Research On Redox Performance Of Solid Oxide Fuel Cells

Solid Oxide Fuel Cell(SOFC)is a new type of energy conversion device with high energy efficiency,zero pollution and great application prospect.The microstructure of SOFC changes after redox cycle,which has an irreversible and harmful effect on the mechanical stability of the overall structure of the battery.In this thesis,by means of scanning electron microscope,transmission electron microscope and X-ray micron CT technique,the electrolyte cracking caused by strain accumulation of fuel cell was studied by analyzing the oxidation resistance of single-layer anode battery,and the Ni diffusion mechanism of Ni-YSZ(nickel-yttrium stabilized zirconia)anode cell during redox process was discussed.The main results are as follows:1.The redox thermal cycle reaction process is used to simulate the operation of the battery,that is,the Ni-YSZ anode is repeatedly oxidized during cooling and reduced during heating.The changes of micro-morphology and energy spectrum of the anode layer of SOFC semi-cell before and after the reaction between Ni and electrolyte were observed,and the fuel cell was studied in terms of phase analysis,residual stress,thermal expansion and so on.The results show that the microstructure of the Ni-YSZ anode has changed,such as Ni particles converging,electrolyte cracking,etc.,and the performance degradation is obvious.2.In the process of redox thermal cycle,it is found that the mobility of Ni is faster than that of oxygen in the oxidation process,and nickel oxide will fo rm a flocculent structure.YSZ is an oxygen conductor material,in which there will be a large number of oxygen vacancies.At the same time,because of the structural characteristics of YSZ,there will be clusters or defects enrichment(Y element doping enrichment and oxygen vacancy enrichment)near the polycrystalline interface.The first thing that occurs in the oxidation process is the adsorption,dissociation and ionization of oxygen molecules,which are diffused by outward cations,accompanied by the formation of isolated pores,and the resulting oxide nuclei grow laterally until hitting the YSZ framework.3.Through the combination of focused ion beam and transmission electron microscope,the microstructure evolution of Ni-YSZ anode in the same region during thermal cycle was studied for the first time.The results show that there are two mechanisms for the reduction of the contact interface of Ni-YSZ during the thermal cycle.the first is that grain growth leads to the diffusion of Ni into the metal,a nd the second is the mismatch of the thermal expansion coefficients of Ni and YSZ,which leads to deformation.in addition,the aggregation effect observed during the thermal cycle operation is relatively small compared with the operation site.After four thermal cycles,the contact area of Ni-YSZ is reduced by about 4%.However,although delamination continues to occur after each thermal cycle,the degree of delamination is not affected by the heating rate.The above research results will be helpful to reveal the degradation mechanism of SOFC and provide a basis for improving battery performance and ultimately prolonging the device life.