Design And Research Of A Novel Triple-conductive Air Electrode Material For Solid Oxide Cells

The rapid development of economy and the fast-consumption of fossil energy have caused serious energy shortage and environmental pollution.In this context,the development and utilization of green sustainable energy is of great significance.As a type advanced energy conversion device famous for high efficiency,fuel flexibility,low emission and no need for precious metal catalysts,solid oxide cells（SOC）are attracting increasing attention.SOCs possess two reversible operating modes,namely fuel cell（FC）mode and electrolysis cell（EC）mode.In FC mode,it converts chemical energy of fuels such as hydrogen directly into electricity,while in EC mode,it can store electricity into high value-added chemicals such as hydrogen.In theory,the two modes can be switched smoothly,making it great convenience for energy storage and utilization.In order to ensure the sufficient ionic conductivity of electrolyte and the high electrochemical reaction activity of electrode,SOC is generally operated at high temperatures,which inevitably raises the problems of considerable cost and poor stability.And thus,the trend toward developing low-to-intermediate temperature SOC is encouraging.Compared to the traditional SOC based on the oxygen-ion conducting electrolyte,protonic ceramic cell（PCC）using proton conducting electrolyte exhibits more favorable performance at lowered temperatures,which results from the much higher mobility of protons than that of oxygen-ions.In addition,the formation or the introduction of water takes place at the air electrode side in PCC,which avoids the dilution of fuel gas（product gas）and the oxidation of Ni-based anodes that happened in traditional SOC.Developing novel air electrode materials with high catalytic activity is the key for the commercialization of PCC,which contributes the main polarization resistance at lowered temperatures.As the right sites for proton involved oxygen reduction reaction（p-ORR）and proton involved oxygen evolution reaction（p-OER）,an ideal PCC air electrode should possess triple conduction of proton,oxygen-ion,and electron,which can expand the active sites to the whole air electrode surface,and thus accelerate reaction rates.Unfortunately,such triple conductive air electrode materials are still rare.Based on this,this thesis focuses on the design and development of new durable triple-conductive air electrode materials.The specific chapters are as follows:The first chapter is an introduction part,in which we firstly introduce the working modes,working principles,polarization losses and corresponding optimization schemes of SOC.Then the key materials of SOC are emphatically summarized.Finally,the research basis and content of this thesis are presented.In second chapter,the key research methods of electrochemical performance are introduced in detail,including conductivity relaxation method,electrochemical impedance spectroscopy and relaxation time distribution.In third chapter,BaCo_0.4Fe_0.4Zr_0.1Sc_0.1O_3-δ（BCFZSc）air electrode material was proposed and prepared by regulating proton conductance via Sc doping.Investigation results show that:（1）BCFZSc possesses a standard cubic perovskite structure,and exhibits good phase stability under high water-containing atmosphere;（2）BCFZSc is bibulous and protonable both at room temperature and operating temperature,and the content of hydroxyl groups and adsorbed oxygen（OH-/O2）on its surface is the highest among all surface oxygen species;（3）BCFZSc shows excellent oxygen chemical surface exchange coefficient and bulk diffusion coefficient,which can be up to 1.8 ×10^-3 cm s^-1 and 1.516 × 10^-4 cm² s^-1 at 700℃ respectively.In the fuel cell mode,the peak power density of a single cell using this air electrode at 700℃ is 1.334 W cm^-2,and the corresponding polarization resistance is only 0.044 Ω cm².In the electrolysis cell mode,the current density of the same cell at 700℃ and 1.3 V can reach the value of 2.43 A cm^-2.These results confirm that BCFZSc is a promising PCC tripleconductive air electrode material.In fourth chapter,SrEu₂Fe₂O_7-δ（SEF）with 0.5 wt.%load Pt air electrode material（0.5Pt-SEF）was designed and fabricated based on SEF triple conductive material combined with Pt single atom loading method.Investigation results show that:（1）0.5Pt-SEF possesses a two-layer R-P phase structure,and shows excellent phase stability in water-containing atmosphere at high temperature;（2）Pt partially enters the lattice by replacing Fe at the B-site,and the rest is anchored to the surface as a single atom;（3）When applied 0.5Pt-SEF as air eletrode,the peak power density of the single cell at 600℃ is increased by 107.1%;（4）Pt loading increases the oxygen adsorption and dissociation capacity of SEF.The fifth chapter briefly summarizes the research content of this paper,and makes a deeper prospect based on the deficiencies in the research.