Analyzing3D Microstructure and Performance of Fuel Cell Anode by LBM
|School||University of Science and Technology of China|
|Course||Synchrotron Radiation and Applications|
|Keywords||X-ray imaging system optical path adjustment lattice Boltzmannmethod solid oxide fuel cell size distribution gas diffusion simulation|
The lattice Boltzmann method is a mesoscopic model, which is a new fluid simulation method between macro-continuity model and microscopic molecular dynamics model. Fluid is a discrete system composed of a large number of particles and each particle does irregular thermal motion. They exchange momentum and energy by frequent collisions among the particles. The fluid, physical area and time are discretized into a series of fluid particles, lattice and time steps by Lattice Boltzmann method, separately. Lattice Boltzmann method describes the statistical behavior of the molecular and obtain macroscopic physical quantities by studying temporal and spatial evolution of the local particle distribution function. Lattice Boltzmann method has been successfully used in many areas, such as porous media flow, chemically reactive flows, biological fluid, micro-and nano-scale flow and so on. The solid oxide fuel cell anode is a kind of micro-and nano-size porous media and anode porous structure directly affects the diffusion of the fuel gas and the electrochemical reaction product, thus affecting the electrochemical performance of the solid oxide fuel cell. Based on the hard X-ray micro-imaging experiment platform constructed by National Synchrotron Radiation Laboratory in Hefei, we have successfully obtained the three-dimensional structure of the solid oxide fuel cell electrode and gradually established a more systematic analysis method. The work carried out in this paper contains the following aspects:1. Design parameters of the main optical devices in hard X-ray imaging station and simulation of X-ray microscopic images for improved alignmentThe design parameters of the main optics devices in nano-S100imaging system established on the U7A experimental station at National Synchrotron Radiation Laboratory were introduced. The adjustment process of the optical path in hard X-ray imaging system needed before experiments was described, as well as some existing problems. Fermat principle combined with ray tracing method was used to simulate the spot distribution in the object plane and image plane after X-ray passing the ellipsoidal focusing mirror. The effect on imaging spot distribution was analyzed when the ellipsoidal focusing mirror was translated or tilted during adjustment process and the simulated results were used to guide the alignment adjustment of the imaging system. The simulation of X-ray imaging system adjustment provided effective guidance for the actual experimental operation, which helped to improve the work efficiency of the optical path adjustments.2. Inner-connectivity and particle size analysis of the solid oxide fuel cell anodeA series of projection data of Ni-YSZ anode could be obtained from X-ray imaging system and a three-dimensional structure after Ni, YSZ, Pore three-phase segmented was achieved by the back-projection reconstruction software and Amira software. Three-dimentional data analysis methods were developed to analyze the volume fractions of connected and unconnected phase as well as the connectivity of each phase. Only the pore phase transporting gas, the YSZ phase transporting ion and the Ni phase transporting electron are all connected, the chemical reaction at the three-phase boundary is able to continue. Therefore, three-dimentional structure parameters such as connectivity can reflect the performance of the solid oxide fuel cell anode.How to use Lattice Boltzmann method analyze the three-dimensional structure of the solid oxide fuel cell anode was introduced and the related software was developed. The data of three-dimensional structure of the Ni-YSZ anode could be obtained by Nano-CT imaging technique. The D3Q19model was established in each of the voxels, so that a set of self-consistent direction vector was formed in the three-dimensional space. These direction vectors could detecte characteristic diameters of each phase and phase size distribution function could be derived from the data statistics and some formula. Study on phase size distribution function could provide guidance for optimization of three-dimensional structure of the anode.3. Lattice Boltzmann method for simulating the gas diffusion in Ni-YSZ anodeSolid oxide fuel cell performance is closely related to the transmission of gas in the battery. The depletion of reactants or the accumulation of products at three-phase boundary will lead to an degradation of the battery performance. Simulating the gas diffusion process in the Ni-YSZ anode by lattice Boltzmann method is able to more realistically reflect the distribution of gas in the three-dimensional structure of the anode and the value of the concentration polarization, and so on.Lattice Boltzmann method for simulating single gas diffusion in a two-dimensional structure, three gas diffusion in the two-dimensional structure and three gas diffusion in the three-dimensional structure was described, including the physical model, calculation formulas, program design and simulated results. And Ni-YSZ anode structures with different connected porosity were simulated. The simulated results of different Ni-YSZ anode structures under the same current density and the same Ni-YSZ anode structure at different current densities were compared. The effects on gas diffusion and concentration polarization caused by connected porosity, operating current density were discussed, respectively.Ni particles gradually gathered into more chunks of solids when Ni-YSZ anode was in the process of thermal cycling, resulting in the reduction of the active three-phase boundary density within the anode. The gas diffusion in Ni-YSZ anode structures undergone different thermal-cycle times was simulated using Lattice Boltzmann method. The three-phase boundary density was used to determine the conditions of the operating current density to make the physical model closer to the real situation.