Hebert Molero

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

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 O(2-) anions at the Ni3S2/YSZ TPB are more reactive towards hydrogen oxidation than is O(2-) 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 O(2-), forming water, prevents sulphur from re-equilibrating to H2S. This may then promote Ni sulphide formation, at least in the TPB region.

The impact of fabrication conditions on the quality of Au nanoparticle arrays on dimpled Ta templates

Highly ordered dimpled Ta (DT) nanotemplates, prepared by electrochemical anodization of Ta, were recently reported to be ideally suited for the fabrication of a Au nanoparticle (NP) array using a Au thin film dewetting method. Here, we provide guidance and understanding of the effect of the DT fabrication and Au film deposition steps on the characteristics of the resulting NP array. Specifically, the optimum anodization time, voltage and solution composition are established, and the thickness of the sputter-deposited metal film is shown to be a very important parameter in achieving the desired single Au NP per dimple. The resulting high quality Au NP arrays are demonstrated to be electrochemically addressable, with the total Au surface area, measured electrochemically for large-scale samples, agreeing with the calculated area, based on scanning electron microscope determination of average particle shape and distribution. As the NP formation process proceeds via confined thin film dewetting, the protocol developed here should be applicable to the formation of NP arrays of a range of other metals and alloys.