Atomic structure of Ni-Nb-Y amorphous alloys and water-surface adsorption characteristics

S Grixti and S Yadav and S Thorpe and CV Singhab, COMPUTATIONAL MATERIALS SCIENCE, 169, UNSP 109095 (2019).

DOI: 10.1016/j.commatsci.2019.109095

Amorphous alloys are a unique class of materials, characterized by their lack of long-range structural ordering while retaining short range chemical ordering. The disordered surface structure of amorphous alloys facilitates a high density of uncoordinated sites, with unique surface chemistries and bond lengths that can render them highly electrocatalytically active. The Ni-Nb-Y system possesses the ability to phase separate into two amorphous phases, amorphous Ni60Nb40 and amorphous Ni57Y43. In this work the atomic structures of amorphous Ni60Nb40 and Ni57Y43 were derived and the binding mechanism of water on their respective surfaces was simulated using computational methods. The Ni60Nb40 amorphous structure was found to be dominated by disordered and pristine nickel centred icosahedral polyhedra, with the most abundant nickel centred Voronoi polyhedra being: <0,2,8,2>, <0,3,6,4>, <0,0,12,0> and <0,1,10,2>. The Ni(57)Y(43)amorphous structure was found to be very disordered, with only the nickel centred <0,3,6,0> Voronoi polyhedra occurring with an abundance greater than 10%. It was found that water binds to the surface of the Ni60Nb40 through the interaction of the oxygen of an intact water molecule with a surface niobium atom through a predominately ionic interaction. On the other hand, water dissociated upon adsorption into a hydroxide ion and lone hydrogen atom on the Ni57Y43 surface, over roughly three quarters of its area while it remained intact for the other quarter. It was found that the hydroxide ion binds preferentially to surface yttrium sites, while the lone hydrogen had a very high surface mobility and preferred to bind to nickel sites. The strong hybridization of the yttrium atoms with the hydroxide ion likely contributes to the greater electrocatalytic activity of the Ni57Y43 alloy in comparison to the Ni60Nb40 alloy as reported in experiments.

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