Sulfur Tolerance of La0.3M0.7Fe0.7Cr0.3O3
-δ (M= Sr, Ca) Solid Oxide Fuel Cell Anodes
Authors: Addo Paul Kwesi , Molero-Sanchez Beatriz , Buyukaksoy Aligul , Paulson Scott , Birss Viola 
Published: February 2020

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 (Sads) 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 La0.3M0.7Fe0.7Cr0.3O3-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 Gd0.9Ce0.1O2-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 La0.3M0.7Fe0.7Cr0.3O3-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 La0.3Sr0.7Fe0.7Cr0.3O3-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.

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