Study Of Electrode Stability For Intermediate Temperature Solid Oxide Fuel Cells

With the rapid development of electronic information industry,the demand for electrical energy has increased dramatically.Unfortunately,the current electrical energy is mainly produced by the combustion of fossil fuels,which not only has a low conversion rate,but also produces a large number of pollutants,causing serious environmental problems.Solid Oxide Fuel Cells?SOFCs?are a kind of clean energy conversion device,which can realize the efficient conversion of chemical energy to electrical energy without the limitation of Carnot cycle.However,the traditional SOFCs device usually works at high temperatures?800-1000??,which limits material selection and cost control.In recent years,researchers are committed to the development of high-performance SOFCs at intermediate temperatures?500-700??to achieve the goal of commercialization.Yet,reducing temperature brings about the rapid decrease of catalysts activity,which leads to large polarization resistance and significant decrease of performance for SOFC.Such polarization loss mainly comes from the sluggish oxygen reduction reactions?ORR?in the cathode.In order to improve the performance at intermediate temperature,researchers are committed to develop new cathode materials,and find that Co based perovskite materials exhibit excellent catalytic activity.Among them,BSCF shows the highest oxygen ion conductivity and high ORR catalytic activity.Nevertheless,the researchers also found that BSCF will change from cubic perovskite structure to hexagonal perovskite structure at intermediate temperature,and generate inert BaCO₃ phase in the atmosphere containing CO₂.The poor stability limits its practical applications.Moreover,one of the advantages of SOFCs is its great fuel flexibility,and the application of methane fuel in SOFCs can improve the energy conversion rate.Unfortunately,for traditional Ni based anodes,the extremely high catalytic activity for methane cracking leads to rapid carbon deposition on the surface of Ni and deactivation of the catalyst.The main challenge is to improve the carbon resistance of direct methane fueled SOFCs with Ni based anodes.Based on the above problems,this thesis used anionic and cationic doping to inhibit the phase transition and thus to improve the stability of BSCF at intermediate temperature.Moreover,nano MgO was exsoluted to improve the stability of Ni based anodes in direct methane fuel.The reaction processes of such anode in methane fuel was studied by synchrotron vacuum ultraviolet photoionization mass spectra?SVUV-PIMS?and temperature changing X-ray photoelectron spectroscopy.This thesis can be divided into five chapters,and the main contents are as follows:Chapter 1 mainly introduces the research background,working principle and possible polarization loss of SOFCs.It also summarizes the basic requirements and research status of cathode materials,anode materials and electrolyte materials of SOFCs,and the research methods of SOFCs are also introduced.In the last,the research themes of this thesis are put forward.In Chapter 2,nano-layer MgO decorated Ni-based anode is prepared by in-situ reduction of Ni0.9-xCu0.1MgxO solid solution.Peak power density of 670 mW cm^-2 at 700? in humid methane is achieved using Ni_0.875Cu_0.1-0.025MgO/Sm_0.2Ce_0.8O₂ anode thanks to the improved active surface and the special modulation effect of MgO nano layer on anode reactions.Interestingly,synchrotron vacuum ultraviolet photoionization mass spectra and high-temperature X-ray photoelectron spectra jointly suggest that the effect of MgO on carbon resistance differs with the operating temperatures,which accelerates the steam reforming of CH4 via improving dissociative adsorption of acidic gas H2O at?500?,while depresses CH₄ cracking to carbon and improves the formation of light olefins at?700?.In addition,possible methane reaction paths over such anode are derived.In chapter 3,anion substitutions,such as F and Cl,can effectively improve the stability of proton conducting electrolytes at no expense of proton conduction.However,during operation,F-and Cl-in electrolytes can transfer to cathodes,which reduces the stability of electrolytes.In this work,F--doped Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-?(Ba_0.5Sr_0.5Co_0.8Fe_0.2O_2.9-?F_0.1)was prepared as potential cathode for proton ceramic fuel cells with BaCe0.8Sm0.2F0.1O2.85 electrolyte.It was found that F-incorporation into cathode can depress F-diffusion from electrolyte and improve the stability of single cells.Temperature changing X-ray photoelectron spectroscopy and electronic conductivity relaxation results demonstrate that F-incorporation enhances oxygen incorporation kinetics at intermediate temperatures and thus improve cathode catalytic performance.Moreover,button cell using this novel cathode operates steadily for 270 hours at current density of 300 mA cm-2 and 700?,much superior to those with Ba_0.5Sr_0.5C_o0.8Fe_0.2O_3-? cathode.In chapter 4,the effect of La³⁺partially substituting for Sr2+on the performance and stability of BSCF is studied.X-ray diffraction?XRD?results indicate that the introduction of La³⁺can reduce the cell parameters of BSCF,which is beneficial to improve its electrical conductivity.The electrical conductivity of La_0.1Ba_0.5Sr_0.4Co_0.8Fe_0.2O_3-??LBSCF?can reach 70 S cm-1 at 700? in air,about 100%larger than that of BSCF.Moreover,La³⁺substitution can obviously reduce thermal expansion coefficient?TEC?of BSCF,which is beneficial to decrease stress caused by the thermal mismatch of cathode and electrolyte.Chemical stability test suggests that LBSCF powders maintain stable and no impurity peaks were detected,however,the sample of BSCF present some peaks corresponding to hexagonal perovskite.The single cell with this novel cathode material presents an excellent electrochemical performance,and the peak power density can reach 550 mW cm^-2 at 700?.In Chapter 5,summarize the whole work,and give suggestions on the future study.