Investigation On The Performance Of Cathode And Proton Conductor Electrolyte Materials For Intermediate-temperature Solid Oxide Fuel Cells

Fuel cell?FC?is an high energy-conversion reactor that directly converts chemical energy into electrical energy by electrochemical reaction.Solid oxide fuel cell?SOFC?have attracted much attention because of its many advantages of all solid state,high energy conversion efficiency,low pollutant emission,fuel flexibility and so on.Traditional SOFC system was operated at high temperature?¹000??,High operating temperature hinder the SOFC commerical application,such as the high costs of boosting SOFC system,the choice of the interconnect and package materials are limited.Moreover,the dramatical decreasing of the cell performance since the seriously chemical reaction of cell components at high temerperature.All above disadvantage factors restrict the commerical application of SOFC.For the purpose of widen the range of materials selection and reduce the costs of SOFC,considerable efforts have been devoted to?reducing?lowing the operating temperature of SOFC to intermediate temperature range?500-800??.However,the interface polarization resistance of cathode's and ohmic resistance of electrolyte lead to cell performance obviously decrease in the intermediate temperature range and restrict its practical application.Therefore,in order to accelerate the practical application of SOFC,it is necessary that developing properly cathodes and electrolytes of SOFC with high electro-catalytic activity.This thesis is aimed to deeply research the cathodes and electrolytes with high electrochemical performance of Intermediate-temperature SOFC?IT-SOFC?.It maybe supply some helpful ideas for accelerate the practical application of SOFC.This thesis is focused on optimize two aspects of the cathodes and electrolytes of IT-SOFC.Firstly,we focused on develop new cathodes of SOFC with high cost-effective,good chemical compatibility and thermal stability with cell components during cycles.On the basis of characterize the physics properties of cathode materials,we fabricate the cell and measure the output power performance under the SOFC working atmosphere.Secondly,in view of the proton conductor electrolyte materials with high ionic mobility and no dilute of fuel gas,this thesis attempt to prepare the Sm and Nd co-doped BaCeO₃-based electrolyte materials Ba Ce1-2xSmxNdxO_3-??abbreviate as BCSN?.The effect of Sm and Nd co-doped on the phase structure,sintering properties and ionic conductivity were investigated.Electrolyte-supported single cell was fabricated and measured the output power density in the intermediate temperature range.Layered perovskite-type Ln BaCo2O_5+? oxides as promising candidate cathode matreias have received much attention since its high oxygen surface exchange coefficient and diffusion coefficient.Unfortunately,high thermal expansion coefficient?TEC?lead to poor thermal cycling stability and limits its practical application.Strontium?Sr?partially substitute Barium?Ba?of A,site and Copper?Cu?partially substitute Cobalt?Co?of B site,which may offer a trade-off between electro-catalytic activity and thermal stability of cathode material.Layered pervoskite Ln Ba_0.5Sr_0.5Co Cu O_5+??Ln=Pr,Sm??abbreviated as PBSCCo,SBSCCo?oxides as SOFC cathode materials were prepared by sol-gel method.XRD data analysis indicate that PBSCCo,SBSCCo are good chemical compatible with GDC,LSGM electrolytes.XPS results suggest that Co4+/3+,Cu2+/+,Sm3+,Ba2+and Sr2+ species co-exist in the SBSCCo sample powder at room temperature.Moreover,the mixed ionic state of Co4+/3+and Cu2+/+ play a key role for improve the electro-catalytic activity and conductivity.The average thermal expansion coefficient?TEC?of PBSCCo,SBSCCo cathode are 17.58×10-6K-1,16.14×10-6K-1 between 30 oC and 850 oC in air,respectively.The average TEC of composite cathode SBSCCo-50 GDC is 13.14×10-6K-1 between 30 oC and 850 oC in air,which is lower 18.5% than single phase SBSCCo cathode.Composite cathode of SBSCCo cathode and GDC electrolyte is beneficial to improve thermal match with common electrolyte.The conductivity of PBSCCo and SBSCCo sample gradually increases with temperature increasing,then decrease with temperature increasing.The conductivity behavior is related to semiconductor-metal transition.And the conductivity of PBSCCo is higher than SBSCCo in the whole test temperature range.Symmetrical cell and electrolyte-supported single cell are prepared by screen-printing method.The polarization resistance?RP?of PBSCCo and SBSSCCo cathodes on GDC electrolyte are 0.0422Wcm and 0.04882Wcm at 800? in air,respectively.At the same temperature,the RP of SBSSCCo cathode on LSGM electrolyte is 0.03782Wcm.The RP results indicate that the electrochemical performance of SBSCCo cathode based on LSGM electrolyte is better than on GDC electrolyte.The maximum power densities of electrolyte-supported single cell with PBSCCo and SBSCCo cathodes on 0.3mm thick GDC electrolytes reach 521 m Wcm-2 and 481 m Wcm-2 at 800 oC,respectively.This primarily results indicate that oxygen reduction reaction catalytic activity of PBSCCo is higher than SBSCCo.The maximum power density of electrolyte-supported single cell with SBSCCo cathode on 0.3mm thick LSGM electrolyte reach 646 m Wcm-2 at 800?.Moreover,Composite cathode of SBSCCo–x GDC?x=20,30,40,50wt%?can evidently decrease the Rp of composite cathode on GDC electrolyte.The lowest Rp of SBSCCo –30wt%GDC composite cathode is 0.0222Wcm at 800? in air.The maximum power density of electrolyte-supported single cell with SBSCCo–30wt%GDC composite cathode on 0.3mm thick GDC electrolyte reach 648 m Wcm-2 at 800?.This above results indicate that composite cathode of single cathode with electrolyte can effectively improve the electrochemical performance.Ba_0.5Sr_0.5Co_0.8Fe_0.1Ni_0.1O_3-??abbreviate as:BSCFNi?is excellent oxygen permeable membrane material.The high oxygen permeability is beneficial to achieve rapid oxygen diffusion and transport rate of SOFC cathode interior.For this purpose,the single phase of simple pervoskite structure sample BSCFNi was prepared by sol-gel method after sintered at 1000? for 10 hours and measured the elcetrochemical properties as IT-SOFCs cathode.XRD spectra show that there are no chemical reaction between BSCFNi cathode and LSGM electrolyte below 950 oC,but weak chemical reaction with SDC electrolyte is observed.XPS results show that Co4+/3+,Fe4+/3+,Ni3+/2+,Ba2+ and Sr2+ metal species exist in the BSCFNi sample powder at room temperature.Mixed ionic states of Co4+/3+,Fe4+/3+ and Ni3+/2+ are beneficial to improve the electrochemical performance of SOFC cathode.The average TEC of BSCFNi cathode is 18.7×10-6K-1 between 30? and 850? in air.The electrical conductivity behavior exhibit P-type small polaron hopping conduction law and the maximum conductivity is 35Scm-1at 425? in air.Electrochemical results indicate that the Rp of BSCFNi cathode on LSGM and SDC electrolyte are 0.033 and 0.0662Wcm at 800? in air,respectively.The maximum output power density of electrolyte-supported single cell with BSCFNi cathode on 0.3mm thick LSGM electrolyte reach 690 m Wcm-2 at 800 oC.La_0.6Ba_0.4Co_0.8Fe_0.2O_3-??abbreviate as:LBFCo?is considered as a stability SOFC cathode mateial because of its good resistance to the nature of interconnect material Cr poisoning.Therefore,single phase simple pervoskite LBFCo cathode is synthesized by sol-gel method and characteriaed the electrocheemical performance as IT-SOFC cathode.XRD results show that between LBFCo cathode and GDC electrolyte is good chemical compatibility below 950?.The average TEC of LBFCo cathode is 22.4×10-6K-1 between 30?and 850? in air.The electrical conductivity of LBFCo sample gradually increases and then decrease with temperature increasing.The conductivity behavior exhibit P-type small polaron hopping conduction law and the maximum conductivity is 666Scm-1at 325? in air.High conductivity is beneficial to improve the electro-catalytic activity of SOFC cathode.The Rp of LBFCo cathode on GDC electrolyte is 0.0352Wcm at 800? in air.The maximum power density of electrolyte-supported single cell of LBFCo cathode on 0.3mm thick GDC electrolyte reach 554 m Wcm-2 at 800?.Proton conductor electrolyte have received much interests because of its some advantages with high ionic conductivity and high energy conversion efficiency.Rare-erath elements doped BaCeO₃ can significantly improve the conductivity and sintering activity of proton conductor electrolyte.Sm and Nd equimolar co-doped Ba Ce1-2xSmxNdxO_3-??2x=0-0.25??abbreviate as :BCSN?series of proton conductor electrolyte material are obtained by EDTA-cirtrate complexing method.XRD results show that series electrolyte of BCSN powders are pervoskite crystal structure after sintered at 1050? for 10 hours.After sintered at 1400? for 10 hours,lattice parameter has a linear growth with the increase of Sm and Nd co-doped molar contents.This result indicated that Sm₂O₃,Nd₂O₃ are completely solid solution in the BaCeO₃ lattice.Raman spectra results show that the vibrational modes associated with oxygen vacanies are presented in the vicinity of 630cm-1 in the BCSN series.With the molar doping ratios increase of Sm and Nd,structural symmetry of BCSN samples are enhanced which may be accompancied by crystal structure from orthorhombic to tetragonal phase transition.Combined with SEM results that Sm and Nd total molar doped content reach 2x?0.15,the electrolyte disk are very dense after sintered at 1400? for 10 hours.It indicate that apropriately content of Sm and Nd co-doped can obviously improve the sintering properties of electrolyte.The conductivity of BCSN series electrolyte are obviously higher than pure BaCeO₃ electrolyte,when the Sm and Nd total molar doping amount of 2x=0.15,the ionic conductivity reach 0.032scm-1which is highest in the series electrolyte samples at 650? in the humid H2?3%H2O?atomsphere.The maximum power density and open circuit voltage of electrolyte-supported Ba Ce0.85Sm0.075Nd0.075O_3-? single cell reach 129 m Wcm-2 and 1.09 V at 650?in the humid H2?3%H2O?atomsphere.The electrochemical performance of Sm and Nd co-doped BaCeO₃ electrolyte are higher than pure BaCeO₃ electrolyte.This above results show that Sm and Nd co-doped can significantly improve the ionic conductivity of BaCeO₃ based electrolyte materials,but deeply undestanding the electrochemical performance of proton conductor electrolyte materials also needs to carry out more detail work.