Development And Evaluation Of Cathode Prepared By Screen Printing For IT-SOFC

A solid oxide fuel cell (SOFC) is an electrochemical device operated at high temperature (600-1000oC), which can directly transform the chemical energy stored in fuels into electrical energy and heat. SOFCs provide many advantages over traditional energy conversion system including highly efficiency, low pollution, quietness, reliability, modularity and fuel adaptability, which show great potential applications in power generation, transport and military and attract great attention worldwide. Under operation conditions, the output voltage of single SOFC is usually less than 1V and the power is limited, in order to gain much more power output, single cells are piled together through serial or parallel or mixed designs. Achievement of stable long-term operation of cell stack requires repeatable and stable electrochemical performances of single cells, which depend on not only the reliable and stable cell fabrication techniques, but also the standardized cell testing process. Based on the goal of 1kW stack in our laboratory, this Ph.D. thesis focuses on development of reliable cathode screen printing technique, optimizing the cathode current collection, establishing fuel cell test platform and standardizing fuel cell test.In the first chapter, we briefly introduced the principle and characteristics of SOFC, and reviewed the key materials, such as: the anode, the electrolyte, the cathode, the interconnector and the seal materials. In addition, we presented the stack designs and the corresponding advantages and disadvantages, summarized how to evaluate the cell performance and what electrochemical processes can influence the performances. We also analyzed the SOFC research situation and development trend. We proposed to establish a stable, reliable, repeatable and standardized cathode preparation and cell testing process.In the second chapter, we firstly introduced the basic principle of screen printing technique and oxygen reduction process in the cathode. On the basis of cathode preparation literature, reasonable parameters about how to increase the amount of three-phase boundaries in LSM/YSZ cathode functional layer (CFL) and improve conductivity and gas distribution in LSM cathode current collecting layer (CCCL) was concluded, and then successfully applied to the screen printing process and developed the cathode.In the third chapter, we mainly focused on preparation and characterization of the screen printing cathode. We proved that this technique had good stability and repeatability via screen printing YSZ powder on sintered substrate. We analyzed the microstructures, porosity, polarization resistance and electrical conductivity of the CFL and CCCL. Single cell achieved the highest power density of 550mWcm^-2 at 750oC. We further investigated the influence of YSZ surface morphology on the cathode polarization resistance. We modified the YSZ electrolyte surface with YSZ particles of different sizes, and studied the relation between the varied surface morphology and cathode polarization resistance. The results showed that: the high-temperature coarsened YSZ particles increased the surface area of the YSZ electrolyte, improved the interface microstructure between the CFL and electrolyte, resulting in reduction of the cathode polarization.In the fourth chapter, we studied the cathode current collector and their influence on the performance of SOFC. Through continuous optimization of the materials and design of cathode current collector, electron conduction from the cathode current collector to the cathode and current distribution in the internal cathode was improved. In addition, concentration polarization was also reduced by the optimized oxygen transport. The highest power density was increased from 120 to 550mWcm^-2 at 750oC. When Ag paste was applied as the cathode current collector, we found that Bi₂O₃ sintering aid in the Ag paste enhanced the sintering of Ag phase and improved the surface morphology, reacted with LSM to generate solid solution and increased the amount of contact points at the interface. These observations helped to suppress the current constriction effect and improve the current distribution, resulting in reduction of both polarization resistance and ohmic resistance of cathode. Moreover, Bi₂O₃ has a low melting temperature of 823oC and may migrate from the Ag paste cathode current collector into the cathode at the operation temperature of SOFC. Due to the high oxygen ion conductivity of Bi₂O₃, additionally formed contact between Bi₂O₃ and LSM increased three-phase boundaries can accelerate the oxygen reduction reaction. Once Bi₂O₃ reached the cathode/electrolyte interface, oxygen ion transfer from the cathode to electrolyte can be improved. Bi₂O₃ also enhanced thermal activity of Ag phase, leading to deposition of Ag into cathode in a short-term test and increase of the catalysis activity. Therefore, the deposited Bi₂O₃ and Ag greatly reduced the cathode polarization resistance and enhanced the power output.In the fifth chapter, we developed GDC interlayer/LSCF cathode structure in order to reduce the working temperature of SOFC. Although LSCF perovskites exhibit higher oxygen catalytic activity than LSM materials, it shows thermal and chemical incompatibility with the state-of-art YSZ electrolyte. Therefore, GDC interlayers were introduced between cathode and electrolyte using screen printing, electron-beam evaporation and ion assisted deposition methods. We found that microstructures of the GDC interlayers greatly depended on the deposition techniques. The screen printing GDC interlayer sintered at 1250oC remained porous, whereas compact microstructure can be prepared at low temperature of 250oC by the electron beam evaporation and ion assisted deposition. We found that energy of deposited species in the physical phase deposition would affect the microstructures of GDC interlayers. In the ion assisted deposition process, the assisted ions impinged on the growing film and transferred their kinetic energy to the deposited species, resulting in enhancement of the deposited species, and a denser and more uniform GDC interlayer. We developed GDC interlayer with densest microstructure, which can contribute to protect the YSZ electrolyte from reacting with LSCF cathode and improve the electrochemical performance of single cell.In the last chapter, we described the basic principle of fuel cell measurement. We established a fuel cell testing system with accurately controlled gas flow rate and temperature, as well as various electrochemical performance measurements. Attempt has been made to standardize the fuel cell testing process.