Solid Oxide Fuel Cells (SOFCs) enable a highly efficient, environmentally-friendly power generation. One major problem of SOFCs working under real conditions are the sulfur compounds contained in almost all available fuels as natural gas, diesel reformate or gas from hydrothermal gasification of biomass (1),(2). If the sulfur is not properly removed by an upstream desulfurization unit even small amounts of sulfur in the ppm range affect the performance of SOFCs. Previous studies showed that the initial performance loss observed after adding sulfur to the fuel is related to a poisoning of the Ni-electrocatalyst in the Ni/YSZ-cermet anode.

In this contribution the impact of sulfur on the short term and long term behavior of Ni-based cermet anodes will be discussed. Sulfur species are chemisorbed on the Ni-surface (3), blocking the adsorption sites and the surface diffusion of reactants and reaction products. By means of electrochemical impedance spectroscopy and a subsequent analysis of the spectra by the distribution of relaxation times (4) two poisoning mechanisms could be decoupled (i) a deactivation of the catalytic watergas-shift-reaction (WGS) at the nickel surfaces and (ii) the electro-oxidation of hydrogen at the three phase boundaries (5),(6). The time constant of these mechanisms depends on the available Ni-surface area and the amount of H₂S supplied to the anode and is typically in the range of several hours. Furthermore a restructuring of the Ni-surfaces accompanied by an enhanced Ni agglomeration results in an enhanced aging in the longer term.

To improve the sulfur tolerance of SOFCs a cermet anode composed of Ni and ceria, a material combination well known as sulfur tolerant catalyst, can be applied. Impedance analysis of electrolyte supported cells exhibiting a Ni/ceria cermet anodes revealed a significantly improved sulfur tolerance. It will be shown that the impact of both poisoning mechanisms can be reduced. Cells with appropriate Ni/ceria anode layers showed the ability to convert carbon monoxide by a sufficiently fast WGS and electrooxidize hydrogen with a polarization resistance below 200 mΩ·cm².

References

(1) Y. Matsuzaki and I. Yasuda, Solid State Ionics, 132 (3-4), p. 261 (2000).

(2) K. Föger and K. Ahmed, Journal of Physical Chemistry B, 109 (6), p. 2149 (2005).

(3) C. H. Bartholomew, Applied Catalysis A: General, 212 (1), p. 17 (2001).

(4) H. Schichlein, A. C. Müller, M. Voigts, A. Krügel and E. Ivers-Tiffée, Journal of Applied Electrochemistry, 32 (8), p. 875 (2002).

(5) A. Kromp, S. Dierickx, A. Leonide, A. Weber and E. Ivers-Tiffée, J. Electrochem. Soc., 159 (5), p. B597 (2012).

(6) A. Weber, S. Dierickx, A. Kromp and E. Ivers-Tiffée, Fuel Cells, 13, p. 487 (2013).