|M.Sc Student||Sakajio Michal|
|Subject||Metal Oxide Cathodes for High Temprature Electrolysis of CO2|
for Solar Fuel Production
|Department||Department of Materials Science and Engineering||Supervisor||Professor Avner Rothschild|
|Full Thesis text|
The growing demand for hydrocarbon fuels of finite reserves and the resultant greenhouse gas emission underline the need for new sustainable technologies that convert abundant and renewable sources to synthetic fuels with minimal carbon footprint. High temperature electrolysis of CO2 and H2O using solid oxide electrolysis cells (SOECs) heated by solar energy is a promising route to produce CO and H2. These gases can be used as feedstock for the production of various synthetic fuels.
From a thermodynamic point of view it is advantageous to perform endothermic reactions such as the dissociation of CO2 to CO and (1/2)O2 at high temperature since the heat, provided by solar energy, reduces the work that must be spent in order to dissociate CO2. Another important feature is the separation of the reaction products, CO and O2, before they recombine. This can be done by high temperature electrolysis wherein oxygen is removed from the reactor via a refractory solid state oxygen separation membrane such as YSZ. The challenge is fitting these membranes with refractory electrodes that are compatible with high temperature operation in CO2/CO mixtures (cathode) or in air (anode) and do not react with the membrane.
In this work I investigated La0.75Sr0.25Cr0.5Mn0.5O3 (LSCM) and La0.4Sr0.4TiO3 (LST) cathodes and a composite of La0.8Sr0.2MnO3 (LSM) and (ZrO2)0.92•(Y2O3)0.08 anode for high temperature electrolysis of CO2. The electrodes were prepared by conventional solid state synthesis routes. The electrodes were coated onto YSZ membranes by screen printing and sintered at 1200°C. The electrodes displayed good compatibility with the YSZ membrane in CO2/CO atmosphere at high temperatures (1000°C). The electrochemical properties of the electrolysis cells were examined at high temperatures (700-1000°C) in different CO2/CO gas mixtures by means of I-V and electrochemical impedance spectroscopy measurements. The LSCM cathodes were found to perform better than Pt/YSZ cermet. In addition, this material demonstrated excellent microstructural and electrochemical stability under long-term experiments.
The power conversion efficiency of an electrolysis cell comprising YSZ membrane with LSCM cathode and LSM/YSZ anode reached as high as 29% at 900°C. The major loss in the system was rather poor Faradaic efficiency. The most probable cause for the low Faradaic efficiency observed in this study is oxygen penetration through the walls of the YSZ tube. In attempt to resolve the oxygen leakage problem we examined an improved cell design where a disk-shaped YSZ membrane was joined to an alumina tube by a high temperature brazing process.