Preparation And Performance Research Of Medium-temperature Reversible Solid Oxide Cells Based On Ceramic Proton Conductors

The development of economic society and ecological protection of the environment have put forward new and higher requirements for energy resources.China has proposed to achieve"Carbon Neutrality"by 2060,and hydrogen energy is involved in the energy development plan as an important clean energy source.Therefore,the development of hydrogen energy technologies such as"green hydrogen"production and fuel cells is particularly important.Proton conductor type reversible solid oxide cell(H-RSOC)is a reversible device that converts chemical energy from hydrogen-containing compounds to electrical energy in fuel cell mode,and produces hydrogen by water-electrolytic process in electrolysis mode.H-RSOCs work at high temperatures and have many advantages such as clean,efficient and reversible,which make them well suited for use in hydrogen energy systems.After years of research and development,important progresses have been made in RSOCs composed of proton or oxygen ion conductors.Currently,the development trends of H-RSOCs are as follows.(1)Lowering the preparation temperature to improve the chemical compatibility between the component materials and reduce manufacturing costs.(2)Reducing the operating temperature of H-RSOCs to alleviate the elemental diffusion and thermal stress inside the single cell,prolong the service life,simplify sealing process and facilitate fast start-stop.(3)Improving robustness of H-RSOCs to meet a variety of complex and variable operating conditions and drive them toward rapid practicalization.In this dissertation,Cu²⁺doping is adopted to promote the sintering activity and conductivity of Ba(Ce,Zr,Y)O_3-?-based proton conductor electrolyte and La_1.2Sr_0.8Ni O_4+?oxygen electrode materials,resulting in reduced preparation and operating temperature of H-RSOC.Robustness of H-RSOC is improved by design and preparation of air-electrode supported single-cell structures which have matched thermal expansion coefficients(TECs)with electrolyte.The effect of microstructural changes of air electrodes on the performance and stability of H-RSOC is investigated.The following research works are carried out in this dissertation.(?)In order to reduce the preparation and operation temperature of H-RSOC,Cu²⁺is doped into Ba(Ce,Zr,Y)O_3-?by Pechini method to obtain Ba Ce_0.7-xZr_0.1Y_0.2Cu_xO_3-?proton conductor material with higher sintering activity and conductivity at low temperatures.The densification temperature of Ba Ce_0.7-xZr_0.1Y_0.2Cu_xO_3-?is reduced to1300? after Cu²⁺doping.At the same time,the average grain size of Ba Ce_0.66Zr_0.1Y_0.2Cu_0.04O_3-?increases to 4.7?m,which reduces grain boundary impedance and results in an increased conductivity of 1.5×10^-2 S·cm^-2 and reduced activation energy of 0.43 e V.However,due to the low solid solubility of Cu in Ba Ce_0.7Zr_0.1Y_0.2O_3-?,high Cu²⁺doping concentration leads to the generation of impurity phases and inhibits grain growth,giving rise to decrease in conductivity and increase in activation energy.(?)To further improve the performance of H-RSOC at low temperatures,Ni²⁺in La_1.2Sr_0.8Ni O_4+?(LSN)is substituted with Cu²⁺over a wide range of concentrations to increase lattice distance to reduce the interstitial oxygen transport resistance.La_1.2Sr_0.8Ni_1-xCu_xO_4+?can maintain a stable crystal structure without secondary phases at a Cu²⁺substitution concentration of 75%.The Cu²⁺substitution expands the cell of La_1.2Sr_0.8Ni_1-xCu_xO_4+?in the c-axis direction and increases the TEC,reducing the conductivity and polarization impedance of La_1.2Sr_0.8Ni_1-xCu_xO_4+?electrode.The polarization impedance of La_1.2Sr_0.8Ni_0.50Cu_0.50O_4+?is 0.24?·cm² and the activation energy is 1.17 e V for the symmetric cell at 700?.The single cell exhibits good electrochemical performance at the low and medium temperature.(?)Based on the proton conductor electrolytes prepared at low temperatures,the robustness of H-RSOC is improved by designing and preparing an air electrode support structure with better thermal stability to replace the Ni-based hydrogen electrode support structure.The air electrode support is prepared by optimizing the ratio of LSN to Ba Ce_0.68Zr_0.1Y_0.1Yb_0.1Cu_0.02O_3-?to match the TEC of the electrolyte and the preparation temperature of H-RSOC is further lowered to 1200? successfully.The air-electrode supported H-RSOC single cell achieves an electrolytic current density of-297 m A·cm^-2at 1.3 V and a maximum power density(MPD)of 120 m W·cm^-2 at700? in fuel cell operation mode.The performance of the single cell is stable during400 h of electrolytic cell and fuel cell mode switching and remains unchanged after 20thermal cycles and 5 redox cycles,making it possible to use the H-RSOC under multiple abuse conditions.(?)The effects of microstructural changes of air electrodes on the performance and stability of H-RSOC are investigated.BCZI@LSC composite air electrode is designed and prepared by a wet chemical procedure containing infiltration of the LSC(La_0.6Sr_0.4Co O₃)oxygen electrode catalyst solution on the Ba Ce_0.5Zr_0.2In_0.3O₃(BCZI)electrolyte skeleton of the tubular single cell and a further heat treatment.The LSC nanoparticles attached to electrolyte skeleton in a continuous film-forming morphology,affording abundant three-phase reaction boundaries(TPBs)and continuous conducting phase for the cell reaction.The electrolytic current density of the single cell at 1.3 V is-248 m A·cm^-2 and the MPD in cell mode is 104 m W·cm^-2 at 650?.Compared with the BCZI+LSC composite air electrode prepared by conventional mechanical mixing,the current density we obtained is increased by 51%and the MPD is nearly doubled.In BCZI+LSC air electrode,the stacked LSC and BCZI particles are dislodged due to differences in thermal expansion properties,resulting in the reduction of electron-conducting phase and TPBs.The LSC catalyst film uniformly coated on BCZI proton conductor skeleton in BCZI@LSC air electrode gradually agglomerates and grows during long-term operation at high temperature,and the increase of LSC particle dispersion distance leads to the reduction of electron-conductivity and the increase of ohmic impedance.