Design,Preparation And Performance Of Cobalt-Free Perovskite Electrode Materials For Symmetrical Solid Oxide Fuel Cells

Solid oxide fuel cells?SOFCs?convert chemical energy directly into electric power in a highly efficient and environmental friendly way.Currently,the bottle-necks of commercialization of SOFC techniques are high cost in manufacturing and application,as well as low durability of devices and components.Symmetrical SOFCs?SSOFCs?have attracted increasing attention due to its remarkable advantages for improved thermomechanical compatibility of the electrolyte and the electrodes,simplified the fabrication process then reduced fabrication cost,and enhanced immunity to coking and sulfur poisoning.While the state-of-the-art anode and cathode?Ni-YSZ,LSM?are not stable under the other electrode condition.The performance of SSOFC is also limited to the electrodes which have stable phase structures under both reducing and oxidizing atmospheres.Moreover,many novel electrode materials show high SSOFC performance are modified cobalt-based elctrodes and still suffer from high thermal expansion coefficient,unsatisfied stability.In this thesis,systematic investigations on the high perfromance and high stability cabalt-free symmetrical electrode materials for SSOFC have been carried out.La_0.75Sr_0.25Cr_0.5-xMn_0.5Cu_xO_3-??LSCMCu_x,x=0,0.05?peroviskite oxides are firstly investigated and Ln_0.5Sr_0.5Fe_0.8Cu_0.2-xM_xO_3-??LnSFCM_x,Ln=Pr,La,M=Cu,Sc,Ti,V,Zr,Nb,Mo,W,x=0-0.15?series materials are prepared,characterized and optimized.The potential of LSFCN_0.05.05 for using as reversible SOFC?R-SOFC?oxygen electrode has also been evaluated.La_0.75Sr_0.25Cr_0.5-xMn_0.5Cu_xO_3-??LSCMCu_x,x=0,0.05?symmetrical electrodes are studied for SSOFC.The optimized sintering temperature for LSCMCu_x?x=0,0.05?electrodes is 1100 ^oC.With Cu substitution,the polarization area specific resistance?R_p?of LSCMCu_x?x=0,0.05?in wet 5%H₂-Ar at 900 ^oC is 0.38 and 0.51?cm²,and the R_p in air at 900 ^oC is 0.93 and 0.29?cm²,respectively.We could see the anodic performance is decreased by 34%while the cathodic performance is enhanced by 69%,which could explain the reason of performance enhancement 19%(203 and 242 mW cm^-2)for SSOFC with LSCMCu_x?x=0,0.05?symmetrical electrodes.Furthermore,the performance of SSOFC could be further improved by inducing a graded composite electrode with ScSZ electrolyte,the peak power density?PPD?reached to 420 and 496mW cm^-2,respectively,and the value is also enhanced by 20%with Cu doping.In order to improve the performance of SSOFC,cobalt-free Pr_1-xSr_xFe_0.8Cu_0.2O_3-??PSFC_x,0?x?0.5?and Pr_0.5Sr_0.5Fe_1-yCu_y O_3-??PSFC_y,0?y?0.3?perovskite oxides are prepared as cathodes for IT-SOFC,and the effects of A-site Sr²⁺and B-stie Cu²⁺content on the electrochemical performance are studied.Sr and Cu substitutions show a synergistic effect on increasing unit cell symmetry,oxygen vacancy generation,electrical conductivity and electrochemical activity,while they demonstrate a competing way in the surface segregation.The composition PSFC5582?x=0.5,y=0.2?exhibits the lowest R_p of 0.018,0.029,0.050,0.129,0.304 and 0.928?cm² at 800,750,700,650,600 and 550 ^oC,respectively.The value is lower than many previously reported cobalt-containing or cobalt-free cathodes.And the cell with PSFC5582cathode shows the highest PPD of 1094 mW·cm^-2 at 800°C.All the results indicate that the PSFC5582 is a promising cathode for IT-SOFC.The increase of the unit cell symmetry,the largest value of oxygen deficiency???and lattice free volume?V_F?,are supposed to be responsible for the improvement of electrochemical performance of PSFC5582 by promoting the generation and migration of oxygen vacancies,the decrease of thermal expansion coefficient and the increment of electrical conductivity.B site cations with different valence sustituted Pr_0.5Sr_0.5Fe_0.8Cu_0.2-xM_xO_3-??PSFCM,M=Cu,Sc,Ti,V,Zr,Nb,Mo,W?perovskite oxides were synthesized by solid state reaction method and applied as both anode and cathode for symmetrical solid oxide fuel cells?SSOFCs?.The compostions can not form a single phase perovskite structure with V and Zr substitution.The electrical conductivity and TEC of decreased with incresing Nb content x,as well as the cathodic performance of PSFCN_x.TG experiments in 5%H₂-Ar at 800 ^oC show the best stability of the Nb doped samples in reducing conditions,suggesting that PSFCN_0.1 may be suitable for both cathode and anode applications.The SSOFC with PSFCN_0.1 as symmetrical elctrodes show a maximum power density of851 and 652 mW cm^-2 at 900 and 850 oC,respectively,using wet hydrogen?³%H₂O?as fuel.The ionic radii and valence lead a different influence on the cyrstal distortion,electrical conductivity,TEC and cathodic performance.The PSFCSc based SSOFC reached the highest PPD of 1030 and 822 mW cm^-2 at 900 and 850 ^oC,respectively.Howerver,the relatively high price of Pr,Sc and Mo is also an important limitation to the practical application of these materials.More affordable La_0.5Sr_0.5Fe_0.8Cu_0.2-xNb_xO_3-??LSFCN_x,x=0-0.20?perovskite oxides were synthesized by solid state reaction method and used for symmetrical electrodes of SSOFC.Nb substitution lead a decreasing of electrical conductivity,TEC and cathodic performance especially at a high content?x>0.05?,while the stability of LSFCNx in wet 5%H₂-Ar is enhanced significantly with increasing Nb content.Nevertheless,all the LSFCN_x samples are reduced to a mixture of SrFeLaO₄,a K₂NiF₄-type layered perovskite oxide,LaFeO₃-typed perovskite oxide and metallic Cu.After treated in air at 900 ^oC for 2 h,intrestingly,the mixture shows a reversible transition to a cubic perovskite phase.Howerver,the redox behavior of LSFCN_x is not the same to PSFCN_x because no La₂O₃ could be found in reduction mixture XRD patterns compared to the Pr₂O₃ for PSFCN_x.TEC of LSFCN_0.05,0.10 is as low as 16.1 and 14.5×10^-66 K^-1 in air between 25-900 ^oC,which is beneficial for the adhesion between electrode and electrolyte,as well as the long-term stability of SSOFC.LSFCN_0.05 based SSOFC reached the highest PPD of 1094 and 907 mW cm^-2 at 900 and 850 ^oC,respectively,and a relatively stable long-term performance is also demonstrated.Finally,the R-SOFC test suggesting the potential application of LSFCN_0.05 as oxygen electrode for R-SOFC.