The Research On Effective Properties Of Solid Oxide Fuel Cell Electrode And The Establishment Of Multiphysics Model

Solid Oxide Fuel Cell(SOFC)is a power generation device that directly converts the chemical energy of fuel into electricity.Due to its low emission and high efficiency,SOFC has drawn wide attention under the background of energy and environmental crisis.With the improving in SOFC output performance and the decreasing of operation temperature,the concentration and activation polarization became more significant than before.The concentration polarization results from the gas transport resistance in the porous electrode and the uneven distribution of species in SOFC stack.The activation polarization,on the other hand,mainly relies on the amount of reaction site and its catalytic activity.Both concentration and activation polarization can be reduced by the modification on electrode microstructure and the optimization of stack design.On microscale electrode,since some microstructure parameters,such as pore tortuosity and reaction site length per unit volume have direct impacts on concentration and activation polarization,respectively,so,they must be correctly evaluated in order to find their decisive factors,which will guide us to reduce the polarization losses through modifying those factors.On the macroscale stack,on one hand,its flow pattern must be optimized in order to evenly distribute species in the stack,so that most reaction sites in the electrode can get enough reactants.On the other hand,its temperature profile must be elaborately controlled to promote the reaction site's catalytic activity,especially when the fuel is hydrocarbon gas,which will cause the cool region in the stack due to the endothermic reforming reaction.Focused on the aforementioned two topics,this master dissertation studied two important SOFC microstructure parameters: the pore tortuosity and the three phase boundary(TPB)length;then a full-scale multi-physics SOFC model was established.1.The tortuosity in traditional electrode: The tortuosity,which is defined as the ratio of the length of the average effective path to the shortest straight distance along the diffusion direction,characterizes the degree of twist and the difficulty while the gas passing through the porous electrode,so it has a significant influence on concentration polarization.The traditional electrode is based on powder sintering technique,the pores are randomly dispersed in the electrode.To experimentally reconstruct the real structure is labor and time consuming;the theoretical 3D spheres packing model is an alternative,however,this model always underestimates the tortuosity.Owing to the absence of a reliable tortuosity formula,the tortuosity values used in open literatures are always unreasonably or randomlydetermined,for instance,the tortuosity is sometimes regarded as adjustable parameters to match the experiment results.Given this fact,from the point of view of electrode microstructure,the 3D cube packing method was introduced to reconstruct the electrode structure.By combing the diffusion simulation with the theoretical derivation,a tortuosity formula was obtained which well agreed with the experimentally measured results.This formula clearly indicates that the tortuosity is depended on porosity so it should not be casually determined.Besides that,it is proved that the scale of the reconstructed representative volume element must be 15 times of the pore's characteristic length in order to be large enough to represent the actual electrode.2.The TPB in electrospun electrode: the reaction in SOFC happens at the TPB site where the electron-conducting phase,ion-conducting phase and pore meet with each other.The TPB length has a direct impact on activation polarization and the reaction rate,which will finally influence the cell output performance,especially with the decreasing of SOFC operation temperature,the activation polarization gradually becomes the bottleneck of cell performance improvement.In order to get more TPB,the impregnation method,which dipping the nanoparticles of one phase on the skeleton surface of the other phase is a typical way.In traditional impregnation,the skeleton particle is sphere-like,however,the fiber-like skeleton fabricated by electrospun method possesses more porosity and specific surface area,which attracts much attention.But,as a newly emerged electrode,the advantage of the electrospun electrode has not been theoretically explained although its superiority has been proved in the experiment,besides that,there is no formula to estimate the TPB length although it is an important input parameter for macro modeling.Based on the TPB calculation methods of other electrodes and the real micro-morphology of the electrospun electrode,the computer program for the percolation rate of impregnated particles was presented at first,then the TPB formula was derived for the electrospun electrode.After that,the relationship between the TPB length and its determining factors was analyzed.The results,on one hand,theoretically justified the superiority of electrospun electrode,on the other hand,proved that smaller particle and fiber size,also their ratio are favorable for increasing TPB length.3.The full-scale multi-physics model for SOFC: in macroscale stack,the uneven distribution of species in the stack will make partial TPB sites suffer from starvation and high activation polarization,which means that not all the reaction sites are fully utilized.Meanwhile,because the exothermal electrochemical reaction and the endothermal reforming reaction may happen in the stack simultaneously,the improper flow field designand species distribution will lead to uneven temperature profile,which will shorten the stack's lifespan.Those problems must be addressed immediately in order to realize the commercialization of SOFC.In the stack design,full-scale multi-physics model is a cost and time efficient tool,however,owing to the complex geometry structure design of SOFC stack and electrochemical reactions with strongly coupled charge,momentum,mass and energy transport processes,there are significant difficulties in establishing such models.So this study seeks to develop the full-scale multi-physics model based on CFD method.On structure,this model consists of the five layers cell,the interconnector,the seal frame and the manifold;on physics,the model includes the charge,momentum,mass and energy conservation equations.Some parameters are taken from the previous two topics.The ANSYS Fluent? together with user define function,user define scalar and user define memory was adopted as the simulation platform.With minimum simplifications,this model presented the distributions of various physical quantities,which opened the door for more upcoming studies.Finally,an innovated SOFC stack design was proposed,in which the fluid paths crossed with each other in order to enhance the heat transfer.With this design,the peak temperature and the temperature gradient can be reduced.