Study On CeO₂-based Electrolytes For Single Component Fuel Cells

In the wake of global climate change and increasing oil prices,the demand for new sustainable energy technologies has dramatically increased.Solid oxide fuel cells（SOFCs）have been considered as one of the most promising energy conversion technologies due to their high-energy conversion efficiency,low emission and excellent fuel flexibility.Conventional SOFCs have a construction of three components:anode,electrolyte and cathode.Among the three components,the electrolyte is commonly an ionic conductor for oxygen ion transport,serving as the key component.However,conventional SOFC technology requires a high operating temperature because electrolytes（e.g.yttrium stabilized zirconia,YSZ）could only reach a sufficiently high oxygen ion conductivity at 800-1000°C,resulting in the mismatch of thermal expansion,improper inter-diffusion and interaction between the electrolyte and the electrodes.Equally imperfectly,high temperature operation restricts the selection of cell component materials,leading to a high cost.Thus,the electrolyte is actually a barrier for SOFC commercialization.To develop economically competitive SOFCs for low temperatures,single component fuel cell（SCFC）as a breakthrough technology was recently proposed and developed,which was constructed by using only one core component,a multifunctional layer based on two-phase nanocomposite materials with mixed ionic and electronic conductivities,instead of three components（anode,electrolyte and cathode）in classical fuel cells.The working principle and mechanisms of the SCFCs were also elucidated,which was based on nano-redox and p-n bulk-heterojunction structures.With the aiming of continuously promoting high performance SCFCs,a possible protocol is to seek highly-effective candidate materials.This work focused on the synthesis and characterization of mixed ionic-electronic conductor nanocomposites and their application for achieving high-performance SCFCs.To be specific,the following are key results I have made in this paper:（1）The mixture of ionic electrolyte Ce_0.8Sm_0.05Ca_0.15O_2-δ（SCDC）and semiconductor Ni_0.8Co_0.15Al_0.05LiO_2-δ（NCAL）layers was used for SCFC applications.Using the as-prepared SCDC-NCAL semiconductor-ionic layer to replace the ionic SCDC electrolyte,following results have been obtained:the SCDC electrolyte fuel cell reached a lower voltage,1.05 V,and lower power output,415 mW cm^-2,compared to that using the semiconductor-ionic layer,1.06 V and 617 mW cm^-2 at 550°C.The electrochemical impedance spectroscopy（EIS）was applied to investigate the electrochemical processes of the device;X-ray diffraction（XRD）and field emission scanning electron microscope（FE-SEM）for the microstructure and morphology of the as-prepared materials.The results have illuminated that the introduction of semiconductor into ionic electrolyte could make extended triple phase boundary（TPB）area,which can provide more active sites to accelerate the fuel cell reactions and enhance the cell performance.Furthermore,we also discovered that the ionic SCDC and electronic NCAL should be in an appropriate composition to achieve a balanced ionic and electronic conductivity,which is the key issue for high performance SCFCs.（2）A novel composite was further fabricated by incorporating the semiconductor NCAL into the ionic electrolyte Ce_0.8Sm_0.2O_2-δ-Na₂CO₃（NSDC）,and developed as a mixed-conducting membrane for SCFC applications.Experimentally,the crystal structure,morphology,chemical composition and thermo-stability of the composite were characterized by XRD,SEM and TGA.The best cell performance was investigated when the NSDC-NCAL membrane was optimized at a weight ratio of 6:4.On this basis,a number of interesting findings were obtained:i)the mixed conducting membrane did not cause any short circuit;on the contrary,the cell reached a decent OCV of¹.0 V.ii)a high power density of 1072 mW cm^-2 was achieved at 550°C for the NSDC-NCAL membrane based cell,which was much better than that using a pure NSDC electrolyte membrane.EIS showed that the NSDC-NCAL composite exhibited significantly improved grain boundary conduction and reduced electrode polarizations,contributing to the resultant performance.To consolidate the usefulness of the device,we also conducted the durability test.The above findings indicate the strategy of introducing mixed NSDC-NCAL membrane is feasible for high-performance SCFCs operating at low temperatures.This work illuminated the use of semiconductor-ionic composite materials as an effective way in designing novel fuel cell devices with improved properties,which are prior to traditional pure ionic electrolyte for SOFC applications.By providing technical advantages of simplified fabrication process,the as-designed SCFC device can be expanded into a wide variety of applications,both from industrial and scientific aspects.