Search

Within the search for new catalysts for the oxygen reduction reaction in polymer electrolyte membrane fuel cells, alloys of Pt with other transition metals are of great interest due to their increased specific and especially mass activity. However, the drawback of these catalysts is their reduced stability due to the dissolution of the less-noble metal from the alloy. To resolve the potential dependence of these dissolution processes, we investigate a material library of Pt-Cu thin-film alloys with compositions ranging from 0 at% Cu up to 60 at% Cu. Utilizing our combinatorial scanning flow cell technique coupled to a mass spectrometer several aspects of dissolution are revealed. The onset of relevant Cu dissolution was found to be around 0.9 VRHE, independent of the composition. Although this is well below the onset potential of the Pt dissolution (1.15 VRHE), the two dissolution processes are clearly correlated, especially when the surface is already depleted of Cu. In contrast to Pt, however, Cu preferentially dissolves during anodic polarization rather than during the cathodic sweep. Additionally, at Cu compositions above the parting limit between 50 and 57 at% Cu a breakdown of passivity and massive Cu dissolution leads to porosity formation. The critical potential for the alloy with 57 at% Cu was detected around 1.3 VRHE, which is above the stability potential of Pt. While the absolute activity during the porosity formation increases due to the formation of more active sites in the pores, the specific activity decays to a value close to pure Pt.

A Nb-Ta thin film compositional spread obtained from a co-sputtering process was analysed. The microstructure and crystallographic investigations revealed the presence of a compositional threshold at Nb-60 at.%Ta where the change from tetragonal to cubic symmetry was evidenced by a mixed tetragonal-cubic phase. The electrochemical properties of the anodic oxides were studied via cyclic voltammetry and the oxide formation factors were mapped along the entire compositional spread. Values ranging from 1.8 nm·V −1 at the Ta-rich side to 2.6 nm·V −1 at the Nb-rich side of the library were measured. All Nb-Ta mixed anodic oxides were found to exhibit a type-n semiconducting behaviour as evidenced by Mott-Schottky analysis. The chemical composition of the surface anodic oxides differed from the composition of the parent metal alloys and no clear trend could be identified regarding their mismatch.

A wide-range thin film Hf–Nb combinatorial library deposited by co-sputtering is studied. The microstructure and crystallographic properties of the thin film alloys locally investigated by SEM and GIXRD are mapped along the entire compositional spread from 14 to 94 at.% Nb. Scanning droplet cell microscopy (SDCM) is used for mapping the electrochemical properties of the naturally oxidised metallic surfaces. Anodisation of the Hf–Nb thin films alloys is achieved with a high throughput due to computer-controlled scanning, made with a composition resolution of 1 at.%. The electrical properties of the anodic oxides are mapped by EIS and a maximum electrical permittivity close to 75 was found for Hf–33 at.% Nb. Semiconducting properties of the mixed anodic oxides are studied using Mott–Schottky analysis and their composition and mixing is investigated by XPS depth profiling.

A co-sputtering technique was used for the fabrication of a thin film combinatorial library (Hf–21 at.% Ta to 91 at.% Ta) based on alloying of Hf and Ta. The microstructure and crystallography of individual metallic alloy compositions were analyzed using SEM and XRD mapping, respectively. Three different zones of microstructure were identified within the range of alloys, going from hexagonal to tetragonal through an intermediate amorphous region. The local oxidation of Hf–Ta parent metal alloys at different compositions was investigated in steps of 1 at.% using an automated scanning droplet cell in the confined droplet mode. Potentiodynamic anodisation cycles combined with in situ impedance spectroscopy provide basic knowledge regarding the oxide formation and corresponding electrical properties. Dielectric constants were mapped for the entire composition range and XPS depth profiles allowed investigation of the oxide compositions.

Composition spread thin film samples of Nb and Ti were prepared by co-sputtering. The composition range from Nb–5 at.% Ti to Nb–78 at.% Ti was achieved and characterized by high resolution field emission scanning electron microscopy and grazing angle X-ray diffraction. Nb stabilized the β-Ti phase over the entire range studied. The structure was cubic with a continuous change in the lattice constants with composition. Several distinct compositional zones were identified in the as-deposited film morphology. Then, anodic oxides were grown potentiodynamically and characterized by electrochemical impedance spectroscopy using a scanning droplet cell to yield a comprehensive description of oxide film properties including dielectric permittivity, resistivity, thickness and film formation factor. Mott–Schottky analysis of potentiostatically grown oxides showed how the alloy composition influences the flat band potential and the donor density of the mixed n-type semiconducting oxides. Complementary X-ray photo electron spectroscopy as a chemical analysis revealed differences in the oxide compositions as compared to the as-deposited metal resulting from the different ion transport numbers.

A comprehensive study of the anodic oxide formation on Hf–Ti alloys over the entire range of composition was conducted. Combinatorial thin film libraries were prepared by co-sputtering. HRSEM and XRD were used to characterize the thin films and to confirm that a continuous change of the composition was obtained for this fully miscible system. Electrochemical investigations were performed by means of an automatic scanning droplet cell with a droplet radius of 100 μm. Subsequent potentiodynamic scans with intermittent impedance measurements allowed a quantitative determination of film formation factor and dielectric constant for each composition with a resolution of 0.5 at.%. Mott–Schottky analysis of the oxides was used to evaluate the relationship between parent metal composition and oxide properties, namely flat band potential and donor density. The structure–property and composition–property relationships are discussed in detail.