The role of cathode in solid oxide electrolyzer cells
Lim, Kevin Chee Kuan
Date of Issue2018-02-07
Interdisciplinary Graduate School (IGS)
Energy Research Institute @ NTU
Solid oxide electrolyzer cells (SOECs) which serve as a highly efficient and viable technology for energy storage, have drawn great attention in the recent years, especially with the continuous rise of renewable energy usage. However, conventional SOECs using state-of-the-art materials are sensitive to contaminants and require ultra-pure feedstock gas which increases their operating cost significantly. Therefore, it is desirable for SOECs to utilize cheaper and more abundant resources such as seawater or industrial flue gas as feedstock gas. This thesis investigates the direct use of seawater electrolysis, the development of a sulfurtolerant cathode/fuel electrode and the modeling of SOECs based on simulated flue gas environment in the cathode. State-of-the-art SOECs are studied for direct electrolysis of water vapor produced from seawater using button cells with Nickel-(Yttria-stabilized-Zirconia) (Ni-YSZ) cathode. Using an ion chromatograph mass spectrometry, water vapor produced from seawater is found to be free of the contaminants, which are present in the seawater. Current-voltage curves, AC impedance spectroscopy and galvanostatic tests are used to evaluate the electrochemical performance and durability of the cell. Similar cell performance is observed using water vapor produced from pure water and seawater. Their short-term degradation rates are similar, which are registered at 15% 1 OOOh- 1 for both cases. A button cell with its cathode impregnated with sea salt is used to investigate the effects of direct sea salt contamination in SOECs. Both the uncontaminated and contaminated cells exhibit rather similar performance as observed from the current-voltage curves and impedance spectra. The difference in area specific resistances between the uncontaminated and contaminated cell are all within a 10% range. Rather similar short-term degradation rates of 15% 1000 h-1 and 16% 1000 h-1 are recorded for the uncontaminated and contaminated cells, respectively. Post-mortem analysis using scanning electron microscopy and energy-dispersive X-ray spectroscopy show that the sea salt impregnated into the cell has been vaporized at a typical SOEC operating temperature of 800°C over the period of operation. Lao.1sSro.2sCro.sMno.sOJ-.S (LSCM)-based composites are studied as the cathode of SOECs for direct co-electrolysis of water vapor and carbon dioxide in industrial flue gas. The electrochemical behaviors of the composite cathodes are investigated using three-electrode electrolysis cells with Pt as counter and reference electrodes. Electrochemical results from current-voltage curves and AC impedance spectroscopy among pure LSCM, LSCM(Gadolinia-doped-Ceria) (GDC), LSCM-YSZ and LSCM-(GDC-YSZ) have shown that LSCM-GDC exhibits the highest water vapor electrolysis performance. The ratio between LSCM and GDC is further optimized and it is shown that the LSCM-GDC with 50-50 wt.% for each component exhibits the highest performance. A 60-40 wt.% Ni-YSZ state-of-the-art cathode is used to benchmark with the optimized LSCM composite cathode. It is shown that the optimized LSCM-GDC cathode exhibits better performance for water vapor electrolysis. Under a cathodic current of -0.1 A cm-2, the optimized LSCM-GDC cathode shows much slower degradation, about 10 times slower as compared to the Ni-YSZ cathode when exposed to 10 ppm of sulfur dioxide for up to 72 h. The optimized LSCM-GDC cathode also shows promising performance for co-electrolysis of water vapor and carbon dioxide at high current densities and stable performance under simulated flue gas condition with 5 ppm of sulfur dioxide and 6.5% oxygen in the feedstock gas. Further impregnation of and Palladium by infiltration into the cathode shows an overall increase in co-electrolysis performance. A two-dimensional SOEC model using COMSOL Multiphysics modeling software is used to investigate the production rates of hydrogen and carbon monoxide as well as the gas molar distributions in the cell during operation under flue gas condition, where the results are being compared with pure co-electrolysis of water vapor and carbon dioxide. Under a certain bias, it is found that the hydrogen and carbon monoxide molar fractions increase along the cathode channel in both cases, but the rate of increase is much slower in the case of simulated flue gas condition as the feedstock gas. This is due to the chemical reaction with oxygen which is present in flue gas, which causes their production rates to be lower than the case of pure water vapor and carbon dioxide. The molar fractions of water vapor and carbon dioxide are of the opposite, where they decrease along the cathode channel, but less significantly in the case of simulated flue gas condition. The oxygen molar fraction decreases along the cathode channel and increases more significantly along the anode channel under simulated flue gas condition.