Lattice Boltzmann method (LBM) simulation of heterogeneous cathode of proton conducting solid oxide fuel cells (H-SOFCs) at pore scale

Abstract

Solid oxide fuel cell (SOFC) is considered to be one of the most promising candidates for future clean energy applications due to its high efficiency and fuel flexibility. Generally, anode supported SOFCs with thin oxygen ion conduction electrolyte (O-SOFCs) are mainly used in engineering applications and experimental tests since they can provide larger power density compared with proton conduction SOFCs (H-SOFC) under the same operating conditions. However, the operation temperature of O-SOFC is very high (750~1000°C), which consumes a large amount of thermal energy to produce certain electrical power. Therefore, it is necessary to develop novel electrode material configuration which has relatively high output capacity and can be operated at lower operating temperature to reduce thermal energy input, long-term operating costs and improve the overall SOFC efficiency. Electrode microstructure parameters (porosity, mean particle size, particle radius distribution and tortuosity factors,etc.) are known to have significant effects on the SOFC performance and durability. Lots of scholars paid close attention to simulations of performance affecting factors, and relationships between factors and performance are discussed in recent decades (section 1.3.2). However, there are almost not enough comprehensive quantitative studies on these affecting factors. In order to improve the performance of SOFC and lower its material cost and thermal energy loss, structure optimization of electrode and proper material configuration are necessary. This work investigates SOFC electrode by a series of experimental tests of H-SOFC cathode materials and 3 kinds of numerical modelings aiming at identifying the factors that affect the performance of electrodes and figuring out optimized SOFC electrode microstructures. The whole work consists of 3 main parts: chapter 2 focuses on porous electrode reconstruction and quantitative and qualitative analysis of performance affecting factors at pore scale; chapter 3 focuses on internal gas transport, momentum transport and interface reaction effects of O-SOFC anode and H-SOFC cathode by LBM; chapter 4 focuses on novel H-SOFC cathode material development. In chapter 2, 3D (three dimensional) microstructures of SOFC cathodes are reconstructed by random packing of spherical binary particles method with MATLAB program to figure out the performance influencing factors. Initial contacts of each two particles are point to point, then two numerical modeling method kMC (kinetic Monte Carlo) and MDM (morphological dilating method) are conducted in this work to simulate sintering process of electrode materials. Sintering conditions and geometrical parameters effects on SOFC electrode performance are presented in this work, and detailed relationships would be discussed. Results of MDM results show great agreement with that of kMC method cases. And this series of studies provide fundamental information on achieving optimized composite cathode microstructure. The results are very valuable for designing high performance SOFC cathodes. The described technique will allow one to design new improved structures and determine optimal SOFC composite cathodes at specific sintering stage. In chapter 3, to understand internal working mechanism of SOFC porous electrode, mass transport and momentum transport with reaction effects at the interfaces between electrode and electrolyte are simulated by lattice Boltzmann method (LBM). 2D (two dimensional) electrode images and 3D electrode geometries are derived from previous 3D electrode reconstructed microstructures slices in chapter 2. To narrow closed pore numbers caused by simulation dimension decrement, appropriate simplification process is necessary to ensure the same porosity with initial 3D structures. Two series of cases are set in 2D LBM simulations: porosity effects and geometry (graded porous electrode microstructures with different particle layers) influences. 2D with 2 species (O-SOFC anode: hydrogen, steam) and 2D with 3 species (H-SOFC cathode: oxygen, nitrogen, steam) are involved in this work. Furthermore, by applying LBM to porous O-SOFC anode (section 3.4.1) and H-SOFC cathode (section 3.4.2) with different porosities and various graded microstructures, effects of structural parameters on mass and momentum transport within electrode are investigated intuitively via 2D color images. Detailed 2D and 3D multicomponent mass transport and momentum transport simulations by LBM are conducted in this chapter. For the same gas inlet conditions, concentration distributions are similar for all porosities cases. Results suggest that the concentration of reactant gas ( hydrogen for anode; oxygen for cathode ) decreases as the porosity of electrode increases, and this phenomenon is more visible toward the interface between electrode and electrolyte (TPB, reaction occurs here). The designed graded electrode provides much wider passage for reacting gas and optimizes gas transport at the interface between electrode and electrolyte, which speeds up the consumption of reactants in TPB sites of interfaces and prompts the efficiency of fuel cells. Simulation results reveal that LBM is a highly efficient and "easy to use, easy to expand" method in complex porous media applications. In chapter 4, to further understand the working mechanism and its electrochemical output performance of proton-conducting solid oxide fuel cells (H-SOFCs) cathode, a series of experimental tests of novel configuration of composite cathode for H-SOFCs based on proton-conducting stable BZCY electrolyte is developed as a trial of new material development. The single button fuel cells with the configuration NiO-BZCY -- BZCY -- SFM-BZCY were manufactured and conducted under the temperature in range of 600~800{493}C with ambient air as oxidant and wet H2 (3 vol% H2O) as the fuel gas fed to cathode. While the symmetrical fuel cells by the structure of SFM-BZCY -- BZCY -- SFM-BZCY were also tested under the same intermediate temperature range. All the single fuel cells and symmetrical fuel cells were sintered under three different situations.