Mathematical Modeling of Solid Oxide Steam Electrolyzer for Hydrogen Production

This dissertation, "Mathematical Modeling of Solid Oxide Steam Electrolyzer for Hydrogen Production" by Meng, Ni, 倪萌, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled Mathematical modeling of solid oxide steam electrolyzer for hydrogen production Submitted by Ni Meng for the degree of Doctor of Philosophy at the University of Hong Kong in July 2007 Water electrolysis is an energy-efficient and viable technology for hydrogen production from water. When integrating electrolyzers with solar or wind power plant, hydrogen can be produced in a renewable and clean manner. Solid oxide steam electrolyzers (SOSEs) can produce hydrogen at a fast electrochemical reaction rate and reduced electrical consumption. This study evaluates the effect of important operating and design parameters on the performance of a SOSE and advances design of SOSEs to enhance their performance. A macro-level electrochemical model is developed to analyze the important current-voltage (J-V) characteristics of a SOSE for hydrogen production. The Butler-Volmer equation, Fick's law, Darcy's law, and Ohm's law are applied to characterize the activation, concentration, and ohmic overpotentials respectively. The theoretical model is validated, as the simulation results agree well with the experimental data recorded in previous studies. Parametric analyses of SOSE performance were conducted. In the study of the component thickness effect, anode- support SOSE configuration is identified as the most favorable design. During operation, the performance of SOSE can be enhanced by increasing the temperature and steam molar fraction, or by regulating the pressure. It is also found that i increasing the electrode porosity and pore size can reduce the concentration overpotentials. A micro-level model is developed to investigate the coupled transport/electrochemical reactions. The generalized Butler-Volmer equation, the Dusty Gas Model (DGM), Darcy's law, and Ohm's law are employed to determine the transport of electrons/ions and gas species as well as the electrochemical reactions. The resulting differential equations are solved numerically. The effect of particle size on SOSE potential is studied, with due consideration being given to SOSE activation and concentration overpotentials. Optimal particle size that can minimize the SOSE potential is obtained. Decreasing the electrode porosity is found to monotonically decrease the SOSE potential. As the electrochemical reactions mainly take place in a thin layer near the EE interface, advanced designs with micro- structurally graded electrodes are proposed. With small particles near the electrode- electrolyte (EE) interface and large particles at the outer layer, the novel design shows significant performance improvement. To further investigate the thermodynamic performance and to pinpoint the major losses of a SOSE system, an energy and exergy analysis is conducted. Under typical operation conditions, the SOSE works in a thermoneutral mode because the heat production due to overpotential losses is equal to the thermal energy needed for steam splitting hydrogen production. With waste heat recovery, the energy and exergy efficiency can be enhanced by regulating the operating current density, the steam conversion rate, and the operating temperature. The study provides an insight into the influence of various operating and design parameters on SOSE performance and can be a useful tool for SOSE design ii optimization. Finally, future research tasks to produce hydrogen cleanly and efficiently by SOSE are sugg