Structure Evolution,Mechanism And Application Of SrFeO₃ Based Perovskite Anode

The demand of energy especially electricity has become more and more urgent since the whole world are experiencing a fast development. Compared with the traditional coalbased power generation, fuel cell can convert the chemical energy in fuels directly into electricity which is an efficient, clean and environmental friendly energy conversion technology. Solid Oxide Fuel Cells（SOFC） possess the advantages of high temperature operation and fuel flexibility, in which the hydrocarbon fuel can be directly used such as natural gas, syngas, coal gas and biogas, etc. SOFC based power generation has the potential to be the most effective approach for China to improve the efficiency of carbonbased fossil fuels and minimize the pollutions. However, the current Ni-YSZ（8% mol Y2O3 stabilized ZrO2） based anode can easily suffer coking when using methane based hydrocarbon fuels in the long term operation. The substitution of Ni by（La,Sr）CrO3,（La,Sr）TiO3 and Sr2MxMo1-xO6 based perovskite oxides can improve the stability of SOFC anode under hydrocarbons, however most of these materials show low catalytic activity, resulting the low cell performance. To obtain stable operation of SOFC under hydrocarbons, the development of perovskite anodes with high electrochemical catalytic activity has become one of the most important missions.The introduction of highly activated catalysts into the perovskite oxide anodes through infiltration is one of the most widely applied methods to prepare the high performance electrodes in SOFC. However, the infiltration technique is time-consuming and is difficult to be well controlled to make the catalysts uniformly deposited. Furthermore, the infiltrated nano catalyst particles can easily suffer serious coarsening in the high temperature operation, resulting the cell performance degradation. Recent research results show that, after treating in the reducing atmosphere, several perovskite oxide materials can in-situ precipitate nano metal particles scattering on the surface, resulting highly active catalyst material system. The in-situ precipitation technique possesses the advantages of cost effective and preparation-time-saving compared with the other surface modification methods like infiltration. The in-situ precipitated nano metal particles have decent catalytic activity towards hydrocarbons, which can decrease the anode polarization resistance and increase the power output of SOFC. However, most of the research work only focus on the（La,Sr）CrO3 and（La,Sr）TiO3 based perovskite material, these material possess high stability and acceptable electrical conductivity in reducing atmosphere but without enough oxygen ion conductivity due to the absence of transition metal doping, which resulting the application of these materials together with YSZ based electrolyte materials. Furthermore, the current work only focus on the precipitataion phenomenon of the transition metals like Fe, Co, Ni, Ru and Pd without showing any precipitation mechanisms nor the influences on the oxide anodes’ structure and performance along with the metal precipitataion process.In this study, we did systematically research on the B site metal precipitation process, material structure evolution, mechanism and application with the selected two SrFeO₃-based perovskite anode materials of La0.4Sr0.6Co0.2Fe0.7Nb0.1O_3-δ and SrTi_0.3Fe_0.7O_3-δ with high electronic/oxygen ionic conductivity by the second transition metal doping with Co or Ni and also the introduction of A-site deficiency. As a result, the high performance Sr0.95Ti_0.3(Fe_0.9Ni_0.1)0.7O_3-δ anode material was developed, the structure and performance of La0.4Sr0.6Co0.2Fe0.7Nb0.1O_3-δ anode was optimized, the hydrogen oxidation mechanism model for perovskite SOFC anodes was developed and was successfully applied to explain some of the results in this study and literatures. Moreover, the CO₂ in-situ reforming of CH₄ was successfully achieved at Co-Fe alloy in-situ precipitated LSCFN based anode, long and stable operation of Ni based anode cell under methane was successfully achieved with new catalyst loading approach.Results show that the B site transition metals in perovskite oxide anode exhibited the exsolution-favorable phenomenon with the doping of a second reducible transition metal. Under the same reducing condition（850 oC, 3% H2O-97% H2）, the perovskite structure of SrTi_0.3Fe0.7O_3-δ（STF） kept stable, however, the exsolution of Co/Fe metal were detected with the doping of Co in SrTi_0.3（Fe,Co）0.7O_3-δ（STFC）. The exsoluted metal was mainly existed as（Co,Fe） alloy under a low doping amount of Co; however, when the Co doping amount was higher, the exsoluted metal only showed the phase of Co. Also, with the increase of Co dopant in STF, the ABO3 perovskite structure of STFC slowly disappeared and finally turned into the structure of A2BO4 layered perovskite. Simultaneously, the precipitated（Fe,Ni） nano alloy particles were observed after the small amount doping of Ni in STF. Results in this study and from the literatures suggested that the transition metal element with higher atomic number in the same period showed the tendency of exsolution-favorable, i.e. Ni>Co>Fe and Pd>Ru.In the La0.8Sr0.2Ga0.83Mg0.17O_3-δ（LSGM） electrolyte supported STF anode cell, with decreasing pH2, the OCV decreased and there appeared to be a decreasing limiting current density and cell performance, which suggested that the hydrogen adsorption could be a possible rate-limiting step in perovskite oxide anode. Compared with STF anode cell, the SrTi_0.3(Fe_0.9Ni_0.1)0.7O_3-δ anode cell with in-situ precipitated（Fe,Ni） nano particles showed small polarization resistance and relatively high cell performance even at very low pH2. A model accounting for hydrogen adsorption and electrochemical oxidation as possible rate-limiting steps is developed and was successfully used to fit current-voltage and EIS results from the cells in this study and literatures. Results show that, the dissociative hydrogen adsorption became an increasingly important rate-limiting step as decreasing temperature and hydrogen partial pressure. The model results also showed that the in-situ precipitated metal particles on the anode surface could accelerate the dissociative H2 adsorption, and promote the cell performance, which was meaningful to the application of perovskite anodes in the large SOFC single cell and SOFC stacks in helping to improve the fuel utilization efficiency and thermal balancing.Results show that, the introduction of A-site deficiency in the perovskite anode materials can suppress the formation of AO phase during the metal precipitation process, improve the anode performance and increase the metal precipitation process simultaneously. Results show that the A-site deficient Sr0.95Ti_0.3(Fe_0.9Ni_0.1)0.7O_3-δ（STFN-951） anode bulk structure kept well after releasing（Fe,Ni） alloy particles without obvious SrO based second phases formation. The maximum power density of STFN-951 anode cell with LSGM electrolyte reached 1.3 and 0.71 W/cm2 at the temperature of 850 and 750 oC respectively. In the case of A-site deficient（La0.4Sr0.6）0.9Co0.2Fe0.7Nb0.1O_3-δ（LSCFN-90） perovskite oxide anode, only small amount of A2BO4 structure phase was formed after releasing Co-Fe alloy in room temperature humidified H2 after the introduction of A-site deficiency compared with the stoichiometric composition by suppressing the formation of（La,Sr）O based phases, which reduced the possible damage to the perovskite structure caused by phase change in the reduction process, and the CoFe alloy precipitation process was also accelerated. The LSCFN-90 anode cell with LSGM electrolyte showed much better performance especially with methane fuel, the maximum power density reached 0.33 W/cm2 under wet methane at 850 oC. Results show that, the introduction of A-site deficiency was demonstrated an effective approach to optimize the perovskite structure and anode performance for the perovskite oxide anodes with B-site metal precipitation. Furthermore, the B-site metal precipitation phenomenon and A-site deficiency method were demonstrated to be generally applicable in the SrFeO₃ based perovskite anodes.Results show that, decent cell performance and stability with in-situ nano catalyst precipitated Sr FeO3 based perovskite anodes were achieved under hydrocarbon fuels. Long term operation of Sr0.95Ti_0.3(Fe_0.9Ni_0.1)0.7O_3-δ anode cell with（Fe,Ni） alloy precipitation was achieved, no obvious degradation was observed during the 200 hours operation. The in-situ reforming of methane using carbon dioxide was successfully achieved at the Co-Fe precipitated LSCFN based anode using 10% CO₂-90% CH₄ as fuel. The maximum power density at 850 oC reached 0.455 W/cm2 compared with 0.28 W/cm2 under pure methane. Furthermore, a new methane reforming catalyst loading method was designed with the segmented-in-series type cell, through which the long term stable operation with methane fuel was achieved using Ni-YSZ based anode cell.