Review pubs.acs.org/CR
Strategies for Carbon and Sulfur Tolerant Solid Oxide Fuel Cell Materials, Incorporating Lessons from Heterogeneous Catalysis Paul Boldrin,*,† Enrique Ruiz-Trejo,† Joshua Mermelstein,‡ José Miguel Bermúdez Menéndez,§ ́ Reina,∥ and Nigel P. Brandon† Tomás Ramırez †
Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, United Kingdom The Boeing Company, 5301 Bolsa Ave., Huntington Beach, CA 92647, United States § Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom ∥ Department of Chemical and Process Engineering, University of Surrey, Guildford GU2 7XH, United Kingdom ‡
ABSTRACT: Solid oxide fuel cells (SOFCs) are a rapidly emerging energy technology for a low carbon world, providing high efficiency, potential to use carbonaceous fuels, and compatibility with carbon capture and storage. However, current state-of-the-art materials have low tolerance to sulfur, a common contaminant of many fuels, and are vulnerable to deactivation due to carbon deposition when using carbon-containing compounds. In this review, we first study the theoretical basis behind carbon and sulfur poisoning, before examining the strategies toward carbon and sulfur tolerance used so far in the SOFC literature. We then study the more extensive relevant heterogeneous catalysis literature for strategies and materials which could be incorporated into carbon and sulfur tolerant fuel cells.
CONTENTS 1. Introduction to Solid Oxide Fuel Cells 2. Scope of the Review 3. Fundamentals of Carbon Poisoning 3.1. Theoretical Studies on Carbon Deposition in Catalysts and Fuel Cell Anodes 4. Fundamentals of Sulfur Poisoning 4.1. Theoretical Studies on Sulfur Poisoning of Catalysts and SOFC Anodes 5. Systems Approaches to Carbon and Sulfur Tolerance 6. Materials Design Strategies for Carbon Tolerance in SOFC Anodes 6.1. Ni/YSZ Cermets 6.2. Alloying with Noble Metals 6.3. Alloying or Replacement of Nickel with Base Metals 6.4. Replacement of Nickel with Nonmetal Electronic Conductors 6.5. Increasing Alkalinity 6.6. Use of Ceria and Other Oxygen Storage Materials 6.7. Replacement of Cermets with Mixed IonicElectronic Conductors (MIECs) 6.7.1. Single Phase MIECs 6.7.2. Addition of Catalytic Metal Nanoparticles to MIECs
© 2016 American Chemical Society
6.8. Regeneration of SOFC Anodes Deactivated by Carbon 7. Materials Design Strategies for Sulfur Tolerance in SOFC Anodes 7.1. Replacement of YSZ with Ceria 7.2. All-Ceramic Anodes 7.3. Alloying of Nickel with Other Metals 8. Strategies from Conventional Catalysis 8.1. Carbon Tolerance in Conventional Catalysis 8.1.1. Sulfur Passivation 8.1.2. Alloying and Bimetallic Systems 8.1.3. Promoters 8.1.4. Regeneration of Catalysts Deactivated by Carbon Deposition 8.2. Strategies against Sulfur Poisoning 8.2.1. Noble Metal-Based Catalysts 8.2.2. Alloys, Bimetallic, and Promoters 8.2.3. Support and Structural Modifications 8.2.4. Regeneration of Sulfur-Poisoned Catalysts 9. Conclusions and Perspectives 9.1. Alloying of Nickel 9.2. Alkaline Promoters and Supports 9.3. Ceria, Doped Ceria, and Oxygen Storage 9.4. Preferential Sulfur Binding Sites
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Received: May 4, 2016 Published: November 9, 2016 13633
DOI: 10.1021/acs.chemrev.6b00284 Chem. Rev. 2016, 116, 13633−13684
Chemical Reviews 9.5. Nonmetal Electronic Conductors 9.6. Infiltration of Nanoparticles 9.7. Regeneration 9.8. Theoretical and Computational Studies 9.9. Reflections on Experimental Work Author Information Corresponding Author Notes Biographies Acknowledgments References
Review
0.13%/1000 h,2 while Bloom Energy, based in California, has a commercially available SOFC capable of generating 100−200 kW aimed at the commercial market, especially data centers, with an installed base of over 30 MW.3 Figure 1 shows a diagram of a combined cycle SOFC with integrated gas turbine.
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1. INTRODUCTION TO SOLID OXIDE FUEL CELLS Solid oxide fuel cells (SOFCs) are electrochemical devices for the direct conversion of fuels into electricity. They operate by the conduction of oxide ions and are capable of using a wide variety of fuels, including hydrocarbons, syngas, biogas, and ammonia, as well as hydrogen. The oxidation of fuel takes place at the anode, which needs to be active for electrochemical oxidation of the fuel species and possess both electronic and ionic conductivity. Typically anodes are made either from ceramic-metallic composites (cermets), where each component provides one aspect of the conductivity, or from a mixed ionicelectronic conductor (MIEC), a ceramic which provides both ionic and electronic conductivity. There are a number of other properties that any materials to be used in SOFC anodes need to possess, including stability toward high temperatures and highly reducing conditions, chemical compatibility with other materials such as electrolytes and interconnect materials, and thermal expansion coefficients matched to the other components during operation and manufacture. The need for these properties places a limitation on which materials can be used. For example, there are materials with high ionic conductivity which are not stable in reducing atmospheres, or which have a large thermal expansion mismatch compared to common electrolyte materials. As well as the direct electrochemical oxidation of fuel species, other relevant reactions which take place in an SOFC anode are water−gas shift, steam reforming, dry reforming, Boudouard reaction, methanation, and hydrocarbon decomposition and cracking, among others. The development of SOFCs has reached an important phase, with rapid technological advancement over the past decade resulting in multiple programs run by governments and/or companies testing systems greater than 100 kW, and installed commercial products in the low kW range combined heat and power market. An initial understanding of the recent progress of multi-kW-scale SOFC development can be gained by studying the U.S. Department of Energy’s SOFC program (Solid State Conversion Alliance, SECA) which is interested in systems of 100 kW and upward operating on syngas from coal or natural gas. For the period of 2005−2007, the SOFC targets were for 1500 h tests on fuel cell stacks with performance degradation targets at steady state of 25 kW stacks with >4 years lifetime and degradation of
