pubs.acs.org/cm Published on Web 10/21/2009 r 2009 American Chemical Society Chem. Mater. 2010, 22, 957–965 957 DOI:10.1021/cm901875u Correlation of Fuel Cell Anode Electrocatalytic and ex situ Catalytic Activity of Perovskites La 0.75 Sr 0.25 Cr 0.5 X 0.5 O 3-δ (X = Ti, Mn, Fe, Co) † Nemanja Danilovic, ‡ Adrien Vincent, ‡ Jing-Li Luo,* ,‡ Karl T. Chuang, ‡ Rob Hui, § and Alan R. Sanger ‡ ‡ Department of Chemical and Materials Engineering, University of Alberta, ECERF Building 7-084H, 107-116 Street, Edmonton, AB, Canada T6G 2 V4, and § NRC Institute for Fuel Cell Innovation, 4250 Wesbrook Mall, Vancouver, B.C. V6T 1W5 Received June 29, 2009. Revised Manuscript Received September 25, 2009 The performance of a series of perovskite oxides having the mutual chemical formula and structure La 0.75 Sr 0.25 Cr 0.5 X 0.5 O 3-δ (X = Co, Fe, Ti, Mn) as solid oxide fuel cell anode electrocatalysts depends on the nature of the substituent element X. The electrocatalytic activity for methane oxidation in a fuel cell correlates well with ex-situ temperature programmed catalytic conversion of CH 4 , X = Co > Mn ∼ Fe > Ti, under temperature programmed reaction conditions in 5% CH 4 /He. The total conductivity of the materials in air decreases X = Co > Fe > Mn > Ti. Within the series of catalysts, the order of maximum fuel cell power density depended on feed: CH 4 , X = Fe > Mn > Ti; H 2 ,X= Fe > Mn > Ti; and 0.5% H 2 S/CH 4 , X = Ti > Fe > Mn. The Co-containing catalyst was unstable under reducing conditions. A process is proposed to explain the difference in catalyst order and enhanced activities in H 2 S/CH 4 as fuel compared to CH 4 alone. 1.0. Introduction Solid oxide fuel cells (SOFCs) are electrochemical devices that directly convert the chemical energy of a fuel into electrical energy. The overall reaction is separated into oxidation of a fuel on the anode catalyst and reduc- tion of oxygen at the cathode catalyst, separated by an impermeable oxygen ion conducting electrolyte. In prin- ciple, a variety of fuels can be used and, through selection of appropriate materials, there can be high tolerance to impurities in the feeds. However, in practice, pure hydro- gen is used to take advantage of the high catalytic reactivity of catalysts such as Ni. 1-3 However, H 2 fuel is produced from CH 4 and/or H 2 O, 4 and the preferred Ni catalysts are intolerant to even small amounts of S-containing impurities that may be present, thus requir- ing expensive purification of H 2 . Similarly, direct use of impure hydrocarbon fuels as feed over conventional Ni-Zr 0.92 Y 0.08 O 2 (YSZ, 8% ytrria stabi- lized zirconia) anode catalysts results in contamination (H 2 S poisoning) and degradation (carbon deposition). 5 A more economical and technically preferable alternative is a SOFC anode catalyst that can utilize CH 4 directly and which is tolerant of impurities such as H 2 S. 6,7 To operate efficiently for a commercially viable lifetime, such an anode catalyst must satisfy the following requirements: high CH 4 electro-oxidation activity, sulfur tolerance, coking resistance, redox stability, and high electronic and ionic conductivity. 7 Herein, we will show that the catalytic and electro- catalytic performance for conversion of H 2 , CH 4 , and CH 4 /0.5% H 2 S, of a series of catalysts having a common perovskite structure depends on the elemental composi- tion of perovskites La 0.75 Sr 0.25 Cr 0.5 X 0.5 O 3-δ (LSCX; X = Co, Fe, Ti, Mn), in which X occupies the B site. The series of perovskites having Cr in the B site was selected as it was expected that Cr would impart redox stability to all of the structures. 8-10 They are each mixed conductors, although they are primarily electronic con- ductors, hence they are not ideal for use as single phase electrode materials. To improve applicability, they can be used in a composite anode with an ionic conducting component (such as gadolinia doped ceria or YSZ). The LSCX perovskites containing Mn, Fe, and Ti are known, but have not been comparatively tested to estab- lish trends across the 3d-series transition metal cations † Accepted as part of the 2010 “Materials Chemistry of Energy Conversion Special Issue”. *E-mail: Jingli.Luo@ualberta.ca. Tel.: 1(780)492-2232. Fax: 1(780)492-2881. (1) Sun, C.; Stimming, U. J. Power Sources 2007, 171, 247. (2) Minh, N. Q. Solid State Ionics 2004, 174, 271. (3) Singhal, S. C., Kendall, K., Eds. High Temperature Solid Oxide Fuel Cells: Fundamentals, Design, and Applications; Elsevier: New York, 2003. (4) Edwards, P. P.; Kuznetsov, V. L.; David, W. I. F. Phil. Trans. R. Soc. A. 2007, 365, 1043. (5) Offer, G. J.; Mermelstein, J.; Brightman, E.; Brandon, N. P. J. Am. Ceram. Soc. 2009, 92, 763. (6) Gong, M.; Liu, X.; Trembly, J.; Johnson, C. J. Power Sources 2007, 168, 289. (7) Gross, M. D.; Vohs, J. M.; Gorte, R. J. J. Mater. Chem. 2007, 17, 3071. (8) Konysheva, E; Irvine, J. T. S. Chem. Mater. 2009, 21, 1514. (9) Sfeir, J. J. Power Sources 2003, 118, 276. (10) Nowotny, J., Sorrell, C. C., Eds. Key Engineering Materials; Trans Tech Publications: Switzerland, 1997; Vol. 125-126, p 187.