Ion Transport Properties Of CeO₂-Based Semiconductor Heterojunction Materials

With the increasing global energy shortage and environmental pollution problems,it is urgent to develop high-efficiency and low-cost new energy technologies to relief the energy and environmental problems.As a typical sustainable clean energy conversion technology,solid oxide fuel cell（SOFC）can directly convert chemical energy in hydro-gen,methane,natural gas,coal gas and other fuels into electricity effectively.These fuels can alleviate the energy effectively and environmental friendliness.However,the current development of SOFCs were limited by the conventional electrolyte-yttrium-stabilized zirconia（YSZ）,which should work at high temperature（>800 ^oC）.So that it resulted in slow startup,difficult sealing,component mismatch and short lifetime.These problems hindered the commercialization process of SOFC.Therefore,it becomes very important to develop high ionic conduction electrolyte under low temperature（400–600 ^oC）.Among various electrolyte replacement materials,the doped CeO₂(such as Ce_0.8Sm_0.2O_2-d,SDC)has attracted much attention due to its higher ionic conductivity than YSZ in the mid-low temperature,which has great potential for further development of low-temperature electrolytes.However,the way of ion doping is limited by the crystal structure and oxygen ion migration number.When the temperature is lower than 600 ^oC,the ionic conductivity of doped CeO₂ sharply decreased.Recent studies have shown that heterostructure systems based on surface/interface engineering are an effective method to enhance the ionic conductivity of SOFC,which can produce interfacial ion-enhancing effects at low temperature.Therefore,in this dissertation,a variety of heterostructured composites were constructed with CeO₂ as the basic unit.By studying their microstruc-tural characteristics,low-temperature electrical/electrochemical properties and ion transport mechanism,the novel electrolytes that can be applied to low-temperature SOFCs.The main conclusions are as follows:1.A fast proton transport channel was constructed on the surface of CeO₂ by on-line reducing atmosphere treatment.It was found that a core-shell structure of CeO₂/CeO_2-δwas formed by an oxygen defect layer on the surface of CeO₂ which was verified through characterization technology.Electrochemical tests show that the heterostructure material has proton transport properties with a high proton conductivity of 0.16 S cm^-1and an out-put power density of 697 m W cm^-2 at at 520 ^oC as the SOFC electrolyte.Theoretical calculations further verified that a space charge region was formed at the CeO₂/CeO_2-δinterface and realized superionic conduction.Based on this,the interfacial shuttle model of protons was established.This work provides a novel CeO₂-based heterostructure elec-trolyte and a novel ion transport mechanism for low-temperature SOFCs.2.Introducing CeO₂with BaZr_0.8Y_0.2O₃（BZY）to construct a semiconducting ionic heterojunction（SIH）electrolyte,which modulates the ionic conduction of BZY from bulk conduction to surface-interface conduction.Through material characterization and elec-trochemical tests,it is found that the built-in electric field at the interface constructed by the heterojunction electrolyte of SOFC which can promote ion conduction.The CeO₂/BZY can obtain 0.23 S cm^-1 ionic conductivity at 520 ^oC,which ionic conductivity is much higher than that of BZY(0.04 S cm^-1).It verifies that the superiority of interfacial conduction.In addition,a SOFC device was constructed based on the heterojunction elec-trolyte,and the water electrolysis test was carried out under the application of 1.2 V volt-age and 31.2 MPa steam partial pressure to a SOEC（solid oxide electrolytic cells）device.It obtained a high electrolysis current output of 3.2 A cm^-2 at 520 ^oC,verifying the dual efficacy of this heterostructured electrolyte in SOFC and SOEC.This work presents a design idea to modulate the bulk conduction of electrolytes for surface-interface super-conductivity.3.The LSCF-CeO₂ heterostructure was designed and constructed according to the energy band matching principle as the electrolyte of SOFC.The wet chemical method was used to combine different compositions of CeO₂ with La_0.8Sr_0.2Co_0.8Fe_0.2O_3-δ（LSCF）,which is commonly used in SOFC cathodes.The optimal SOFC performance reaches power density of 501 m W cm^-2 at 520 ^oC.Through element valence and surface potential analysis,it is found that the surface oxygen vacancy concentration of this sample is higher than that of other components.There is a built-in electric field at its heterointerface,which provides favorable conditions for ion transportation.This work provides a feasible refer-ence for the design of fuel cell electrolytes.4.Using LaNiO₃（LNO）as an electrolyte layer in SOFC,electrochemical tests found that undergoes a phase transition in the fuel cell,making its semiconductor electronic properties change to semiconductor ionic properties.Furthermore,different content of CeO₂ were introduced into LNO（LNO:CeO₂=9:1,8:2,7:3,6:4）to construct LNO/CeO₂semiconductor heterostructure materials,which greatly improved the power output to 983m W cm^-2（7:3）at 520 ^oC.According to characterization tests,found that the main reason for the transition from electronic conduction to ionic conduction.It is because the reduc-ing atmosphere leads to the lowering of the energy band position and the increase of the forbidden bandgap.After the construction of the LNO/CeO₂ heterojunction,the degree of phase transition is promoted due to the matching of the energy bands and the establish-ment of the built-in electric field.This work sheds new light on the development of novel semiconducting heterostructure electrolytes.In this paper,different types of CeO₂-based semiconductor heterojunction materials were constructed,and investigated their ion transport properties,mechanisms,and feasi-bility of applying to low-temperature SOFC electrolyte.Excellent ionic conductivity and battery performance were obtained.The new electrolyte for low temperature SOFC pro-vides experimental basis and theoretical basis.