the metallic electrode and improving its thermal stability are challenges for using metallic electrode in LT-SOFCs. Due to the lack study on oxygen reduction reaction mechanism of Ag cathode for LT-SOFCs, the fundamental study of oxygen reduction reaction, mainly on the diffusion behavior and surface adsorption of oxygen was carried out to understand the rate-limiting step in either surface or bulk diffusion and the contribution of oxygen adsorption/dissociation on Ag cathode at low temperature (300-500 ºC) by electrochemical impedance spectroscopy (EIS) analysis. The preliminary results showed the surface area for dissociative adsorption of oxygen molecules, surface path for atomic oxygen diffusion, and triple phase boundary (TPB) for charge transfer played the important roles in oxygen reduction reaction of Ag cathode at the temperature from 300 to 500 °C. As compared to the bulk path, bulk diffusion of oxygen atoms through Ag is assumed as a rate-limiting step. Therefore, the micro/nanoporous structure of Ag cathode with high TPB at the interface and surface area is considered to be the optimum microstructure. Modifications of Ag cathode were used to improve thermal stability and electrochemical performance. Three different modification methods, including infiltration, nanocomposite, and core-shell structure were developed, studied, and characterized in this dissertation. Ion-conducting samarium-doped ceria (SDC) was selected to modify the nanoporous silver for its high ionic conductivity at low temperatures and relatively lower phase formation temperatures than zirconia-based materials. These three methods effectively and significantly enhanced the thermal stability and electrochemical performance of nanoporous Ag by confining the Ag nanoporous structure and extending TPB. The highest thermal stability and electrochemical performance were obtained from the Ag@SDC core-shell cathode due to the better coverage of SDC on the Ag nanoparticles and more electrochemical reaction sites were provided by SDC shell.