| With the fast development of human society,energy shortage and environmental pollution have become problems that human beings must face.Hydrogen energy is a clean and renewable secondary energy with high combustion calorific value,which has attracted much attention because of its advantages such as wide sources,storable property,and various applications.The large-scale preparation of hydrogen energy is the basis of its application.As the reverse process of solid oxide fuel cell?SOFC?,solid oxide electrolysis cell?SOEC?is considered as one of the most promising energy conversion devices.It can use electric energy and thermal energy to electrochemically reduce water,carbon dioxide and other raw materials to chemical energy.Hydrogen production with high temperature steam electrolysis by SOEC shows the advantages of high energy conversion efficiency,zero pollution and large-scale production.At present,it mainly has such issue as the delamination at oxygen electrode/electrolyte interface under high oxygen partial pressure,which causes the delamination SOEC and limits its life time.Aiming for the exploration of SOEC oxygen electrode materials with excellent stability to overcome the problems of Sr segregation and thermal expansion coefficient?TEC?mismatch of traditional oxygen electrode materials containing Sr and Co,we take Sr and Co-free Ln1-xCaxFe O3-??Ln=La,Pr?oxygen electrode materials as the research object.The preparation method is studied and their basic properties are characterized.The optimum sintering temperature of single-phase oxygen electrode materials La0.6Ca0.4Fe O3-??LCF64?and Pr0.6Ca0.4Fe O3-??PCF64?are obtained.And the optimum composite ratio of LCF64-GDC and PCF64-GDC are also explored.Moreover,hydrogen electrode supported single cells with LCF64,PCF64,LCF64-GDC and PCF64-GDC as oxygen electrodes are prepared,and their electrochemical properties at different operating temperatures and steam concentrations are investigated.The main contents are as follows:?1?A series of Ln1-xCaxFe O3-??Ln=La,Pr?materials are prepared by glycine-nitrate combustion,and the phase structure and electrical conductivity of the materials are studied.All materials have a single perovskite structure.The conductivity increases at first and then decreases with the increase of Ca doping in A site.According to the electrical conductivity of materials,the TEC and chemical compatibility of LCF64 and PCF64 with high conductivity are further studied.The results show that the TEC of LCF64 and PCF64 in the range of 25?1000?are 15.38×10-6 K-1 and 14.88×10-6K-1,respectively,which are closer that of SSZ.And the chemical compatibility of the two materials with GDC is better.?2?According to the principle of oxygen pump,the oxygen pumping test of symmetrical cells are conducted in air.And the optimal sintering temperature of LCF64and PCF64 as SOEC oxygen electrode materials are discussed.The results indicate that LCF64 oxygen electrode sintered at 1050?and PCF64 oxygen electrode sintered at1100?illustrate low polarization impedance.?3?The SSZ|Ni O-YSZ hydrogen electrode supported half-cell is successfully prepared by tape casting and co-firing.Among them,SSZ is the electrolyte,Ni O-YSZ is the support layer,respectively.Two kinds of hydrogen electrode supported single cell with LCF64 and PCF64 as oxygen electrode are obtained by screen printing.It is found that the performance of PCF64 single cell is better,the maximum power density of462.93 m W cm-2 and the polarization impedance of 0.82?cm2 are obtained at 800?in SOFC mode.In SOEC mode,with the increase of operating temperature and steam concentrations,the polarization impedance decreases.The electrolytic current densities at 650?,700?,750?and 800?are 33.67 m A cm-2,66.12 m A cm-2,139.26 m A cm-2,277.14 m A cm-2,respectively under the atmosphere of 60%H2O-H2 and 1.3 V applied voltage.The cell is operated at 750?,60%steam concentrations and 1.3 V applied voltage for 10 hours without degradation.The ohmic impedance and polarization impedance are basically unchanged,indicating that the cell had good stability.?4?In order to further optimize the performance of oxygen electrode,the best composite ratio of LCF64-GDC and PCF64-GDC composite oxygen electrodes are discussed.And we also prepare two kinds of hydrogen electrode supported single cells with LCF64-GDC?6:4?and PCF64-GDC?7:3?as oxygen electrodes,respectively.The results show that the PCF64-GDC?7:3?single cell has better performance.The PCF64-GDC?7:3?single cell has a maximum power density is 452.38 m W cm-2 and the polarization impedance is 0.53?cm2 at 800?in SOFC mode.In SOEC mode,the electrolysis current density is 349.58 m A cm-2 and the hydrogen production rate is146.12 m L cm-2 h-1 at 800?,60%steam concentrations and 1.3 V applied voltage.PCF64-GDC?7:3?shows excellent cell stability and no obvious degradation after running for 10 h at 750?,60%steam concentrations and 1.3V applied voltage,which indicates that PCF64-GDC?7:3?is a potential SOEC oxygen electrode material.Through the research of this paper,it is found that perovskite materials LCF64 and PCF64 show great performance as SOEC oxygen electrode materials for hydrogen production by high temperature steam electrolysis.Especially PCF64 is a promising SOEC oxygen electrode material.The performance of single cell can be further improved by using composite oxygen electrode.PCF64-GDC composite electrode has wide application prospect in SOEC oxygen electrode materials.This paper has 51 figures,6 tables and 136 references. |
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