EXAFS of Nickel Oxide in the Ionic Liquid Emic/AlCl3
Authors: Roeper Donald , Cheek Graham T. , Pandya Kaumudi I. , O'Grady William E. 
Published: February 2020

Introduction

Earlier research has shown that nickel metal and nickel alloys can be deposited from nickel chloride dissolved in the IL AlCl3/EMIC but this depended on Lewis acidity of the IL which in turn affects the structure of the Ni ions that form in the solution (1, 2). The structures of several transition metal chlorides have been studied in situ in ILs using EXAFS (3-6). However, the oxides of these metals have not received as much attention. When working with the water and air sensitive ILs, there is always a concern about oxygen and moisture contamination and its effect on the properties of the IL. Many of the transition metal chlorides are studied in the anhydrous state and thus they are also sensitive to oxygen and moisture contamination in the IL. In this study, we examine the structure of the nickel ions that form in the AlCl3/EMIC IL when nickel(II) oxide is added to simulate the effect of oxygen contamination. The structure of NiO was studied with CVs and EXAFS in both Lewis acidic and Lewis basic AlCl3/EMIC ILs.

Experimental

Solutions were prepared in a Vacuum Atmosphere nitrogen filled drybox. Voltammetric experiments were carried out at ambient temperature. The ionic liquid solutions were sealed in 2 mil thick polyethylene bags and were examined with EXAFS. The electrochemical experiments were conducted with an EG&G PARC model 283 potentiostat controlled with the EG&G PARC PowerSuite software package.
The EXAFS experiments were conducted on beamline X-11A at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). All of the EXAFS data were measured at room temperature with the storage ring operating at 2.8 MeV and beam currents in the range of 150 to 260 mA. The EXAFS data were analyzed using the standard XDAP data analysis package.

Results and Discussion

Data were collected for NiO and anhydrous NiCl2 powder samples for reference standards to be used in the analysis of the IL samples. The Fourier transform for the NiO powder standard shows several shells. The first shell is the nickel - oxygen interaction and the second shell is the nickel - nickel interaction. The shells that are at greater distances are additional oxygen and nickel interactions, but they also include multiple scattering contributions. The Fourier transform of the NiCl2 powder standard shows two primary shells. The first shell is the nickel - chlorine interaction and the second shell is the nickel – nickel interaction.

The Fourier transform for NiO dissolved in the N=0.43 basic IL shows a single peak, indicating a single shell around the central nickel atom. The Fourier transform was phase-corrected with both the chloride shell from the NiCl2 standard, and oxide shell from the NiO standard, in order to determine which element was contributing to the backscattering. When the FT is phase-corrected with the correct backscattering atom, the peak in the imaginary part corresponds to the maximum of the positive peak in the magnitude of the FT. The phase corrected FT shows that the backscattering atoms must be primarily from a Ni–Cl interaction where the two peaks are mostly in phase.
Acknowledgements
The authors would like to acknowledge the financial support of the Naval Research Laboratory, the American Society for Engineering Education and the U.S Department of Energy. This research was carried out at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886.
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