Fabrication And Eletrochemical Performance Of Solid Oxide Cells Supported On Electrodes With Straight Pores

Solid oxide cells（SOCs）are all-solid-state electrochemical energy conversion devices with the advantages of safety,high efficiency,clean,and ultra-quietness.They have two operating modes:in the solid oxide fuel cells（SOFCs）mode,the fuels can be used to generate electricity;in the solid oxide electrolysis cells（SOECs）mode,the greenhouse gas CO₂ can be converted into CO and O2 by using renewable energy.Nevertheless,traditional Ni-based cermet fuel electrodes exhibit some shortcomings,such as poor oxidation resistance and low mechanical strength,limiting the development of SOCs.Perovskite oxide electrode-supported SOCs are widely studied because of their excellent redox stability and high mechanical strength.However,compared to Ni-based electrode,the catalytic activity and electronic conductivity of perovskite oxide electrode are relatively low.Therefore,in this thesis,the phaseinversion tape casting method was used to prepare ceramic-based electrodes with welloriented pores,and the electrodes were modified with catalysts to obtain highperformance all-ceramic SOCs.Furthermore,the oxidation resistance of Ni-based electrodes was improved.Chapter 1 mainly introduces the development history,operating principles of SOCs and the electrode preparation methods.Finally,the main contents of this thesis are proposed.In Chapter 2,the CO₂ electrolysis performance of the lanthanum chromium ferrite cathode was improved through modification with metal nanoparticle catalysts.NiFe nanopartciels were exsolved fromLa_0.75Sr_0.25Cr_0.5Fe_0.5O_3-δ（LSCrF） SOECs (La_0.75Sr_0.25)_0.95Cr_0.5Fe_0.35Ni_0.15O_3-δ（LSCrFN）upon exposure to hydrogen at 800℃.For comparison,NiFe nanoparticles were also be deposited on La0.7sSr0.25Cr0.5Fe0.5O3-δ（LSCrF）using the infiltration method.When used as a supporting cathode for SOECs,the one with the exsolved nanoparticles exhibited a higher CO₂ electrolysis current density than the one with the infiltrated nanoparticles than the one without the nanoparticles,e.g.,their corresponding current densities at 1.5 V and 800℃ were 1.15,0.80,0.59 A cm^-2.The electrode with the exsolved nanoparticles also demonstrated much better durability than that with the infiltrated nanoparticles.Tested at 1 V and 800℃,the current density of the former decreased from 0.66 to 0.63 A cm^-2 during a period of 260 h,i.e.,0.012%/h,and the nanoparticles remained well dispersed after the test.In contrast,for the latter,the current density dropped from 0.33 to 0.29 A cm^-2 within 26 h,i.e.,0.15%/h,and severe agglomeration of the nanoparticles occurred.It is concluded that the perovskite oxides modified with the exsolved metal nanoparticles possess both high electrocatalytic activity and stability,promising for use as the supporting cathode for SOECs.In Chapter 3,the effects of the infiltrated nano-catalyst and the pore structure of the electrode supports on the CO₂ electrolysis performance were investigated.Two LSCrF-YSZ electrode supports with different structures were prepared by phaseinversion tape casting method and conventional tape casting method,respectively.The former contained open straight pores,while the latter contained tortuous pores.On this basis,two electrode-supported symmetric cells were prepared.The cell configuration was LSCrF-YSZ/YSZ/LSCrF-YSZ.The electrode supports were modified with Sr2Fe1.5Mo0.5O6-δ（SFM）nanoparticles.The current density of the bare cell with open straight pores was 0.32 A cm^-2 at 800℃ and 1.5 V,higher than the cell with tortuous pores(0.25 A cm^-2).After infiltrated with 15 wt.%SFM,the current density of the former was 1.54 A cm^-2,which was significantly higher than that of the latter(1.18 A cm^-2).Apperantly,the infiltrated SFM nanoparticles and the open straight pores in the electrodes significantly improved CO₂ electrolysis performance.In addition,the cell infiltrated with SFM operated stably in CO₂-CO gas,showing superior coking resistance.Chapter 4 demonstrated that cells with the configuration of LSCrFYSZ/YSZ/LSCrF-YSZ can also be used for electrochemical pump oxygen（EOP）device.The electrochemical performance of EOP was significantly improved through infiltration with SFM nanoparticles.At 1 V and 750℃,the current density of the SFMmodified EOP was 1.48 A cm^-2,much higher than the unmodified EOP(0.30 A cm^-2).Moreover,the SFM nanoparticles greatly enhanced the sulfur resistance of EOP by preventing the direct contact of SO2 with LSCrF-YSZ electrode.At 750℃,the current density of the SFM-modified EOP could still reach 1.33 A·cm^-2 in an atmosphere containing 10 ppm SO2,and the performance only decreased by about 10%after a period of 120 h.However,the performance of the unmodified EOP decayed by about 45%after running for about 14 h.In Chapter 5,the enhancement effects of the infiltrated Cu nanoparticles on the electrochemical performance of SOFCs were investigated.A 3Y-TZP electrodesupported symmetric cell was prepared by phase-inversion tape casting and multilayer lamination technology.The air electrode was modified with SFM nanoparticles;and the fuel electrode was modified with SFM and Cu nanoparticles.The cell infiltrated with Cu nanoparticles exhibited a maximum power density of 898 mW cm^-2 at 700℃,which was much higher than the cell without Cu nanoparticles(525 mW cm^-2).Such a large performance improvement was owing to the enhanced electronic conductivity of the ceramic-based fuel electrode by infiltrating Cu.In addition,after 16 redox cycles,the performance of the symmetric SOFCs only degraded by about 13.5%,showing excellent redox cycling performance.In Chapter 6,a Ni-Fe alloy layer in combination with a cermet layer composed of Ni and yttria-stabilized zirconia（YSZ）cermet layer was explored as an anode for solid oxide fuel cells（SOFCs）.The cell supported on the dual-layered anode with straight pore paths showed a maximum power density of 1070 mW cm^-2 at 800℃,while 737 mW cm^-2 for the one supported on the anode with tortuous pore paths.Electrochemical impedance measurement and distribution of relaxation time（DRT）analysis revealed that the straight pore paths allowed fast gas phase transport thus mitigating the concentration polarization,and improved the accessibility of electrochemical reaction sites hence reducing the activation polarization.The cell supported on the Ni-YSZ/Ni-Fe dual-layered anode remained intact after 8 redox cycles,whereas the cell supported on the Ni-YSZ single layered anode failed after one redox cycle.It is concluded that the Ni-YSZ/Ni-Fe dual-layered composite explored in the present study is suitable for use as the supporting anode for SOFCs.In Chapter 7,the summary and prospects of this thesis are presented.In addition,CO₂ was used to modify the commercial graphite anode of lithium-ion batteries in Appendix A to enable it to have super fast charging capability.