This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys., 2013, 15, 6769--6772 6769 Cite this: Phys. Chem. Chem. Phys., 2013, 15, 6769 Origin of electrolyte-dopant dependent sulfur poisoning of SOFC anodes† ZhenHua Zeng, a Mårten E. Bjo ¨rketun, a Sune Ebbesen, b Mogens B. Mogensen b and Jan Rossmeisl* a The mechanisms governing the sulfur poisoning of the triple phase boundary (TPB) of Ni–XSZ (X 2 O 3 stabilized zirconia) anodes have been investigated using density functional theory. The calculated sulfur adsorption energies reveal a clear correlation between the size of the cation dopant X 3+ and the sulfur tolerance of the Ni–XSZ anode; the smaller the ionic radius, the higher the sulfur tolerance. The mechanistic study shows that the size of X 3+ strongly influ- ences XSZ’s surface energy, which in turn determines the adhesion of Ni to XSZ. The Ni–XSZ interaction has a direct impact on the Ni–S interaction and on the relative stability of reconstructed and pristine Ni(100) facets at the TPB. Together, these two effects control the sulfur adsorption on the Ni atoms at the TPB. The established relationships explain experimentally observed dopant- dependent anode performances and provide a blueprint for the future search for and preparation of highly sulfur tolerant anodes. Sulfur poisoning is a serious problem in many important catalytic processes, e.g. in solid oxide fuel cell (SOFC) based power production, hydrogenation, methanation, Fischer– Tropsch synthesis, hydrocarbon steam reforming, and the water-gas shift reaction. 1–3 Hence, it has been the subject of extensive investigations in the past. The primary focus of these studies has been the poisoning of nickel metal catalysts by H 2 S, which is the main source of sulfur poisoning in the above catalytic processes. 4 Adsorption studies indicate that H 2 S adsorbs strongly and dissociatively on nickel surfaces. 5–7,25 This explains the severe poisoning problem in commonly used SOFC anodes. For example, at typical SOFC operating tempera- tures (>927 K), even trace amounts of H 2 S (a few ppm) severely limit the performance of Ni–YSZ (yttria stabilized zirconia) anodes. 8,9 However, the strong sulfur adsorption on nickel itself is not sufficient to explain why some anodes with other electrolyte components, e.g. Ni–ScSZ (Scandia stabilized zirconia), exhibit higher sulfur tolerance than Ni–YSZ anodes. 10–13 Appar- ently, the electrolyte component plays an important role, but the exact origin of the increased sulfur tolerance is still unclear. 8 Recently, it has been suggested that it may be related to the better size match between Sc 3+ and Zr 4+ (3% mismatch) than between Y 3+ and Zr 4+ (21% mismatch) in the electrolyte component. 14 Until now, to the best of our knowledge, however, no direct evidence in support of this hypothesis has been presented. In this communication we report density functional theory (DFT) data that not only disclose a correlation between the size of cation dopants X 3+ and the strength of the sulfur adsorption on Ni atoms at the triple phase boundary (TPB), but also reveal the mechanism behind this effect. Besides confirming the importance of the size match, possible routes for further improvement of the sulfur tolerance are proposed. An atomic-scale TPB model composed of a Ni nano-rod supported on a YSZ(111) or ScSZ(111) substrate has been used to model Ni–YSZ and Ni–ScSZ anodes (see Fig. 1 for Ni–YSZ and Fig. S1 (ESI†) for Ni–ScSZ, which is essentially identical to the Ni–YSZ model except for the optimized positions of Sc 3+ and O vacancies). This model closely resembles those used previously in DFT calculations on Ni–YSZ and Au–TiO 2 , 15,16 except that the metal nano-rod is wider in the present model. The dopant (Y 2 O 3 or Sc 2 O 3 ) concentration is fixed at 9% mol, which compares well with the normal experimental range of 8% to 10%. 8,10,14 We note that when modeling the electrochemical interface between an electron-conducting electrode (e.g. Ni) and an electrically insulating but ion-conducting electrolyte (e.g. YSZ) it is essential that the work function of the electrode is smaller than the ionization potential and larger than the electron affinity of the electrolyte. Only then can artificial charge transfer across the interface, due to DFT’s notorious underestimation of the band gaps of oxide electrolytes, be avoided. 17 We have checked that the present systems fulfill these requirements. Initially, the TPB contains two types of Ni facets: Ni(111) and Ni(100). However, the bare (100) facet is found to reconstruct into a (111) type upon geometric optimization. a Center for Atomic-scale Materials Design, Department of Physics, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark. E-mail: jross@fysik.dtu.dk b Department of Energy Conversion and Storage, DTU Risø Campus, Technical University of Denmark, P.O. Box 49, 4000 Roskilde, Denmark † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cp51099a Received 13th March 2013, Accepted 13th March 2013 DOI: 10.1039/c3cp51099a www.rsc.org/pccp PCCP COMMUNICATION