Font Size: a A A

Preparation And Properties Of Anode And Interconnect Materials For Solid Oxide Fuel Cells

Posted on:2012-01-21Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y ShenFull Text:PDF
GTID:1101330335952970Subject:Condensed matter physics
Abstract/Summary:PDF Full Text Request
Compared with other fuel cells, solid oxide fuel cell (SOFC) has a high operating temperature (600-1000℃), which also makes it has a high tolerance to typical catalyst poisons, produces high-quality heat for reforming of hydrocarbons, and offers the possibility of direct utilization of hydrocarbon fuels. Direct utilization of hydrocarbons as fuel is a major advantage of SOFC. However, the commonly used Ni-based composite anode will suffer from carbon deposition and sulfur poisoning in the operating temperature which makes the cell performance reduce dramatically. To overcome these problems, substantial effort has been devoted to the development of new anode materials and novel electrode structures, such as Cu-CeO2-YSZ, La4Sr8Ti11Mn0.5Ga0.5O37.5,(La0.75Sr0.25)1-xCr0.5Mn0.5O3 and Sr2MgMoO6-δ. However, there is no report on LnBaFe2O5+δ(LnBFO, Ln=Pr, Nd, Sm, Gd) as an anode in SOFCs in the literatures. In addition, as the intermediate-temperature SOFC developed, there is a need for the development of new interconnect materials with good performance at reduced temperatures.The perovskite structure La0.75Sr0.25Cr0.5Mn0.5O3 (LSCM) anode material was synthesized using the glycine nitrate process. The thermogravimetric analysis result showed that the main phase composition LSCM in the sample begins to form after heating at 835℃. The phase evolution in the sample calcined at various temperatures was characterized by powder X-ray diffraction. The single-phase LSCM anode material with perovskite-structure was obtained by calcining the samples at 1100℃for 6h. The synthesize temperature in this study is lower than that in the sol-gel technique made by other researchers. The sinterability of the LSCM sample increases with the increase of the sintering temperature. The electrical conductivity of the LSCM sample sintered at 1400℃for 10 h is 28.6 S cm-1 in air and 1.3 S cm-1 in hydrogen at 900℃. respectively. Thermal expansion coefficient of the LSCM sample is 11.7×10-6 K-1 in the temperature range from 30 to 1000℃in air, which is close to that of the LSGM electrolyte (11.3×10-6 K-1). For the single-cell with LSCM|LSGM |La0.6Sr0.4Co0.2Fe0.8O3-Gd0.1Ce0.9O1.95, the maximum power density is 375 and 352 mW cm-2 at 850℃in humidified H2 and humidified CH4 fuels, respectively. However, the maximum power density of the single-cell in ethanol fuel is only 73 mW cm-2 at 850℃, indicating that the catalytic activity of the LSCM anode in the ethanol fuel needs to be improved.LnBFO (Ln=Pr, Nd, Sm, Gd) materials were synthesized using sol-gel technique and investigated as novel anode materials for SOFCs. The single-phase LnBFO samples with orthorhombic double-perovskite structure were synthesized after sintering at 1050℃for 10 h in 5% H2/Ar. XRD results showed that no chemical reaction was observed between the LnBFO anodes and the La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) (or SDC) electrolytes calcined at 950℃for 2 h, indicating that a good chemical compatibility between the two materials. The LnBFO materials showed a good semiconducting behavior in the reducing atmosphere in the temperature range studied. The electrical conductivity of the samples is 28.1, and 22.5 S cm" at 850℃in H2 for PBFO. NBFO. SBFO and GBFO. respectively. The electrical conductivity values were higher than those of the La0.75Sr0.25Cr0.5Mn0.5O3 and Sr2MgMoO6-δanode materials reported by other authors. The thermal expansion coefficients (TECs) of LnBFO material decrease from PBFO to GBFO. The average TEC value in the temperature range 30-900℃is 16.3×10-6 K-1,15.1×10-6 K-1,14.2×10-6 K-1 and 12.4×10-6 K-1 for the PBFO. NBFO, SBFO and GBFO samples, respectively, which is higher than those of the LSGM or SDC electrolytes. The electrochemical impedance spectroscopy results showed that the LnBFO anodes exhibited a good electrochemical performance in H2 at SOFC operating temperature. For example, the polarization resistance of the PBFO on LSGM electrolyte is 0.50, 0.56 and 0.70Ωcm2 at 850.800 and 750℃, respectively, which is slightly higher than that of the (La0.75Sr0.25)0.95Cr0.5Mn0.5O3-δanode in the similar conditions reported by Tao et al. For the single-cell with a configuration of PBFO|LSGM|SmBaCo2O5+δ, the maximum power densities reach 674 mW cm-2 at 850℃in H2 and 422 mW cm-2 at 900℃in humidified CH4 (with 3% H2O), respectively. The good electrochemical performance of the PBFO anode material in humidified CH4 indicates that the PBFO is a promising anode for direct utilization of hydrocarbon in SOFCs.The open-circuit voltage of an individual SOFC is low (around 1 V), thus interconnect material is used to connect individual cells in series and in parallel to realize high output voltages, currents and powers. We prepared Nd1-xCaxCrO3 (0≤x≤0.25) oxides using the solid-state reaction, and the results indicated that the composition Nd0.75Ca0.25CrO3 is a promising candidate as an interconnect material for application in SOFCs. The electrical conductivity of the Nd0.75Ca0.25CrO3 is 28.8 S cm-1 and 1.1 S cm-1 at 850℃in air and the H2 atmosphere, respectively. The average TEC of the Nd0.75Ca0.25CrO3 sample is 9.19×10-6K-1 in the temperature range from 30 to 1000℃. Because the sintering can be improved by using a chromium-deficient composition along with either calcium or strontium doping, in the same way, we prepared Cr-deficient Nd0.75Ca025Cr1-xO3-δ(0.02≤x≤0.06) oxides. Among these materials, the Nd0.75Ca0.25Cr0.98O3-δsample with a relative density of 96.3% showed the best electrical conductivity of 39.0 and 1.6 S cm-1 at 850℃in air and hydrogen, respectively. The TEC of the Nd0.75Ca0.25Cr0.98O3-δsample is 9.29×10-6 K-1 in the temperature range from 30 to 1000℃in air, which is close to those of the YSZ electrolyte and the other cell components. The obtained data indicate that Nd0.75Ca0.25Cr0.98O3-δis a potential interconnect material for application in SOFCs.
Keywords/Search Tags:Solid oxide fuel cell, Anode, Interconnect, Electrical property, Electrochemical performance
PDF Full Text Request
Related items