SOFC anode

Solid oxide fuel cell, SOFC anodes, 97 catalysis in, 98,410 cathodes, 96... 

SOFC Anode, cathode, electrolyte Powder synthesis... 

In the search of high-performance SOFC anode, doped ceria have been evaluated as possible anode materials [9,10]. Comparing Ni-samaria-doped ceria (SDC) with Ni-YSZ, the Ni-SDC anode exhibits higher open-circuit voltages and a lower degree of polarization with either methanol as the fuel, as shown in Fig. 5, or methane as the fuel, as shown in Fig. 6. It was found that the depolarization ability of the anode is associated with the catalytic activity, the electrical conductivity, and the oxygen ionic conductivity of the anode materials [9]. It was also found that the anodic polarization and electro-catalytic activity strongly depend on the Ni content in the anode, and the optimum result for the Ni-SDC anode is achieved with 60... 

There are a number of informative reviews on anodes for SOFCs [1-5], providing details on processing, fabrication, characterization, and electrochemical behavior of anode materials, especially the nickel-yttria stabilized zirconia (Ni-YSZ) cermet anodes. There are also several reviews dedicated to specific topics such as oxide anode materials [6], carbon-tolerant anode materials [7-9], sulfur-tolerant anode materials [10], and the redox cycling behavior of Ni-YSZ cermet anodes [11], In this chapter, we do not attempt to offer a comprehensive survey of the literature on SOFC anode research instead, we focus primarily on some critical issues in the preparation and testing of SOFC anodes, including the processing-property relationships that are well accepted in the SOFC community as well as some apparently contradictory observations reported in the literature. We will also briefly review some recent advancement in the development of alternative anode materials for improved tolerance to sulfur poisoning and carbon deposition. 

The general requirements for an SOFC anode material include [1-3] good chemical and thermal stability during fuel cell fabrication and operation, high electronic conductivity under fuel cell operating conditions, excellent catalytic activity toward the oxidation of fuels, manageable mismatch in coefficient of thermal expansion (CTE) with adjacent cell components, sufficient mechanical strength and flexibility, ease of fabrication into desired microstructures (e.g., sufficient porosity and surface area), and low cost. Further, ionic conductivity would be beneficial to the extension of... 

Ni-YSZ cermet anodes satisfy most of the basic requirements for SOFC anodes. The effective conductivity of a Ni-YSZ cermet anode increases with the Ni to YSZ volume ratio, relative density, and decreasing the particle size ratio of NiO to YSZ. While coarse YSZ powders may result in poor mechanical strength and low stability, coarse NiO powders may lead to poor effective conductivity. The effective conductivity increases with the temperature at which the NiO is reduced to Ni metal in a reducing atmosphere. Further, very low reduction temperatures (e.g., below 400°C) may result in not only low electrical conductivity, but also poor mechanical strength. 

Lee J-H, Moon H, Lee H-W, Kim J, Kim J-D, and Yoon K-H. Quantitative analysis of microstructure and its related electrical property of SOFC anode, Ni-YSZ cermet. Solid State Ionics 2002 148 15-26. 

Yu JH, Park GW, Lee S, and Woo SK. Micro structural effects on the electrical and mechanical properties of Ni-YSZ cermet for SOFC anode. J Power Sources 2007 163 926-932. 

Matsuzaki Y and Yasuda I. The poisoning effect of sulfur-containing impurity gas on a SOFC anode part I. dependence on temperature, time, and impurity concentration. Solid State Ionics 2000 132 261-269. 

Marina O and Stevenson J. Development of ceramic composites as SOFC anodes. Presented at the SECA core technology program review meeting, Albany NY, September 2003, see the link at DOE SECA program website http //www.netl.doe. gov/publications/proceedings/03/seca-review/marina.pdf... 

Marina O and Stevenson J. SOFC Anode Materials Development at PNNL. In 2004 Office of Fossil Energy Fuel Cell Program Annual Report, Office of Fossil Energy, U.S. Department of Energy, 2004 90-92. 

Vashook V, Muller R, Zosel J, Ahlbom K, Ullmann H, and Guth U. Catalytic and electrical properties of SOFC anode material based on lanthan-chromite-titanate. Mat.-wiss. U. Werkstofftech. 2002 33 335-338. 

To meet the requirements for electronic conductivity in both the SOFC anode and cathode, a metallic electronic conductor, usually nickel, is typically used in the anode, and a conductive perovskite, such as lanthanum strontium manganite (LSM), is typically used in the cathode. Because the electrochemical reactions in fuel cell electrodes can only occur at surfaces where electronic and ionically conductive phases and the gas phase are in contact with each other (Figure 6.1), it is common... 

Suda S, Itagaki M, Node E, Takahashi S, Kawano M, Yoshida, H et al. Preparation of SOFC anode composites by spray pyrolysis. J. Eur. Ceram. Soc. 2006 26 593-597. 

Liu Y and Liu M. Porous SOFC anodes prepared by sublimation of an immiscible metal oxide during sintering. Electrochem. Solid State Lett. 2006 9 B25-B27. 

Gorte RJ, Kim H, and Vohs JM. Novel SOFC anodes for the direct electrochemical oxidation of hydrocarbon. J. Power Sources 2002 106 10-15. 

Gross MD, Vohs JM, and Gorte RJ. A study of thermal stability and methane tolerance of Cu-based SOFC anodes with electrodeposited Co. Electrochim. Acta 2007 52 1951-1957. 

La O GJ, Hertz J, Tuller H, and Shao-Hom Y. Microstructural features of RF-sputtered SOFC anode and electrolyte materials. J. Electroceram. 2004 13 691-695. 

Catalysis Issues in SOFC Catalytic processes play in important role in the operation of in particular, the SOFC anode and cathode, and the major challenges for the further development of SOFC towards a commercial project are largely related to improving catalytic activity and robustness. The critical technical issues for SOFC are reliability and lifetime, while economic requirements impose low manufacturing and materials costs. 

The main catalysis challenges related to SOFC anodes can be summarized as the specification of an anode material or mix of materials having the following properties ... 

O. Marina, M. Walker, J. Stevenson, Development of ceramic composites as SOFC anodes, Proceedings Fuel Cell Seminar, Miami, Florida, 2003. Courtesy Associates, Washington DC, USA (2003). S. Zha, W. Rauch, M. Liu, Solid State Ionics 166 (2004) 241. 

J. Sfeir, P A. Buffat, P. Mockli, N. Xanthopoulos, R. Vasquez, H. J. Mathieu, J. Van herle, and K. Ravindranathan Thampi, Lanthanum chromite based catalysts for oxidation of methane directly on SOFC anode, J. Catal. 202, 229-244 (2001). 

To provide some perspective on the viability of SOFCs in APU applications from a cost perspective, NETL sponsored an estimate of the cost structure of small-scale (5 kW), simple-cycle SOFC anode-supported system, operated on gasoline. The estimated manufacturing cost for such systems (see Figure 1-16) could well be close to that estimated for comparable PEM systems, while providing somewhat higher system efficiency. 

Figure 9. Temperature programmed oxidation (TPO) data showing CO2 evolution m/e = 44) of thermally deposited carbon from a Cu-Ce02-YSZ SOFC anode material after exposure to n-butane for 30 min (solid line) and a graphite powder sample (dashed line). (Reprinted with permission from ref 172. Copyright 2003 The Electrochemical Society, Inc.)...

The previous discussion has focused on the properties of perovskite materials rather than on their performance as anodes. The number of actual fuel-cell studies is more limited, but this literature has been reviewed recently by Irvine. Various perovskites have been investigated as potential SOFC anode materials however, these early efforts were hampered by low electrochemical activity toward methane oxidation,poor anode structure,or insufficient electrode conductivity. Most recently, Tao and Irvine demonstrated that an anode based on (Lao.75Sro.25)o.9Cro.5Mno.503 can provide reasonable power densities at 1173 K in 3% humidified CH4. Barnett and co-workers also reported stable power generation with methane and propane fuels on an anode based on LaCr03 however, they reported that the addition of Ni, in levels too small to affect the conductivity, was crucial in providing activity for the electrochemical oxidation reactions. 

Lehnert W., Meusinger J., Thom F., 2000. Modelling of gas transport phenomena in SOFC anodes. Journal of Power Sources 87, 57-63. 

Suwanwarangkul R., Croiset E., Fowler M.W., Douglas P.L., Entchev E., Douglas M.A., 2003. Performance comparison of Fick s, dusty-gas and Stefan-Maxwell models to predict the concentration overpotential of a SOFC anode. Journal of Power Sources 122, 9-18. 

To model direct internal reforming in SOFC stacks additional source terms need to be added to the specie and energy equations in addition to including the equations for the extra species. The source terms account for a set of chemical reactions, the chemical mechanism, that is assumed to occur. A simple and commonly used mechanism for steam methane reforming on SOFC anode is ... 

The IP-SOFC bundle is then the basic unit of the IP-SOFC stack (Figure 6.2), where several bundles are coupled to a reformer in which the fresh fuel (CH4) reacts with a part of the water-rich SOFC anode off-gas, in order to produce an H2 and CO-rich gas mixture. The fuel produced by the reformer is fed in parallel to the various electrochemical bundles. The oxidant stream crosses all the bundles of the stack in series. The hot oxidant stream exiting the last electrochemical bundle flows through the reformer as well, in order to provide the heat necessary for the endothermic reforming reaction. 

Tikekar, N. M., Armstrong, T. J., and Virkar, A. V. Reduction and Reoxidation Kinetics of Nickel-based SOFC Anodes, Journal of the Electrochem. Soc., 153, A654 (2006). 

A.-L. Sauvet, J. Fouletier, F. Gaillard, M. Primet, Doped lanthanum chromites as SOFC anode materials , Journal of Catalysis 209[1],25-34 (2002). 

SOFC cathode air stream co2 + h2o Combustion of remaining fuel in SOFC anode exhaust gas... 

Anode exhaust gas SOFC containing unconverted fuel h2o + co2 h2 Combustion of SOFC anode off-gas with cathode air stream (SOFC-GT concept). Recycling of fuel or for PEM, and so on. 

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