Preparation And Performance Of LnBaCo₂O_5+? Cathode And Proton Conductor Electrolyte For Intermediate-temperature Solid Oxide Fuel Cells

The operating temperature of most solid oxide fuel cells is in the range of800-1000?,which is quite high and usually causes high cost and preparation difficulties.Therefore,lowering the operating temperature but keep the same performance of SOFC is the key of the research of SOFC for practical application.The purpose of this paper is to explore the cathode materials and electrolyte materials which have high electrochemical reaction activity in the intermediate temperature(IT)range(500-800?),and to study the further improvement and application of IT-SOFC.This paper focuses on the study of cathode and electrolyte materials for SOFC at IT.Firstly,the double perovskite cathodes with high conductivity and oxygen reduction activity were developed,and the oxygen reduction process of the cathodes was studied in details.Secondly,BaCeO₃-based electrolytes were studied and we found that they have high ionic conductivity in the intermediate temperature range.Moreover,Ba Ce_1-2xPr_xGd_xO₃(BCPG)electrolytes were prepared by double doping Pr and Gd,and the effects of their chemical composition on the phase structure and ionic conductivity were studied,and the power density of the battery was tested.The major results of this study are highlighted as following:1.Increasing the oxygen vacancy concentration and improving the electrochemical performance by introducing Ba²⁺vacancy.NdBa_1-xCo₂O_5+?(NB_1-xCO,x=0.0,0.02,0.04,0.06),the oxidation state of each metal ion in the cathode material is Nd³⁺?Ba²⁺?Co⁴⁺/Co³⁺,the existence of these Co⁴⁺/Co³⁺ions will play an important role in the electrochemical reaction of the electrode.In the whole test temperature range,NB_0.96CO shows the minimum interfacial polarization resistance.The high and low frequency end conductivity of NB_0.96CO cathode is higher than that of NB_1.0CO cathode material.The results show that the introduction of Ba²⁺vacancy promoted the reduction of oxygen.The results show that the oxygen reduction reaction on the electrode is composed of the high-frequency process of oxygen atom generating oxygen ion and the low-frequency process of oxygen ion combining with oxygen vacancy to form lattice oxygen,the low frequency process is the speed limiting step.2.The addition of a certain amount of Cu can be reduced the coefficient of thermal expansion of the material and improves the thermal matching between the electrolyte and the cathode.With the increase of Cu content,the content of oxygen decreases gradually and the concentration of oxygen vacancy increases in NdBaCo_2-xCu_xO_5+?(NBC_2-xC_xO,x=0.0,0.1,0.3,0.5).The polarization resistance(Rp)decreases first and then increases with the increase of Cu doping content,and the Rp value is the smallest when the Cu=0.1.The results show that the reaction on the NBC_1.9C_0.1O electrode contains oxygen ions which combine with oxygen vacancy at the three-phase interface to form lattice oxygen,and oxygen atoms are adsorbed to form adsorbed oxygen ions,dissociative adsorption and diffusion of oxygen.Compared with pure NBCO cathode,the reaction rate of each step is faster.With the increase of Cu doping amount,the power of NBC_2-xC_xO/SDC/Ni_0.9Cu_0.1-SDC single cell increases first and then decreases.The maximum power density of NBC_1.9C_0.1O single cell is 334 m W/cm²at 700°C.The Composite cathode is made by adding SDC with high ionic conductivity,which reduces the coefficient of thermal expansion of the material,increases the length of the three phase interface,and reduces the interfacial polarization resistance between the electrode and the electrolyte.When x=30wt.%,the polarization resistance is the minimum.The maximum power density of single cell at 800°C is 568.17 m W/cm².The results show that the electrochemical performance of composite cathode is better than that of single-phase Cathode material.3.The stability of BaCeO₃-based electrolyte materials can be improved by introducing some oxides.Ba Ce_1-2xPr_xGd_xO_3-?electrolytes were prepared by sol-gel method with citric acid-EDTA complex.The samples were calcined at 1400°C for10h,the electrolyte substrate is smooth,the crystallinity of the samples is good,and the samples with Pr and Gd Perovskite(structure)are formed single phase.The conductivity of Pr and Gd doped BCPG samples are higher than that of pure BaCeO₃sample.The ionic conductivity of Ba Ce Pr_0.05Gd_0.05O_3-?ion conductor is the highest when the content of Pr and Gd was 10 mol%.With the further increase of Pr and Gd doping amount,the conductivity decreased.The conductance activation energy of the sample is the lowest when the doping amount is 2x=0.10.Ba Ce_0.9Pr_0.05Gd_0.05O_3-?with the highest conductivity is selected as the electrolyte to prepare the electrolyte-supported single cell.High OCV and high output power were obtained.The OCV of the single cell is about 1.1 V at 650°C,the maximum power density of the battery is 105 m W/cm²,about 15 times that of the pure BaCeO₃.4.With an aim to decrease the cathode polarization resistance,and further improve the electrocatalytic activity of PBCO,the PBCO-BCS-SDC composite cathodes were prepared by mixing PBCO,SDC and BCS.The XRD results demonstrate that there are no obvious reactions between PBCO,BCS and SDC,this indicates that there is very good chemical compatibility between them.The microstructure of 0.7BCS-0.3SDC is characterized by more appropriate grain size and more adequate porosity.The composite containing 0.7BCS-0.3SDC cathode exhibited the smallest interfacial resistance(Rp).At 600 and 700?,the interfacial resistance Rp was about 0.4766 and 0.0991?cm²for 0.7BCS-0.3SDC,meanwhile,the interfacial resistance Rp of PBC-BCS was about 2.2169 and 0.7006?cm².Using0.7BCS-0.3SDC cathodes,Ni_0.9Cu_0.1-SDC anode,SDC electrolyte achieves excellent performance.The maximum power densities of the single cell is much better than that of the others at the same work temperature.