Activation of H2 oxidation at sulphur-exposed Ni surfaces under low temperature SOFC conditions

Abstract

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 °C. However, they are expected to be increasingly degraded as the operating temperature is lowered in the presence of H₂S (5–10 ppm) in the H₂ fuel stream. However, at 500 to 600 °C, a temperature range rarely examined for sulphur poisoning, but of great interest for next generation SOFCs, we report that H₂S-exposed Ni–YSZ anodes are catalytic towards the H₂ oxidation reaction, rather than poisoned. By analogy with bulk Ni₃S₂/YSZ anodes, shown previously to enhance H₂ oxidation kinetics, it is proposed that a thin layer of Ni sulphide, akin to Ni₃S₂, is forming, at least at the triple point boundary (TPB) region under our conditions. To explain why Ni₃S₂/YSZ is so active, it is shown from density functional theory (DFT) calculations that the O²⁻ anions at the Ni₃S₂/YSZ TPB are more reactive towards hydrogen oxidation than is O²⁻ at the Ni/YSZ TPB. This is accounted for primarily by structural transformations of Ni₃S₂ during H₂ oxidation, rather than by the electronic properties of this interface. To understand why a thin layer of Ni₃S₂ 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 H₂ with O²⁻, forming water, prevents sulphur from re-equilibrating to H₂S. This may then promote Ni sulphide formation, at least in the TPB region.