Electrochemical Performance of La and Ni co-doped SrTiO3 as an Alternative Anode of Solid Oxide Fuel Cell

Abstract

The generation of energy by clean, efficient and environmental-friendly means is now one of the major challenges. Interest in low carbon and renewable power generation has grown massively in the last 20 years after the acceptance of the idea that greenhouse gases are causing significant damage to the environment. Combustion of fossil fuels, by an overwhelming degree the most utilized power generation method in use today, is less efficient and causes significant environmental pollution. Fuel cells, devices which convert chemical energy to electrical energy with a higher efficiency than heat based power generators, are considered to be a promising technology. There are many kinds of fuel cells including polymer electrolyte membrane fuel cell (PEMFC), solid oxide fuel cell (SOFC) and etc. One of them, SOFCs using oxide-ion conducting ceramic material as the electrolyte of the cell have many advantages such as high efficiency, eco-friendly system and good fuel flexibility. SOFCs are constructed from anode and cathode electrodes which sandwich a solid oxide electrolyte. The most common anode materials of SOFCs are Ni-based cermet anodes such as Ni-yttria-stabilized zirconia (Ni-YSZ) or Ni-gadolinia-doped ceria (Ni-GDC). However, these anode materials suffer from low carbon deposition resistance by use of direct methane fuel. In addition, sulfur poisoning in hydrocarbon fuels, Ni coarsening during the cell fabrication and/or operation, and poor redox stability by repeating oxidation and reduction cycling are major problems of these Ni-cermet anodes. In this study, La (donor)-doped SrTiO3 (LST) perovskite oxide was used as an alternative anode material. LST with high electrical conductivity has shown good carbon coking or sulfur poisoning resistance and good redox stability compared with conventional Ni-cermet anodes. Exsolution generating electro-catalytic nanoparticles on the surface of perovskite oxides in reducing atmosphere was studied in LST by incorporating Ni to improve the electrochemical properties. The degree of Ni solubility in La0.2Sr0.8TiO3 and exsolution of Ni from La0.2Sr0.8Ti1-xNixO3-δ as well as kinetics parameters of exsolution (temperature and time) was studied (Chapter Ⅲ). The electrochemical performance (i.e., the impedance spectra and power density) of the cell was also measured. About half of the Ni observing the size of ~ 7 nm was exsolved in La0.2Sr0.8Ti0.9Ni0.1O3-δ (LSTN) at 800 °C in H2. While the Ni exsolution reaction is relatively fast, the Ti reduction reaction is slow and thus a high temperature or a long reduction time is required. The maximum power density (MPD) after exposure to H2 for 2h at 800 °C is relatively low, ~150 mW/cm2, due to the thick (~300 μm) electrolyte and low anodic performance. Improvement of the anodic performance of LSTN was investigated by varying the temperature (1100, 1250 oC) and atmosphere (air, H2) during LSTN anode firing to control the degree of Ni exsolution and anode microstructure (Chapter Ⅳ). The best anodic performance of LSTN was obtained by firing anode at 1250 oC in H2 gas due to the optimized microstructure and enhanced Ni exsolution. However, the MPD value in methane was still low due to low quantity of exsolved Ni particles. The effect of anode interlayer with Gd0.2Ce0.8O2−δ (GDC) and composite phase with electrolyte was investigated in order to further enhancement of anodic performance (Chapter Ⅴ). The LSTN-GDC composite anode with GDC anode interlayer showed dramatically improved anode performance by reducing the interfacial reaction between anode and electrolyte and by increasing the ionic conductivity. It showed high MPD value ~510 mW∙cm-2 after 100 h at 800 °C under an air/wet H2 gradient. Poor performance in CH4 should be still improved. Composition effect of A-site and/or B-site for La and Ni co-doped SrTiO3 anodes was investigated to improve the exsolution property by use of A-site deficiency (Chapter Ⅵ). LaxSr1-3x/2Ti1-yNiyO3-δ (x=0.2, y=0.1 for LSTN2791 / x=0.3, y=0.2 for LSTN3582) was used as A-site deficient composition by considering charge neutrality. On the other hand, LaxSr1-xTi1-yNiyO3-δ (x=0.2, y=0.1 for LSTN2891) was used as A-site stoichiometric composition. LSTN3582 anode showed the best performance in both H2 and CH4 fuels due to the largest amount of Ni exsolution. Redox stability and performance in H2 and CH4 of the cell with 3-phase as well as 2-phase anode were finally investigated (Chapter Ⅶ). Some part of GDC in LSTN-GDC was substituted to Ni to form LSTN-GDC-Ni 3-phase anode to improve the methane reforming reaction. The influence of Ni addition in LSTN-GDC anode on redox stability as well as the performance in H2 was also studied. 3-phase anode showed good performance in H2 and CH4 as well as good redox stability although high amount of Ni (~40 wt%) is in LSTN-GDC-Ni anode. In conclusion, La and Ni co-doped SrTiO3 material with Ni exsolution capability was investigated as an alternative anode for SOFC. The anodic performance was improved by controlling the anode microstructure, exsolution property, interfacial reaction and composite phase. From the results by stability of anode polarization resistance, power density measurement at 800oC in H2 and in CH4, and redox stability test at 600oC in H2, La and Ni co-doped SrTiO3-based anode can replace the conventional Ni-cermet anode.