Redox Kineticsof Ni-based Anode Materials For Solid Oxide Fuel Cells

Anodes manufactured from NiO and yttria-stabilized zirconia(Y2O3 doped ZrO2,YSZ)powders are state-of-the-art for solid oxide fuel cells(SOFCs)because they are easy to manufacture and havehigh performance.The SOFCs were required to work on a long-term basis for the commercialization.However,when the fuel supply was interrupted,oxygen would continue to diffuse through the electrolyte,or the imperfect seals,which result in the reoxidation of the metallic nickel anode.This reoxidation is not only associated with a volume expansion,but also with significantly structural changes in the anodemicrostructure,generating stresses in the anode and the electrolyte.These stresses can exceed the stability of the components,potentially promoting crack growth,which leads to degradation of the SOFC or complete failure.Hence,the single cell was expected to go through several redox cycles when used on a long-term basis.In order to promote the redox performance of SOFC,this Ph.D.thesis focuses on three aspects,namely,studying the redox mechanism and process of NiO particles,probing of the reoxidation stability of Ni-Fe anode material and studying the effects of Fe addition in SOFC anode functional layer(AFL)on the redox performance.According to our researches,the main results are obtained as follows:In the first chapter,we briefly introduced the principles and characteristics of SOFC,and listed the key materials,such as anode,electrolyte,cathode,interconnector and so on.Hot issues on development of SOFC anode were mentioned and main researches in this dissertation were shown.In addition,we presented the redox stability of SOFC,summarized how to evaluate the cell performance and what electrochemical processes can influence the performances.At the same time,we briefly introduced the high temperature oxidation kinetics of metal,and the development of Wagner theory was mentioned emphatically.We focused on the oxidation process of Ni and introduced that the rate of oxidationof nickel is controlled by the outward diffusion of nickel cations by a vacancy mechanism through the NiO barrier layer.Ni ions diffuse in NiO scale through the different diffusion paths(the lattice,grain boundaries or dislocations)during the oxidation process.The dominating diffusion path of Ni in NiO scale depends on the temperature.In thesecond chapter,we focused on the oxidation process and redox stability of Ni particles reduced at the temperature from 600 to 750?C.Meanwhile,Thermal gravimetric analysis(TGA)in combination with microstructure analysis is used for probing the oxidation processes and oxidation kinetics of Ni particles.The modified parabolic law equation was used to describe the oxidation process of particles,which is not only a good expression of relationship between the degree of oxidation and time changing,but also can be better accord with the experiment results.In the third chapter,we mainly studied the oxidation and diffusion mechanisms of small permalloy particles with different Fe contentby using thermal gravimetric analysis(TGA)and microstructure characterizations.Microstructure analysis indicates Fe2O3/(Ni,Fe)3O4 plays a key role in the morphology evolution and diffusion mechanisms of small NiFe particles upon oxidation.Constant temperature TGA indicates that the grain boundary diffusion dominates the oxidation process at low temperatures.The activation energy of grain boundary diffusion for the NiFe alloys increases as the Fe content increases from 0 to 50 wt.%.We have developed a diffusion process resolved temperature programmed oxidation(DPR-TPO)analysis method,and three diffusion mechanisms are recognized by using this method.In addition to the well-known grain boundary diffusion and lattice diffusion,our TGA analysis suggests that the phase conversion from Fe2O3 to(Ni,Fe)3O4 will also induce diffusion changes and affect the diffusion process at the intermediate temperatures.NiFe alloys with?5-10 wt.%Fe content have the lowest oxidation rates among all the samples.This work not only gives an insight knowledge into the oxidation and diffusion processes of small NiFe alloy particles,but also presents an analysis method as a useful tool for analyzing solid-gas reactions of other materials.In the fourth chapter,we mainly introduced the effects of Fe addition in the Ni-YSZ anode functional layer(AFL)on the redoxstability performance of SOFC.During the experiment,we firstly study the effects of Fe2O3 addition in the AFL on the structure and composition before and after reduction.XRD and SEM analyses confirmed that NiFe alloys were formed under the operation conditions of the SOFC.Meanwhile,we also study the oxidation process of NiFe-YSZ particles.The result indicated that the redox stability was enhanced by doping of Fe,which inhibit the diffusion of Ni2+outward through the oxide scale.The result of redox cycles and powder density curves show that performance and redox stability of SOFC was improved by addition of-5%Fe content.