Sulfur Tolerance of La0.3M0.7Fe0.7Cr0.3O3 -δ (M= Sr, Ca) Solid Oxide Fuel Cell Anodes

Solid oxide fuel cells (SOFCs) are highly efficient electrochemical devices that also demonstrate excellent fuel flexibility, functioning on fuels such as H₂, CO, methane. Traditionally, SOFCs are based on a Ni-YSZ (yttria stabilized zirconia) cermet anode, a YSZ electrolyte, and a lanthanum strontium manganite (LSM) cathode. Although Ni-YSZ cermets are excellent SOFC anodes, largely because of their  excellent catalytic activity towards fuel oxidation, work in our group and by others has shown that Ni is susceptible to  poisoning in low levels (1-100 ppm) of H₂S exposure at SOFC operating temperatures (700-1000 ^oC) [1-4]. It has been suggested that H₂S inhibits the H₂ oxidation reaction (HOR) rates because it readily dissociates to form a surface adsorbed Ni-S layer (S_ads) on catalytic sites normally involved in H₂ dissociation and subsequent oxidation [4], thereby decreasing the performance of the SOFC. As a result,  extensive research has been carried out to develop sulfur tolerant SOFC anode materials based on Ni-free conducting metal oxides, such as perovskites.

Previous and current studies in our group employing La_0.3M_0.7Fe_0.7Cr_0.3O_3-δ (M= Sr, Ca) perovskite oxides as both the anode and cathode electrode material, screen printed onto both sides of a YSZ electrolyte with a Gd_0.9Ce_0.1O_2-δ (GDC) buffer layer separating the electrodes and the YSZ, showed very good electrochemical performance in both H₂ and CO/CO₂ atmospheres in the absence of H₂S [5-6].  Therefore, in this work, extensive electrochemical studies, focusing on the performance of La_0.3M_0.7Fe_0.7Cr_0.3O_3-δ (M= Sr, Ca) in a symmetrical SOFC configuration and operating on H₂ and/or CO/CO₂ fuels containing varying concentrations of H₂S  (5- 30 ppm), were carried out at 500 - 800 ^oC. Also, the effect of the dc bias on the sulfur tolerance of these materials has been examined.

Our preliminary results have shown that La_0.3Sr_0.7Fe_0.7Cr_0.3O_3-δ  (LSFCr) electrodes, operated as an anode in humidified H₂ plus up to  9 ppm H₂S (balance in H₂)  at 800 ^oC showed no H₂S poisoning. The polarization resistance (Rp) observed for the cell in 30% humidified H₂ was 1.20 Ω.cm² at 800 ^oC and, with the addition of  9 ppm H₂S, there was no observed change in Rp. However, in dry CO/CO₂, the addition of H₂S showed a ca. 3% increase in Rp, but without any further degradation after longer term exposures to H₂S.  Importantly, when the H₂S was removed from this gas mixture, the performance recovered fully.

Acknowledgements – We are very grateful to the SOFC Canada NSERC Strategic Research Network, as well as Carbon Management Canada, for the support of this work.

References

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