Hydrogen Solubility and Atomic Structure of Graphene Supported Pd Nanoclusters.

We investigated the atomic structure of graphene supported Pd nanoclusters and their interaction with hydrogen up to atmospheric pressures at room temperature by surface X-ray diffraction and scanning tunneling microscopy. We find that Ir seeded Pd nanocluster superlattices with 1.2 nm cluster diameters can be grown on the graphene/Ir(111) moiré template with high structural perfection. The superlattice clusters are anchored through the rehybridized graphene to the Ir support, which superimposes a 2.0% inplane compression onto the clusters. During hydrogen exposure at 10 mbar pressure and room temperature, a significant part of the clusters gets unpinned from the superlattice. The clusters in registry undergo an out-of-plane expansion only, whereas the detached clusters expand in in- and out-of-plane directions. The formation of a hydrogen rich PdHx α' phase was not observed. After exposure to 1 bar, the majority of the clusters are unpinned from superlattice sites, due to their surface interaction with hydrogen and possible spill over to the graphene support. Only minor sintering was observed, which is more pronounced for the unpinned clusters. The results give evidence that ultrasmall Pd clusters on graphene are a stable hydrogen storage system with reduced hydrogen storage hysteresis and maintain a large surface area for hydrogen chemisorption.

Role of Precursor Carbides for Graphene Growth on Ni(111).

Surface X-ray Diffraction was used to study the transformation of a carbon-supersaturated carbidic precursor toward a complete single layer of graphene in the temperature region below 703 K without carbon supply from the gas phase. The excess carbon beyond the 0.45 monolayers of C atoms within a single Ni2C layer is accompanied by sharpened reflections of the |4772| superstructure, along with ring-like diffraction features resulting from non-coincidence rotated Ni2C-type domains. A dynamic Ni2C reordering process, accompanied by slow carbon loss to subsurface regions, is proposed to increase the Ni2C 2D carbide long-range order via ripening toward coherent domains, and to increase the local supersaturation of near-surface dissolved carbon required for spontaneous graphene nucleation and growth. Upon transformation, the intensities of the surface carbide reflections and of specific powder-like diffraction rings vanish. The associated change of the specular X-ray reflectivity allows to quantify a single, fully surface-covering layer of graphene (2 ML C) without diffraction contributions of rotated domains. The simultaneous presence of top-fcc and bridge-top configurations of graphene explains the crystal truncation rod data of the graphene-covered surface. Structure determination of the |4772| precursor surface-carbide using density functional theory is in perfect agreement with the experimentally derived X-ray structure factors.

Operando X-ray Investigation of Electrode/Electrolyte Interfaces in Model Solid Oxide Fuel Cells.

We employed operando anomalous surface X-ray diffraction to investigate the buried interface between the cathode and the electrolyte of a model solid oxide fuel cell with atomic resolution. The cell was studied under different oxygen pressures at elevated temperatures and polarizations by external potential control. Making use of anomalous X-ray diffraction effects at the Y and Zr K-edges allowed us to resolve the interfacial structure and chemical composition of a (100)-oriented, 9.5 mol % yttria-stabilized zirconia (YSZ) single crystal electrolyte below a La0.6Sr0.4CoO3-δ (LSC) electrode. We observe yttrium segregation toward the YSZ/LSC electrolyte/electrode interface under reducing conditions. Under oxidizing conditions, the interface becomes Y depleted. The yttrium segregation is corroborated by an enhanced outward relaxation of the YSZ interfacial metal ion layer. At the same time, an increase in point defect concentration in the electrolyte at the interface was observed, as evidenced by reduced YSZ crystallographic site occupancies for the cations as well as the oxygen ions. Such changes in composition are expected to strongly influence the oxygen ion transport through this interface which plays an important role for the performance of solid oxide fuel cells. The structure of the interface is compared to the bare YSZ(100) surface structure near the microelectrode under identical conditions and to the structure of the YSZ(100) surface prepared under ultrahigh vacuum conditions.