Electronic Absorption and Voltammetric Analysis of Ni(II) Coordination in the Room Temperature Ionic Liquid: 1-Ethyl-3-Methylimidizolium Chloride/Aluminum Chloride

Abstract

The coordination chemistry of Ni(II) in the ionic liquid aluminum chloride : 1-ethyl-3-methyl imidazolium chloride (EMIC) has been determined using visible absorption spectrophotometry. By dissolving nickel chloride (NiCl2) in Lewis acidic and basic ionic liquids and observing the resultant spectra and voltammetric information, one can determine the coordination and electron-transfer properties of Ni(II) at the various acidities. The optical data indicate the formation of tetrahedral NiCl42- in the ionic liquid with excess EMIC (basic melt, NAlCl3 < 0.5) with absorption maxima at 659 and 707 nm. In acidic melts (NAlCl3 > 0.5), the wavelength of maximum absorbance shifts to 462 nm. This dramatic change in the visible spectrum is indicative of a change in the coordination of Ni(II) from tetrahedral (basic) to octahedral (acidic) geometry, changes which are mirrored in the cyclic voltammetry data of the same samples. Studies in the neutral melt are also presented here.

Oxidation of Nickel in AlCl3-1-Butylpyridinium Chloride at Ambient Temperature

“…A strong anodic peak was observed at 1.17 V vs. Al/Al 3+ in the first sweep and weakly broad cathodic peaks were observed at around 0.5 and 0.2 V vs. Al/Al 3+ in the second and later sweeps. The peak at 1.17 V vs. Al/Al 3+ may be attributed to the nickel oxides, which were dissolved as [NiCl 6 ] 4− ions, 26 and the weak broad cathodic peaks at around 0.5 and 0.2 V vs. Al/Al 3+ are arrtibuted to Ni plating and under potential deposition of Al. 23,26 A new anodic peak appeared at around 1.0 V vs. Al/Al 3+ in the second and third sweeps.…”

“…The peak at 1.17 V vs. Al/Al 3+ may be attributed to the nickel oxides, which were dissolved as [NiCl 6 ] 4− ions, 26 and the weak broad cathodic peaks at around 0.5 and 0.2 V vs. Al/Al 3+ are arrtibuted to Ni plating and under potential deposition of Al. 23,26 A new anodic peak appeared at around 1.0 V vs. Al/Al 3+ in the second and third sweeps. We performed the following tests to reference the peak.…”

“…12,13 Gilbert et al investigated the electrochemical properties of nickel in molten salts consisting of AlCl 3 and KCl at 175-210 • C. 14 On the other hand, electrochemical properties for molten salts containing AlCl 3 and organic salts have also been widely studied. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] Such chloroaluminate salts vary their melting points, decreasing to lower temperature as they approach, by changing the ratio of AlCl 3 and organic salts, although their ion conductivity has been reported to be well maintained. Accordingly, many studies have been done on electrochemical properties for chloroaluminate salts with organic salts at around 100 • C or ambient temperature in view of application for batteries as well as electro-deposition of metals and sensors.…”

“…O'Grady et al also intensively investigated the electrochemical properties of NiCl 2 with cyclic voltammetry and other metals that were first dissolved in AlCl 3 -EMIC and then deposited on to the Pt plate at room temperature. [23][24][25][26] They analyzed the electrolyte after oxidizing nickel but did not identify the product of NiCl 2 on the Pt electrode. 23 Kohl et al also studied some metals behavior in the NaCl-buffered AlCl 3 -EMIC salt.…”

“…Few reports, however, have been published on the detailed results on electrochemical properties such as cyclic voltammograms depending on the molar ratio of AlCl 3 /organic salt. 23,26 Furthermore, few studies on oxidation of Ni have been reported in spite of several studies on reduction of NiCl 2 . 23 Specifically, no reports relate any identified products deposited on the electrode by oxidizing Ni.…”