Structure And Properties Of Perovskite-Based Electrode Materials For Intermediate-Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell(SOFC)is a type of all-solid-state power generation device that can directly convert chemical energy into electrical energy efficiently and environmentally,which is playing an increasingly important role in alleviating the energy crisis and environmental pollution.However,the commercialization of SOFC technology is severely hampered by high costs and limited system life due to high operating temperatures(>850℃).Thus,it is urgent to reduce the operating temperature of SOFC to intermediate-temperature range(IT;600～800℃).However,the decrease in operation temperature is always accompanied by a sharp increase in the electrode polarization resistance,leading to a deterioration in the performance of the entire cell.Therefore,the development of high-efficiency,stable,and low-cost electrode materials in the intermediate-temperature region is critical to the commercialization of SOFC.This thesis mainly focuses on the perovskite-based electrode materials,exploring the main factors affecting the catalytic activity and stability of the electrodes by the combination of experiments and theoretical calculations,and further develops the potential electrode materials for IT-SOFC.Firstly,to investigate the effect of rare earth doping on the crystal structure and oxygen reduction reaction(ORR)activity of the simple perovskite cathodes BaCo0.7Fe0.3O3-δ(BCF),a series of rare earth-doped simple perovskite cathodes LnxBa1-xCo0.7Fe0.3O3-δ(Ln=La,Pr,Nd,x=0.1,0.2)are synthesized successfully.The experimental results show that an appropriate amount of rare earth doping can transform the low symmetrical hexagonal phase of BCF into a highly symmetrical cubic phase.The obtained compounds exhibit significantly enhanced ORR activity in comparison with the parent compound BCF.Among them,the sample has the highest electrocatalytic activity when the doping amount of Pr is x=0.1.This cathode reaches area specific resistance of 0.026 and 0.038 Ω cm2 at 700 and 650℃,respectively.The anode-supported single cell using H2 as fuel reaches peak power densities of 1236.4 and 905.9 mW cm-2 at 700 and 650℃,respectively,while mataining excellent stability(600℃,～150 h).Therefore,Pr0.1Ba0.9Co0.7FeO.3O3-δ is a promising cathode material for IT-SOFC.The first-principles calculation results reveal that the enhancement of the ORR activity can be ascribed to the promoted oxygen vacancy formation and oxygen adsorption-dissociation process due to rare earth doping,which are consistent with the experimental results.This work can provide theoretical guidance and experimental basis for the application of rare earth-doped single perovskite cathodes in SOFC.Next,to improve the ORR activity and stability of the typical perovskite cathode La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF)in the intermediate-temperature range,we develop a heterosrtuctured composite nanofiber cathode LSCF/CeO2.The cathode is composed of LSCF and CeO2 nanoparticels,and more heterointerfaces are formed between the two phases,which exhibits remarkably enhanced ORR activity and durability as compared to single LSCF powder and nanofibers.This cathode achieves a polarization resistance of 0.031 Ω cm2 at 700℃,only 1/5 of that for the LSCF powder cathode(～0.158Ωcm2).The maximum power density of the anode-supported single cell using H2 as fuel can reach 1221.6 mW cm-2 at 700℃,which is much higher than that of the LSCF powder cathode(861.8 mW cm-2).Such enhancement can be attributed to the continuous paths provided by nanofibers for efficient mass/charge transport and the interdiffusion of La and Ce at the heterointerface which leads to more oxygen vacancy formation.Furthermore,the anode-supported cell with the LSCF/CeO2 composite cathode shows excellent long-term stability(600℃,～200 h)because of suppression of Sr segregation in LSCF by introducing CeO2 and the structure of heterogeneous nanofibers.This finding may provide a new strategy for the microstructure design of highly active and robust ORR catalysts in SOFCs.Then,to investigate the effect of the amount of B-site elements on the crystal structure,thermal expansion behavior and electrochemical performance of the double perovskite cathode,a series of the double perovskite cathodes Sr2Co1+xMo1-xO6-δ(x=0,0.3,0.5)are synthesized successfully.The experimental results show that the obtained compounds have tetragonal structure with space group 14/m(No.87).The introduction of more Co ions decreases the ordering of B-site cations and increases the oxygen vacancy concentration,significantly enhancing the electrical and ionic conductivity of the material.Among them,the sample has the lowest polarization resistance of 0.03 Ωcm2 at 800℃ when the amount of Co introduced was x=0.5.The electrolyte-supported single cell using H2 as fuel reaches peak power density of 602 mW cm-2 at 800℃,while mataining excellent stability(700 ℃,～60 h).First-principles calculation results reveal that the introduction of Co obviously decreases the oxygen vacancy formation energy of the material,facilitating the formation of oxygen vacancies,which is consistent with the experimental results,and further explain the enhanced electrochemical performance.Finally,to develop high-efficiency and stable Ni-free anode materials,series of the metal nanoparticle-decorated double perovskite anodes Sr2CoMo1-xFexO6-δ(x=0,0.05,0.1)are synthesized by in situ exsolution.Under a high-temperature reducing condition,Co-Fe alloy nanoparticles with rich multiple-twinned defects are exsolved in situ on the Fe-doped double perovskite anode surface,which significantly enhances its catalytic activity towards the oxidation of fuel gas.When the doping amount of Fe is x=0.05,the sample exhibits the highest catalytic activity.The maximum power densities of the single cell using H2 and CH4 as fuel can reach 992.9 and 652.3 mW cm-2 at 800℃,respectively,while maintaining excellent stability(700℃,～50 h).Furthermore,the reduced anode also demonstrates excellent coking resistance in CH4,which can be attributed to the increased oxygen vacancies due to Fe doping and the effective catalysis of multiple-twinned Co-Fe alloy nanoparticles for reforming of CH4 to H2 and CO.In summary,in situ exsolved multiple-twinned Co-Fe nanoparticles can dramatically enhance the anode performance and coking resistance and may provide a new insight for the design of highly active and robust electrocatalysts in many fields,including SOFCs,steam reforming of natural gas,water splitting,and so on.