3 research outputs found
Porous, robust highly conducting Ni-YSZ thin film anodes prepared by magnetron sputtering at oblique angles for application as anodes and buffer layers in solid oxide fuel cells
Uniform, highly porous, columnar thin films incorporating YSZ and NiO prepared by magnetron sputtering with deposition at glancing incidence exhibited stoichiometries close to that of the Y–Zr–Ni sputter target. Characterization by means of SEM, XRD, XPS and RBS revealed that the uniformly distributed nickel component in the as-deposited films consisted of NiO, and that the YSZ component was essentially amorphous. Annealing such films at 850 °C in hydrogen resulted in crystallization of the YSZ phase with preservation of the columnar morphology, while the NiO underwent reduction to metallic Ni, which partially segregated to the film surface. The hydrogen-annealed thin film anodes exhibited high conductivity, comparable to that of conventionally-prepared anodes, in both hydrogen and hydrogen/water mixtures at temperatures relevant to SOFC operation. They were also robust against strain-induced separation from the substrate under limited thermal cycling in both oxidizing and reducing atmospheres and are promising candidates for use as anodes in their own right and as strain-accommodating buffer layers between conventional anodes and the electrolyte for use in SOFC applications.Peer reviewe
Ternary Pt-Ru-Ni catalytic layers for methanol electrooxidation prepared by electrodeposition and galvanic replacement
Ternary Pt-Ru-Ni deposits on glassy carbon substrates, Pt-Ru(Ni)/GC, have been formed by initial electrodeposition of Ni layers onto glassy carbon electrodes, followed by their partial exchange for Pt and Ru, upon their immersion into equimolar solutions containing complex ions of the precious metals. The overall morphology and composition of the deposits has been studied by SEM microscopy and EDS spectroscopy. Continuous but nodular films have been confirmed, with a Pt÷Ru÷Ni % bulk atomic composition ratio of 37÷12÷51 (and for binary Pt-Ni control systems of 47÷53). Fine topographical details as well as film thickness have been directly recorded using AFM microscopy. The composition of the outer layers as well as the interactions of the three metals present have been studied by XPS spectroscopy and a Pt÷Ru÷Ni % surface atomic composition ratio of 61÷12÷27 (and for binary Pt-Ni control systems of 85÷15) has been found, indicating the enrichment of the outer layers in Pt; a shift of the Pt binding energy peaks to higher values was only observed in the presence of Ru and points to an electronic effect of Ru on Pt. The surface electrochemistry of the thus prepared Pt-Ru(Ni)/GC and Pt(Ni)/GC electrodes in deaerated acid solutions (studied by cyclic voltammetry) proves the existence of a shell consisting exclusively of Pt-Ru or Pt. The activity of the Pt-Ru(Ni) deposits towards methanol oxidation (studied by slow potential sweep voltammetry) is higher from that of the Pt(Ni) deposit and of pure Pt; this enhancement is attributed both to the well-known Ru synergistic effect due to the presence of its oxides but also (based on the XPS findings) to a modification effect of Pt electronic properties
Insights into the Surface Reactivity of Cermet and Perovskite Electrodes in Oxidizing, Reducing, and Humid Environments
Understanding
the surface chemistry of electrode materials under gas environments
is important in order to control their performance during electrochemical
and catalytic applications. This work compares the surface reactivity
of Ni/YSZ and La<sub>0.75</sub>Sr<sub>0.25</sub>Cr<sub>0.9</sub>Fe<sub>0.1</sub>O<sub>3</sub>, which are commonly used types of electrodes
in solid oxide electrochemical devices. In situ synchrotron-based
near-ambient pressure photoemission and absorption spectroscopy experiments,
assisted by theoretical spectral simulations and combined with microscopy
and electrochemical measurements, are used to monitor the effect of
the gas atmosphere on the chemical state, the morphology, and the
electrical conductivity of the electrodes. It is shown that the surface
of both electrode types readjusts fast to the reactive gas atmosphere
and their surface composition is notably modified. In the case of
Ni/YSZ, this is followed by evident changes in the oxidation state
of nickel, while for La<sub>0.75</sub>Sr<sub>0.25</sub>Cr<sub>0.9</sub>Fe<sub>0.1</sub>O<sub>3</sub>, a fine adjustment of the Cr valence
and strong Sr segregation is observed. An important difference between
the two electrodes is their capacity to maintain adsorbed hydroxyl
groups on their surface, which is expected to be critical for the
electrocatalytic properties of the materials. The insight gained from
the surface analysis may serve as a paradigm for understanding the
effect of the gas environment on the electrochemical performance and
the electrical conductivity of the electrodes