2, Éole polytechnique Fédérale de Lausanne, Lausanne, , Switzerland
3, ZHAW, Winterthur, , Switzerland
Sulfur poisoning is a common problem for Ni catalysts and various industrial processes, including solid oxide fuel cells (SOFC). Especially in the case of biomass derived syngas feeds sulfur species like H2S are present in large concentrations and can cause irreversible damage to a SOFC anode. It is widely accepted that this is largely due to the poisoning of Ni sites active for the water gas shift reaction (WGS, CO + H2O → CO2 + H2), which provides additional H2 as fuel to the cell. Conventional Ni/Yttria-stabilized zirconia (Ni/YSZ) anode cermets cannot be regenerated from poisoning by reoxidation due to fatal anode fracturing. This work addresses anode regenerability by substituting the conventional cermet with a self-regenerating perovskite type metal oxide (PMO). The development of such a material can significantly increase overall device lifetime and reduce costs of feed gas desulfurization.
Some PMO compositions exhibit the remarkable reversible segregation of catalytically active metals from and back into the host lattices. This was found to be a highly efficient way to inhibit catalyst deactivation by metal particle sintering in alternating red/ox conditions [2-4]. A material was developed in this work which is completely regenerable from S-poisoning. Simultaneously, vital SOFC application targeted material requirements such as high ionic and electronic conductivity are fulfilled.
Phase pure materials were prepared by a citrate-gel method and characterized using X-ray diffraction (XRD), N2-physisorption and electron microscopy (SEM and STEM). La0.3Sr0.55Ti0.95Ni0.05O3-δ (LSTN) and impregnated 1.6 wt% Ni/La0.3Sr0.55TiO3-δ (Ni/LST) are compared to a conventional 50 wt% Ni/YSZ SOFC anode material. Ni reduction, segregation and reincorporation were followed by means of H2-temperature programmed reduction as well as XRD and X-ray absorption spectroscopy (XAS). Catalytic activity towards WGS was determined on powder samples. It is shown that at 800 °C Ni can be selectively reduced and segregated from the bulk LSTN to the material surface where it forms catalytically active, metallic Ni particles of few tens nm in size . After reoxidation Ni is fully reincorporated into the host lattice as shown by Ni K-edge XAS. Catalytic data obtained over a number of redox cycles and Ni particle size analysis show the excellent redox stability of this material and demonstrates inhibition of Ni particle sintering due to the reversible Ni reincorporation during oxidation. This is displayed by neither reference material (Ni/LST and Ni/YSZ). Catalytic activity after sulfur poisoning could be completely restored by two redox cycles thus demonstrating catalyst regenerability. Furthermore, the material was successfully implemented and tested in a redox stable SOFC anode.
1. Kuhn, J.N., et al., J. Mol. Catal. A 282 (2008)
2. Nishihata, Y., et al., Nature 418 (2002)
3. Steiger, P., et al., ChemSusChem 10 (2017)
4. Burnat, D., et al., A. J. Mater. Chem. A 4 (2016)