Design And Characterization Of Iron-based Perovskite Fuel Electrode For Solid Oxide Cells

Solid oxide cell(SOC)is an all-solid-state energy converter device that can realize direct and efficient conversion between chemical energy and electrical energy,showing the advantages of flexible operation,environmental friendliness,low CO2 emissions,etc.When operated in solid oxide fuel cell(SOFC)mode,the device can convert the chemical energy of fossil fuel into electrical energy;when operated reversely in solid oxide electrolysis cell(SOEC)mode,it can directly electrolyze CO2 and H2O using electrical energy generated from renewable energy to synthesize fuels such as CO and H2.Therefore,SOC is one of the important technologies to help China achieve carbon emissions peak,carbon neutrality,and the rational use of energy.However,the commercial application of SOC still faces many challenges,especially when operating at high temperatures.Developing high-performance and stable electrode materials is an important method to reduce the working temperature of SOC and promote its commercial application.Traditional Ni-based cermet fuel electrodes exhibit high catalytic activity,but carbon deposition can easily occur when using hydrocarbon fuels in SOFC mode.In SOEC mode,additional reducing gas is required to prevent Ni from being oxidized.In addition,the Ni-based cermets show poor redox cycling stability.In recent years,perovskite oxide fuel electrodes with ABO3 structures,showing excellent resistance to carbon deposition and redox cycle stability,have been widely studied.Among them,iron-based perovskite oxides have great advantages in catalytic activity,thermal compatibility and raw material cost,and they are the most promising to replace Ni-based cermets.However,the electrochemical performance of iron-based perovskite fuel electrodes should be further improved for commercial applications.Based on this,the representative iron-based perovskite fuel electrodes Sr2Fe1.5Mo0.5O6-δ(SFM)and La0.5Sr0.5FeO3-δ(LSF)are studied in this dissertation,aiming to further improve their electrochemical performance and stability.The main contents are as follows:(1)Nontransition metal element Sb doping at the B-site to enhance the electrochemical performance of SFM in hydrogen and hydrocarbon fuels.(2)F anion doping at the Osite combined with in-situ exsolution to optimize the bulk and surface properties of SFM to improve the electrochemical performance for direct CO2 electrolysis.(3)F anion doping at the O-site to improve the electrochemical performance and stability of LSF perovskite fuel electrode for direct CO2 electrolysis.(4)Mo doping at the B-site to improve the electrochemical performance and enhance the stability of LSF for CO2H2O co-electrolysis,and attempts to couple the Fischer-Tropsch reaction in a tubular SOEC to directly and efficiently synthesize hydrocarbon chemicals such as CH4.The first chapter introduces the working principle and basic theory of SOC,systematically summarizes the functional characteristics,advantages and disadvantages,and research status of key SOC materials(air electrode,electrolyte and fuel electrode),and finally proposes the topic selection and research content of this dissertation.In chapter 2,the non-transition metal element Sb is adopted to partially replace Mo element at the B-site of SFM to improve its catalytic activity as a fuel electrode.DFT calculations and experiments show that Sb doping reduces the oxygen vacancy formation energy and increases the oxygen vacancy concentration by～1 1.2%at 800 ℃.In addition,for H2 oxidation reaction,Sb doping increases the oxygen exchange coefficient(kchem)by～64.8%,increases the oxygen diffusion coefficient(Dchem)by～44.9%,and reduces the interface polarization resistance(Rp)by～59%at 800℃.Therefore,the SOFC performance is significantly improved by doping Sb,increasing by 35%to 920 mW cm-2 at 800℃ with H2 as the fuel,increasing by 47%to 765 mW cm-2 with syngas as the fuel,and increasing by 45%to 557 mW cm-2 with ethanol as the fuel,all of which achieve stable operation.In chapter 3,F doping at the O-site of SFM perovskite combined with in-situ exsolution strategy is designed to improve the performance of SFM fuel electrode for direct CO2 electrolysis in SOEC.F doping and the exsolved Ni-Fe nanoalloy enhance the CO2 adsorption capacity by 2.4 times,significantly improve the kchem and Dchem of CO2 reduction reaction(CO2RR),decrease the Rp by approximately 52%from 0.64 Ωcm2 to 0.31 Ω cm2,and achieve a high current density of 2.66 A cm-2(800 ℃,1.5 V)and stable operation for 140 h.In chapter 4,aiming at the problems of low performance and insufficient stability of LSF perovskite as a fuel electrode,the strategy of F doping at the O-site is used to improve the catalytic activity and chemical stability of SOEC for direct CO2 electrolysis.F doping promotes the formation of oxygen vacancies,improves the CO2 adsorption capacity and CO2RR kinetics,significantly increases the kchem from 3.49 × 10-4 cm s-1 to 6.24 ×10-4 cm s-1 and the Dchem from 4.68 ×10-5 cm2 s-1 to 9.45 ×10-5 cm2 s-1,and decreases Rp from 0.226 Ω cm2 to 0.108 Ω cm2 at 800℃.The cell with F-doped LSF as the fuel electrode achieves a high performance of 2.58 A cm-2(800℃.1.5 V)and stable operation for 200 h.In chapter 5,the strategy of high-valence Mo doping at the B-site is adopted to improve the chemical stability of LSF perovskite as a fuel electrode in various complex atmospheres containing CO2,CO and H2,etc.The Mo-doped LSF(Mo0.125-LSF)is proven to be a perovskite oxide electrode for CO2-H2O co-electrolysis with suitable electrochemical performance and excellent stability.Furthermore,the Mo0.125-LSF fuel electrode is coupled with Ni@ZrO2 nanocatalyst in a tubular SOEC supported by YSZ electrolyte(length≈10 cm)prepared by a dip-coating method for direct synthesis of CH4.In the long tube reactor,the SOEC co-electrolysis reaction at a high-temperature zone and the Ni@ZrO2 catalytic methane reaction at a low-temperature zone are separated to realize the efficient synthesis of CH4 with high stability.Through a special coupling design,the CO2 conversion and CH4 selectivity are nearly 100%,and the CH4 yield is as high as 30.4%in the SOEC reactor.The performance and stability are significantly better than those reported in the literature.Chapter 6 systematically summarizes the major conclusions and deficiencies of this dissertation,and proposes possible research fields in SOC.