Ionic Conduction Mechanism Of BaCeO₃ Based Electrolyte In SOFC With Lithium Compounds Electrodes

In recent years,a new solid oxide fuel cell（SOFC）prepared by the co-pressing method which uses lithium compounds as symmetrical electrodes and traditional SOFC materials as electrolytes has achieved high maximum output power density in low temperatures（400～600℃）.The new structured SOFC provides a potential solution to reduce operating temperature.At present,there are many reports indicate that this new SOFC has obtained very high electrochemical performance in low-temperature,but the reasons for its high performance and the ion conduction mechanism of electrolyte are not clear.Our recent studies have found that lithium compounds own excellent catalytic activity to the hydrogen oxidation reaction on the anode side and the oxygen reduction reaction on the cathode side,but the reason that ion conductivity of the electrolyte in this new type of SOFC has significantly improved is not very clear.In order to study the mechanism of electrolyte ion conduction in the new SOFC,the traditional proton-type conductor BaCe0.9Y0.1O3（BCY）was used as the electrolyte,and Ni0.8Co0.15Al0.05LiO2（NCAL）coated nickel foam（Ni-NCAL）was used as the electrode.A symmetric cell with a structure of Ni-NCAL\BCY\Ni-NCAL was studied.The electrochemical performance test and a series of characterization of the electrolytes before and after the performance test were performed to study the ion conduction mechanism of the electrolyte in this new structure SOFC.The main contents as follows:（1）Two BCY powders with different particle sizes and crystallinity were prepared by changing the sintering temperature of the BCY gel during the preparation of BCY powders,and button cells were prepared with these two powders as electrolytes.The maximum power densities of cells using BCY powders with sintering temperatures of 1000 and 1200℃ as electrolytes operated at 550℃ in H2 are 405.2 and 350.4 mW·cm-2,respectively.According to the EIS results,at 550℃,the ionic conductivity of BCY electrolyte with a sintering temperature of 1000℃ is 0.476 S·cm-1,which is significantly higher than that of a traditional BFC electrolyte sintered at 1600℃.Ce0.9Gd0.1O2-δ（GDC）which is the pure oxygen ion conductor and SrCe0.95Y0.05O3（SCY）with high proton transference numbers were used as filters to study the BCY electrolyte through the electrochemical performance test of the double-layer electrolyte cells.It is found that the carriers of BCY electrolyte in this new SOFC should be a mixed conduction of oxygen ions and protons.HRTEM of the BCY electrolyte before and after the electrochemical performance test found that a 1.2 nm thick amorphous layer was formed on the surface of the BCY electrolyte particles after the electrochemical performance tests.The XPS results show there is a high oxygen vacancy concentration in this amorphous layer,and there are a lot of Li₂CO₃ and LiOH.The formation of this amorphous layer should be the reason why the BCY electrolyte has a very high ionic conductivity at 550℃.（2）Through reduction experiments of NCAL powder in H2 at 550℃ it was found that NCAL was reduced to Ni,LiOH,Li₂CO₃ and CoAl alloys in a hydrogen atmosphere.It was proved that the Li₂CO₃ and LiOH detected in the BCY electrolyte after the electrochemical performance test should originated from Ni-NCAL anode.The influence of the density,grain size and microstructure of the BCY electrolyte on its ionic conductivity in the new structure SOFC was studied by changing the sintering temperature of the BCY electrolyte.The results show that in the four BCY electrolyte pellets the grain size of BCY increased significantly with increasing of sintering temperature,and the density of the electrolyte also increased.At 550℃the maximum power densities of SOFC with four BCY-based electrolytes sintered at 900,1100,1300,and 1600℃ are 454.98,375.68,275.85,and 239.89 mW·cm-2,respectively,the corresponding ionic conductivity of BCY electrolyte are 0.470,0.36,0.21,and 0.19 S·cm-1,respectively,which proves that the ionic conductivity of the BCY electrolyte in the new structure SOFC decreases with the increase of the grain size and the density of electrolyte.The XPS results show that as the sintering temperature of the BCY electrolyte increases,the amount of lithium carbonate and lithium hydroxide entering the electrolyte decreases,it proves that the Li₂CO₃ and LiOH from the anode side during the electrochemical performance tests are responsible for the significant increase in the electrolyte ion conductivity.The amount of Li₂CO₃/LiOH entered in the electrolyte has a decisive effect on the increase of the electrolyte ion conductivity.（3）In this paper,BCY-x%Li₂CO₃（x=0,5,10,and 30）and BCY-y%LiOH（y=0,2,5 and 8）composite electrolytes were prepared by mechanical mixing.Ni-NCAL and porous Pt were used as symmetrical electrodes to prepare Ni-NCAL\BCY-x%Li₂CO₃ or y%LiOH\Ni-NCAL and Pt\BCY-x%Li₂CO₃ or y%LiOH\Pt cells.The fuel cell performance test results at 550℃ and H2/Air show when Li₂CO₃ and LiOH are added to the BCY electrolyte by mechanical mixing,the maximum power density of the cell decreases.While the cell without Li₂CO₃ and LiOH mixed in the electrolyte achieved the highest maximum power density.It indicates the Li₂CO₃ and LiOH produced from Ni-NCAL anode play a key role in improving the electrolyte ion conductivity.The EIS results show that the ionic conductivity of the BCY-30%Li₂CO₃ composite electrolyte and the pure BCY electrolyte at 550℃ is 0.170 and 0.00123 S·cm-1,respectively,while the ionic conductivity of the BCY-8%LiOH composite electrolyte is only 0.0022 S·cm-1 which proves that the addition of LiOH does not improve the ionic conductivity of the BCY electrolyte,and the addition of Li₂CO₃ does it.It indicates that Li₂CO₃ in the amorphous layer is the key to improve the ionic conductivity of the electrolyte in the new structure SOFC.