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Releasing Metal Catalysts via Phase Transition: (NiO)0.05-(SrTi0.8Nb0.2O3)0.95 as A Redox Stable Anode Material for Solid Oxide Fuel Cells Guoliang Xiao, Siwei Wang, Ye Lin, Yanxiang Zhang, Ke An, and Fanglin Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/am5055417 • Publication Date (Web): 21 Oct 2014 Downloaded from http://pubs.acs.org on October 28, 2014
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ACS Applied Materials & Interfaces
Releasing Metal Catalysts via Phase Transition: (NiO)0.05-(SrTi0.8Nb0.2O3)0.95 as A Redox Stable Anode Material for Solid Oxide Fuel Cells Guoliang Xiao,a Siwei Wang,a Ye Lin,a Yanxiang Zhang,a,b Ke Anc and Fanglin Chen*a a
Department of Mechanical Engineering, University of South Carolina, Columbia, South
Carolina 29208, United States. Phone: +1 803 777 4875; E-mail:
[email protected] b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin,
Heilongjiang 150001, China c
Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak
Ridge, TN 37831, United States
ABSTRACT
Donor doped perovskite-type SrTiO3 experiences stoichiometry changes at high temperatures in different Po2 involving formation of Sr or Ti-rich impurities. NiO is incorporated into the stoichiometric strontium titanate, SrTi0.8Nb0.2O3-δ (STN), to form an A-site deficient perovskite material, (NiO)0.05-(SrTi0.8Nb0.2O3)0.95 (Ni-STN), for balancing the phase transition. Metallic Ni nano particles can be released upon reduction 1
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instead of forming undesired secondary phases. This material design introduces a simple catalytic modification method with good compositional control of the ceramic backbones, by which transport property and durability of solid oxide fuel cell anodes are largely determined. Using Ni-STN as anodes for solid oxide fuel cells, enhanced catalytic activity and remarkable stability in redox cycling have been achieved. Electrolytesupported cells with the cell configuration of Ni-STN-SDC anode, LSGM electrolyte and LSCF cathode produce peak power densities of 612, 794, and 922 mW cm-2 at 800, 850 and 900oC, respectively, using H2 as the fuel and air as the oxidant. Minor degradation in fuel cell performance resulted from redox cycling can be recovered upon operating the fuel cells in H2. Such property makes Ni-STN a promising regenerative anode candidate for solid oxide fuel cells.
Keywords: SOFC, catalysis, ceramics, electrode, nano particles, redox
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1. INTRODUCTION Solid oxide fuel cells (SOFCs) possess high conversion efficiency and fuel flexibility, and have been considered as one of the most versatile clean energy technologies1, 2. Conventional Ni-cermet anode materials for SOFCs have unique merits such as low cost, ease of processing and excellent electrochemical activity for typical fuels, but suffer from several instability issues3-5. Among them, the redox cycling instability, arisen from the substantial volume change of Ni, causes severe performance degradation and even failure of SOFCs, and redox cycling may be unavoidable during the long-term operation of SOFCs6. Considerable efforts have been devoted to solve such issue by developing alternative redox stable anode materials7. Although ceramic materials have been proposed to address the redox instability issues of SOFC anodes, their generally low catalytic activity for fuel oxidation significantly limits fuel cell performance compared to Ni-cermet anodes8-11. Additional catalyst loading through infiltration has been generally applied to enhance the anode catalytic activity12-16. Comparing with conventional deposition method, ceramic materials that can self-extract catalysts during operation become very attractive for their simplicity to be scaled up17-21. Recently, some other materials can also be designed to form catalyst particles by introducing large amounts of cation vacancy and reducing the ceramic phase stability22, 23. Some of these materials are summarized in Table 1. However, the major limitation for these materials is the less-controlled ceramic composition change. Extracting catalysts from the ceramic phase causes compositional change, by which the transport property and durability of the anode are largely determined24. In some cases, the ceramic phase gradually decomposes while generating metallic catalysts. It causes the 3
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catalyst particles being blocked from connecting to the conductive network and becoming electrochemically inactive. The cell performance may not be improved and may even degrade21,
23, 25
. Therefore, besides obtaining nano-catalysts, carefully tailoring the
composition of the reduced ceramics is essential for achieving active and stable performance in applications. However, so far little attention has been paid to the reduced ceramic phases in these material systems. Here we demonstrate a strategy to store Ni catalysts via phase transition of Nb doped SrTiO3. Nano-sized metallic catalysts are extracted under anode conditions while the ceramic composition gradually approaches the anticipated stoichiometry without forming undesirable secondary phases. Enhanced catalytic activity and remarkable stability in redox cycling have been achieved. Table 1 Ceramic materials self-extracting catalysts upon in situ reduction Catalysts (size), reduction temperature Ru (
