| Solid oxide fuel cells (SOFCs) have attracted much attention due to their high energy efficiency, modularity, and excellent fuel capability. To study and develop intermediate temperature solid oxide fuel cells (IT-SOFCs), which operate at 600-800℃, is the developmental direction of SOFCs in order to widen the selectivity of component materials and improve the reliability of SOFC system. On the other hand, the overall electrochemical performance of SOFC will decrease as temperature drops. This is mainly due to the increased polarization resistances for electrode reactions and the decreased electrolyte conductivity at lower temperature.The resistive contribution of the electrolyte can be reduced by decreasing the thickness of electrolyte to the microscale on an anode-supported structure. Thus, the overall performance loss is increasingly dominated by the polarization loss of the electrode reactions, and especially the cathode one. There are several approaches which can be employed to decrease the cathodic polarization resistance:(1) using mixed ionic-electronic conductor materials with higher catalytic activity at lower temperature; (2) adding an ionically conducting second phase to form a composite cathode; (3) optimizing the microstructure of the cathode, such as the pore size, porosity and grain size. During recent years, many research efforts have been made to develop alternative cathodes or composite with high performance but few studies have been conducted to optimize properties by microstructure design.La0.6Sr0.4Co0.2Fe0.8O3-δperovskite oxide has been extensively investigated and utilized as a cathode material for IT-SOFCs due to its high ionic and electronic conductivity at lower temperatures. In this thesis, the effects of pore formers on microstructure and electrochemical performance of LSCF were studied firstly, then, LSCF powders with different particle sizes were synthesized using different synthesis methods. Based on these results, the 3-layer LSCF cathodes with graded grain size, pore size and porosity were fabricated by low-cost tapecating technique. It was showed that when graphite content for inner layer is 10wt% and corn starch content for outer layer is 35wt%, the graded LSCF fired at 1050℃exhibited the best performance. The lowest interfacial resistances at 800-650℃were 0.053,0.11,0.27 and 0.65Ω·cm2, respectively, which were much lower than that reported in literatures. The maximum power density of Ni-YSZ| YSZ| SDC| LSCF cell with graded LSCF cathode is 1.0 and 0.58 Wcm-2 at 800 and 700℃, respectively. SEM micrographs showed that 3-layer porous structure was good bonded to the electrolyte support and no cracking can be found among different layers. Impedance spectra analysis showed that both charge transfer reactions and the oxygen mass transport were enhanced for graded porous LSCF cathode.While cells with (La, Sr)(Co, Fe)O3-δcathodes offer significantly higher power densities than those with La(Sr)MnO3 cathodes, long-term stability of LSCF cathodes seem to be a concern. Thus, the following part in this thesis would focus on modification of LSCF surface by infiltration process to improve the electrochemical performance and long term stability. In first part, LSCF-Sm0.2Ce0.8O1.95 (SDC) composite cathode was fabricated by infiltration using SDC as infiltrant. XRD spectra showed that the phase formation temperature of SDC was 900℃and no other phase was found in SDC infiltrated LSCF cathode at this temperature. The diameters of new formed SDC nanoparticles were around 20-80nm, which are distributed uniformly on the surface of LSCF backbone with narrow size distribution. The SDC particle size can be controlled by changing concentration of infiltrated solution. Impedance analysis indicates that the SDC infiltration has dramatically reduced the polarization of LSCF cathode, reaching the lowest interfacial resistances of 0.074, and 0.44Ω·cm2 at 750 and 650℃, respectively, which was more than 50% improvement as compared to measurement on the blank LSCF cathode. Meanwhile, ohmic resistance of the symmetrical cell at 800 and 650℃did not show ant increase when the concentration of infiltrant increasing from 0 to 0.35mol/L. The dramatic decrease in the electrode polarization resistance is mainly attributed to the extension of triple phase boundary (TPB) reaction area. The maximum power density of anode-supported cell with SDC infiltrated LSCF cathode was about 1.11 and 0.85 Wcm-2 using H2+3%H2O as fuel. Furthermore, the infiltrated cell showed improved long-term stability within the time range of testing although the detailed mechanism is yet to be determined. These results indicated that the potential promise of ionic conductor infiltration as a method for enhancing the electrocatalytic activities and long-term stability of MIEC cathodes such as LSCF.The last part in this thesis studied La0.85Sr0.1503-δ(LSM) infiltrated LSCF cathodes. The electrochemical performance of LSCF-LSM electrodes and long term stability was investigated under cathodic polarization. At last, the mechanism of LSCF degradation and LSM enhanced LSCF cathode stability was analyzed. The resistance impedance spectra of blank LSCF and LSM infiltrated LSCF cathode under different overpotentials at 700℃showed that under open circuit, the interfacial resistance of LSM infiltrated LSCF cathodes was 0.35Ω·cm2, higher than that of blank LSCF cathodes 0.24Ω·cm2. This was because LSM was pure electronic conductor with less TPB length comparing with mix-conducting LSCF materials. While continuing increasing the cathodic overpotentials, the interfacial resistance of LSM infiltrated LSCF cathodes decreased dramatically. In contrast, the interfacial resistance of LSCF cathodes decreased slowly at the same conditions. The interfacial resistance of LSM infiltrated LSCF cathodes was 0.18Ω·cm2 comparing that of 0.21Ω·cm2 for blank LSCF cathodes. The cell Ni-YSZ| YSZ| SDC| LSCF-LSM showed good stability during 120h test and the output voltage (at 400 mA/cm2 constant current) was increasing all the time. The output voltage was 0.753V operation and the maximum power density was 0.401 Wcm-2 at 700℃after 120h operation, implying about 27% improvement.A new electrode prepared by radio frequency (RF) sputtering is designed to study the electrochemical reactions on the air-exposed surface. It was found that LSM thin film enhanced surface activity for oxygen reduction reaction and the electrochemical performance was improved for LSM coated LSCF cathode under polarization conditions. Raman spectroscopy showed that no obvious main peaks shift were experienced for LSM coated LSCF cathode before and after polarization, representing superior stability of this new cathode.
|
PDF Full Text Request