Understanding the Effect of Temperature on the Sulfur Tolerance of a Ca Rich Ferrite SOFC Electrode

Previously, we have shown that a symmetrical La0.3Ca0.7Fe0.7Cr0.3O3-δ (LCFCr) SOFC full cell (LCFCr used at both electrodes) exhibits very good electrochemical performance in H2 and CO at the fuel cell and also has good sulfur tolerance in wet H2 with a low ppm H2S content at 800 oC [1]. In fact, a somewhat enhanced performance of the LCFCr anode towards the hydrogen oxidation reaction was observed under these conditions [1]. However, the effect of temperature, which is known to have a significant impact on the sulfur tolerance of conventional Ni-based anodes [2-6], has not yet been studied. Therefore, in this work, both a symmetrical full cell and a three electrode half cell configuration, using LCFCr for all of the electrodes, were constructed and tested to better understand the sulfur tolerance of this Ca rich ferrite based perovskite in humidified H2 at temperatures ranging from 600-800 oC. Our preliminary potentiostatic data have shown that, in both the full and half-cell mode in low ppm H2S, the LCFCr electrode exhibits an enhancement in the hydrogen oxidation reaction (HOR) kinetics at 750-800 oC, while between 600 and 700 oC, the LCFCr electrode shows some form of deactivation. For instance, in the three electrode half-cell study at a constant anodic overpotential of 100 mV, the current density of the LCFCr electrode increased by 7-10% with the addition of 9 ppm H2S to 50% humidified H2 in the higher temperature range, while at 600 oC and 650 oC, the current density decreased by 2% and 15%, respectively. This deactivation was found to be fully reversible when H2S was removed from the gas stream, however, indicating that there was no permanent damage to either the electrodes, the interfaces, or the current collectors. Similarly, open circuit impedance data for the symmetrical full cell study showed that the polarization resistance (Rp) decreased from 1.0 to 0.95 Ωcm2 with the addition of 9 ppm H2S to 30% humidified H2 at 800 oC, while at 650 oC, Rp increased from 15.3 to 17.6 Ωcm2. To better understand these results, we are employing various surface characterization techniques, such as X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), to better understand the interaction between H2S and LCFCr, as well as the type of S species formed, over the full temperature range in low ppm H2S. References [1] P. K. Addo, B. Molero-Sánchez, A. Buyukaksoy, S. Paulson and V. Birss, ECS Trans., 66 (2), 219 (2015). [2] L. Debeeleeck et al, Phys.Chem.Chem.Phys., 16, 9383(2014). [3] J.-H. Wang, M. Liu, Electrochem. Commun., 9, 2212 (2007). [4] D.G. Ivey et al, J. Power Sources, 195, 6301 (2010). [5] C. Bartholomew, Appl. Catal. A: Gen., 212, 17 (2001).

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