Research On Proton Transport Properties Of ZnO-based Semiconductor Heterostructure Composites

Solid oxide fuel cell（SOFC）is a kind of device that converts fuel chemical energy into electric energy directly.It has the advantages of wide fuel flexibility and clean efficient energy utilization.SOFC operating in the low temperature（below 600℃）is the general trend of its large-scale commercial application.However,low temperature brings about many more challenges,focusing on the ionic conductivity of critical component electrolyte materials.Proton transport process has lower activation energy and migration energy compared with oxygen ions,so the electrolyte material based on proton conduction is more suitable to deal with the low temperature challenge.Considering semiconductor membrane fuel cells reported in recent years,the built-in electric field at the heterogeneous interface can facilitate ion transport together with the effect of ion concentration difference,which provides a good reference for the design of new low-temperature electrolyte materials.In this dissertation,the effects of different heterojunction composite materials on the performance and proton transport of the battery are studied by constructing three different heterojunctions,namely pn junction（Zn O-Ni O）,pin junction（Zn O-Al₂O₃-Ni O）and nn junction（Zn O-Sm₂O₃）.The main research results are as follows:（1）The pn junction composites of p-type semiconductor Ni O and n-type semiconductor Zn O are prepared by solid-state method,and the optimum weight ratio is Zn O:Ni O=7:3.The optimum Zn O-Ni O heterojunction cell achieves a maximum output power density of 644 m W cm^-2 and ohmic resistance of 0.4482Ωcm² at 550℃,both of which are better than pure Zn O.However,the stability of the material needs to be further improved,and this study only focuses on the proton transport mechanism and performance analysis.The reason for optimized performance of pn junction composites is theoretically analyzed and the band configuration of Zn O-Ni O is established.The contribution of pn junction materials to ion transport and electron barrier is explained from the perspective of space charge region and band bending of the heterojunction.（2）Based on the optimum composition of pn junction Zn O-Ni O,the pin junction composites are constructed by introducing intrinsic semiconductor Al₂O₃ through solution method.The optimum composition of Al₂O₃ is 10%weight ratio of Zn O composed.The optimized pin-junction Zn O-Al₂O₃-Ni O cell delivers the maximum output power density of 271～917 m W cm^-2 at 460～550℃,corresponding ionic conductivity of 0.06～0.24 S cm^-1,and corresponding activation energy of 0.34e V at 490～550℃.The power output of pin junction material can be further improved,which is385～1096 m W cm^-2 at 460～550℃,by adding the buffer layer.Herein,the buffer layer is obtained by mixing electrode material and electrolyte material according to the weight ratio ratio of 6:4.The pin junction material with amorphous Al₂O₃ layer coated on Zn O and Ni O has good stability.The single cell delivers a stable power output of 98 m W cm^-2 for over 35h at 500℃.The difference between pin junction and pn junction is verified by rectification characteristic test.The reverse saturation current of pin junction material is 3 orders of magnitude lower than that of the other.Combined with energy band configuration and space charge region,the reason why pin junction improves battery performance and proton transport is comprehensively analyzed from the mechanism.On the one hand,pin junction increases space charge region.On the other hand,the energy barrier generated by band bending of pin junction better separates charge carriers.（3）Heterojunction Zn O-Sm₂O₃ composites are prepared by samarium carbonate and zinc acetate precursor,and the best weight ratio is Zn O:Sm₂O₃=7:3.The Hall coefficient obtained by Hall effect test is negative,indicating that most carriers are electrons,which verifies that Sm₂O₃ is a n-type semiconductor,and the nn heterojunction type is determined.Phase analysis and microscopic morphology show that the addition of Zn O inhibits the formation of monoclinic Sm₂O₃,while the mainly cubic phase Sm₂O₃ shows better stability.The maximum output power density of the optimal Zn O-Sm₂O₃ cell is 252～790 m W cm^-2,ionic conductivity is 0.06～0.22 S cm^-1.The single cell based on Zn O-Sm₂O₃ heterojunction material runs stably for 40h at a constant discharge current of 154 m A cm^-2 at 490℃,and the cell degradation rate is 0.002 V h^-1.Sm₂O₃ is a wide-band gap semiconductor and the effect of charge depletion region and band configuration of heterojunction can contribute to carrier separation and proton transport.In this dissertation,the battery performance and proton transport of pn junction（Zn O-Ni O）,pin junction（Zn O-Al₂O₃-Ni O）and nn junction（Zn O-Sm₂O₃）are studied by combining the microstructure,multispectral characterization and electrochemical performance test.The results show that the heterojunction material improves the performance,stability and ionic conductivity of fuel cells at low temperatures.The construction of heterojunction materials provides a new idea and technical routes to cope with the low temperature challenge of electrolytes.