A First-principles Study On Divergent Reactions Of Using Sr₃Fe₂O₇ Cathode In Solid Oxide Fuel Cells

Solid oxide fuel cell(SOFC)is a cleaning energy conversion equipment with high efficiency,which can directly transfer chemical energy from fuels to electrical energy.Application of SOFC is helpful for solving the energy and environmental problems,which has attracted widespread attention.However,the commercialized steps of SOFC system are always sluggish,resulting from the large polarization resistance of electrode(especially cathode),which greatly impedes the improvement of electrical performance.In this paper,the reaction processes of over the surface of Sr3Fe2O7(SFO)cathode with R-P structure were studied for both O-SOFCs and P-SOFCs on the atomic scale.It aims to determine the reaction speed control step and provide theoretical guidance for the development of effective oxide cathode materials for SOFCs.The main results of this work are as follows:Chapter 1:The related theories of SOFCs were summarized and the basic concepts of theoretical calculation were introduced.Firstly,we introduced the characteristics of SOFCs and the research status of anode,electrolyte and cathode materials.Then,from the perspective of experiment and calculation,we described the research status of the cathode reduction reaction on O-SOFCs and P-SOFCs in detail.Next,we briefly introduced the theory and basic concept of the calculation method used in our study.Finally,the research topics of this thesis were introduced.Chapter 2:using the first-principles approach,we studied systematically the reduction reactions on Sr3Fe2O7(SFO)cathode surfaces and compared with those over surface of perovskite cathode,La0.5Sr0.5Co0.25Feo.75O3(LSCF).The effect of different crystal structures on the surface reactions was investigated.Compared with LSCF,the low formation energies of oxygen vacancies and low migration energy barriers for oxygen ions in SFO made it more preferable for oxygen conduction which was essential for cathode reactions in O-SOFCs.Nevertheless,a large energy barrier(2.19 eV)was predicted for oxygen dissociation reaction over SFO(001)surface,while zero barrier over LSCF(001)surface.This result clearly indicated that SFO showed a weaker activity toward the oxygen dissociation,which may root in its low surface energies,low vacancy formation energies and specific R-P structure.Interestingly,in P-SOFCs,the presence of protons on SFO(001)surface could largely depress the energy barriers to around 1.46～1.58 eV Moreover,surface protons benefited the oxygen adsorption over SFO(001)surface.This result together with the extremely low formation energies and migration energy barriers for protons seem to suggest that SFO could work more effectively in P-SOFCs than in O-SOFCs.It also suggested that too many protons at SFO surface will lead to high energy barriers for the water formation process,and thus that the proper concentration of proton,oxygen vacancy and electron defects in the test environment may be crucial to the battery performance.Chapter 3:we summarized the innovation and shortcoming of this thesis and prospected the future research on the SFO surface reaction.