The most common solid oxide fuel cell (SOFC) anode material, nickel - yttria stabilized zirconia (YSZ), displays excellent catalytic properties for fuel oxidation and good electronic conductivity. However, nickel-based anodes can cause carbon deposition when hydrocarbon fuels are used [1]. Nickel catalyzes the formation of carbon filaments in the presence of hydrocarbons under reducing conditions, resulting in coverage of the active sites of the anode, and potentially also to dissolution, precipitation, and metal dusting [2]. It is possible to avoid carbon deposition if sufficient steam is present such that the rate of carbon removal is comparable to or faster than the rate of carbon deposition. However, there is a corresponding decrease in electrical efficiency associated with dilution of the fuel. Metals with electronic structures similar to that of carbon may form an alloy with nickel at the surface, thus reducing carbide formation, and as a consequence, coking [3]. Tin, for example, has been reported to reduce carbon deposition on a Ni-YSZ cermet when present in low amounts such as 1 wt.% of tin with respect to nickel [4,5]. However, volatilization of a metallic tin phase at SOFC operating temperatures may present a problem for long term operation.

To increase the stability of tin-containing anodes, we fabricated Ni-Ni₃Sn-YSZ anodes using a hybrid solution-precursor / suspension plasma spray (SPPS-SPS) process and compared them to nickel-YSZ anodes made using the same process parameters. Thermogravimetric analyses in a 4% CH₄ (balance nitrogen) atmosphere showed that the presence of tin in the anode reduced the rate of carbon deposition. Furthermore, higher amounts of tin resulted in less coking.

Ni-Ni₃Sn-YSZ and Ni-YSZ anodes were incorporated into metal-supported SOFCs that consisted of a porous metal support (Sandvik Sanergy), samaria-doped ceria (SDC) barrier layer, anode as previously described, YSZ electrolyte, and La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ-SDC cathode. Cells were tested at 750°C with air as the oxidant and fuel consisting of 65% CH₄, 32% H₂, and 3% H₂O. The cells were held at a current density of 0.15 A·cm^-2 for 500 hours. The cell without tin in the anode began to degrade after approximately 200 hours and catastrophically failed at approximately 425 hours. After testing, we observed that the anode had disintegrated, and as a result, the electrolyte and cathode were completely detached from the metal support. On the other hand, the cell with an anode containing tin did not catastrophically fail, and the cell remained intact after 500 hours of operation. Over 500 hours, the cell potential degraded from 0.841 V to 0.743 V, and scanning electron micrographs showed that the anode had delaminated from the electrolyte in some areas. Nonetheless, the cell with the Ni-Ni₃Sn-YSZ anode degraded significantly less than the cell containing a Ni-YSZ anode, suggesting that the addition of Ni₃Sn to an anode could be a useful strategy for improving durability of SOFC anodes using hydrocarbon fuels.

References

1. S. P. Jiang and S. H. Chan, J. Mater. Science, 39, 4405 (2004).

2. A. Atkinson, S. Barnett, R.J. Gorte, J.T.S Irvine, A.J. McEvoy, M. Mogensen, S.C. Singhal and J. Vohs, Nature Materials, 3, 17 (2004).

3. D.L. Trimm, Catal. Today, 49, 3 (1999).

4. Y. Shiratori, Y. Teroka and K.Sasaki, Solid State Ionics, 177, 1371 (2006).

5. K. Tomishige, Y. Chen and K. Fujimoto, J. Catalysis, 181, 91 (1999).