Struture And Properties Of Perovskite-based Electrode Materials For Solid Oxide Fuel Cells

As electrochemical devices that directly convert the chemical energy of fuels into electricity, solid oxide fuel cells (SOFCs) have attracted lots of attention due to their cleanness, high efficiency, fuel flexibility and all solid-state structure, and been regarded as the most commercial potential energy conversion technology. Ni/YSZ cermet is the state-of the-art anode material for SOFCs, which tends to degrade with the direct use of hydrocarbon fuels due to the fatal problems including carbon deposition and sulphur poisoning. Therefore, novel alternative anode materials with strong structural stability, high peroformance and good tolerance to carbon deposition/sulfur poisoning are urgently in demand. Perovskite oxides have been widely investigated as potential anode materials due to suitable structural stability under reducing atmosphere and good resistance against carbon deposition and sulfur poisoning. Unfortunately, there are still many problems for perovskite anode materials, including poor electrocatalytic activity, low ionic conductivity, structural instability towards redox cycling and insufficient power output. To address these issues, systematical investigations were carried out in this work.From the viewpoint of defect chemistry, crystallography and element chemistry, we designed and prepared a novel anode material Sr2FeMo2/3Mg1/3O6-δ, which shows high redox stability, suitable thermal expansion coefficient, good electrical conductivity, fast oxygen surface exchange kinetics and excellent electrochemical performance. It has a structure type of Sr2(Mg1/3Fe2/3)(Fe1/3Mo2/3)O6-δ where Mg and Mo occupy different B-sites in double perovskite lattice due to the big difference between Mg and Mo ions in valence and radius. These structural characteristics render the transition metal Fe to take simultaneously two different B-sites, B and B’in A2BB’O6 double perovskite. The first-principles computation reveals that the presence of FeB-O-FeB-bonds decreases the formation and migration energy of oxygen vacancies, which is the key to boosting oxygen ion diffusion and oxygen surface exchange kinetics. In a electrolyte (300μm) supported single cell, the Sr2FeMo2/3Mg1/3O6-δ anode demonstrates excellent cell performance with maximum power density of 637,866 and 1044 mW cm-2 at 800,850 and 900℃, respectively. The designed Sr2FeMo2/3Mg1/3O6-δ is proven to be an attractive anode candidate for SOFCs.To get high performance anode material, in situ precipitation strategy was applied for the first time to the the double perovskite. We design and prepared Sr2FeMoo.65M0.3506-δ (SFMM, M-Co, Ni,) in air, which acts as a host ceramic to exsolve catalytic active metallic Co-Fe and Ni-Fe nanoparticles on its particle surface under cell operation condition. At the same time, the host ceramic partially transforms into Ruddlesden-Popper (RP) phase Sr3FeMoO7 and perovskite Sr(FeMo)O3. The host ceramic partially transforms into Ruddlesden-Popper (RP) phase Sr3FeMoO7 and perovskite Sr(FeMo)O3. This leads to the in situ generatation of metallic nanoparticle decorated perovskite anode. The prepared anode Sr2FeMoo.65M).3506 (M=Co, Ni) shows excellent catalytic activity towards the oxidation of H2 and CH4 fuels. The maximum power densities of electrolyte supported single cells with SFMCo and SFMNi anodes reach 820, and 960 mW cm-2 in H2, and 430 and 500 mW cm-2 in CH4 at 850℃, respectively. The prepared SFMCo and SFMNi materials are promising high performance anodes for SOFCs.The study reveals that SrMoO4, an insulating impurity commonly observed in preparation process of Mo-based double perovskites, can be converted into highly conductive SrMo03 under anode operating condition. By combining with electrolyte GDC, the SrMoO3-60GDC composite exhibits good electrochemical performance and structural stability as anode material.In order to fully understand the correlation between structure and properties for SOFC electrode materials, we try to convert anode material La0.3Sr0.7TiO3 to cathode material by substituting Co for Ti, forming La0.3Sr0.7Ti1-xCoxO3-δ(LSTC, x =0.3-0.6) materials. The experimental results demonstrate that LSTCs (x= 0.45-0.6) are potenal cathode materials. They have high electrical conductivity, good chemical compatibility with LSGM electrolyte and excellent catalytic activity towards oxygen reduction reaction (ORR). The polarization resistance of LSTCs (x= 0.45,0.6) is only 0.0575 and 0.0233Ω cm2 at 800℃, respectively. First-principles computation reveals that there is a linear interrelationship between the catalytic activity of LSTC and O 2p-band center, which can be used to predict the catalytic activity of cathode materials towards ORR, and further guide the design of high performance new electrode materials.