Study On Application And Electrical Conductivity Of Zinc Oxide Based Solid Electrolyte In Fuel Cells

With the depletion of worldwide fossil fuels and the increasing environmental pollution,it is urgent to develop new clean energy conversion technologies.Solid oxide fuel cell(SOFC)is a kind of power generation device that can convert chemical energy stored in fuel directly into electric energy via redox reaction.Due to its features of high energy conversion efficiency and low emission,SOFC has attracted extensive attention.Current research and development of SOFC have still been hampered by its electrolyte layer,yttrium stabilized zirconia(YSZ)which requires high operating temperature,resulting in high cost to impeding its commercialization.In order to promote the commercialization of SOFC,it is necessary to reduce its operating temperature as much as possible to develop low-temperature SOFC(LT-SOFC).However,the ionic conductivity of the conventional electrolytes always drop sharply as the temperature decreases,leading to a significant loss in the fuel cell power output.To address this problem,it is needed to develop new electrolytes with high ionic conductivity at low temperatures.In this thesis,the conduction property and application of ZnO based solid electrolytes in SOFC are studied by applying the materials in a new type of fuel cell-semiconductor ionic membrane fuel cell(SIMFC).By using the approaches of heterostructure composite and doping,the electrolyte functionality of ZnO has been optimized,and the developed ZnO based electrolytes have been successfully demonstrated in LT-SOFC,providing new materials and strategies for realizing cost-effective and high-performance SOFC.The main work is as follows:(1)Nano-sized ZnO powder is selected as study case in this chapter,which is treated at different temperatures and in different atmospheres for oxygen vacancy characterization and DC conductivity measurement,to verify the ionic conducting capability and electrolyte functionality of ZnO.The treated ZnO samples are applied in SIMFC for performance measurement and electrochemical impedance analysis,to initially assess the electrolyte functionality of ZnO in LT-SOFC.Our findings show that the ZnO sample sintered at 650?gains enriched surface oxygen vacancies in fuel cell atmosphere,and reveals considerable ionic conduction with non-negligible electronic conduction.When applied in SIMFC,the fuel cell obtains a peak power density of 530 m W cm^-2with an open circuit voltage(OCV)of 1.06V at 600?,confirming the feasibility of ZnO electrolyte in LT-SOFC and determining the pre-treating method for nano-sized ZnO electrolyte.(2)Further investigation has been carried out in this chapter to study the material structure,ionic conductivity,and LT performance of nano ZnO,and a semiconductor-ionic conductor heterostructure approach is used to introduce ionic conductor La/Pr doped Ce O₂(LCP)into ZnO,to design a new composite electrolyte ZnO-LCP,followed by comparison study of ZnO-LCP and ZnO from the perspectives of material properties,fuel cell structure and electrochemical performance.The obtained results show that ZnO possesses hybrid proton and oxygen ion conduction capability,reaching a high ionic conductivity of 0.09 S cm^-1at at 550?which includes a proton conductivity of 0.05 S cm^-1,and the corresponding fuel cell achieves power density of 482 m W cm^-2at 550?.Comparatively,ZnO-LCP composite shows an remarkable ionic conductivity of 0.156 S cm^-1at the same temperature along with significantly improved fuel cell performance of 864 m W cm^-2.Via impedance analysis,it is found the performance enhancement is mainly a result of reduced grain boundary resistance of electrolyte and lowered electrode polarization resistance of the cell.The results reflect that the ionic conduction and fuel cell performance of ZnO based electrolyte can be effectively promoted by heterostructure composite approach,which deserves further study for optimization.(3)On the basis of chapter(2),the ZnO-LCP composite is further studied to seek the impact of composition modulation on its fuel cell performance and hybrid ion-electron conduction,before investigating the Schottky junction(SJ)effect to interpret the avoidance of short circuit by the ZnO-LCP fuel cell.Based on material and device characterization,the ionic and electronic conductivities of ZnO-LCP with various mass ratios are calculated according to I-V polarization curve and electrochemical impedance spectra(EIS)fitting results.It is found the 3ZnO-7LCP sample achieves the highest ionic conductivity with balanced electronic conductivity in the same order of magnitude,which is probably the reason to lead to the highest performance of 3ZnO-7LCP cell.Moreover,the SIMFCs based on various ZnO-LCP composites all demonstrate good performance at 550?with power density of 780?1055 m W cm^-2and OCVs of 1.04?1.07 V.On basis of which,the rectifying curve of an“anode/3ZnO-7LCP”half cell is acquired under fuel cell working condition,certifying the existence of SJ barrier at anode/electrolyte interface region,which interpret how the fuel cell get rid of short circuit risk.These findings reflect an obvious impact of ZnO/LCP mass ratio on the ionic conduction and fuel cell performance of ZnO-LCP,indicating that the electrolyte functionality of ZnO based heterostructure composites can be optimized by composition modulation,while metal-semiconductor junction effect play a crucial role in ZnO based fuel cell.(4)On the basis of chapter(1),structural doping approach is used to attempt to optimize the LT electrolyte functionality of ZnO.A new electrolyte,Li doped ZnO(Li_0.15Zn_0.85O,LZO)is prepared by co-precipitation method,which is then characterized in terms of crystal structure,morphology,optical properties,ionic conductivity and I-V performance,to study the effect of Li doping on the fuel cell performance and electrical conduction of ZnO electrolyte.In the meanwhile,the I-V characteristics of the metal-semiconductor contact for LZO-Ni and LZO-Ag are measured for investigating the function of SJ effect in ZnO based electrolyte fuel cell.As a result,Li doping presents few influence on the ionic conductivity of ZnO,but facilitates the catalytic reaction at LZO electrolyte/electrode interface region,resulting in a decreased polarization resistance of the cell.This promotes the fuel cell performance to a higher level(165?290 m W cm^-2)at 450?500?as compared to ZnO electrolyte fuel cell,and enables a LT operability of LZO fuel cell at 425?450?.In addition,our measurements reveal that LZO can form an Ohmic contact with Ag,while establishing a SJ contact with the Ni that produced by the anode.This suggests that the LZO electrolyte can form in-situ SJ at the anode/electrolyte interface of the LZO fuel cell,thus guaranteeing the high OCV and normal operation of the cell.These results reflect that the LT performance and LT operability of ZnO electrolyte fuel cell can be promoted by Li doping,and the SJ should be the main reason to avoid short circuit risk of ZnO based SIMFCs,which provide a new strategy to further optimize ZnO based solid electrolytes.In summary,ZnO is a semiconductor ionic material with great promise for LT-SOFC electrolyte uses,which can be effectively improved by heterostructure composite and doping approaches to hit the mark of enhanced performance of SOFC at low temperatures.The developed ZnO based electrolytes in this thesis provide new candidate materials for LT-SOFC,and the utilized methods point out new strategies to develop semiconductor ionic electrolytes.