Highly Active and Thermally Stable Core-Shell Catalysts for Solid Oxide Fuel Cells Ju-Sik Kim, a Noah L. Wieder, a Ashley J. Abraham, a Matteo Cargnello, b Paolo Fornasiero, b Raymond J. Gorte, a, * and John M. Vohs a, * ,z a Department of Chemical and Biomolecular Engineering University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA b Department of Chemical and Pharmaceutical Sciences, ICCOM-CNR, INSTM, Center of Excellence for Nanostructured Materials, University of Trieste, Trieste, Italy The effect of catalyst nanostructure on the performance of solid oxide fuel cell (SOFC) anodes prepared by infiltration of an elec- tronic conductor (45 wt % La 0.8 Sr 0.2 Cr 0.5 Mn 0.5 O 3 , LSCM) and a catalyst (1 wt % Pd and 9 wt % CeO 2 ) into porous yttria-stabi- lized zirconia (YSZ) scaffolds was examined. When Pd and CeO 2 were added by classical infiltration with nitrate salts, the initial electrode impedance in 97% H 2 -3% H 2 O at 973 K was 0.1 X cm 2 ; however, the impedance was found to increase significantly with time at 973 K and with heating to 1173 K. SEM images showed that the loss of performance coincided with a large increase in the size of the Pd crystallites. When Pd@CeO 2 dispersible core-shell structures obtained through self-assembly were infiltrated into the anode and used as the catalytic component, the initial performance was excellent and the activity was remarkably stable with time at 973 K and upon heating to 1173 K. The improved stability is shown to be the result of greatly suppressed particle-size growth for Pd@CeO 2 within the electrode structure. This study highlights the potential use of core-shell materials as stable struc- tures in various fields of materials science and heterogeneous catalysis. V C 2011 The Electrochemical Society. [DOI: 10.1149/1.3571039] All rights reserved. Manuscript submitted January 12, 2011; revised manuscript received March 3, 2011. Published March 31, 2011. Solid oxide fuel cells (SOFC) and electrolyzers (SOE) offer an attractive means for converting between electrical and chemical energy. Because they operate at high temperatures and are usually based on electrolytes that are oxygen-ion conducting ceramics, such as yttria-stabilized zirconia (YSZ), they are equally capable of con- verting between CO and CO 2 as H 2 and H 2 O. 1–4 When operated in the SOFC mode, they are also able to utilize hydrocarbon fuels so long as there are no materials within the anode that catalyze carbon formation. 5–7 Although Ni-based composites remain the standard for the fuel electrodes, there would be significant advantages to using electrodes based on ceramic conductors. 5,8–10 Oxides do not catalyze the for- mation of carbon fibers when exposed to hydrocarbons under reduc- ing conditions as do Ni and most other transition metals, 11–13 and they are expected to exhibit better redox stability. While one cannot use the same methods to achieve optimal electrode-electrolyte inter- faces with ceramic electrodes as one does with Ni-composite elec- trodes, recently developed ceramic electrode fabrication procedures that infiltrate the conductive ceramic components into a preexisting, porous scaffold of the electrolyte material have been shown to yield excellent performance. 14,15 Another difference between conductive oxides and Ni is that oxides do not exhibit the same high catalytic oxidation activity as Group VIII metals, like Ni. 15 Therefore, it is necessary to add dop- ant levels of catalytic metals to ceramic-based, fuel electrodes in order to achieve high performance. 14,15 Although there are some claims in the literature that certain perovskite-based ceramics when used in SOFC anodes exhibit high oxidation catalytic activity, 16,17 all of the examples of which we are aware that make this claim have used current collectors with high catalytic activity, such as Pt. Fur- thermore, Kim et al. 15 have reported that the maximum power den- sity of SOFC with La 0.8 Sr 0.2 Cr 0.5 Mn 0.5 O 3 (LSCM) anodes increased by a factor of 4 when a Ag paste (a material with low catalytic activ- ity) current collector was replaced with a Pt paste current collector; thus when evaluating the catalytic performance of ceramic-based anodes one must consider how the choice of current collector may influence the overall performance. In order for a catalytic metal to remain effective in only dopant amounts, the catalyst must remain well dispersed. This is a difficult to achieve in an SOFC, however, due to their high operating temper- atures which helps facilitate the tendency of transition metals to ag- glomerate into large particles in order to minimize their high surface energies. The use of large loadings of catalytic materials can be a solution when cheap transition metals are used, but this is not eco- nomically viable in the case of noble metals (such as Pt or Pd) which posses much higher oxidation activity. The primary goal of the work described here was to determine whether novel core-shell catalysts consisting of a nano-particle metal core surrounded by a porous oxide shell could be used to enhance the thermal stability of highly dispersed noble metal catalysts in ceramic-based SOFC ano- des using low loadings of the catalytic metals. Core-shell materials have received significant attention recently 18 as composites with novel catalytic properties. Much of this work has focused on using metal-metal core-shell systems com- posed of a metal nanoparticle encapsulated by a monolayer or multi- layer film of a second metal to alter catalytic properties. 19–28 Some attempts have also been made to use shells of thermally stable SiO 2 or other oxides to tailor catalytic properties and enhance thermal sta- bility. 24,29–32 This latter trait makes such materials attractive for use in SOFC electrodes where, as noted above, the low thermal stability of supported metal nanoparticles is problematic. The synthesis methods typically employed for metal-metal oxide core-shell catalysts involve the use of microemulsions which are not amenable to producing catalysts that can be easily dispersed into the pores of a highly porous support material. 24,32,33 Several of us, how- ever, have recently developed a method to produce easily dispersi- ble Pd@CeO 2 core-shell oxidation catalysts 34 based on self-assem- bly concepts. These Pd@CeO 2 catalysts consist of Pd nanoparticle cores (average diameter, 1.8 nm) encapsulated in a porous CeO 2 shell which is approximately 10 nm thick. In the work reported here we show that these Pd@CeO 2 catalysts can be easily incorporated into LSCM/YSZ composite SOFC anodes and allow the fine tuning of the amount of catalytic materials incorporated. Furthermore, core-shell structures significantly enhance the catalytic oxidation ac- tivity of these anodes, and exhibit unusually high thermal stability relative to traditional infiltrated Pd and CeO 2 catalysts. Experimental Synthesis of Pd@CeO 2 .— The synthesis of the Pd@CeO 2 core- shell structures is described in detail by Cargnello et al. 34 and involved initially synthesizing 11-mercaptoundecanoic acid (MUA) stabilized Pd nanoparticles in tetrahydrofuran (THF) solvent. The THF solution of MUA-Pd nanoparticles was then mixed with a * Electrochemical Society Active Member. z E-mail: vohs@seas.upenn.edu Journal of The Electrochemical Society, 158 (6) B596-B600 (2011) 0013-4651/2011/158(6)/B596/5/$28.00 V C The Electrochemical Society B596 Downloaded 04 Apr 2011 to 158.130.76.242. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp