Abstract
High-entropy alloys (HEAs) have intriguing material properties, but their potential as catalysts has not been widely explored. Based on a concise theoretical model, we predict that the surface of a quaternary HEA of base metals, CoCrFeNi, should go from being nearly fully oxidized except for pure Ni sites when exposed to O2 to being partially oxidized in an acidic solution under cathodic bias, and that such a partially oxidized surface should be more active for the electrochemical hydrogen evolution reaction (HER) in acidic solutions than all the component metals. These predictions are confirmed by electrochemical and surface science experiments: the Ni in the HEA is found to be most resistant to oxidation, and when deployed in 0.5 M H2SO4, the HEA exhibits an overpotential of only 60 mV relative to Pt for the HER at a current density of 1 mA/cm2.
Highlights
Platinum-group metals are used in a variety of critical catalytic processes in petrochemical, automotive, pharmaceutical, and other industries
The properties of the bulk CoCrFeNi Highentropy alloys (HEAs), which crystalizes in an A1 face-centered cubic structure,[29] were calculated using two methods: (1) the Korringa−Kohn−Rostoker coherent potential approximation (KKR-CPA), which requires only one-atom unit cells for homogeneous fcc alloys, and (2) the plane-wave pseudopotential method as implemented in the Vienna Ab Initio Simulation Package (VASP),[30] which involves the use of large, explicit unit cells with an equimolar composition
The results suggest that the HEA surface should exist in a partially oxidized state when it is exposed to an acidic solution in the hydrogen evolution reaction (HER) potential regime
Summary
Platinum-group metals are used in a variety of critical catalytic processes in petrochemical, automotive, pharmaceutical, and other industries. We predict, through a theoretical analysis of the surface reactivity of this HEA using an approximate surface model[23,24] instead of a parameterization approach,[9,12,25,26] that one particular HEA, CoCrFeNi, shows activity for the electrochemical HER that is closer to that of Pt than all of the individual component metals. This is confirmed through electrochemical testing of the HEA in a 0.5 M aqueous solution of H2SO4. The surface oxidation behavior of the HEA is probed using X-ray photoelectron spectroscopy (XPS) and is likewise found to be consistent with our theoretical findings
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