Date of Award

1-1-2012

Document Type

Campus Access Dissertation

Department

Mechanical Engineering

First Advisor

Fanglin Chen

Abstract

Solid oxide fuel cells (SOFCs) are considered one of the most promising technologies for future energy conversion, and can operate on a wide range of fuels including fossil-based hydrocarbon fuels such as petroleum, diesel and natural gas as well as bio-derived hydrocarbons. The primary challenge in realizing direct oxidation of hydrocarbon fuels in SOFCs lies in the design and development of materials and catalysts for the SOFC anode. The objective of this study is to explore ceramic-based anode materials to meet the basic requirements of SOFCs operating in sulfur-containing hydro-carbon fuels. In this study, both LaGaO3 and LaCrO3-based¬ systems have been explored as anode materials for SOFCs. The results have shown that these two systems are promising anode materials by judicious selection of dopants on both A- and B- sites of the perovskite structure.

For the LaGaO3-based system, Sr- and Mn-doped LaGaO3 (La0.8Sr0.2Ga0.5Mn0.5O3-δ, LSGMn) has been synthesized by both GNC (glycine nitrate combustion) and SSR (solid state reaction) methods. XRD examination shows that the synthesized LSGMn has a pure single primitive cubic phase. Four-probe DC conductivity characterization indicates that GNC method is effective to synthesize single-phase perovskite LSGMn with relatively high electronic conductivity. The conductivity of the sintered LSGMn samples is lower in H2 than that in air while the activation energy in H2 is higher than that in air. Such an expected p-type conduction mechanism is the result of valence change of manganese, enabling the LSGMn a mixed ionic and electronic conductor. The fuel cell performance evaluation shows that LSGMn can be a potentially good anode material for SOFCs using La0.8Sr0.2Ga0.83Mg0.17O3-δ (LSGM) as the electrolyte. Peak power density of the LSGM electrolyte-supported all-perovskite ceramic cell can reach 460 mW cm-2 at 800 oC using H2 as fuel and ambient air as oxidant. LSGMn anode also has shown reasonably good sulfur tolerance when tested in H2 with 100 ppm H2S and good long-term stability at a constant cell voltage of 0.7 V at 800 oC.

For the LaCrO3-based system, the samples have been synthesized by a sol-gel combustion method and evaluated as anode materials for SOFCs. For LaCrO3 doped with calcium and cerium on the A-site in the series of La0.9-xCaxCe0.1CrO3-δ (LCCC3060, LCCC4050, LCCC5040, LCCC6030 with x = 0.6, 0.5, 0.4, and 0.3 respectively), relatively higher Ca-doping is found to improve both electronic and ionic conductivity. LCCC samples have demonstrated good chemical stability in reducing atmospheres. Evaluation of the LCCC material as anode in symmetrical cell configuration shows that the highest Ca-doping composition results in the lowest activation energy and the lowest polarization resistance. Performance evaluation of LSGM electrolyte-supported single cells with LCCC3060 as the anode and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) as the cathode show that LCCC3060 is a potential anode material when using H2 as fuel, but shows relatively poor performance when using CH4 as fuel. In order to further improve the electronic conductivity and catalytic activity of LaCrO¬3-based system doped with calcium and cerium on the A-site, 50% manganese has been used to substitute chromium on the B-site. XRD patterns of LCCCM (La0.5Ca0.4Ce0.1Cr0.5Mn0.5O3-δ) show that this material is stable in reducing atmospheres at the typical SOFC operating temperatures. Both conductivity measurement and symmetrical cell test indicate that doping manganese to the B-site can significantly improve the electrochemical properties of the LaCrO3-based materials. LSGM electrolyte-supported cells with LCCCM as anode and LSCF as cathode have been evaluated in H2 (with 3 vol% H2O), H2 with 50 ppm H2S and CH4. The cell output power density can reach 830 mW cm-2 at 900 oC using H2 as fuel and ambient air as oxidant. The stability test of the cells using fuel of either H2 with 50 ppm H2S or CH4 indicates that LCCCM material can be a promising anode for SOFCs operating on hydrocarbon fuels with ppm level H2S. EDS analysis and XPS characterization demonstrate that sulfur exists as SO42- and SO32- when sulfur poisoning takes place on the LCCCM anode and these sulfur containing species can be removed by removing sulfur containing species in the fuel stream at the cell operating temperature.

In conclusion, ceramic-based perovskite type materials have been fabricated and studied as anode materials for SOFCs. Various characterization techniques have been used to characterize the potential anode materials for SOFCs. Sr- and Mn-doped LaGaO3 shows reasonable cell performance in H2 and H2 with 100 ppm H2S, but the performance using CH4 as fuel is relatively low. As for the LaCrO3-based system, doping higher amount of calcium on the A-site can improve both electronic and ionic conductivities, and 50% manganese doping on the B-site can further improve electronic conductivity and catalytic activity, resulting in relatively promising cell performance in H2 and fairly stable cell performance in CH4 and H2 containing 50 ppm H2S. In summary, by judicious selection of the doping elements and doping concentration, both LaGaO3 and LaCrO3-based systems can be potential anodes for SOFCs directly operating on sulfur-containing hydro-carbon fuels.

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