Study On The Material Preparation And Properties Of Solid Oxide Fuel Cells Based On Layered Cobalt-Based Oxides

Strengthening the use of renewable energy is conducive to getting rid of the threat of energy and economic security,fuel cell is one of the most attractive technologies in the field of renewable energy.In this dissertation,Ca_2.5Bi_0.1Pr_0.4Co₄O_9-δ(CBP_0.4C,CBPC for short)material is taken as the main research target.The cathode of traditional structure intermediate temperature solid oxide fuel cell（IT-SOFC）and the composite electrolyte of semiconductor-ion low temperature solid oxide fuel cell（LT-SOFC）are systematically studied.The research object is the mismatching layered cobalt oxide Ca₃Co₄O_9-δ（CCO）with Bi and Pr double doping at the A-position.By studying the structure of the material and the minimum phase formation temperature,the optimal doping amount and the optimization of the preparation process of the half cell,the high performance traditional structure intermediate temperature SOFC is obtained.The controlled variable method is used to study to realize the transition optimization from traditional structure of intermediate temperature SOFC to semiconductor-ion heterostructured of low-temperature SOFC based on CBP_0.4C,and find the best composite ratio of CBP_0.4C and Ce_0.8Sm_0.2O_1.9（SDC）.In order to further optimize the electrochemical performance of the semiconductor-ion SOFC,the lanide metal oxides Sm₂O₃ and Pr₆O₁₁ are respectively composited with the target material CBP_0.4C.The specific research results are as follows:1.The performance optimization of the cells is achieved by double doping layered cobalt based oxides as cathode materials for intermediate temperature traditional structure SOFC.Ca_2.9-xBi_0.1PrxCo₄O₉-δ（CBPxC,x=0,0.1,0.2,0.3,0.4,0.5,0.6）are prepared by sol-gel method.Through testing and analyzing the structure and electrochemical performance of the materials,we found that:the peak of（004）shifts to low angle after Ca²⁺（1.00 ?）replaced by Bi³⁺（1.03 ?）.For the size of Pr³⁺（0.99?）is smaller than that of Ca²⁺,the peak shifts to high angle gradually with the increasing of Pr³⁺ doping.It indicates that doped Bi³⁺ and Pr³⁺ are involved in the crystallization of CCO crystal.And the minimum phase forming temperature is 700℃.The optimal process conditions for the preparation of the symmetric cells are CBP_0.4C sintered at 950℃ for 5h to obtain the minimum polarization impedance.When SDC is used as the electrolyte material,the electrochemical performance of single cell Ni_0.95Cu_0.05O_x-SDC/SDC/CBP_0.4C supported by electrolyte is prepared by screen printing method.At 800℃,the maximum power density is 0.423 W/cm²,the OCV is 0.759 V,and the polarization impedance is 0.100 Ω·cm².The electrochemical performance of single cell Ni_0.95Cu_0.05O_x-SDC/SDC/CBP_0.4C supported by anodes is prepared by co-pressure co-firing method.The maximum power density is 0.705 W/cm²,the OCV is 0.673 V,and the polarization impedance is 0.051 Ω·m² at 800℃.2.Based on the material CBP_0.4C,the electrochemical performance of SOFC with traditional structure and new semiconductor heterostructure is studied and tested by the control variable method.When Ni₍0.8_Co_0.15Al_0.05LiO_2-δ（NCAL）is coated on nickel foam as an anode,the maximum power density of Ni-CBP_0.4C/SDC/Ni-NCAL cell is 0.711 W/cm² and the OCV is 1.234 V after testing at 550℃.The power density and OCV decrease rapidly at 400-450℃,which further indicates that the catalytic performance of CBP_0.4C is weak in the low temperature region.The maximum power density of the Ni-NCAL/SDC/Ni-NCAL cell is 0.725 W/cm² and the OCV is 1.111 V after testing at 550℃.LiOH/LiCO3 is generated in the reaction of NCAL electrode,which can promote ion transport performance.The semiconductor-ion cell is prepared.The best mass composite ratio of 1:9 gains the maximum power density 0.791 W/cm² and the OCV 1.101 V.Experiments have shown that after the composition of two materials,a large number of p-n heterostructures with bulk phase distribution are formed inside the electrolyte.The field effect generated by the heterostructures promotes the improvement of oxygen ion conductivity,leading to excellent fuel cell performance.CBP_0.4C is an ion electron hybrid conductor,so as the composite ratio increases,the existing electronic conductivity will form a short-circuit current,resulting in a decrease in the open circuit voltage of the cell and a deterioration in cell performance.3.In order to further explore the heterostructure composite electrolytes based on CBP_0.4C,we composite the lanthanide metal wide bandgap oxide Sm₂O₃ with the target material CBP_0.4C as electrolytes to explore their potential applications in semiconductor ion type SOFC.After testing and characterizing the performance of the cells and material,it is shown that:When the operating temperature is 550℃,the maximum power density is 0.880 W/cm²,the OCV is 1.108 V,the conductivity of cell is 0.327 S/cm,and the polarization impedance ASR value is 0.111 Ω·m².After the XRD test of 1CBP_0.4C-9Sm₂O₃ material,it was found that the heterostructure composite material directly physically mixed would not have the second phase and impurity phase,indicating that the two materials would not react,proving its chemical stability.And at the basic level of the TG curve,there is no loss,which proves that the composite material will not decompose at the working temperature of the fuel cell,and the loss is due to the relaxation loss of lattice oxygen.The lost lattice oxygen,due to the material itself maintaining electrical neutrality,generates oxygen vacancies that serve as transport jump sites for oxygen ions and promote oxygen ion conductivity,thereby improving the output performance of the fuel cell.UV testing has shown that Sm₂O₃ is a broadband gap semiconductor,resulting in a large bandgap difference at the interface between the heterostructure and CBP_0.4C,which creates energy barriers that hinder electron migration.At the same time,electrons gather due to the presence of these barriers,forming a local electric field that is conducive to oxygen ion migration.Ultimately,the Sm₂O₃-CBP_0.4C composite electrolyte exhibits excellent performance in fuel cell applications.4.We composite the lanthanide metal narrow bandgap oxide Pr₆O₁₁ with the target material CBP_0.4C to prepare a composite electrolyte,in order to explore their potential application in semiconductor ion type SOFC.At 550℃,the optimal composite ratio of CBP_0.4C and Pr₆O₁₁ is 1:9.And the maximum power density is 0.989 W/cm²,the OCV is 1.020 V,the conductivity of cell is 0.189 S/cm,and the polarization impedance ASR value is 0.179 Ω·cm².Pr ions in Pr₆O₁₁ have multiple valence states in oxides,which has been shown to have good catalytic activity.In UV testing,we obtained a band gap of 1.54 eV for Pr₆O₁₁,which is similar to the band gap of the target material CBP_0.4C.So we designed and synthesized a composite heterostructure of Pr₆O₁₁ and CBP_0.4C,and found that both p-n and Schottky junctions exist in the cell to achieve the optimal electrochemical performance.Because both materials have good catalytic activity,due to the reduction of Ni metal by the NCAL electrode under the action of H₂,there will be a Schottky type heterostructure composed of metal Ni and composite materials on the electrode side.Ultimately,at the cost of a small loss of the open circuit voltage of the cell,the three-phase interface of the electrode electrochemical reaction is greatly increased.This resulted in excellent fuel cell performance with a maximum power density of 0.989 W/cm² and a conductivity of 0.189 S/cm.