Design And Characterization Of Sr₃Fe₂O₇ Based Air Electrode For Solid Oxide Cells

Energy and environment are two hot issues today.Consumption of fossil fuels,such as coal and oil,in traditional energy conversion systems,not only leads to low energy conversion efficiency,but also causes serious environmental pollution.And therefore,developing new energy sources and/or new energy conversion devices is urgently needed.Solid Oxide Cell（SOC）is a kind of green and efficient energy conversion device with two operating modes.One is the Fuel Cell（FC）mode,which directly converts chemical energy of fuels into electric energy for utilization;and the other is the Electrolysis Cell（EC）model,which directly converts electrical energy into chemical energy for storage.FC and EC are the inverse processes,and the two working modes can be realized on the same cell,which brings great convenience for energy utilization and storage.For cells with Ni-based fuel electrode,the polarization loss in both FC and EC modes is mainly from the air electrode.And thus,to improve the performance of cells and to lower the operating temperatures,it is of great significance to design excellent air electrode materials.In recent years,proton-conducting SOC has been widely concerned because it is more suitable for low and medium temperature operation.Conventional perovskite and double perovskite materials suffer from the large polarization loss due to the absence of proton defects and the instability in EC mode.Ruddlesden-Popper（R-P）material Sr₃Fe₂O_7-δ（SFO）is a candidate for intermediate temperature SOC air electrode materials thanks to its special crystal structure and excellent electrochemical performance,yet also has low structure stability in high-steam involved atmosphere and low activity toward oxygen reduction reaction.Based on these properties,this thesis focuses on improving Sr₃Fe₂O_7-δ’s structure stabilities and catalytic activities by A and/or B site substituting.Main results are listed as follows:Chapter 1 introduces the research background,the working principle of SOC,and the development of electrode and electrolyte materials.The performance of the air electrode materials for proton conducting solid oxide cell（P-SOC）are summarized,and the research methods and analytical methods of electrochemical performance are described.At last,the theme of this thesis is proposed.In Chapter 2,oxygen ion conduction and electrochemical properties of SFO were characterized using both density functional theory（DFT）and experimental methods.Density functional theory calculation suggested that SFO has extremely low oxygen vacancy formation energy at the intersection of perovskite layer and considerable energy barrier for O^2-diffusion,especially when along the[010]direction.Unfortunately,Sr-O surface,the most stable surface of SFO,was demonstrated to be inert to O₂ adsorption and dissociation reaction,and thus restricts its catalytic activity toward oxygen reduction reaction（ORR）.Based on this observation,Co partially substituted SFO（SFCO）was then synthesized and applied to improve its surface vacancy concentration to accelerate the oxygen adsorptive reduction reaction rate.The oxygen surface exchange coefficient（k^δ）and oxygen bulk diffusion coefficient（D^δ）are 1.2×10^-4 cm s^-1 and 6.0×10^-6 cm² s^-1,respectively,meausred 650 ℃.Electrochemical performance results suggested that the cell using the SFCO single phase cathode has a low polarization resistance（0.106 t cm² at 650 ℃）and high power density(peak power density of 685 mW cm^-2 at 650 ℃).In chapter 3,SFO has extremely low structure stability in high steam involved atmosphere.Based on the idea of basicity regulation,Eu₂O₃ has higher acidity than SrO and thus Eu was selected to partially substitute Sr in Sr₃Fe₂O_7-δ(SrEu₂Fe₂O_7-δ,SEF)to improve its stability in high steam involved atmosphere.The XRD refinement results showed very limited oxygen vacancy detected in SEF,and therefore,Co substitution strategy was used to improve oxygen vacancies and enhanced its catalytic activity.Scanning transmission electron microscopy（STEM）pictures demonstrated that Eu elements take the rock salt layer,while Sr occupy the center of perovkiste slab.Using BaZr_0.3Ce_0.5Y_0.2O_3-δ（BZCY）as electrolyte and SEFC as air electrode,our P-RSOC demonstrates great electrochemical performance,offering a good discharging current of 432 mA cm^-2 at 0.7 V and 700℃,and a high electrolysis current of 2058 mA cm^-2 at 1.5 V and 700 ℃.Espcially,our P-RSOC presents outstanding stability.Neglectable performance loss is observed during a 230 hours’long term operation a wet H₂（～3%H₂O）/10%H₂O-air atmospheres as a electrolysis cell.Moreover,the cell operates smoothly for 135 hours switching between FC mode and EC mode with no degradation observed,and sustains over 10 thermal cycling tests.All these indicate that SEFC is a great air electrode material for SOC.It should be also noted that both bulk and polarization resistances of cells in EC mode are lower than those in FC mode.Enhanced electronic conduction in electrolyte under electrolysis voltages should account for this phenomenon,which leads to the depressed Faraday efficiency.In chapter 4,based on the studies in chapter 3,serious electronic leakage were detected in the BZCY electrolytes when operating in EC mode,which resulted in low Faraday efficiency.Unfortunately,no studies about the effects of such electronic leakage on electrode polarization reestances were ever reported.In this work,we put forward an equivalent circuit model which takes into account the electronic conduction in electrolyte and can be used to evaluate the intrinsic electrode polarization resistances.With this model,the corresponding Rp,r of each rate-determining step can be calculated.Comparing Rp,r with the apparent polarization resistance（RP）,the influence of electronic leakage on each rate-determining step and electrolysis efficiency can be evaluated.With Sr_2.5La_0.2Fe₂O_7-δ（SLF）as the stable cathode material,the polarization resistances of an electrolysis cell were systematically studied using this equivalent circuit model.The results indicated that the electronic leakage is more serious with the increase of testing temperatures and applied electrolysis voltages,and that the increase in pH₂O in air electrode benefits electrolysis performance.Importantly,the rate－determining steps change correspondingly with the different working conditions,indicating that electrolysis voltages have different impact factors on each rate-limiting steps.In Chapter 5,to further improve the electrocatalytic activity of air electrode materials,Sr₃EuFe₃O_10-δ（3-SEF）,a new three-layer R-P material,was successfully prepared.And Sr₃EuFe_3-xCo_xO_10-δ（x=0.0,0.5,1.0,1.5）（3-SEFC,）were prepared to investigate the influence of Co contents on the structure and properties.The results demonstrated that the 3-SEFCx materials indeed have a three-layer structure,and that with the increase of Co doping amount,material particles become larger suggesting improved sintering activity.Symmetric cells with BZCY as electrolyte and 3-SEFCx as air electrodes were prepared and measured in the wet air（～3%H2O）.It is found that the Rp decreases sharply when the Co doping amount increases to 0.5,and then decreases slowly with the increased Co content.Taking 3-SEFC0.5 as air electrode,the peak power density of the single cell reaches around 900 mW cm^-2,and the Rp is 0.030Ω cm².This result seems to suggest that 3-layered R-P oxide is more efficient than 2-layered R-P oxide as air electrode in SOC,which may root in the anisotropy in electronic and ionic conduction in R-P structures.