Study On The Mechanism For Major Conductivity Increase Of GDC Electrolyte In SOFC Using Iithium Compounds As Electrode

Solid oxide fuel cells(SOFCs)are all-solid-state energy conversion devices that directly convert chemical energy stored in fuel and oxidants into electrical energy.Their high energy conversion efficiency,flexibility of fuels and environmental friendliness have attracted wide attention in recent years.However,high operating temperature makes it difficult for SOFC to maintain long-term stability and reduce cost,which limits its commercialization.However,to reduce the operating temperature of SOFC,there will be some problems,such as the low conductivity of electrolyte and the poor catalytic activity of electrode.In recent years,a new type of fuel cell with lithium oxide as electrode,oxide or carbonate and oxide composite as electrolyte has attracted wide attention due to its excellent low temperature(400～600℃)electrochemical performance.Our recent research found that the fuel cell with this new structure of Ni0.8Co0.15Al0.05LiO2(NCAL)as electrode and oxide as electrolyte has excellent electrochemical performance at 550℃,and the ionic conductivity of electrolyte is far higher than that of traditional SOFC electrolyte materials.However,the mechanism of the large increase of the electrolyte ionic conductivity of the fuel cell with the new structure of lithium compounds electrode is not clear.In this thesis,NCAL was used as the electrode to study the mechanism of major conductivity increase of GDC electrolyte,the details are as follows:(1)The GDC electrolyte supported SOFC with NCAL/GDC/Pt and Pt/GDC/Pt structure were prepared,the results of electrochemical performance test show that the maximum power density of the cell with NCAL anode is 32 times that of the cell with Pt anode.It is found that the ionic conductivity of GDC electrolyte in the cell with NCAL anode is 3.3 times higher than that of the cell with Pt anode.Through the characterization of electrolyte and electrode materials before and after the electrochemical performance test,it is found that the main reason for the increase of ionic conductivity of GDC electrolyte in the cell with NCAL anode is due to the diffusion of LiOH/Li2CO3 formed by the reduction of NCAL in H2 into the electrolyte to form GDC-LiOH-Li2CO3 composite electrolyte.EPR results show that the oxygen vacancy concentration in GDC-LiOH-Li2CO3 composite electrolyte is significantly higher than that in pure GDC electrolyte.Therefore,we propose that a space charge layer with cation enrichment is formed at the interface between GDC and LiOH/Li2CO3,and the high oxygen vacancy concentration area around it is the channel for high-speed ion transport in the electrolyte,which is also the main reason for the large increase of ion conductivity in the GDC electrolyte of NCAL anode cell.(2)It is found that the formation of LiOH and Li2CO3 in NCAL anode during performance test should be driven by chemical potential difference and enter into GDC electrolyte by chemical diffusion.In this thesis,four kinds of GDC electrolytes with thickness of 1mm and density and microstructure were prepared at different sintering temperatures of 900,1100,1300 and 1550℃,respectively.The electrolyte supported SOFC of Ni-NCAL/GDC/NCAL-Ni was prepared with NCAL coated nickel foam(Ni-NCAL)as symmetrical electrode.It is found that with the increase of the sintering temperature of electrolyte,the grain size of GDC in the electrolyte is increasing,and the electrolyte is becoming denser and denser,while the amount of LiOH/Li2CO3 entering the electrolyte is gradually decreasing,which indicates that the reason for the decrease of the ionic conductivity of electrolyte is directly related to the amount of LiOH/Li2CO3 entering the electrolyte from NCAL anode.Due to the smaller GDC grain size and larger specific surface area,the GDC electrolyte with lower sintering temperature has more effective interfaces after compounding with LiOH/Li2CO3,so its ionic conductivity is much higher than the GDC-LiOH/Li2CO3 composite electrolyte with large GDC grains and a small amount of LiOH/Li2CO3.(3)Single cells with structure of Ni-NCAL\GDC-x%LiOH(x=0,3,5,7)or GDC-y%Li2CO3(y=0,5,10,30)\NCAL-Ni were prepared by using GDC with different proportions of LiOH or Li2CO3 as electrolyte and Ni-NCAL as symmetrical electrode.Through the electrochemical performance test,it was found that directly mixing LiOH or Li2CO3 in the GDC electrolyte could not effectively improve the electrochemical performance of the cell,while the LiOH/Li2CO3 diffused from the NCAL anode into the GDC electrolyte during the electrochemical performance test played a key role in improving the ionic conductivity of the GDC electrolyte.