Fabrication And Electrochemical Study Of Nb-doped Cathode Materials For IT-SOFCs

Solid oxide fuel cells(SOFCs)is a new energy conversion device.SOFCs can convert chemical energy stored in fuel into electrical energy,and it is recognized as an environment-friendly power conversion device with high efficiency.The operational temperature of traditional SOFC is too high,which makes it difficult to maintain long-term stability in the operation process,and also causes material damage and performance degradation.These problems hinder the large-scale commercialization of SOFCs.Therefore,reducing the operating temperature of SOFCs is a major challenge.In this field,in order to achieve the high performance of SOFCs in intermediate temperature range,it is vital to develop suitable cathode materials.In addition,since proton conducting electrolyte is more suitable for intermediate and low temperature conditions,the development of cathode materials for proton conducting fuel cells is also an important research direction.In this dissertation,the research and analysis of suitable cathode materials for intermediate temperature SOFCs(IT-SOFCs)were carried out.In the first chapter,we introduced our research background.The definition,working principle and the research progress of oxygen ion conducting solid oxide fuel cells(O-SOFCs)and proton conducting solid oxide fuel cells(H-SOFCs)were introduced.We also focused on the mechanism of cathode reaction kinetics and the development of SOFCs materials.In the second chapter,we optimized the traditional high-performance cathode Ba0.5Sr0.5Co0.8Fe0.2O3-?.(BSCF)by doping niobium,and synthesized a series of composite cathodes Ba0.5Sr0.5Co0.8-xFe0.2NbxO3-?-SDC(x=0,0.05,0.1).This series of cathodes were applied for O-SOFCs.We investigated the effects of different Nb-doping level from different aspects,including the crystal structure,thermal expansion coefficient,electrical conductivity,solid surface composition,electrochemical performance and stability in CO2 atmosphere.With the increase of Nb-doping level from 0 to 0.1,we found that the thermal expansion coefficients of cathodes decreased from 21.74*10-6 K-1 to 18.74*10-6 K-1,and the chemical stability in CO2 atmosphere increased significantly.Through the electrochemical tests of symmetrical-cells and single-cells,we can know that Ba0.5Sr0.5Co0.75Fe0.2Nb0.05O3-? has the minimum polarization impedance value,and can reach the maximum power density of 882mW·cm-2 at 700?.A series of experimental results showed that the chemical stability and the electrochemical performance of Ba0.5Sr0.5Co0.75Fe0.2Nb0.05O3-?(BSCFNb0.05)formed by 5%Nb-doping is obviously improved compared with undoped BSCF.The results revealed that BSCFNb0.05 is a potential cathode of IT-SOFCs.In the third chapter,SrFe0.9Nb0.1O3-?-BaZr0.1Ce0.7YO.2O3-?(SFN-BZCY)composite cathode for proton-conducting SOFCs(H-SOFCs)was developed by infiltration technology.Through infiltration technology,we obtained the SFN deposition layer with nanostructure.We used BaZr0.1Ce0.7Y0.2O3-? as a backbone material,and synthesized SFN-BZCY composite cathodes with different SFN infiltration amount.We applied the SFN-BZCY composite cathodes to BZCY-based H-SOFCs.The electrochemical test results showed that the polarization impedance of SFN-BZCY composite cathode reached the minimum value of 0.05 ?.Cm2,and the power density reached the maximum value of 1186 mW·cm-2 when the infiltration amount was 47.4wt%.It is worth mentioning that the single-cell with?47.4w%SFN infiltrated cathode exhibits the power density of 198 mW·cm-2 at 450?,which means that this cathode material may even be used in low temperature range.SFN-BZCY cathode prepared by infiltration is a kind of cathode material with excellent performance for IT-SOFCs.In the fourth chapter,we proposed a new single-phase cathode BaFe0.4Sn0.2Zr0.1Y0.3O3-?(BFSZY4213),and applied it to H-SOFCS.The cubic structure of BaFeO3-? can be stabilized at room temperature by the Co-doping of Sn,Zr and Y.At 700?,the maximum power density of the single-cell with the BFSZY4213 cathode is 771 mW·cm-2,and the polarization impedance is 0.09 ?·cm2,which indicates that BaFe0.4Sn0.2Zr0.1Y0.3O3-? has the potential to be used in H-SOFCS.The fifth chapter summarized the whole dissertation.At the same time,the shortcomings of our research work and the possible research direction in the future are prospected.