Preparation and evaluation of symmetrical solid oxide electrolysis cells for ammonia feedstock production

• Main Author: Ong, Yong Kiat
• Other Authors: Chan Siew Hwa
• Format: Final Year Project
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Materials::Ceramic materials; DRNTU::Engineering::Materials::Material testing and characterization
• Online Access: http://hdl.handle.net/10356/78765
• Institution: Nanyang Technological University

Solid oxide electrolysis cells (SOECs) have gained increasing interest in recent years due to their promising potential as a viable candidate for the production of hydrogen gas in an environmentally sustainable way. Hydrogen gas is an important industrial commodity in the hydrogen economy and for the synthesis of the chemical ammonia. Current industrial methods to produce hydrogen gas for ammonia synthesis have a significant negative impact on the environment. As SOECs have shown excellent hydrogen production efficiency, it is touted as the next alternative direction in producing sustainable hydrogen gas. This project is meant to develop an efficient SOEC system towards wet air co-electrolysis for production of ammonia feedstock, i.e. mixture of N2 and H2. This report presents an examination of the performance behaviour of SOECs and investigates the influence of the microstructure and composition of its porous electrodes on its electro-catalytic activity under wet air co-electrolysis operation. A symmetrical electrode configuration was primarily adopted in this project to develop thin electrolyte-supported SOECs. Scandia-stabilized Zirconia (SSZ) was used as the electrolyte while three different sets of electrodes were evaluated: (La,Sr)(Cr,Mn)O3 doped with GdCeO2 (LSCM-GDC) electrodes, Pd-SSZ electrodes and LSCM-GDC electrodes with a transition layer of LSCM-GDC mixed with platinum. Wet impregnation approach was also used to dope LSCM-GDC electrodes with palladium metal catalyst to improve their electrochemical performance. Electrochemical characterization results of the cells showed a general trend of decrease in the cell impedance and increase in the current density as the operating temperature rises. Also, increase in the inlet air flowrate contributes to more oxygen electrolysis which delays the onset of limiting current behaviour. It was also observed that increase in the water vapour concentration in the feedstock gas resulted in a higher current density due to more water electrolysis occurring. Tested cells that had sub-par performance were analysed and several plausible reasons relating to the electrode’s microstructure and compositions were put forth. The LSCM-GDC electrode with platinum transition layer exhibited the best performance out of the three, with an average polarisation resistance of ~ 0.42 Ωcm2 at 900°C. However, its performance is still considered underwhelming according to the hydrogen yield in the produced gas. Gas chromatography analysis on this sample at 900°C, 5sccm air flowrate, 70% water vapour concentration and at a constant current of 0.55A measured a H2:N2 ratio of 1:2.2, which is still far from the desired ratio of 3:1 necessary for ammonia synthesis. Hence, the electrode performance still has to be significantly enhanced to achieve the required ratio.