Corrosion resistance and catalytic activity toward the
oxygen reduction
reaction (ORR) in an alkaline environment are two key properties for
water recombination applications. In this work, CoCrFe
x
Ni (0 ≤ x ≤ 0.7) thin
films were deposited by magnetron sputtering on polished steel substrates.
The native passive layer was 2–4 nm thick and coherent to the
columnar grains determined by transmission electron microscopy. The
effect of Fe on the corrosion properties in 0.1 M NaCl and 1 M KOH
and the catalytic activity of the films toward ORR were investigated.
Electrochemical impedance spectroscopy and potentiodynamic polarization
measurements indicate that CoCrFe0.7Ni and CoCrFe0.3Ni have the highest corrosion resistance of the studied films in
NaCl and KOH, respectively. The high corrosion resistance of the CoCrFe0.7Ni film in NaCl was attributed to the smaller overall grain
size, which leads to a more homogeneous film with a stronger passive
layer. For CoCrFe0.3Ni in KOH, it was attributed to a lower
Fe dissolution into the electrolyte and the build-up of a thick and
protective hydroxide layer. Scanning Kelvin probe force microscopy
showed no potential differences globally in any of the films, but
locally, a potential gradient between the top of the columns and grain
boundaries was observed. Corrosion of the films was likely initiated
at the top of the columns where the potential was lowest. It was concluded
that Fe is essential for the electrochemical activation of the surfaces
and the catalytic activity toward ORR in an alkaline medium. The highest
catalytic activity was recorded for high Fe-content films (x ≥ 0.5) and was attributed to the formation of platelet-like
oxide particles on the film surface upon anodization. The study showed
that the combination of corrosion resistance and catalytic activity
toward ORR is possible for CoCrFe
x
Ni,
making this material system a suitable candidate for water recombination
in an alkaline environment.
A variety of bulk high-entropy alloy superconductors have been recently discovered; however, for thin films, only the TaNbHfZrTi high-entropy alloy system has been investigated for its superconducting properties. Here, (TiZrNbTa)1−xWx and (TiZrNbTa)1−xVx superconducting films have been produced by DC magnetron sputtering at different growth temperatures. The phase formation and superconducting behavior of these films depend on the content of alloying x and deposition temperature. A single body-centered cubic (bcc) phase can be formed in the low x range with enough driving energy for crystallinity, but phase transition between amorphous or two bcc structures is observed when increasing x. The highest superconducting transition temperature Tc reaches 8.0 K for the TiZrNbTa film. The superconducting transition temperature Tc of these films deposited at the same temperature decreases monotonically as a function of x. Increasing deposition temperature to 400 °C can enhance Tc for these films while retaining nearly equivalent compositions. Our experimental observations suggest that Tc of superconducting high entropy alloys relate to the atomic radii difference and electronegativity difference of involved elements beyond the valence electron number.
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