Quantitative Interpretation of K-Edge NEXAFS Data for Various Nickel Hydroxides and the Charged Nickel Electrode

Abstract

A quantitative interpretation of Ni K-edge NEXAFS spectral line shapes for β-Ni(OH)2, β-NiOOH, and BaNiO3 and of in situ data for a charged Ni electrode are reported. We have performed curve−wave multiple scattering calculations utilizing the FEFF6 code on clusters approximating these materials. These theoretical results reproduce the experimental line shape changes with oxidation remarkably well. Our interpretation of the NEXAFS line shape for the charged electrode indicates that large amounts of Ni coordinated with six oxygen atoms exist along with Ni coordinated with two OH and four oxygen atoms, giving an average oxidation state slightly greater than 3.5, consistent with that shown elsewhere from the edge shift data.

The importance of nickel oxyhydroxide deprotonation on its activity towards electrochemical water oxidation

“…It has been proposed that the formal oxidation state of nickel in the γ-NiOOH lies in the range 3.5–3.67,9,14 which suggests that some nickel sites in this compound have NiO 2 -like character that may be seen as tetravalent nickel sites; this hypothesis has been supported with X-ray adsorption spectroscopy (XAS), by matching the position of the Ni K-edge of the γ-NiOOH samples with the K-edge of reference compounds in which nickel was thought to be in the Ni IV state (BaNiO 3 or KNiIO 6 ) 15–17. However, the values reported for the oxidation state of nickel in those reference compounds did not consider the possibility of oxygen vacancies, which affect the formal valence of nickel in the compounds and make the conclusions derived from the XAS data uncertain 18,19…”

“…15 – 17 However, the values reported for the oxidation state of nickel in those reference compounds did not consider the possibility of oxygen vacancies, which affect the formal valence of nickel in the compounds and make the conclusions derived from the XAS data uncertain. 18 , 19 …”