Understanding the Structural Evolution of a Nickel Chalcogenide Electrocatalyst Surface for Water Oxidation

Recently, nickel-selenide- and -telluride-based electrocatalysts have shown promising results toward water electrolysis, exhibiting very low overpotential. However, a major challenge for these chalcogenide-based electrocatalysts has been correct identification of catalytically active species on the surface, with common concern being that the surface is totally being converted to nickel oxide, which becomes the true catalytically active species. In this Article, we have attempted to understand evolution of the active surface composition for nickel-selenide- and -telluride-based electrocatalysts by intentionally creating nickel-oxide-coated Ni₃Se₂ and Ni₃Te₂ surfaces and comparing their electrocatalytic activity with pristine and aged (subjected to a KOH electrolyte for an extended period) Ni₃Se₂ and Ni₃Te₂ surfaces, respectively. From such a comparison, it could be confirmed that catalytically active Ni₃Se₂ and Ni₃Te₂ surfaces were in fact stable in alkaline medium and were not coated with nickel oxide even after prolonged exposure to KOH under anodic potential. Rather, the active surface for these electrocatalysts can be described as a mixed anionic (hydroxo)chalcogenide surface. The nickel-oxide-coated nickel selenide and nickel telluride samples were prepared through electrodeposition and characterized with various bulk and surface analytical techniques such as powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), line scan analysis, and soft X-ray absorption spectroscopy (sXAS). The electrochemical properties of these oxide-coated chalcogenide surfaces were measured in 1 M KOH under an anodic potential scan and compared with that obtained from pristine nickel selenide and nickel telluride films. It was observed that the electrochemical properties were influenced by the coordinating anion composition and showed a significant difference between oxide, selenide, and telluride surfaces. More importantly, it also revealed that an oxide-coated chalcogenide surface showed a significantly different electrocatalytic response, indicating that electrochemical properties can be an appropriate tool for investigating change in the composition of the chalcogenide surfaces. This study provides conclusive evidence that surface of the nickel-selenide- and telluride-based OER electrocatalysts evolves into a mixed anionic (hydroxo)chalcogenide surface, retaining its predominant chalcogenide coordination.