Activation of H sub(2) oxidation at sulphur-exposed Ni surfaces under low temperature SOFC conditions

• Published in: Physical chemistry chemical physics : PCCP 2014-04, Vol.16 (20), p.9383-9393
• Main Authors: Deleebeeck, Lisa, Shishkin, Maxim, Addo, Paul, Paulson, Scott, Molero, Hebert, Ziegler, Tom, Birss, Viola
• Format: Article
• Language: English
• Subjects: Anodes; Nickel; Oxidation; Solid oxide fuel cells; Sulfides; Sulfur; Sulphur; Yttria stabilized zirconia
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c3cp53377h

Ni-YSZ (yttria-stabilized zirconia) cermets are known to be very good anodes in solid oxide fuel cells (SOFCs), which are typically operated at 700-1000 degree C. However, they are expected to be increasingly degraded as the operating temperature is lowered in the presence of H sub(2)S (5-10 ppm) in the H sub(2) fuel stream. However, at 500 to 600 degree C, a temperature range rarely examined for sulphur poisoning, but of great interest for next generation SOFCs, we report that H sub(2)S-exposed Ni-YSZ anodes are catalytic towards the H sub(2) oxidation reaction, rather than poisoned. By analogy with bulk Ni sub(3)S sub(2)/YSZ anodes, shown previously to enhance H sub(2) oxidation kinetics, it is proposed that a thin layer of Ni sulphide, akin to Ni sub(3)S sub(2), is forming, at least at the triple point boundary (TPB) region under our conditions. To explain why Ni sub(3)S sub(2)/YSZ is so active, it is shown from density functional theory (DFT) calculations that the O super(2-) anions at the Ni sub(3)S sub(2)/YSZ TPB are more reactive towards hydrogen oxidation than is O super(2-) at the Ni/YSZ TPB. This is accounted for primarily by structural transformations of Ni sub(3)S sub(2) during H sub(2) oxidation, rather than by the electronic properties of this interface. To understand why a thin layer of Ni sub(3)S sub(2) could form when a single monolayer of sulphur on the Ni surface is the predicted surface phase under our conditions, it is possible that the reaction of H sub(2) with O super(2-), forming water, prevents sulphur from re-equilibrating to H sub(2)S. This may then promote Ni sulphide formation, at least in the TPB region.