Computational modeling investigation of electrolyte materials in solid oxide electrolyser cell
Date of Issue2019-01-28
School of Materials Science and Engineering
Energy Research Institute @NTU
Solid oxide electrolyser cell (SOEC) is regarded as a promising device for future energy storage and for the deduction of carbon dioxide emission. This thesis aims at finding ways to help improve the long-term stability of SOEC from theoretical perspective. With the background and motivation discussed, we focus on improving the cell performance and seeking for better understanding in the electrolyte materials, especially in the electrolyte material yttria stabilized zirconia (YSZ) and gadolinium doped ceria (GDC). We find that the maximum ionic conductivity of YSZ may come from the competitive interaction between the positive effect from the increase of oxygen vacancies and the negative effect from the increasing of yttrium ions. We also find that oxygen ion diffusion is hindered by the existence of GB in YSZ. With further investigation, we find that the oxygen ions diffuse faster along the GB interface than across it. The results suggest that GB shall be avoided in synthesizing if possible, and that the direction of GB interface shall be aligned parallel to the ion conducting direction to improve cell performance. A minimal layer thickness requirement is found on the thickness of the GDC interlayer, which blocks reactions between electrolyte and electrode. The minimal thickness is found to be ~10nm and ~2µm under 700℃ and 1000℃, which provides useful advices for researchers in the designing and synthesizing of SOEC components. A relationship between the lattice parameter and overall thickness of CeO_x laminated structure is given as a tool for researchers to estimate the oxygen conducting property based on the thickness of GDC interlayer. Electronic structures and work function in ceria (CeO_2) and GDC are studied. Doping of gadolinium significantly decreases the band gap of ceria, while thin film structure of GDC raises up the band gap a lot. The existence of oxygen vacancy is found to be responsible for the decrease in band gap and helps explain the electronic conductivity of GDC under low oxygen partial pressure. In conclusion, the results of this thesis help improve the understanding in SOEC electrolyte materials and provide useful information and advices to researchers who are interested in designing and making SOECs with improved performance.