| Reversible Solid Oxide Cells(RSOCs)are the devices that can directly and reversibly convert chemical energy into electricity.There are two operation modes in SOCs,one is Solid Oxide Fuel Cell(SOFC)mode,in which the chemicals such as hydrogen and hydrocarbons can be converted into electricity,and the other is Solid Oxide Electrolysis Cell(SOEC)mode,which can convert electricity into high value-added chemicals such as hydrogen,oxygen,and carbon monoxide.RSOCs have several advantages,including high conversion efficiency,no noise,great fuel flexiablity,and no need of noble catalysts.Unfortunately,when practically adapting hydrocarbons as fuels,SOFCs encounter the challenge of the low out-put power density,the severe carbon deposition and/or the sulfur poisoning in fuel electrodes,which call for further research in developing novel catalysts and optimizing the microstructure of fuel electrodes to improve the cell performance and operating stability.Besides,high operation temperatures are generally needed when traditional oxygen ion conducting SOECs(O-SOECs)are used to electrolyze CO2.Such high operating temperatures lead to high requirements concerning the componets in RSOC stacks and thus increase the costs.Considering the fact that proton electrolytes are more suitable for operating at intermediate temperatures,researchers have been devoted to develop proton conducting RSOCs and some promising results have been achieved.Yet,the large polarization loss in the fuel electrode greatly impact the cell performance.As discussed above,the fuel electrodes is the right place to convert hydrocarbons,and plays key role in improving the electrochemical performance and operation stability of RSOCs.And therefore,revealing the working principles of fuel electrode,optimizing the electrode configuration and improving the catalytic activity of fuel electrodes are of key importance in future RSOCs'developments.To solve the problems of low performance,poor stability,and low tail gas utilizations when SOFC fueled with hydrocarbons,as well as the high working temperatures of electrolyze CO2 through SOECs.We optimizated the fuel electrode microstructures,improved catalysts performance through bulk dopings,and decorated electrode with nano catalysts.As a result,we obtained series promising results including obtaining unique fuel electrode microstructures with finger like open open pores,high performance SrTiO3 and CeO2 based catalysts,successfully electrolyzing CO2 through proton conducting solid oxide electrolysis cells(P-SOECs),and revealing working principles of fuel electrode based on electrochemical and catalytic tests results.This thesis can be divided into six chapters and the specific contents are as follows:The first chapter is an introduction part,in which we introduced the classifications of RSOCs,the working modes of RSOCs,the working principles,the fundamental materials,and the obstacles of fuel electrode when dealing with the hydrocarbons.We also summarized the strategies to improve the fuel electrode performance.In addition,the methods concerning electrochemical and catalytic characterizations were also discussed.At the end of this chapter,the theme of this thesis was proposed.The second chapter discussed the fabrication of novel anode supports using a phase inversion combined with tape casting method.The micro-structures of anode supports were characterized by scanning electron microscope(SEM)method.With this unique anode,a peak power density of 700 mW cm-2 was achieved at 700? when fueled with humidified hydrogen.For the first time,methanol was adapted as the fuel in proton conducting solid oxide fuel cells(P-SOFCs),a peak power density of 496 mW cm-2 was achieved at 700?,which is about 40%higher than that prepared via cold-pressed method.Importantly,the cells with such special anode microstructure worked stable in methanol fuels.The elementary reactions of such cell were also analyzed using distribution relaxation time(DRT)method as functions of hydrogen partial pressures,oxygen partial pressures,and operating temperatures.The tests results indicate that the anode polarization loss contributes greatly to the button cell performance.In the third chapter,proton conducting solid oxide electrolysis cells(P-SOECs)was used to electrolyze CO2.The electrochemical investigations suggested that CO2 injection in fuel electrode brought a slight decrease of polarization resistance,indicating that the addition of CO2 in fuel electrode actually facilitated the electrolysis reactions.A high electrolysis current density of 1.23 A cm-2 was achieved at 1.5V electrolysis voltage and 550?,which was much better than previous reports.The compositions of out-let gases of fuel electrode were confirmed and compared to the theoretical results via the thermal equilibrium calculations.We found that concentration of CO were higher than the theoretical one,suggesting that CO was more prone to be formed in SOECs.To reveal the reasons for this improvement,active species on the catalysts during CO2 conversions were explored using high temperature Raman,high resolution transmission electron microscope(HRTEM),in-situ diffuse reflectance FTIR spectroscopy(in situ DRIFTS),and the possible reaction routes for CO2 conversion were proposed based on the testing results.In the fourth chapter,the co-generation of electricity and olefins in P-SOFCs were explored.To improve the electrochemical performance and stability of button cell when fueling with hydrocarbons,a new catalyst of(Pr0.3Sr0.7)0.9Ni0.1Ti0.9O3 was developed.Nano nickel particles were exsolved after treating(Pr0.3Sr0.7)o.9Ni0.1Ti0.9O3 in reducing atmospheres.Such catalyst was proved to have high catalytic activity toward propane conversions and hydrogen production.Applying(Pr0.3Sr0.7)0.9Ni0.1Ti0.9O3 as the reforming layer over the fuel electrode,the button cell had a great peak power density of 450 mW cm-2 at 750? in 90%C3H8-10%H2O fuel,and operated almost stable for 50 hours.Compositions of the out-let gases as functions of operating temperatures,water contents,and discharge current densities were also investigated.Synchrotron vacuum ultraviolet photoionization mass spectrometer(SVUV-PIMS)was applied to reveal the gas phase free radicles during propane conversions,the tests results indicate that the propane conversions are mainly through free-radical cracking mechanism.And the operations of P-SOFC should accerelate the dehydrogenation reaction.In the last,the propane conversion processes as well as the improving mechanisms of such catalyst layer were proposed based on these investigations.In the fifth chapter,a Y0.08Zr0.92O2(YSZ)anode support was fabricated using phase inversion combined with tape casting method,and a new catalyst Ni0.08Co0.02Ce0.9O2 was impregnated over the anode support after Ni-Co particles were impregnated as current collector.Using YSZ as electrolyte and(La0.8Sr0.2)0.95MnO3-?-YSZ as cathode,the button cell showed a high peak power density of 730 mW cm-2 in humidified methane fuel,which was very close to that in hydrogen fuel.Moreover,the cell operated stable for 140 hours in humidified methane fuel,demonstrating remarkable stability in hydrocarbon fuels.In addition,the oxygen contained species during the conversions were characterized using in-situ DRIFTS and SVUV-PIMS,the results indicate that the oxygen species should be the key to the high catalytic performance of Ni0.08Co0.02Ce0.9O2,and the unique microstructures of anode not only benefits infilitration processes,but also facilitates anode reactions and improves button cell performance in the end.In the sixth chapter,the innovation points and research results of this thesis were summarized,and the future research directions as well as hotspots in this field were also discussed. |
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