Study On The Performance Of Semiconductor Ionic Membrane Fuel Cells

It's of great concern to reduce the operating temperature and develop cryogenic fuel cells(300-600?)because of the material instability and large commercialization cost under high temperature conditions in solid oxide fuel cells.The conventional solid oxide fuel cells adopts Yttrium Stabilized Zirconia(YSZ)as electrolyte,which greatly restricts the cells performance since the lower temperature would induce the increased ohmic and electrode polarization.Therefore,it's necessary and urgent to develop new electrolytes with high conductivity especially for low temperature application.Compared to traditional fuel cells system,Semiconductor ionic membrane fuel cells has attracted more attentions due to their better performance and moderate cost.The adopted semiconductor ionic materials has multi-ion transport path and internal electric field generated by the heterogeneous structure,which greatly enhances the ionic conductivity of cells.In this paper,three kinds of semiconductor ionic electrolyte materials are prepared.The effects of material preparation technology and composition ration on the composite electrolyte'structure and cell performance are researched.In addition,the ionic conduction mechanism of this semiconductor ionic composite electrolyte materials is also discussed based on the energy band theory.The main contents and research results are as follows:(1)In this paper,nanometer ZnO-carbonate nanocomposite were prepared.The effects of carbonate(Na₂CO₃/K₂CO₃)and sintering temperature on the material structure and cells properties are studied.The characterization results of X-ray diffraction and scanning electron microscope indicate that the carbonate in ZnO-carbonate material is coated with nano zinc oxide particles,showing amorphous structure.The zinc oxide co-doped with Na₂CO₃ and K₂CO₃(ZNK-0)is agglomerated after sintering at 600?.The current-voltage-power curve test reveals that the performance of cell with ZnO-carbonate composite electrolyte is closely related to the carbonate type.At 550?,the maximum power density of ZnO cells reaches up to 118 mWcm^-2 with the open-circuit voltage of 0.77V.Although ZnO contains certain electronic conductance,there is no obvious short circuit in the cells due to the formation of schottky junction in hydrogen side.ZNK-0 cells perform best with the power density and open-circuit voltage being 162 mWcm^-2and 0.77 V respectively,on the contrary,ZN-0/ZK-0 cells has poor performance,indicating that the double doped is capable of improving the battery performance.The heterogeneous interface formed between carbonate and nano zinc oxide can not only effectively inhibit electrical conduction but also promote interface ionic conduction,greatly improving the electrochemical performance of zinc oxide cells,in addition,carbonate dopinh will change the band structure of ZnO,making the ZnO bandgap slightly reduced,so it is easier for electrons to transition from valence band to conduction band,enhancing the conductivity of the composite materials,but also resulting in the reduction of open circuit voltage.(2)In this paper,ZnO-NiO heterojunction electrolyte materials were prepared,and also the influence of the volume ratio of the semiconductor phase material on cells performance is researched.Compared to the nanometer ZnO cells and NiO cells suffered from significant internal electronic short circuit,the performance of cells adopted ZnO-NiO composite electrolyte material has been significantly improved.Results show that the performance of the nanocomposite electrolyte cells is closely related to the volume ratio of ZnO/NiO.Specially,the cells has the best electrochemical performance when the volume ratio of ZnO-NiO is 70:30.Moreover,at 550?,the open-circuit voltage and peak power density of70ZnO-30NiO cells rise to 0.879 V and 283 mWcm^-2 respectively.Due to the difference in energy band structure between n-type ZnO and p-type NiO materials,p-n heterojunction is formed at the heterogeneous interface through band bending and alignment.The corresponding built-in electric field isolates electron conduction but promotes interfacial ion conduction,enabling ZnO-NiO heterojunction cells to exhibit good electrochemical performance.(3)LSCF-LTO composite materials were prepared by mixing La_0.6Sr_0.4Co_0.2Fe_0.8O₃(LSCF)and Li₄Ti₅O₁₂(LTO)according to different mass ratios.The effects of temperature and phase ratio on the composite electrolyte conductivity and cells performance are studied through electrochemical impedance spectroscopy.High resolution transmission electron microscopy and X-ray diffraction analysis show that a large number of heterogeneous interfaces with good chemical compatibility are formed between p-type LSCF and n-type LTO.The open circuit voltage of this LSCF-LTO cells is greater than 1.0 V,indicating that p-n heterojunction can effectively inhibit electron hole conduction of traditional LSCF cathode materials.At 550?,the cells with LSCF/LTO mass ratio of 1:1 performs the best.The conductivity and the peak power density of this LSCF-LTO cells reaches up to 0.677s cm^-1 and 343 mWcm^-2 respectively,further confirming that the semiconductor ionic membrane fuel cells based on the energy band is promising in developing high-performance low temperature solid oxide fuel cells.